SECM Laboratory Experiments - The Molecular Materials Research

SECM Laboratory Experiments - The Molecular Materials Research
Table of Contents
Table of Contents…………………………………….…………….……...1
Lab Instructors…………………………………………………..….……..2
Laboratory Procedures…………………………………………….…….3
Basic Safety and Waste Disposal Procedures……………………….4
Experiments
1. Tip Fabrication and Characterization: 25 µm Pt disk I…………………….5
2. Operation of the CHI900 SECM I: Learning the Basic Functions of
the Instrument…………...……………………………………….……..10
3. Operation of the CHI900 SECM II: Learning How to Obtain Approach
Curves and Fit Them to Theory and How to Calibrate the Distance.….24
4. Tip Fabrication and Characterization II: Polishing, Sharpening and
Testing……………………………………………………………..…...31
5. Basics of Imaging……………………………………………………..…...41
6. Determination of Kinetics Parameters of Heterogeneous Electron
Transfer………………………………………………………………...48
7. Copper Etching and Imaging………………………………………….......53
8. Imaging Electrochemical Activity by Generation-Collection Modes….....59
9. Biological System: SECM Studies of the Reaction Center of
Rhodobacter Sphaeroide Chromatophores……………………….…...67
Appendix: Basics of Cyclic Voltammetry………………………………..….77
Acknowledgements…………………………………………………..……....87
Laboratory Experiment Schedule…………………………….…...Back Cover
1
Laboratory Instructors
Fu-Ren F. Fan
The University of Texas at Austin
Chemistry and Biochemistry
(a) 1 University Station A5300
Austin, TX 78712-0165
Phone: 471-6890, Welch 2.144
José Fernandez
The University of Texas at Austin
Chemistry and Biochemistry
1 University Station A5300
Austin, TX 78712-0165
Phone: 471-1323, Welch 2.404
Francois Laforge
Queens College - CUNY
Department of Chemistry
(b) Flushing, NY 11367
Biao Liu
The University of Texas at Austin
Chemistry and Biochemistry
1 University Station A5300
Austin, TX 78712-0165
Phone: 471-1323, Welch 2.120
Janine Mauzeroll
The University of Texas at Austin
Chemistry and Biochemistry
1 University Station A5300
Austin, TX 78712-0165
Phone: 471-1323, Welch 2.120
2
Laboratory Procedures
Pre-lab Preparation
Prepare for each experiment before you come to the lab session by
reading your lab manual and the references recommended reading. If you like,
prepare some pre-lab notes, this will help you finish your lab more quickly and
efficiently.
Experiment Session
Your lab manual will indicate what chemicals and equipment you need
for your experiments. It will also guide you through a series of detailed steps to
the end of the experiment.
Shut Down
At the end of each lab period, you need to clean up the equipment, e.g.
cells, electrodes, glassware, etc. based on the Shut Down procedures described
in each experiment, although no detailed Shut Down procedures are described
in some experiments, they are basically the same.
It is your responsibility to save all the data you obtain for further data analyses.
Data Analyses
You might need to process some of your data after each experiment
session.
3
Basic Safety and Waste Disposal Procedures
Basic Safety and Protection
•
Wear safety goggles, gloves and protective clothing for dealing with
toxic and hazardous chemicals.
•
Make sure you know the exact locations of the safety features of the lab;
e.g., eyewash fountains, safety showers, chemical spill kits, fire extinguisher,
fire alarms and etc. Emergency contact numbers are listed near the phone.
•
Keep the instrumentation areas clean and organized, so groups that
follow can use them and also to reduce the possibility of accidents.
•
Avoid unnecessary exposure to chemicals. Use hoods when necessary.
•
Take appropriate precautions. Keep flammables away from open flames.
Disposal of Chemical Waste
It is important to properly dispose of the chemical waste you generate to
avoid contaminating our environment.
•
Generate as little waste as possible.
•
Never return unused portions of chemicals to the reagent bottle. At the
end of the experiment, unused reagent must be disposed of as waste, so do not
pour out more than you need.
•
Use the clearly marked glass containers to dispose of broken glass and
Pasteur pipettes.
•
Place chemical waste only in the appropriate container. Often, more than
one waste container is provided for different waste chemicals. Pay attention to
the waste disposal information for each experiment in this manual, and use the
waste container indicated.
•
Do not discard chemicals down the sink or chemicals in the wastebasket.
4
1. Tip Fabrication and Characterization: 25 µm Pt disk I.
REFERENCES
“Scanning Electrochemical Microscopy”, Bard A. J.; Mirkin M. V. (eds.), Marcel Dekker
Inc, N.Y., 2001, Chapter 3, p. 75.
OBJECTIVES
1. SEALING THE CAPILLARY TUBE
2. MANIPULATING THE PLATINUM WIRE
3. SEALING THE ELECTRODE
INTRODUCTION
The fabrication of an ultramicroelectrode (UME) will be separated into two stages
for the purpose of these laboratory sessions. In the first stage, the end of a glass capillary
will be sealed such that a conical shape is obtained. A straight 25 µm Pt wire will be
positioned at the bottom of the sealed capillary, and then it will be put under vacuum for 30
minutes. The capillary will slowly be sealed onto the wire using a heated resistor coil. The
sealed wire will then be electrically connected to a larger wire using a conducting silver
epoxy. The connected tip will be placed in the oven at 120 °C overnight to cure the epoxy.
This procedure can also be applied to 10 µm tips of platinum, gold (although the use
of soft glass capillaries is sometimes preferred) and carbon nature. For smaller tips (1-2 µm
diameter), a Wollaston wire, a metal wire covered by a silver layer, is often first dissolved
with a weak nitric acid solution prior to sealing the tip. A laser puller can also be used with
small diameter quartz capillaries to make submicron sized electrodes. Such sealing
techniques require a great deal of practice and patience.
In laboratory experiment 4, the tip fabrication will continue as the electrodes are
polished, sharpened and characterized. The voltammetric behavior of the electrodes will be
recorded to evaluate the tip radius and compare that value to the one observed optically.
Conducting and insulating approach curves will also be acquired and fitted to theory in order
to evaluate the quality of the ultramicroelectrode.
1. Sealing the Capillary Tube
Take a clean and dry Pyrex (borosilicate) capillary tube (at least 15 cm long to
manipulate in the flame, inner diameter 1 mm, outer diameter 2 mm). The pre-cleaned
capillary should be moisture and dust free so as to avoid poor sealing of the glass onto the
wire. Polluting substances and water can lead to bubble formation next to the wire as the
glass is melted. To circumvent this, the glass tubes can be soaked in 1:10 diluted HNO3,
rinsed with copious amounts of distilled water, oven dried, and stored in an enclosed vial.
Use a gas oxygen flame to seal the capillary tube. If you are not familiar with
gas/oxygen torches ask a lab instructor to show you how to safely operate the torch. Choose
an adequate flame temperature, not too hot to avoid bending of the glass. Rotate the tube
continuously on the side of the flame to obtain a conic shape (Figure 1). This shape must be
obtained to adequately place the Pt wire in the capillary.
5
Check the capillary under the microscope and make sure that it is completely sealed
at the base.
Cut the tube to about 5 cm length using a file.
Figure 1: The end of a sealed glass capillary presenting a conical shape.
2. Manipulating the Platinum Wire
Now that the capillary is sealed and presents a conic shape, a straight Pt wire must be
inserted at the bottom of the capillary in the crack of the cone. Straightening the wire and
positioning it is time consuming work and it is possible that you might have to take the wire
out and straighten it several times before getting the desired result.
Using gloves, cut a piece of approximately 1.5 cm of hard (i.e. not annealed)
platinum wire (10 or 25 µm diameter, 99.9% purity) and rinse with acetone. The gloves are
necessary to prevent the oils of the fingers from contaminating the wire.
Carefully, straighten the wire with your fingers without twisting it on a white piece
of paper. This can be done by rolling the wire on a sheet of white glossy paper with your
finger. You might have to try several times.
Bring the straight wire to the edge of the paper and introduce the wire into the sealed
capillary tube. At this point, special care must be taken not to bend the wire. The wire must
go in straight. The use of tweezers tends to crimp the end of the wire making it difficult to
fall into the tube properly.
Using the microscope, position the wire at the bottom of the capillary in the crack of
the cone (Figure 2) by gently tapping the capillary on the bench top. If you tap the capillary
too forcefully against the bench, the wire can sometimes bounce out of the capillary or curl
up at the end. Checking under the microscope, you should see the wire aligned as shown in
Figure 2. If this is not the case, then you can take the glass/wire assembly and let it fall
through an approximately 10 cm glass tube of larger diameter, which is sitting on a layer of
Kimwipes to cushion the fall. Approximately 2 or 3 falls should align the wire in the
capillary. If this does not center the wire, then it is often easiest to remove the wire from the
capillary and start again.
6
Figure 2: The 25 µm Pt wire is inserted at the base of the cone and remains straight.
3. Sealing the Electrode
To seal the glass capillary onto the Pt wire, the setup presented in Figure 3 is used. It
is composed of a Nickel-Chromium resistor coil (gauge 18) that can be electrically heated
up to 800 oC. By turning the screw on the right-hand side of the vertical metal support, the
coil can be moved vertically.
Figure 3: LHS: Diagram of the Setup where the capillary is aligned with the resistor
coil. RHS: Picture of the sealing setup.
The power supply is an old pipette puller that has a 2.5 Amp fuse, a 120 V output
and a maximum power of 300 W. The Nickel-Chromium wire used has a thickness of
approximately 1 mm. It is coiled to a length of about 1 cm, with 5 or 6 turns, and an inner
and outer diameter of 5 mm and 7 mm respectively. The alligator clip above the coil is used
to hold the glass capillary and to help align it perpendicularly to the bench top.
Connect the capillary tube to the vacuum line by inserting it into the white rubber
tube and clamping it to the alligator clip. Align the tip such that the capillary is
perpendicular to the bench top and at the center of the resistor coil as depicted in Figure 3 on
the left hand side. If the capillary is not in the center of the coil and is closer to the sides, it
7
will cause the capillary to bend during sealing. Take the time to make a good alignment and
to move the coil up and down the capillary.
To Set-up
To Vacuum
To Air
Before turning on the vacuum, check that the T joint is in the gray position such that
the capillary is not yet directly connected to the vacuum line. Turn on the vacuum on the
right hand side of the setup on the floor.
Slowly turn the T joint clockwise to the red position. This must be done slowly in
order not to displace the Pt wire.
Leave the capillary under vacuum for 30 minutes. If the pump used is bad or the
evacuation time too short, air bubbles occur during the sealing process.
Center the capillary tube in the coil with the bottom of the capillary just inside the
coil. This means that the bottom of the tip should be approximately 1mm (or one coil
diameter) inside the coil. At this point ask a lab instructor to look at your set-up.
Turn on the power at the bottom. Push the yellow button in the middle right hand
side of the panel that says cycle. Turn the heater knob at the upper left-hand panel to
approximately 85-90%. The color of the coil should be orange yellow but not bright yellow.
Leave the capillary at the bottom of the coil for 20 minutes so that volatile compounds and
residual moisture can be evacuated.
Move up the coil along the capillary tube very slowly to assure proper sealing. Take
steps of 1 mm every 5 minutes to seal approximately 1 cm of the capillary tube. Using a
Kimwipe placed behind the capillary, make sure that you are not sealing past the wire. Also,
be very careful not the bump the setup while manipulating. If the capillary touches the hot
sides of the coil, the process has to be repeated from the start.
Turn the power supply and vacuum off. Wait for the capillary to cool down. Remove
the capillary tube and check under the microscope (Figure 4) that it has been properly
sealed.
8
Figure 4: Sealed 25 µm Pt wire in a Pyrex capillary. A small air pocket is observed at
the beginning of the wire but the rest of the body is properly sealed. This is not unusual
and can be shaved off during the polishing steps.
Once sealed, electrical contact between the Pt wire and a lead wire must be made.
Using a hypodermic syringe (for example, a 3 ml syringe with a 22G1.5 gauge needle works
best for this size glass capillary) found in the freezer of the chemical refrigerator, inject the
pre-mixed silver epoxy mixture into the capillary tube around the sealed wire. The epoxy
must be inserted all the way down where the Pt wire is sealed. Then, introduce a conductive
wire (30AWG wire for this size capillary) into the tube and put the electrode into the oven
(120 °C) overnight (i.e. for 10-12 hours) to cure the epoxy.
9
2. Operation of the CHI900 SECM I: Learning the Basic
Functions of the Instrument.
REFERENCES
“Scanning Electrochemical Microscopy”, Bard A. J.; Mirkin M. V. (eds), Marcel Dekker
Inc, N.Y., 2001, Chapters 1, 2 and 5.
User’s manual, CHI900 SECM, CH Instruments.
Bard, Allen J; Faulkner, Larry R. Electrochemical Methods, Fundamentals and
Applications, second ed., John Wiley and Sons, Inc., N.Y., 2001. Chapters 5 and 6.
OBJECTIVES
1.
2.
3.
4.
5.
6.
7.
USING THE SOFTWARE
EXPERIMENTAL AND PARAMETER SET-UP
SCANNING LATERALLY AND VERTICALLY
DATA ACQUISITION: VOLTAMMOGRAMS AT THE TIP AND SUBSTRATE
POSITIONING THE TIP OVER THE SUBSTRATE ELECTRODE
STOPPING WITHOUT BREAKING THE TIP
FILE MANAGEMENT ISSUES
INTRODUCTION
A scanning electrochemical microscope is composed of a bipotentiostat that can
apply a bias and measure the currents at a working and a substrate electrode simultaneously.
The potentials applied are always measured with respect to a non-polarizable reference
electrode (Ag/AgCl, Hg/Hg2SO4, K2SO4 sat'd or Hg/Hg2Cl2, KCl sat'd) and the current
observed is measured between the working electrode of interest and an auxiliary electrode
(0.5 mm Pt wire). Usually the auxiliary electrode is chosen such that it does not produce
electrolysis species that would reach the working electrode and interfere with the studied
reaction. SECM uses ultramicroelectrodes (UMEs) as working electrodes and typically large
electrodes as a substrate. When recording the electrochemistry at the working electrode the
investigator can often use a two electrode system involving the UME and non-polarizable
reference electrode. But when large substrate electrodes are used in combination with the
UME, as is the case in many SECM applications, the auxiliary electrode must be used to
prevent complications arising from solution iR drop.
The electrodes are positioned in the SECM cell as depicted in Figure 1. The UME is
placed in an electrode holder that is attached to the SECM head where three positioning
inchworms allow for the 3D movement of the electrode within the cells. In the recent
commercially available instruments, the Burleigh inchworms have a total distance capacity
of 2.5 cm and a resolution that is below 1 nm. The inchworms used in this instrument are not
operated at close-loop and can suffer from front and back hysterisis (i.e. backlash). Such
hysterisis can be corrected using a calibration factor.
The CHI900 used in these labs offers a wide range of electrochemical techniques,
such as sweep techniques (cyclic and linear sweep voltammetry), step techniques
(chronoamperomentry, chronocoulometry, differential pulse or normal pulse voltammetry
and polagraphy, square wave voltammetry), stripping techniques (linear, differential pulse,
normal pulse and square wave pulse stripping voltammetry), scan probe techniques (probe
10
Figure 1: Conventional SECM set-up
scan curve, probe approach curve and SECM imaging) and other techniques. A maximum
voltage of ± 3.275 V can be applied at the working and substrate electrode and it can
measure currents between 1pA and 1mA.
The SECM minimum sytem requirements are:
Operating Sytem: Microsoft Windows 95/98 NT
Processor: Pentium
RAM: 16 M bytes
Monitor: SVGA
Mouse: PS/2
Serial communication port: RS-232
Output device: any compatible printer
1. Using the Software
•
•
Turn on the bipotentiostat and positioner and the back of the SECM instrument.
Double click on the CHI900 icon to start the software. If the SECM was not turned
on prior to opening the software an error message will appear indicating that the
communication between the computer and machine was not made.
11
The software displays a menu for file management, setup, cell control, graphic
preferences, data processing, mathematical analysis available, simulations, view format,
window options and a help section. The user’s manual provided by CH Instruments has a
comprehensive explanation of all menu functions.
• Please take the time to look at the different options available in the menu.
FILE:
The usual open, save, close file operations. This usually pertains to CHI files that are
in the .bin extension as saved by the machine at the end of an experiment.
The convert to text and text file format are the commands used to export the .bin files to a
desired location as a text file. The text format can be changed in the text file format
command. Multiple files can be selected at once and converted. Such text-files are easily
exportable into Excel, Origin and other spreadsheet programs for further treatment. The
print setup must always be engaged as landscape in order to print directly from the SECM
software.
12
SETUP:
The techniques command lists all the available electrochemical techniques as a
dialog box. This command is also located on the toolbar as the T button. By selecting the
command either from the setup menu or from the toolbar, you can select a technique using
the mouse.
To modify the parameters of the selected technique choose the parameters
command in the setup or the list button on the toolbar. Depending on the technique selected,
the options will be different in terms of the available controllable parameters. All the
parameter dialog boxes are displayed in the user’s manual with a description and range of all
variable parameters.
The hardware test command verifies that the ROM, RAM and analog circuitry is in
working order.
CONTROL
This lists the different commands available for running the experiment, to setup
repetitive runs and macros, measure the open circuit potential, impose step functions for
electrode conditioning, filter settings and the SECM probe command for positioning. The
important icons can be found as toolbar buttons.
GRAPHICS
These commands allow you to plot the current data recorded, overlay single or
multiple plots, display parallel plots and format the graphs. Since most people tend to treat
their data using spreadsheet software, the most commonly used function is the overlay plots
and parallel plots commands.
13
The rest of the CHI menu involves mathematical processing of the data, analysis and
simulation that one could also do using a spreadsheet. We invite the investigator to go
through these options at a later time.
2. Experimental and Parameter Set-up
Inspect the UME to be used under the microscope. The metal surface should be
smooth and clean. If the tip appears dirty a soft hand polishing with 0.05 µm alumina and
rinsing with Milli-Q water can be done prior to the experiment. When the metal surface is
satisfactory, the RG of the tip should be determined. The RG is defined as the ratio of the
diameter of the tip that includes the glass sheet to that of the diameter of the metal wire. This
value should always be as small as possible and values ranging from 2-5 are usually
acceptable.
Take the SECM Teflon cell and firmly place the clean and polished substrate
electrode in the hole, as shown in (1) in Figure 2. If the seal between the cell and the
substrate electrode is not tight enough, a small quantity of Teflon tape can be used to ensure
a snug fit.
Screw the SECM cell to the SECM metal plate (Figure 2, #2). Connect the substrate
electrode to the black lead.
Screw this metal plate onto the SECM head such that the cell is over the tip holder
(Figure 2, #3).
Fill the SECM with the prepared 1mM ferrocene methanol solution. Inspect the base
of the cell to make sure that the substrate electrode is snug and that the solution is not
leaking out.
Place first the auxiliary electrode and the reference electrode in their respective holes
(Figure 3, #1 and 2).
14
SECM cell
3
3
1
Metal plate
2
SECM Head
Figure 2: Inserting the substrate electrode in the SECM cell (1); Screwing the cell onto
the SECM metal plate (2); Screwing the metal plate onto the SECM head setup (3).
tip
3
Ag/AgCl
reference
Tip holder
Pt wire
1
2
SECM cell
Figure 3: SECM cell and electrodes.
•
Put the UME in the electrode holder (Figure 3, #3). Turn the screw to fix the UME in
position. The inchworm has a maximum distance range of 2.5 cm so you want the UME to
be in solution close to the substrate but not touching the substrate.
•
Connect the UME, reference and auxiliary electrode to their respective leads. The
green lead is the working, the white the reference the red the auxiliary. At this point your
experimental set-up is ready.
•
Select the techniques command in the setup menu or click on the T toolbar button.
Select cyclic voltammetry. Press OK.
15
Select the parameters command in the setup menu or the list button on the toolbar.
The dialog box should pop up. Set the initial potential to -0.2 V, the high potential to 0.5 V,
the low potential to – 0.2 V, the initial scan polarity to positive, the scan rate to 0.02 /s, the
segments to 6, the quiet time to 10, the sensitivity to 1e-8. Press OK.
16
3. Scanning Laterally and Vertically
At this point you have completed the experimental and parameter setup. Before
acquiring the voltammetric response of the UME and substrate electrode, the UME must be
roughly positioned over the Pt substrate.
•
In the control menu select the SECM probe command or the xyz coordinate axis
on the toolbar. A dialog box will pop up that allows you to manually control the x, y, z
direction, calibrate the inchworms and calibrate for the back and forth hysterisis on the x, y
inchworms.
At the bottom of the dialog box change the travel distance of the x inchworm to +500
and then press the move button to the right of that axis. Observe how the tip is moving in the
SECM and notice that a clicking noise is heard coming from that inchworm. By using
negative values the tip can be placed back to its original position.
Using only the x and y axis, place the tip approximately in the center of the substrate.
Then using the z axis, carefully bring down the tip closer to the substrate. Positive
values move the tip down and negative values retract the tip. You must watch as you move
the tip since this is a manual approach that requires that the moving and stopping be carried
out manually. If one fails to stop the tip in time when it is moving towards the substrate, it
will crash and will be damaged. When the reflection image of the tip can be seen in the Pt
of the substrate, the tip should be stopped.
4. Data Acquisition: Voltammograms at the Tip and Substrate
To summarize, we have set-up all connected electrodes in solution and have defined
the parameters such that we can record a cyclic voltammogram of the oxidation of ferrocene
methanol at the ultramicroelectrode and positioned the tip near to the substrate.
This means, as seen in Figure 4a, that for 10 sec the tip will be held at -0.2 V and
then will be linearly scanned to 0.5 V at a rate of 0.02 V/s. This will correspond to the first
segment. At 0.5 V the scan direction will be reversed so as to scan from 0.5 V to -0.2 V at
the same rate of 0.02 V/s. As the potential varies from negative to positive, the ferrocene
methanol will be oxidized to the ferrocenium ion at a rate that is entirely governed by the
17
rate at which the molecular species diffuse to the UME. The resulting current potential
curve, also called a CV, will be this S shaped curve, as seen in Figure 4b. The curve starts
with a current close to zero and decreases to a more and more anodic current until it reaches
a plateau that corresponds to the mass transfer limited diffusion of the ferrocene methanol to
the UME. This plateau is directly related to the concentration of the redox mediator in
solution. Because of the small size of the electrode, very high scan rates are required to see
any change in steady state current.
a
E applied (V)
0.5
0
-0.2
time
Quiet
time
1 segment
1 voltammogram
b
Figure 4: (a) Applied potential behavior with time for a voltammetric experiment.
(b) Typical steady-state voltammogram for a 25 µm Pt UME in a 1 mM
ferrocenemethanol solution. Potentials are given versus a Ag/AgCl reference electrode.
Start data acquisition by pressing the play button on the toolbar or by selecting the
run experiment command in the control menu.
18
Once the acquisition completed, save the voltammetric response of the UME in your
designated folder in the lab2 folder. Do not close the active window. If you do, reopen the
saved document from your folder.
Select the parameters command; select the swap electrode 1 and 2. Change the
sensitivity to 1e-5 A. You have now selected the substrate electrode as the working
electrode.
Start the data acquisition again to measure the voltammetric response of the large
substrate electrode. This voltammogram should look like a “duck” as seen in Figure 5.
6.00E-06
4.00E-06
2.00E-06
I (A)
0.00E+00
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
-2.00E-06
-4.00E-06
-6.00E-06
-8.00E-06
E (V vs. Ag/AgCl)
Figure 5. Cyclic voltammogram of a 3 mm Pt disk substrate electrode in a 1 mM
ferrocene methanol solution. The potentials are given vs. an Ag/AgCl reference
electrode.
Once the acquisition is completed, save the voltammetric response of the substrate in
your designated folder in the lab2 folder. Do not close the active window. If you do, reopen
the saved document from your folder.
Go to the graphic menu and select present data plot to readjust the scale of the
substrate electrode CV. In the graphic menu then select parallel plots to display the saved
UME CV next to the substrate CV. Do not close the windows.
5. Positioning the Tip Over the Substrate Electrode
Engage the techniques dialog box using the T button on the toolbar or the
techniques command in the setup menu. Select the probe approach curve technique. Press
OK. Then display the parameters for the probe approach curve by either selecting the list
button on the toolbar or the parameters command in the setup menu.
19
The probe approach curve allows you to approach the tip quickly to the substrate
based on the increases or decreases in the tip current relative to the steady state current in
solution. The increase or decrease in current is related to how the substrate electrode is
biased relative to the UME reaction. Suppose that the UME is poised such that the reaction
R - ne Æ O occurs. In our case, R would be ferrocene methanol that we would oxidize to O,
the ferrocenium ion. When the tip is far away from the substrate the UME will measure the
mass transfer limited steady state current. As the tip approaches the substrate electrode, the
processes occurring at the substrate electrode become important. If the electrode is an
insulating surface where the O + ne Æ R reaction cannot occur, the tip will record a smaller
current with distance relative to the steady state current as a result of the blocked diffusion
of R. This behavior is called SECM negative feedback. If on the other hand, the substrate
electrode is a conductor poised such that O + ne Æ R is taking place, an increase current
with distance relative to the steady state current will occur. This is called positive feedback
(Figure 6).
20
Far from the Substrate
Steady State Current
R
R
Negative Feedback
Positive Feedback
ETip: R - ne Æ O
ESub: no reaction
ETip: R - ne Æ O
ESub: O + ne Æ R
R
O
R
Close to the Substrate
R
R
R
R
O
R
R
R
R
R
O
Substrate
Insulator
Conductor
Hemispherical Diffusion
Hindered Diffusion
Regeneration Reaction
Figure 6. Fundamental principles of SECM.
From the parallel plot, identify the potential that needs to be applied such that
positive feedback takes place. This means that the oxidation of the ferrocene methanol
should occur at the tip and the reduction of the ferrocenium ion should take place at the
substrate electrode.
Input theses potential values in the respective probe potential of the probe electrode
and substrate potential of the substrate electrode. The sensitivity of the probe electrode
should be set to 1e-8 A and that of the substrate at 1e-5 A. The probe pulse probing program
is not used in this experiment. The ratio for the probe stop at current level should be set at
120% since this is a positive feedback experiment. The maximum increment has to do with
the distance the tip moves for each step. Leave the increment at 5 µm. The greater the
distance the faster the tip will approach and the easier it is to break the tip. The withdraw
distance is the distance that the tip retracts from the substrate prior to starting the approach.
This prevents breaking the tip that would have initially been placed too close to the substrate
electrode. Set this distance to 50 µm. The quiet time is applied following the withdraw
distance but before the approach in order to stabilize the tip current. Set this value to 10 sec.
Finally, to apply the potential at the substrate electrode select the E2 on button. This means
that the substrate potential will be applied. In the event that the substrate current would be of
interest, the I2 on current would also be selected.
Although this program stops the tip automatically when it reaches the current ratio
desired, it is always good practice to keep the mouse close to the stop button in the case of
too quick of an approach.
Start data acquisition by pressing the play button.
Once program has stopped automatically or manually, make sure that the curve you
have recorded is a flat current that eventually increases at very long distances. Save this file
in your designated folder.
Select the SECM probe command from the control menu or from the xyz axis
button on the toolbar. Retract the tip from the substrate by putting -300 as the travel distance
for the z inchworm and than pressing move. MAKE SURE THAT THE INPUTED
21
DISTANCE IS A NEGATIVE VALUE. From this approach you know that the tip to
substrate distance is now 300 ± 25 µm.
6. Stopping Without Breaking the Tip
Now that you have done a fast approach to the surface and that you know that you
are about 300 µm away from the substrate, we use a slower approach to get more points in
order to fit this approach to theory and evaluate the tip to substrate distance.
In the techniques command select the probe scan curve option. Then select the
parameters of this technique. Enter the same potentials for the probe and substrate
electrode as the ones used in the previous fast approach and the same sensitivity for each
probe electrode. Select E2 on. We do not need to change anything in the pulse option or the
probe stop current level. Select the amperomentric mode, and the z direction for the scan
direction. The travel distance should be set to 1000 µm. This is the maximum distance that
the tip will travel prior to stopping. The rate at which the tip will approach the substrate is
given by the ratio of the increment distance (incr. dist.) and the increment time. Set the
increment distance to 0.0666 µm so that the rate of approach is equal to 2 µm/s. The quiet
time should be set to 30 sec. Press OK.
The manual approach is now set to acquire. THIS APPROACH IS A MANUAL
APPROACH THAT MUST BE STOPPED BY THE MANIPULATOR USING THE
STOP BUTTON. Once you are in close proximity to the substrate the current will be
increasing straight up. As soon as you see a small discontinuity in the slope of the current,
stop the instrument. Usually this means that the insulating glass sheet of the UME has hit the
substrate electrode. When performing your first approach curve ask the lab instructor for
assistance.
Start the acquisition
Once you stop the tip, retract the tip from the substrate by 300 µm. Save the file in
the appropriate folder. Rescale the approach and ask the laboratory instructor to look at the
22
curve and confirm the best approach. Usually, a well aligned tip should be able to at least
triple the steady state current value. If the value is only two times that of the steady state
current, poor alignment of the tip to the substrate could explain this.
To get a better approach curve retract the tip 1000 µm, loosen the tip screw and rotate the tip
in the holder and repeat the fast and slow approach procedures.
7. File Management Issues
All the previously saved files are binary data files (bin) that can be opened on the
same or newer versions of the CHI900 software. Newer files cannot be opened on past
versions of the software. CHI can manipulate the files using the graphics menu. To evaluate
the tip to substrate distance using the developed theory, the files have to be converted to text
format in order to open them on a spreadsheet application.
Go to the file menu in the text file format command. Check the memo box if you
wish to include the date, time, technique, label and notes. Checking the parameter box will
list the experimental parameters. Check the Results box for experimental results such as
peaks, wave potential, current and area to be listed in the text file. The numerical data points
can also be listed if need be. You can define the separator to be used to separate the x,y
parameters in the text file. The precision of the data can be controlled via the number of
significant figures where the default is 4 and the maximum is 10. The data point interval
allows you to store only part of the data to reduce the file size but also causes loss of data as
a result. By choosing any of the 3 bottom boxes, an application specific format can be
selected either for Digisim purposes or Excel 3D formats.
Once your text file preferences have been selected, the convert to text command can
be selected in the file menu. One or multiple files can be selected to be converted to text.
This option only converts .bin files and stores the new text file in the same folder as the
original .bin files.
Convert all the .bin files collected in this experiment into text format. The tip to
substrate distance for the curves can be fitted to SECM theory using a spreadsheet program.
This will be part of the focus in laboratory experiment 3.
-
23
3.
Operation of the CHI900 SECM II: Learning How to
Obtain Approach Curves, to Fit Them to Theory and How to
Calibrate the Distance.
REFERENCES
“Scanning Electrochemical Microscopy”, Bard, A. J. and Mirkin, M.V. (eds.), Marcel
Dekker, N.Y., 2001, Chapter 5, p. 145-199.
User’s manual, CHI900 SECM, CH Instruments.
OBJECTIVES
In this exercise, you will learn how to obtain and process SECM approach curves with
CHI900 instrument. You will practice the procedures for calibration of the instrument and
for fitting the approach curves to the theory. You will also perform experiments where both
tip and substrate currents are monitored (T/S and S/T).
MATERIALS REQUIRED
- CHI900 SECM Instrument
- Caliper
- Teflon SECM cell
- Pt tip, diameter = 25 µm
- Reference electrode: Ag/AgCl (3 M KCl for inner solution)
- Counter-electrode: Pt wire
- Substrate: Pt disk (diameter = 2 mm) embedded in Kel-F
- Solution: 1 mM Ferrocene methanol (FcMeOH) in 0.2 M KCl deareated
1. Calibration of the Piezo-inchworms
At the moment of extracting information from the approach curves by fitting them
with the theoretical equations it is very important that the value of the read distance is the
real one. The piezo-inchworm moves through steps (or “clicks”) and the software converts
the number of clicks into a distance value by using a calibration factor. This factor must be
established by a calibration procedure before performing any serious experiment with a new
instrument, and should be checked periodically, depending on how frequently the instrument
is used.
The CHI900 software allows one to perform this operation in a straightforward way.
It will allow the inchworm to travel a pre-established distance (normally the whole possible
distance) and will ask you to measure this length with some external precise equipment, like
a micro-meter or caliper.
Calibrate the instrument through the following steps:
1. Open the window “SECM Probe Control” from the menu Control/SECM Probe…
or by clicking the icon.
2. Identify the frame “Inchworm Motor Calibration”. It contains three rows with the
calibration parameters of each inchworm. The first parameter (Number of Clicks) is the total
number of clicks the inchworm moves; the second parameter (Distance) is the length that the
24
software assigns to the total number of clicks. You will refresh this length after the
calibration.
3. Start the calibration procedure for the Z-axis inchworm by clicking the control
button Calib Z Motor. The inchworm will retreat completely to its first position and will ask
you to measure its position.
4. Take a fixed reference point on the Z-axis and measure with the caliper the
distance between it and the bottom end of the inchworm. Write down this value and click
OK.
5. The inchworm will travel the total number of steps and will ask you to measure
the last position. Do that with the caliper by taking the same previous reference point, and
write down this value. Click OK.
6. Subtract the first value from the second one and write down its absolute value (in
micrometers) in the Distance box of the Z motor. This distance value will be assigned to the
total number of clicks for this inchworm.
7. The calibration procedure is finished, although you can repeat it several times to
get a more exact mean value. Note that if you change the distance value without following
this procedure, the new value will not be stored.
2. Processing and Fitting Approach Curves
In the following experiments you will obtain approach curves on conductive and
insulating substrates, and will fit them to the theoretical equations.
Obtaining Approach Curves
Place the substrate electrode in the substrate holder of the SECM cell, fix the cell on
the SECM stage and fill it with deareated 1 mM FcMeOH solution. Put the reference and
counter electrodes into their respective slots, and insert the Pt tip in the tip holder. Connect
the four electrodes to the bipotentiostat.
Insulating surface: Using the “SECM Probe Control” window, place the tip over the
Kel-F substrate sheath. Follow the procedure described in laboratory experiment 2 to obtain
a negative-feedback approach curve for the oxidation of FcMeOH, with a total travel
distance of ~300 µm at a scan rate of 3 µm/s. Save the curve.
25
Conductive surface: Using the “SECM Probe Control” window, withdraw the tip ~
500 µm and position it over the substrate Pt disk. Follow the procedure described in
laboratory experiment 2 to obtain a positive-feedback approach curve for the oxidation of
FcMeOH, with a total travel distance of ~200 µm at a scan rate of 3 µm/s. Save the curve.
In both cases, identify the end point of the curve as a change in the slope.
Fitting Approach Curves with the Theoretical Equations
You will need to use calculus software to process the approach curves. Microsoft
®
Excel and Microcal Origin® are very suitable for this purpose. Thus, you must export the
curves as .txt files from the menu File/Convert to Text….
The raw data contains two columns: one with the tip current values (iT) and the other
with the distances from the scan initial point (dexp). You need to normalize these values
according to eqs. (1) and (2) to make them compatible with the theoretical equations for
conductive (eq. 3) and insulating (eq. 4) substrates (see Table A for the parameter values).
iT
dexp
→
→
I = iT / iT,∞
L = d/a = (do – dexp)/a
(1)
(2)
I = A + B/L + C exp(D/L)
I = 1/[A + B/L + C exp(D/L)]
(3)
(4)
where iT,∞ is the tip current, a is the UME tip radius, d is the tip-substrate distance, do is the
distance from the scan first point to the substrate surface. In addition, the experimental value
of I must be corrected by an RG-dependant factor. Note that the only unknown parameter,
which will require adjustment in the fitting process, is do.
Table A
- Parameter Values for Equation (3) (Positive Feedback) at different RG Values
RG
1.1
1.5
2.0
5.1
10
A
0.5882629
0.6368360
0.6686604
0.72035
0.7449932
B
0.6007009
0.6677381
0.6973984
0.75128
0.7582943
C
0.3872741
0.3581836
0.3218171
0.26651
0.2353042
D
-0.869822
-1.496865
-1.744691
-1.62091
-1.683087
- Parameter Values for Equation (4) (Negative Feedback) at different RG Values
RG
1.1
1.5
2.0
10
A
1.1675164
1.0035959
0.7838573
0.4571825
B
1.0309985
0.9294275
0.877792
1.4604238
C
0.3800855
0.4022603
0.424816
0.4312735
D
-1.701797
-1.788572
-1.743799
-2.350667
You can search the do value either manually or using a regression procedure. In the
latter case, a calculus machine (calculator, computer software, etc.) will find the do value
that best correlates the experimental curve with the theoretical function. As this procedure is
26
not general (since it depends on the software you use), it will not be described here.
However, a general sequence of steps for a manual search of do, which can be performed in
any calculus program, is described below.
1. Open the curve .txt file. Identify the first column as dexp and the second as iT.
2. Plot the two columns and determine the iT,∞ value reading iT when the tip is far
away from the substrate and the tip current is stable. In a new column, calculate the
normalized current through eq. (1) and correct it with the correction factor. This value will
be called Iexp.
3. Estimate visually the do value. In a new column, calculate L using this estimated
value.
4. Using equation (3) or (4) for conductive or insulating substrate, respectively, and
the parameters that correspond to the RG of your tip (tabulated in Table A), calculate the
theoretical approach curve (ITheor vs. L) in a new column.
5. Plot the values of Iexp as a function of L and overlap the Itheor values. Compare and
analyze the differences. Change the do value until a good correlation between both is found.
The examples below show good correlations.
Figure 1. Experimental and theoretical approach curves on an insulating substrate.
27
Figure 2. Experimental and theoretical approach curves on a conductive substrate.
Note that the end point of the experimental approach curve has an offset respect to
the zero-distance coordinate. The real value of this particular position can only be obtained
after the fitting procedure.
3. T/S and S/T Experiments
In these experiments, after positioning the tip close to the substrate (L < 0.5), you
will either cycle the substrate potential while holding the tip potential constant to collect the
species generated at the substrate (T/S), or cycle the tip potential while keeping the substrate
potential constant to collect the tip-generated products (S/T). Using the same setup and
solution that is used in the previous experiments, position the tip at 5 µm from the Pt-disk
substrate.
Obtaining S/T Voltammograms
1. Open a Cyclic Voltammetry experiment and define the parameters as is shown
below. Note that the substrate (Electrode 2) is “On”, and its potential is held at a value
where any tip-generated FcMeOH+ (oxidized) reaching the substrate is reduced. The tip
potential will be cycled around the formal potential of the FcMeOH+/FcMeOH couple. The
Quiet Time will allow the substrate background current to be stable. If needed, the solution
should be deareated to minimize oxygen reduction.
2. Run the experiment, save the results and export them as .txt file. This file contains
three columns: tip potential (ET), tip current (iT) and substrate current (iS). Plot both iT vs. ET
and iS vs. ET.
28
3. From the graphs obtain the tip current for the FcMeOH oxidation and the substrate
current for the reduction of the tip-generated FcMeOH+. This last value has to be calculated
subtracting the background substrate current from the total reduction current.
4. Calculate the S/T Collection Efficiency, CES/T = iS / iT, and discuss the results.
Obtaining T/S voltammograms
1. Use the same technique and parameters as used in the previous experiment, but
now swap the substrate and tip electrodes by checking the option “Swap Electrode 1 and 2”,
as it is shown below. Increase and decrease the sensitivity values of the electrodes 1 (which
now is the substrate) and 2 (which now is the tip) respectively. Note that the tip potential is
held at a value where any substrate-generated FcMeOH+ reaching the tip is reduced. The
substrate potential will be cycled around the formal potential of the FcMeOH+/FcMeOH
couple.
29
2. Run the experiment, save the results and export them as .txt file. The three
columns now are substrate potential (ES), substrate current (iS) and tip current (iT). Plot both
iT vs. ES and iS vs. ES.
3. Obtain the substrate current for the FcMeOH oxidation and the tip current for the
reduction of the substrate-generated FcMeOH+.
4. Calculate the T/S Collection Efficiency, CET/S = iT / iS, and discuss the results.
Repeat both voltammograms at a larger tip-substrate separation (L = 2.5). Compare
the new CE values with those previously obtained.
30
4. Tip Fabrication and Characterization II: Polishing,
Sharpening and Testing
REFERENCES
“Scanning Electrochemical Microscopy”, Bard, A. J.; Mirkin, M. V. (eds.), Marcel Dekker
Inc, N.Y., 2001, Chapters 3 and 5.
OBJECTIVES
1.
2.
3.
4.
POLISHING THE ELECTRODE
OBTAIN A STEADY STATE VOLTAMMOGRAM
ELECTRODE SHARPENING
OBTAIN CONDUCTING AND INSULATING APPROACH CURVES AND
THEORETICAL FITTING
5. EVALUTATE TIP GEOMETRY
INTRODUCTION
This laboratory will guide you through the last finishing steps of tip preparation. In
laboratory experiment 1, glass capillaries were sealed, cleaned and a straight 25 µm Pt wire
was inserted in the crack at the bottom of the capillary. Following a 30 minute vacuum
evacuation, the capillary was sealed onto the Pt wire using a resistor coil. Electrical contact
was ensured between the Pt and lead wire by a conducting silver epoxy that was left to cure
in the oven over night.
In this laboratory experiment, the lead of the cured electrode will be sealed with
epoxy to strengthen the connection wire. The glass at the end of the electrode will be ground
off with sandpaper until the Pt wire is exposed. Successively finer sandpaper and alumina
solutions on micropolishing cloths will then be used to smoothly polish the electrode
surface. At that point, a good steady-state voltammogram should be obtained. From this
response and using the theoretical expression for the steady state current, the radius of the
electrode should be back calculated. The extracted electrode radius should be very close to
the optically determined one. Finally, the electrode will be sharpened such that the RG (ratio
of the outer glass diameter to the diameter of the wire) is between 2 and 10.
The sharpened tip should be tested by acquiring conductive and insulating approach
curves and comparing them to the theoretical behavior of a microdisk. A short discussion
concerning the response of non-disk UME will also follow.
1. Polishing the Electrode
A small amount of 5-minute epoxy should be patched between the connection wire
and the end of the capillary to restrict the strain put on the connection wire. Allow to dry.
To polish the electrode, a polishing wheel can be used as shown in Figure 1. The
sandpaper or micropolishing cloths are put at the center of the wheel. The wheel is turned on
at the base and has two speed levels. Only the first speed level is used to polish the
electrodes.
31
Figure 1: Polishing wheel
Using sandpaper (# 240), remove the glass from the bottom of the tip until the sealed
platinum wire is exposed and observed under a microscope (Figure 2). Using water on the
sandpaper while polishing sometimes reduces the strain on the glass.
Figure 2: Shaved capillary exposing the 25 µm Pt wire at the center.
Polish the tip by gradually increasing the grid size of the sandpaper (240, 600, 1200).
Try to maintain the electrode as vertical as possible so that the tip stays completely flat.
Observe the polishing progress under a microscope. Always wash the surface of the
electrode with Milli-Q water before changing from one grid to the other.
Use polishing cloth and solutions of alumina with different particle size (typically
1.0, 0.30 and 0.05 micron) to perform the final polishing. Always go from the larger grain
size to the smallest one and use a different polishing cloth for each alumina size grain, but
do not discard them, since they are quite expensive. Make sure to wash the electrode surface
extensively between alumina solutions. Decrease the particle size gradually and check under
the microscope. A smooth surface must be obtained as shown in Figure 3.
32
Figure 3: Smoothly polished 25 µm Pt UME that has been slightly sharpened.
2. Obtain a Steady State Voltammogram
At this point, the electrochemical behavior of the tip must be checked. No matter
how good the tip looks under the microscope, an acceptable electrochemical signature must
be obtained.
A solution of ferrocenemethanol (1 mM in 0.1 M KCl) will be used. The cyclic
voltammograms of the 25 µm diameter electrode at scan rates of 10 and 25 mV/s are shown
in Figure 4.
Figure 4: Steady state voltammogram at a 25 µm Pt disk UME in a solution of
approximately 1 mM ferrocenemethanol in 0.1 M KCL electrolyte. The potentials are
given with respect to Ag/AgCl.
Turn on the CHI900 and open the software.
Take the SECM Teflon cell and firmly place the clean and polished substrate
electrode in the hole as seen in (1) in Figure 5. If the seal between the cell and the substrate
electrode is not tight enough, a small quantity of Teflon tape can be used to ensure a snug
fit.
33
SECM cell
3
3
1
Metal plate
2
SECM Head
Figure 5: Inserting the substrate electrode in the SECM cell (1); Screwing the cell onto
the SECM metal plate (2); Screwing the metal plate onto the SECM head setup (3).
Screw the SECM cell to the SECM metal plate (Figure 5, #2). Connect the substrate
electrode to the black lead of the potentiostat.
Screw this metal plate onto the SECM head such that the cell is over the tip holder
(Figure 5, #3).
Fill the SECM with the prepared 1 mM ferrocenemethanol solution. Inspect the base
of the cells to make sure that the substrate electrode is snug and that the solution is not
leaching out.
Place first the auxiliary electrode and the reference electrode in their respective holes
(Figure 6, # 1 and 2).
-
tip
3
Ag/AgCl
reference
Tip holder
Pt wire
1
2
SECM cell
Figure 6: SECM cell and electrodes.
34
• Put the UME in the electrode holder (Figure 6, #3).
• Connect the UME, reference and auxiliary electrode to their respective leads. The green
lead is the working, the white the reference and the red the auxiliary. At this point your
experimental set-up is ready.
• Select the techniques command in the setup menu or click on the T toolbar button.
Select Cyclic Voltammetry. Press OK.
• Select the parameters command in the setup menu or the list button on the toolbar. The
dialog box should pop up. Set the initial potential (Init E) to -0.2 V, the high potential (High
E) to 0.5 V, the low potential (Low E) to –0.2 V, the initial scan polarity to positive, the scan
rate to 0.02 V/s, the segments to 6, the quiet time to 10, the sensitivity to 1e-8. Press OK.
35
-
Acquire the voltammogram.
If the CV does not resemble that in Figure 4, one has two choices: to throw away the
tip or to repeat the polishing again to remove more glass to find a better sealed segment of
electrode. The most common cause for electrode failure is an unsuccessful sealing.
If the CV is satisfactory, the quantitative behavior of the tip can be checked by back
calculating the radius of the electrode using the theoretical expression for the steady state
current. The steady state current, Iss (A = coulombs/s), for a microdisk electrode can be
expressed as:
Iss = 4nFDaC
(1)
Where n is the number of electrons involved in the electrochemical reaction (n = 1
equ./mol.), F is the Faraday’s constant (9.64853*104 coulombs/equ.), D is the diffusion
coefficient of the reacting species (for ferrocenemethanol D= 7*10-6 cm2/s), C is the bulk
concentration of the specie (1*10-6 mol/cm3) and a is the radius of the electrode (in cm).
From the optical measurements, we know that the radius of the electrode should be
12.5*10-4 cm, the value extracted from the voltammogram using equation 1 should be very
close to that value.
When micron or sub-micron sized electrodes are used, it can sometimes be very
difficult to optically define the radius of the electrode. It is useful to use available analytical
expressions to determine these values or to monitor decreases in electrode active areas as a
result of adsorption processes.
3. Electrode Sharpening
Electrode sharpening is a delicate process. The goal is to reduce the ratio of the
diameter of the glass to the diameter of the electrode (the so-called RG value) to 10 or less,
36
with 2-5 being optimum. Everyone has a preferred way to sharpen a tip. We describe one of
the ways below but we suggest that you try and find what is most comfortable for you.
Use 600-grit sandpaper on the polishing wheel and hold the electrode at about a 45
degree angle while rotating the electrode in your fingers. Check on the RG value frequently
under a microscope that is equipped with a ruler in the eyepiece. When RG ≈ 20, change to
1200-grit sandpaper and continue using the polishing wheel, but stop more frequently to
check under the microscope. At about RG=10, it is advisable to sharpen the electrode
manually using the 1200 grit sandpaper with frequent trips to the microscope.
The desirable final shape of the electrode tip is shown in Figure 7.
Figure 7: Left: 25 µm Pt tip with RG=4; Right: 25 µm Pt tip with RG=3
Depending on how one does the final polishing, the overall appearance of the tip can
be different as seen in Figure 7. On the left, 0.3 µm alumina was used to finish sharpening
the tip. This yields a much smoother glass rim that is often hard to photograph. The use of
small grid sandpaper in the last instance yields a more defined edge that is easy to observe.
Using sandpaper is faster, but it is also more likely that the electrode area will be scratched
up by glass debris or that the side of the Pt wire will be exposed. There is no one way of
making a tip and many people end up developing their own style to polish the tip.
Before collecting the approach curves it is always good practice to manually repolish the electrode with 0.05 µm alumina and to take a steady-state voltammogram of the
tip.
4. Obtain Conducting and Insulating Approach Curves and Theoretical
Fitting
Figures 8 and 9 present the fitted insulating and conductive approach curves,
respectively. The experimental data obtained are fitted to the theory of approach curve for a
microdisk and a RG=10:
(2)
37
(3)
For a complete discussion on the theory supporting SECM please refer to Chapter 5
of the SECM monograph.
Figure 8: Insulating approach curve
Figure 9: Conductive approach curve
During this course, you will have plenty of opportunity to look at perfectly fitted
approach curves. Figure 8, is not one of them but it shows effectively that the insulating
approach curve is more sensitive to the tip shape and alignment than the conductive
approach curve.
If the RG is smaller than 10, as is the case here, deviations from the RG=10 theory
can be observed. The steady state current is higher than the one calculated from equation 1
38
because of the added effect of the diffusion of the mediator from the back of the tip. Fitting
the curve to the RG dependent theory, as is discussed in Chapter 5 (p. 152-157) of the
monograph, is often advantageous.
Acquire the insulating and conducting approach curves. Follow the same procedures
as was used in laboratory experiments 2 and 3 to obtain and treat the data.
5. Evaluation of the Tip Geometry
The microdisk electrodes are the most commonly used tips for SECM because of
their well-defined geometry, theory and ease of preparation. There are, however, advantages
in using different geometry electrodes like ring, conical, spherical and hemispherical tips.
With conventional size (1-25 µm) UME, the use of a different electrode material than Pt is
often interesting. The use of mercury, for example, has always been a favorite amongst
electrochemists because of the atomically smooth surface, the well-behaved
electrochemistry and the high proton overpotential of mercury. Hg UME electrodes can be
made via electrodeposition of mercury onto a Pt microdisk from a mercury salt solution. It
can also be formed by simple contact with a mercury pool. The resulting electrode has a
hemispherical geometry. Spherical gold UMEs have been made from gold nanoparticles,
conical electrodes made of Pt, C and Ir have been made via etching.
All the above geometries have the advantage of having the electrode material
protruding from the glass surface. This means that their electroactive area is more accessible
to the substrate plane. Their approach to the substrate is less hampered by the glass sheet
than the microdisk, allowing them to be positioned closer to the substrate electrode and
suffer less from misalignment problems.
The current measured at such electrodes is, however, inherently smaller than that of
the microdisk electrode because of the effects of enhanced radial diffusion of the redox
species to the tip. The conductive approach curve of a mercury hemispherical tip is fitted to
hemispherical theory and compared to the theoretical response of the disk UME in Figure
10. A systematic lower current is observed for such tips.
39
Positive feedback of a Hg Hemispherical tip in 2mM Hexamine ruthenium Chloride
4.50E+00
4.00E+00
3.50E+00
3.00E+00
I/Io
Experimental
Hem. Theory
Disk theory
2.50E+00
2.00E+00
1.50E+00
1.00E+00
0
0.2
0.4
0.6
0.8
1
1.2
1.4
L
Figure 10: Conductive approach curve of a Hg hemispherical UME, fitting to
hemispherical theory and comparison to microdisk theory.
Figure 10 demonstrates that the SECM behavior is dependent on the geometry of the
probe. To extract useful information from the approach curve, theories and analytical
approximations for the different geometries are needed. This is particularly the case when
trying to characterize sub-micron electrodes where an optical characterization is difficult and
the assessment of the tip geometry must be made electrochemically.
For a more detail description of how to develop such theories please refer to p. 162
of the SECM monograph.
40
5. Basics of Imaging
REFERENCES
“Scanning Electrochemical Microscopy”, Bard, A. J.; Mirkin, M. V. (eds.),
Marcel Dekker, N.Y., 2001, Chapter 4, pp. 111-143.
User’s Manual, CHI900 SECM, CH Instruments
OBJECTIVES
In this exercise, you will become acquainted with the procedures for obtaining topographic
and chemical reactivity images of conductive and insulating surfaces by SECM in the
amperometric constant height mode. You will learn how to chose the proper tip-substrate
distance and tip scan rate, and study the effects of the distance on the resolution of image
and tip-substrate alignment (sample tilt). You will also learn how to store, view and analyze
images.
MATERIALS REQUIRED
CHI 900 SECM Instrument
Teflon SECM cell with acrylic base
Platinum tip, diameter = 25 µm
Reference electrode: Ag/AgCl (3 M KCl for inner solution)
Counter-electrode: Platinum wire
Substrates:
# SBS1: Gold pattern on a gold surface
# SBS2: Nylon membrane filter, pore diameter = 20 - 25 µm, on a glass slice
# SBS3: Gold lines supported on a glassy carbon plate
Solutions:
# 1 mM ferrocenemethanol (FcMeOH) in 0.1 M KCl
# 2 mM Fe(III) in 0.1 M H2SO4 deareated with argon
1. General Procedure to Obtain an SECM Image
A SECM image is the combination of, basically, a sequence of tip scans in one
direction (long travel direction) done at different successive positions in the other direction.
The CHI900 can perform the whole operation, provided you define some necessary
parameters.
Once the cell is placed on the stage, all electrodes are connected and the tip is moved to the
region where the image will be taken, you are ready to start the x-y scan that will produce
the SECM image. Prior to that, you need to establish the tip potential, the substrate potential
(for a conductive substrate), the area to be scanned and the scan rate. With CHI900, all these
parameters can be selected through the following steps:
• In the menu Setup/Technique, first select the Scanning Electrochemical Microscopy
technique.
41
•
Then open the Parameters window (Setup/Parameters…).
•
Make sure that the amperometric mode (Amperometry) is selected in the “SECM Mode”
frame. Potentiometric images can also be obtained by measuring the tip rest potential as
a function of the tip position, but this will not be done in this practice.
Introduce the tip potential in the “Probe Electrode” frame. If a conductive substrate is
imaged, turn its potential on by checking “E On” in the “Substrate Electrode” frame, and
write the substrate potential. In both Probe and Substrate frames check the Sensitivity of
the current.
Write the scan distances in the x and y directions into the “Probe Travel” frame. In the
“Long Travel Direction” frame it is possible to choose the fast scan axis. Furthermore, in
the “Data Sampling Scheme” you can select the possibility to read the tip current when
just scanning in one direction (forward), or to do the measurements in both directions
(forward and reverse). The former mode is slower, but the quality of the image is better.
The scan rate is defined in the “Probe Travel” frame by selecting the distance increment
(distance that the tip will move continuously in each voltage ramp of the driver) and the
time increment (time the driver will stay on the ramp). The scan rate results from the
ratio between them.
•
•
•
42
•
•
•
There are two ways for positioning the tip at a desired tip-substrate distance:
-Approach the tip to the surface using the Probe Scan Curve technique as an
independent operation. Use the extent of positive or negative feedback, for
conductive and insulating substrates respectively, to know the tip-substrate
separation, and position the tip at the desired distance.
-Get the tip close to the surface by performing an automated approach curve before
starting the x-y tip scan. You need to check the option “Approach Surface before
Imaging” and to define the parameters for the approach in the “Reposition Probe
before Run” frame. You can choose to stop the approach using the criteria of the
change in the tip current with respect to the infinite value or just using the criteria of
a maximum (minimum) tip current value. The tip will be withdrawn a given distance
(defined in the “Withdraw distance” box) before starting the approach curve, and
will wait the defined Quiet time. The scan rate during the approach will change: it
decreases when the tip current approaches to the Stop criteria. The x-y scan will start
immediately after the Stop criterion is accomplished.
You may want to return the tip to its original position after performing the whole frame
of imaging. To do that, check the option “Return to Origin after Run”.
Before starting any imaging of a large area, it is necessary to calibrate the
Forward/Reverse distance ratio of each inchworm, besides the conventional calibration
already described. Refer to Section 5 for the procedures.
2. Imaging Morphology of Homogeneous Conductive and Insulating
Surfaces
From the SECM theory of feedback mode, we know the tip current that results from
an electrochemical reaction at an UME tip operating under diffusion-controlled conditions
can give us information about the exact tip-substrate distance. The theory relating the tip
current with the distance has been developed, so a map of the tip current as a function of the
x-y tip position can be easily transformed into a two-dimensional height map, which is a
topographic image. For this to be true, the whole surface must behave homogeneously to the
mediator reaction.
In the following experiments you will obtain topographical images of two different
kinds of surfaces, one totally conductive (Au) on which the mediating reaction
(FcMeOH/FcMeOH+) is very fast, and another totally insulating (nylon membrane).
FcMeOH in solution will be oxidized under diffusion-controlled condition at the tip. The
FcMeOH+ will be reduced back to FcMeOH at the conductive substrate, generating a
positive feedback at the tip. The diffusion of FcMeOH will be partially hindered when the
insulating substrate is used, generating a negative feedback at the tip.
Conductive Surface
The positive feedback current will be used, so higher tip currents will reflect lower
tip-substrate distances.
Place the electrode SBS1 in the substrate holder of the SECM cell, as indicated in
Figure 1, exposing the Au array to the solution. Fix the cell on the SECM stage and fill it
with deareated 1 mM FcMeOH solution. Put the reference and counter electrodes into their
respective slots. Finally, insert the Pt tip and position it near to the array to be imaged.
Following the procedure described in Section 1, set the instrument in the Scanning
Electrochemical Microscopy mode. Select the tip and substrate potentials to observe a total
43
positive feedback. Use a scan rate of 30 µm/s to image an area of 200 x 200 µm at two
different tip-substrate distances (about 13 and 5 µm (L/a ≅ 1 and 0.4)). Observe and discuss
the effect of the tip-substrate distance. Save the files to be processed later.
Figure 1. Scheme of the SECM cell for imaging experiments.
Insulating Surface
The negative feedback current will be used, so lower tip currents will reflect lower
tip-substrate distances.
Place the electrode SBS2 in the substrate holder of the SECM cell as indicated in
Figure 1. Fix the cell on the SECM stage and fill it with deareated 1 mM FcMeOH solution.
Put the reference and counter electrodes into their respective slots. Insert the Pt tip and
position it at 3 µm from the substrate using the negative feedback for the oxidation of
FcMeOH. Set the instrument in the Scanning Electrochemical Microscopy mode, and select
the tip potential at a value where the tip reaction is totally diffusion-controlled. Image an
area of 100 x 100 µm at two different scan rates (60 and 30 µm/s). Observe and discuss the
effect of this parameter. Save the files to be processed later.
3. Imaging Heterogeneous Surfaces
The surface to be imaged can be a composite of two or more components, such as an
inter-digitated array (IDA) of gold on carbon. It is possible that all of the components
present the same activity for the probe reaction, on which a topographic image can be
obtained without problems in the estimation of the tip-substrate distance. If a different
component presents different activity for the probe-generated species, for a given tipsubstrate distance the feedback tip current will be different on different component. (This
concept is demonstrated better in laboratory experiment 6). Thus, the image obtained is
convoluted by topographic and electrochemical-activity effects. If a smooth array, where
topographic effects are negligible, is imaged with a material-sensitive mediator, an image of
chemical reactivity can be obtained. This is an exclusive and very useful property of SECM.
In the following experiment you will image the electrochemical activity of an array
of Au on glassy carbon for the oxidation of Fe(II) to Fe(III). Fe(III) in solution will be
reduced under diffusion control at the tip, and the substrate potential will be held at two
different values: one at which the reaction is equally fast on both materials (just imaging
44
morphology) and another at which the reaction is significantly faster on gold than on glassy
carbon (imaging reactivity).
Assemble the SECM cell as was done in Section 2 (Figure 1), placing the electrode
SBS3 in the substrate holder. Fill the cell with the 2 mM Fe(III) solution and insert the
reference and counter electrodes. Position the tip near to the array. Approach the tip to the
GC surface by using the negative feedback of the Fe(III)/Fe(II) reaction by selecting ET = 0.2 V vs. Ag/AgCl. Prepare the instrument for scanning an area of 200 x 50 µm at 30 µm/s.
Select the tip potential at -0.2 V and the substrate potential at 1.3 V (first scan) and 0.7 V
(second scan) vs. Ag/AgCl. Perform the scans at a tip-substrate separation of 7 µm.
Compare these images and discuss the observed differences.
4. Viewing, Storing and Analyzing Images
The image will be displayed during the experiment as a color scale x-y plot, where
each different color or color tone represents a range of tip currents. For example, high tip
currents correspond to dark brown colors, and low tip currents are dark green. At the end of
the experiment you can see the whole image with an updated scale by clicking the “Data
Plot” icon. Also you can expand the x-y scale in a particular region by using the “Zoom”
icon.
An image can be stored in three different ways:
- Save the image as a .bin file from the menu File/Save As…. This file can be opened
and processed by the CHI900 software.
- Copy the image into the clipboard as a bitmap picture from the menu
Graphics/Copy to Clipboard (the image window must be maximized first). Then
paste it to wherever the image processor locates to edit it.
- It is possible to get an ASCII file (.txt) whose format is basically a matrix
consisting of tip current values arrayed in a two-dimensional grid where columns and
rows correspond to different distances in each direction. In the menu File/Text File
Format… check the option “Excel 3D Format for SECM data”.
For exporting the file, go to File/Convert to Text and select the files. You will be
able to open (or import) these files with any of the most known calculus software like
Microsoft Excel® or Microcal Origin®. Each particular program has its own facilities to
process and plot these data, like to make color map or 3D graphs, overlap images, etc.
5. Calibration of the Forward/Reverse Ratio
The length of a piezo-inchworm step is slightly different when it is moving in the
forward direction than when moving in the reverse direction. This fact is almost
45
imperceptible when small areas are scanned, but the effect is noticeable in large-area scans,
leading to deformation in the images.
You can solve this problem by measuring the Forward/Reverse ratio for each
inchworm using a shape-known substrate, and inserting this value into the software to
correct the inchworm motion. It is necessary to obtain this ratio for each desired scan rate,
since its value depends on this parameter. A possible way to do this operation is described
below.
Set up the typical SECM cell using a Pt microdisk (diameter < 150 µm) embedded in
glass as substrate. Use a 1 mM FcMeOH solution. Place the tip near the Pt disk, and be
ready to get a 500 x 500 µm image. Position the tip at a known tip-substrate distance
(typically 10 µm) using the negative feedback over the glass sheath.
Before starting the image acquisition, check that the Forward-Reverse Distance Ratio
to be corrected is 1 in the SECM Probe Control window (“Forward and Reverse Distance
Ratio” frame).
In the Parameters window, select the options “Return to Origin after Run” and
“Single Direction”, choose the desired scan rate and start the imaging experiment. The
contrast between the negative feedback onto the glass sheath and the positive feedback onto
the Pt disk will allow you to clearly identify the disk. Immediately after finishing acquiring
the image, save it and start another similar scan. Comparing both images, obtain the distance
that the image shifted, from which it will be possible to calculate the ratio. The following
example illustrates the calculation:
500 µm
1
2
3
.
.
.
2nd scan
.
.
.
1st scan
100 µm
.
.
49
50
46
In this experiment you can see that the tip shifted -100 µm in 50 scans, which is -2
µm/scan. As each scan is 500 µm in length, you have a shift of -0.004 µm by micrometer.
Thus, the forward-reverse distance ratio is calculated by the equation
RF-R = 1 + Shift = 1 - 0.004 = 0.996
Insert the calculated ratio value in the “Forward and Reverse Distance Ratio” frame in the
SECM Probe Control window, and repeat the experiment. If there is still some shift, correct
again the ratio value. Perform this procedure for each inchworm.
47
6. Determination of Kinetics Parameters of
Heterogeneous Electron Transfer
REFERENCES
“Scanning Electrochemical Microscopy”, Bard A. J.; Mirkin M. V. (eds), Marcel Dekker
Inc, N.Y., 2001, Chapters 5 and 6.
OBJECTIVES
1.
2.
3.
4.
OBTAIN CV OF FE(III) REDUCTION AT CARBON TIPS AND SUBSTRATE
POSITION CARBON TIP OVER SUBSTRATE
OBTAIN APPROACH CURVES AT DIFFERENT SUBSTRATE POTENTIALS
EXTRACT THE RATE CONSTANT OF THE IRREVERSIBLE REACTION
INTRODUCTION
SECM is a powerful tool for determining kinetic parameters of electron transfer (ET)
reactions. Its advantages include high mass transfer, which can be varied by changing the
tip/substrate distance (d) and the absence of problems such as charging current and IR drop.
The objective of this work is to measure the rate constant of an irreversible redox reaction.
The Fe3+/Fe2+ couple is used as a redox mediator. At the tip surface, the reduction of Fe3+
occurs at diffusion-controlled rate (see Figure 1):
Tip reaction:
Fe3+ + e → Fe 2+
(1)
2+
The substrate re-oxidizes Fe produced by the tip (Figure 1) at a kinetically controlled rate
determined by its potential:
Substrate reaction:
Fe2+ → Fe3+ + e
(2)
The ET at the substrate must be irreversible so that the rate of the backward reaction
(reduction) would be negligible. The reaction of Fe2+ at a glassy carbon electrode fits this
criterion.
UME tip
+1e
Fe2+
3+
Fe
kb,s
Fe3+
diffusion
Fe2+
kf,s
Glassy-carbon substrate
Figure 1. Positive SECM feedback
48
PROCEDURES
CV and SECM Experiments
The scheme of the SECM cell is shown in Figure 2. Before obtaining approach
curves, make sure that the tip and the glassy carbon substrate are well polished (50 nm
alumina) and washed to remove any particles from the surface.
to bipotentiostat output A
y
x
Carbon Fiber Tip
Ag/AgCl // KCl (0.3M)
ref. electrode
z
Pt counter electrode
Container for Fe(III) solution
to bipotentiostat output B
Glassy Carbon substrate
Figure 2: Scheme of the SECM setup.
1. Obtain CV of Fe(III) Reduction at Carbon Tips and Substrate
Check the substrate and the UME by obtaining a CV in a 10mM Fe3+ in 1 M H2SO4
solution (Init E (V) = +0.800, High E (V) = +0.800, Low E (V) = - 0.1, Initial Scan Polarity
= negative, Scan Rate (V/s) = 0.02, Sweep Segments = 2, Sample Intervals (V) = 0.02, Quiet
time (sec) = 20, for the substrate; Init E (V) = +0.200, High E (V) = +0.200, Low E (V) = 0.800, Initial Scan Polarity = negative, Scan Rate (V/s) = 0.02, Sweep Segments = 2,
Sample Intervals (V) = 0.02, Quiet time (sec) = 20, Sensitivity (A/V) = 1.e-009, for the tip).
The obtained CV should be similar to those of Figures 3 and 4. If necessary, repolish the
electrode surface until you obtain an acceptable CV.
49
50
40
I(nA)
30
20
10
0
200
0
-200
-400
-600
-800
E(m V) vs Ag/AgCl
Figure 3. Cyclic voltammogram of the reduction of 10 mM Fe(NO3)3 in 1M H2SO4
at a carbon fiber tip (a = 5.5 µm).
32
27
22
17
I(µ A)
12
7
2
-3
-8
-13
-18
1000
800
600
400
200
0
-200
E(mV) vs Ag/AgCl
Figure 4. Cyclic voltammogram of the reduction of Fe3+ at a 3-mm glassy-carbon
(10 mM Fe(NO3)3 in 1M H2SO4).
2. Position Carbon Tip Over Substrate
1. Install the cell, fill it with the solution, and introduce the reference and auxiliary
electrodes in solution. Secure the UME in its holder, use SECM Probe Control software to
50
roughly (but safely) position the tip at ~1 mm above the substrate. Attach all electrodes to
the bipotentiostat.
2. Bias the substrate at a potential at which reaction (2) is diffusion controlled (e.g.
+800 mV). Bias the tip at -800 mV and record the diffusion limiting current (it,∞) value.
Choose Probe Scan Curve technique to move the tip towards the substrate at 10 µm/s
(forward on the z axis). Stop the approach when the diffusion limited current is increased by
20%. Be very careful to avoid crashing the tip on the substrate.
3. When the tip current is 1.2 it,∞, the UME tip is about 2.5 radii (~14 µm) away from
the substrate surface. Choose Probe Scan Curve and set appropriate parameters, move the
UME tip back at 5 µm/s (reverse on the Z axis) until the diffusion limited current decreases
to 1.05 it,∞.
4. Set the new parameters to: scan rate = 0.2 µm/s, and approach the tip toward the
substrate on the z-axis. If Probe Scan Curve technique is chosen, start the scan and stay
alert in order to stop the motion of the tip when it touches the substrate surface (the tip
current will either reach the overload limit or significantly deviate from the ideal approach
curve), otherwise, you can choose Probe Approach Curve technique and set appropriate
parameters for this experiment. Stop the motion and save the data in a file appropriately
named.
5. If the maximum feedback obtained is below 5, the tip has to be moved laterally to
a smoother area. Move the tip away from the surface until the current decreases to 1.05it,∞.
Change the scanning direction to either y or x forward, change the scan rate to10 µm/s. Set
the travel distance to 200 µm.
6. Using Probe Scan Curve technique, start the lateral motion (x or y-direction) of
the tip. Two situations can arise: (1) the tip gets closer to the surface or (2) the tip moves
away from the surface. In the first case, stop the motion, move the tip backward (z reverse)
a few, and then resume the lateral motion having set the axis to y or x again. In the second
case, bring the tip a few microns closer to the surface after the scan is finished.
7. Scan laterally until a flat region is reached. Over a flat region, a constant feedback
(for example it = 1.05it,∞) should be observed during the lateral scan.
8. Repeat steps 3 and 4 to collect an approach curve. The maximum feedback
current should be at least 5it,∞. Now you are ready for data collection.
3. Obtain Approach Curves at Different Substrate Potentials
The UME is now a few radii away from the substrate. The reduction of Fe(III) at the
tip and oxidation of Fe(II) at the substrate are diffusion controlled. Use the following
procedures to obtain approach curves at different substrate potentials:
1. Bring the electrode to the starting position (id = 1.05it,∞).
1. Decrease the substrate potential by 50 mV.
2. Wait for ~1 min for the substrate surface to equilibrate (the diffusion layer will form
only when the potential of the substrate is sufficiently low to allow for reduction of
Fe3+).
3. Move the tip in the z forward direction at a scan rate of 0.2 µm/s.
4. Save the data in an appropriately named file.
5. Repeat steps 1 through 5 until the substrate potential is +350 mV.
4. Extract the Rate Constant of the Irreversible Reaction
Use the provided spreadsheet to analyze the data. Open the Finite_Kinetics.wks
document which will be our template for fitting the data.
51
1. Paste the data from the first file (substrate potential at +800 mV) into the appropriate
columns i.e. “Z” and “i_diff”. Copy and paste the formulas for the normalized current and
distance.
Fit the simulated and experimental approach curves by modifying the values of the three
parameters in their corresponding cells. These parameters are (1) the offset = value of Z at
the substrate surface (in microns); (2) it,∞; (3) the value of the dimensionless parameter Λ =
kf,sa/D, i.e., the normalized rate constant of the substrate reaction.
2. Save the file with an appropriate name and record the value of Λ. Close the file and
reopen the template Finite_Kinetics.wks. Repeat steps 1 through 3 with all data files
obtained.
3. Open a new spreadsheet. In the cell $A$2 write “E0 =”; fill the appropriate value in
the right adjacent cell.
4. Type “substrate potential” in cell A9. Fill cells $A$10:$A$19 with the different
substrate potentials (+800 mv to +350 mV).
5. Type “over potential” in cell B9. Fill cells $B$10:$B$19 with the formula “=A10$A$2”.
6. Type “Λ” in cell C9. Copy the Λ values obtained from the fitted curves into the
$C$10:$C$19 cells. Type “ln[Λ]” in cell D9. And fill cells $D$10:$D$19 with the
corresponding formula.
7. Plot ln(Λ) vs. over potential. Fit the curve with a linear trend line. According to the
k 0 a (1 − α )nF
formula
ln[Λ ] = ln(
)+
( E substrate − E 0′ )
D
RT
the slope and the intercept should give the values of α and ko. The values for a and D are: a
= 5.5x10-4cm, D = 4.8x10-6cm2/s.
lnΛ
Compare your Tafel plot and the values of kinetic parameters to those in Figure 5.
Figure 5. Tafel plot [lnΛ vs. (Es – Eo′)].
(Adapted from Bard, A. J.; Mirkin, M. V.; Unwin, P. R.; Wipf, D. O. J. Phys. Chem. 96,
1861 (1992).)
52
7. Copper Etching and Imaging
REFERENCES
“Scanning Electrochemical Microscopy”, Bard, A. J. and Mirkin, M. V. (eds.), Marcel
Dekker, N.Y., 2001, Chapter 13, p. 593-627.
OBJECTIVES
To etch micrometer-sized pits in Cu and image them by SECM; to produce a pattern (lines)
and image them; to see how the tip/substrate distance and the tip size affect the size of
generated features.
INTRODUCTION
High-resolution etching and modification of metal and semiconductors is of
technological importance in the fabrication of microelectronic devices and is usually carried
out by such techniques as photolithography and wet etching. SECM in the feedback mode
has recently been used as an alternative approach for high resolution etching of metal and
semiconductors. The principle of etching via SECM feedback is shown in Figure1.
UME
Fe(phen)32+
Fe(phen)33+
Fe(phen)32+
Fe(phen)33+
Cu2+
Cu
Fig. 1. Principles of copper etching by the SECM in the feedback mode.
An ultramicroelectrode (UME), which is immersed in a solution containing a redox
couple, e.g., Fe(phen)32+ (phen = 1,10-phenanthroline), is moved close to a substrate to be
etched (Cu in this case). The potential of the UME is controlled so that the redox couple is
oxidized at the tip at a diffusion-controlled rate. When the tip is far away (several tip
diameters) from the substrate surface, a constant, steady-state current is established within
several seconds. However, as the tip is brought close to the substrate, i.e., within a few tip
radii, an increase in the steady-state current (a positive feedback current) is observed due to
the regeneration of the redox mediator via electron transfer at the surface. As a result, copper
dissolution occurs, which is limited to the diffusion range of the oxidized mediator. Since
53
the feedback current depends on the tip/substrate separation, the distance between the tip
and the substrate can be determined and controlled by measuring the changes in the tip
current. The resolution of the etched patterns is governed by the tip diameter and
tip/substrate distance. Other parameters affecting the etching size and shape include the
concentration of the redox couple and the dwell time of the tip.
In this experiment, SECM is used to produce a micrometer-sized pattern (pits or
lines) on Cu substrate and image them. The effect of the tip/substrate distance and the tip
size on the size of generated patterns is studied.
Reagents and Equipment
Etching solution: 1 mM Fe(phen)3(ClO4)2 in 10 mM acetate buffer (pH 4.0)
i) Copper substrates: Sputtered copper films (thickness ~1,500 Å) on silicon
wafer
Tips: Platinum wire (10 and 25 µm diameter) heat-sealed in glass tube under vacuum and
then beveled to produce SECM tips
Reference electrode: Ag/AgCl (3 M KCl)
Counter electrode: Platinum wire
CHI900 SECM
Optical Microscope: Olympus BH-2
Experimental Procedures
Instrument Set-up
The SECM cell consists of Teflon machined to accept an O-ring in a cylindrical
cavity surrounding a hole in the cell bottom (see Figure 2). Rinse the cell with Millipore
water. Sonicate the copper substrate in acetone for 3 min and rinse it with Millipore water,
then dry it in a steam of nitrogen and mount it in the cell with the O-ring and the cell base.
Mount the cell on a cell-holder plate and place it on the SECM stage. Insert a Pt counterelectrode and a reference electrode into the holes in the SECM Teflon cell.
Pressure
Figure 2. The SECM cell used in etching experiments.
54
Etching Micrometer-sized Pits in Cu
(1) Mount a clean 25-µm diameter tip on the z-piezoelectric driver. Bring the tip
manually close to the copper substrate. Then add the etching solution into the cell. Record
CV waves of the tip in the etching solution with the tip potential scanned between 0.7 and
1.0 V vs. Ag/AgCl at a scan rate of 100 mV/s. The potential corresponding to the diffusionlimited plateau in the CV wave can be used as the etching potential (~ 1 V).
(2) Follow the procedure described in lab experiment 2 to position the tip so that it is
~ 8 µm from the substrate. Briefly, approach the tip quickly to the substrate with the Probe
Approach Curve technique. Once the program has stopped automatically, move the tip
upwards from the surface a distance of 100 µm, using the SECM Probe Control icon. Then
let the tip approach the substrate at 0.5 µm/s using the Probe Scan Curve technique.
Stopping the tip manually when the tip current is 2 it,∞ (d ≈ 8 µm). Be very careful to avoid
crashing the tip on the substrate during this process!!!
(3) Scan the tip laterally over the substrate forward and backward (in both x and y
directions) for 200 µm with tip based at the etching potential. Record the tip current as a
function of the tip position. Gently adjust the screws under the cell holder plate until the tip
current versus tip position curve is flat in both x and y direction. In this way, the optimum
alignment of the tip to the substrate can be achieved.
(4) Keep the tip ~ 8 µm from the substrate. Choose the Amperometric i-t Curve from
the Electrochemical Techniques window. See Figure 3 for the parameter setting. Click the
RUN icon to etch Cu for 10 min.
(5) In order to etch a pit at a different position on the Cu surface, move the tip
backward in x-direction for 40 µm. Then repeat step (4) to etch a new pit.
(6) Repeat step (5) until 4 pits are obtained. In order to study the effect of the
tip/substrate separation on the etching shape, the last two pits can be etched with the tip held
at ~ 6 and 10 µm instead of ~ 8 µm from the Cu substrate.
Figure 3. Parameters for copper etching.
55
Imaging the Etched Pits in Copper
(1) Current vs. tip position dependencies
Position the tip so it is ~ 8 µm above the Cu surface. Choose Probe Scan Curve from
the Electrochemical Techniques window. This technique allows scanning of the tip laterally
over the pits to obtain a one-dimensional current profile. The tip is set at the etching
potential and scans above the pits in Cu substrate in the x-direction at the scan rate of 10
µm/s (see Figure 4 for all parameters). Save the file in your folder after running the program.
Figure 4. Parameters for One-dimensional scan
(2) Topographic Images
Keep the tip ~ 8 µm from the substrate. Choose Scanning Electrochemical
Microscope from the Electrochemical Techniques window. Set the parameters as shown in
Figure 5. By moving the tip laterally, determine the starting point for imaging so that all pits
are inside the imaging range (200 × 100 µm). Click the RUN icon to obtain the topographic
images of the etched pits. Save the data to your folder.
56
Figure 5. Parameters for topographic imaging.
Etching of Cu with Tips of Different Diameters
Move the tip upwards in z-direction for 500 µm. Take out the 25-µm-diameter tip
from the tip holder. Disconnect the electrodes. Take the Cu substrate out of the cell, rinse it
with Millipore water, dry it in a steam of nitrogen, and store it in a vial.
Mount a new Cu substrate in the cell and use a 10-µm-diameter platinum electrode
as the tip to etch Cu and image the pits by following the same procedure as described above.
Note that for a tip with smaller diameter, the etching and imaging should be performed at
closer tip/substrate distance (e.g., 4 µm from Cu surface for a 10-µm-diameter Pt tip).
Correspondingly, some parameters shown in Figures 4 and 5 should be changed (e.g., the tip
approaches Cu at a slower rate of 0.25 µm/s, the imaging range may be 80 × 40 µm if three
pits are in a line and the distance between two pits is 20 µm).
Etching Lines in Cu (optional)
Bring a 10-µm-diameter tip close (~ 4 µm) to the copper substrate and scan it over
the copper surface (e.g., in x-direction) at 0.1 µm/s to etch a line that is 100 µm long using
the parameters shown in Figure 6. Save your file after the etching is finished.
Shutdown Procedures
Move the tip upwards in z-direction for 500 µm. Disconnect the electrodes and rinse
them with Millipore water. Take the Cu substrate out of the cell, rinse it with Millipore
water, dry it in a steam of nitrogen, and store it in a vial. Dispose the waste etching solution
in the waste bottle. Clean up your work space.
Optical image of the etched Cu
Observe the topography of the Cu pits with an optical microscope and take a picture
of the pits.
57
Figure 6. Parameters for etching a line in Cu.
Discussion and Questions
(i)
Estimate the diameters of the pits from the one-dimensional current profiles or
from the topographic images of the pits. Are the data identical to those measured with
optical microscopy? If not, explain why.
(ii)
Based on the results obtained in the experiment, discuss the parameters that
affect the etching size and shape.
58
8. Imaging Electrochemical Activity by GenerationCollection Modes
REFERENCES
“Scanning Electrochemical Microscopy”, Bard, A. J.; Mirkin, M. V. (eds.), Marcel Dekker,
N.Y., 2001, Chapters 4 and 6.
OBJECTIVES
In this exercise, you will learn how to use other modes of operation of SECM rather than the
feedback mode to obtain images of electrochemical activity for material-sensitive electrode
reactions. You will see how to visually evaluate and compare the activity of catalysts for
hydrogen evolution reaction (HER) using the Substrate Generation-Tip Collection (SG-TC)
mode, and for oxygen reduction reaction (ORR) using the Tip Generation-Substrate
Collection (TG-SC) mode.
MATERIALS REQUIRED
- CHI900 SECM Instrument and X-Y-Z piezo-inchworm stage.
- External floating power source.
- Teflon SECM cell.
- Pt tip, diameter = 25 µm.
- Reference electrode: Hydrogen Reference Electrode (HRE) in 0.5 M H2SO4 and PH2 = 1
atm.
- Counter-electrode: Pt wire.
- Substrate: Pt (diameter = 127 µm) and Au (diameter = 100 µm) disks electrically
connected and embedded in glass.
- Solution: 0.5 M H2SO4 deareated with argon.
INTRODUCTION
You have seen in previous experiments that the SECM feedback mode is highly
sensitive to the intrinsic activity of the electrode material for the probe reaction. This allows
one to perform kinetic studies of electrode reactions on varied materials as well as to image
different catalytic activity of heterogeneous electrodes. A limitation of this operation mode
is that the experimental conditions must guarantee that the feedback loop can be detected.
This happens only when the tip-reactant (substrate-product) concentration is low enough to
keep the tip reaction controlled by diffusion of the reactant, and when there is no tip-product
(substrate-reactant) initially present in the solution. If those conditions are not fulfilled, the
feedback mode can not be used. However, other SECM configurations allow one to study
electrode reactions that are inaccessible through the feedback mode.
Substrate Generation-Tip Collection (SG-TC) Mode
In this mode the tip behaves as a passive sensor probing the concentration profile of
the product of a process occurring at the substrate. The imaging capability of this mode has
been mainly exploited to image activity of biological materials (cells, enzymes, etc.) and
molecular transport across membranes. It can also be used to image the concentration profile
of electro-generated species at the substrate, as illustrated in Figure 1 for the HER reaction
in acidic medium that will be studied in this experiment. The substrate is held at a potential
to reduce H+ to H2 (though forming no bubbles), and the tip is held at a potential such that
59
the substrate-generated H2 is oxidized back to H+. As the reaction at the tip is diffusioncontrolled, the tip current is proportional to the concentration of H2. On the other hand, the
concentration profile of H2 around the substrate is a function of its electrocatalytic activity
for the HER. Thus, imaging this profile using the tip current will give us a visual image of
the activity of the substrate material as catalyst for this reaction.
Figure 1. Scheme of the SG-TC mode for imaging the HER activity.
Although the concentration profile changes in time, it reaches a quasi steady-state in a short
time if the substrate electrode is sufficiently small.
The expected behavior of the tip current during a long-direction scan is shown in
Figure 2. An anodic contribution (negative current) will be added to the background tipcurrent during the oxidation of the substrate-generated H2. Successive long-direction scans
will define an image of the H2 concentration profile.
60
Figure 2. Expected behavior of the tip current during a long-direction scan.
Tip Generation-Substrate Collection (TG-SC) mode
Although this mode has rarely been used for imaging purposes, you will apply it to
image the activity of different materials for the ORR in acidic medium, basically doing what
is shown in Figure 3.
Figure 3. Scheme of the TG-SC mode for imaging the ORR activity.
The substrate potential is held at a value where oxygen can be reduced to water.
Since the solution initially contains no oxygen, the substrate current is negligible. When a
UME tip is placed close to the substrate and a constant oxidation current (iT) is applied to
the tip, water is oxidized to oxygen on the UME, and a constant flow of oxygen is generated
61
at the tip. The tip current must be sufficiently small to prevent the saturation of the solution
by oxygen and the subsequent formation of bubbles. When the oxygen reaches the substrate
surface, it is reduced at a reaction rate that depends on the substrate potential and its
electrocatalytic activity. The substrate current is governed by the flow of oxygen from the
UME tip surface, reacting at the substrate and being lost by lateral diffusion toward the bulk
solution.
The expected behavior of the substrate current during a long-direction scan is shown
in Figure 4. In this case, a reduction current (positive) will be added to the background
substrate current when the substrate reduces the tip-generated O2. Successive long-direction
scans recording the substrate current will produce an image of how good the substrate
material is for this reaction.
Figure 4. Expected behavior of the substrate current during a long-direction scan.
In the following experiment you will evaluate and compare the electrocatalytic
activities of Pt and Au for the HER and the ORR in strongly acidic medium by using the two
modes of operation described. Note that we can not use the feedback mode to study these
reactions in these conditions. A substrate containing two disks (~ 100 µm diameter), one Pt
and the other Au, separated ~ 500 µm will be used. Both disks will be simultaneously
polarized to verify the reaction under study, which will proceed at a different reaction rate
on each material. That difference will be detected during the imaging procedure.
1. General Experimental Steps
1- Assemble a 4-electrode SECM setup. Place the Pt tip, a Pt-Au substrate, the Pt
counter-electrode and the HRE in their respective holders and slots. Align the two substrate
microdisks either vertically or horizontally, since this will allow you to scan a smaller area.
Fill the cell with 0.5 M H2SO4 solution that is not deaerated. Make sure that the reference
electrode is in contact with the solution.
2- Perform an electrochemical cleaning of the substrate by cycling its potential (CV)
between 0 V and 1.9 V vs. HRE at 0.5 V/s for 10 minutes.
62
3- Move the tip laterally as close to the microdisks as you can by using the “SECM
Probe Control” window.
4- Approach the tip to the glass surface by using the negative feedback for the
reduction of O2 in solution. Set the following parameters for the approach curve experiment:
ET = 0.1 V vs. HRE, substrate off and Scan Rate = 3 µm/s. Position the tip at 40 µm from
the glass surface. Note that the tip-substrate distance is much larger than those used for
feedback imaging. Remove the air from the solution by bubbling with argon or change the
solution to a deareated one.
5- Visually finding the right initial position for the scan is difficult, so you will need
to find a good starting position by doing preliminary images (using one of the described
modes) at high scan rates.
2. Imaging the HER activity using the SG-TC mode
The cathodic responses of Pt and Au electrodes in an acidic solution are qualitatively
shown in Figure 5.
Figure 5
Pt is more active than Au for the HER, which is reflected in higher cathodic currents.
You will try to image this difference since there will be a change in the concentration profile
of H2 around Pt respective to Au due to the fact that the flux of H2 coming from Pt will be
higher than the one coming from Au. As a sensing probe you will use a Pt tip polarized at a
potential anodic enough to oxidize H2 under total mass-transfer control.
1- Connect the “Electrode 1” socket (green cover) to the tip and the “Electrode 2”
socket (black cover) to the substrate.
2- Select the “Scanning Electrochemical Microscopy” technique and open the
“Parameters” window.
3- Select the parameter values as shown in Figure 6. Note that the Scan Rate is really
high (600 µm/s) and the area to be scanned is large (1000 x 1000 µm) to search for a good
starting location prior to obtaining the final images. If you already found one, you do not
need to use these conditions. Note also that the substrate potential is set at a value where H+
reduces only at a slow rate to prevent bubble formation. Remember to check the option
“Return to Origin after Run” to always start the scans from the same point.
63
Figure 6
Aquire images as many times as necessary until you can find the disks and position the tip at
a good starting point.
4- Change the parameters to scan in a single direction an area of 500 x 700 µm (or
700 x 500 µm depending on the orientation of the disks) at a scan rate of 300 µm/s.
Section 1.02 5- Obtain images at three different substrate potentials: 0 V, -0.05 V
and -0.065 V vs. HRE. Analyze and discuss the results.
3. Imaging the ORR activity using the TG-SC mode
The i vs. E responses of Pt and Au UMEs for the ORR in an acidic solution saturated
with O2 are qualitatively shown in Figure 7.
While Pt is already active for the reduction of O2 at potentials lower than 0.8 V vs.
HRE, it requires much more cathodic potentials (lower than 0.5 V vs. HRE) to see a
significant activity of Au for this reaction. You will try to image this difference by using the
TG/SC mode: generating O2 at a Pt tip and keeping the Pt-Au substrate at potentials where
both are equally active, Pt is more active than Au, or just Pt is active.
To perform this experiment you need to keep the tip current constant while
measuring the substrate current for a given substrate potential. To do that, you will use a
floating power source (a 9 V battery) and a big resistor to supply the constant current
flowing between tip and counter electrodes. You can change the tip current by just changing
the resistor.
64
Figure 7
1. Connect the electrodes as shown in Figure 8. “Electrode 1” socket (green cover)
is connected to the substrate, while “Electrode 2” socket will remain disconnected. Connect
the positive pole of the power source to the tip and the negative pole to the counter
electrode.
Figure 8
2. Prior to starting each imaging scan, clean the substrate electrode by cycling its
potential between 0 V and 1.9 V vs. HRE at 0.5 V/s for 2 minutes. This procedure is
65
required because Pt and Au lose the activity for ORR very fast due to adsorption of
impurities in the solution.
3. Select the “Scanning Electrochemical Microscopy” technique and open the
“Parameters” window.
4. Select the parameter values as shown in Figure 9. The Scan Rate is high (600
µm/s) and the area scanned is large (1000 x 1000 µm), but this is done primarily to find a
good starting position prior to obtaining the final images. If you already found one, you do
not need to use these conditions. Note that the “Substrate Electrode” frame is not activated
because we are going to use a normal, three-electrode configuration. In fact the substrate is
connected to the probe cable, and its parameters are defined in the “Probe Electrode” frame.
The potential of the substrate is set at a value where O2 can be reduced on both disks. The
Quiet Time is set at 120 s to allow the background substrate current to stabilize. Remember
to check the option “Return to Origin after Run” to always start the scans from the same
point.
Figure 9
5. You need to turn the tip on to evolve O2 at a current of approximately -50 nA. By
using a 9 V battery this current can be obtained with a 100 MΩ resistor. Start the imaging
experiment and after a while during the quiet time turn the power source on. When the scan
is finished, turn the power source off. Aquire images as many times as necessary until you
can find the disks and position the tip at a good location.
6. Change the parameters to scan in a single direction an area of 500 x 700 µm (or
700 x 500 µm, depending on the orientation of the disks) at a scan rate of 300 µm/s.
7. Following the same procedure and always cleaning (by potential cycling) the
substrate as described before, obtain images at three different substrate potentials: 0.0 V, 0.1
V and 0.5 V vs. HRE. Analyze and discuss the results.
66
9.
Biological System: SECM of the Reaction Center of
Rhodobacter Sphaeroide Chromatophores
REFERENCES
-Cai, C.; Liu, B.; Mirkin, M. V.; Frank, H. A.; Rusling, J. F. Anal. Chem. 2002, 74, 114.
-Munge, B.; Pendon, Z.; Frank, H. A.; Rusling, J. F. Bioelectrochemistry 2001, 54, 145
-Ishikita, H.; Morra, G.; Knapp, E.-W. Biochemistry 2003, 42, 3882
-Okamura, M. Y.; Paddock, M. L.; Graige, M. S.; Feher, G. G. Biochimica et Biophysica
Acta 2000, 1458, 148
-Yasukawa, T.; Uchida, I.; Matsue, T.; Biophysical J. 1999, 76, 1129.
OBJECTIVES
1. LEARN TO WORK WITH OXYGEN SENSITIVE SYSTEMS
2. EXTRACT HETEROGENOUS RATE CONSTANT OF REACTION FROM
APPROACH CURVES
3. STUDY THE EFFECT OF THE PH ON THE RATE CONSTANT OF
REACTION
4. DISCUSS THE EFFECT OF REDOX POTENTIAL ON THE RATE
CONSTANT
5. FIT APPROACH CURVES TO HETEROGENOUS KINETIC THEORY
INTRODUCTION
In this experiment, Rhodobacter spaheroide chromatophores will be studied.
Rhodobacter sphaeroides are nonsulfur bacteria that differ from cyanobacteria and other
eucaryotic photosynthesizers in that they lack photosystem II. This means that they are
anoxygenic: they do not use water as an electron source to produce O2 photosynthetically.
Instead they use organic molecules.
In the bacterial membrane several transmembrane proteins (L, M and H) form a
scaffolding structure upon which unbounded co-substrates are organized. These cosubstrates are directly involved in the electron transport events and are also responsible for
the establishment of a transmembrane proton gradient that is required for ATP synthesis.
The rhodobacter reaction center is probably the best understood photosynthetic reaction
center since it was the first to be purified, to offer solved X-ray structures and on which
pisecond kinetic methods were used. Here we offer an oversimplified description of the
workings of the reaction center (Figure 1). The reaction center has two pairs of each cosubstrates that can undergo electron transport (A and B) across the membrane. Branch A is
the dominant pathway and so Figure 1 only describes this path.
67
e-
2H+
Fe
hν
Periplasm
QbH2
e-
P+
P*
Fe
QbH2
Qb
Cytoplasm
Cytochrome bc1
e-
B
B-
Qa
eQa-
Qb
e- (2)
QbH2
Reaction Center
P
e- (1)
QbH
Qb-
H+
H+
Qb
Figure 1. Diagram of the electron and proton transfer in photosystem I of rhodobacter
sphaeroide.
Briefly, when light excites the special reaction center chlorophyll P870, (PÆ P*), an
electron is then donated to a bacteriopheophytin (B) and results in P+. From there, the
electron flows down to a first ubiquinone molecule, Qa, and then is transferred to a second
ubiquinone, Qb, situated 15 Ån away from the first one. Upon electron transfer, the
semiquinone (QbH), is formed from the extraction of a proton from the cytoplasm. A second
electron transfer process is repeated via the same pathway to form the ubiquinol (QbH2)
molecule and at the same time cause a gating conformation change. The second
ubiquinone/ol moiety is positioned in a more hydrophilic pocket and is more loosely held to
the reaction center then Qa. Upon reduction to ubiquinol, QbH2 diffuses across the
membrane to the periplasmic surface where it is oxidized back to the quinone and releases
the protons in the periplasm. Such movement of protons from the cytoplasm media across
the membrane generates a transmembrane proton gradient that is required for ATP synthesis.
Finally, the electron flows back to the special reaction center to regenerate the ground state
(P+ + 1e- Æ P).
Reaction centers are interesting because they help us understand how photosynthetic
systems convert and store energy. Fundamental questions still remain about this
photosystem I. For example, how it is possible for the strong reducing electron acceptors, Qa
and Qb to avoid the autooxidation by oxygen. Also, what is the effect of pH on the overall
rate of the reaction?
In this experiment, the electron acceptors, Qa and Qb, face outward into the bulk
solution and the special pair reaction center (P) is facing inward. Figure 2 presents the
experimental scheme. By choosing the redox mediator (O) carefully, one can reduce it at the
tip to generate, R, such that it will spontaneously react with Qb (and possibly Qa), to
regenerate R and form Qb(H2).
68
Figure 2
From such experiments three things can be studied:
1)
The effect of pH on the reaction rate
2)
The effect of the redox potential of the couple on the reaction rate
3)
The possible autooxidation of Qb(H2) to Qb in presence of trace oxygen.
1. Working with Oxygen Sensitive Couples
The redox couples used in this experiment are reduced at very negative potentials.
To measure a well-behaved response, the solutions need to be free of oxygen. Oxygen can
be reduced around these potentials and this second faradaic process can lead to the
superimposition of a background current onto the desired voltammogram. To remove
oxygen from solution one can either replace oxygen by an inert gas such as argon or
nitrogen or use sodium sulfite to react with dissolved oxygen.
This option was often used in polarography but cannot be used when studying
oxidations. It also has the disadvantage of possibly interacting with the redox couple of
interest, therefore, great care must be taken. The former option is the preferred option, but
contrary to the latter one, it is very difficult to remove all traces of oxygen from the solution
and then perform the measurement under the same conditions.
Today’s experiment will allow you to work with the first (inert gas) oxygen removal
approach and also to use a Au UME as a working electrode. When using very negative
couples, Pt is not always the material of choice since it catalyzes the hydrogen evolution
reaction. This reaction occurs at higher overpotentials for gold, carbon and mercury.
2. Extract the Heterogenous Rate Constant of Reaction from the
Approach Curves
-
-
Turn on the SECM instrument and software.
Screw in the metal plate onto the SECM head without the SECM cell (Figure 3, #1).
69
4
1
1
3
Metal plate
2
SECM Head
Figure 3. Fixing the petri dish on SECM setup
Put a fair amount of double sided tape where the SECM cell would usually go
(Figure 3, #2). Fix the top of the petri dish such that the hollow end is facing down in the
center of the hole (Figure 3, #3). Using double sided tape fix the reaction center substrate
onto the top of the petri dish (Figure 3, #4) such that the chromatophore film is under the tip
holder.
Place the Au tip in the holder and place it 1 mm above the film. Make a horizontal
mark on the side of the tip using a marker to remember this position. Remove the tip from
the holder.
Cover the petri dish with a piece of parafilm such that the cover is tightly sealed
around the petri dish and stretched flat (Figure 4, #1).
Heat the end of a glass pipette tip with a cigarette lighter and make a small hole in
the parafilm to accommodate the tip. Referring to the mark made on the tip, put the UME in
the holder at the appropriate height (Figure 4, #2).
-
Gas
Pipet
tip
2
5
Ag/AgCl
reference
Tip holder
Pt wire
4
3
1
Petri Dish
Figure 4. SECM set-up for biological experiment.
70
Make similar holes for the auxiliary and electrode. Using the universal support and
clamps secure the two electrodes and insert them in the holes (Figure 4, # 3 and 4). Make
sure that the reference electrode is not touching the bottom of the petri dish.
Connect the UME, reference and auxiliary electrode to their respective leads. The
green lead is the working, the white the reference the red the auxiliary.
Make a last hole for the glass pipette gas inlet connected to the inert gas tank (Figure
4, # 5). Secure and insert the pipette in its designated hole.
Using a small Pasteur pipette and a bulb, fill the ¾ of the petri dish with the prepared
0.1 mM menadione solution.
Gently, bubble inert gas through the petri dish for 7 minutes. If the flow is too hard,
the bubbles can damage the chromatophore film, and can also push the solution out onto the
parafilm or leak all the way to the metal base. If the latter occurs, a short circuit can occur
and the set up will need to be dismantled, dried and put back again.
Select the techniques command in the setup menu or click on the T toolbar button.
Select cyclic voltammetry. Press OK.
•
• Select the parameters command in the setup menu or the list button on the toolbar. The
dialog box should pop up. Set the initial potential to -0.1 V, the high potential to –0.1 V, the
low potential to –0.6 V, the initial scan polarity to negative, the scan rate to 0.02 V/s, the
segments to 6, the quiet time to 10, the sensitivity to 1e-9. Press OK.
71
After bubbling, move the gas pipette over the solution such that an inert gas blanket
is maintained over the solution. Acquire the voltammogram.
If the shape of the CV does not resemble that in Figure 5a, one has two choices: to
bubble some more inert gas through the solution and try again or hand polish the tip with 0.5
µm alumina. Usually, it is the oxygen level that is the main cause of distortion. For the 25
µm Au electrode that you are using, the limiting current of your CV should be about twice
as that seen in Figure 5a.
7.00E-10
Menadione + 2e- +2H+ --> Menadiol
(0.1 mM)
At 10 µm Pt tip
Co(III) Sepulchrate + 1e- --> Co(II) Sepulchrate
(0.1 mM)
At 25 µm Au tip
I (A)
5.00E-10
B
A
3.00E-10
1.00E-10
-0.1
-1.00E-10
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
-0.9
E (V vs. Ag/AgCl)
Figure 5. Voltammogram of 0.1 mM menadione at 10 µm Pt tip (A) and of 0.1 mM
cobalt sepulchrate trichloride at 25 µm Au tip (B).
72
Once a good CV is recorded, perform a fast approach to the surface using the probe
scan curve technique. The tip is much further away then in past experiments so you might
have to approach for a long time. From your CV, evaluate a good potential to apply at the
tip, set the sensitivity to 1*10-9 A and the % ratio to 120.
Approach the tip to the surface and save the fast approach.
Using the SECM probe control, retract the tip 300 µm away from the film. Move the
tip 500 µm in the x direction. Lower the gas pipette into the solution and bubble inert gas
through the solution for 5 minutes.
Because of time restraints, we will not make you perform a control approach curve
of the Au tip to the clean plastic petri. In such a control, complete negative feedback is
expected. Whenever a different substrate support is used, it is good practice to measure such
a control to be certain that the new support behaves like an insulator. Figure 6 presents this
previously measured control.
-
Negative feedback of 100 µM Menadione
1.2
1
0.8
I/Io
RG = 5.09
0.6
04/10/03
working: 10 um Pt
Ref: Hg/Hg2SO4
aux: 0.5 mm Pt wire
subs: petri dish
0.01M PBS pH=7.5
degass argon
0.4
0.2
experimental
0
0
2
4
d/a
6
8
10
Figure 6. Insulator feedback approach of a UME to a clean petri dish for a 0.1 mM
menadione solution. The red line represents the experimental approach curve while the
blue line is the microdisk theory for a tip with an RG of 5.09.
Open the probe scan curve program to measure a slow approach curve. Use the same
potential as in the fast approach, same sensitivity, select the amperometric mode, the z
direction, a travel distance of 500 µm, an increment distance of 0.0999 for a total rate of
approach of 3 µm/s and a 10 sec quiet time.
Record the approach curve. (Remember that you must stop the approach curve
manually.)
Figure 7 presents a typical approach curve.
-
73
Feedback fom chromatophore using 100 µM menadione
1.2
1.15
I/Io
1.1
1.05
1
theory
0.95
Experimental
0.9
0
0.5
1
1.5
d/a
2
2.5
3
3.5
4
Figure 7. Approach of a 25 µm Au tip to the chromatophore film in 0.1 mM menadione
solution, at a rate of 3 µm/s. The experimental curve (red) is fitted to the heterogenous
kinetics theory (blue) for a microdisk.
Retract the tip, save the approach and move to another location. Bubble inert gas for
five minutes between each approach. You will notice that the starting limiting current is not
always at the same value. This is the effect of trace oxygen in the solution.
You will then be able to fit them to the heterogeneous kinetics theory to extract a rate
constant for the reaction of menadiol and the electron acceptor (Qb and possibly Qa).
Since you have recorded many curves in different spots, the extracted constants can be
averaged and the standard deviation can be computed. This can be done at the end of the
session.
Dismantle the set-up, rinse all the electrodes with ample amounts of Milli Q water
and gently hand polish the tip with 0.05 µm alumina.
3. Study the Ph Dependence of the Rate Constant
As seen in Figure 1, the formation of ubiquinol is dependent on the availability of
protons in solution. Therefore, taking the approach curves in lower pH solutions should
yield a faster rate constant of reaction.
Repeat the same procedure as in Section 2 for the prepared menadione solutions that
are at different pH. Start with pH 4 and then repeat the experiment with pH 9. For each new
pH use a new chromatophore substrate.
4. Discuss the Effect of Redox Couple Potential on the Rate Constant
How fast the reduced mediator reacts with the electron acceptor ubiquinone depends
on the redox potential of the mediator. In the case of the menadione, complete positive
feedback was not observed, which means that the rate of reaction of menadiol with
ubiquinone is slower than the mass diffusion of menadiol to the chromatophores. In the case
of cobalt sepulchrate trichloride, Figure 5b, the redox potential of this mediator is shifted
about 300 mV more negative than that of menadione. We should expect the response to be
faster and so a stronger feedback than menadione and a faster rate constant of reaction.
Experimentally, this is the case as seen from Figure 8.
74
Feedback fom chromatophore using 100 µM menadione vs cobalt
sepulchrate trichloride
1.5
theory
I/Io
1.4
menadione
1.3
Cobalt sepul.
1.2
contheory
1.1
1
0.9
0
0.5
1
1.5
d/a 2
2.5
3
3.5
4
Figure 8. Comparison of the feedback observed with a 25 µm gold tip while
approaching rhodobacter chromatophores in 0.1 mM menadione (red) and 0.1 mM
cobalt sepulchrate trichloride. Both experimental approaches agree well with theory.
5. Fit Approach Curves to the Heterogenous Kinectics Theory
Convert the approach curves of Section 2 into text format
Open the kinetics Excel program in the lab8 folder on your computer
Open one of the text format approach curves. Select the columns and copy them.
Paste them under the columns d(exp) and I(exp) highlighted in yellow in the kinetics
worksheet.
Go down to the end of your paste values to see if there are superfluous values from
another fit and if so, remove the unwanted data. If the pasted column is longer than the
previously saved one, extend the two columns with the red heading such that it is at equal
length to the experimental values. To extend it, copy two rows, select the area to fill in with
the mouse and paste. The formulas included in the original rows should be transferred to the
empty selected ones.
The red label column normalizes the experimental distance and current such that
they can be fitted to the heterogeneous kinetic theory. The light blue parameters are the
adjustable parameters where iss is the steady state current used to normalize all the
experimental currents, a is the radius of the metal wire of your tip (in this case 12.5), offset
is the zero distance adjustment, ka/D1 is the dimensionless kinetic parameter.
At the end of the fit, the true rate constant of reaction, k (cm/s), will be extracted by
multiplying the fitted parameter by the diffusion coefficient of the mediator (cm2/s) and
divided by the radius of the metal wire (12.5*10-4 cm).
Copy the first I(exp) from the yellow column into the iss value. This is your first
approximation of the steady state current.
Copy the last value of the d(exp) column into the offset value.
a is the radius of the electrode and that is not an adjustable parameter.
By changing the kinetic parameter, the offset and only if absolutely necessary the iss
(but this must be a very small adjustment), the experimental curve should be fitted to theory.
Monitor your progress on the graph.
This particular program requires for the fit to be done manually.
75
Take the fitted kinetic parameter multiply by 3.7E10-6 cm2/s, the diffusion coefficient
of menadione in PBS buffer and divide by 12.5E-4 cm, the radius of the Au wire. This will
give you the rate constant of reaction in cm/s.
76
Appendix: Basics of Cyclic Voltammetry
REFERENCES
Bard, Allen J; Faulkner, Larry R. Electrochemical Methods, Fundamentals and
Applications, second ed., John Wiley and Sons, Inc., N. Y., 2001.
Principle
Since mass transfer, that is, the movement of material from one location in solution
to another, plays a big role in electrochemical dynamics, we shall describe briefly its three
modes. Mass transfer arises either from difference in electrical or chemical potential at the
two locations or from movement of a volume element of solution. The modes of mass
transfer include convection (stirring or hydrodynamic transport, temperature, or density
gradient) and migration (movement of a charged species under the influence of an electric
field). We minimize convection by not stirring the solution and keeping it at a constant
temperature. We also minimize migration by using a supporting electrolyte at a
concentration much higher than that of the redox couple studied. The supporting electrolyte
carries the bulk of the current through the solution and makes it less probable that any
charged redox species will travel to or from the vicinity of the electrode surface by
migration. With the first two mass transfer modes effectively minimized, the redox species
reaches the electrode surface mainly by diffusion, and so the current is diffusion-controlled
Cyclic voltammetry is one type of potential sweep electrochemical (EC) method.
We shall first describe CV on a large plane electrode (e.g., an inlaid Pt disk of diameter >
100 µm). By changing the potential of the working electrode vs. a reference electrode at a
constant rate, one can scan over a wide potential range while measuring the resulting
current, and determine the potential where electrode reactions occur. After scanning
through the potential region in which one or more electrode reactions take place, the
direction of the linear sweep is reversed and the electrode reactions of intermediates and
products, formed during the forward scan, can often be detected. Because of this reversal,
this technique is referred to as cyclic voltammetry (CV) rather than linear sweep
voltammetry (LSV) where no reversal takes place. CV has become a very popular technique
for initial EC studies of new systems and has proven very useful in obtaining information
even for fairly complicated electrode reactions. For an example of the current response to
this potential scan, see Figure 1.
As an example, we shall start by looking at a simple, reversible, diffusion-controlled,
electron transfer reaction in a solution containing ferrocenyltrimethylammonium (FcTMA+)
hexaflurophosphate. We start a CV scan from an initial potential, Ei, below the oxidation
potential of FcTMA+ and scan toward higher potential, no (or very small as compared with
the desired value) current will flow until the potential is high enough (in the vicinity of E0’)
to oxidize FcTMA+ to ferroceniumtrimethylammonium (FcTMA2+). As the potential
continues to grow more positive, the surface concentration of FcTMA+ drops; hence the flux
to the electrode surface (and the current) increases. As the potential moves pass E0’, the
concentration of FcTMA+ at the electrode surface approaches zero, mass transfer of
FcTMA+ to the surface reaches a maximum rate and the magnitude of the current will
increase rapidly and reach a peak anodic current, ipa, at the potential Epa, when the
concentration of FcTMA+ at the electrode surface approaches zero. This peak
77
(b)
(a)
Epc
E (+) →
Eτ
Epa
0
1 mM FcTMA+
in
0.2 M KCl
Switching time, τ
Time →
Figure 1 (a) Cyclic potential sweep. (b) Resulting cyclic voltammogram.
results from competition between two processes: the increase in the oxidation rate of
FcTMA+ as the electrode potential continues to increase and the decrease in the amount of
FcTMA+ close to the electrode surface. Consequently, the anodic current peaks at a
potential slightly higher than the oxidation potential of FcTMA+. The size of the peak
anodic current depends on a number of variables, such as the area of the working electrode,
A, the number of electrons involved in the electrode reaction, n, the bulk concentration of
the reacting species in solution, CR, its diffusion coefficient, DR, and the scan speed of the
potential, v.
Meanwhile, we are still scanning toward higher (positive) potential. After the anodic
current has peaked, the magnitude of the current drops back down due to depletion of the
electroactive species near the working electrode surface. At faster potential scan rates, a
higher current peak is observed since there is less time for depletion before reaching the
maximum electrode reaction rate. Now we reverse the direction of the potential sweep. At
first, the potential is still higher than the oxidation potential of FcTMA+, so there will still be
anodic current until the potential becomes low enough to reduce FcTMA2+ back to FcTMA+.
A peak cathodic current, ipc, is observed at Epc. Once the potential returns to the point at
which the cyclic voltammogram was started, the cycle is complete.
For a reversible electrode reaction, the potentials at which we see the anodic and
cathodic peaks of a cyclic voltammogram are independent of scan rate. We define the halfwave potential, E½, as the potential halfway between the anodic and cathodic peaks:
E½ = (Epa + Epc)/2
(1)
The half-wave potential is within a few mV of the formal reduction potential Eo’ for a
reversible couple, as long as the reduced and oxidized forms have approximately equal
diffusion coefficients (see Eq. 2.)
E½ = Eo’ + (RT/2nF)ln(DR/DO)
(2)
In which R is the gas constant and F is the Faraday constant. The difference between Epa
and Epc of a reversible reaction is a fixed quantity, dependent on n.
Epa – Epc = (2.22RT/nF) = (59.0/n) mV at 25oC
78
(3)
Thus, a one-electron reversible process should ideally exhibit a peak separation of 0.059 V
at 25oC. The peak current of a diffusion-controlled reversible cyclic voltammogram is
described by the Randles-Sevcik equation:
ipa = kACR(n3DRv)½ at 25oC
(4)
Where ipa is the anodic peak current (Amps), k is a constant (= 2.69 x 105 Cmol-1V-½); A
(cm2), n, DR (cm2s-1), CR (molcm-3) and v (Vs-1) have defined above. Thus, ip (as well as the
current at any other point on the wave) is proportional to v½. This property indicates
diffusion control.
For linear sweep voltammetry (LSV) on an ultramicroelectrode (UME), the resulting
current is
i = i(plane) + 4nFDRCRaφ(σt)
(5)
Where i(plane) is the current for a macro-plane electrode, a is the radius of the UME and
φ(σt) is a tabulated function (for a reversible electrode reaction, see Table 6.2.1 in Allen J.
Bard and Larry R. Faulkner. Electrochemical Methods, Fundamentals and Applications,
second ed., John Wiley and Sons, Inc., New York, 2001, which is a function of σt =
(nF/RT)(E – E½). For large scan rate and with macro disk, the i(plane) term is much smaller
than the spherical diffusion correction term (the second term in Eq. 5, and the electrode can
be considered as planar under such condition. However, for a UME where a is small, the
second term will dominate at a sufficiently small scan rate. This is true when
v << RTDR/nFa2
(6)
Under such conditions, the voltammogram will be a steady-state response independent of
scan rate (See Figure 2 for a typical steady-state CV).
Figure 2. A steady-state CV at a 10-µm diameter Pt UME in 0.2 M KCl solution
containing 1 mM FcTMA+.
79
For a = 5 µm, DR = 10-5 cm2/s at room temperature (298oK), the right side of Eq. 6
has a value of 1000 mV/s, thus a scan rate of ~ 100 mV/s or slower will permit an accurate
recording of state-state voltammograms and the steady-state current is given by
ist = 4nFDRCRa
(7)
Since the limit described in Eq. 6 depends on the square of the disk radius, with very
small UMEs, one requires a high sweep rate to obtain a peak-shaped voltammogram. You
should study the transition from typical peak-shaped voltammograms at a fast sweep rate in
the linear diffusion region to steady-state voltammograms at a slow scan rate with UME in
the experimental section.
Instrumentation
Three electrodes will be used: working, reference, and auxiliary (sometimes called
counter electrodes). The bipotentiostat module of a CHI900 SECM is used to control the
potential of the working electrode relative to a reference electrode. We use a silver/silver
chloride electrode, consisting of a silver chloride-coated silver wire suspended in an aqueous
solution of potassium chloride (3.0 M) and silver chloride, as the reference electrode. An
inert polarizable metal, e.g., platinum, coil with a much greater surface area than the
working electrode, is used as the auxiliary electrode to carry most of the current from the
working electrode. We use either a 25-µm-diameter platinum microdisk or a Pt disk of 1-2
mm diameter as the working electrode, since it is polarizable and unreactive in the desired
working potential range.
Chemicals
Alumina (0.3 and 0.05 µm) slurry
1 M sulfuric acid
1 mM (~ its solubility) ferrocenemethanol (FcMeOH) in 0.2 M KCl
Equipment
Electrodes: Pt macrodisk and UME, Pt wire, and Ag/AgCl reference electrode (stored in 3.0
M KCl)
Floppy disk or zip disk
Polishing cloth
CHI900 SECM
Sample vials
Waste Disposal
Waste Chemicals
All solutions containing FcMeOH
Sulfuric acid solutions
Waste Container
FcMeOH/0.2 M KCl
Acidic Electrochem
Location
Hood
Hood
Experimental Procedures
There are various components to these procedures. You should divide the work
amongst you so that the experiment is completed more quickly; for example, the working
electrodes can be polished while the instrument is being setup.
80
1. The CHI900 SECM bipotentiostat is used to apply varying potential to the
electrodes. Turn on the SECM bipotentiostat module without turning on the inchworm
translator controller module.
2. Awaken the computer if it’s sleeping and double click the SECM900 icon on the
desktop to display the SECM Command window. Click the T(echniques) icon to display the
Electrochemical Techniques window as shown in Figure 3. Choose Cyclic Voltammetry
and press the OK button to accept it.
Figure 3 Electrochemical
techniques listed in CHI900
Polishing of platinum disk electrodes (see also the section about SECM
Tip Preparation)
3. The platinum disk electrodes are gently polished successively with 0.3 and 0.05µm alumina slurry placed on a polishing cloth as follows: Shake the bottle of 0.3 µm in
Millipore water to mix well. Put a small amount of slurry onto the polishing cloth. Polish
the electrode by making small circles in the alumina paste on the polishing cloth. Keep the
electrode as vertical as possible and do not press too hard, or you could rip the polishing
cloth or break the microelectrode. Polish the electrode in this manner this for a few min.
Use the Millipore water bottle to rinse off all alumina from the electrode. Aim the water
directly at the electrode surface.
4. Repeat step 3 using the second polishing cloth and 0.05 µm alumina slurry. Rinse
the electrode very thoroughly with water as in step 3. After sonicating the polished electrode
very well in water, store it in Millipore reagent water before the final electrochemical
cleaning.
81
Electrochemical cleaning of platinum macrodisk electrode
5. Rinse the SECM Teflon cell (attached with a platinum coil as the counter
electrode) well with Millipore water. Gently push the polished and cleaned Pt disk
electrode through the central hole of the SECM cell. If necessary, wrap the electrode with a
thin piece of Teflon tape to prevent the leakage of solution. After mounting the cell on a
cell-holder plate, place it on the SECM stage. Three-quarter fill the cell with 1 M sulfuric
acid.
6. Remove the reference electrode from its storage vial and rinse well with Millipore
water. Do not leave the Ag/AgCl electrode out of liquid for very long. Position it in the hole
for the reference electrode in the SECM Teflon cell.
7. Make connections from the SECM to the electrodes as follows: white to the
reference electrode, red to the counter electrode and black to the macro platinum disk.
Leave the green line unconnected.
8. Click the Parameters icon to set up experimental parameters for CV as shown in
Figure 4.
Figure 4. CV parameters for
electrochemical cleaning of a Pt
macrodisk electrode.
High E = 2.0 V
Low E = -2.0 V
Scan Rate = 0.1 V/s
9. Click the RUN icon in the window. This will scan the potential of the macro Pt
disk forwards and backwards between -3 and +3 V at a scan rate of 100 mV/s. As the
potential of the electrode is scanned relative to the reference electrode, materials on the
surface of the electrode are reduced and oxidized. This will help to clean up the electrode
surface. One may be able to see small bubbles forming on the electrode surface as water is
oxidized and reduced. Tap the electrode to shake the bubbles off the Pt disk to keep the
electrode surface exposed for cleaning. Leave the electrode cleaning for 5 min.
82
10. Stop the scan by clicking the STOP icon. Disconnect the reference electrode and
remove it from the cell. After, rinse it thoroughly with Millipore water and store it in the
storage vial.
11. Rinse the EC-cleaned electrode (without removing it from the cell) and the
SECM Teflon cell very well with Millipore water, so there is no acid remaining on the
electrode and cell. Dispose the waste in the appropriate waste container.
Cyclic voltammetry of ferrocenemethanol at Pt macrodisk electrode
12. Three-quarter fills the cell with 1 mM FcMeOH in 0.2 M KCl. Remove the
reference electrode from its storage vial and rinse well with Millipore water. Position it in
the hole for reference electrode in the SECM Teflon cell.
13. Make connections from the SECM to the electrodes as step 7.
14. Set the CV parameters for Pt macrodisk electrode
Init E [V] ---------------------- 0
High E [V] -------------------- +0.5
Low E [V] --------------------- 0
Initial Scan Polarity ---------- Positive
Scan rate [V/s] ---------------- 0.1
Sweep Segments -------------- 2
Sample Intervals [V] ---------- 0.02
Quiet Time [sec] --------------- 20
Sensitivity [A/V] --------------- 1.e-005
Electrode 2
Potential [V] --------------------default
Sensitivity [A/V] ---------------- default
----- Scan
-
----
Check --- swap Electrode 1 and 2
Check --- Scan Complete Cycles
15. Click the RUN icon in the window to start the potential scan. After waiting for
20 sec, the potential will start to scan anodically from an initial potential of 0 V. The
computer starts collecting data, while the potential of the macrodisk electrode starts
scanning from Init E up to High E, which can be seen on the computer screen. When the
potential has scanned all the way to High E, it will reverse and scan back to Low E, which is
the same as Init E in this example. When it has completed this cycle, the program will stop
itself and display the collected data.
16. The x-axis of the displayed voltammogram is the potential vs. the Ag/AgCl
reference electrode, so it varies between Low E and High E, the two potential extremes of
the potential scan. The y-axis shows the current flow at the working electrode. Your cyclic
voltammogram should look like that shown in Figure 1 with the curve shifting to a less
positive potential, since FcMeOH has a less positive E0’ than FcTMA+; if not, talk to your
instructor. You may have to re-clean your electrode (If re-cleaning the electrode is
necessary, disconnect the working and reference electrodes; rinse them with Millipore
water; and store the reference electrode in the storage vial; dispose the waste in the
appropriate waste container, and re-clean the electrodes based on steps 5 – 11). If your
cyclic voltammogram has extra bumps, you have some contaminants in your solution,
probably from the un-cleaned electrode or cell.
83
on
17. Once your cyclic voltammogram looks good, save your data (ask your instructor
for help for saving data, if necessary).
18. Record cyclic voltammograms over the same potential range, using scan rates of
200, 100, 70, 50, 20, and 10 mV/s. Save your data.
19. Disconnect the electrodes; rinse them with Millipore water and store the
reference electrode in the storage vial. Dispose the waste in the appropriate waste containers
and rinse the cell thoroughly.
20. Measure the diameter of the Pt macrodisk electrode with micrometer or using
optical microscopy. Calculate its area (in cm2). You need it for the calculation of the
diffusion coefficient of FcMeOH.
Electrochemical cleaning of platinum microelectrode
21. Mount a polished and cleaned (see steps 3 and 4 for the procedures) 25-µm
platinum microelectrode (if available) in the tip holder of the SECM. Three-quarter fill the
cell with 1 M sulfuric acid. Adjust the vertical height of the microelectrode to immerse it in
the solution. Gently tighten the electrode using your fingers (Beware that the glass tube of
the microelectrode is very brittle and fragile!)
22. Set the CV parameters as shown in Figure 5 and repeat the same procedures as
used
Figure 5. CV parameters for
electrochemical cleaning of a
25-µm diameter Pt
microelectrode.
High E = 2.0 V
Low E = - 2.0 V
for the Pt macrodisk (steps 9-11) for electrochemical cleaning of the Pt tip.
23. After EC-cleaning the microelectrode, remove it from its holder and rinse it very
well with Millipore water. Do not let the microelectrode touch any surfaces. Rinse also the
SECM Teflon cell and the reference electrode very well with Millipore water, so there is no
84
acid remaining on them. Dispose the waste in the appropriate waste container. Store the
reference electrode in its storage vial.
24. Re-install the setup to run cyclic voltammetry at Pt microdisk electrode: mount
the polished and cleaned 25-µm platinum microelectrode in the tip holder of the SECM.
Properly adjust the vertical height of the microelectrode and tighten the electrode with your
fingers very gently.
Cyclic voltammetry of ferrocenemethanol at Pt microdisk electrode
25. Three-quarter fills the cell with 1 mM FcMeOH in 0.2 M KCl. Remove the
reference electrode from its storage vial and rinse well with Millipore water. Position it in
the hole for reference electrode in the SECM Teflon cell.
26. Make connections from the SECM to the electrodes as follows: white to the
reference electrode, red to the counter electrode and green to the platinum microdisk.
Leave the black clip unconnected.
27. Carry out CV on 25-µm diameter Pt microelectrode by setting
Init E [V] ---------------------- 0
High E [V] -------------------- +0.5
Low E [V] --------------------- 0
Initial Scan Polarity ---------- Positive
Scan rate [V/s] ---------------- 0.1
Sweep Segments -------------- 2
Sample Intervals [V] ---------- 0.02
Quiet Time [sec] --------------- 20
Sensitivity [A/V] --------------- 1.e-009
Electrode 2
Potential [V] --------------------default
Sensitivity [A/V] ---------------- default
----- Scan
-
----
uncheck --- swap Electrode 1 and 2
Check --- Scan Complete Cycles
28. Following steps 15-17, once your cyclic voltammogram looks well, save your
data.
29. Record cyclic voltammograms over the same potential range, using scan rates of
1000, 500, 200, 100, 50, 20, 10 and 5 mV/s. Save your data.
Shutdown Procedures
30. Disconnect the electrodes; rinse them with Millipore water and store the
reference electrode in the storage vial. Dispose of waste in the appropriate waste
containers and rinse the cell thoroughly. Leave the cleaned cell, working and counter
electrodes to air dry. Cover the polishing cloths. Clean and dry all glassware. Clean up
your work area.
31. Measure more precisely the diameter of the Pt UME, which will be used in data analysis.
Data Analyses and Discussions
1. Determine the potentials of the anodic and cathodic peaks, Epa and Epc in volts vs.
E ’ of the Ag/AgCl reference electrode (+ 0.197 V vs. the standard hydrogen electrode at
0
85
on
25oC). From your measured Epa and Epc, calculate your experimental half-wave potential,
E½, using Eq. 1. Compare it to the accepted formal reduction potential E0’.
2. Calculate the number of electrons involved in this redox reaction from Epa and Epc
using Eq. 3. Round your answer to the nearest integer.
3. Measure the anodic peak current, ipa, for various scan rates and plot ipa vs. the
square root of the scan rate. Find the equation of the line best fitting your data and use the
slope of your fit to calculate the diffusion coefficient of FcMeOH in cm2/s, using the
Randles-Sevcik equation, Eq. 4, In this calculation, you need to know the electrode area, the
concentration of FcMeOH and the n value you calculated from step 2. Watch out for the
units in Eq. 4.
4. Measure the steady-state anodic current at UME and calculate the diffusion
coefficient of FcMeOH from Eq. 7. You need to know n (calculated from step 2), the radius
of the microdisk (in cm) and the concentration of FcMeOH (in mol/cm3) for this
measurement. Compare your diffusion coefficient of FcMeOH from the two different CVs
(for macro- and microdisk electrodes).
5. Explain your observation of the effects of scan rate on CV, in particular the
change in the anodic and cathodic currents for macroelectrodes. Discuss the changes in the
shapes of CVs (from steady-state to peak-shaped or vice versa) at UME with different scan
rate. Estimate the upper v for permitting the accurate recording of a steady-state CV for a
25-µm diameter Pt disk.
6. Explain qualitatively what the effect of stirring would be on the observed cyclic
voltammograms for macrodisk and UME. This is one of those interesting questions
frequently inquired in the operation of the SECM.
86
ACKNOWLEDGEMENTS
Acknowledgment is made to the Donors of the American Chemical Society Petroleum
Research Fund for support of this program.
Special thanks to CH Instruments for use of the SECMs and associated materials.
87
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