NANOSIZED ALKALINE EARTH METAL TITANATES: EFFECTS OF SIZE ON by

NANOSIZED ALKALINE EARTH METAL TITANATES: EFFECTS OF SIZE ON
PHOTOCATALYTIC AND DIELECTRIC PROPERTIES
by
DMYTRO V. DEMYDOV
M.S., Ukrainian State University of Chemical Technology, Dnepropetrovsk, Ukraine, 1996
M.B.A., Ukrainian State University of Chemical Technology, Dnepropetrovsk, Ukraine, 1997
M.S., Pittsburg State University, Kansas, USA, 2002
AN ABSTRACT OF A DISSERTATION
submitted in partial fulfillment of the requirements for the degree
DOCTOR OF PHILOSOPHY
Department of Chemistry
College of Arts and Sciences
KANSAS STATE UNIVERSITY
Manhattan, Kansas
2006
ABSTRACT
A new approach to synthesize nanosized strontium titanate (SrTiO3) and barium titanate
(BaTiO3) has been developed. Nanocrystals of mixed metal oxide were synthesized by a
modified aerogel procedure from alkoxides.
The textural and surface characteristic properties were studied by nitrogen BET analysis,
transmission electron microscopy, and powder XRD. The crystallite sizes of aerogel prepared
powders can vary from 6 to 25 nm by the use of different solvents. A mixture of ethanol and
toluene was found to be the best binary solvent for supercritical drying, which produced a SrTiO3
sample with a surface area of 159 m2/g and an average crystallite size of 8 nm, and a BaTiO3
sample with a surface area of 175 m2/g and an average crystallite size of 6 nm.
These titanates have been studied for photocatalytic oxidation of volatile organic
compounds and acetaldehyde (CH3CHO) in particular. The big band gaps of the bulk (3.2 eV for
SrTiO3 and 3.1 eV for BaTiO3) limit their application to a UV light region only. The
modification of titanates by doping with transition metal ions (partial substitution of Ti ions with
metal ions) creates a valence band or electron donor level inside of the band gap, narrows it, and
increases the visible light absorption.
The enhanced adsorption of visible light was achieved by the synthesis of nanosized
SrTiO3 and BaTiO3 by incorporating Cr ions during the modified aerogel procedure. Gaseous
acetaldehyde photooxidation has been studied on pure SrTiO3 and BaTiO3, and on chromium
doped Cr-SrTiO3 and Cr-BaTiO3 under UV and visible light irradiation, and compared with the
photoactivity of P25 TiO2.
SrTiO3 doped with antimony/chromium shows absorption in visible light and show
photocatalytic activity for CH3CHO oxidation. The reason for the codoping of SrTiO3 with Sb/Cr
was to maintain the charge balance and to suppress oxygen defects in the lattice. This
photocatalyst shows high photoactivity under visible light irradiation even after several
continuous runs. The photoactivity under visible and UV light irradiation was almost identical
for the Sb/Cr-SrTiO3 photocatalyst.
Dielectric properties of aerogel prepared barium titanate samples have being studied and
the bulk resistance values of AP-BaTiO3 were significantly lower than that of commercial
BaTiO3, by several orders of magnitude.
NANOSIZED ALKALINE EARTH METAL TITANATES: EFFECTS OF SIZE ON
PHOTOCATALYTIC AND DIELECTRIC PROPERTIES
by
DMYTRO V. DEMYDOV
M.S., Ukrainian State University of Chemical Technology, Dnepropetrovsk, Ukraine, 1996
M.B.A., Ukrainian State University of Chemical Technology, Dnepropetrovsk, Ukraine, 1997
M.S., Pittsburg State University, Kansas, USA, 2002
A DISSERTATION
submitted in partial fulfillment of the requirements for the degree
DOCTOR OF PHILOSOPHY
Department of Chemistry
College of Arts and Sciences
KANSAS STATE UNIVERSITY
Manhattan, Kansas
2006
Approved by:
Major Professor
Kenneth J. Klabunde
ABSTRACT
A new approach to synthesize nanosized strontium titanate (SrTiO3) and barium titanate
(BaTiO3) has been developed. Nanocrystals of mixed metal oxide were synthesized by a
modified aerogel procedure from alkoxides.
The textural and surface characteristic properties were studied by nitrogen BET analysis,
transmission electron microscopy, and powder XRD. The crystallite sizes of aerogel prepared
powders can vary from 6 to 25 nm by the use of different solvents. A mixture of ethanol and
toluene was found to be the best binary solvent for supercritical drying, which produced a SrTiO3
sample with a surface area of 159 m2/g and an average crystallite size of 8 nm, and a BaTiO3
sample with a surface area of 175 m2/g and an average crystallite size of 6 nm.
These titanates have been studied for photocatalytic oxidation of volatile organic
compounds and acetaldehyde (CH3CHO) in particular. The big band gaps of the bulk (3.2 eV for
SrTiO3 and 3.1 eV for BaTiO3) limit their application to a UV light region only. The
modification of titanates by doping with transition metal ions (partial substitution of Ti ions with
metal ions) creates a valence band or electron donor level inside of the band gap, narrows it, and
increases the visible light absorption.
The enhanced adsorption of visible light was achieved by the synthesis of nanosized
SrTiO3 and BaTiO3 by incorporating Cr ions during the modified aerogel procedure. Gaseous
acetaldehyde photooxidation has been studied on pure SrTiO3 and BaTiO3, and on chromium
doped Cr-SrTiO3 and Cr-BaTiO3 under UV and visible light irradiation, and compared with the
photoactivity of P25 TiO2. SrTiO3 doped with antimony/chromium shows absorption in visible
light and show photocatalytic activity for CH3CHO oxidation. The reason for the codoping of
SrTiO3 with Sb/Cr was to maintain the charge balance and to suppress oxygen defects in the
lattice. This photocatalyst shows high photoactivity under visible light irradiation even after
several continuous runs. The photoactivity under visible and UV light irradiation was almost
identical for the Sb/Cr-SrTiO3 photocatalyst.
Dielectric properties of aerogel prepared barium titanate samples have being studied and the bulk
resistance values of AP-BaTiO3 were significantly lower than that of commercial BaTiO3, by
several orders of magnitude.
TABLE OF CONTENTS
TABLE OF CONTENTS ............................................................................................................v
LIST OF FIGURES ...................................................................................................................ix
LIST OF TABLES ...................................................................................................................xiv
ACKNOWLEDGEMENTS......................................................................................................xvi
DEDICATION...................................................................................................................... xviii
PREFACE................................................................................................................................xix
Chapter 1 : Introduction ..............................................................................................................1
1.1 Literature review on titanates.............................................................................................3
1.2 Advantages of nanosized titanates ...................................................................................10
1.3 Possible applications .......................................................................................................11
1.4 References.......................................................................................................................13
Chapter 2 : Synthesis of titanates...............................................................................................18
2.1 Introduction.....................................................................................................................18
2.2 Preparation of titanates ....................................................................................................18
2.2.1 Solid-state reaction ...................................................................................................19
2.2.2 Gas phase reaction ....................................................................................................21
2.2.3 Sol-gel technique ......................................................................................................29
2.2.4 Aerogel procedure ....................................................................................................32
2.3 Temperature treatment.....................................................................................................33
2.3.1 Heat treatment ..........................................................................................................35
2.3.2 Calcination ...............................................................................................................35
2.3.3 Drying ......................................................................................................................36
2.4 Conclusions.....................................................................................................................38
2.5 References.......................................................................................................................39
Chapter 3 : Characterization of titanates ....................................................................................41
3.1 Introduction.....................................................................................................................41
3.2 Synthesis of strontium and barium titanates .....................................................................41
3.2.1 Solid–state reaction...................................................................................................42
v
3.2.2 Modified aerogel procedure ......................................................................................44
3.3 Structural studies .............................................................................................................47
3.3.1 UV-visible spectroscopy ...........................................................................................47
3.3.2 Braunauer-Emmer-Teller analysis (BET)..................................................................50
3.3.3 Powder X-ray diffraction ..........................................................................................54
3.3.4 Transmission electron microscopy ............................................................................65
3.3.5 Elemental analysis ....................................................................................................71
3.3.6 Thermogravimetric analysis ......................................................................................71
3.4 Discussion .......................................................................................................................71
3.5 Conclusions.....................................................................................................................73
3.6 References.......................................................................................................................75
Chapter 4 : Photooxidation of acetaldehyde by titanates ............................................................76
4.1 Introduction.....................................................................................................................76
4.2 Photoactivity under light irradiation.................................................................................77
4.2.1 Design of photocatalysts of high activity...................................................................82
4.2.2 UV light irradiation...................................................................................................84
4.2.3 Visible light irradiation .............................................................................................87
4.3 Acetaldehyde photodecomposition studies.......................................................................88
4.3.1 Experimental setup for photodecomposition reactions...............................................90
4.3.2 Photoactivity of commercially available and synthesized samples.............................96
4.4 Conclusions...................................................................................................................102
4.5 References.....................................................................................................................103
Chapter 5 : Modification of titanates by doping .......................................................................106
5.1 Introduction...................................................................................................................106
5.2 Doping process..............................................................................................................108
5.2.1 Doping with transition metals in solid-state reaction ...............................................112
5.2.2 Doping with transition metals in aerogels................................................................112
5.3 Photoactivity of doped titanates .....................................................................................115
5.3.1 Cr doping and Sb/Cr codoping of aerogel prepared catalysts...................................115
5.3.2 Cr doping and Sb/Cr codoping of solid-state prepared samples ...............................134
5.3.3 Cr doping of SrTiO3 and BaTiO3 catalysts ..............................................................147
vi
5.4 Discussion and conclusions ...........................................................................................157
5.5 References.....................................................................................................................159
Chapter 6 : Surface studies of titanates by FTIR spectroscopy.................................................160
6.1 Introduction...................................................................................................................160
6.2 Acetaldehyde decomposition on the surface of aerogel prepared SrTiO3 ........................163
6.2.1 Acetaldehyde adsorption over SrTiO3 at 243 K: evidence of H-bonding .................165
6.2.2 Warm up effect of adsorbed acetaldehyde: evidence of aldol condensation and
formation of α, β-unsaturated aldehyde (crotonaldehyde, CH3-CH=CH-CHO) ................168
6.2.3 Dark oxidation: influence of dioxygen exposure over preadsorbed acetaldehyde at 243
K .....................................................................................................................................170
6.2.4 Spectral development during photooxidation reaction: influence of dioxygen exposure
over preadsorbed acetaldehyde at 243 K..........................................................................173
6.3 Acetaldehyde decomposition on the 2% Cr doped and 2.5% Sb/2% Cr codoped AP-SrTiO3
............................................................................................................................................176
6.3.1 Dehydroxylation on 2% Cr doped AP-SrTiO3 .........................................................176
6.3.2 Dehydroxylation on 2.5% Sb/2% Cr doped AP-SrTiO3 ...........................................177
6.3.3 Adsorption, evacuation, and warming prior to dark oxidation of acetaldehyde on 2%
Cr doped AP-SrTiO3........................................................................................................177
6.3.4 Adsorption, evacuation, and warming prior to dark oxidation of acetaldehyde on 2.5%
Sb/2% Cr codoped AP-SrTiO3 .........................................................................................181
6.3.5 Attempted dark oxidation of acetaldehyde on 2% Cr-SrTiO3 ...................................181
6.3.6 Attempted dark oxidation of acetaldehyde on 2.5% Sb/2% Cr codoped AP-SrTiO3.184
6.3.7 Photooxidation of acetaldehyde on 2% Cr doped AP-SrTiO3 ..................................184
6.3.8 Photooxidation of acetaldehyde on 2.5% Sb/2% Cr doped AP-SrTiO3 ....................187
6.4 Mass spectrometry studies on reaction products.............................................................187
6.5 Conclusions...................................................................................................................191
6.6 References.....................................................................................................................192
Chapter 7 : Dielectric studies on titanates ................................................................................193
7.1 Introduction...................................................................................................................193
7.2 Dielectric properties of titanates ....................................................................................193
7.3 Aerogels for electrical applications ................................................................................196
vii
7.4 Synthesis of Ba0.5Sr0.5TiO3 aerogel ................................................................................197
7.5 Dielectric measurements................................................................................................203
7.5.1 Impedance analysis .................................................................................................203
7.5.2 Raman spectroscopy ...............................................................................................210
7.6 Conclusions...................................................................................................................212
7.7 References.....................................................................................................................213
APPENDIX A: XRD analysis .................................................................................................215
APPENDIX B: Elemental analysis ..........................................................................................217
APPENDIX C: TGA analysis..................................................................................................218
APPENDIX D: in situ FTIR....................................................................................................220
APPENDIX E: Dielectric measurements .................................................................................223
APPENDIX F: Permission to reproduce materials ...................................................................226
viii
LIST OF FIGURES
Figure ‎1.1 Perovskite (ABO3) Unit Cell ......................................................................................3
Figure ‎1.2 Perovskite Structure (BO6 and A2+ Layers).................................................................4
Figure ‎2.1 Unit Cell of YBa2Cu3O7-x Oxide...............................................................................20
Figure ‎2.2 Principal Scheme for Chemical Transport (Adapted and Modified from Reference 5)
..........................................................................................................................................22
Figure ‎2.3 Principal Scheme of a CVD Reactor for Oxide Film Deposition (MFC – Mass Flow
Controller) (Adapted and Modified from Reference 5).......................................................25
Figure ‎2.4 Production Options for the Sol-gel Process (Adapted and Modified from Reference 5)
..........................................................................................................................................31
Figure ‎2.5 Sintering by diffusion (path 1 - surface diffusion, path 2 - volume diffusion)............34
Figure ‎2.6 Temperature-pressure Diagram for Supercritical Drying, where C - Critical Point,
SCF – Super Critical Fluid, Tc – Critical Temperature, Pc – Critical Pressure (Adapted and
Modified from Reference 5 and 13) ...................................................................................37
Figure ‎3.1 Solid-state Reactions for SrTiO3 and BaTiO3 Synthesis ............................................43
Figure ‎3.2 Modified Aerogel Procedure (MAP) from Alkoxides for SrTiO3 and BaTiO3
Synthesis ...........................................................................................................................46
Figure ‎3.3 UV-visible Spectra of TiO2 P25 Degussa and Aerogel Prepared SrTiO3 ...................48
Figure ‎3.4 UV-visible Spectra of Different SrTiO3 Samples ......................................................49
Figure ‎3.5 Five Types of Adsorption Isotherms [2] ...................................................................52
Figure ‎3.6 Powder XRD Patterns of Commercial and Synthesized SrTiO3 with Different
Alcohols Used in Synthesis (CM-SrTiO3 – Commercial, NCM-SrTiO3 – Commercial
Nanosized, AP-SrTiO3 – Aerogel Prepared Samples) ........................................................57
Figure ‎3.7 Powder XRD Patterns of Solid-state (SSR-SrTiO3) and Aerogel Prepared (APSrTiO3) Samples................................................................................................................58
Figure ‎3.8 Powder XRD Patterns of AP-SrTiO3 (Ethanol) Calcined in Air at Different
Temperatures.....................................................................................................................60
Figure ‎3.9 Powder XRD Patterns of Commercial and Synthesized BaTiO3 with Different
Alcohols Used in Synthesis (CM-BaTiO3 – Commercial, NCM-BaTiO3 – Commercial
ix
Nanosized, AP-BaTiO3 – Aerogel Prepared Samples). The XRD Studies of the Samples
were Conducted using a Shimadzu XRD 6000 (NanoScale Materials, Inc.) .......................61
Figure ‎3.10 Powder XRD Patterns of AP-BATiO3 (Ethanol) Calcined in Air at 500 °C with a
BaCO3 Phase vs. Freshly Prepared Sample of AP-BaTiO3 (Ethanol) .................................63
Figure ‎3.11 Calcination of AP-BaTiO3 (Ethanol) in Air and Oxygen at 500 °C..........................64
Figure ‎3.12 Transmission Electron Micrographs of SSR-SrTiO3 Prepared at 1100 °C................67
Figure ‎3.13 Transmission Electron Micrographs of AP-SrTiO3 after Synthesis (left) and after
Calcination in Air at 500 °C (right) ....................................................................................68
Figure ‎3.14 Transmission Electron Micrographs of SSR-BaTiO3 Prepared at 1100 °C...............69
Figure ‎3.15 Transmission Electron Micrographs of AP-BaTiO3 (Isopropanol) Samples after
Synthesis (left), Heat Treated in Vacuum at 500 °C (middle), and Calcined in Air at 500 °C
(right) ................................................................................................................................70
Figure ‎4.1 Semiconductor Photocatalyst for Water Photolysis ...................................................78
Figure ‎4.2 Semiconductor Oxide Band Gaps and Potentials ......................................................80
Figure ‎4.3 Photocatalytic Reactions: Photoinduced Reaction (down hill) and Photon Energy
Conversion Reaction (up hill) [17].....................................................................................81
Figure ‎4.4 Photocatalytic Oxidation of Various Organic Compounds on TiO2 Surface under UV
Light Irradiation [33].........................................................................................................84
Figure ‎4.5 Oxidation of Acetic Acid on the TIO2 under UV Irradiation and in Oxygen-free
Environment [33] ..............................................................................................................86
Figure ‎4.6 UV-visible Absorbance of Titanium Based Semiconductor Oxides...........................89
Figure ‎4.7 Experimental Setup for Photocatalysis Reactions......................................................92
Figure ‎4.8 Peak Areas versus Time for CH3CHO Decomposition and CO2 Evolution under UV
Light for AP-SrTiO3 ..........................................................................................................93
Figure ‎4.9 Concentration versus CH3CHO Decomposition and CO2 Evolution under UV Light
for AP-SrTiO3 ...................................................................................................................94
Figure ‎4.10 Photocatalytic Decomposition of Acetaldehyde under UV Light for P25 TiO2 ........95
Figure ‎4.11 CO2 Evolution under UV Light Irradiation for Different Catalyst Samples .............97
Figure ‎4.12 CH3CHO Decomposition under UV Light Irradiation for Different Catalyst Samples
..........................................................................................................................................98
Figure ‎4.13 Defuse Reflectance Spectra of Different SrTiO3 Samples .....................................100
x
Figure ‎5.1 Transition Metal Doping of UV Photocatalysts.......................................................107
Figure ‎5.2 UV-Visible Absorption Spectra of a) Pure TiO2 and b) –d) Cr Ion-implanted TiO2
with Cr of 2.2, 6.6, and 13 x10-7 mol/g [11] .....................................................................109
Figure ‎5.3 Diffuse Reflectance Spectra of Doped SrTiO3:M (0.5%) with a) Mn, b) Ru, c) Rh, d)
Pd, e) Ir, f) Pt [12] ...........................................................................................................110
Figure ‎5.4 Metal Doping of SrTiO3 Photocatalyst by Solid-State Reaction ..............................113
Figure ‎5.5 Metal Doping of SrTiO3 Photocatalyst by Modified Aerogel Procedure..................114
Figure ‎5.6 Powder XRD Patterns of Aerogel Prepared Strontium Titanate Samples (AP-SrTiO3 –
Pure, AP-2%Cr-SrTiO3 – 2% Chromium Ion Doped, AP-2.5%Sb/2%Cr-SrTiO3 – 2.5%
Antimony and 2% Chromium Ion Codoped)....................................................................117
Figure ‎5.7 Defuse Reflectance Spectra of Pure SrTiO3, TiO2 P25 Degussa, 2% Cr Doped and 2%
Cr/2.5% Sb Codoped Aerogel Prepared Strontium Titanate Samples ...............................119
Figure ‎5.8 Defuse Reflectance Spectra of 2% Cr Doped and 2% Cr/2.5% Sb Codoped Aerogel
Prepared Strontium Titanate Samples Freshly Prepared and Calcined at 773 K................120
Figure ‎5.9 Transmission Electron Micrographs of 2% Cr Doped (left) and 2% Cr/2.5% Sb
Codoped (right) Aerogel Prepared Strontium Titanate Samples .......................................122
Figure ‎5.10 UV and Visible Photoactivity of Cr-SrTiO3 Aerogel for CO2 Production..............125
Figure ‎5.11 UV and Visible Photoactivity of Sb/Cr-SrTiO3 Aerogel for CO2 Production.........126
Figure ‎5.12 UV and Visible Photoactivity of Cr-SrTiO3 Aerogel for CH3CHO Degradation....127
Figure ‎5.13 UV and Visible Photoactivity of Sb/Cr-SrTiO3 Aerogel for CH3CHO Degradation
........................................................................................................................................128
Figure ‎5.14 CO2 Evolution for Aerogel Prepared Catalysts under UV Light ............................129
Figure ‎5.15 CO2 Evolution for Aerogel Prepared Catalysts under Visible Light ......................130
Figure ‎5.16 CH3CHO Degradation for Aerogel Prepared Catalysts under UV Light ................131
Figure ‎5.17 CH3CHO Degradation for Aerogel Prepared Catalysts under Visible Light ..........132
Figure ‎5.18 2.5% Sb/2% Cr Codoped SrTiO3 Prepared by Solid-State Reaction and Aerogel
Modified Procedure.........................................................................................................135
Figure ‎5.19 Transmission Electron Micrographs of 2% Cr Doped (left) and 2% Cr/2.5% Sb
Codoped (right) Solid-State Prepared Strontium Titanate Samples...................................138
Figure ‎5.20 Diffuse Reflectance Spectra of 2.5% Sb/2% Cr SrTiO3 Prepared by Solid-State
Reaction ..........................................................................................................................139
xi
Figure ‎5.21 Diffuse Reflectance Spectra of 2%Cr SrTiO3 and 2.5% Sb/2% Cr SrTiO3 Prepared
by Solid-state Reaction and by Aerogel Modified Procedure ...........................................140
Figure ‎5.22 UV and Visible Photoactivity of Cr Doped SSR-SrTiO3 for CO2 Production ........143
Figure ‎5.23 UV and Visible Photoactivity of Cr/Sb Codoped SSR-SrTiO3 for CO2 Production144
Figure ‎5.24 UV and Visible Photoactivity of Cr Doped SSR-SrTiO3 for CH3CHO
Decomposition ................................................................................................................145
Figure ‎5.25 UV and Visible Photoactivity of Cr/Sb Codoped SSR-SrTiO3 for CH3CHO
Decomposition ................................................................................................................146
Figure ‎5.26 Diffuse Reflectance Spectra of Pure AP-BaTiO3 and 2%Cr Doped AP-SrTiO3 .....148
Figure ‎5.27 CO2 Evolution for Aerogel Prepared SrTiO3 and BaTiO3 Under UV Light Irradiation
........................................................................................................................................151
Figure ‎5.28 CH3CHO Degradation for Aerogel Prepared SrTiO3 and BaTiO3 Under UV Light
Irradiation........................................................................................................................152
Figure ‎5.29 CO2 Evolution for Aerogel Prepared SrTiO3 and BaTiO3 Under Visible Light
Irradiation........................................................................................................................153
Figure ‎5.30 CH3CHO Degradation for Aerogel Prepared SrTiO3 and BaTiO3 Under Visible Light
Irradiation........................................................................................................................154
Figure ‎5.31 CO2 Evolution for Aerogel Prepared SrTiO3 and BaTiO3 Under Light Irradiation
(UV or Visible Light) ......................................................................................................155
Figure ‎5.32 CH3CHO Degradation for Aerogel Prepared SrTiO3 and BaTiO3 Under Light
Irradiation (UV or Visible Light) .....................................................................................156
Figure ‎6.1 Acetaldehyde Reaction on CeO2 and M/CeO2 (M is Po, Co and Po-Co) [3]............161
Figure ‎6.2 Acetaldehyde Adsorption on AP-SrTiO3 and Sequential Change of Temperature ...166
Figure ‎6.3 Dark Oxidation of Acetaldehyde on AP-SrTiO3 Surface.........................................171
Figure ‎6.4 Kinetics Study of Acetaldehyde Dark Oxidation Using Oxygen .............................172
Figure ‎6.5 Spectral Development during Photooxidation of Acetaldehyde on the AP-SrTiO3
surface.............................................................................................................................174
Figure ‎6.6 Dehydroxylation as a Function of Temperature and Calcination on 2%Cr-SrTiO3
Nanoparticles ..................................................................................................................178
Figure ‎6.7 Dehydroxylation as a Function of Temperature and Calcination on 2.5%Sb/2%CrSrTiO3 Nanoparticles.......................................................................................................179
xii
Figure ‎6.8 Adsorption, Evacuation, and Warming prior to Dark Oxidation of Acetaldehyde on
2%Cr-SrTiO3 Nanoparticles ............................................................................................180
Figure ‎6.9 Adsorption, Evacuation, and Warming prior to Dark Oxidation of Acetaldehyde on
2.5%Sb/2%Cr-SrTiO3 Nanoparticles ...............................................................................182
Figure ‎6.10 Attempted Dark Oxidation of Acetaldehyde as a Function of Time at Constant
Temperature on 2%Cr-SrTiO3 Nanoparticles ...................................................................183
Figure ‎6.11 Dark Oxidation of Acetaldehyde as a Function of Time at Constant Temperature on
2.5%Sb/2%Cr-SrTiO3 Nanoparticles ...............................................................................185
Figure ‎6.12 Visible and UV-Visible Photooxidation of Acetaldehyde on 2%Cr-SrTiO3
Nanoparticles ..................................................................................................................186
Figure ‎6.13 Visible and UV-Visible Photooxidation of Acetaldehyde as a Function of Time on
2.5%Sb/2%Cr-SrTiO3 .....................................................................................................189
Figure ‎6.14 Temperature Programmed Desorption/ Mass Spectroscopy after Photooxidation of
Acetaldehyde on 2.5%Sb/2%Cr-SrTiO3 ...........................................................................190
Figure ‎7.1 BaTiO3 Structure....................................................................................................194
Figure ‎7.2 Modified Aerogel Procedure (MAP) from Alkoxides for Ba0.5Sr0.5TiO3 Synthesis..199
Figure ‎7.3 Modified Aerogel Procedure (MAP) from Alkoxides for Ba0.5Sr0.5TiO3 Synthesis..200
Figure ‎7.4 UV-visible Spectra of Different Aerogel Prepared Samples ....................................201
Figure ‎7.5 Transmission Electron Micrographs of AP-Ba0.5Sr0.5TiO3 Freshly Prepared (left and
middle) and Calcined in Oxygen at 500°C (right) ............................................................202
Figure ‎7.6 Powder XRD of NCM-BaTiO3 Powder and Powder of Crashed NCM-BaTiO3 Pellet
........................................................................................................................................205
Figure ‎7.7 Complex Impedance Spectrum of Aerogel Prepared AP-Ba0.5Sr0.5TiO3 at Room
Temperature ....................................................................................................................207
Figure ‎7.8 Resistance of Commercial BaTiO3 and Aerogel Prepared BaTiO3 and Ba0.5Sr0.5TiO3
........................................................................................................................................208
Figure ‎7.9 Relative Permittivity of Commercial BaTiO3 and Aerogel Prepared BaTiO3 and
Ba0.5Sr0.5TiO3 ..................................................................................................................209
Figure ‎7.10 Raman Spectra of AP-BaTiO3 and NCM-BaTiO3 .................................................211
Figure ‎7.11 Raman Spectra of BaTiO3 and Extraction of Soft Mode Frequencies [31].............212
xiii
LIST OF TABLES
Table ‎1.1 Wet Chemical Methods for the Synthesis of Mixed-Metal Oxides ...............................5
Table ‎2.1 CVD Related Methods [5] .........................................................................................26
Table ‎2.2 Oxide Powders Prepared by Aerosol Method [5] .......................................................27
Table ‎2.3 Critical Points of Some Solvents [13] ........................................................................37
Table ‎3.1 Characteristic Properties of Different SrTiO3 Samples ...............................................56
Table ‎3.2 Characteristic Properties of AP-SrTiO3 (Ethanol) Calcined at Different Temperatures
in Air.................................................................................................................................59
Table ‎3.3 Characteristic Properties of Different BaTiO3 Samples ..............................................62
Table ‎3.4 Characteristic Properties of AP-BaTiO3 (Ethanol) Calcined at Different Temperatures
in Air.................................................................................................................................65
Table ‎4.1 Rate Constant (k) for the Reaction of OH• Radical with Air Pollutants at 298 K [33] .85
Table ‎4.2 Initial Rates for CH3CHO Decomposition and CO2 Production under UV Light
Irradiation for the Different Catalyst Samples....................................................................96
Table ‎4.3 Initial Rates for CH3CHO Decomposition and CO2 Production under Visible Light
Irradiation for Different Catalyst Samples .......................................................................101
Table ‎5.1 Textural Properties of Photocatalysts .......................................................................121
Table ‎5.2 Initial Rates of Acetaldehyde Degradation and Carbon Dioxide Production .............123
Table ‎5.3 Photocatalysts for Acetaldehyde Degradation and Carbon Dioxide Production ........133
Table ‎5.4 Textural Properties of AP- and SSR-SrTiO3 Samples...............................................136
Table ‎5.5 Photocatalytic Properties of AP- and SSR-SrTiO3 Samples for Acetaldehyde
Decomposition ................................................................................................................142
Table ‎5.6 Photocatalytic Properties of Chromium Doped Aerogel Prepared and Solid-state
Prepared Strontium Titanate and Barium Titanate Catalysts.............................................149
Table ‎6.1 Vibrational Frequencies & Assignments of Adsorbed Acetaldehyde and Related
Surface Species ...............................................................................................................169
Table ‎6.2 Spectral Changes during the Photooxidation of Acetaldehyde..................................175
Table ‎6.3 Desorbed Species Identified by Mass Spectroscopy from the SrTiO3 Surface after
Photooxidation of Acetaldehyde ......................................................................................188
xiv
Table ‎7.1 Dielectric Constants of Titanates [7] ........................................................................195
Table ‎7.2 Textural properties of pelletized BaTiO3 ..................................................................204
xv
ACKNOWLEDGEMENTS
First of all, I would like to express my deep appreciation to my high school teacher of
chemistry, Lubov Shvedova. She opened my eyes to the beauty of chemistry and helped to
choose a career of chemist. Secondly, I would like to express my deep gratitude to my
undergraduate adviser at Ukrainian State University of Chemical Technology, Dr. Boris
Melnikov who introduced me to the world of nanomaterials and broadened my research interests
to include the synthesis of nanoparticles.
Thirdly and foremost, I would like to thank my doctorate advisor at Kansas State
University, Dr. Kenneth J. Klabunde, for the privilege of working in his research group. In this
group, I gained invaluable experience in the synthesis of different nanomaterials (metals, oxides,
sulfides, and nitrides) and familiarized myself with the equipment, methods used for synthesis,
and the characterization and applications of these materials. Under his guidance, I was given the
freedom to pursue my research in my own way and I greatly appreciated that freedom.
My colleagues in our research group, Dr. Alexander Bedilo, Dr. Jeevanandam Pethaiyan,
Dr. Uma Sitharaman, Dr. Ranjit Koodali, Dr. Shalini Rodriguez, Dr. Igor Martyanov, Dr Savka
Stoeva, Dr. Peter Stoimenov, Dr. Gavin Medine, Dr. David Heroux, Dr. Aldo Ponce, Alexander
Smetana, Johanna Haggstrom, Aaron Yang, Dambar Hamal, Luther Mahoney, Erin Beavers,
Sreeram Cingarapu and Kevin Quinn, are gratefully acknowledged for helpful discussions,
valuable suggestions, interesting collaborations, strong research group traditions, numerous
travels for conferences, hard working and pleasant times. Thank you for memorable graduate
school years.
I would also like to express extreme gratitude to Dr. Dilip K. Paul and students (Brian
and Owen) in his research group at Pittsburg State University for the intense collaboration on the
FTIR Surface studies.
The financial support provided by the Department of Defense through the MURI grant
and National Science Foundation are gratefully acknowledged. I would also like to express my
sincere appreciation to the Chemistry Department at KSU for their constant support and
resources provided.
xvi
I would like to express my appreciation to the members of my Dissertation Committee,
Dr. Kenneth J. Klabunde, Dr. Duy H. Hua, Dr. Stefan Bossmann, Dr. James H. Edgar, Dr. Daniel
A. Higgins, and Dr. Kevin Lease. I whould like to thank them for their service, for useful
suggestions and valuable discussions.
Last, but certainly not least, I would like to thank my family and friends for always
supporting me. You do not know how much your support means to me and without you standing
by me I would never have written this dissertation.
xvii
DEDICATION
To my beloved family and friends for always supporting, helping, and standing by me.
xviii
PREFACE
Minima maxima sunt.
The smallest things are most important.
Nanometer is 10-9 meter
xix
Chapter 1 : Introduction
The new developments in modern industries have generated an increasing need for new
types of materials. In this respect, the application of mixed metal oxides as catalytic,
photocatalytic, photoelectric, and dielectric materials has shown tremendous promise [1-4].
Metal oxides exhibit very diverse properties, including optical, electrical, magnetic and etc. They
have high hardness, high chemical resistance, and high thermal stability. These oxides have been
widely used as ceramics, electronics, catalysts, and coatings. For example, silicon oxide (SiO2) is
a widely used oxide material. It has found numerous applications because of its use as an optical
material, thermal and electrical insulator, and material with high hardness and chemical stability.
Other oxides can be used as semiconductor (TiO2) or superconductor (YBa2Cu3O7) materials.
Ferroelectric and dielectric oxides of perovskite structure (BaTiO3, PbTiO3) are widely used in
electronics. The perovskite lead zirconate titanate (PbZr1-xTixO3) is a ferroelectric and
piezoelectric material. SrTiO3 is used for high Curie temperature superconducting films.
Highly porous oxide materials like silica [5] and titania [6, 7] synthesized by sol gel
processing have also received much attention recently because of their application in low-loss
dielectrics, catalysis, filtering and photonics. High surface areas of the porous materials yield
good candidates for catalysis. These materials have very low density and can be used for lowloss electronic devices due to a very low dielectric constant.
Semiconducting oxides have band gaps in the UV light spectrum. Titanium oxide (TiO2)
is one of the most promising wide-band-gap (3.2 eV for anatase phase and 3.0 eV for rutile
phase) materials which has been used as a pigment because of its refractive index, absence of
absorption of visible light, stability, and nontoxicity [8]. Titania did not find applications for
structural ceramics due to its poor fracture toughness. However, it is widely used as the catalyst
support for different oxides such as molybdena, tungsta, and vanadia [9]. This catalyst has been
used for selective oxidation of hydrocarbons, decomposition of isopropanol, ammoxidation of
aromatic hydrocarbons, and reduction of nitric acid [9, 10].
UV light illumination of the TiO2 surfaces using light with higher energy than the TiO2 band
gap (shorter than 410 nm for rutile phase and shorter than 385 nm for the anatase phase) causes
the photoexitation of electrons from the valence band (O2- 2p-orbitals) to the unoccupied
1
conduction band (Ti4+ 3d-orbitals) [8]. Light irradiation causes a production of the highly active
holes and increases the photocatalytic and photooxidative activity of titanium oxide.
Some mixed-metal oxides (BaCeO3-δ, BaPb1-xBixO3-δ, CaTiO3)) can conduct ions and
transport oxygen owing to oxide ion mobility [11, 12]. They conduct electrons and oxide ions
and supply oxygen from the oxide to surface. This allows for oxidizing different hydrocarbons
(oxidative coupling of methane) [13, 14]. Different methods can be used for the synthesis of
metal oxides. The solid-state reaction or the ceramic method is the oldest and most commonly
used process for the synthesis of bulk oxide material, but chemical vapor deposition can be
applied for metal oxide film depositions on to different substrates. Finally, the aerosol process
gives highly dispersed metal oxide (TiO2, SiO2, and ZrO2) through controlled flame hydrolysis.
The conventional preparation of oxide powders is based on solid-state reactions and
requires repeated cycles of milling and calcination at high temperature. However, the powders
are agglomerated grains of different sizes and contaminated due to incomplete reactions. In
contrast to a solid-state method, wet chemical syntheses provide nanosized oxides of high purity.
Sol-gel processing is the most widely used route and involves a colloidal sol that is
converted into a gel through aging. The gel is subsequently calcined, giving rise to a crystalline
product with a homogeneous composition and a large surface area.
An aerogel method can be successfully applied for the production of nanosized metal
oxides of high purity [15]. This process involves the production of a gel of a three-dimensional
polymeric network from alkoxides. Solvent removal by supercritical drying prevents the collapse
of the network and preserves the unique properties of the product with a high porosity, small
crystallite sizes, and a large surface area. Different mixed oxide aerogels (ZrO2-MgO, Al2O3MgO, TiO2-MgO) have been synthesized [16].
Oxide nanomaterials have unique optical and electrical properties due to a very small size
of the particles and the quantum confinement phenomenon. Oxide nanomaterials are good
catalysts due to their high surface to volume ratio. Additionally, nanosized oxide powders are
used for the production of novel ceramics and composites.
2
1.1 Literature review on titanates
Metal and mixed-metal oxides are of extreme interest because of their unique properties
as well as already known industrial uses. They possess high mechanical, thermal, and chemical
stability. Some of them prepared by aerosol or aerogel methods have low densities and are
potential candidates for light-weight applications.
The application of mixed metal oxides as photoelectric, ferroelectric, piezoelectric, and
dielectric materials is an area of tremendous promise in research currently. Many of these oxides
have a perovskite structure and behave as semiconductors. Barium titanate (BaTiO3), lead
titanate (PbTiO3), lead zirconate titanate (PbZr1-xTixO3), and lead ferrate niobate (PbFe1-xNbxO3)
are used in multilayer capacitors [17]. Strontium (SrTiO3) is a well known photocatalyst for the
decomposition of some organic compounds and production of hydrogen by water splitting under
UV light irradiation [18, 19]. Some of them exhibit electro-optic or non-linear optical properties
[20].
Perovskites are solid mixed metal oxides with a general formula of ABO3 (Figure 1.1). In
these compounds, the A2+ metal ions occupy the corners of the rock salt structure and B4+ metal
ions form octahedral structure with oxygen anions (Figure 1.2).
Figure 1.1 Perovskite (ABO3) Unit Cell
3
High purity perovskite oxide powders, ABO3 (where A is an alkaline-earth metal and B is
a transition metal), are widely used for the preparation of dense ferroelectric, thin film electronic
components and electro-optical materials. Perovskites are also known to be good catalysts in
processes such as oxidation and hydrogenation [21, 22]. However, the small surface areas (≈ 1
m2/g) and the lack of homogeneity of solids, owing to solid-state reactions and to incomplete
reaction between the precursors, are not attractive for the preparation of catalysts.
Figure 1.2 Perovskite Structure (BO6 and A2+ Layers)
Many of the already mentioned perovskites belongs to the titanate compound family. The
work described herein will emphasize the synthesis, characterization and applications of
alkaline-earth metal titanates and in particular strontium titanate (SrTiO3), barium titanate
(BaTiO3), and strontium barium titanate (Sr1-xBaxTiO3).
Traditional preparation of titanates (MTiO3, where M is a bivalent metal) by solid-state
synthesis includes repeated grinding and calcination above 1000ºC of metal carbonate and
titanium oxide. Voorhoeve et al. synthesized a series of 20 perovskite oxides by solid-state
reaction [23]. The synthesis of these oxides is inexpensive, simple and easy to perform.
However, the prepared materials have very small surface areas of approximately 1 m2/g and lack
of homogeneity owing to incomplete reaction between solid precursors.
4
The preparation of barium titanate by solid-state reaction causes the formation of the
pyrochlore phase and grinding contaminates the sample with the grinding medium. These
impurities have a significant influence on the dielectric performance of this material [24]. High
sintering temperatures are required for capacitor production. The examined literature on the topic
shows that different research groups have attempted to synthesize titanate using other methods
and techniques to obtain perovskite structure phase at lower temperature, with higher purity and
smaller particle sizes. Some of those groups have used wet chemical methods for the synthesis of
mixed-metal oxides (Table 1.1) [25]. Oxides prepared by these methods have several significant
advantages over the traditionally solid-state prepared oxides. The wet chemically prepared
oxides have higher homogeneity, higher surface area, and relatively higher reactivity.
Table 1.1 Wet Chemical Methods for the Synthesis of Mixed-Metal Oxides
Process
Thermal Decomposition
Method
1.
Oxalate
2.
Citrate
3. Peroxide
4. Acetate
5. Complex Cyanide
Evaporative Decomposition
1. Spray Pyrolysis
2. Liquid Mix
1. Mixed Alkoxide
Sol-Gel Processing
2.
Carboxy-Alkoxide
3. Hydroxide-Alkoxide
Hydrothermal Synthesis
Hydroxides
5
The perovskite oxides prepared by coprecipitation methods have higher surface areas: up to 10
m2/g irrespectively of the precipitating agent [26].
The sol-gel process is the most widely used technique where gel is produced by the aging
of the colloidal sol. Mixed-metal oxides can be synthesized by the sol-gel technique using a
mixture of two metal alkoxides, a mixture of metal salt and metal alkoxide, or a mixture of metal
hydroxide and metal alkoxide. The final crystalline products are obtained by calcination of the
dried gels. These materials have homogeneous composition and high surface areas. These mixedmetal oxides can segregate during calcination to produce a mixture of two separate phases
(V2O5-TiO2, SiO2-TiO2, and Nb2O5-TiO2) or form one chemical phase (MgTiO3, CaTiO3,
SrTiO3, and BaTiO3). The alkaline-earth metal titanates do not segregate and consist of one
perovskite phase during synthesis or calcination.
Vanadia-titania mixed oxides were synthesized by the aerogel process with a nominal
loading of V2O5 up to 30 wt% and surface areas up to 200 m2/g [27]. The vanadium phase was
uniformly distributed in the titania aerogel. The advantages of this aerogel catalyst included a
higher possible loading of vanadia in comparison with traditional preparation methods and
higher reaction rates per catalyst weight.
Silica-titania aerogels also suffered from phase segregation [28]. The pore structure
consisted of mesopores and macropores with a pore size distribution up to 60 nm and specific
pore volume of 3.6 cm3/g. This oxide has interesting physical and chemical properties including
acoustic impedance matching, transparency, thermal insulation and low coefficients of thermal
expansion [29, 30].
Niobia-titania aerogel was synthesized by the hydrolysis of niobium (V) pentaethoxide
and titanium (IV) tetrabutoxide in methanol [31]. The prepared samples with low concentration
of titania were amorphous, while titania-rich samples segregated and contained poorly crystalline
anatase.
Metal titanates of magnesium (MgTiO3), calcium (CaTiO3), strontium (SrTiO3), and
barium (BaTiO3), were synthesized by the sol-gel technique by interaction of titanium (IV) nbutoxide (Ti(OC4H9)4) with a nitrate salt of Mg, Ca, Sr, or Ba metal (M(NO3)2) in alcoholic
solution at room temperature [32]. High-purity crystalline titanates were produced by calcination
of the obtained amorphous gels at temperatures 700 – 900ºC.The surface area for SrTiO3 was
40.8, BaTiO3 – 7.3, MgTiO3 – 2.2, and CaTiO3 – 0.9 m2/g respectively. Inhomogeneity in the
6
chemical composition of the titanates was caused by the difference in the precipitation speed of
the precursor salt and alkoxide.
Magnesium titanate has many interesting properties and can be used for many
applications. MgTiO3 is widely used in low cost capacitors, high-frequency capacitors and
temperature-compensating capacitors due to its dielectric properties [33]. Contrarily to other
alkaline-earth metal titanates, conventionally prepared magnesium titanate has an ilmenite
structure instead of the perovskite structure. Ilmenite can be transformed into the perovskite
structure at very high pressure (40GPa) [34]. In ilmenite, the MgTiO3 structure has octahedral
coordination of magnesium and titanium with oxygen atoms and consists of alternative layers of
magnesium and titanium octahedra. While in the perovskite structure, the structure consists of
the TiO6 octahedra layers and 12-fold coordinated Mg. The conventional magnesium titanate
synthesis includes the solid-state reaction between MgO and TiO2 at high temperature (1400ºC)
[35]. The obtained product powders have relatively large particle sizes and high concentration of
impurities contaminated by the grinding media and not-fully reacted precursors. Wet chemical
methods allow synthesizing more pure MgTiO3 powders at lower reaction temperatures. The
coprecipitation of Mg/Ti from the sulfuric solution by the reaction with sodium hydroxide at
room temperature and following calcination at 600ºC gave a pure MgTiO3 product [36]. In
addition, magnesium titanate powders were prepared by the peroxide method [37] and sol gel
process [38, 39].
Calcium titanate, with its perovskite structure, has interesting refractory, chemical
resistance and n-type semiconductor properties (band gap of 3.5 eV) [40, 41]. Traditionally,
calcium titanate is prepared by the solid-state reaction of calcium carbonate (CaCO3) and
titanium oxide at a temperature above 1300ºC [42]. The calcination at high temperature caused a
significant porosity decrease due to sintering.
Several wet chemical methods for the synthesis of CaTiO3 with lower temperatures are
known. Precipitated calcium titanyl oxalate was thermally degraded at 600ºC to produce calcium
titanate [43]. An aqueous slurry of calcium oxide and hydrated titanium gel was prepared at 150200ºC in an autoclave to produce fine particles of CaTiO3 consisting of 0.1-0.5 µm size
crystallites by the hydrothermal method [44]. High-purity CaTiO3 was synthesized using the
peroxide method [45, 46]. Calcium chloride (CaCl2), titanium chloride (TiCl4), hydrogen
peroxide (H2O2) and ammonia (NH4OH) were mixed together to prepare the CaTiO2(O2)·3H2O
7
precursor which gave CaTiO3 powder during calcination at 800ºC. It was possible to use titanium
tetraisopropoxide instead of TiCl4 [47]. Similar peroxide methods were used to synthesize
BaTiO3 and SrTiO3 powders [48, 49].
Mixed calcium strontium titanate (Ca1-xSrxTiO3) was prepared by the Pechini method
using calcium and strontium carbonates, tetraisopropyl orthotitanate, ethylene glycol, and citric
acid [50]. The solvent was evaporated first, and the sample was calcined at 850ºC.
CaTiO3-based oxides (Ca1-xSrxTi1-yMyO3-δ, M=Fe, Co, Cr or Ni, x = 0-1, y = 0-0.6) were
synthesized by a modified sol-gel method using citrate [51]. Substitution with 10% Sr for Casites in calcium titanate and substitution of Ti-sites with lower valent metal ions of iron, cobalt,
chromium, or nickel, were performed to increase the electrical and ionic conductivity. This
method still requires high temperature of 900ºC and gives a powder with small surface area of 20
m2/g and big crystallite sizes.
Calcium titanate powder was studied and found active for the decomposition of water
into hydrogen and oxygen under UV light irradiation [52]. This powder was prepared by the
hydrothermal method. Particles were in the range of 0.1 - 0.4 µm and surface area was 11 m2/g.
Besides, powder was impregnated with noble metal (Pt, Au, Ir, or Ru) co-catalyst to separate the
holes and electrons and increase production of H2 and O2.
Strontium titanate is a perovskite with a cubic structure whose dielectric constant
increases upon cooling [53]. Non-stoichiometric or doped n-type semiconducting SrTiO3 is used
as a dielectric and photoelectric material [54]. Conventionally, strontium titanate is synthesized
by the solid-state reaction between strontium carbonate and titanium oxide at temperatures above
1000°C [55].
Wet chemical methods give pure strontium titanate phase at lower temperatures. SrTiO3
was prepared by the thermal decomposition of SrTiO(C2O4)2⋅4H2O precursor [56] or controlled
hydrolysis of strontium titanium carboxylates [57].
Application of the sol-gel process gives high-purity, homogeneous, stoichiometric and
ultrafine strontium titanate [58]. Strontium acetate and titanium isopropoxide were hydrolyzed to
form a gel which was dried at 110ºC first and then calcined at 900ºC. After calcination, particles
sizes were up to 500 nm and surface area of 20 m2/g.
8
Nanosized powders of SrTiO3 with the size of crystallites in the range of 5 - 13 nm were
synthesized by gel to crystallite conversion [59]. The prepared material was found
photocatalytically active for the mineralization of phenol.
Barium titanate is a dielectric material with good dielectric properties such as high
dielectric constant, small dielectric loss, low leakage current, and large dielectric breakdown
strength. It is a ferroelectric perovskite with Curie temperature Tc at 123ºC [60]. In this material,
the ferroelectric and dielectric properties critically depend on the particle sizes [61, 62]. Different
methods have been used for the barium titanate synthesis including hydrothermal [63],
microemulsion [64], hydrolytic decomposition [65], precipitation from solutions [66], gascondensation process [67], and sol-gel methods [68].
Barium titanate particles in the range from 10 nm to 1.5 µm were prepared by thermal
decomposition of polymeric barium titanate methacrylate [70]. The precursor was obtained from
the reaction of metallic barium, titanium isopropoxide and methacrylic acid in boiling methanol.
The BaTiO3 product was obtained by the pyrolysis of the precursor at different temperatures and
oxidative atmospheres. High temperature (1350ºC) and oxidative atmosphere (O2) causes
production of big particles, while at lower temperature (600ºC) and inert atmosphere (N2)
smaller particle sizes were obtained.
Mixed strontium barium titanate was synthesized using a spark plasma sintering
technique from a mixture of barium and titanium titanates [71]. The Ba0.6Sr0.4TiO3 structure
consisted of grain size of 1-3 µm surrounded by submicron barium titanate and strontium titanate
grains. This technique gives ceramics with reduced dielectric losses and a significant diffuse
transition in permittivity. However, it gives inhomogeneous composition and requires high
pressure (50 MPa) and temperature (1100°C).
Also, barium strontium titanate (BST) was synthesized by a sol-gel technique to give
BaxSr1-xTiO3 in the range of x = 0 to 1.0 [72]. The synthesis includes drying of the mixture of the
barium acetate, strontium acetate, and titanium isopropoxide in dehydrated acetic acid at 90°C.
After that, the white solid was ground and calcined at 1000°C.
9
1.2 Advantages of nanosized titanates
The unique properties of nanosized materials are determined by surface structures and
sizes. High surface areas and small sizes can lead to unexpectedly new or different properties of
the material [72, 73]. Nanomaterials are defined as nanostructured or nanoparticulate materials
with sizes up to 100 nm. These materials are intermediates between single atoms and crystalline
solids, single elements and single crystalline structures.
Currently, researchers in chemistry, physics, biochemistry, and engineering are exploring
a new class of nanosized materials for applications in electronics, optics, catalysis, and for solar
light conversion [74]. Size effects and processes for the synthesis of solids with desired size,
shape and surface structure are currently being examined. For now, a growing knowledge in
nanocrystals research allows the production of different materials at the nanometer scale, a better
understanding of their properties, and control of their synthesis.
The novel and unique properties of nanomaterials are a result of the finite-size effect
when the electronic bands are converted into molecular orbitals with the decrease in particle
sizes. In nanoparticles, a high number of particle atoms are located on the surface of the grains or
interface boundaries, which is why the physical and chemical properties of nanomaterials are
different from the isolated atoms or bulk materials.
One of the specific properties of nanomaterials is the surface-to-volume ratio. This sizedependent effect includes a high percentage of surface atoms. The high distribution of electrons
confines the quantized energy levels. This quantum confinement is used in semiconductors and
optoelectronics. A high percentage of surface atoms lead to an increase in surface activity. In
addition, the highly exposed surface stimulates the chemical reactions and promotes the
catalysis.
The semiconductor nanoparticles have interesting optical properties that are different
from the bulk. With the decrease in sizes, the spacing between energy levels becomes larger and
the absorption band progressively shows a blue shifting. In addition, these nanoparticles can
exhibit non-linear optical properties caused by the dependence of material induced polarization
on the intensity of the incident light.
The electric properties of the nanomaterials also change with size decreases [75]. The
electrical conductivity within nanoparticles decreases with the transition from the electronic band
10
structure of the bulk into separate molecular orbitals. Small sizes of nanoparticles can affect the
sintering or miscibility properties of the material. The sintering of nanomaterials or achieving
homogeneity of the phase requires relatively lower temperatures.
The surface area increase and particle size decrease can influence chemical properties of
nanoparticles. Catalysis, sensing, adsorption, and separation processes are size and surface
dependant. In heterogeneous catalysis, chemical reactions have traditionally higher reaction rates
with the smaller particle sizes.
Several special methods are known for the synthesis of nanomaterials [76]. Milling, using
mechanical attrition, can produce small particles from the bulk material. The drawback of this
method includes the non-uniform size and shapes of obtained particles and severe contamination
with the milling media (hardening steel or tungsten carbide in ball mills).
Nanosized materials can be synthesized using gas-phase reactions such as aerosol,
chemical vapor deposition (CVD), solvated metal atom dispersion method (SMAD) or others.
The aerosol process gives solid nanoparticles by heat treatment of the dispersed droplets of
precursor [77]. SMAD technique is mainly used to prepare wide array of metal nanoparticle (Au,
Ag, Cu, Pd, and others) by the metal vaporization and trapping of metal particles in the frozen
organic solvent matrix [78].
The most common synthesis approach for the synthesis of nanoparticles involves the wet
chemical method such as microemulsions, sol-gel, aerogel and other techniques. In
microemulsion, each droplet is a small nanosized reactor where small nanoparticles can be
synthesized. In sol-gel technique, small sol particles are formed first and then aggregate to form
gel network of particles with uniform size and shape. Supercritical drying of the solvent preserve
the network structure from collapse and the bulk from the densification.
1.3 Possible applications
High purity perovskite oxide powders are widely used for the preparation of electronic
components, electro-optical and photocatalytic materials.
Their possible photocatalytic applications include
1. Oxidation and hydrogenation [79-80],
2. Photocatalytic surfaces (films, coated metals, laminated composite materials),
11
3. Antifouling and antibacterial materials (fibers, sanitary clothes, uniforms, mops),
4.
Photocatalytic decomposition of NOx and SOx for sound-absorbing walls on highways,
5. Odor elimination (air purifier fans, vacuum cleaners, dishwashers, garbage disposers),
6. Decomposition of water and dioxin [81],
7. Photocatalytic self-cleaning glass windows and plasma screens.
Their possible electronic applications include
1. Dielectric devices (flexible film capacitors, dielectric bolometers, IR focal plane arrays)
[82-85],
2. High-permittivity thin films (DRAMs, pyroelectric sensors, gas detection sensors) [86],
3. Superconducting microwave tunable devices, tunable phase shifters, and ring resonators
[87, 88],
4. Acoustic imaging arrays (real-time acoustic cameras for underwater divers, medical
imaging, and acoustic microscopy) [89].
Titanate ceramics are used in capacitors, piezoelectric ceramics, and PTCR (positive
temperature coefficient resistors). Barium titanate is the main dielectric material component of
ceramic capacitors. SrTiO3 has applications as a dielectric and photoelectric material. Owing to
its high dielectric permittivity that increases on cooling and to its low microwave losses, SrTiO3
is an attractive material for many high-frequency and MW applications, particularly at low
temperatures. Strontium titanate photocatalyst has a possible application in water electrolysis,
hydrogen production, toxic water remediation, and in organic synthesis [90, 91].
The excellent dielectric properties of BaxSr1-xTiO3 dielectric material make it useful for a
wide range of applications, including phase shifters, phased array antennas, thermistors, and
pyroelectric detectors [92].
To summarize, the variety of applications for alkaline-earth metal titanate perovskites
makes them the subject of intense investigations. Different production methods of titanate are
available and depend on the required textural and industrial characteristics. The solid-state
method is widely used for the production of cheap bulk titanates. However, growing demands for
high-pure, homogeneous and nanosized titanate move the research investigations toward wet
chemical methods and synthesis of nanosized titanate powders by the sol-gel process and aerogel
process.
12
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17
Chapter 2 : Synthesis of titanates
2.1 Introduction
There are three main steps in the development of new titanate materials: synthesis of
titanates, their property characterization, and application. This chapter offers the discussion of
the available methods for the synthesis of titanates. The synthesis and characterization of
titanates will be presented and discussed in Chapter 3. Property studies applications of
synthesized titanates will be investigated and discussed in Chapters 4, 5, 6, and 7.
For a long time, solid materials were produced with traditional techniques that did not
allow synthesis control and desirable textural properties. The novel techniques presented in this
dissertation allow the production of solids with controllable size and shape. These materials can
be produced with well-defined structure and morphology.
2.2 Preparation of titanates
To design a new method for the synthesis of new materials in oxide chemistry, we
explored the traditional synthetic methods in this field. The majority of oxide ceramics and
powders have been synthesized by high temperature solid state reactions, and oxide films and
coatings have been made by deposition from gas phase reactions. With such techniques as solid
state reaction or chemical vapor deposition, researchers may have difficulties controlling particle
size, shape, and size distribution. In these methods, the process requires high temperatures for the
reaction of the precursors as solids or in gas phase.
In comparison with traditional methods, wet chemical techniques such as sol-gel,
colloidal or cluster solutions, the synthesis of particles and the material structure can be
controlled. Solvent plays an important role, while the process requires lower temperatures.
Control of the synthesis can be achieved by mixing different precursors, choosing the right
solvent, monitoring the nucleation, following stoichiometric ratio, and by using appropriate
ligands.
18
2.2.1 Solid-state reaction
Different inorganic solids can be synthesized through the reaction of a solid with another
solid (“solid/solid” reaction), a liquid (melt) or a gas (“solid/gas” reaction) at high temperature.
Also, solids can turn into melt or gaseous intermediates through the ongoing reaction. Solid/solid
reaction aka ceramic method has been widely applied for the production of oxides for a long
time. The combination of solid and gaseous reactants gives a better contact between them in
comparison to that between just two solids.
The ceramic method is the oldest method for the synthesis of solid-state materials [1].
High temperatures are required for solids to react with each other. The advantages of this method
include low cost of production, available precursors, and simplicity of the processes. The
shortcomings include formation of undesirable phases, nonhomogeneous distribution of dopants,
and a difficulty in monitoring the reaction progress. High temperatures in this method require the
application of chemically inert materials as containers (platinum, silica, stabilized zirconia, and
alumina).
The reaction goes through the formation of product nuclei at the interface of the solids.
The growth of the product layer slows down the counter diffusion of ions from the reacting
solids to each other and decreases the rate of the reaction. Only very high temperatures provide
enough energy for ions to diffuse through. To maximize the reaction rates, the starting precursors
need to be well ground to increase surface areas and contact between particles.
The solid-state reaction depends on the reactivity of the starting precursors. The final
oxide products can be produced by the oxidation of precursors by oxygen from the air or by the
reaction during the decomposition of the precursor compounds. For example, the mixed-metal
oxide of yttrium, barium, and cupper (YBa2Cu3O7-x) can be prepared by the heating of a mixture
of yttrium oxide (Y2O3), barium peroxide (BaO2), and cupric oxide (CuO) and reacting with
oxygen from the air. Moreover, YBa2Cu3O7-x (YBCO) can be synthesized using barium
carbonate (BaCO3) as the source of barium. In this case, barium carbonate decomposes at high
temperature and gives highly reactive barium oxide (BaO). Other precursors that can decompose
at a high temperature and can be used for solid-state reactions are oxy salts (citrates, acetates) or
nitrates.
19
The YBa2Cu3O7-x oxide is a well known supercoductor (-181ºC) and has a structure
similar to a perovskite structure. In YBCO, three perovskite (ABO3) unit cells are stacked above
each other. Barium ions in the II oxidation state and yttrium ions in the III oxidation state are
sharing the B4+ sites (Figure 2.1). Copper ions are equally in the II and III oxidation states and
occupy the A2+ sites in the unit cell. The oxygen deficiency in this material is derived from the
presence of oxygen vacancies in the lattice to maintain the charge balance.
Figure 2.1 Unit Cell of YBa2Cu3O7-x Oxide
Barium titanate can be produced by the solid-state reaction of barium carbonate (BaCO3)
and titanium oxide (TiO2). Barium carbonate decomposes into barium oxide (rock salt structure)
that reacts with the titanium oxide (rutile structure). BaO reacts with titanium oxide to form an
intermediate Ba-rich layer of the Ba2TiO4 phase. This phase allows the migration of Ba ions
from the BaO phase into the TiO2 phase to form the BaTiO3. During synthesis of barium
titanium, other undesirable phases can be produced such as BaTi2 O5. The same approach can be
used for the synthesis of other mixed metal oxides; however, the reactions between two solids
may not occur due to unfavorable thermodynamic conditions (slow reaction rates, slow diffusion,
or very high temperatures).
20
There are several other ways to synthesize mixed metal oxides through the heating and
decomposition of the solid precursors that contain metal cations in an intimately mixed atomic
dispersion. Different metal oxides can be synthesized by coprecipitation or precursor methods
[2]. The cations are closer to each other so lower temperatures are required for the decomposition
of precursors.
In the coprecipitation method, the precipitate of two salts of different metal cations from
the solution is heat-treated to give the oxide products. Metal salts (carbonates, oxalates, formates,
or citrates) are dissolved in water and coprecipated by the concentration of the solution.
Solid solutions of carbonates are coprecipated first and then heat treated to produce
corresponding metal oxides:
(1-x) MCO3 + x M’CO3 → M1-xM’x(CO3) → M1-xM’xO
(2.1)
where M, M’ are metals such as Mg, Ca, Sr, Ba, Zn, Cd and some others.
In the precursor method, the two different metal cations are incorporated in the same
solid precursor. For example, barium titanate can be synthesized by slowly heating of the mixedmetal barium titania citrate Ba[TiO(C2O4)2] in air to burn off the organic matter. This method
gives products with higher purity and more accurate stoichiometry in comparison with other
ceramic methods. However, all oxide products synthesized by ceramic methods have a dense
structure, big particle sizes, and small surface areas. The solid-state preparation of oxides
requires repeated cycles of milling and calcination at high temperatures. The resulting powders
from this preparation are agglomerated crystallites of different sizes and contain impure phases
due to incomplete reactions [3, 4]. These disadvantages can be overcome by using other
methods, as discussed below.
2.2.2 Gas phase reaction
It is also common to use gaseous precursors or intermediates for solid material
production. Several techniques are used for synthesis of solid products from the gas phase,
including chemical transport, chemical vapor deposition (CVD), and aerosol methods.
21
The chemical transport technique is mainly used for crystal growth and for the
purification of solids. The purification is achieved by the reaction of a solid (A) with a gaseous
agent (B), transportation of a gaseous product (AB), and following decomposition back to solid
(A) and gaseous agent (B) (Figure 2.2).
Figure 2.2 Principal Scheme for Chemical Transport (Adapted and Modified from
Reference 5)
A non-volatile solid (A) reversibly reacts with a transport gaseous agent (B), and the
equilibrium constant of this reaction is temperature dependent. Diffusion transportation from one
site (the source) to another (the sink) is achieved by different temperatures by changing the
temperature gradient for the AB product. The chemical equilibrium shifts from the product AB at
the source site to A and B at the sink site.
Chemical transport can be applied for the synthesis of new compounds also if the
transport reaction is followed by an additional reaction at the sink site. Transport of metal oxides
(for example chromium oxide) is achieved by the use of a mixture of halides and water as
transport agents.
Chemical Vapor Deposition (CVD) is a method for the preparation of thin films and
coatings for a variety of inorganic materials. Solid thin films of different materials, including
oxides, can be deposited on the substrate [5]. The formation of a material for the film occurs at
significantly lower temperatures than its melting temperature, which is why this technique is
intensively used on an industrial scale for hard coatings such as those on cutting tools or engine
22
parts [6]. CVD is also used to make three-dimensional microelectronic components, coatings on
glass, and catalyst coatings on the support material, layers in solar cells.
In the CVD process, vapor phases of volatile precursors react on a heated substrate in the
reaction chamber and the films of new materials are produced. Precursors go through adsorption
onto the substrate surface, and the following chemical reaction leads to the deposition of a solid
film. These films of metal oxides are mainly produced for dielectric and optical purposes. CVD
is widely used for the deposition of oxide films (TiO2, SnO2, or SiO2) on glasses. Besides,
coatings and layers of oxides are used in solar cells, membranes and catalysts.
The most common precursors are metal hydrides (SiH4), metal alkoxides (Si(OEt)4),
metal alkyls (AliBu3, GaEt3), and volatile metal halides (TiCl4) in the presence of the oxygen
source (O2, N2O, or water). The application of organometallic precursors is more preferable as
they are more volatile than the inorganic ones. However, the organometallic precursors are less
thermally stable; they can decompose and contaminate the product.
Several individual metal precursors can be decomposed together to produce mixed-metal
oxide films. However, it is difficult to keep the stoichiometry in these films. In this system,
precursors and intermediates have different volatilities and reactivity, and this causes the excess
of one precursor and the deficiency of another on the surface of the substrate. For example, a
lead deficient material is produced by CVD during the synthesis of lead titanate (PbTiO3). The
deficiency is caused by higher volatility of the lead oxide (PbO) and its higher desorption rate
from the surface of the substrate.
The formation of the mixed metal oxide films can be achieved also by the use of a
precursor which has all required elements for the product in the stoichiometric ratio. This is
simplifying the delivery system of the precursor. However, in this case, precursors have higher
molecular weights, and they are less volatile.
Silicon oxide (SiO2) films produced by CVD are widely used in electronics for insulation
and passivation layers. These layers separate and protect semiconductor layers in microcircuit
electronic devices. The application of silicon alkoxides (tetraalkoxysilane, TEOS) as the
precursor gives the SiO2 growth without an external oxygen source by the decomposition of
alkoxides:
Si(OEt)4 → SiO2 + 2 C2H4 + 2 EtOH
(2.2)
23
The formation of the Si-O-Si bond goes through the reaction of the silanol groups with TEOS
and the release of ethanol:
(EtO)3Si-OEt → (EtO)3Si–OH + C2H4
(2.3)
(EtO)3Si-OH + EtO-Si(OEt)3 → (EtO)3Si-O-Si(OEt)3 + 2 EtOH
(2.4)
Decomposition of the alkoxides goes at 750ºC without the oxygen source. Adding oxygen from
air does not have an influence on the decomposition temperature. However, oxygen removes
carbon contamination in the films and improves the quality of the films. Contrary, the addition of
ozone can decrease the decomposition temperature down to 300ºC and increase the quality of the
film even further.
The typical CVD reactor consists of the precursor delivery system, reactor, and pumping
system to remove by-products of the reaction (Figure 2.3). 200 - 800°C is the typical range of
temperature in the reactor, and 0.1 mbar to 1 bar is the typical pressure.
24
Figure 2.3 Principal Scheme of a CVD Reactor for Oxide Film Deposition (MFC – Mass
Flow Controller) (Adapted and Modified from Reference 5)
Heating the precursor vessel can increase the volatility of the precursor. Several alternative
methods of precursor delivery (aerosol, spray pyrolysis) can be used for precursors that are not
sufficiently volatile or thermally unstable.
Many different techniques are based on the CVD process and the most frequently used
and applied for oxide production are summarized in Table 2.1. The vapor phase epitaxy (VPE)
technique is used to grow epitaxial crystalline films that have crystallographic orientation and
lattice parameters similar to those of a single crystal. The OMCVD and OMVPE techniques use
organometallic compound precursors with metal-carbon bonds. In contrast, the metal-organic
precursors without any metal-carbon bonds (alkoxide, amines) are applied in the MOCVD and
MOVPE techniques.
25
Table 2.1 CVD Related Methods [5]
Acronym
Full Name of the Techniques
CVD
Chemical Vapor Deposition
VPE
Vapor Phase Epitaxy
OMCVD
Organometallic Chemical Vapor Deposition
OMVPE
Organometallic Vapor Phase Epitaxy
MOCVD
Metal-organic Chemical Vapor Deposition
MOVPE
Metal-organic Vapor Phase Epitaxy
PACVD
Plasma-assisted Chemical Vapor Deposition
LCVD
Laser-induced Chemical Vapor Deposition
Other vapor deposition methods include Physical Vapor Deposition (PVD), Atomic-layer
Epitaxy (ALE), Chemical-beam Epitaxy (CBE) and Metal-organic Molecular-beam Epitaxy
(MOMBE).
The main disadvantages of the CVD method include the incorporation of carbon into films
using organometallic precursors, the contamination with chlorine by using halide precursors, and
contamination with unwanted by-products. The by-product must be removed from the deposition
zone. The other problem is the non-uniform stoichiometry in the films due to different volatility
and reactivity of precursors and intermediates.
The main difference between the chemical vapor deposition and aerosol methods is the
place of the solid product formation. Gaseous precursors are adsorbed first on the substrate
surface and then react to form the solid product in CVD, while by the aerosol method the solid
particles are produced right in the gas phase. The products of CVD are films, while the products
of the aerosol technique are fine powders.
In an aerosol, an atomized solution in the form of droplets or solid powders in a carrier
gas in the form of suspension is passed through a heated region [7]. The product particles are
obtained by the precursor pyrolysis or by the reaction with a gas. The solvent in the droplets
evaporates, and solid particles form through the densification. However, if the solute precipitates
at the supersaturated surface first, a crust forms, and this particle is hollow inside.
26
The aerosol process is widely used for the preparation of ultrafine particles (< 100 nm) of
different oxides (Table 2.2). The process is performed in the presence of oxygen to favor the
oxide production. Different oxide powders can be synthesized in thermal (combustion), laser
(convection or radiation) or plasma (energy of highly ionized gases) reactors. In some cases,
mixed-metal oxides can be prepared using a precursor mixture (for example SiCl4 and TiCl4).
Aerosol powders are usually in the form of agglomerates of primary particles held together by
weak van der Waals forces.
Table 2.2 Oxide Powders Prepared by Aerosol Method [5]
Oxides
Precursors
SiO2
SiCl4
TiO2
TiCl4
Al2O3
AlCl3
Bi2O3
BiCl3
Cr2O3
CrO2Cl2
Fe2O3
FeCl3 or Fe(CO)5
GeO2
GeCl4
NiO
Ni(CO)4
MoO2
MoCl5
SnO2
SnCl4 or SMe4
V2O5
VOCl3
WO3
WOCl4
ZrO2
ZrCl4
AlBO3
AlCl3 and BCl3
Al2TiO5
AlCl3 and TiCl4
The aerosol process can be also used for the synthesis of mixed metal oxides (spinels) by
the spray pyrolysis method.
27
Typical examples of the aerosol process are the synthesis of highly dispersed silicon
oxide (silica) and titanium oxide (titania). This Aerosil® process was developed by the German
company Degussa, so the silica product has the name Aerosil A200 fumed silica, and the titania
product has the name of titania P25 Degussa [8]. These oxides are produced by flame hydrolysis.
Titanium is produced by the reaction of titanium tetrachloride with water in the oxygenhydrogen flame according to the following chemical reactions:
2 H2 + O2 → 2 H2O
TiCl4 + 2 H2O → TiO2 + 4 HCl
(2.5)
(2.6)
The overall reaction gives highly dispersed titanium oxide and hydrogen chloride as a
byproduct:
TiCl4 + 2 H2 + O2 → TiO2 + 4 HCl
(2.7)
The produced titania is widely used as a pigment for paints. This material is a light powder
consisted of agglomerated spherical particles of up to 40 nm diameter. It also has a high surface
area of 50 m2/g.
Fumed silica is produced by the similar flame hydrolysis reaction:
SiCl4 + 2 H2 + O2 → SiO2 + 4 HCl
(2.8)
Fumed silica and aerosol titania have found wide applications as the filler for rubber, additive for
drilling fluids, plastics, paints, creams, tablets, cosmetic powders, toners, and toothpastes.
The high surface areas and low density of aerosol materials can be compared to the
aerogel materials which will be discussed later in this chapter. However, the surface of aerosols
originates only from the outer surface of particles, while the aerogels have traditionally higher
surface areas originating not only from outer surfaces but also from the high porosity of aerogels.
28
In the aerosol process, products are produced at the expense of energy. Other
disadvantages of the aerosol technique include the production of hollow particles, which can be
explained by the differences in nucleation and growth rate of the precursors.
2.2.3 Sol-gel technique
The sol-gel technique is one of the most widely used soft chemical methods and mainly
applied for the synthesis of metal and semimetal oxides. In this process, oxides are synthesized
by the formation of an oxide network directly in solution by hydrolysis of alkoxides, followed by
gelation and finally by removal of the solvent [9]. In contrast to the solid-state method, a wet
chemical synthesis can provide homogenous nanosized oxides of high purity at lower reaction
temperatures. Sol-gel and aerogel processes are the most widely used routes and involve a
colloidal sol which is converted into a gel during aging [10, 11].
The main principle of the classical sol-gel process is the controlled hydrolysis of metalloorganic compounds (alkoxides) in an organic solvent. The sol-gel process involves olation
(formation of hydroxyl bridges) and oxolation (formation of oxygen bridges) reactions during
hydrolysis (reactions 2.9-2.11). The oxolation a.k.a. condensation reaction is responsible for the
formation of colloidal agglomerates, and the oloation a.k.a. addition reaction is responsible for
their aggregation into a polymeric gel.
≡M-OR + H2O → ≡M-OH + HOR
≡M-OH + RO-M≡ → ≡M-O-M≡ + HOR
2 ≡M-OH → ≡M-O-M≡ + H2O
(Hydrolysis)
(2.9)
(Condensation)
(2.10)
(Addition)
(2.11)
where R is alkyl group, and M is metal or semimetal (IV).
The gel formation depends on different parameters including the nature of starting
material(s) (precursor[s]), kind of solvent, precursor concentration in the solvent, alkoxy to water
ratio, temperature of the reaction, pH, kind of catalyst, stirring and aging time. Metal alkoxides
29
serve as starting materials and can be hydrolyzed by water. The alkoxides have been extensively
used for the production of oxides and glasses. During hydrolysis, alkoxy groups are replaced by
strong OH
–
nucleophiles, and the following condensation and addition steps lead to the
formation of oxide chains.
The sol-gel synthesis goes through the formation of a sol of colloidal particles or units in
a solution, gelation of the sol by the agglomeration of these particles or sub-units into a big gel
network structure, removing of the solvent, and heat treatment to transfer gel into solid.
Depending on reaction conditions, the sol particles may grow further or form gel.
The sol-gel process can be used for the preparation of a variety of materials (Figure 2.4).
The drying of the sol gives powders. The application of dip-coating or spin-coating leads to the
preparation of the thin films.
The removal of the solvent by drying causes the shrinking of the gel and significant
reduction in the volume due to increasing capillary forces. The high capillary pressure in the
pores causes the collapse of the gel network structure and the production of less porous powder
(xerogel). In contrast, the supercritical extraction when the solvent is removed above its critical
temperature preserves the structure of the gel network and yields a highly porous material
(aerogel). Dense ceramic material or glass can be produced by sintering the xerogel or aerogel.
30
Figure 2.4 Production Options for the Sol-gel Process (Adapted and Modified from Reference 5)
31
2.2.4 Aerogel procedure
Samples prepared by the sol-gel method are porous and have a three-dimensional
polymeric network. However, the liquid-vapor interfacial tension during conventional drying can
cause destruction of this porous network, but the removal of the solvent by supercritical drying
prevents the network from collapse [12-15]. Under the supercritical extraction of the solvent, the
gel is processed with heat and pressure, giving rise to a nanocrystalline product with
homogeneous composition, high porosity, and large surface area [16, 17].
The synthesis of mixed oxides by the sol-gel method can be achieved by cogelation of a
metal precursor mixture (hydrolysis of metal alkoxides). The hydrolysis step (2.12) uses the
metal alkoxide and releases the alcohol. The partially hydrolyzed intermediate undergoes a
condensation reaction (2.13) forming a metal-oxygen-metal bridge (M-O-M) by either the
removal of water (X is H) or alcohol (X is R).
M(OR)n + m H2O → M(OR)n-m(OH)n + m ROH
MOH + MOX → MOM + XOH
(2.12)
(2.13)
When synthesizing mixed-metal aerogels, the combination can yield condensation sites
of MOM, MOM’, and M’OM’ (where M and M’ are different metals). The final products are
three dimensional crosslinked networks with oxo-bridged metals. In mixed-metal aerogels, each
metal site has several possible combinations of bridged neighbors.
Often, mixed metal oxides can consist of two separate oxide phases (oxide segregation),
or of one phase with cationic substitution in the lattice, depending on the solubility of the metal
oxides in each other [18].
The aim of the research described herein was to substitute titanium for strontium or
barium in a 1:1 stoichiometric ratio, and to synthesize single-phase nanosized strontium titanate
and barium titanate powders (SrTiO3 and BaTiO3).
Commonly, metal alkoxides in different alcohols are used for the aerogel synthesis. For
mixed metal oxide systems, the traditional way is the acid-alkoxide route through the mixing of
32
one metal (A) alkoxide with the salt (nitrate, acetate, acetylacetonate, citrate, or carbonate) of the
second metal (B) or the hydroxide-alkoxide route through the mixing of metal (A) alkoxide with
metal (B) hydroxide in alcohol followed by hydrolysis (gelation). The presence of acid anions
from the salt or hydroxide supports faster gelation, but also causes impurities in the samples that
must be removed by calcination [10, 18, 19]. In our research, the mixed alkoxide route through
the mixing of two metal (A and B) alkoxides was used to achieve a better gelation and prevent
the formation of impurities. Since strontium and barium oxides tend to produce a carbonate
phase, conditions to prevent reaction with CO2 during hydrolysis, supercritical drying, and
calcination must be provided for. To accomplish this, supercritical solvent extraction was
executed in a N2 atmosphere.
The influence of solvent on the aerogel properties has been widely studied in our research
group [20, 21]. It was found that a solvent mixture of alcohol and toluene affected the hydrolysis
of the aerogel, and resulted in higher surface areas of aerogels compared to pure alcohol
solvents. The presence of the hydrophobic solvent may reduce surface tension in gel pores and
thereby prevent sintering [20].
The presence of toluene in the solvent also accelerated the hydrolysis and gelation
processes; it also yielded more porous and, therefore, less dense wet gels with a lower mass
fractal dimension [21]. Finally, the excess of toluene in solvent and nitrogen gas during
hypercritical drying in the autoclave prevented the gel from stress and shrinkage, which results in
products with higher surface areas.
2.3 Temperature treatment
Drying and/or heat treatment processes are important final steps for the preparation of the
oxide products. The drying of the wet samples causes the collapse of the network structure and
shrinkage to produce materials with smaller surface areas. Heating the powders at high
temperatures causes the particles of the powders to fuse together, as well as decreasing the pore
sizes and voids between particles. Further heat treatment leads to the production of dense solid
material in a process called sintering.
33
Sintering is an important process for producing different materials like ceramics and
ironware. However in case of the nanoparticles, the sintering process is undesirable because it
will cause the increase of the particle sizes and the decrease of surface areas.
Particles in the powder have an excess of surface free energy. During heating, they try to
decrease this free surface energy by decreasing the total surface area. Particles join together and
transport mass to decrease the surface of particles (Figure 2.5). Surfaces within particles have
different chemical potentials based on the curvature. The convex surface has a positive surface
energy, while the concave surface has a negative free energy. Mass transport goes through the
diffusion process from the concave particle surface to the concave interparticle surface (necks or
pores). The driving force of sintering is greater for smaller particles, and nanoparticles undergo
significant increase in particle sizes and decrease of surface areas.
Figure 2.5 Sintering by diffusion (path 1 - surface diffusion, path 2 - volume diffusion)
The driving force for mass transport is the diffusion of mass flow depending on different
vacancy concentrations. Diffusion goes through two main paths, which are surface diffusion and
volume diffusion. In the surface diffusion, vacancies go from the neck surface to the convex
surface, so the mass flow goes oppositely from the convex surface to the neck (Figure 2.1, path
1). In the volume diffusion, vacancies in the grain boundaries and/or dislocations in the grain
matrix move in the volume to the surface while the mass flow tend to fill the place of moved
vacancies (Figure 2.1, path 2).
34
2.3.1 Heat treatment
During the heat treatment of powders, the sintering of particles goes through three main
steps. The first step includes the fusing of particles together and the mass transfer from the
convex to concave surfaces. The total pore volume and distance between particles decreases
insignificantly. During the second step, diffusion and mass transfer processes occur and cause an
increase in density and growing of interparticle necks and decrease in pore diameters and
distances between particle centers. Significant shrinkage and densification occurs also. In the
final step, a dense material is produced, and all remaining pore are gradually eliminated.
Heat treatment has a significant influence on particle size, particle shape, and particle
packing. For smaller particles, the mass transport goes faster and sintering starts at lower
temperatures. High temperatures also cause an increase of the average size of particles and
decrease of the particle size distribution. The coarsening process can be explained by larger
particles growing at the expense of smaller particles. The sintering process goes faster for
particles with a high surface area to volume ratio. In aggregates of particles, the high number of
contacts between particles favors the sintering and densification of the material.
2.3.2 Calcination
Calcination is a heat treatment process where the decomposition of the precursor or the
degradation of the impurities in the product occurs with the liberation of gases (H2O and CO2, or
other gaseous products of degradation) in the oxidizing atmosphere (air, oxygen, nitrous oxide,
or ozone). Metal oxides prepared by wet chemical methods are usually contaminated with
organic residues from the organic components involved in the synthesis, such as physisorbed
solvent, ligands, or modifiers. Studies on different aerogels showed that calcination at 300-500ºC
is necessary to remove these organic residues, but it is still possible that a small amount is
present in pores and on the surface of the aerogel [22]. Application of oxidative atmospheres
such as air or oxygen helps to burn off the organic residuals and remove them in the form of CO2
gas.
The oxide powder characteristics such as particle size, particle shape, surface area,
crystallinity, phase purity are dependent on the conditions of calcination. The presence of
35
organic residuals on the surface can significantly influence the physical and chemical properties
of an aerogel. They can cover active sites of the material and decrease the reactive and catalytic
activity of the aerogel.
2.3.3 Drying
The freshly prepared wet gels obtained by the sol-gel method can be dried by several
techniques. Drying of the wet gel in air results in gel network collapse and strongly shrunken gel
(xerogel). To preserve the inorganic network structure from shrinkage, high-temperature
supercritical drying, low-temperature supercritical drying, and freeze-drying techniques have
been developed [23].
In the supercritical drying process, the solvent in pores is transferred into a supercritical
fluid (SCF). In this state, solvent is above its critical pressure and critical temperature, so it has
properties of liquid and gas together, so there is no liquid-vapor interface and no capillary
pressure.
For supercritical drying, a gel is placed into an autoclave and heated following the steps
showed in Figure 2.6. The gel is heated in such way that the solvent does not cross the liquid –
gas phase boundary. Over the critical point, solvent is vented out in the form of gas, so the gel
structure is preserved from collapsing by capillary forces. After that the aerogel is cooled down
to room temperature in an inert atmosphere (N2).
36
Figure 2.6 Temperature-pressure Diagram for Supercritical Drying, where C - Critical
Point, SCF – Super Critical Fluid, Tc – Critical Temperature, Pc – Critical Pressure
(Adapted and Modified from Reference 5 and 13)
Water can not be used as the solvent because many oxides dissolve in water at
supercritical conditions and the oxide peptizes and causes production of a dense crystallized
oxide bulk. Instead of water, alcohols are the most widely used solvents for the supercritical
drying and preparation of the aerogels. However, high critical temperature and pressure of the
alcohol solvents [Table 2.3] in combination with solvent flammability requires strong
precautions.
Table 2.3 Critical Points of Some Solvents [13]
Solvent
Critical temperature Tc, °C
Critical pressure Pc, bar
H2O
374
221
CO2
31
74
Methanol
240
80
Ethanol
243
64
Isopropanol
235
47
1-Butanol
290
43
37
It was found that a mixture of alcohol and organic solvent such as benzene or toluene is
essential for obtaining mixed metal oxides with high surface areas and small crystallite sizes
[24]. Toluene in the mixture reduces the surface tension at the gas-liquid interface, resulting in
the formation of more open porous network products with higher surface areas. Alcohol is
important for the prevention of phase separation of the alkoxide precursors.
Carbon dioxide has a very low critical temperature of 31ºC, and it is non-flammable. These
advantages can be used for low-temperature supercritical drying. This process is energy efficient
and non-explosive. The solvent in gel is exchanged by liquid CO2 before the drying step which
gives dried aerogels (carbogels). This method can not be used for the preparation of some oxides,
which react with carbon dioxide to form carbonates, such as alkaline-earth metal oxides.
In the freeze-drying method, gel is quickly frozen first to prevent the solvent from
crystallization, and after that it slowly evaporates by sublimation under vacuum. The final
product (cryogel) is a highly porous solid with a structure similar to an aerogel [25].
2.4 Conclusions
Many methods have been used for the synthesis of these materials, and new methods are
been developed continually. Some of these methods allow synthesizing nanosized materials with
unique characteristics and possible advantages over bulk materials.
The sol gel method in combination with supercritical drying is one of these methods that
offer an improved way to prepare nanosized materials with unique properties. Aerogels are the
lightest inorganic solid materials available today; they are of great value for thermal insulation,
for catalysis and as electrode materials. Their unusual high surface areas make them attractive
for different applications.
Oxide and mixed oxide aerogels are interesting for their textural, structural and chemical
properties. They have morphological and structural properties that differ from the conventionally
prepared by the solid-state technique.
Titania-based oxides synthesized buy the aerogel technique have high porosity, high
surface areas, homogeneity, and thermal stability. In particular, nanosized alkaline-earth metal
titanates offer new possibilities for different applications including catalysis, photocatalysis,
38
electronics, water electrolysis, organic synthesis and toxic waste remediation. Drawbacks of the
aerogel technique include high cost of preparation, safety and technical risk due to high
temperature, and high pressure.
2.5 References
[1]
W. Lengauer, Surface and Interface Analysis, 15 (1990) 377.
[2]
N.R. Rao, Mater. Sci. Eng., B18 (1993) 1-21.
[3]
J. Petzelt, T. Ostapchuk, I. Gregora, I. Rychetsky, S. Hoffmann-Eifert, A. V. Pronin, Y.
Yuzyuk, B. P. Gorshunov, S. Kamba, V. Bovtun, J. Pokorny, M. Savinov, V. Pokhonskyy, D.
Rafaja, P. Vanek, A. Almeida, M. R. Chaves, A. A. Volkov, M. Dressel, and R. Waser, Phys.
Rev. B, 64 (2001) 184111.
[4]
J. Moreno, J. M. Dominguez, A. Montoya, L. Vicente, and T. Viveros J., Mater. Chem.,
5 (1995) 509.
[5]
U. Schubert and N. Hüsing, Synthesis of Inorganic Materials, Wiley-VCH, 2000, pp. 396.
[6]
H. E. Hintermann, Thin Solid Films, 84 (1981) 215.
[7]
M. Drygaś, C. Czosnek, R. T. Paine, and J. F. Janic, Materials Research Bulletin, 40
(2005) 1136.
[8]
E. Wagner and H. Brünner, Angew. Chem., 72 (1960) 744-750.
[9]
J. Gopalakrishnan, Chemistry of Materials, 7 (1995) 1265.
[10]
L. G. Hubert-Pfalzgraf, S. Daniele, and J. M. Decams, J. Sol-Gel Sci. Technol., 8 (1997)
49.
[11]
X. Wang, Z. Zhang, and S. Zhou, Mater. Sci. Eng. B: Solid-State Mater. Adv. Technol.,
B86 (2001) 29.
[12]
S. S. Kistler, J. Phys. Chem., 36 (1932) 52.
[13]
C. J. Brinker and G. W. Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-
Gel Processing, Academic Press, San Diego, 1990.
[14]
L. C. Campbell, B. K. Na, and E. I. Ko, Chem. Mater., 4 (1992) 1329.
[15]
C. L. Carnes, P. N. Kapoor, and K. J. Klabunde, Chem. Mater, 14 (2002) 2922.
[16]
K. J. Klabunde, J. Stark, O. Koper, C. Mohs, D. G. Park, S. Decker, Y. Jiang, I. Ligadic,
and D. Zhang, J. Phys. Chem., 100 (1996) 12142.
39
[17]
T. Lopez, J. Hernandez, R. Gomez, X. Bokhimi, J. L. Boldu, E. Munoz, O. Novaro, and
A. Garcia-Ruiz, Langmuir, 15 (1999) 5689.
[18]
W.-S. Cho and E. Hamada, J. Alloys Compounds, 266 (1998) 118.
[19]
S. Ahuja and T. R. N. Kutty, J. Photochem. Photobiol. A: Chem., 97 (1996) 99.
[20]
S. Utamapanya, K. J. Klabunde, and J. R. Schlup, Chem. Mater, 3 (1991) 175.
[21]
Y. Diao, W. P. Walawender, C. M. Sorensen, K. J. Klabunde, and T. Rieker, Chem.
Mater., 14 (2002) 362.
[22]
M. Scneider and A. Baiker, Catal. Rev.-Sci. Eng., 37 (1995) 515.
[23]
P.H. Tewari, A.J. Hunt, and K.D. Lofftus, Mater. Lett., 3 (1985) (9-10) 363-367.
[24]
Utamapanya, S., Klabunde, K.J., and Schlup, J.R., Chemistry of Materials, 3 (1991) 175-
81.
[25]
W. Mahler and U. Chowdhry, in Ultrastructure Processing of Ceramics, L.L. Hench and
D.R. Ulrich (Editors), Glasses and Composites, Wiley, (1984) p. 207.
40
Chapter 3 : Characterization of titanates
3.1 Introduction
Synthesis of materials has been achieved by different chemical techniques.
Characterization of the materials was conducted by using structural analysis and measurements
of material properties. The structural analysis included the application of different instrumental
techniques such as microscopy, spectroscopy, and X-ray diffraction. The properties measured for
the large quantity of particles in powders are the average of all the individual particles.
These measurements were done using powder XRD, UV-visible spectroscopy, elemental
analysis, and surface BET analysis. The property studies of individual particles are more
complicated due to extremely small sizes of each of them and the lack of suitable techniques and
handling procedures. The properties of the individual particles were studied mainly by
transmission electron microscopy. Additionally, data on the properties of prepared materials
characterized by Infrared spectroscopy can be found in Chapter 6 and by Raman spectroscopy in
Chapter 7.
3.2 Synthesis of strontium and barium titanates
Strontium and barium titanate samples have been synthesized by using solid-state
reaction and aerogel techniques. The textural and structural properties of these materials were
characterized by different available techniques and compared with each other and with available
commercial (SrTiO3 and BaTiO3 powder samples from Aldrich, and nanosized SrTiO3 and
BaTiO3 powders from Aldrich). The comparison of novel, new samples with materials prepared
by conventional methods allows one to observe the differences between them and possible
advantages for future applications.
The available and prepared samples have the following abbreviations for future
references:
CM – commercial samples purchased from a normal vendor or retailer,
41
NCM – commercial nanosized samples purchased from retailer,
AP – aerogel prepared samples synthesized by the Modified aerogel procedure,
XP – xerogel prepared samples synthesized by the Sol-gel process,
SSR or SCR – samples prepared by Solid-state chemical reaction.
Additionally, the prepared samples were compared with titanium oxide (P25, Degussa).
3.2.1 Solid–state reaction
Titanium (IV) oxide (Aldrich; 99.8% anatase), strontium carbonate (Aldrich; 98+%), and
barium carbonate (Aldrich; 99 . 9 8 %) were used as received. Commercially available oxides,
CM-SrTiO3 (99+%) from Alfa-Aesar, nanosized powder NCM-SrTiO3 (99.5+%) from Aldrich,
CM-BaTiO3 (99 %) from Aldrich, and nanosized powder NCM-BaTiO3 (99+%) from Aldrich
were also used as received.
Titanium oxide (TiO2, anatase) and strontium carbonate (SrCO3) powders were used for
the preparation of the SSR-SrTiO3, and titanium oxide (TiO2, anatase) and barium carbonate
(BaCO3) powders were used for the preparation of the SSR-BaTiO3.
The procedures for the synthesis of strontium titanate and barium titanate were identical
(Figure 3.1). The starting materials were mixed in the molar stoichiometric ratio 1:1 of Sr/Ti or
Ba/Ti according to the composition of SrTiO3 or BaTiO3. The powders were mixed together and
calcined at 1100°C and 1200°C for 36 hours in air using alumina crucibles (99.7% purity). After
18 hours of calcination, each sample was cooled down, the sample ground and calcined again 18
hours. The resulting monoliths of fused particles were crushed, and the obtained powders were
used for further analysis and characterization. The structure of the final titanate products was
confirmed by X-ray diffraction.
42
Figure 3.1 Solid-state Reactions for SrTiO3 and BaTiO3 Synthesis
43
3.2.2 Modified aerogel procedure
All manipulations during synthesis were performed under argon. Special consideration
was given to the reactions of strontium and barium metals with alcohols: they are hazardous and
must be conducted with precaution under an inert atmosphere only. All autoclave treatments for
supercritical drying were performed on a Parr 4843.
Titanium (IV) isopropoxide (Aldrich; 97 %), absolute ethanol (Aaper Alcohol and
Chemical Co.; 200 proof), methanol (Fisher Scientific; certified ACS), 2-propanol (Fisher
Scientific; certified ACS), toluene (Fisher Scientific; certified ACS), strontium metal (Aldrich;
99 % dendritic pieces), and barium metal (Aldrich; 99 % dendritic pieces) were used as received.
Commercially available oxides, CM-SrTiO3 (99+%) from Alfa-Aesar, nanosized powder NCMSrTiO3 (99.5+%) from Aldrich, CM-BaTiO3 (99 %) from Aldrich, and nanosized powder NCMBaTiO3 (99+%) from Aldrich were also used as received.
The Modified Aerogel Procedure (MAP) for mixed metal oxides employs the preparation
of metal alkoxide mixtures in an alcohol-toluene solvent, hydrolysis, co-gelation of alkoxide
mixture, and supercritical drying of the solvent. The final oxide products depend on the size and
shape of the sol particles in the gel. The scheme for aerogel-prepared strontium and barium
titanium oxides (AP-SrTiO3 and AP-BaTiO3) by a modified aerogel procedure are presented in
Figure 3.2.
Different alcohols mixed with toluene were used as solvents for synthesis. Strontium
titanate aerogels were prepared in either a methanol, ethanol, or isopropanol mixture with
toluene in a toluene-to-alcohol volume-ratio of 1.5. Barium titanate aerogels were prepared in
ethanol or isopropanol mixtures with toluene in the same ratio. All liquids were deaerated by
bubbling with argon for 30 minutes.
The procedures for the synthesis of strontium titanate and barium titanate were identical. In
a 250 ml flask, 0.02 mol of Sr (or Ba) metal was allowed to react with 40 ml of alcohol under a
flow of argon and constant stirring. In a separate 250 ml flask, 0.02 mol of titanium isopropoxide
was added to 40 ml of alcohol with similar stirring under Ar, allowing a 1:1 stoichiometric ratio
between Sr (or Ba) and Ti.
44
After this, 60 ml of toluene was added into each flask forming clear solutions. The alkoxide
solutions were mixed together and stirred, and then this mixture was hydrolyzed with a
stoichiometric amount of doubly distilled water by slow, dropwise addition. The slow water
hydrolysis transformed a solution of alkoxides into a slightly milky wet gel (gel in alcoholtoluene solution). The viscous solution was left for aging by vigorously stirring at room
temperature for 1 hour.
The wet gel was transferred to a 600 ml glass liner and placed in an autoclave. The
autoclave was flushed with nitrogen, and then pressurized to 100 psi (6.9 bar, 6.9 × 105 Pa). The
reactor was slowly heated to 265 °C. The pressure increased from 100 to 1000 psi (68.9 bar,
68.9 × 105 Pa) upon heating. At 265 °C, the solvent vapors were removed by quick venting to the
atmosphere, followed by flushing the reactor with nitrogen to remove any remaining solvent
vapors. Finally the autoclave was cooled to room temperature.
The resulting slightly yellowish AP-SrTiO3 and white AP-BaTiO3 powders were dried in
air at 120 °C. Aerogel products contained some residual solvent that was removed by heat
treatment in vacuum and calcination in air or oxygen at different temperatures.
45
water
SrTiO 3
or BaTiO3
Supercritical
Drying
Co-Gellation
1:1
Release of
pressure
4h under N 2,
≈1000psi, 265°C
alcohol
toluene
Ti alkoxide
Figure 3.2 Modified Aerogel Procedure (MAP) from Alkoxides for SrTiO3 and BaTiO3 Synthesis
toluene
Sr alkoxide
or Ba alkoxide
alcohol
Sr or Ba metal
Alcohols:
Methanol
Ethanol
Isopropanol
46
3.3 Structural studies
3.3.1 UV-visible spectroscopy
Diffuse reflection spectra were obtained using a UV-visible-NIR spectrometer (Varian) and
were converted from reflection to absorbance by the Kubelka-Munk method. The comparison of
TiO2 Degussa and aerogel prepared SrTiO3 is presented in Figure 3.3. Both materials have a
bright white color, absorb light in the ultra violet region, and reflect visible light. There are some
insignificant differences in the line slopes in the spectra that are due to the different band gap of
materials. The extrapolation of the titania slope gives an intersection with the x axis at 410 nm
and the extrapolation of the strontium titanate slope gives intersection with the x axis at 380 nm.
The conversion into energy of band gaps gives 3.0 eV for titanium oxide and a 3.2 eV for
strontium titanate correspondently. This means that TiO2 start to absorb light at higher
wavelengths (from 410 nm and shorter) while SrTiO3 absorbs at 380 nm and below.
Figure 3.4 shows the absorption spectra for several strontium titanate samples prepared by
different methods. Comparison of aerogel prepared sample and solid-state prepared samples
showed the same trend in the absorption of UV light (<380 nm). However, solid-state prepared
samples are not pale white, but have a slightly pinkish gray color. The interesting color of these
materials can be explained by the low purity of the SrTiO3 product and by the presence of
impurity phases produced by incomplete reaction of precursors (Sr2TiO4) and oxygen deficiency
(SrTiO2.7) (for more details see Section 3.3.3 and Appendix A). This causes the appearance of
the small absorbance response in the visible light region.
47
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
300
400
600
Wavelength (nm)
500
AP-SrTiO3
TiO2 P25 Degussa
700
800
Figure 3.3 UV-visible Spectra of TiO2 P25 Degussa and Aerogel Prepared SrTiO3
Absorbance
48
0.00
200
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
2.50
o
600
AP-SrTiO3 500 C
o
SSR-SrTiO3 1200 C
o
SSR-SrTiO3 1100 C
SSR-SrTiO3 commercial
W avelength (nm)
400
Figure 3.4 UV-visible Spectra of Different SrTiO3 Samples
Absorbance
800
49
3.3.2 Braunauer-Emmer-Teller analysis (BET)
The textural properties of solid material can be characterized by porosity, surface area,
pore volume, and pore size. Porous solids can have pores in the form of cavities, channels, or
interstices. Some of these pores are closed, while open pores may have different shapes including
cylindrical, blind with one open end, inkbottle or funnel shaped.
Porosity is the ratio of the total pore volume to the apparent volume of the particles in the
powder. Pore volume is the volume of the pores in the investigated material. Pore size or pore
diameter is the distance between two opposite walls of the pore.
Pore size is important for the application of materials. They are divided into three poresize regions and characterized according to their sizes: microporous (diameter < 2 nm),
mesoporous (2 nm < pore diameter < 50 nm), and macroporous (pore diameter >50 nm). Porosity
of these solids can be characterized by gas adsorption studies.
The adsorption of gas over the surface of the material allows determination of the specific
surface area, total pore volume, and pore size distribution of the sample. The free gas and
absorbed gas are in dynamic equilibrium, and the surface coverage depends on the pressure of
the gas. The fractional surface coverage measured at different pressures at a set temperature
gives adsorption isotherms. The mathematical analysis of the adsorption isotherm gives
necessary data for calculations of surface areas and porosity characteristics of the samples.
Nitrogen is the gas traditionally used as the adsorbate since it exhibits intermediate values for the
C constant (50-250) on solid surfaces and allows calculating the cross-sectional area of an
adsorbate.
The Branauer-Emmet-Teller equation (3.1) is widely used for determining of the surface
area of solid materials [1]
1
1
C −1 P

=
+
P
W
C
W
C


m
m
 P0
W  0 − 1
P




(3.1)
50
in which W is the weight of the absorbed gas at relative pressure P/P0, C is a constant related to
the energy of adsorption in the first adsorbed layer, and Wm is the weight of a monolayer of
adsorbate.
The total surface area of the sample can be calculated by
St =
Wm NAcs
M
(3.2)
where N is an Avogadro number (6.023 ×1023 molecules/mol) and M is a molecular weight of
the adsorbate, and Acs is a cross-sectional area (16.2 Å at 77 K).
The specific surface area can be calculated from the total surface area and the weight of
the material sample:
S=
St
w
(3.3)
The total pore volume can be calculated from the amount of vapor adsorbed at relative
pressure close to unity. Besides, the pore size distribution can be calculated from the desorption
branch of the isotherm.
Pore size is calculated using the Kelvin equation:
rK =
− 2γVm
P
RT ln 
 P0 
(3.4)
where γ is a surface tension of nitrogen at its boiling point (8.85 ergs/cm2 at 77 K), Vm is a molar
volume of liquid nitrogen (34.6 cm3/mol), R is a gas constant (8.314 × 107 ergs/deg mol), T is a
boiling point of nitrogen (77 K), P/P0 is a relative pressure of nitrogen, and RK is a Kelvin radius
of the pore.
51
The analysis of gas adsorption on the surface of the material can provide information on
the surface area and porosity. The adsorption isotherm is obtained by introducing known
volumes of nitrogen and measuring the equilibrium pressure. The desorption isotherm is
obtained by measuring the quantities of released gas from the samples as the relative pressure is
lowered.
All isotherms may be divided into five types according to the way the adsorption of
nitrogen occurs.
Figure 3.5 Five Types of Adsorption Isotherms [2]
52
Type I isotherm is associated with microporous materials where the adsorption of
nitrogen is on the external surface area and is limited by the accessibility of the micropores. Type
II isotherms represents nonporous and macroporous materials where unrestricted multilayers of
nitrogen can be adsorbed. Type III isotherm is characterized by the heat of adsorption which is
less than the heat of adsorbate liquefaction and there is an additional adsorption through the
interaction with the adsorbed layer. Type IV isotherm represents mesoporous materials where the
mesopores are filled at higher elevated pressures. Finally, Type V isotherm is similar to the Type
III isotherm but this type is common for mesoporous materials.
The adsorption and desorption branches in Type I, Type II, and Type III isotherms are
identical and hysteresis occurs very rarely. However, in Type IV and Type V isotherms there is a
hysteresis effect between adsorption and desorption. The bottleneck shape of pores and
differences in the meniscus of the condensing and evaporating nitrogen causes this effect [3, 4].
Isotherms from aerogels usually fall into the Type IV category for mesoporous solids.
Aerogels are usually meso- to macroporous with little microporosity. These textural properties
are catalytically favorable for easy accessibility to the internal surface and high availability of
active sites.
Surface area studies were conducted using a Quantachrome NOVA 1200 instrument.
Each sample was outgassed, and then the surface area, pore volume, and pore diameter data were
measured by the BET method from the amount of N2 absorbed at 77 K.
Traditionally, solid-state prepared materials at high temperatures can be characterized by
absent or small porosity and high density, while aerogels are materials with extremely high
porosity and low density. The pore structure of aerogels mainly corresponds to the mesoporous
structure with interconnected pores in the range of 2 to 50 nm.
The surface areas of SSR-SrTiO3 and SSR-BaTiO3 have not exceeded over 1 m2/g
(Tables 3.1, 3.3). Compared to aerogel, solid-state prepared samples are not porous materials and
contain significantly smaller pore volumes as is evident from the smaller amount of N2
adsorption.
The surface area of AP-SrTiO3 (ethanol) was 160± 1 0 m2 /g in comparison with 1 m2 /g of
the commercial CM-SrTiO3, and 17 m2 /g of nanosized commercial NCM-SrTiO3 (Table 3.1).
Calcination in air at 300 °C did not significantly affect the specific surface area. At higher
temperatures (400 °C and 500 °C) the surface area decreased by one third and almost half
53
respectively (Table 3.2). The AP-SrTiO3 (Methanol) and NCM-SrTiO3 had similar crystallite
sizes, but the surface area of the aerogel sample was five times higher due to much larger pore
volume and pore accessibility.
The surface area of AP-BaTiO3 (Ethanol) was 175 ± 15 m2 /g in comparison with 3 m2 /g of
commercial CM-BaTiO3, and 19 m2 /g of nanosized commercial NCM-BaTiO3 (Table 3.3).
During calcination in air, the surface area was dramatically reduced from 175 m2 /g to 89 m2 /g
at 400 °C, and at 500 °C it decreased further to 45 m2 /g (Table 3.4). Sintering during heating
caused an increase in crystallite size and average pore size, and decreased the total pore volume.
3.3.3 Powder X-ray diffraction
Powder X-ray diffraction (XRD) is a powerful technique for determining the structure of
materials with long-range order [5]. However, for disordered and amorphous materials this
technique finds only limited applications. The XRD pattern of each pure substance is unique, so
for different substances there are no identical patterns. The availability of numerous standard
patterns in data libraries allows identifying pure substance or crystalline phases in this substance
by using simple search and matching procedures.
When an X-ray beam heats the atoms in the solid sample the electrons of these atoms start
to oscillate and form constructive interference, which is characteristic of the arrangement of
atoms in the crystal. The diffracted beam consists of in phase X-rays which mutually reinforce
one another.
The diffraction can be explained by incident and reflected rays and theta angle (θ) between
them. Using Bragg’s Law, the unit cell dimensions (d) can be determined:
2d sin θ = nλ
(3.5)
The average crystallite sizes of samples can be calculated from the XRD spectra using the
Debye-Scherrer equation:
54
D=
Kλ
B cosθ B
(3.6)
where D is the thickness of the crystal (Å); K = 0.9 is a constant related to the crystallite shape; λ
is the X-ray wavelength (1.54051 Å for CuKα); θB is the Bragg angle (between incident and
diffracted beam); and B is the line broadening, measured from the peak width at half the peak
height (radians).
Spectrometric studies were conducted using a Brucker D8 Advance and Scintag XDS 2000
(Chemistry Department, KSU), and a Shimadzu XRD 6000 (NanoScale Materials, Inc.;
Manhattan, Kansas). Copper Kα was the radiation source used with an applied voltage of 40 kV
and a current of 40 mA. The 2θ angles ranged from 20 to 85° with a scanning rate of 2°/min.
In comparison to commercial samples, synthesized SrTiO3 aerogels have significantly
broader peaks, corresponding to smaller crystallite sizes. The XRD patterns show the welldefined crystalline structure of the powders after supercritical drying (Figure 3.6). These samples
do not require high-temperature treatments to initiate a phase change from an amorphous to
crystalline state. The removal of a residual organic solvent can be achieved by heating
procedures.
Solid-state prepared strontium titanate samples have narrow peaks of well-defined
crystalline structure of the SrTiO3 phase with crystallites of big sizes (Figure 3.7). The presence
of additional peaks in the XRD patterns can be assigned to the impurity phases produced by
incomplete reaction of precursors (Sr2TiO4) and oxygen deficiency (SrTiO2.7) (Appendix A).
The crystallite sizes of different SrTiO3 samples prepared by the aerogel procedure in
different alcohol-toluene mixtures, prepared by the solid-state reaction, and available
commercial SrTiO3 samples are summarized in Table 3.1.
Ethanol mixed with toluene was the best solvent for AP-SrTiO3, yielding products with
the highest surface area and the smallest crystallite sizes in comparison with other alcoholtoluene mixtures.
55
Table 3.1 Characteristic Properties of Different SrTiO3 Samples
SrTiO3 sample
Average
(alcohol in solvent
crystallite
for aerogels)
sizes (nm)
Commercial
CM-SrTiO3
Nanosized
NCM-SrTiO3
SSR-SrTiO3
AP-SrTiO3
(methanol)
AP-SrTiO3
(ethanol)
AP-SrTiO3
(isopropanol)
Surface area
Total pore
Average pore
(m2 /g)
volume (cm3 /g)
sizes (Å)
145
1
0.003
93
25
17
0.12
290
150
1
0.009
125
25
82
0.58
280
8
159
0.62
160
20
121
0.59
190
56
0
1000
2000
3000
4000
5000
6000
7000
20
30
40
2 theta (deg)
50
60
70
CM-SrTiO3
NCM-SrTiO3
80
AP-SrTiO3 (Isopropanol)
AP-SrTiO3 (Methanol)
AP-SrTiO3 (Ethanol)
SrTiO3 – Commercial, NCM-SrTiO3 – Commercial Nanosized, AP-SrTiO3 – Aerogel Prepared Samples)
Figure 3.6 Powder XRD Patterns of Commercial and Synthesized SrTiO3 with Different Alcohols Used in Synthesis (CM-
Intensity (arb)
57
0
200
400
600
800
1000
20
30
40
AP-SrTiO3
o
60
70
o
80
SSR-SrTiO3 1100 C
SSR-SrTiO3 1200 C
2 theta (deg)
50
o
AP-SrTiO3 500 C
Figure 3.7 Powder XRD Patterns of Solid-state (SSR-SrTiO3) and Aerogel Prepared (AP-SrTiO3) Samples
Intensity
58
The crystallite sizes of the SSR-SrTiO3 sample have non-uniform shapes. Sizes and shapes
of the particles for this sample are similar to commercially available CM-SrTiO3. This sample
has a small surface area of 1 m2/g and the total pore volume of 0.009 cm3/g.
The crystallite sizes of AP-SrTiO3 synthesized in an ethanol/toluene solvent were an
average of 8 ± 2 nm in diameter, and were three times smaller than commercial nanosized NCMSrTiO3 sample (25 nm), and almost twenty times smaller than commercial CM-SrTiO3 (145 nm)
(see Table 3.1). The information on sizes and shapes of AP-SrTiO3 are also confirmed by
transmission electron microscopy. The XRD showed no change in pattern for calcined SrTiO3
aerogels although calcination caused narrowing of the peak width due to sintering and, therefore,
to a slight increase in crystallite sizes (Figure 3.8 and Table 3.2).
Table 3.2 Characteristic Properties of AP-SrTiO3 (Ethanol) Calcined at Different
Temperatures in Air
Temperature
Average
of calcination
crystallite size
(ºC)
(nm)
265
Surface area Total pore volume Average pore
(m2 /g)
(cm3 /g)
size (Å)
8
159
0.62
160
300
6
156
0.57
150
400
9
114
0.54
190
500
10
93
0.45
190
Identical to the synthesized SrTiO3 aerogels, a well-defined crystalline structure of the
powders was observed after supercritical drying for AP-BaTiO3 (ethanol) and AP-BaTiO3
(isopropanol) aerogels (Figure 3.9). The broad peaks in the spectra corresponded to smaller
crystallite sizes in comparison with CM- and NCM-BaTiO3 samples.
59
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
29
30
31
2 theta (deg)
32
33
34
o
265 C
o
300 C
o
400 C
o
500 C
35
Figure 3.8 Powder XRD Patterns of AP-SrTiO3 (Ethanol) Calcined in Air at Different Temperatures
Intensity (arb)
60
0
1000
2000
3000
4000
5000
6000
20
30
40
60
2 theta (deg)
50
70
CM-BaTiO3
NCM-BaTiO3
80
AP-BaTiO3 (Isopropanol)
AP-BaTiO3 (Ethanol)
the Samples were Conducted using a Shimadzu XRD 6000 (NanoScale Materials, Inc.)
61
BaTiO3 – Commercial, NCM-BaTiO3 – Commercial Nanosized, AP-BaTiO3 – Aerogel Prepared Samples). The XRD Studies of
Figure 3.9 Powder XRD Patterns of Commercial and Synthesized BaTiO3 with Different Alcohols Used in Synthesis (CM-
Intensity (arb)
Due to the high temperature synthetic method, the solid-state prepared barium titanate
samples have similar textural properties to commercially available barium titanate. These
samples have very small surface areas and total pore volumes.
For AP-BaTiO3 samples, the use of ethanol/toluene solvent provided the smallest
crystallites: an average of 6 ± 2 nm, compared to commercial CM-BaTiO3 (76 nm) and
commercial nanosized NCM-BaTiO3 (36 nm) (Table 3.3).
Table 3.3 Characteristic Properties of Different BaTiO3 Samples
BaTiO3 Sample
(alcohol in solvent)
Average
Surface
Total pore
Average pore
area (m2 /g)
volume (cm3 /g)
sizes (Å)
76
3
0.009
110
36
19
0.16
330
200
1
0.02
125
6
175
0.43
100
12
101
0.23
92
crystallite
sizes (nm)
Commercial
CM-BaTiO3
Nanosized
NCM-BaTiO3
SSR-BaTiO3
AP-BaTiO3
(ethanol)
AP-BaTiO3
(isopropanol)
In contrast to AP-SrTiO3, the calcination of AP-BaTiO3 in air caused the formation of a
carbonate phase that can clearly be seen in the XRD pattern (Figure 3.10). The organic residues
in the aerogel-prepared materials were removed by heat-treating the aerogels in vacuum,
followed by calcination in air or oxygen (Figure 3.11). Calcination aerogels showed an increase
in crystallite size (Table 3.4).
62
0
100
200
300
400
500
600
700
800
900
1000
BaCO3
20
30
40
60
2 theta(deg)
50
o
70
80
AP-BaTiO3 (Ethanol)
calcined at 500 C in air
AP-BaTiO3 (Ethanol)
Prepared Sample of AP-BaTiO3 (Ethanol)
Figure 3.10 Powder XRD Patterns of AP-BATiO3 (Ethanol) Calcined in Air at 500 °C with a BaCO3 Phase vs. Freshly
Intensity (arb)
63
Intensity (arb)
20
30
40
60
2 theta (deg)
50
o
70
80
Aerogel BaTiO3
Aerogel BaTiO3
o
calcined at 500 C air
calcined at 500 C oxygen
Aerogel BaTiO3
Figure 3.11 Calcination of AP-BaTiO3 (Ethanol) in Air and Oxygen at 500 °C
0
200
400
600
800
1000
1200
64
Table 3.4 Characteristic Properties of AP-BaTiO3 (Ethanol) Calcined at Different
Temperatures in Air
Temperature
Average crystallite
Surface area
Total pore
Average pore
size (nm)
(m2 /g)
volume (cm2 /g)
size (Å)
265
6
175
0.43
100
400
9
89
0.37
170
500
11
45
0.39
350
of calcination
(ºC)
Significant crystallite size increase was observed for the aerogel prepared BaTiO3 powders
during calcination; the average crystallite sized increased almost double after calcination in air at
500°C.
3.3.4 Transmission electron microscopy
The characteristic property of powder materials is their particle sizes. Although the X-ray
diffraction reveals the structure and the average size of the particles, it doesn’t provide shape and
size distribution of the crystallites. Fortunately, transmission electron microscopy (TEM) can
give a real space image on the distribution of particles, their surface and shape [6]. With a finely
focused electron probe, not only imaging of materials is possible, but also a single particle can be
identified. Besides, electron microscopy shows the shape and state of agglomeration of particles.
Samples were placed onto a carbon-coated copper grid by physically interacting the grid
and powders, and analyzed to see the particles that remained adhered to the grids. The TEM
studies were performed using a Philips CM 100 (Biology Department, KSU).
The crystallite morphology of the solid-state and aerogel prepared strontium titanate
samples was observed by TEM (Figures 3.12-3.15). The agglomeration of primary particles
65
(crystallites) is caused by van der Waals forces; and this tends to form secondary particles
(agglomerates). In aerogel samples, crystallite sizes after calcination were slightly larger in
comparison with crystallites of freshly prepared aerogels. All crystallites had uniform spherical
shapes with a size distribution near 8 nm for synthesized AP-SrTiO3 (ethanol) and near 10 nm
for calcined AP-SrTiO3 (ethanol) at 500 °C in air. The shapes of aerogel samples have a defined
spherical form and sizes are relatively monodispersed, while solid-state prepared samples have
non-uniform shapes and polydispersed sizes.
The crystallite morphology of AP-BaTiO3 (isopropanol) after heat treatment in vacuum and
calcination in air is presented in Figure 3.15. The heat treatment in vacuum did not cause
significant changes in crystallite sizes; however, the calcination in air or oxygen increased the
AP-BaTiO3 crystallite size to almost double the original size.
Heat treatment in vacuum of AP-BaTiO3 caused the formation of pure carbon (dark grey
color). Calcination in air removed the coloring, but also formed the BaCO3 phase. The formation
of a carbonate phase was inhibited by calcination of AP-BaTiO3 in oxygen. This procedure was
done in a Schlenk tube under a flow of pure oxygen at 500°C for oxidation and removal of
organic residuals. The XRD pattern confirmed the absence of a carbonate phase (Figure 3.11).
66
Figure 3.12 Transmission Electron Micrographs of SSR-SrTiO3 Prepared at 1100 °C
67
(right)
68
Figure 3.13 Transmission Electron Micrographs of AP-SrTiO3 after Synthesis (left) and after Calcination in Air at 500 °C
Figure 3.14 Transmission Electron Micrographs of SSR-BaTiO3 Prepared at 1100 °C
69
Vacuum at 500 °C (middle), and Calcined in Air at 500 °C (right)
70
Figure 3.15 Transmission Electron Micrographs of AP-BaTiO3 (Isopropanol) Samples after Synthesis (left), Heat Treated in
3.3.5 Elemental analysis
The content of organic residues in aerogel products, which arise from the organic
components involved in the sol-gel process, was studied by CH analysis at Galbraith
Laboratories, Inc. (Appendix B). Elemental analyses for carbon and hydrogen content were
performed on freshly prepared and calcined aerogel samples.
The presence of organic residues after supercritical drying can significantly influence the
physical and chemical properties of aerogel-prepared samples. In order to remove the organic
residues, aerogel-prepared samples were heat treated in vacuum and/or calcined in air. After
heat-treating AP-SrTiO3 in vacuum, the sample color changed to dark grey, due to the pure
carbon formed from the degradation of residual organics. Calcination in air allows nearly
complete removal of residuals. The elemental carbon content in fresh AP-SrTiO3 was 1.86 %,
and the elemental H content was 0.70 %. The presence of carbon and hydrogen in a 500 °Ccalcined sample significantly decreased to < 0.5 % for both C and H.
3.3.6 Thermogravimetric analysis
Thermogravimetric studies (TGA-50, Shimadzu) shows that heating a freshly prepared
aerogel sample in air causes the desorption of physically adsorbed solvent and water from the
surface at 100-200ºC and decomposition of residual organic groups into carbon dioxide at 300500ºC (Appendix C). This method was used to determine the weight loss of organic residuals
during the heat treatment. Attention must be directed to the removal of organics to avoid
undesirable blackening of the sample from the reduction of organic residuals into pure carbon.
Two major weight losses were observed. The first weight loss occurs during heating to
100°C and can be attributed to the remove of physisorbed water. The second weight loss occurs
at 400°C which is due to the removal of chemisorbed water and organic residuals of solvent.
3.4 Discussion
The alcohol-toluene mixture was essential for obtaining mixed metal oxides with high
surface areas and small crystallite sizes. The usefulness of a binary solvent (methanol-benzene)
71
was first noticed by Teichner and co-workers almost 30 years ago [7]. From long and continuous
study of aerogel syntheses, we have found that a mixture of alcohol and hydrophobic solvent
favors the production of high-quality aerogels. The use of alcohol without hydrophobic solvent
yields an aerogel with significantly lower surface area [8]. Toluene in the binary solvent
apparently serves to speed up gelation and perhaps to reduce surface tension at the gas-liquidpore walls. This causes less dense and more porous wet gels to form of lower mass fractal
dimension, leading to products with higher surface areas. Metal alkoxides are soluble in toluene
and although alcohol is needed to aid hydrolysis, rates are faster in the presence of toluene. The
role of alcohol is also important for the preparation of metal alkoxide, which is based on the
zero-valent metal reacting directly with the alcohol.
Different alcohols in mixture with toluene lead to aerogels with different properties crystallite size, surface area, pore-size distribution - thereby requiring studies of the alcohol
effect on aerogel products. Ethanol in mixture with toluene was the best solvent choice to
synthesize SrTiO3 and BaTiO3 aerogels with the highest surface areas and smallest crystallite
sizes. The influence of this binary solvent on the aerogel process is not clear and requires further
investigations. However, the gel stability was altered by the use of an organic solvent such as
ethanol and powders prepared in ethanol exhibited a more uniform particle size distribution.
By the modified aerogel procedure, it is possible to produce pure powders of strontium
and barium titanates with very small crystallite sizes and high surface areas. The morphology, as
determined by powder XRD and TEM, indicates a crystalline nature with very small crystallites,
and XRD and TEM results are in good agreement with respect to crystallite size. The high
surface areas of aerogel-prepared samples correspond to their high total pore volume. The
comparison of two samples (NCM-SrTiO3 and AP-SrTiO3 (methanol)) with similar crystallite
sizes and pore sizes accentuates the big difference in total pore volume (Table 3.1).
Temperature treatment is a crucial step and must be done carefully in order to preserve
the high surface area and total pore volume of the powders. Heat treatment in vacuum does not
greatly influence crystallite size, while calcination in air increases the AP-BaTiO3 crystallite size
significantly. The surface area, however, is a parameter that is noticeably affected by heating. It
decreases significantly due to a decrease in total pore volume. With calcination in air at 500 °C,
the AP-SrTiO3 surface area decreases by 40 % and the AP-BaTiO3 surface area decreases to
25 % of the initial value (Tables 3.2 and 3.4). The formation of the BaCO3 phase during
72
calcination in air is not desirable. Heat treatment in vacuum prevents formation of carbonate, but
produces carbon impurities in the sample. Calcination in O2 solves the carbonate formation
problem although the surface area also decreases significantly.
3.5 Conclusions
Alkaline-earth metal titanates (SrTiO3 and BaTiO3 ) were synthesized by solid-state
reaction through the reaction of alkaline-earth metal carbonate and titanium oxide, and by an
aerogel procedure through the sol-gel process by hydrolysis of alkaline-earth metal alkoxide and
titanium alkoxide with followed by supercritical drying.
The solid-state reaction produces materials with non-uniform shapes and polydispersed
sizes. This morphology of crystallites arises from the high temperature calcination synthesis
where the big particles grow by sintering and densification of small ones. The sol gel process is
particularly attractive for the synthesis of multicomponent materials with binary or ternary metal
composition using several metal alkoxides. Multicomponent gels can be thermochemically
converted to form nanocomposite oxide powders.
The aerogel method for synthesis of nanosized SrTiO3 and BaTiO3 powders has been
satisfactorily developed. This method permitted larger surface areas and smaller crystallite sizes
to be obtained in comparison with the traditional solid-state reaction method. Another big
advantage of the aerogel procedure is the significantly lower calcination temperature for the
preparation of pure crystalline titanates.
Different alcohols mixed with toluene were used as solvents for synthesis. AP-SrTiO3
and AP-BaTiO3 were prepared in either an ethanol or isopropanol mixture with toluene in a
toluene-to-alcohol volume-ratio of 1.5. The mixture of metal alkoxides and alcohol-toluene was
hydrolyzed with water by a slow, dropwise addition, to transform it into a gel. Supercritical
conditions were achieved by heating in an autoclave under a nitrogen atmosphere. The solvent
vapors were removed by quick venting off, and the residual solvent in the prepared aerogel
powders was removed by heat treatment (calcination in air for SrTiO3, and calcination in oxygen
for BaTiO3).
A binary mixture of alcohol-hydrophobic solvent (ethanol-toluene) was the best solvent
for obtaining high-surface-area SrTiO3 and BaTiO3 by the aerogel-modified procedure. The
73
presence of toluene in the mixed solvent changes gelation rates and lowered the surface tension
in the gel pores, yielding more porous, less dense wet gels that were processed into aerogelprepared products with a higher surface area and a higher pore volume. The presence of ethanol
in the mixture solubilized the water added for hydrolysis in the homogeneous ethanol-toluenemetal alkoxides mixture.
Transmission electron micrographs confirmed the crystallite sizes of aerogel-prepared
samples derived from XRD. Strontium titanate prepared by the above method has a surface area
of 160± 1 0 m2 /g and a crystallite size of 8 ± 2 nm in comparison with 1 m2 /g and 145 nm of
the commercial CM-SrTiO3, and 17 m2 /g and 25 nm of nanosized commercial NCM-SrTiO3.
Barium titanate has 175 m2 /g of surface area and 6 nm crystallite size compared to commercial
CM-BaTiO3, which is 3 m2 /g and 76 nm, and 19 m2 /g and 36 nm for commercial nanosized
NCM-BaTiO3.
Aerogel products contain some residual solvents after synthesis, which can be removed
by high-temperature oxidation. Heat treatment in vacuum of AP-SrTiO3 and AP-BaTiO3 caused
the sample color to change to dark grey signaling the appearance of pure carbon from the
degradation of the residual organics. Calcination in air almost fully removed residuals for APSrTiO3 at 500 °C and caused a slight increase in crystallite sizes. Aerogel products contain some
residual solvents after synthesis, which can be removed by simple calcination at 500 °C in air for
AP-SrTiO3. Calcination of AP-BaTiO3 in air, however, generates the BaCO3 phase; to prevent
this, calcination in oxygen is necessary.
The high surface area in titanates tends to impart interesting properties as catalysts,
sorbents, and electronic materials. Details concerning this surface chemistry will be reported
later in the Chapter 6.
It should also be pointed out that the modified aerogel synthesis (MAP) can be used to
prepare intimately mixed metal oxide nanomaterials of various compositions, including MgOAl2O3, CaO-Al2O3, SrO-Al2O3, BaO-Al2O3, MgO-SrO, and others [9]. These materials
invariably possess superior capacities and kinetics as sorbents for gases and toxic chemicals, and
represent a new family of mesoporous inorganic mixed metal oxides [10].
74
3.6 References
[1]
S. Braunauer, P. Emmet, and E. Teller, J. Am. Chem. Soc., 60 (1938) 309.
[2]
S. Braunauer, L.S. Deming, W.S. Deming, and E. Teller, J. Am. Chem. Soc., 62 (1940)
1723.
[3]
J.W. Mc Bain, J. Am. Chem. Soc., 57 (1935) 699.
[4]
L.H. Cohan, J. Am. Chem. Soc., 60 (1938) 433.
[5]
B.D. Cullity, Elements of X-Ray Diffraction, Addison-Wesley Publishing Co. Inc., CNM
book, 1978.
[6]
P.B. Useck, J.M. Cowley and L. Eyring eds. High Resolution Transmission Electron
Microscopy and Associated Techniques, New York, London, Amsterdam, Oxford University
Press, 1988.
[7]
S. J. Teichner, G. A. Nicolaon, M. A. Vicarini, and G. E. E. Gardes, Adv. Coll. Interface
Sci., 5 (1976) 245.
[8]
S. Utamapanya, K. J. Klabunde, and J. R. Schlup, Chem. Mater., 3 (1991) 175.
[9]
C. L. Carnes, P. N. Kapoor, and K. J. Klabunde, Chem. Mater., 14 (2002) 2922.
[10]
G.M. .Medine, V. Zaikovskii, and K. J. Klabunde, J. Mater. Chem., 14 (2004) 4, 757.
75
Chapter 4 : Photooxidation of acetaldehyde by titanates
4.1 Introduction
Aerogel materials are potential candidates for catalysis due to their unique morphological
characteristics and chemical properties. A major advantage of them is their high surface area and
high number of active sites per gram of aerogel material respectively. The porous structure of
aerogel oxides gives good access for molecules and resistance to thermal coarsening.
The mixed metal oxide aerogels of NiO-Al2O3 and NiO-SiO2-Al2O3 were studied for
selective oxidation [1, 2], NiO-Al2O3 for nitroxidation [3], Fe2O3-Cr2O3-Al2O3 for selective
reduction [4], Fe2O3-SiO2 and Fe2O3-Al2O3 selective hydrogenation [5, 6]. For this organic
synthesis, aerogel catalysts give high selectivity; however, the activity is not outstanding.
Aerogel materials can be used for the protection of the environment and solving
environmental problems by the reduction or decomposition of hazardous wastes, toxic air
contaminants, toxic chemicals (volatile organics, solvents, pesticides, chlorophenols, heavy
metals etc.), decrease of pollutant emissions or exhaust gases from automobiles, and purification
of contaminated groundwaters [7].
It is desirable to decompose these toxic compounds and pollutants so that only CO2, H2O,
and N2 are released in the atmosphere. Traditionally, to fully decompose these compounds the
catalysis at a temperature from 200 to 1200°C is necessary. This catalytic process is energy
intensive and not economically feasible for low pollutant concentrations. Besides, high
temperature causes oxide sintering and decrease of the active catalytic surface. Thus, processes
that can decompose low pollutant concentrations under ambient conditions are needed.
The photocatalytic process can be used for the detoxification of air pollutants at low
concentration
and
achieve
complete
contaminant
mineralization.
Aerogel
prepared
semiconductor metal mixed oxides of strontium titanate with a surface area of 160 m2/g and
barium titanate with a surface area of 175 m2/g may have advantages for the photocatalytic
decomposition of volatile organic compounds or pollutants, so further investigations are
necessary.
76
4.2 Photoactivity under light irradiation
The discovery of photoinduced water splitting on titanium dioxide electrodes by Fujishima
et al. in 1972 opened new possibilities for the application of semiconductor solid materials for
catalysis induced by light irradiation [8, 9]. The production of hydrogen from water was based
on a photoelectrochemical cell (PEC) where TiO2 single crystal was a photoanode and Pt metal
was a cathode. The UV light irradiation of the titanium oxide surface causes the separation of
holes and electrons by electron migration to the conduction band and hole production in the
valence band (Figure 4.1). The generation of electrons and holes in the conduction and valence
bands occurs only when light with energy larger than the band gap (hν ≥ Eg) is incident on the
catalyst and excites it. A wider bang gap requires shorter wavelengths.
The photogenerated electrons and holes act as reducers and oxidizers and split water by
reduction of water molecules with electrons to form H2 and by oxidation with holes to produce
O2:
TiO2 + hυ → h+vb + e-cb
(4.1)
Valence band holes (h+vb) are powerful oxidants (+1.0 to +3.5V versus Normal Hydrogen
Electrode, NHE) and can split water forming oxygen gas molecules and/or hydroxyl radical
species.
2 h+vb + H2O → 2 H+ + ½ O2
(4.2)
h+vb + H2O → H+ + OH•(ads) (E = 2.85eV)
(4.3)
For H2 gas conduction band electrons are good reductants (+0.5 to –1.5V versus NHE) and can
form a hydrogen gas molecule and/or superoxide species.
2 e-cb + 2 H+ → H2
(4.4)
77
e-cb + O2 → O2-
(E = -0.13eV)
(4.5)
For H2 gas evolution the conduction band potential of the semiconductor must be more negative
than the H2 evolution potential (- 0.4 V (Standard Hydrogen Electrode, SHE) in acid solution or
– 1.2 V (SHE) in alkaline solution). Similar, for the O2 gas evolution the valence band potential
of the semiconductor must be more positive.
Figure 4.1 Semiconductor Photocatalyst for Water Photolysis
Holes and electrons tend to recombine, so to prevent recombination and a decrease in
photoactivity, platinum metal cocatalyst was deposited on the titania. In this case, electrons were
migrating to the metal and were trapped there, while holes were staying on the surface of
titanium dioxide. Hydrogen gas was produced on the surface of Pt metal while oxygen bubbles
were formed on the titania surface.
Four years later in 1976 it was found that strontium titanate (SrTiO3) is also an efficient
photocatalyst for the H2 production from water and for the decomposition of organic compounds
under the UV irradiation [10]. The SrTiO3 sintered powder electrodes and the single crystal
electrodes showed almost the same quantum efficiency, action spectra and potential of
photocurrent. It was also found that SrTiO3 had higher negative potential than that of TiO2 and
was more efficient for hydrogen production.
The desirable band gap of the semiconductor for optimum utilization of solar energy
(visible light) for water splitting should be around 2 eV [11]. A non-oxide material such as
78
cadmium sulfide (CdS) has a matching band gap of 2.4 V and is a very good photocatalyst
candidate for water photolysis and hydrogen production. However, this chalcogenide
semiconductor undergoes easy photocorrosion, and rapidly loses photoactivity due to the surface
covering with reduced sulfur. Oxide materials have higher stability during photocatalysis, but
have wide band gaps and do not absorb visible light. Among conventional semiconductor oxides,
only several show activity for water photolysis (Figure 4.2) and all of them requires UV light
irradiation due to wide band gaps.
With time, the PEC cells were replaced by particulate systems [12-16]. The application of
the monoccrystalline system (semiconductor single crystal) was very expensive, while the
polycrystalline system (semiconductor powders in suspension) is simpler, requires low
construction and maintenance cost and gives high light absorption efficiency.
The cocatalysts of noble metals deposited on the semiconductor oxide prevent the
recombination of holes and electrons and increase the photoactivity toward water splitting. A Pt
metallized powdered TiO2 semiconductor (Pt/TiO2) can work like a TiO2-Pt
photoelectrochemical cell (PEC). Pt/TiO2 forms small amount of H2 and no O2 in pure water
under UV irradiation in the absence of external bias. Pt/TiO2 can not photodecompose gas-phase
water in so called “dry state”; however, water was decomposed in a “wet state” when Pt/TiO2
was moistened with a small amount of water.
Pt/SrTiO3 decomposes gas-phase water into H2 and O2 when it is coated with basic material
(NaOH). Rh/SrTiO3 shows activity for water photolysis with 1.2 % maximum quantum yield.
79
Figure 4.2 Semiconductor Oxide Band Gaps and Potentials
Photocatalytic reactions can be divided into two categories: down hill and up hill reactions.
Water photolysis is an up hill reaction and has a high positive change in the Gibbs free energy
(∆G0 = 237 kJ/mol) [17], while the photodegradation of organic compounds in the presence of
oxygen is a down hill reaction (Figure 4.3). In down hill reactions, the catalyst produces active
species of O2-, H+, and OH• (See reactions 4.3 and 4.5) for the oxidation of organic compounds.
The studies of down hill reactions revealed that TiO2 is an excellent catalyst for the
photocatalytic degradation of organic compounds [18].
80
Figure 4.3 Photocatalytic Reactions: Photoinduced Reaction (down hill) and Photon
Energy Conversion Reaction (up hill) [17]
TiO2 was extensively studied for the photocatalytic degradation of volatile organic
compounds (VOCs), such as aldehydes, aromatic compounds and chlorinated hydrocarbons [1922]. The full photocatalytic degradation of these compounds gives simple CO2 and H2O (HCl for
chlorine containing organic compounds) reaction products. The full degradation of acetaldehyde
into CO2 and H2O was achieved in batch type reactors [23, 24]. The degradations of
trichloroethylene (TCE) and benzene on TiO2 under UV irradiation were also reported [22, 25,
26]. All reported investigations on the photodecomposition of organic compounds were mostly
performed at elevated temperatures.
TiO2 is not active for the oxidation of CO which was formed during the oxidation of
benzene. The addition of Pt to TiO2 by the photodeposition from H2PtCl6/6H2O enhances its
reactivity for the CO oxidation under UV light irradiation. The difference in the reactivity of
photocatalyst depends on the kind of an active oxygen species formed on the Pt/TiO2 and
reactants. It was also found that the addition of Pt enhances the stability of oxygen species (Oand O3 -) on the TiO2 surface:
CO + O3- → CO2 + O2- (UV light and presence of O2)
(4.6)
While extended investigations were performed on titanium oxide, some studies have been
done on other oxide semiconductor powders. It was found that they also exhibit photoactivity in
the oxidation of alcohols, hydrocarbons, and CO [27]. Nanosized SrTiO3 powders (5-13 nm, 46.7
81
m2/g) were used for the photodegradation of phenol in water [28]. The NiO supported SrTiO3
was studied for the photocatalytic production of H2 and acetone from mixture of H2O and 2propanol in a gas phase. The photooxidation of C2H6 in the presence of O2 went through the
production of alcohol and aldehyde intermediates to the final products of CO2 and H2O. BaTiO3
photochemicaly reduced aqueous silver cations and oxidized thin films of steric acid converting
it into CO2 and H2O [29].
From the comparison of catalytic activity of catalyst samples prepared by different
methods, it was found that their photocatalytic activity strongly depends on the preparative
source. The conventional solid-state reaction gives materials with small surface areas and large
particle, while novel wet methods (sol-gel, aerogel) can offer catalyst materials with significantly
higher surface areas which can benefit the photocatalysis. Besides, the xerogel or aerogel
materials have higher purity and less crystal defects. The study of these novel materials for
photocatalysis and comparison with already available samples prepared by conventional methods
will bring more understanding of the complex process of catalysis induced by light irradiation on
the surface of the semiconductor oxide.
4.2.1 Design of photocatalysts of high activity
Many factors influence the activity of a photocatalysts including the number of active sites
in the catalyst, mobility and life time of electrons and holes, charge separation and recombination
rate. These factors are strongly affected by bulk properties of material. The activity can be
significantly influenced by the textural characteristics of the catalyst. The high specific surface
area of the catalyst favors catalysis due to presence of more active reaction sites and higher rate
of surface reaction of electrons or holes. However, the surface where the light irradiation does
not reach does not contribute to the photoactivity.
Small particle size of catalyst is also in favor of higher photoactivity due to smaller
distances for electrons and holes to migrate to the surface. It takes longer time for the
photogenerated e- and h+ to reach the surface in larger size particles and recombination can occur
more often. When the dimensions of semiconductor particles decrease, the energy levels shift
according to the quantum size effect. The shift of the conduction band may accelerate the
reduction, while that of the valence band may increase the oxidation reaction. However, when
82
the size of the particle decreases, the transfer efficiency of electron-hole separation decreases
too.
Crystal structure of the catalyst material is an important factor and can influence catalytic
activity by the rate of crystallinity and presence of crystal defects in the catalyst material. Higher
crystallinity and less crystal defects allows a slower rate for the recombination of the holes and
electrons. However, highly defective surface materials may also have high photocatalytic activity
due to the electron or hole charge transfer between defects.
Good catalysts have high activity and selectivity toward promoting a desired reaction, high
stability toward resistance to degradation, and they stay active over many catalytic cycles. Very
often high activity and selectivity correlate with surface structure of the catalyst and the amount
of active sites on this surface.
Textural properties can significantly influence the photocatalytic properties of a catalyst.
Surface defects such as surface oxygen vacancies can act as photocatalytic sites and under UV
irradiation [30]. O2 can be absorbed by surface defects and form superoxide ions (O2-) by
interaction with electrons [31, 32].
The uniqueness of high surface nanosized materials and possible advantages for the
decomposition of organics under ambient conditions when illuminated with light attracts
attention toward application of aerogel prepared samples for photocatalysis and investigation of
their photoactivities. The designed photooxidation process includes the decomposition of volatile
organic compounds on the surface of the catalyst in the presence of atmospheric oxygen at low
temperature (room temperature, RT=25 °C) and under light irradiation (visible or UV). The
oxidation of volatile organic compounds undergoes the following reactions:
hν (UV or visible)
VOC + O2
catalyst
hν (UV or visible)
intermediates
catalyst
CO2 + H2O
(4.7)
The importance of these studies is based on possible numerous applications for water and air
purification, self cleaning surfaces, and toxic chemical deactivation.
83
4.2.2 UV light irradiation
Titanium oxide has strong photooxidative ability and can be used for decomposition of
volatile organics and air pollutants. The active oxygen species of hydroxyl radicals (OH•), oxide
ions (O-, O2-, O3-), and other active species produced from the reaction of holes and electrons
with oxygen and water from air and OH- groups and lattice oxygen of TiO2, participate in the
decomposition of organic compounds to form simple reaction products (Figure 4.4).
Figure 4.4 Photocatalytic Oxidation of Various Organic Compounds on TiO2 Surface
under UV Light Irradiation [33]
The active species on the surface of the catalyst can oxidize different air pollutants and
volatile hydrocarbons, and halogenated hydrocarbons to form carbon dioxide, water and/or
hydrogen halides. Besides, different sulfur oxides and nitrogen oxides can be oxidized to form
sulfuric and nitric acids (H2SO4 and HNO3).
The activity of OH• radicals toward oxidation of different air pollutants is presented in
Table 4.1. Common air pollutants like mercaptan (CH3SH), hydrogen sulfide (H2S),
acetaldehyde (CH3CHO) and formaldehyde (HCHO) can be easily decomposed. In addition,
titanium oxide radicals have high activity toward the oxidation of some nitrogen and halogen
84
containing compounds (NO2 and CHCl=CCl2). In some cases however, titanium oxide was
almost inactive for the oxidation of ammonium (NH3), ethanol (C2H5OH), or trichloroethane
(CH3CCl3).
Table 4.1 Rate Constant (k) for the Reaction of OH• Radical with Air Pollutants at 298 K
[33]
Pollutant
k
Pollutant
k
Pollutant
k
CO
1.3
CH3OH
7.9
t-2-C4H8
700
NO2
670
C2H5OH
1.6
CH3CCl3
0.1
NH3
1.6
CH3COOH
8.0
CHCl=CCl2
21
SO2
20
CH4
0.06
CCl2=CCl2
1.7
CH3SH
330
C2H6
2.5
C6H6
10
H2S
48
C3H8
11
Toluene
61
HCHO
92
C2H4
90
m-Xylene
240
CH3CHO
200
C3H6
300
C6H5Cl
6
Photooxidation of organic compounds is a complex process which undergoes several
simultaneously running reactions with the production and consumption of different species on
the surface, and this process is not fully understood. The adsorption and desorption steps when
the reactants approach and adsorb on the surface and reaction products leave the surface can
slow down the oxidation reaction and limit the whole process.
In aqueous suspensions of titanium oxide, the oxidation of organic compounds can go
through direct and indirect oxidation. These two processes can go on simultaneously and the
decomposition of acetic acid on titania particles under UV irradiation in deaerated environment
is a good example (Figure 4.5). Indirect oxidation goes through the production of hydroxyl
radicals (OH•) by the oxidation of water with holes. These radicals react with organics to form
oxidized species or decomposed products. By direct oxidation, the organic compounds are
oxidized on the surface by photogenerated holes.
85
Figure 4.5 Oxidation of Acetic Acid on the TIO2 under UV Irradiation and in Oxygen-free
Environment [33]
It was found that strontium titanate (SrTiO3) powders act similar to titanium oxide and is
also photoactive for the oxidation of volatile organic compounds, alcohols, and hydrocarbons. A
mixture of H2O and 2-propanol were photocatalytically converted into H2 and acetone on the
NiO/SrTiO3 surface. The photooxidation of C2H6 with O2 over SrTiO3 went via alcohol and
aldehyde to final products of CO2 and H2O. Pt-SrTiO3 cocatalyst assisted in the photoreduction
of CO2 with H2 to CO in the presence and absence of water vapors, and side products such as
methane, formaldehyde, and methanol (CH4, CH2O and CH3OH) were also formed.
It is convincing that semiconductor oxide materials and aerogels in particular can
demonstrate a wide range of applications for different catalytic reactions [34-36]:
86
1. Oxidation of air pollutants,
2. Photocatalytic reactions of volatile hydrocarbons (propylene, toluene, benzene),
3. Photocatalytic reactions of halogenated hydrocarbons (trichloroethylene,
tetrachloroethylene, trichloroethane),
4. Oxidation of Nitrogen Oxides (NOx).
4.2.3 Visible light irradiation
Unfortunately, all oxide semiconductors have wide band gaps and do not absorb visible
light. In practice, they can not be used for the utilization of solar light energy. For efficient use of
visible light from solar irradiation and to have high photoactivity, the doping of this oxide with
metals (transition, noble) or nonmetals is required to create a bathochromic shift of the band gap
energy. Another option is to induce external sensitization with dyes in order to harvest the visible
light photons and transfer them to the catalyst.
The increase in photoactivity can be achieved by the application of organic photosensitizers
(Ru(bipy)32+) [37, 38] or by addition of platinum, palladium or nickel oxides [39, 40]. The
distribution of a noble metal is very important and can be achieved by different deposition
methods (co-precipitation, impregnation, photoplatinization) [41]. Doping with transition metal
ions creates d donor and acceptor states in the forbidden area (between conduction and valence
bands) of catalyst. This also improves trapping of the electrons and inhibits electron-hole
recombination. Doped TiO2 with Fe, Mo, V, Cr, transition metal ions are promising
photocatalysts under visible light irradiation [42-44]. Nitrogen-doped TiO2 catalysts showed
photoactivity under visible light [45]. TiO2-xNx was active for the acetaldehyde decomposition to
CO2 under visible light irradiation [46].
In this chapter, emphasis will be mainly put on the UV photoactivity of different catalysts.
In the following chapter (Chapter 5), the doping of catalysts and their photoactivity properties
under visible light irradiation will be studied more extensively. The results for visible light
induced photoactivity for the degradation of acetaldehyde by different doped catalysts will also
be discussed later.
87
4.3 Acetaldehyde photodecomposition studies
Acetaldehyde is a “probable human carcinogen” and hazardous air pollutant according to
the EPA and is “ubiquitous” due in part to its appearance in vehicle exhaust, building materials
(polyurethane foams, coatings), consumer products (adhesives, lubricants, inks, and nail polish)
fruits, and etc. [47]. The U.S. Occupational Safety and Health Administration (OSHA)
Permissible Exposure Limit (PEL) for acetaldehyde is 200 ppm.
The safe removal of or detoxification of these chemicals, possibly through decomposition
on a catalytic nanoparticle surface, is therefore, a prime concern with regard to household safety.
In addition, nanoparticles offer great opportunities for the development of reusable, solid
catalysts in industries to replace the toxic liquid catalysts [48].
Titania based semiconductor oxides (TiO2, SrTiO3, and in limited cases BaTiO3) are good
UV photocatalysts that decompose volatile organic compounds and acetaldehyde in particular.
The big band gap of this materials (3.2 eV for SrTiO3, 3.1 eV for BaTiO3, 3.0 eV for TiO2
anatase) limits its application to a UV light region only (λ < 400 nm) (Figure 4.6). In this
chapter, the results on acetaldehyde photooxidation in the gas phase over the solid-state and
aerogel prepared strontium and barium titanate samples (SSR-SrTiO3, SSR-BaTiO3, AP-SrTiO3,
and AP-BaTiO3) under UV light irradiation will be presented, discussed, and compared with the
photoactivity of available samples of P25 Degussa (TiO2), commercial and nanocommercial
strontium and barium titanates (CM-SrTiO3, CM-BaTiO3, NCM-SrTiO3, and NCM-BaTiO3).
88
0.0
0.5
1.0
1.5
2.0
2.5
300
350
Wavelength (nm)
400
450
AP-BaTiO3
AP-SrTiO3
TiO2 P25 Degussa
of TiO2 is 3.0 eV
of BaTiO3 is 3.1 eV
Band gap of SrTiO3 is 3.2 eV
Figure 4.6 UV-visible Absorbance of Titanium Based Semiconductor Oxides
Absorbance
500
89
4.3.1 Experimental setup for photodecomposition reactions
The experimental setup included a light source, a static reactor and a circulating water
thermostat (Figure 4.7). The light irradiation from the light of a 1000 W High pressure Hg lamp
was employed to drive the photocatalytic oxidation of acetaldehyde over catalyst samples.
Different filters (Oriel Spectra-Physics 57396, 59062, 57346, and 59680 filters) were used to
pass UVA (320 nm < λ < 400 nm) or visible (420 nm < λ) light radiation. The IR irradiation was
cut by passing it through the Newport 6123 liquid infrared filter. The light intensity for visible
light was 7.6 mW/cm2 (1.2 × 10 16 photon/(s×cm2)) and 8.4 mW/cm2 for UV light, and it was
measured with a Power Max 500D laser power meter from Molectron Detector, Inc.
The prepared photocatalysts were studied for visible and UV activity for gaseous
acetaldehyde decomposition, which was carried out at room temperature. The powdered
photocatalyst sample (100 mg) was placed into a circular glass dish to have a uniform surface
and then mounted in an air filled cylindrical 305 mL glass reactor. The reactor was made of glass
and had a quartz window for the passing of light irradiation. 100 µL of liquid acetaldehyde were
introduced into the reactor. After that the reactor was closed and stirred continuously. All
experiments were carried out at a constant temperature of 298 K by cooling with circulation
water from a thermostat.
Before irradiation, the reaction was equilibrated for 30 minutes to allow the vaporization of
the injected liquid acetaldehyde. The reactor is equipped with septum cups allowing multiple
gaseous samples to be extracted. Gaseous samples (35 µL) were periodically (every 5-10
minutes) extracted from the reactor and injected into a GCMS to monitor the gaseous
environment in the reactor.
A gas chromatograph with a mass-selective detector (GCMS-QP5000 from Shimadzu
equipped with a phase XTI-5 capillary column, Restek Corp.) was used to follow the
concentrations of acetaldehyde degradation and carbon dioxide evolution. The products were
identified by following the characteristic masses and comparison of retention times of products
with pure compounds. The separation of products and reagents was achieved in column, which
was maintained at 313 K. The intensity of m/z = 44 peak was used to analyze the acetaldehyde
and carbon dioxide. Concentrations of the two were calculated using calibration curves for the
90
pure compounds of known concentrations. The calibration curves are based on the known peak
areas of known concentrations of carbon dioxide and acetaldehyde. Using this calibration, the
peak areas for an unknown concentration can be recalculated into known concentrations (Figure
4.8). The concentrations of acetaldehyde decomposition and carbon dioxide evolution were
plotted using Origin 6.1 software. The calibration parameters were incorporated into the Origin
program data sheets to convert the peak areas into concentration of CO2 and CH3CHO (Figure
4.9).
The analysis in Figures 4.8 and 4.9 clearly shows the photoactivity towards the
decomposition of acetaldehyde and carbon dioxide production under UV light irradiation.
Without light, there was no activity toward CH3CHO consumption or CO2 production. Similarly,
under visible light irradiation photoactivity was almost negligible, which can be explained by the
inefficiency of the lower energy photons of visible light to excite electrons and holes within the
wide band gap of SrTiO3.
The linear fit for initial several experimental points can be used to calculate the slope which
was later used for the initial reaction rate estimation. The comparison of the initial rates of
different catalyst samples allows comparing their photoactivity. The slope from the linear
equations for reaction curves are used to define the initial rates for the reaction. The standard
deviation in parentheses shows how good the linear fit is (Figure 4.10).
Each catalyst sample has been studied for acetaldehyde photooxidation and the data from
the GC experiment was plotted similar to Figure 4.9. For the sake of space, not all of them will
be presented separately; for more clarity the results from these graphs will be combined in
multicomponent graphs or summarized in tables.
The obtained photoactivity for different catalyst samples are compared with activity of P25
TiO2 Degussa which is an unofficial standard catalyst for photoreactions under UV light (Figure
4.10). This comparison will allow seeing the efficiency and possible advantage over P25.
91
Figure 4.7 Experimental Setup for Photocatalysis Reactions
Water circulator
Reactor
Light source
92
6
6
0.0
2.0x10
5
4.0x10
5
6.0x10
5
8.0x10
5
6
1.0x10
1.2x10
6
1.4x10
0
20
light off
60
Time (min)
Visible light on
40
light off
80
100
UV light on
120
6
1.2x10
6
1.4x10
6
1.6x10
1.8x10
6
6
2.0x10
6
2.2x10
6
2.4x10
6
2.6x10
Figure 4.8 Peak Areas versus Time for CH3CHO Decomposition and CO2 Evolution under UV Light for AP-SrTiO3
Peak area of CO2
1.6x10
Peak area of CH3CHO
93
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
20
light off
40
Time/min
60
80
100
y = -0.0664 - 0.00278*x (R=-0.9995)
UV light on
y = 23.7151 - 0.0525*x (R=-0.9873)
18
20
22
Figure 4.9 Concentration versus CH3CHO Decomposition and CO2 Evolution under UV Light for AP-SrTiO3
[CO2] / mM
0.22
[CH3CHO] / mM
94
0.0
0.2
0.4
0.6
20
light off
40
Time/min
60
80
100
y = -0.545 - 0.0131*x (R=-0.999)
UV light on
26
28
30
32
34
Figure 4.10 Photocatalytic Decomposition of Acetaldehyde under UV Light for P25 TiO2
[CO2] / mM
0.8
y = 37.995 - 0.1023*x (R=-0.996)
[CH3CHO] / mM
95
4.3.2 Photoactivity of commercially available and synthesized samples
Different catalyst samples including commercially available, solid-state prepared and
aerogel prepared, were studied for the decomposition of acetaldehyde under light irradiation. The
initial reaction rates for different catalyst samples were calculated and compared. Titania P25
shows very high activity toward the acetaldehyde decomposition and carbon dioxide production
under UV light irradiation (Table 4.2). It has the highest initial reaction rates in comparison with
all other studied catalyst samples (Figures 4.11 and 4.12).
Table 4.2 Initial Rates for CH3CHO Decomposition and CO2 Production under UV Light
Irradiation for the Different Catalyst Samples
r [CH3CHO]
r [CO2]
P25 TiO2 Degussa
0.102
1.3 × 10 -2
AP-SrTiO3
0.053
2.8 × 10 -3
AP-BaTiO3
0.041
1.4 × 10 -3
SSR-SrTiO3
0.116
1.1 × 10 -3
SSR-BaTiO3
0.093
4.9 × 10 -4
CM-SrTiO3
0.037
7.5 × 10 -4
NCM-SrTiO3
0.034
6.6 × 10 -4
Catalyst Sample
96
0.0
0.1
0.2
0.3
0.4
0
light off
20
40
Time/min
P25 TiO2 Degussa
AP-SrTiO3
AP-BaTiO3
COM-SrTiO3
NCM-SrTiO3
UV light on
60
Figure 4.11 CO2 Evolution under UV Light Irradiation for Different Catalyst Samples
[CO2]/mM
97
8
10
12
14
16
18
20
22
0
20
40
Time, min
P25 TiO2 Degussa
AP-SrTiO3
AP-BaTiO3
CM-SrTiO3
NCM-SrTiO3
light off
60
UV light on
Figure 4.12 CH3CHO Decomposition under UV Light Irradiation for Different Catalyst Samples
[CH3CHO]/mM
98
The initial rate for carbon dioxide production on titanium oxide P25 catalyst is
significantly higher than that of other competitors (in the range of 4-25 times). The nearest
competitor is an aerogel prepared strontium titanate which has activity for acetaldehyde
decomposition two times lower and activity for carbon dioxide production four times less than
P25. Aerogel prepared barium titanate is also photoactive for acetaldehyde decomposition, but
less active than AP-SrTiO3.
All synthesized samples including solid-state prepared and aerogel prepared were more
active for acetaldehyde decomposition than those commercially available (commercial SrTiO3 or
nanosized commercial SrTiO3).
Solid-state prepared samples of SrTiO3 and BaTiO3 have photoactivities similar to titania
for acetaldehyde decomposition, but carbon dioxide production on these samples was
significantly lower not only in comparison with P25 (10-25 less), but also in comparison with
aerogel prepared SrTiO3 and BaTiO3 samples (3 times less).
The diffuse reflectance spectra of solid-state prepared, aerogel prepared, and commercial
SrTiO3 samples (Figure 4.13) show an interesting feature of a shallow shoulder adsorption in the
400-700 nm range for SSR-SrTiO3 and CM-SrTiO3. Besides, having similar textural
characteristics (See Chapter 3, Table 3.1) these two samples also have a light purplish white
color in comparison with the bright white color of AP-SrTiO3. The similarity in properties and
characteristics of them allows speculation that commercial SrTiO3 was probably produced by a
solid-state reaction. The absorbance of some visible light and color of these samples can be
explained by the presence of impurity phases produced by the incomplete reaction of precursors
(Sr2TiO4) and oxygen deficiency (SrTiO2.7), and oxygen defects due to oxygen deficiency (See
Appendix A).
The presence of oxygen defects can accelerate the adsorption of the acetaldehyde species
on the surface of the catalyst giving a higher rate of acetaldehyde disappearance from the
gaseous phase, or produce more superoxide species to decompose acetaldehyde and produce
carbon dioxide. Since there is no significant increase in the carbon dioxide production in
comparison with other samples, it is possible to postulate that the higher rate of decrease in the
acetaldehyde concentration is due to the faster absorption of acetaldehyde molecules on the
active sites of the catalyst. More surface studies are needed to prove this finding (FTIR studies
on surface in Chapter 6).
99
0.00
200
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
2.50
400
Wavelength (nm)
600
CM-SrTiO3
SSR-SrTiO3
AP-SrTiO3
Figure 4.13 Defuse Reflectance Spectra of Different SrTiO3 Samples
Absorbance
800
100
Aerogel prepared catalyst samples, despite their very high surface areas, in comparison
with solid-state catalyst samples with lower surface areas, have not shown a significant increase
in acetaldehyde decomposition. This proves one more time that photocatalysis is a complex
process that depends on many different parameters. High surface area is important for
heterogeneous catalysis but not a decisive factor.
Nevertheless, aerogel prepared samples are the most active samples for carbon dioxide
production under UV light irradiation and AP-SrTiO3 is second in photoactivity after P25 TiO2.
The available samples were also tested for the photodecomposition of acetaldehyde under
visible light irradiation (420 nm < λ). It is clear that under visible light all samples lose
photoactivity toward the decomposition of acetaldehyde and carbon dioxide production (Table
4.3). The titania P25 photocatalyst is also inactive under visible light.
Table 4.3 Initial Rates for CH3CHO Decomposition and CO2 Production under Visible
Light Irradiation for Different Catalyst Samples
Catalyst Sample
r [CH3CHO]
r [CO2]
P25 TiO2 Degussa
0.015
1.6 × 10 -4
AP-SrTiO3
0.008
2.1 × 10 -4
AP-BaTiO3
0.005
1.4 × 10 -4
SSR-SrTiO3
0.032
1.4 × 10 -4
SSR-BaTiO3
0.017
2.0 × 10 -5
The level of produced carbon dioxide is similar for all catalyst samples. In case of solidstate prepared catalyst samples, the initial rates for acetaldehyde consumption were slightly
higher in comparison with others, but again, there were no increases in carbon dioxide
production.
101
A balance between the consumed acetaldehyde concentration and the amount of CO2
produced is not achieved. While CO2 is the main gaseous product, other non-volatile or lessvolatile intermediates are probably formed and stay absorbed on the surface of the catalyst; that
is why they were not detected in the gaseous phase by gas chromatography.
4.4 Conclusions
Volatile Organic Compounds and acetaldehyde in particular in the presence of
photocatalyst can be decomposed from the gaseous phase into CO2 and H2O through different
intermediates under light irradiation. Active oxygen species (O2-, .OH, O-, O3-) are important
species to promote the photoreaction. Surface holes and oxygen defect can also significantly
influence the activity of catalyst.
Titanium based catalysts are known as materials that can be applied for the photooxidation
of volatile organic compounds under light irradiation. Acetaldehyde photooxidation has been
studied on aerogel prepared strontium and barium titanates (AP-SrTiO3 and AP-BaTiO3), solidstate prepared strontium and barium titanates (SSR-SrTiO3 and SSR-BaTiO3) under UV and
visible light irradiation, and compared with the photoactivity of available commercial samples
(CM-SrTiO3 and NCM-SrTiO3) and P25 TiO2 (Degussa).
The degradation of CH3CHO over AP-SrTiO3 is lower than that of P25 under UV.
Nevertheless, aerogel prepared titanates showed high activity for the acetaldehyde
decomposition and carbon dioxide production in the UV region of irradiation compared to other
available commercial and synthesized catalysts.
The comparison of aerogel prepared samples with differently prepared samples shows the
influence of morphology and textural properties on photocatalytic activity of SrTiO3 particles.
The solid-state prepared samples with low surface areas showed reletively high initial rates for
acetaldehyde consumption in comparison with aerogel prepared samples which have
significantly higher surface areas. The higher consumption of acetaldehyde from the gaseous
phase by the solid-state prepared samples can be explained by the higher rates of acetaldehyde
adsorption on the surface of catalyst, but not by the higher rates of the acetaldehyde
decomposition since the increase in carbon dioxide was not noticed. It seems that highly oxygen
102
defective surface of solid-state prepared samples, plays some role in this process, and the surface
studies of catalyst during photooxidation would be useful.
TiO2 and titanates are active only under UV light irradiation, and this limits their
application. To make photocatalyst more efficient for solar light, it must work in the visible light
region. The modification of photocatalysts by doping can solve this problem.
4.5 References
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M.A. Anderson, S.Y. Nishida, and S.C. March, in Photocatalytic Purification and
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105
Chapter 5 : Modification of titanates by doping
5.1 Introduction
The search for new, inexpensive, environmentally friendly, and easy access energy sources
is crucial in today’s society. Doubtlessly, solar radiation is the most favorable candidate as an
energy source for a variety of applications.
The annual consumption of energy on a world scale is 10 17 kcal and this number
constantly increases [1]. The annual incident solar energy on the Earth is 10 21 kcal. 10 16 kcal of
solar energy is converted into chemical energy through photosynthesis in plants and
microorganisms. Only a small part of solar energy is utilized by people through fuel and
materials (10 16 kcal), and food (10 15 kcal).
The utilization of incident solar light can cover human needs for energy, which is why
intensive efforts are going on for the discovery of materials which can convert the energy of light
into other forms of energy like electricity (solar cells) and heat (solar collectors). Additionally,
materials which can transfer it into chemical (photocatalysts) or mechanical (solar sails) work are
also of high interest..
Photocatalytic processes can utilize light energy for water splitting, toxic waste
mineralization, air and water purification [2, 3]. Different semiconductor titanium based oxide
catalysts (TiO2, and perovskites such as CaTiO3, SrTiO3, and BaTiO3) were studied for the
degradation of organic compounds and/or water splitting [4 - 6]. All these oxides were active
only under UV irradiation and this is conditioned by large band gaps. Since UV light is only 3%
of solar light, the semiconductor oxide materials with lower band gaps need to be developed to
absorb visible light and to perform more effective catalysis.
Unlike titanium oxide, the combination of A and B cation sites can be varied and
substituted with other metal cations in the perovskite structure of ABO3. The substitution or
doping with cations that are compatible with the lattice sizes of cations in perovskite (AxA’1xBO3
or ABxB’1-xO3) can change its electronic properties [7]. Doping with different cations (A’
or B’) can also decrease the band gap size of the perovskites and thus will increase the
106
absorption of light in the visible light region and enhance photocatalytic properties of doped
perovskites.
The modification of UV photocatalysts by doping (incorporation) with transition metal ions
(partial substitution of Ti ions with metal ions) creates a valence band or electron donor level
inside the band gap, narrows it, and increases the visible light absorption (>420 nm).
Recently, our group has developed a way to modify the UV photocatalytic materials so
they will respond to visible irradiation [8-10]. These materials were doped with transition metals,
which decreased the band gap and enhanced the absorption of visible light (Figure 5.1).
Figure 5.1 Transition Metal Doping of UV Photocatalysts
The enhanced absorption of visible light in titanate photocatalysts can be achieved by the
synthesis of nanosized powders doped with metal ions. These novel nanosized catalysts may
have higher photocatalytic activity in comparison with bulk catalyst powders. Nanoparticles
offer advantages for catalysis by making available more surface area and active sites on the
surface.
Therefore, the purpose of the research studies in this chapter are to answer the following
questions:
107
a) Can the doping of perovskite affect the photocatalytic activity in visible light region?
b) Are the doped nanosized perovskite catalysts more efficient than other conventionally
prepared doped perovskite catalysts?
5.2 Doping process
Modification of semiconductor photocatalysts to shift the light absorption into the visible
light region can be achieved using several methods [11]:
1. Phosensitization by dyes (sensitizers) and transition metal complexes of the type [Ru
(2,2’-bipiridyl)3],
2. Ion-implantation by surface bombardment with V, Cr, Mn, Fe, or Ni ions,
3. Doping with V, Cr, Rh, Mn, Fe, Co, Ni, or Cu metals,
4. Doping with S, N, or F nonmetals,
5. Impregnation or incorporation with fullerenes (C60).
Implantation is a very effective method and can significantly enhance the adsorption of
visible light by the sample. Figure 5.2 shows that Cr implanted TiO2 has an increase in
absorption of visible light with increasing of the concentration of implanted chromium. The big
disadvantage of this method is the high cost of catalyst production.
108
Figure 5.2 UV-Visible Absorption Spectra of a) Pure TiO2 and b) –d) Cr Ion-implanted
TiO2 with Cr of 2.2, 6.6, and 13 x10-7 mol/g [11]
Doping with transition metals is another effective method to increase the absorption of
visible light by a semiconductor catalyst. Different transition and noble metals were used for
doping of photocatalysts to enhance the visible light absorption (Figure 5.3). The visible light
response of these catalysts can be explained by the creation of electron donor levels on bands
formed by metal ions in the forbidden area and transition of electrons from them to the
conduction band of Ti 3d orbitals.
109
Figure 5.3 Diffuse Reflectance Spectra of Doped SrTiO3:M (0.5%) with a) Mn, b) Ru, c)
Rh, d) Pd, e) Ir, f) Pt [12]
All doped photocatalysts were mainly prepared to study water splitting under visible light,
and information on application of them for organic compound photodecomposition is very
limited. The main method to synthesize them is the solid-state reaction when all solid precursors
are mixed together and calcined at very high temperatures (>1000°C). Not surprisingly, these
materials are very dense powders with large particle sizes. In any case, they show good activities
for H2 or O2 production during water photolysis process. Mn and Ru doped SrTiO3 showed good
photoactivity for O2 production, while Ru, Rh, and Ir doped SrTiO3 shows activity for H2
production under visible light irradiation (λ >440 nm) [12].
The absorption of visible light does not mean that the doped material will be a good
photocatalyst, but it indicates that harvested photons of visible light may be used for the
photocatalytic process. For example, Cr 3+ doped TiO2 and SrTiO3 absorb visible light; however,
there is no increase in photoactivity and sometimes it even decreases [13-15]. The substitution of
110
Ti 4+ with Cr 3+ causes charge imbalance and formation of oxygen defects and/or Cr 6+ ions to
stabilize the charge balance. Oxygen defects and Cr 6+ ions may act as recombination centers for
holes and electrons and cause a decrease in photoactivity. In contrast, chromium and antimony
codoped SrTiO3 showed an increase in photoactivity for H2 production in visible light range
[16]. In this system two O 2- ions are replaced with one Cr 3+ ion and one Sb 5+ ion in such a way
that charge balance is preserved and no defects were formed. In a similar manner, the Cr and Ta
codoped, Cr and Nb codoped, Sb and Ni codoped SrTiO3 catalysts were synthesized by the solidstate reaction method and showed an increase in photocatalytic activities [17].
Doping TiO2 with Fe 3+, Mo 5+, Ru 3+, Os 3+, Re 5+, V 4+ and Rh 3+ showed a significant
increase for photoactivity for carbon tetrachloride reduction and chloroform oxidation, while
doping with Co 3+ and Al 3+ decreased the photoactivity [18].
In all previously mentioned cases, doping was done with cations. There are a few cases when
the doping of titania with anions has been done, but has been achieved by the substitution of
nonmetal ion for O 2- in the lattice of TiO2 to form TiO2-xFx, TiO2-xNx, or TiO2-xSx [19-21].
Doping can also be done with several nonmetals together. Doping of SrTiO3 with nitrogen only
where the O 2- is replaced with N 3- causes the formation of anion defects and forms charge
imbalance. As already mentioned above the defects can act as electron-hole recombination
centers. Charge balance can be preserved if two O 2- ions are replaced with one N 3- ion and one
F– ion simultaneously [22]. The photoactivity of N and F codoped SrTiO3 increased significantly
in comparison with N doped SrTiO3 under visible light irradiation. It is necessary to mention that
almost all doping with metal is done by solid-state reaction method or techniques similar to it.
Keeping in mind the work already done by other research groups, it would be interesting to
prepare doped aerogel catalysts to study their photoactivity and compare with conventionally
solid-state prepared samples. In this chapter doped aerogel catalyst will be investigated:
1) Cr doped AP-SrTiO3 to increase photoactivity in a visible light region,
2) Cr doped AP-BaTiO3 to increase photoactivity in a visible light region,
3) Sb/Cr codoped AP-SrTiO3 to decrease the amount of recombination centers,
4) Cr doped SSR-SrTiO3 to compare photoactivity with Cr doped AP-SrTiO3,
5) Sb/Cr codoped SSR-SrTiO3 to compare photoactivity with Cr and Sb codoped APSrTiO3.
111
5.2.1 Doping with transition metals in solid-state reaction
The traditional way to prepare solid-state samples is to mix solid precursors in the
necessary stoichiometric ratio and to calcine them at high temperatures. Solid-state prepared
SrTiO3 or BaTiO3 doped with a transition metal are synthesized by mixing solid starting
materials of titanium oxide, strontium or barium carbonate, and transition metal oxide according
to the ratio SrMxTi1-xO3 or BaMxTi1-xO3 (where M is dopant metal) (Figure 5.4). The precursors
are mixed and ground first, and these samples are packed in high purity alumina crucibles and
calcined at 1100°C for 36 hours. After 18 hours, the samples are cooled down, ground, packed,
and calcined for another 18 hours (More detailed information on the solid-state method operation
procedure can be found in Chapter 3.2.1).
5.2.2 Doping with transition metals in aerogels
The sol-gel or aerogel methods for the synthesis of nanoparticles can also be used for
doping of samples with transition metals (Figure 5.5). Titanium alkoxide with strontium or
barium alkoxides (Ti(OR)4 with Sr(OR)2 or Ba(OR)2) are mixed first, and solvent soluble salt of
transition metal (nitrate or acetate) is added afterwards according to the ratio SrMxTi1-xO3 or
BaMxTi1-xO3 (where M is dopant metal). The hydrolysis of this mixture gives the mixed metal
oxides where some of the Ti ions in the lattice are substituted with transition metal ions (More
detailed information on the aerogel method operation procedure can be found in Chapter 3.2.2).
112
Figure 5.4 Metal Doping of SrTiO3 Photocatalyst by Solid-State Reaction
113
Figure 5.5 Metal Doping of SrTiO3 Photocatalyst by Modified Aerogel Procedure
114
5.3 Photoactivity of doped titanates
The doped photocatalysts were studied for visible and UV activity for gaseous
acetaldehyde decomposition at room temperature. Acetaldehyde photooxidation was performed
on chromium ion-doped AP-SrTiO3, chromium/antimony ion-codoped AP-SrTiO3, chromium
ion-doped SSR-SrTiO3, chromium/antimony ion-codoped SSR-SrTiO3, chromium ion-doped
AP-BaTiO3 under UV and visible light irradiation, and compared with the photoactivity of pure
aerogel prepared strontium titanate and barium titanate (AP-SrTiO3 and AP-BaTiO3) and P25
TiO2 (Degussa).
The experimental setup included a light source (1000W high pressure Hg lamp), a static
reactor and a circulating water thermostat. Different filters (Oriel Spectra-Physics 57396, 59062,
57346, 59680 filters, Newport 6123 liquid infrared filter) were used to pass UV (320 nm < λ <
400 nm) or visible (420 nm< λ) light radiation.
The powdered photocatalyst sample (100 mg) was placed into a circular glass dish to have
a uniform surface and then mounted in an air filled cylindrical 305 mL glass reactor. 100 µL of
liquid acetaldehyde were introduced into the reactor. After that the reactor was closed and stirred
continuously. All experiments were carried out at a constant temperature of 298 K. Before
irradiation, the reaction was equilibrated for 30 minutes to allow the vaporization of the
aldehyde. Gaseous samples (35 µL) were periodically extracted from the reactor and injected
into a GCMS (gas chromatograph with a mass detector GCMS-QP5000 from Shimadzu
equipped with a phase XTI-5 capillary column, Restek Corp.) to monitor the concentrations of
acetaldehyde degradation and carbon dioxide evolution (More detailed information on operation
procedures for photocatalytic study can be found in Chapter 4.3.1).
5.3.1 Cr doping and Sb/Cr codoping of aerogel prepared catalysts
Chromium and antimony/chromium doped strontium titanates aerogels were prepared with
2% Cr and 2% Cr/2.5% Sb loadings by molar weight. The addition of metal ions from salts
(chromium (III) nitrate and/or antimony (III) acetate) was in such a way so that the molar ratio of
115
SrTi0.98Cr0.02O3 and SrTi0.954Cr0.02Sb0.025O3 would be preserved. Metal salts were dissolved in
alcohol and added under stirring to the Sr-Ti alkoxides mixture before hydrolysis.
Doping of SrTiO3 with metal(s) is based on the substitution of Ti ions with metal ions.
Substitution of Ti 4+ with Cr 3+ ions causes the formation of oxygen defects (SrTiIV1-2xCrIII2xO3-x)
and/or Cr 6+ ions (SrTiIV1-3xCrIII2xCrVIxO3) to keep the charge balance. This increases the number
of recombination centers where photogenerated electrons and holes can recombine. Contrarily,
the charge balance can be preserved by the use of charge compensation. The application of
charge compensators like antimony (Sr Ti IV1-2xCrIIIxSbVxO3), tantalum (Sr Ti IV1-2xCrIIIxTaVxO3),
or niobium (Sr Ti IV1-2xCrIIIxNbVxO3) has been studied for the Sb/Cr, Ta/Cr and Nb/Cr doped
SrTiO3 systems [16-17]. Until now, these materials were synthesized only by solid-state
reactions and mainly studied as photocatalysts for the water splitting. To our knowledge, until
the work described herein, such systems have not been synthesized by an aerogel method and
studied as photocatalysts to destroy volatile organic compounds.
The textural properties of the aerogel prepared samples were studied by different available
techniques (More detailed information on sample characterization operation procedures can be
found in Chapter 3.3). Powder X-ray diffractions patterns were obtained on a Bruker D8
Advance spectrometer with a CuKα radiation source with an applied voltage of 40 kV and a
current of 40mA. Scans were made in the 2θ range of 20-85º with a scanning rate of 2 º/min. The
crystallite size was calculated from the XRD patterns using the Debye-Scherrer equation.
Peaks from the diffractograms patterns were assigned to the strontium titanate phase, and
no additional peaks corresponding to the dopants were observed (Figure 5.6). The absence of the
dopant peaks indicates the high dispersion and incorporation of the doped metal ions into the
SrTiO3 lattice. The average crystallite sizes of different AP-SrTiO3 samples are summarized in
Table 5.1.
116
600
700
800
900
1000
20
30
40
2 theta (deg)
50
60
70
80
AP-SrTiO3
AP-2%Cr-SrTiO3
AP-2.5%Sb/2%Cr-SrTiO3
2% Chromium Ion Doped, AP-2.5%Sb/2%Cr-SrTiO3 – 2.5% Antimony and 2% Chromium Ion Codoped)
117
Figure 5.6 Powder XRD Patterns of Aerogel Prepared Strontium Titanate Samples (AP-SrTiO3 – Pure, AP-2%Cr-SrTiO3 –
Intensity (arb)
The surface areas, pore size distributions, and pore volumes of different samples were
measured on a Nova 1200 gas sorption analyzer (Quantachrome Corp.) from the amount of N2
absorbed at 77K and calculated according to the Brunauer-Emmett-Teller (BET) method. The
samples were degassed at 423 K for 1 hour prior to the analysis.
Light absorption spectra of the samples were obtained on a Cary 500 Scan UV-Visible
Spectrometer with an integrating sphere attachment for diffuse reflectance in the range 200-800
nm.
Transmission electron micrographs were obtained on a Philips CM 100. Samples were
placed onto a carbon-coated copper grid by the physical interaction of the grid and powder in
such a way that the particles remained adhered to the grids.
Pure strontium titanate (SrTiO3) and titanium oxide (TiO2 P25 Degussa) absorbed only
UV light and did not absorb any visible light (Figure 5.7). Chromium doping has increased the
absorbance in the visible light region compared with undoped samples. Cr and Cr/Sb doped
strontium titanate samples had intense absorption in the 400-600 nm.
The freshly prepared Cr doped SrTiO3 aerogels needed calcination in air at 773 K for five
hours to remove the organic residuals of solvent in the pores and on the surface of the samples.
The color of the samples changed from light green for freshly prepared chromium doped
aerogels to light yellow for calcined Cr-SrTiO3 aerogels.
The comparison between freshly prepared aerogel and calcined aerogel samples (Figure
5.8) showed the incorporation of the Cr 3+ ions into the lattice of the SrTiO3 samples (intense
absorption band at 400-500 nm). Some of Cr 3+ surface ions (absorption band at 610 nm) were
oxidized into Cr 6+ ions (absorption band at 380 nm) during calcination. The antimony/chromium
doped exhibited more intense absorption compared with chromium doped strontium titanate.
118
0.0
0.5
1.0
1.5
2.0
2.5
300
400
600
Wavelength (nm)
500
700
TiO2 P25 Degussa
AP-SrTiO3
AP-2% Cr-SrTiO3
AP-2.5% Sb/2% Cr-SrTiO3
800
Aerogel Prepared Strontium Titanate Samples
Figure 5.7 Defuse Reflectance Spectra of Pure SrTiO3, TiO2 P25 Degussa, 2% Cr Doped and 2% Cr/2.5% Sb Codoped
Absorbance
119
0
1
2
3
4
o
o
300
500
600
Wavelength (nm)
400
700
800
AP-2.5% Sb/2% Cr-SrTiO3 calcined in air at 500 C
AP-2% Cr-SrTiO3 calcined in air at 500 C
AP-2.5% Sb/2% Cr-SrTiO3 fresh prepared
AP-2% Cr-SrTiO3 fresh prepared
Samples Freshly Prepared and Calcined at 773 K
120
Figure 5.8 Defuse Reflectance Spectra of 2% Cr Doped and 2% Cr/2.5% Sb Codoped Aerogel Prepared Strontium Titanate
Absorbance
The crystallite morphology of the synthesized chromium and chromium/antimony doped
SrTiO3 aerogels was analyzed by TEM (Figure 5.9). All crystallites had uniform spherical shapes
with a size distribution near 12 nm for Cr-SrTiO3 and near 10 nm for Sb/Cr-SrTiO3 samples. The
average crystallite sizes were calculated from XRD studies using the Debye-Scherrer equation.
The nanosized crystalline primary particles tend to stay together and form aggregates of
secondary particles.
The BET surface areas of the samples are shown in Table 5.1. The small loss in surface
areas for doped samples could be due to sintering favored by the presence of dopants. Besides,
the addition of the dopant (Cr or Cr/Sb) caused an increase in the total pore volume and average
pore size. The average crystallite sizes of all prepared SrTiO3 samples were around 10 nm in
diameter.
Table 5.1 Textural Properties of Photocatalysts
Sample
Crystallite
sizes, nm
Surface
2
area, m /g
Total pore
Average
3
volume, cm /g pore size, Å
TiO2 P25 Degussa
25
50.2
0.182
145
Pure Aerogel SrTiO3
11
93.0
0.450
190
2%Cr doped SrTiO3
12
81.9
0.488
236
2%Cr/2.5%Sb doped SrTiO3
10
68.8
0.662
384
121
Strontium Titanate Samples
122
Figure 5.9 Transmission Electron Micrographs of 2% Cr Doped (left) and 2% Cr/2.5% Sb Codoped (right) Aerogel Prepared
All doped aerogel samples were studied and compared with a typical photocatalyst
standard, Degussa P25 TiO2 particles. The undoped samples of titanium oxide (P25 TiO2) and
pure aerogel prepared strontium titanate (AP-SrTiO3) showed significant activity for the
degradation of acetaldehyde (CH3CHO) and carbon dioxide (CO2) production utilizing UV light
(Table 5.2). When these samples were irradiated using visible light, the conversion of CH3CHO
into CO2 was negligible. After doping with 2% chromium, the SrTiO3 sample was active, not
only under UV light irradiation, but also under visible light irradiation (Table 5.2). The
introduction of Cr ions into the SrTiO3 lattice caused an increase in the initial reaction rate for
the acetaldehyde decomposition in the visible region and a slight decrease in this reaction rate
under UV irradiation in comparison with undoped strontium titanate. The initial reaction rate for
the carbon dioxide was significantly lower for the Cr doped AP-SrTiO3 in the UV light region.
This dramatic decrease in the rate can be explained by the stronger production of the not fully
oxidized byproducts and less formation of the carbon dioxide final product. Nevertheless, Crdoped AP-SrTiO3 showed a higher activity in the visible region for carbon dioxide production
and acetaldehyde decomposition compared to pure AP-SrTiO3 and P25 TiO2.
Table 5.2 Initial Rates of Acetaldehyde Degradation and Carbon Dioxide Production
Initial rate
Initial rate
r[CH3CHO]
r[CO2]
To
Sample
Dopant
Light source
TiO2 Degussa
--
UV (320<λ<400 nm)
0.1023
0.01310
photocatal
AP-SrTiO3
--
UV (320<λ<400 nm)
0.0525
0.00280
ytic
AP-SrTiO3
2% mol. Cr
UV (320<λ<400 nm)
0.0498
0.00084
properties
TiO2 Degussa
--
Visible (420 nm<λ)
0.0147
0.00016
AP-SrTiO3
--
Visible (420 nm<λ)
0.0077
0.00021
AP-SrTiO3
2% mol. Cr
Visible (420 nm<λ)
0.0350
0.00039
study the
and
reproduci
bility of
the CrSrTiO3
and Sb/Cr-SrTiO3 samples, three consecutive experiments of each sample for acetaldehyde
decomposition were performed and compared. The photoactivity of 2.5%Sb/2%Cr codoped
SrTiO3 aerogel was significantly higher than that of the SrTiO3 sample doped with only Cr. The
123
activity of antimony/chromium doped strontium titanate was almost twice as high for carbon
dioxide production in UV and visible light regions (Figures 5.10, 5.11).
The photoactivity of 2.5%Sb/2%Cr codoped SrTiO3 aerogel was relatively similar to the
activity of chromium doped strontium titanate for acetaldehyde decomposition under UV and
visible light irradiation (Figure 5.12, 5.13). There was an induction period for the Sb/Cr-SrTiO3
sample during the first experiment. The lower activity could be assigned to the initial presence of
the Cr 6+ species. During the experiments, these species were reduced by organic material to Cr 3+
ions, and this might be the reason for the increase in activity in visible and UV light regions. The
Sb/Cr-SrTiO3 aerogel also showed almost equal activity for acetaldehyde decomposition under
UV and visible irradiation.
Strontium titanate aerogel was also doped with antimony to study the influence of Sb
doping on the photoactivity of the catalyst. The addition of antimony did not have any effect on
the photoactivity of the catalyst for carbon dioxide production under UV or visible irradiation
(Figures 5.14, 5.15)
Also, the addition of antimony did not have a significant effect on the photoactivity of the
catalyst for acetaldehyde decomposition under UV or visible irradiation (Figures 5.16, 5.17), and
only a slight increase in initial reaction rates for acetaldehyde decomposition was noticed.
124
0.0000
0.0004
0.0008
0.0012
0.0016
1
Experimental Repetitions
2
Figure 5.10 UV and Visible Photoactivity of Cr-SrTiO3 Aerogel for CO2 Production
Initial Rate, mM/min
3
vis activity
UV activity
125
0.0000
0.0004
0.0008
0.0012
0.0016
1
Experimental Repetitions
2
Figure 5.11 UV and Visible Photoactivity of Sb/Cr-SrTiO3 Aerogel for CO2 Production
Initial Rate, mM/min
3
vis activity
UV activity
126
0.00
0.02
0.04
0.06
0.08
1
Experimental Repetitions
2
Figure 5.12 UV and Visible Photoactivity of Cr-SrTiO3 Aerogel for CH3CHO Degradation
Initial Rate, mM/min
3
vis activity
UV activity
127
0.00
0.02
0.04
0.06
0.08
1
Experimental Repetitions
2
3
vis activity
UV activity
Figure 5.13 UV and Visible Photoactivity of Sb/Cr-SrTiO3 Aerogel for CH3CHO Degradation
Initial Rate, mM/min
128
0.0
0.2
0.4
20
light off
60
Time, min
40
80
UV light on
P25 TiO2 Degussa
AP-SrTiO3
2.5% Sb /2% Cr AP-SrTiO3
2.5% Sb /2% Cr AP-SrTiO3 after 3 runs
2% Cr AP-SrTiO3
2% Cr AP-SrTiO3 after 3 runs
2% Sb AP-SrTiO3
Figure 5.14 CO2 Evolution for Aerogel Prepared Catalysts under UV Light
[CO2] / mM
100
129
0.02
0.03
0.04
0.05
0.06
0.07
0
20
light off
40
T ime, min
60
80
100
visible light on
P25 TiO2 Degussa
AP-SrTiO3
2.5% Sb /2% Cr AP-SrTiO3
2.5% Sb /2% Cr AP-SrTiO3 aftr 3 runs
2% Cr AP-SrTiO3
2% Cr AP-SrTiO3 aftr 3 runs
2% Sb AP-SrTiO3
Figure 5.15 CO2 Evolution for Aerogel Prepared Catalysts under Visible Light
[CO2] / mM
120
130
40
42
44
46
48
UV light on
20
Time, min
40
60
80
P25 TiO2 Degussa
AP-SrTiO3
2.5% Sb /2% Cr AP-SrTiO3
2.5% Sb /2% Cr AP-SrTiO3 aftr 3 runs
2% Cr AP-SrTiO3
2% Cr AP-SrTiO3 aftr 3 runs
2% Sb AP-SrTiO3
light off
Figure 5.16 CH3CHO Degradation for Aerogel Prepared Catalysts under UV Light
[CH3CHO] / mM
100
131
42
44
46
48
visible light on
20
Time, min
40
60
80
P25 TiO2 Degussa
AP-SrTiO3
2.5% Sb /2% Cr AP-SrTiO3
2.5% Sb /2% Cr AP-SrTiO3 aftr 3 runs
2% Cr AP-SrTiO3
2% Cr AP-SrTiO3 aftr 3 runs
2% Sb AP-SrTiO3
light off
Figure 5.17 CH3CHO Degradation for Aerogel Prepared Catalysts under Visible Light
[CH3CHO] / mM
132
The initial reaction rates for all samples used for acetaldehyde decomposition are
summarized in Table 5.3. Undoubtedly, Cr doping and Sb/Cr codoping increased the
photoactivity of SrTiO3 aerogels towards the visible light region. The Sb/Cr-SrTiO3
photocatalyst gave the highest activity for CO2 production under visible light irradiation and is a
promising candidate to be studied for other photocatalytic reactions.
Table 5.3 Photocatalysts for Acetaldehyde Degradation and Carbon Dioxide Production
UV
Sample
Dopant
Visible
Initial rate
Initial rate
r[CO2]
r[CH3CHO]
r[CO2]
r[CH3CHO]
no
0.1023
0.01310
0.0147
0.00016
no
0.0525
0.00280
0.0077
0.00021
Cr6+- Cr3+
0.0498
0.00084
0.0350
0.00039
Cr6+- Cr3+
0.0490
0.00085
0.0370
0.00039
Cr3+- Sb5+
0.0380
0.00147
0.0102
0.00073
2.5%Sb/ 2%Cr after Cr3+- Sb5+
0.0529
0.00120
0.0477
0.00083
0.0775
0.00241
0.0181
0.00016
P25 TiO2 Degussa
AP-SrTiO3 dopant
free
AP-SrTiO3 with
2%Cr
AP-SrTiO3 with
2%Cr after 3 runs
AP-SrTiO3 with
2.5%Sb/2%Cr
AP-SrTiO3 with
3 runs
AP-SrTiO3 with
2%Sb
Sb3+ - Sb5+
A balance between the amount of consumed acetaldehyde and the amount of formed
carbon dioxide was not achieved since other less volatile byproducts besides carbon dioxide
were also formed. To identify the nonvolatile byproducts, which were absorbed on the catalyst
133
surface and were not seen in the gas phase by gas chromatography, the FTIR studies were
performed.
5.3.2 Cr doping and Sb/Cr codoping of solid-state prepared samples
To evaluate the photoactivity of chromium doped and chromium/antimony codoped
aerogel prepared SrTiO3 catalysts, the chromium doped and chromium/antimony codoped solidstate prepared SrTiO3 were synthesized and activity of these catalysts was compared.
Chromium and antimony/chromium doped solid-state strontium titanates were prepared
with 2% Cr and 2% Cr/2.5% Sb loadings by molar weight. The addition of metal ions from salts
(chromium (III) nitrate and/or antimony (III) acetate) was in such a way that the molar ratio of
SrTi0.98Cr0.02O3 and SrTi0.954Cr0.02Sb0.025O3 would be preserved. The solid precursors of strontium
carbonate (SrCO3), titanium oxide (TiO2), chromium oxide (Cr2O3) and additionally antimony
oxide (Sb2O3) in case of Cr/Sb codoping) were mixed and calcined at 1100°C to synthesize Cr
and Cr/Sb doped materials (SrTiIV1-2xCrIII2xO3-x and/or SrTiIV1-3xCrIII2xCrVIxO3, and SrTiIV12xCr
III
V
xSb xO3)
according to method used in [16-17] and following the operational procedure for
the preparation of solid state samples (Chapter 3.2.1). These materials have been already
synthesized and extensively studied as photocatalysts for the water splitting[16, 17]. However,
these catalysts have not been yet studied as photocatalysts to destroy volatile organic
compounds.
The textural properties of the aerogel prepared samples were studied by different available
techniques (More detailed information on sample characterization operation procedures can be
found in Chapter 3.3). Powder X-ray diffractions patterns were obtained on a Bruker D8
Advance spectrometer with a CuKα radiation source with an applied voltage of 40 kV and a
current of 40mA. Scans were made in the 2θ range of 20-85º with a scanning rate of 2 º/min. The
crystallite size was calculated from the XRD patterns using the Debye-Scherrer equation. The
comparison of powder diffractograms of chromium/antimony codoped solid-state and aerogel
prepared strontium titanate is presented in Figure 5.18).
134
0
200
400
600
800
1000
1200
20
30
40
60
2 theta (deg)
50
70
80
2.5% Sb/2% Cr doped SSR-SrTiO3
2.5% Sb/2% Cr doped AP-SrTiO3
Figure 5.18 2.5% Sb/2% Cr Codoped SrTiO3 Prepared by Solid-State Reaction and Aerogel Modified Procedure
Intensity
135
The peaks of solid-state prepared samples are significantly narrow, which can be attributed
to larger particle sizes. Besides, even after long calcination, the solid-state prepared sample is not
pure and has some impurities caused by incomplete reaction of precursors and/or by oxygen
deficiency. This problem is common for all samples prepared by the solid-state reaction method
and has been already discussed in Chapter 3.3.3 (also see Appendix A). The absence of the
dopant peaks indicates high dispersion and incorporation of the doped metal ions into the SrTiO3
lattice. The application of the Debye-Scherrer equation for calculation of the average crystallite
sizes of SSR-SrTiO3 samples is limited by big particle sizes of these materials; the real sizes can
be obtained from TEM graphs.
Surface areas, pore size distributions, and pore volumes of different samples were
measured on a Nova 1200 gas sorption analyzer (Quantachrome Corp.) from the amount of N2
absorbed at 77K and calculated according to the Brunauer-Emmett-Teller (BET) method. The
samples were degassed at 423 K for one hour prior to analysis.The smaller surface areas of solid
state prepared samples in comparison with aerogel samples can be explained by the preparation
method. During the heating at high temperature, sintering causes the formation of nonporous or
microporous dense material with large particle sizes and small surface area.
Table 5.4 Textural Properties of AP- and SSR-SrTiO3 Samples
Sample
Surface
Total pore
Average Pore
area, m2/g
volume, c3/g
Diameter, Å
AP-SrTiO3
93
0.450
194
SSR-SrTiO3
0.5
0.016
125
2% Cr doped AP-SrTiO3
82
0.483
236
2% Cr doped SSR-SrTiO3
1.1
0.009
229
2.5% Sb/2% Cr codoped AP-SrTiO3
69
0.662
384
2.5% Sb/2% Cr codoped SSR-SrTiO3
3
0.011
153
136
The addition of dopants (chromium oxide and antimony oxide) during solid-state reaction
causes a slight increase in the porosity of the products and surface area. However, the total pore
volume does not increase by doping; the higher surface area is caused by an increase in pore
sizes.
Transmission electron micrographs were obtained on a Philips CM 100. Samples were
placed onto a carbon-coated copper grid by the physical interaction of the grid and powder in
such a way that the particles remained adhered to the grids. The solid-state prepared samples
have polydispersed sizes up to 500 nm and nonuniform shapes due to grinding and sintering.
Light absorption spectra of the samples were obtained on a Cary 500 Scan UV-Visible
Spectrometer with an integrating sphere attachment for diffuse reflectance in the range 200-800
nm. Chromium doping has increased the absorbance in the visible light region compared with the
undoped solid state prepared sample (Figure 5.20). Solid-state prepared samples of Cr and Cr/Sb
doped strontium titanate had intense absorption in 400-700 nm. The main differences between
chromium doped and antimony/chromium codoped samples are the shapes of the adsorption
peaks in the visible light region. The Cr doped SSR-SrTiO3 has two combined peaks located at
400-500 nm and 500-600 nm regions, while Sb/Cr codoped SSR-SrTiO3 has one steep peak at
400-500 nm. Besides, the colors of the samples are different, too. The color of Cr doped SSRSrTiO3 sample is dark purple, while the color of the Sb/Cr sample is beige.
Confirming findings of Kudo et al. [16], the Cr doped SSR-SrTiO3 sample has incorporated
Cr 3+ ions (400 - 500 nm) and Cr 6+ (500 - 600 nm), and the shallow shoulder (up to 700 nm)
shows the presence of oxygen defects. Cr/Sb codoped SSR-SrTiO3 has only incorporated Cr 3+ at
400-500 nm. Antimony plays a role of a charge compensator and prevents it from the Cr 6+ ions
and oxygen defects production. The comparison of DRS between aerogel prepared and solidstate prepared doped samples (Figure 5.21) showed that aerogel prepared samples, besides
having Cr 3+ ions incorporated in the lattice of samples, also have some Cr 6+ ions. The presence
of Cr (VI) in both aerogel samples (Cr doped and Cr/Sb codoped) can be explained by the
oxidation of some Cr (III) into Cr (VI) on the surface during the calcination of samples after
aerogel preparation to remove the organic residuals.
137
Prepared Strontium Titanate Samples
Figure 5.19 Transmission Electron Micrographs of 2% Cr Doped (left) and 2% Cr/2.5% Sb Codoped (right) Solid-State
138
0.0
200
0.5
1.0
1.5
2.0
2.5
Wavelength (nm)
400
600
AP-SrTiO3
SSR-SrTiO3
2.5% Sb/ 2% Cr SSR-SrTiO3
2% Cr SSR-SrTiO3
Figure 5.20 Diffuse Reflectance Spectra of 2.5% Sb/2% Cr SrTiO3 Prepared by Solid-State Reaction
Absorbance
800
139
0.0
200
0.5
1.0
1.5
2.0
2.5
300
500
600
Wavelength (nm)
400
700
AP-SrTiO3 2% Cr
SSR-SrTiO3 2% Cr
AP-SrTiO3 2.5% Sb/2% Cr
SSR-SrTiO3 2.5% Sb/2% Cr
800
Aerogel Modified Procedure
140
Figure 5.21 Diffuse Reflectance Spectra of 2%Cr SrTiO3 and 2.5% Sb/2% Cr SrTiO3 Prepared by Solid-state Reaction and by
Absorbance
To study the photocatalytic properties of solid-state prepared samples and reproducibility
of them, three consecutive experiments of each sample were performed and compared with
aerogel prepared catalysts, and standard catalyst (TiO2 P25) for acetaldehyde decomposition
under UV and visible light irradiation (Table 5.5).
All solid-state prepared samples including pure and doped catalysts show high activity for
acetaldehyde decomposition under UV irradiation which is comparable with TiO2 P25 and
almost twice higher than the respective aerogel prepared catalysts. However, the production of
the carbon dioxide under UV light for solid-state and aerogel prepared were on the same level.
There is no direct dependence between the decomposition of acetaldehyde and the production of
carbon dioxide due to the formation of intermediates and the adsorption of them on the surface.
Nevertheless, if a catalyst decomposes twice more of the reagent, but forms twice less of the
product, it means that additional reactions goes on the surface of the catalyst or a different
mechanism of CH3CHO decomposition takes place. From the analysis of the photoactivity data
after several consecutive runs under UV irradiation, the first case is more likely. While the
activity for acetaldehyde decreases significantly, the production of carbon dioxide stays on the
same level (Figures 5.22-5.25). The comparison of Cr doped and Sb/Cr codoped solid-state
prepared SrTiO3 catalyst for carbon dioxide does not show any difference; however, for
acetaldehyde decomposition, Cr doped SSR-SrTiO3 sample losses activity twice faster than
Cr/Sb codoped SSR-SrTiO3 after three consecutive experimental runs (Figures 5.24, 5.25).
Similar in the visible region, the Cr doped SSR-SrTiO3 sample loses activity for
acetaldehyde decomposition twice faster than Cr/Sb codoped SSR-SrTiO3 after three consecutive
experimental runs. In contrast, aerogel prepared catalysts of Cr doped AP-SrTiO3 and Cr/Sb
codoped AP-SrTiO3 after three consecutive experimental runs do not lose their activity.
The production of carbon dioxide on the surface of Cr doped and Cr/Sb codoped solid-state
prepared catalysts is smaller than on the respective Cr doped and Cr/Sb codoped aerogel
prepared samples.
141
0.1160
0.0525
0.1027
0.0526
0.0498
0.0490
0.1080
0.0884
0.0380
0.0529
SSR-SrTiO3
AP-SrTiO3
2% Cr doped SSR-SrTiO3
2% Cr doped SSR-SrTiO3 3runs
2% Cr doped AP-SrTiO3
2% Cr doped AP-SrTiO3 3 runs
2.5% Sb/2% Cr codoped SSR-SrTiO3
2.5% Sb/2% Cr codoped SSR-SrTiO3 3runs
2.5% Sb/2% Cr codoped AP-SrTiO3
2.5% Sb/2% Cr codoped AP-SrTiO3 3 runs
0.00120
0.00147
0.00102
0.00127
0.00085
0.00084
0.00108
0.00134
0.00280
0.00113
0.01310
r [CO2]
r [CH3CHO]
0.1023
Initial rate
Initial rate
P25 TiO2 Degussa
Sample
UV light activity
0.0477
0.0102
0.0347
0.0376
0.0370
0.0350
0.0297
0.0489
0.00770
0.03230
0.01470
r [CH3CHO]
Initial rate
0.00083
0.00073
0.00034
0.00022
0.00039
0.00039
0.00031
0.00032
0.00021
0.00014
0.00016
r [CO2]
Initial rate
Visible light activity
Table 5.5 Photocatalytic Properties of AP- and SSR-SrTiO3 Samples for Acetaldehyde Decomposition
142
0.0000
0.0004
0.0008
0.0012
0.0016
0.0020
1
Experimental Repetitions
2
Figure 5.22 UV and Visible Photoactivity of Cr Doped SSR-SrTiO3 for CO2 Production
Initial Rate, mM/min
3
vis activity
UV activity
143
0.0000
0.0004
0.0008
0.0012
0.0016
1
Experimental Repetitions
2
3
vis activity
UV activity
Figure 5.23 UV and Visible Photoactivity of Cr/Sb Codoped SSR-SrTiO3 for CO2 Production
Initial Rate, mM/min
144
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
1
Experimental Repetitions
2
3
vis activity
UV activity
Figure 5.24 UV and Visible Photoactivity of Cr Doped SSR-SrTiO3 for CH3CHO Decomposition
Initial Rate, mM/min
145
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
1
Experimental Repetitions
2
3
vis activity
UV activity
Figure 5.25 UV and Visible Photoactivity of Cr/Sb Codoped SSR-SrTiO3 for CH3CHO Decomposition
Initial Rate, mM/min
146
Finally, the activity of Cr/Sb codoped aerogel prepared AP-SrTiO3 catalyst was highest
among all synthesized and studied catalyst under UV or visible irradiation. This proves that
novel nanosized Cr/Sb codoped AP-SrTiO3 is a very effective catalyst for the decomposition of
volatile organic compounds.
5.3.3 Cr doping of SrTiO3 and BaTiO3 catalysts
In the previous chapter, it was described that the nanosized barium titanate catalyst is also
active for decomposition of acetaldehyde under UV light irradiation, which is similar to
nanosized strontium titanate. While the activity of barium titanate is smaller than that of
strontium titanate, it has the potential to be a good photocatalyst for visible light irradiation after
doping with transition metals.
2% chromium doped barium titanate catalysts were prepared by solid-state reaction and
aerogel procedure. The addition of chromium ion dopant (chromium (III) nitrate for aerogel, and
chromium (III) oxide for solid-state prepared) was done in such a way that the molar ratio of
BaTi0.98Cr0.02O3 would be preserved.
Light absorption spectra of the samples were obtained on a Cary 500 Scan UV-Visible
Spectrometer with an integrating sphere attachment for diffuse reflectance in the range 200-800
nm. Chromium doping of BaTiO3 increased the absorbance in the visible light region compared
with undoped solid state prepared sample (Figure 5.27). Solid-state and aerogel prepared samples
of Cr doped barium titanate had intense absorption in the 400-500 nm due to incorporated Cr 3+
ions. Besides, the aerogel prepared sample had an additional peack at 630 nm which can be
assigned to surface chromium oxide (Cr2O3). Some of chromium did not incorporate into the
lattice and stayed as a separate phase.
Surface areas of different samples were measured on a Nova 1200 gas sorption analyzer
(Quantachrome Corp.) from the amount o N2 absorbed at 77K and calculated according to the
Brunauer-Emmett-Teller (BET) method. The samples were degassed at 423 K for one hour prior
to analysis. The BET surface areas of the samples are shown in Table 5.6.
147
0.0
200
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
300
400
600
Wavelength (nm)
500
700
AP-BaTiO3
2% Cr SSR-BaTiO3
SSR-BaTiO3
2% Cr AP-BaTiO3
Figure 5.26 Diffuse Reflectance Spectra of Pure AP-BaTiO3 and 2%Cr Doped AP-SrTiO3
Absorbance
800
148
1.2
81.9
70.0
2% Cr doped AP-SrTiO3
2% Cr doped AP-BaTiO3
49.0
AP-BaTiO3
2% Cr doped SSR-BaTiO3
93.0
AP-SrTiO3
1.1
5.4
SSR-BaTiO3
2% Cr doped SSR-SrTiO3
0.5
SSR-SrTiO3
0.0348
0.0498
0.0930
0.1027
0.0415
0.0525
0.0930
0.1160
0.1023
r [CH3CHO]
area, m2/g
50.0
Initial rate
0.00089
0.00084
0.00058
0.00134
0.00141
0.00280
0.00049
0.00113
0.01310
r [CO2]
Initial rate
UV light activity
Surface
P25 TiO2 Degussa
Sample
Barium Titanate Catalysts
0.02660
0.03500
0.01670
0.03130
0.00492
0.00770
0.01680
0.03230
0.01470
r [CH3CHO]
Initial rate
0.00025
0.00039
0.00003
0.00028
0.00014
0.00021
0.00002
0.00014
0.00016
r [CO2]
Initial rate
Visible light activity
149
Table 5.6 Photocatalytic Properties of Chromium Doped Aerogel Prepared and Solid-state Prepared Strontium Titanate and
The difference in surface areas for aerogel prepared catalysts of barium titanate and
strontium titanate can vary significantly, while for the solid-state prepared catalysts of BaTiO3
and SrTiO3, textural properties and surface areas are similar for both chemicals. The comparison
of two different chemicals with similar surface areas helps to see more clearly the difference in
their photoactivity.
Doping of strontium titanate or barium titanate with chromium causes the formation of
impurity levels or bands in the forbidden area of the semiconductor band gap. While the purpose
of this is to decrease the band gap and to achieve the absorption of visible light, the doping can
influence the absorption of UV light also. It is common, when the doping causes a decrease in
UV photoactivity, to explain this by more facile recombination of electrons and holes. This case
is characteristic for the Cr doped AP-SrTiO3 and AP-BaTiO3 when under UV irradiation they
have less activity for acetaldehyde decomposition and carbon dioxide production than that of
pure aerogel prepared SrTiO3 and BaTiO3 (Figures 5.27, 5.28). The solid-state prepared samples
of a doped catalyst have fewer tendencies for photoactivity decrease under UV irradiation (Table
5.6).
In the visible region, the photoactivity of doped catalysts is significantly higher in
comparison with pure catalysts. The electrons and holes are excited by the visible light photons
on the incorporated dopant bands, so electrons go to the conduction band and less recombination
between holes and electrons is possible. This case can be seen in Figures 5.29 and 5.30 where Cr
doped SrTiO3 and Cr doped BaTiO3 samples are more active for acetaldehyde decomposition
and carbon dioxide production under visible light in comparison with undoped catalysts.
Finally, the complete comparison of photoactivity for different prepared samples of barium
titanate or strontium titanate showed that the SrTiO3 catalyst was more effective for acetaldehyde
photodecomposition than the BaTiO3 catalyst, and each SrTiO3 sample was more photoactive
than the corresponding BaTiO3 sample (Figures 5.31 and 5.32, Table 5.6).
150
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0
20
light off
80
UV light on
60
Time, min
40
AP-SrTiO3
AP-BaTiO3
2% Cr AP-SrTiO3
2% Cr AP-BaTiO3
Figure 5.27 CO2 Evolution for Aerogel Prepared SrTiO3 and BaTiO3 Under UV Light Irradiation
[CO2] / mM
100
151
42
44
46
48
0
20
light off
60
Time, min
40
80
AP-SrTiO3
AP-BaTiO3
2% Cr AP-SrTiO3
2% Cr AP-BaTiO3
UV light on
100
Figure 5.28 CH3CHO Degradation for Aerogel Prepared SrTiO3 and BaTiO3 Under UV Light Irradiation
[CH3CHO] / mM
152
0.02
0.04
0.06
0
20
light off
40
AP-SrTiO3
AP-BaTiO3
2% Cr AP-SrTiO3
2% Cr AP-BaTiO3
Time, min
60
vis light on
80
100
Figure 5.29 CO2 Evolution for Aerogel Prepared SrTiO3 and BaTiO3 Under Visible Light Irradiation
[CO2] / mM
153
43
44
45
46
47
48
0
20
light off
Time, min
40
60
80
AP-SrTiO3
AP-BaTiO3
2% Cr AP-SrTiO3
2% Cr AP-BaTiO3
vis light on
100
Figure 5.30 CH3CHO Degradation for Aerogel Prepared SrTiO3 and BaTiO3 Under Visible Light Irradiation
[CH3CHO] / mM
154
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0
20
light off
Time, min
40
60
AP-SrTiO3 (uv light)
AP-SrTiO3 (vis light)
AP-BaTiO3 (uv light)
AP-BaTiO3 (vis light)
2% Cr:AP-SrTiO3 (uv light)
2% Cr:AP-SrTiO3 (vis light)
2% Cr:AP-BaTiO3 (uv light)
2% Cr:AP-BaTiO3 (vis light)
80
100
Figure 5.31 CO2 Evolution for Aerogel Prepared SrTiO3 and BaTiO3 Under Light Irradiation (UV or Visible Light)
[CO2] / mM
155
44
45
46
47
48
49
0
20
light off
Time, min
40
60
80
100
AP-SrTiO3 (uv light)
AP-SrTiO3 (vis light)
AP-BaTiO3 (uv light)
AP-BaTiO3 (vis light)
2% Cr:AP-SrTiO3 (uv light)
2% Cr:AP-SrTiO3 (vis light)
2% Cr:AP-BaTiO3 (uv light)
2% Cr:AP-BaTiO3 (vis light)
156
Figure 5.32 CH3CHO Degradation for Aerogel Prepared SrTiO3 and BaTiO3 Under Light Irradiation (UV or Visible Light)
[CH3CHO] / mM
5.4 Discussion and conclusions
The goal of this research was to develop a photocatalyst (doped with transition metal),
which would be active in the visible region for the decomposition of VOCs and acetaldehyde in
particular. SrTiO3 or BaTiO3 doped with transition metals (Cr or Sb/Cr) showed absorption in
visible light and photoactivity for CH3CHO oxidation.
Doping with chromium decreased the band gap of the semiconductor by the incorporating
of Cr donor bands or levels into the forbidden area of the band gap. The decrease in band gap
allowed the excitement of electrons and holes by visible light photons. The visible light response
was due to the transition from the electron donor level formed by Cr ions to the conduction band
of SrTiO3.The produced holes formed active species on the surface of the catalyst which
participated in catalysis and decomposed organic compounds.
The introduction of Cr (III) ions in to the lattice to substitute Ti (IV) ions caused charge
imbalance. The charge transfer between Cr and Ti ions induced by light allowed forming oxygen
defects
Cr 3+ + Ti 4+ + hν => Cr 4+ + Ti 3+
(5.1)
The doping of SrTiO3 with Sb/Cr was based on the ability to keep the charge balance and
suppresses oxygen defects in the lattice. Codoping with Sb 5+ ions suppressed the formation of Cr
6+
ions and oxygen defects in the lattice.
Sb/Cr codoped samples had a wider absorption range in the visible region than that of Cr
doped ones. Probably, the Sb/Cr codoped samples had more stabilized Cr 3+ in the SrTiO3 lattice
in a comparison with Cr doped samples.
Novel aerogel prepared and conventional solid-state prepared SrTiO3 and BaTiO3 samples
were used for photocatalytic studies. Codoping of SrTiO3 with Sb/Cr was studied and compared
with pure, Cr-, and Sb-doped SrTiO3 samples and the standard catalyst of TiO2 P25. The doped
photocatalyst showed high photoactivity under visible light irradiation during several continuous
experiments and photoactivities in visible and UV regions were similar. The photocatalytic
157
activity for CO2 evolution of the antimony/chromium doped AP-SrTiO3 was twice higher than
that of doped with only chromium under UV or visible light irradiation.
Solid-state samples with a high amount of lattice defects gave a higher reaction rate of the
acetaldehyde decomposition. However, there was no increase in the carbon dioxide production.
After three consecutive experiments with the same catalyst, a decrease in acetaldehyde
decomposition was noticed while the production of carbon dioxide stayed on the same level. It is
possible to speculate that acetaldehyde decomposition went through several reactions on the
surface of the catalyst. One of them was the photooxidation of acetaldehyde, while others are
using the presence of high amount of lattice defects for the production of other products which
were less photoactive and less volatile and stay absorbed on the surface. The case at hand, only
carbon dioxide was monitored by gas chromatography and other intermediates and/or product are
undetected because they are less volatile and stay absorbed on the surface. IR surface studies of
the catalyst during photocatalysis would help to prove this hypothesis.
This hypothesis is more plausible considering the presence of a large amount of oxygen
defects and Ti 3+ ions (reaction 5.1) on the surface of solid-state prepared samples. They can
absorb and hold more organic species on the small surface area in comparison with the large
surface area of the catalysts which do not have surface defects.
Nevertheless, for a photocatalytic reaction of acetaldehyde decomposition on the SrTiO3
catalyst under visible light irradiation, high activity can be achieved by the doping with Cr 3+ and
its charge compensation to prevent the formation of lattice defects.
The comparison of photoactivity for different barium or strontium titanate catalysts showed
that SrTiO3 catalysts are more effective for acetaldehyde photodecomposition than the BaTiO3
catalysts.
The activity of Cr/Sb codoped aerogel prepared AP-SrTiO3 catalyst is highest among all
synthesized and studied catalysts under UV or visible irradiation for carbon dioxide production.
The highest rate of mineralization of acetaldehyde proves that novel nanosized Cr/Sb codoped
AP-SrTiO3 is a very effective catalyst for decomposition of volatile organic compounds.
158
5.5 References
[1]
K. Zamaraev and V. Parmon, Energy Resources through Photochemistry and Catalysis,
Ed. M. Grätzel, Mir, Moscow, 1986.
[2]
J.M. Herrmann, Catal. Today, 53 (1999) 115-129.
[3]
N. Serpone and E. Pelizzetti, Photocatalysis. Fundamentals and Applications, John Wiley
and Sons, New York, 1989, p. 650.
[4]
H. Mizoguchi, K. Ueda, M. Orita, S.C. Moon, K. Kajihara, M. Hirano, and H. Hosono,
Mater. Res., 37 (2002) 2401-2406.
[5]
F.T. Wagner and G.A. Somorjai, Nature 285 (1980) 559-560.
[6]
J.L Giocondi and G.S. Rohrer Mat. Res. Soc. Symp. Proc., 654 (2001) AA7.4.1-10.
[7]
R.W. Schwartz, Chem. Mater., 9 (1997) 2325-2340.
[8]
J. Wang, S. Uma, and K.J. Klabunde, Applied Catalysis, 48 (2004) 151-154.
[9]
S. Uma, S. Rodrugues, I.N. Martyanov, and K.J. Klabunde, Microporous and
Mesoporous Materials, 67 (2004) 181-187.
[10]
I.N. Martyanov, S. Uma, S. Rodrigues, and K.J. Klabunde, Langmuir, 21 (2005) 2273-
2280.
[11]
M. Kaneko and I. Okura, Photocatalysis, Springer, 2002
[12]
R. Konta, T. Ishii, H. Kato, and A. Kudo, J. Phys. Chem. B, 108 (2004) 8992-8995.
[13]
J.-M. Herrmann, J. Disdier, and P. Pichat, Chem. Phys. Lett., 108 (1984) 618.
[14]
G. Campet, M.P. Dare-Edwards, A. Hamnett, and J.B. Goodenough, Nouv. J. Chim., 4
(1980) 501.
[15]
A. Mackor and G. Blasse, Chem. Phys. Lett., 77 (1981) 6.
[16]
H. Kato and A. Kudo, J. Phys. Chem. B, 106 (2002) 19, 5029-5034.
[17]
T. Ishii, H. Kato, and A. Kudo, J. Photochem. Photobiol. A, 163 (2004) 181.
[18]
W. Choi, A. Termin, and M.R. Hoffmann, J. Phys. Chem., 98 (1994) 13669.
[19]
R. Asahi, T. Morikawa, T. Okwaki, K. Aoki, and Y. Taga, Science, 293 (2001) 269.
[20]
J.C. Yu, J. Yu, W. Ho, Z. Jiang, and L. Zhang, Chem. Mater., 14 (2002) 38008-3816.
[21]
T. Umebayashi, T. Yamaki, S. Tanaka, and K. Asai, Chem. Lett., 4 (2003) 330-331.
[22]
J. Wang, S. Yin, M. Komatsu, Q. Zhang, F. Saito, and T. Sato, Transaction of the
Materials Research Society of Japan, 29 (2004) 6, 2693-2696.
159
Chapter 6 : Surface studies of titanates by FTIR spectroscopy
6.1 Introduction
The studies of oxygenated hydrocarbon reactions on metal oxide surfaces have a great
deal of significance from both fundamental and practical perspectives. These reactions are rich
and complex (oxidation, reduction, condensation, esterification etc.).
Aldehydes are a common representative of these oxygenated hydrocarbons and are the
products of alcohol oxidation, carboxylic acid reduction, or can be found in automotive exhaust
gases due to an incomplete combustion of fuel in the engine. The reactions of acetaldehydes have
been extensively investigated on different oxide surfaces (TiO2, CeO2, Al2O3, and UO3),
although strontium titanate has not yet been examined. The results from these investigations can
be useful as guidelines for the acetaldehyde reaction on the SrTiO3 surface.
The studies on TiO2 surfaces give the most important information due to similarity of the
surface with strontium titanate and the presence of titanium-oxide layers in both materials. The
studies dedicated to acetaldehyde reactions and photocatalysis on the TiO2 P25 Degussa surface
by Lue et al. showed the aldol condensation of acetaldehyde and production of the
crotonaldehyde [1].
2 CH3CHO → CH3CH=CHCHO + H2O
(6.1)
Thermal programmable desorption for acetaldehyde on P25 surface gave crotonaldehyde, butane,
butadiene, and acetone. It was found that after adsorption about, 90% of the CH3CHO molecules
were staying on the surface.
The acetaldehyde reaction on uranium oxide surfaces (UO2, UO3, and U3O8) was studied
by H. Idriss et al. [2]. Different uranium oxides gave different product of the acetaldehyde
reaction on the surface of the oxide in the presence of oxygen. Reductive coupling of two
aldehydes to form CH3CH=CHCH3 was the dominant pathway for UO2 oxide. The aldolization
of two aldehydes gave crotonaldehyde on the surface of U3O8. Over UO3, furan (C4H4O) was the
160
dominant reaction product at low surface coverage in the presence of oxygen. In the absence of
oxygen, the reaction products were furan and crotonaldehyde and with time the production of
furan decreased significantly. Also it was found that without oxygen the UO3 transformed into
U3O8 (mild reduction) and UO2 (deep reduction). From IR analysis, acetaldehyde was absorbed
only in η1 (O) configuration over UO3, while the η2 (C,O) configuration was present over UO2.
It is clear that the aldol condensation reaction is common on the surface of different
oxides. It was found that silica supported alkali metal containing solid bases catalyzes the aldol
condensation with 90% selectivity. Titanium oxide was found active for aldol condensation with
93% selectivity, and cerium oxide gave 48% selectivity [3]. The basicity of the oxidized CeO2
favored aldolization reactions, while the reduced CeO2 was more active for cross-reductive
coupling between the absorbed aldehyde molecules (Figure 6.1).
Figure 6.1 Acetaldehyde Reaction on CeO2 and M/CeO2 (M is Po, Co and Po-Co) [3]
161
The reduction of CH3CHO to ethanol was observed on CeO2 and Pd doped CeO2, and the
oxidation of acetaldehyde caused the production of acetates. The production of acetates was also
observed on zinc oxide (ZnO) and titanium oxide (TiO2). Adsorbed acetate species decompose to
carbon dioxide (CO2) and methane (CH4), or can react by bimolecular ketonization to give
CH3COCH3 and CO2. Acetaldehyde can also decompose through the formation of carbon
monoxide (CO). Linear and bridging modes of CO were observed on Pd-CeO2 while only the
bridging modes were observed on Co-CeO2. Tungsten oxide on a silica support (WO3/SiO2) was
also active for aldol condensation while molybdenum oxide on a silica support (MoO3/SiO2) was
not active.
The acetaldehyde reaction on the surface of strontium titanate has not been studied
intensely. Only in Wang et al. work, it was found that the surface of stoichiometric SrTiO3 was
not sufficiently active for aldol condensation [4]. However, the decomposition of acetaldehyde
occurred in the presence of surface defects created by Ar+ sputtering and gave H2, C2H4, C4H6,
and C4H8 products.
In situ FTIR studies in combination with photooxidation reactions of acetaldehyde on the
surface of SrTiO3 will bring more understanding to the process of photooxidation of CH3CHO
and help determine the products of reaction.
The following steps can be done to study acetaldehyde interaction with the surface of the
strontium titanate
1.
The activation of the SrTiO3 sample (the removal of organic residuals from the
surface in vacuum or oxygen),
2. IR studies of adsorbed acetaldehyde and evacuation of gas phase CH3CHO,
3. IR studies on the dark oxidation of CH3CHO in oxygen,
4. IR studies on the photooxidation of acetaldehyde in oxygen under UV light
irradiation
5. IR studies on the photooxidation of acetaldehyde in oxygen under visible light
irradiation
6. MS studies on reaction products.
The comparison of IR results for pure strontium titanate samples (AP-SrTiO3) and
chromium doped strontium titanate samples (Cr-AP-SrTiO3 and Sb/Cr-AP-SrTiO3) will help to
162
explain the advantages of doped samples for acetaldehyde decomposition under visible light
irradiation.
6.2 Acetaldehyde decomposition on the surface of aerogel prepared SrTiO3
For the in situ infrared studies, a Mattson Research Series RS_10000 FTIR with a liquid
nitrogen cooled detector and manually translatable stage was used. WinFirst v.2.10 software was
used to operate the spectrometer and for some data plotting purposes. Origin 6.1 was also used as
a plotting application. Microsoft’s Excel software was used for some data manipulation and
calculations.
Reactions were carried out in a stainless steel reaction cell with KBr windows for the
passage of the IR beam and a quartz window for the UV exposure (See Appendix D Figure D.1).
The cell utilized differential pumping from a Pfeiffer-Balzers turbomolecular pump and a Duo
Seal mechanical vacuum pump to reach pressures as low as 10 nanoTorr. Temperatures were
monitored using an E-type thermocouple. Heating was done using electrical resistivity with a 50
amp power driver built by the University of Pittsburgh and Honeywell controller. Dosing of the
gases and evacuation was controlled and directed using a bakeable, stainless-steel UHV
manifold. Pressures were monitored via an ion gauge and dual, capacitance manometer gauges.
Samples were pressed onto 100 x 100, plain, 0.002 inch (0.0508 mm thick, with 0.22
mm2 holes) tungsten mesh from the Unique Wire Weaving Company and held in the chamber
with a nickel clamp. The powdered SrTiO3 samples were hydraulically pressed at 12000 lbs/in2
onto a tungsten mesh [5] as a circular spot 7 mm in diameter, typically weighing 1-1.5 mg (1.31.9 mg/cm2) (See Appendix D Figure D.2). The transparency of the grid to the IR beam is about
80%, so that infrared radiation can pass through the sample efficiently. The grid is supported in
the center of the stainless steel cell by nickel clamps along the grid edges. A chromel/costantan
thermocouple spot-welded to the top center region of the grid is used for temperature control.
Electrical heating and cooling with liquid nitrogen and a power supply/temperature programmer
permit the temperature of the grid to be maintained to about ±2 K within the range 100-1500 K.
The IR cell is connected through a gas port to a high vacuum system, equipped with a
quadrupole mass spectrometer (See Appendix D Figure D.3). The base pressure of the system is
kept below 10-8 Torr by a Pfeiffer Vacuum 60 1/s turbomolecular pump backed by an oil-free
163
diaphragm pump. The pressure was measured by a capacitance manometer (Baratron, 116A,
MKS, range 10-3-103 Torr) or by the ion current drawn by the ion pump. The cell windows were
KBr single crystals mounted on concentric Viton O-rings which are differentially pumped to
prevent leaks.
The IR cell is mounted on a computer-controlled translation system (Newport
Corporation), capable of moving the cell to ±1 µm accuracy in the horizontal and vertical
directions. Thus, it is possible to study two samples, i.e. SrTO3 and TiO2 at the middle and the
bottom positions on the same grid for comparison of their spectroscopic and adsorptive
properties under identical conditions of temperature and gas exposure. The upper one third
positions on the grid was empty and was used for the background absorbance measurements in
the same experiments.
Light exposures were done with a water-cooled, PTI short arc xenon lamp passing
through a Newport 6123 liquid infrared filter. The light source was a 100W Xenon Arc Lamp
(Photon Technology International) which was focused onto the sample through a quartz window.
The energy range for the radiation was 2.1-5 eV. In later trials, Oriel Spectra-Physics model
59044 and 59480 long pass filters were added to limit the light to the visual and the UV-visible
ranges of 400 nm to 700 nm and 300 nm to 700 nm, respectively. A Cole-Parmer 8852 sonicator
was used to clean the clamp and grid prior to sample mounting. Fisher-Scientific certified
spectra-analyzed acetone was used in the sonications and for cleaning the probe on which the
clamp is mounted in the cell. A MKS PPT Residual Gas Analyzer/quadrapole mass spectrometer
was used for the mass spectral studies. Thermocouple wire leads, chromel and constantan, were
welded to the mesh and thermocouple using a Rocky Mountain 506 Dial-a-Weld pulsation
welding device. Dry ice and acetone were used as cryogenic agents for the cooling of the cell
through thermal conduction. Compressed nitrogen was used when needed as a purging agent.
Infrared spectra were obtained with a dry CO2-free air purged Mattson Research Series I
FTIR spectrometer equipped with a narrow band HgCdTe detector operating at 77 K. The
sample and background spectra shown here were recorded in the ratio mode with a resolution of
4 cm-1 using 300 scans.
Acetaldehyde (99%) and crotonaldehyde (99%) used for this work were obtained from
Aldrich. These liquids were stored in glass bulbs and purified by five freeze-pump-thaw cycles
164
and attached to the ultra high vacuum system. The oxygen used is obtained from Matheson and
was 99.999% pure.
The oxide samples were heated in vacuum at 473 K for ~10 h and then the temperature
was raised gradually to 773 K for calcination with 20.0 Torr oxygen followed by evacuation. The
evacuated sample was then heated at 873 K for 30 min before cooling. This procedure allows the
removal of traces of residual organic species which could come from remnant alkoxy groups
retained by the oxide nanoparticles after the preparation procedure. Before exposure to
adsorbates the samples were cooled in vacuum to 233 K. The adsorption of acetaldehyde,
crotonaldehyde, and FTIR measurements were carried out at different temperatures beginning
233 K.
6.2.1 Acetaldehyde adsorption over SrTiO3 at 243 K: evidence of H-bonding
Figure 6.2 (A) shows the background spectrum of the SrTiO3 sample at 233 K before
exposure to acetaldehyde. The OH region shows an infrared feature at 3689 cm-1 which can be
assigned to the ν(OH) mode for isolated hydroxyl groups bound to Ti atoms. However, in a
recent TiO2 study, Panayotov and Yates identified the small features at 3700 cm-1 as due to
isolated Ti-OH groups [6]. This 11 cm-1 red-shift may be due to electron donating effect of the
alkaline earth metal ion, Srn+. The low frequency band at 3403 cm-1 can be assigned to either TiOH perturbed by nearby Sr atom or to Sr-OH groups.
165
4000
0 .0 5
0 .1 0
0 .1 5
0 .2 0
3403 ν(Sr-OH)
3500
ν (CH3)
0.1 Torr
243K
263K
253K
273K
2500 1800
-0 .1 0
-0 .0 5
0 .0 0
0 .0 5
0 .1 0
0 .1 5
-1
1500
W a v e n u m b e r (c m )
3000
B ackground Sam ple (*1/8)
(a)
(c)
(b)
(d)
(e)
(f)
(g)
2867 ν(CH)
evacuate
0 .2 0
ν (C -C)crot.
1770
1707 ν(C=O)acet.
1667 ν(C=O)crot.
1644
1564 νas(OCO)
1555
1400δa(CH3)acet.
Figure 6.2 Acetaldehyde Adsorption on AP-SrTiO3 and Sequential Change of Temperature
Absorbance
0 .2 5
ν(Ti-OH)
3689
3
s
0 .3 0
2973 a
2928 ν (CH )
2932
1140
1119
1200
1162
B
ρ (CH 3 )crot.
ν (C-O )
ν (C -C H 3 )crot.
ν (C -C)acet.
1067
1071
0 .2 5
1375 δs(CH3)acet.
1348 δ(CH)acet.
1333 δ(CH)crot.
1289
1435
900
964
940
1007
A
855
860
SrT iO 3
720
600
166
The strong broad feature at 1435 cm-1 can be assigned to νasym(OCO) groups for CO32groups formed during preparation of nanoparticles. All other spectra shown in Figure 6.2 were
obtained by the subtraction of the background spectrum (A). Upon the exposure of 0.1 Torr
acetaldehyde at 233 K, the evolution of ν(CH) and δ(CH) modes along with the concomitant
depletion of ν(OH) absorbance are clearly observed as shown in spectrum (B).
The dominant spectral features in the ν(CH) region at 2973 and 2928 cm-1 can be
assigned to νa(CH3) and νs(CH3) of acetaldehyde adsorbed onto the surface mainly through Hbonding, whereas the mode at 2867 cm-1 is due to the overtone for δ(CH) mode. The depletion of
isolated Ti-OH intensity at 3689 cm-1 with an increase in intensity of the band in the low
frequency side (3550-3350 cm-1) indicates the formation of H-bond through the oxygen of the
carbonyl group [TiOH···O=C(H)CH3]. Hydrogen bonding is generally characterized by (a)
frequency shifts to lower frequency of the absorption bands due to ν(OH) stretching vibration
and (b) broadening of the shifted OH band and the increase in the O-H absorbance. Upon
evacuation for 15 minutes at 1x10-6 Torr, no apparent changes in spectral behavior were observed
indicating the irreversibility of H-bonding. The configuration of irreversibly bound CH3-CHO
species is most likely those in which the partially negatively charged oxygen moiety of
acetaldehyde interacts with positively charged hydrogen moiety of the surface hydroxyl groups,
leaving the methyl moiety largely unperturbed relative to its behavior in the gas phase. This
configuration is most probable since gas phase CH3 CHO possesses a substantial dipole moment
of 2.7 Debye. The relatively minor frequency and intensity changes in the observed ν(CH3) and
δ(CH3) vibrational modes are a consequence of the local mode character of the CH3 groups. A
fraction of acetaldehyde may also be adsorbed through >C=O →Ti4+ and/ or Ti3+ ion, which
forms by the thermal treatment of TiO2 under evacuation. The fingerprint region shows a variety
of deformation modes in addition to the strong ν(C=O) mode at 1707 cm-1 which are explained
below. The infrared features at 1375 and 1348 cm-1 can be assigned to δa(CH3) and δs(CH3)
respectively, whereas the strong signature at 1119 cm-1 indicate the ν (C-C) mode. The strong
feature at 940 cm-1 may be assigned to ρ(CH3) consistent with other studies. The weak feature at
1564 cm-1 along with a shoulder at ~1420 cm-1 (on the high frequency side of 1375 cm-1) is
attributed to νas(OCO) and νs(OCO) respectively for the bidentate acetate species [νas(OCO) -
167
νs(OCO) =144 cm-1]. The formation of acetate species from acetaldehyde molecules was
previously observed on several oxide species including TiO2, ZnO, and CeO2.
6.2.2 Warm up effect of adsorbed acetaldehyde: evidence of aldol
condensation and formation of α, β-unsaturated aldehyde (crotonaldehyde,
CH3-CH=CH-CHO)
As the temperature is raised at an interval of 10ºC (Figure 6.2 (B), b-e), the decrease in
intensities of spectral bands of surface bound acetaldehyde at 1707-ν(C=O), 1348-δ(CH), 1119ν(C-C) and 940 –γr(CH3) cm-1 can clearly be noticed. In addition, the new infrared spectral
features appeared at 1667(s), 1644(shoulder), 1333, 1162, 1067, and 964 cm-1. This observation
is accompanied by a slight decrease in intensity of ν(O-H) mode at 3403 cm-1 feature along with
an increase in intensity of an unresolved broad ν(O-H) feature ~3250 cm-1 indicating the
probable formation of new aldol surface species formed by the condensation of two acetaldehyde
molecules according to the reactions shown below having ν(C-O) mode at 1007 cm-1 and the
unresolved ν(C-C) mode at 1140 cm-1. In addition, δ(CH)ald mode of acetaldehyde at 1348 cm-1
decreased in intensity forming a new band at 1307 cm-1 which can be assigned to δ(CH)ald for
aldol.
CH3CHO + -Ti-O-Ti- → CH2 CHO(a) + -Ti2OH
(6.1)
CH2 CHO(a) + CH3CHO(a) + H(ad)→ CH3-CH (OH) –CH2 CHO
(6.2)
β-hydroxy butanal (aldol)
CH3-CH (OH) –CH2 CHO
→ CH3CH=CH-CHO(a) + H2O(a)
(6.3)
2-butenal (crotonaldehyde)
The aldol ultimately dehydrates on the surface forming crotonaldehyde (an α,βunsaturated aldehyde) surface species. The dehydration is clearly be evidenced by a gradual
decrease in intensity of >C=O mode at 1707 cm-1 with the sequential growth of 1667 cm-1
feature accompanied by a weak ν (C=C) mode at 1644 cm-1. Idriss and Barteau reported
168
acetaldehyde coupling over both oxidized and reduced TiO2(001) forming self condensation
products, crotonaldehyde and crotyl alcohol. The infrared frequencies and the vibrational
assignments of surface species formed for acetaldehyde conversion during warm-up are given in
Table 6.1.
Table 6.1 Vibrational Frequencies & Assignments of Adsorbed Acetaldehyde and Related
Surface Species
Assignment Gas Phase Ar Matrix TiO2 Anatase
SrTiO3
Crotonaldehyde
/SrTiO3
νas (CH3)
3014
3022
2969
2973
2973
νs(CH3)
2968
2921
2914/2846
2928/2867
2928/2867
ν(CHald)
2716
2736
2759
2755
ν(C=O)
1743
1729
1718
1707
ν(C=C)
1667
1644
δas(CH3)
1433
1427
1400
1455
δs(CH3)
1395
1399
1375
1394/1371
δ(CH)
1352
1349
1348
1311
ν(C-C)
1114
1111
1119
1158
1355
ν(C-CH3)
1105
γr(CH3)
920/867
γ(CH3)
764
873
940
968
169
6.2.3 Dark oxidation: influence of dioxygen exposure over preadsorbed
acetaldehyde at 243 K
Figure 6.3 shows a series of subtracted (for clarity purposes) infrared spectra (for the
duration of 85 minutes) obtained upon introduction of oxygen gas over preadsorbed acetaldehyde
at 273 K. The reference spectrum used for subtraction purposes was Figure 6.2 (g), which has
been evacuated before introduction of 14.0 Torr O2 in order to observe the effect of oxygen
exposure without the irradiation of UV light. It may be noted that at this temperature, below
1800 cm-1 region (Figure 6.3) (B) (a-g)), a number of bands (1668, ~1610, 1580, 1018, and 972
cm-1) gained intensities as a function of time, while the other bands (1716, 1373, 1167, 1122,
1064, and 936 cm-1 ) lost intensities within the same time period.
Here, the decrease in intensity of 1716 cm-1 feature along with simultaneous decrease in
intensities of ν(C-C) mode at 1122 cm-1 and ν(C-CH3) mode at 1064 cm-1 indicate that both
preadsorbed acetaldehyde and crotonaldehyde (formed during warm-up) are continually being
oxidized forming carboxylate surface species showing strong infrared feature ~1580 cm-1 and a
medium intense peak at 1424 cm-1 for νa(OCO) and νs(OCO) surface species respectively.
However, there is no evidence confirming the formation of CO2 for νa(OCO) in 23002400 cm-1 region. In addition there was no overall change in the integrated absorbance for both
νa(OCO) and ν(CH) for the entire duration (85 minutes) of dark oxidation as observed by in the
plot given in Figure 6.4. The decrease in the ν(CH) (at 2980 cm-1) absorbance may be due to a
slight shift in CH stretching mode due to the partial oxidation of adsorbed aldehyde forming
acetate.
170
-0.08
4000
-0.07
3500
(a)
(b)
-0.05
-0.06
(c)
-0.04
(e)
2980
2958
3000
(f)
(d)
20min.
85min.
(g)
νa(CH3)
-0.03
-0.02
-0.01
0.00
ν(C=O)crot.
Wavenumber (cm )
-1
20min.
CO2
85min.
2400
-0.06
-0.05
-0.04
-0.03
-0.02
-0.01
0.00
0.01
0.02
0.03
2500
νa(CH3)
Figure 6.3 Dark Oxidation of Acetaldehyde on AP-SrTiO3 Surface
Absorbance
0.01
2919
2905
2906
0.02
δ (CH )acet.
B
1500
1373
δ(CH)acet. 1351
A
1260
1194
2
SrTiO3
T=243K
PO =14Torr
1000
ν(C-C)acet.
1167
1122
1064
0.04
1648
ν(C=O)acet.1716
1018
972
0.03
1668
1612
1580νa(OCO)
1564
1424
1334 δ(CH)crot.
3
s
893
936
171
Integrated Absorbance
-1
0
1
2
6
7
8
9
0
0
2
SrTiO3
T = 258K
PO = 14Torr
2
SrTiO3
T = 258K
PO = 14Torr
20
20
Time(min.)
60
60
Time(min.)
Figure 6.4 Kinetics Study of Acetaldehyde Dark Oxidation Using Oxygen
Integrated Absorbance
80
ν(CO2)
80
ν(CH3)
172
6.2.4 Spectral development during photooxidation reaction: influence of
dioxygen exposure over preadsorbed acetaldehyde at 243 K
Figure 6.5 shows that the intensity of CH3 vibrational modes (at 2964, 2919, and 2862
-1
cm ) of adsorbed acetaldehyde is significantly attenuated during 149 minutes of photooxidation.
The fact that new CHx stretching modes in this spectral region are not clearly observed to be
produced during photooxidation indicates that C-H bond oxidation is occurring extensively, and
that any intermediate species containing CHx bonds are of low surface coverage. Simultaneously
various carbonyl stretching modes due to aldehyde, carboxylate, and formate are developed, as
shown in Table 6.1.
The gradual production of gas phase CO2 is observed for CH3CHO photooxidation for a
continuous period of 150 minutes at 273 K as shown in Figure 6.5 (A). The formation of CO2 is
the final oxidation product, often referred to as mineralization product because of the extreme
level of oxidation which it represents. It represents the result of multiple elementary
photooxidation steps and these aldehydes are destroyed by a sequence of intermediate oxidation
products which ultimately reach CO2. However, no CO formation is observed during
photooxidation.
As photooxidation takes place over a period of 149 minutes, the intensity of the Hbonded Ti-OH mode at 3680 cm-1 decreases slightly as acetaldehyde/crotonaldehyde molecules
are gradually destroyed. This suggests that the photooxidation products containing carbonyl or
water are also H-bonded which caused the red shift of the Ti-OH mode ~200 cm-1.
The spectral region below 2000 cm-1 contains a complex overlap of vibrational modes
which undergo systematic changes in absorbance as photooxidation takes place. The observation
of modes which increase or decrease in absorbance can be observed most conveniently by
presenting the difference spectra obtained over the course of the photooxidation experiments,
and this has been indicated in Figure 6.5 (B). Table 6.1 summarizes these results and presents a
tentative assignment of each mode.
173
3480 ν(Sr-OH)
3500
3006
νa(CH3) 2964
-1
1800
10 min.
CO2
149 min.
-0.10
2500 2400
-0.05
0.00
0.05
0.10
0.15
0.20
0.25
Wavenumber (cm )
3000
νs(CH3) 2919
ν(CH) 2862
1564
1596
1500
1426 νs(OCO)
1387 δa(CH3)
3680 ν(Ti-OH)
1200
2
SrTiO3
T=270K
PO =14 Torr
Figure 6.5 Spectral Development during Photooxidation of Acetaldehyde on the AP-SrTiO3 surface
4000
0.00
0.05
Absorbance
νas(OCO)
1700 ν(C=O)acet.
1676
1320
1375
1243
1190
1144
B
1018
1063
0.10
A
900
894
0.30
ρ(CH3) 938
174
The simultaneous growth of bands, attributed to various carbonyl stretching modes due to
aldehyde, carboxylate, formate, and carbonate are observed, as shown in Figure 6.5 and Table
6.2.
Table 6.2 Spectral Changes during the Photooxidation of Acetaldehyde
Frequency (cm-1)
(species formed during oxidation)
2353
CO2
1700
Carbonyl ν(C=O)
1596/1564
Carbonyl ν as (OCO)
1424/1378
Carbonyl ν(OCO)s
3680 decrease
Ti-OH···OCO
3480 increase
Frequency (cm-1)
(spectral features depleted during oxidation)
1676
1144
1063
938
2964
2919
2862
Carbonyl ν(C=O)as
ν(C-C)
ν(C-O)
γr(CH3)
νas (CH3)
νs(CH3)
ν(CH)
Adsorption of acetic acid on this catalyst showed bands at 1560 cm-1 (νa OCO), 1458 cm1
(δa CH3), 1391 cm-1 (νs OCO), 1340 cm-1 (δa CH3), and 1021 (ρ CH3) (Spectrum not shown).
The shape and location of these bands are similar to the spectra obtained during photooxidation
experiments indicating that a large fraction of acetaldehyde formed acetate species including
other minor products. These frequencies and assignment of carboxylate species are consistent
with studies of acetic acid over different metal oxide surfaces such as CeO2, TiO2, Fe2O3, and
UO2. One of the products suspected to have formed during photochemical oxidation was
CH3CH2OH; however, the key infrared features for ethoxide species on TiO2 such as ν(C-O)
band at 1119 and 1042 and ω(CH2) at 1356 cm-1 are apparently missing.
175
6.3 Acetaldehyde decomposition on the 2% Cr doped and 2.5% Sb/2% Cr
codoped AP-SrTiO3
The samples were pressed onto a 1” x 1.125” section of tungsten mesh under 12,000 lbs
of pressure between two stainless steel cubes and mounted into the reaction vessel [6]. The
vessel was then attached to the UHV line and heated to 373 K internally and 323 K externally
while under evacuation. After several hours at this temperature, the sample was taken to 473 K
and left overnight. The next day the external heating was removed and the grid and sample
spectra were taken at 473 K. The sample was then heated to 573, 673, and 773 K. Each
temperature was held for approximately an hour and followed by a cooling to 473 K for the
taking of an IR spectrum. At 773 K, 15-20 Torr of oxygen was dosed and left in the reaction
vessel for 15 minutes. The gas was then evacuated and the sample heated another 50 to 100 K for
a half hour. The sample was then cooled to 473 K for a final dehydroxylation spectrum and then
to room temperature.
The cell was cooled to approximately 233 K and exposed to acetaldehyde or
crotonaldehyde in a series of 3-5 dosings. IR spectra, 250-300 scans per, were taken one after
another during the exposure period and for approximately 30 minutes after the final exposure.
The gas phase aldehydes were then evacuated and the sample was warmed in 5 K increments to
273 K. 15-20 Torr of oxygen was then introduced to the cell and spectra were taken over the next
hour or so. The arc lamp was then ignited and the sample exposed to it through the quartz
window of the cell with spectra being continuously taken. The temperature of the cell was
controlled as much as ambient conditions would allow. Temperatures were usually maintained
within 5 K of 273 K. Ambient heat, the presence of gas in the reaction cell, imperfections in the
thermocouple weld, and the heat of the lamp were the primary causes of this variation in
temperature.
6.3.1 Dehydroxylation on 2% Cr doped AP-SrTiO3
The dehydroxylation as a function of temperature and calcination on chromium
incorporated SrTiO3 can be seen in Figure 6.6. Decreasing negative peaks at 3667 cm-1 and 3391
cm-1 can be seen forming as the temperature was increased and calcination was undergone.
176
These correspond to the stretching modes of the titania hydroxyl groups and strontium/chromium
perturbed titania hydroxyl groups, respectively.
6.3.2 Dehydroxylation on 2.5% Sb/2% Cr doped AP-SrTiO3
When antimony is incorporated into the nanoparticle, three negative features develop
with increasing temperature and calcination. The first is at 3659 cm-1 and represents the
stretching mode of the titania hydroxyl groups. The peak at 3438 cm-1 represents the strontium
and/or chromium perturbed titania hydroxyl groups, and the last peak at 3361 cm-1, which is
absent on the Cr-SrTiO3, appears to be either an antimony perturbed titania hydroxyl group or a
surface antimony hydroxyl group (Figure 6.7).
6.3.3 Adsorption, evacuation, and warming prior to dark oxidation of
acetaldehyde on 2% Cr doped AP-SrTiO3
As acetaldehyde was sequentially dosed onto the Cr-SrTiO3 nanoparticle surface a
number of distinct bands can be seen. There is a slight negative feature at 3403 cm-1, which
corresponds to the chromium/strontium perturbed titania hydroxyl groups. This indicates that
they play some role, though apparently not a large one, in acetaldehyde adsorption. The peaks
from 2975 cm-1 to 2702 cm-1 represent the stretching modes of the methyl groups and the C-H of
the aldehyde. The carbonyl stretches of acetaldehyde and in the later spectra crotonaldehyde can
be found at 1709 cm-1 and 1669 cm-1, respectively. At 1645cm-1, the C=C stretch of
crotonaldehyde can be seen forming as a shoulder peak. Another shoulder peak at 1588 cm-1 is
believed to be an adsorbed acetate species’ O-C-O stretching mode. The bending/deformation
modes of acetaldehyde’s and crotonaldehyde’s CH3’s and CH aldehydes appear between 1373
cm-1 and 1267 cm-1. The carbon-carbon stretching modes of both acetaldehyde and
crotonaldehyde occur in the 1161 cm-1 to 1066 cm-1 region and form a large peak complex. The
final peak is the rocking deformation mode found at 966 cm-1 (Figure 6.8).
177
3800
Surface prior to
calcination ratioed
(a)
against grid
(b)
(c)
(d)
'b', 'c', and 'd'
ratioed
against 'a'
2% Cr-SrTiO3
Tscan = 473 K
(except for 298 K)
0.000
4000
0.025
0.050
0.075
0.100
-1
3400
3200
3391 ν Sr-OH
and/or Cr-OH
Wavenumber (cm )
3600
3667 Ti-OH
3000
298 K
573 K
473 K
after calcination
at 773 K
Figure 6.6 Dehydroxylation as a Function of Temperature and Calcination on 2%Cr-SrTiO3 Nanoparticles
Absorbance
178
3800
Sample at 297 K
573 K
623 K
673 K
723 K
773 K
0.0
4000
0.1
0.2
0.3
3400
-1
Wavenumber (cm )
3600
3200
3000
(a)
Sample at 297 K
ratioed against grid
(b)
'b' through 'g'
ratioed against 'a'
ν(Ti-O-H) ν(Sr-OH and/or
(g)
Sr or Cr-Ti-OH)
(f)
ν(Sb-OH or Sb-Ti-OH)
3659
(e)
(d)
3361
3438
(c)
After Calcination @ 773 K
and Heated to 873 K
2.5%Sb-2%Cr-SrTiO3
Tscan= 473 K
Figure 6.7 Dehydroxylation as a Function of Temperature and Calcination on 2.5%Sb/2%Cr-SrTiO3 Nanoparticles
Absorbance
179
238 K
243 K
3500
273 K After
Evacuation
and Warming
0.000
4000
0.025
0.050
0.075
2975
2932
2866
3000
0.022 Torr
0.057 Torr
0.034 Torr
0.078 Torr
0.1
0.2
2%Cr-SrTiO3
-1
1600
Wavenumber (cm )
0.0
2500 2000
ν(CHald.)
0.3
1709 ν(C=Oacet.)
1669 ν(C=Ocroton.)
1645 ν(C=C)
1588 ν(O-C-O)
δ(CH3)
δ(CHald.)
ν(C-C)
1200
800
180
Figure 6.8 Adsorption, Evacuation, and Warming prior to Dark Oxidation of Acetaldehyde on 2%Cr-SrTiO3 Nanoparticles
Absorbance
ν (Cr/Sr-Ti-OH)
3403
3
2746
2702
ν(sp -CH3)
1329
1373
0.100
1161
1287
1267
1142
1119
1066 ν(C-CH3)
966 γr(CH3)
The fact that the 966 cm-1 peak does not depreciate significantly upon evacuation
indicates that the species observed after evacuation are surface bound and that the adsorption is
to an extent irreversible. No significant formation of crotonaldehyde was seen until the sample
was warmed above ~252 K.
6.3.4 Adsorption, evacuation, and warming prior to dark oxidation of
acetaldehyde on 2.5% Sb/2% Cr codoped AP-SrTiO3
The analogous spectra for the Sb/Cr-SrTiO3 sample can be found in Figure 6.9. Similar
trends are seen here as were seen on Cr-SrTiO3 with a few exceptions. There is a significant
negative feature between the 1719 cm-1 ν(C=O) of acetaldehyde and 1667 cm-1 ν(C=O) of
crotonaldehyde. This is a surface artifact, not shown, that was not removed by calcination. In
addition, the hydroxyl features not only appear at 3666cm-1 and 3401 cm-1, but are more
significant than on the chromium incorporated sample. This is most likely a function of the
amount of sample present on the grid and not a real difference between the adsorption of
acetaldehyde on the two samples. The rest of the major groups that were present on the
chromium-incorporated sample can be seen on the antimony/chromium sample as well. A
significant difference between the two experiments was that the antimony/chromium sample was
taken to room temperature, 298 K, prior to dark oxidation while the chromium only sample was
stopped at 273 K.
6.3.5 Attempted dark oxidation of acetaldehyde on 2% Cr-SrTiO3
The most noticeable aspect during the entire course of the dark oxidation in presence of
oxygen was the lack of change in the spectrum. The 1669 cm-1 crotonaldehyde carbonyl stretch
did become predominant over the 1705 cm-1 acetaldehyde carbonyl stretch, but this was mostly
due to the temperature increase prior to the oxygen introduction and not to the oxidation itself.
This indicates that the presence of an oxidative atmosphere is not enough to engage the catalytic
properties of the nanoparticles (Figure 6.10), without the presence of light.
181
0.040
Torr
0.065
Torr
0.084
Torr
Evacuation
273 K
0.0
4000
0.1
0.2
0.3
3681
3500
3000
ν(Sb/Cr/Sr-Ti-OH)
ν(Ti-OH)
3666
Wavenumber (cm )
-1
0.040 Torr
0.065 Torr
0.084 Torr
Evacuation
273 K
298 K
0.0
2500 2000
0.1
0.2
0.3
0.4
0.5
0.6
1600
1719 ν(C=O )
acet.
1667 ν(C=Ocrot.)
1639 ν(C=C)
1576 ν(O-C-O)
CHald.)
δ(CH3 and
ν(C-C)
1200
800
Nanoparticles
Figure 6.9 Adsorption, Evacuation, and Warming prior to Dark Oxidation of Acetaldehyde on 2.5%Sb/2%Cr-SrTiO3
Absorbance
298 K
3401
3
2967
2929
2865
ν(sp -CH3)
2749 ν(CHald.)
2.5%Sb-2%Cr-SrTiO3
1377
1334
1291
0.4
1158 1135
1121
1087
1066
967 γr(CH)
182
0.00
0.05
0.10
0.15
3500
ν(sp -CH3)
3
3000
2969
2935
2855
0.20
2
2748 ν(CHald.)
2000
-1
1800
7 min.
59 min.
Wavenumber (cm )
2500
-0.10
-0.05
0.00
0.05
0.10
0.15
0.20
0.25
0.30
1668 ν(C=O)crot.
1705 ν(C=O)acet.
0.35
B
1600
1636 ν(C=C)
1583 ν(O-C-O)
δ(CHx)
1400
ν(C-C)
1200
1000
800
Nanoparticles
Figure 6.10 Attempted Dark Oxidation of Acetaldehyde as a Function of Time at Constant Temperature on 2%Cr-SrTiO3
Absorbance
2%Cr-SrTiO3
PO = 19 torr
T = 298K
1372
1329
1288
0.25
A
1162
0.40
1122 1142
0.30
973 γr(CH3) 1066 ν(C-CH3)
183
6.3.6 Attempted dark oxidation of acetaldehyde on 2.5% Sb/2% Cr codoped
AP-SrTiO3
The antimony/chromium sample also experienced no significant changes with the
addition of oxygen (Figure 6.11). It should be noted that this section of the reaction was carried
out on this sample at 298 K while the same section on the chromium only sample was done at
273 K.
6.3.7 Photooxidation of acetaldehyde on 2% Cr doped AP-SrTiO3
There were two parts to the photooxidation. In the beginning, filters were used to restrict
the spectrum of the incoming light from the lamp to between 400 nm and 700 nm, the visible
range. After 2 hours, some of the filters were removed to open up the spectrum of incoming light
to include all the way down to 300 nm, thus including a portion of the ultraviolet spectrum. As
expected, with photooxidation there was evolution of carbon dioxide, but for reasons concerning
space, that section of the spectra are not shown.
Once the light was turned on, a reaction can be seen taking place. The crotonaldehyde
carbonyl stretch at 1671 cm-1 can be seen to decrease in relationship to an incoming peak at 1600
cm-1. This peak is believed to be the O-C-O asymmetric stretch of an acetate species that is
forming. A methyl group deformation mode for this species appears at 1313 cm-1 and its C-O
stretching mode can be seen coming in at 1024 cm-1. At 1723 cm-1, an incoming feature was
detected that can be attributed to a surface bound acetaldehyde species with the hydrogen
abstracted from the aldehyde carbon, CH3C-=O. In Figure 6.12, the decrease in the
crotonaldehyde carbonyl stretch is accompanied by decreases in all the acetaldehyde and
crotonaldehyde carbon-carbon, single-bond stretches between 1162 cm-1 and 1066 cm-1 and the
methyl deformation mode at 1377 cm-1. Note that the spectra were cut off above 2000 cm-1 due
to the lack of any changes in the features beyond the range shown.
184
0.00
4000
0.05
0.10
0.15
0.20
0.25
3500
-1
0.00
2500 2000
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
1667 ν(C=Ocrot.)
1600
1703
1717 ν(C=Oacet.)
0.55
Wavenumber (cm )
3000
3
ν(sp -CH3)
2968
2929
2866
2751 ν(CHald.)
ν(Sb/Cr/Sr/Ti-OH)
ν(Ti-OH) 3402
2
3668
0.30
2.5%Sb-2%Cr-SrTiO3
T = 273 K
PO = 17.6Torr
1627 ν(C=C)
1568 ν(O-C-O)
ν(C-C)
1200
800
ν(C-CH3)
Nanoparticles
Figure 6.11 Dark Oxidation of Acetaldehyde as a Function of Time at Constant Temperature on 2.5%Sb/2%Cr-SrTiO3
Absorbance
δ(CH3 & CHald.)
1379
1335
1290
0.35
1136
1117
1088
1070
965 ρ(CH3)
185
2000
0.00
0.05
0.10
2
1723
1709 ν(C=O)acet.
1800
ν(CH3C=O)
1671 ν(C=O)
crot.
1600 ν(O-C-O)acetate
1377 δ(CH3)
1400
1313 δ(CH3)acetate
-1
ν(C-CH3)
ν(C-C)
1200
Wavenumber (cm )
1600
1281 δ(CH)ald.
1000
800
(b)
(a)
(c)
(d)
(e)
(f)
(g)
UV-Vis.
d - 3 min.
e - 55 min.
f - 94 min.
g - 118 min.
Figure 6.12 Visible and UV-Visible Photooxidation of Acetaldehyde on 2%Cr-SrTiO3 Nanoparticles
Absorbance
Visible
a - 8 min.
b - 84 min.
c - 127 min.
1162
1114
1087
1066
2%Cr-SrTiO3
PO = 19 Torr
T = 273-283 K
1024 ν(C-O)acetate
0.15
944 ρ(CH3)
186
6.3.8 Photooxidation of acetaldehyde on 2.5% Sb/2% Cr doped AP-SrTiO3
A similar reaction to what occurred on the chromium only sample occurred on the
antimony/chromium sample but with some significant differences. Negative features were seen
for the sp3-hybridized carbon-hydrogen stretches between 2969 cm-1 and 2868 cm-1 and for the
carbonyl stretches of acetaldehyde and crotonaldehyde at 1708 cm-1 and 1669 cm-1, respectively
(Figure 6.13). This is an area of difference between the two samples. The chromium sample saw
no decrease in the carbonyl stretch of acetaldehyde, but the sample with antimony and chromium
experience a significant decrease in both acetaldehyde and crotonaldehyde carbonyls. Though
the basic reaction taking place does not appear to change with the addition of the ultraviolet
light, the fact that after the initial concentrations of reactants has decreased due to visible light
photooxidation, the reaction did not slow down. On the contrary, it appears to have increased in
rate and suggests that ultraviolet light is probably more effective than visible light only.
6.4 Mass spectrometry studies on reaction products
Thermal Programmed Desorption (TPD) has been used to identify the reaction product
adsorbed on the surface of the SrTiO3 samples. The samples after photooxidation have been
heated up to 500ºC to achieve the desorption of the reaction products. The gaseous mixture of
product was sequentially introduced into the Mass Spectrometer to identify the chemical
formulas of desorbed products. The desorbed species detected by MS after photooxidation are
summarized in Table 6.3. Additionally, the monitoring of the evolved species upon heating up to
623 K from the 2.5% Sb/2% Cr codoped AP-SrTiO3 sample surface is presented in Figure 6.14.
For thermal oxidation, the acetate species were the main products, while during
photochemical oxidation, ethanol, pentane, furan, and benzene species were also formed. Carbon
oxide was formed only starting at 623 K during thermal oxidation, whereas it was observed
during photooxidation at 273 K.
187
Table 6.3 Desorbed Species Identified by Mass Spectroscopy from the SrTiO3 Surface after
Photooxidation of Acetaldehyde
Fragments
Possible Compounds
44, 29, 43
Acetaldehyde
70, 69, 42, 41, 40, 39 38, 29, 27
Crotonaldehyde
70, 55, 54, 42, 41, 39, 29, 27
2-Pentane
79, 78, 77, 50, 51, 52
Benzene
68, 42, 40, 39, 38, 37, 29
Furan
46, 45, 31, 29, 27
Ethanol
188
0.000
4000
0.025
0.050
0.075
0.100
0.125
2804
3013 ν(sp -CH)
2
3500
3000
(e)
(f)
(a)
(b)
(c)
(e)
(d)
(f)
0.0
2000
0.1
0.2
0.3
0.4
1729 ν(CH3-C=O)
-1
Wavenumber (cm )
2500
(a)
(b)
(c)
(d)
Visible
UV-Visible
a - 3 min.
d - 3 min.
b - 55 min. e - 76 min.
c - 115 min. f - 105 min.
3410
ν(Cr/Sr-Ti-OH)
2969
2920
2868
2964
2921
0.150
1708 ν(C=O)acet.
1600
2
ν(C-CH3)
and
ν(C-C)
1200
800
2.5%Sb-2%Cr-SrTiO3
T = 283-293 K
PO = 17.6 Torr
Figure 6.13 Visible and UV-Visible Photooxidation of Acetaldehyde as a Function of Time on 2.5%Sb/2%Cr-SrTiO3
Absorbance
1595 νa(O-C-O)
1669 ν(C=O)crot.
0.5
1387 νs(O-C-O)acetate
1329 δs(CH3)acetate
3
1022 ρ(CH3)acetate
ν(sp -CH3)
1140
1119
1071
0.175
937 ρ(CH3)
189
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
m
75
100
175
o
200
-C6H5
225
250
-CHO
275
300
2.5%-Sb/2%-Cr/SrTiO3
benzaldehyde
C6H7 from benzene
Temperature ( C)
150
furan
2-vinylfuran
CH3CH=CHCH2-
-CH2OH
125
2-pentene;
Crotonaldehyde
CO2;
CH3CHO
furfural;
2,4-hexadienal
/z[scale]
29[x1]
[x100]31
44[x1]
55[x250]
68[x250]
70[x250]
77[x250]
94[x250]
96[x100]
105[x250]
106[x250]
79[x250]
2.5%Sb/2%Cr-SrTiO3
Figure 6.14 Temperature Programmed Desorption/ Mass Spectroscopy after Photooxidation of Acetaldehyde on
Partial Pressure (µTorr)
190
6.5 Conclusions
The unique properties of aerogels, such as high porosity, small crystallite size, and large
surface area can be utilized to photocatalytically oxidize volatile organic compounds. IR vacuum
is a useful technique to study the photooxidation of CH3CHO on the SrTiO3 surface in
combination with Mass Spectrometry for the identification of reaction products. IR spectra gave
a complex pattern of peaks, which show the production of crotonaldehyde (crotyl alcohol) during
exposure to acetaldehyde for dark oxidation in the presence of oxygen.
Acetate species were produced by photooxidation in O2 under UV irradiation while
acetaldehyde and crotonaldehyde were consumed. Formation of ethanol, pentane, furan, and
benzene were observed by MS after thermal programmed desorption. During thermal oxidation,
CO2 formation was observed at 623 K, whereas for photooxidation reaction it was observed at
273 K.
In Chapter 5 it was found that SrTiO3 doped with chromium ions showed absorption in
the visible light region and photoactivity for acetaldehyde decomposition. The photocatalytic
activity for CO2 evolution of the antimony/chromium incorporated SrTiO3 aerogels was twice as
high as that doped with only chromium under visible light irradiation. As predicted by the
literature, the addition of antimony to the chromium doped strontium titanate nanoparticles did
offer greater catalytic activity. The IR studies reported herein of the doped SrTiO3 samples, and
comparison with pure aerogel prepared SrTiO3 samples, clarified the process of acetaldehyde
adsorption, dark decomposition, and photooxidation under light irradiation on the SrTiO3
surface.
From FTIR studies for pure aerogel and doped aerogel prepared SrTiO3 samples, it was
found that the products of the reactions appeared to be the same or at least within the same
chemical group. Dark oxidation resulted in no significant catalysis indicating that the electron
promotion via visible and/or UV light is necessary to activate the catalyst. While visible light
alone was found to be sufficient to enable catalysis, the addition of ultraviolet wavelengths
appears to increase the photocatalysis noticeably.
Upon exposure to ~15 Torr of oxygen in the absence of anything but minimal ambient
light, no change in reaction was recorded. This indicates the necessity of light as an activating
191
factor for the catalytic properties of these nanoparticles. Upon exposure to the visible spectrum,
reactions resulting in acetate species were observed on both samples with some differences. It
was generally found that some additional reactivity was afforded by the incorporation of a charge
balancing species such as antimony. Significant additional reactivity was observed when the
spectrum of incoming light was widened to include wavelengths as low as 300 nm for samples
containing Sb and Cr.
6.6 References
[1]
S. Luo and J. Falconer, Catalysis Letters, 57 (1999) 89-93.
[2]
H. Idriss and Madhavaram, Journal of Catalysis, 224 (2004) 358-369.
[3]
H. Idriss, C. Diagne, J.P. Hindermann, A. Kiennemann, and M.A. Barteau, Journal of
Catalysis, 155 (1995) 219-237.
[4]
L. Wang and K.J. Ferris, J. Phys. Chem. B, 108 (2004) 1646-1652.
[5]
C.-A. Chang, B. Ray, D.P. Paul, D. Demydov, and K.J. Klabunde, Photocatalytic
Oxidation of Acetaldehyde over SrTiO3 Nanoparticles: An in situ FTIR Study, in press.
[6]
D.K. Paul, D.A. Panayotov, and J.T. Yates, Abstract, 39th Midvest Regional Meeting of
the American Chemical Society in Manhattan, KS, United States, October 20-22, 2004.
[7]
D. Demydov, K.J. Klabunde, C.-A. Chang, B. Ray, and D.P. Paul, in press.
192
Chapter 7 : Dielectric studies on titanates
7.1 Introduction
Barium titanate is widely used in electronic applications [1-3] as a material for
1) Multilayer ceramic capacitors,
2) Dielectric devices (flexible film capacitors, dielectric bolometers, IR focal plane arrays),
3) High-permittivity thin films (DRAMs, pyroelectric sensors, gas detection sensors)
4) Superconducting microwave tunable devices, tunable phase shifters, and ring resonators,
5) Acoustic imaging arrays (real-time acoustic cameras for underwater divers, medical
imaging, and acoustic microscopy).
The application of BaTiO3 for ceramic capacitors requires small diameter of particles,
narrow size distribution and high purity of the BaTiO3 phase. Nanosized BaTiO3 samples
prepared by wet chemical methods (sol-gel or aerogel) are highly pure powders of small
monodispersed particles. The application of nanosized BaTiO3 for ceramic capacitors can be
beneficial, and it is important to determine if dielectric properties depend on the size of particles.
7.2 Dielectric properties of titanates
Barium titanate is the main titanate being used for capacitors. It is usually made by solidstate reaction of BaCO3 with TiO2 at temperature of 1100° C. Recently, the production of
BaTiO3 by wet chemical methods using alkoxide chemistry is becoming popular. Temperatures
for calcination of BaTiO3 prepared from alkoxides are significantly lower (500-700°C), and wet
chemical methods allow better control of purity, Ba/Ti ratio, and particle sizes [4]. The textural
properties of BaTiO3 are important for consistent dielectric behaviors and reproducible structures
in the sintered product.
The dielectric constant of a material can be calculated using the following equation
Ct
K=
ε0 A
(7.1)
193
where - K is a dielectric constant of the dielectric material at specific frequency and specific
temperature, C is capacitance (farads), t is distance between electrodes (thickness in cm), εo is
permittivity of vacuum (8.854 x 10-14 F/cm), and A = πr2 is an area of the electrode (cm2).
BaTiO3 is available in several phases including tetragonal, cubic, and hexagonal crystal
structure (Equation 7.2). Cubic phase (Pm3m space group) has an ideal perovskite structure
where a=b=c (Figure 7.1). At high temperature (>1432°C) it transforms into hexagonal structure
(P63mmc space group) where c>b=a and γ =120°C.
Tetragonal →
Cubic
132°C
→
Hexagonal
1432°C
→ Liquid
(7.2)
1625°C
Figure 7.1 BaTiO3 Structure
The tetragonal structure (P4mm space group and c>a=b) has atomic displacement for Ti
and O atoms with respect to Ba atom and exhibits ferroelectricity. The transition from cubic to
tetragonal occurs at 132°C (Curie temperature).
By cooling of BaTiO3 through 132°C causes the cubic phase to undergo dipole charge
changes on minor faces and transfers into the tetragonal phase. For grains larger than 3 µm, the
194
formation of domain walls relieves the stress from phase transformation and the material has a
dielectric constant of 1500-1900 at room temperature. In materials with grains smaller than 1.5
µm, the 90° domain walls are not formed and the dielectric constant is significantly higher
(2500-3500).
SrTiO3 has a cubic perovskite structure and is paraelectric at room temperature. Its
dielectric constant is smaller than BaTiO3 and is about 300 at room temperature. The SrTiO3
dielectric constant increases with cooling and is about 20000 near 0 K. Strontium titanate based
materials are used for high-voltage capacitors with applied voltage up to 5 kV/mm [5].
Calcium titanate has similar dielectric properties to strontium titanate, but its dielectric
constant is twice lower. Magnesium titanate has an ilmenite structure and a very low dielectric
constant of 20. The advantage of MgTiO3 is that its dielectric constant has a positive temperature
coefficient. It can be added to SrTiO3 or CaTiO3 to adjust their negative temperature dependence.
These materials give good high-frequency performance in multilayer capacitors [6].
Dielectric constant depends on the arrangement and bonding of atoms in the material, and
the values for dielectric constant of different titanates are summarized in Table 7.1. The high
value of dielectric constant and high thermal stability makes titanates good candidates for use in
capacitors.
The dielectric constant of barium titanate is highly dependent on temperature, so cationic
substitution is used to make the barium titanate suitable for capacitor applications and less
dependent on temperature. The partial substitution of Ba2+ with Sr2+ allows decreasing the Curie
temperature of the BaTiO3 material. Strontium titanate and barium titanate can form a continuous
series of solid solutions with different Sr/Ba ratios. These materials (Ba1-xSrxTiO3) have a very
high dielectric constant (>5000) at room temperature. The linearity of the SrTiO3 paraelectricity
gives more stable capacitor performance in comparison with the pure BaTiO3 material.
Table 7.1 Dielectric Constants of Titanates [7]
Material
Dielectric constant
K, at 25°K
Titanium Oxide, TiO2
100
Magnesium Titanate, MgTiO3
20
195
Calcium Titanate, CaTiO3
160
Strontium Titanate, SrTiO3
320
Barium Titanate, BaTiO3
Barium Strontium Titanate, BaxSr1-xTiO3
1000-2000
>5000
The dielectric constant in BaTiO3 depends on the purity of the materials. The presence of
impurities such as lattice hydroxyl (OH-) and carbonate (CO32-) groups decreased the dielectric
constant [8]. The dielectric constant also decreased with decreasing particle sizes. The critical
size for BaTiO3 particles has been extensively studied by different research groups [9, 10]. It was
found that tetragonality of barium titanate particles decreased at ~ 40 nm of particle diameter and
changed to cubic phase at 20 nm [11, 12]. It was also found that the decrease of grain sizes from
50 µm to 0.8 µm causes increasing of dielectric constant [13]. With the decrease of grains
smaller than 0.8 µm, the dielectric constant started to decrease too. This chapter aims to
understand the size effect of BaTiO3 nanoparticles on the dielectric properties.
7.3 Aerogels for electrical applications
Advances toward nanoscale electronics require application of nanosized materials for
dielectric, piezoelectric, pyroelectric, and electro-optic ceramics. Due to their unique properties,
including high surface areas and small particle sizes, aerogel oxide materials can be used for
these electrical applications. The electrical applications of aerogel materials already include
rechargeable batteries, capacitor electrodes, piezoelectric materials, and dielectric materials.
A larger amount of ions can reversibly intercalate into a porous structure of aerogel
prepared material in comparison with conventionally prepared material. The pore size
distribution of aerogels is more effective for enhancing ionic conductivity in comparison with
xerogels [14]. Several oxide aerogels have already been studied for lithium ion intercalation,
including vanadium oxide [15], manganese oxide [16], and molybdenum oxide [17] aerogels.
Vanadium oxide aerogels with high surface area up to 450 m2/g and a specific pore volume as
much as 2.3 cm3/g were able to intercalate up to 5.8 equivalents of lithium per mol of V2O5
aerogel [18]. This material can be used as a positive electrode in lithium batteries with high
196
capacity (500-600 mAh/g) [19]. Additionally, aerogels with high thermal insulation properties
can be used for high-temperature electrical batteries [20, 21].
Aerogel materials can reversibly immobilize a large quantity of electrical charge carriers.
Carbon aerogels which are good conductors of electricity were studied as double layer electrodes
in supercapacitors, pseudocapacitors, capacitive deionization units, and fuel cells [22-24]. Due to
the lower electric resistance and higher specific area of aerogels, they can store more electrical
energy than that of conventional capacitors [25]. These materials are also good for ion storage
and deionization applications and in particular can be used for metal removal from water (Cu,
Zn, Ni, Cd, Cr, Pb, U) [26].
Highly porous piezoceramic materials can be prepared by the aerogel process and be used
for the production of porous piezoelectric transducers with low acoustic impedance [27-29].
Aerogel prepared lead zirconium titanate (PZT) with composition PbZr0.53Ti0.47O3 has porosity
up to 90 % by volume, specific surface area above 300 m2/g, high polarization and low coercive
field strength. Thin films of silica aerogels have very low dielectric constants and can be applied
for integrated circles to increase the computer speed [30].
7.4 Synthesis of Ba0.5Sr0.5TiO3 aerogel
Barium strontium titanate aerogel was prepared in such a way that the molar ratio of
Ba0.5Sr0.5TiO3 would be preserved. Sr and Ba metals were separately dissolved in alcohol first
and then mixed in 1:1 ratio and added to Ti alkoxide (Figure 7.2). The textural properties of this
sample were characterized by XRD, TEM, and UV-visible spectrometry (More detailed
information on sample characterization operation procedures can be found in Chapter 3.3).
A powder X-ray diffraction pattern was obtained on a Bruker D8 Adpvance spectrometer
with a CuKα radiation source with an applied voltage of 40 kV and a current of 40mA. Scans
were made in the 2θ range of 20-85º with a scanning rate of 2 º/min. The crystallite size was
calculated from the XRD patterns using the Debye-Scherrer equation. The peaks from the
diffractograms patterns were assigned to the pure barium strontium titanate phase, and no
additional peaks were observed (Figure 7.3).
The surface area was measured on a Nova 1200 gas sorption analyzer (Quantachrome
Corp.) from the amount o N2 absorbed at 77K and calculated according to the Brunauer-Emmett197
Teller (BET) method. The samples were degassed at 423 K for 1 hour prior to the analysis. The
surface area of AP- Ba0.5Sr0.5TiO3 after calcination in oxygen for five hours was 59 m2/g.
Light absorption spectra of the Ba0.5Sr0.5TiO3 sample were obtained on a Cary 500 Scan
UV-Visible Spectrometer with an integrating sphere attachment for diffuse reflectance in the
range 200-800 nm. Similar to the SrTiO3 and BaTiO3 samples, Ba0.5Sr0.5TiO3 absorbed only UV
light and did not absorbed any visible light (Figure 7.4).
Transmission electron micrographs were obtained on a Philips CM 100. Samples were
placed onto a carbon-coated copper grid by the physical interaction of the grid and powder in
such a way that the particles remained adhered to the grids. The calcination of Ba0.5Sr0.5TiO3
samples in oxygen at 500°C caused almost a double increase of crystallite sizes (Figure 7.5). The
average crystallite sizes were 20 nm for a Ba0.5Sr0.5TiO3 sample calcined in oxygen at 500°C for
5 hours.
198
Figure 7.2 Modified Aerogel Procedure (MAP) from Alkoxides for Ba0.5Sr0.5TiO3 Synthesis
199
Figure 7.3 Modified Aerogel Procedure (MAP) from Alkoxides for Ba0.5Sr0.5TiO3 Synthesis
200
-0.5
300
0.0
0.5
1.0
1.5
2.0
2.5
3.0
400
o
Wavelength (nm)
500
600
AP-SrTiO3 500 C
o
AP-Sr0.5Ba0.5TiO3 500 C
o
AP-BaTiO3 500 C
AP-Sr0.5Ba0.5TiO3
Figure 7.4 UV-visible Spectra of Different Aerogel Prepared Samples
Absorbance
700
201
Oxygen at 500°C (right)
Figure 7.5 Transmission Electron Micrographs of AP-Ba0.5Sr0.5TiO3 Freshly Prepared (left and middle) and Calcined in
202
7.5 Dielectric measurements
Dielectric properties of barium titanate samples were studied by Impedance analysis and
Raman spectroscopy.
7.5.1 Impedance analysis
For comparison, the dialectic response from aerogel prepared barium titanate, aerogel
prepared barium strontium titanate, and commercial barium titanate samples were studied. The
impedance behaviors of these samples in the form of pressed pellets were studied. Impedance
measurements (Solatron SI-1260 Impedance/ Gain-phase Analyzer in combination with Solatron
1296 Dielectric Interface) from 10 mHz to 1 MHz were performed on barium titanium powders
that were pressed into pellets (12 mm × 2 mm) using a IR pellet die and a press with pressure of
5000 lbs. The impedance analyzer was controlled by a personal computer with GPIB interface.
A special dielectric cell was built where the pellets were held in-between two alumina disks
using platinum foil as the electrode contacts (See Appendix E). The sample cell was placed into
furnace and heated in air. Dielectric measurements were made starting at room temperature and
in 50°C steps between 50 and 500°C.
The disk-shaped pellets were calcined at 700°C for 7 hours for hardening. After
calcination, the pellet surfaces were coated with silver metallic film for better contact with
platinum electrodes. Coating was done by simply painting on the external flat faces with silver
paste and following calcination at 400°C to get silver metallic coating (See Appendix E).
Before dielectric studies, the effect of pressing powder sample into pellets was studied.
Surface area and pore volume measurements were made on the pellet of commercially available
(CM-BaTiO3) and nanosized commercially available (NCM-BaTiO3) barium titanate samples
and summarized in table 7.2. The surface area of pellets made from nanosized sample decreased
significantly losing half of its total value, while the surface area of low surface area samples
(pellets of commercial BaTiO3) did not change. The powder X-ray diffraction (Figure 7.6) also
shows the narrowing of the peaks which corresponds increasing in particle sizes for nanosized
samples after pelletization.
203
Table 7.2 Textural properties of pelletized BaTiO3
BaTiO3 Sample
Surface area, m2/g
CM-BaTiO3
3
CM-BaTiO3 pellet (700ºC)
1
CM-BaTiO3 pellet (700ºC)
3
NCM-BaTiO3
19
NCM-BaTiO3 pellet (700ºC)
10
NCM-BaTiO3 pellet (700ºC)
10
204
Figure 7.6 Powder XRD of NCM-BaTiO3 Powder and Powder of Crashed NCM-BaTiO3 Pellet
205
The impedance response was studied for commercial BaTiO3, aerogel prepared BaTiO3,
and aerogel prepared Ba0.5Sr0.5TiO3 pellets. The complex impedance spectrum for APBa0.5Sr0.5TiO3 pellet is presented in Figures 7.7. The imaginary impedance part (Z”) is plotted
against the real impedance part (Z’). The semicircle in the graph stands for the contribution from
the bulk and grain boundaries. The contribution from the bulk and grain boundaries can not be
separated in the high-frequency arc. The additional feature next to the semicircles is attributed to
the interfacial effect at the contacting Ag electrodes.
The semicircles from resulting impedance spectra can be used for the calculation of the
capacitance (C) and resistance (R) by the modulation of equivalent circuit (Equation 7.3) with
two parallel RC elements where first element represents the conduction in the bulk and the
second element represents the conduction of along grain boundaries:
ωmaxRC = 1
(7.3)
where ωmax =2πf max, and f max is the frequency at the arc maxima.
The resistance and relative permittivity of different BaTiO3 samples were plotted and
compared (Figures 7.8 and 7.9). The aerogel prepared BaTiO3 bulk resistance values were
significantly lower than that of commercial BaTiO3, by several orders of magnitude. This can be
explained by higher density of grain boundaries in aerogel samples.
The dielectric constants for BaTiO3 samples at room temperature were calculated using
Equation 7.1. The dielectric constant for AP-Ba0.5Sr0.5TiO3 sample was 2250, for AP-BaTiO3
sample - 2150, and CM-BaTiO3 - 1890, respectively. There was no significant increase for
aerogel prepared barium strontium titanium oxide in comparison with aerogel prepared barium
titanate; however, the dielectric constant of the former sample was less dependent on the
temperature. All aerogel samples had a higher dielectric constant, higher permittivity, and
smaller resistance at room temperature in comparison with commercial barium titanate samples.
206
Figure 7.7 Complex Impedance Spectrum of Aerogel Prepared AP-Ba0.5Sr0.5TiO3 at Room Temperature
207
1.0
6
7
8
9
10
11
12
1.5
2.0
1000/T (K )
-1
2.5
3.0
3.5
CM-BaTiO3
AP-BaTiO3
AP-Ba0.5Sr0.5TiO3
Figure 7.8 Resistance of Commercial BaTiO3 and Aerogel Prepared BaTiO3 and Ba0.5Sr0.5TiO3
log R (Ω)
208
50
100
150
200
250
300
350
300
400
500
T (K)
600
700
CM-BaTiO3
AP-BaTiO3
AP-Ba0.5Sr0.5TiO3
800
Figure 7.9 Relative Permittivity of Commercial BaTiO3 and Aerogel Prepared BaTiO3 and Ba0.5Sr0.5TiO3
Relative permittivity, κ
209
7.5.2 Raman spectroscopy
It is difficult to use XRD to analyze nanosized samples for phase structure in BaTiO3 and
almost impossible to distinguish between tetragonal and cubic phases when the peaks are broad.
Raman spectroscopy is a significantly more sensitive as a method and can be used for detecting
the presence of tetragonal or cubic phases in BaTiO3. Some bands are active in the tetragonal and
inactive in cubic phase. The intense band at 307 cm-1 indicates the presence of tetragonal phase
in nanosized commercial BaTiO3, while the aerogel prepared BaTiO3 sample does not have a
tetragonal phase and consists of only cubic phase (Figure 7.10).The average crystallite sizes of
NCM-BaTiO3 particles are 36 nm and the average crystallite sizes of AP-BaTiO3 are 11 nm.
This confirms that the critical size for the cubic-tetrahedral transformation lays in-between 11-36
nm size range.
While Impedance analysis is used to calculate the extrinsic dielectric constant which
includes the influence of BaTiO3 compacting into a dense pellet, the effect of pores, space
charge, and boundaries; Raman spectroscopy can be used for the calculation of intrinsic
dielectric constant without the effect of pores, space charge, and boundaries. It can be calculated
by Lyddane-Sachs-Teller (LST) equation from the Raman spectra [31]
ε ω1 LO ω2 LO ω3 LO
= 2 * 2 * 2
ε ∞ ω1 TO ω2 TO ω3 TO
2
2
2
(7.4)
where ε is a dielectric constant at zero frequency, ω is a mode frequency for each phonon modes,
ε∞ is an optical dielectric constant for BaTiO3 (a = 5.22, c = 5.07).The dielectric constant can be
determined for c-axis using frequencies of the A1 modes, and for a-axis using frequencies of the
E modes. To use these modes for calculation of dielectric constant, they need to be extrapolated
from the Raman spectra (Figure 7.10).
210
0
2
4
6
8
10
12
14
16
0
200
-1
600
800
AP-BaTiO3
NCM-BaTiO3
Raman shift (cm )
400
Figure 7.10 Raman Spectra of AP-BaTiO3 and NCM-BaTiO3
Intensity
1000
211
Unfortunately, the traditional mathematical software programs are not able to separate and
extract the exact peak values of the modes. A special program that uses the sum of damped
harmonic oscillators and a Debye relaxation mode may be useful for this purpose (Figure 7.12).
Figure 7.11 Raman Spectra of BaTiO3 and Extraction of Soft Mode Frequencies [31]
7.6 Conclusions
Barium titanate is a well-known ferroelectric material with a high dielectric constant. The
electrical properties of BaTiO3 aerogels were evaluated from room temperature up to 500°C and
compared to those of commercially available barium titanate samples. The pressed pellets of
aerogel prepared barium titanate lost some of its surface area during pellet preparation.
The dielectric permittivity of BaTiO3 samples was determined as a function of frequency
up to 1MHz. The aerogel samples show higher permittivity than the commercial sample.
212
Permittivity in nanosized samples was higher due to higher density of grain boundaries. The
aerogel prepared BaTiO3 bulk resistance values were significantly lower than that of commercial
BaTiO3, by several orders of magnitude.
The dielectric constant for AP-Ba0.5Sr0.5TiO3 sample was 2250, for AP-BaTiO3 sample it
was 2150, and for CM-BaTiO3 sample it was 1890. There was no significant increase for aerogel
prepared barium strontium titanium oxide in comparison with aerogel prepared barium titanate;
however, the dielectric constant of AP-Ba0.5Sr0.5TiO3 sample was less dependent on temperature.
All aerogel samples had higher dielectric constant, higher permittivity, and smaller resistance at
room temperature in comparison with the commercial barium titanate sample.
A further exploration of the aerogel prepared BaTiO3 materials is necessary. The studies on
BaTiO3 samples with the high concentration of oxygen vacancy defects and doped with
transition metals will bring more understanding in the dielectric behaviors of barium titanate and
finding of advantages in possible application of these novel materials.
7.7 References
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M. Kahn, D.P. Burks, I. Burn, and W.A. Schulze, in Electronic Ceramics, L.M. Levinson
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Figure A.1 Powder XRD Pattern of Solid-State Prepared Strontium Titanate
APPENDIX A: XRD analysis
215
Figure A.2 Powder XRD Pattern of Solid-State Prepared Strontium Titanate
216
APPENDIX B: Elemental analysis
Figure B.1 CH Elemental Analysis for AP-SrTiO3 and AP-BaTiO3 (Galbraith
Laboratories, Inc.)
217
Figure C.1 Thermogravimetric Analysis of AP-SrTiO3 (Ethanol)
APPENDIX C: TGA analysis
218
Figure C.2 Thermogravimetric Analysis of AP-SrTiO3 (Methanol)
219
APPENDIX D: in situ FTIR
Figure D.1 Infrared Cell for Powdered Photocatalysis
220
Figure D.2 Catalyst Sample Preparation on a Tungsten Grid
221
Figure D.3 Setup of Mattson Research Series RS-10000 FTIR, Vacuum Line, and MKS
PPT Residual Gas Analyzer/Quadruple Mass Spectrometer
222
Dielectric Cell
Figure E.1 Solatron SI-1260 Impedance/ Gain-phase Analyzer in Combination with Solatron 1296 Dielectric Interface and
APPENDIX E: Dielectric measurements
223
Figure E.2 Dielectric Cell with a Pellet between Platinum Electrodes
224
Figure E.3 AP-Ba0.5Sr0.5TiO3 Pellet Coated with Silver
225
APPENDIX F: Permission to reproduce materials
1. Reprinted from Journal of Non-Crystalline Solids Dmytro Demydov and Kenneth J.
Klabunde, “Characterization of mixed metal oxides (SrTiO3 and BaTiO3) synthesized by a
modified aerogel procedure,” 350, 165 -172, Copyright (2004), with permission from Elsevier.
2. Reprinted from Nanostructured and Advanced Materials, Vaseashta, A.; DimovaMalinovska, D.; Marshall, J.M. (Eds.), Proceedings of the NATO Advanced Study Institute, held
in Sozopol, Bulgaria, 6-17 September 2004, Series: NATO Science Series II: Mathematics,
Physics and Chemistry, D.V. Demydov and K.J. Klabunde, “Synthesis, characterization,
photocatalytic and dielectric properties of nanosized strontium and barium titanates,” 204, 327330, Copyright (2004), with kind permission of Springer Science and Business Media.
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