Fabrication Iridium Oxide pH Electrode by using Lift-Off
Method and Electrodeposition
(Physics 390A final project report)
Done by Na Zhang
2003.12.12
The aim of this project is to fabricate Iridium Oxide pH electrode through Lift-Off
Method and electro-deposition method.
Because of the time limit and lack of material, the first part which is to fabricate the
electrode substrate through lift-off method cannot be finished on time. But the basic
procedures are illustrated in the following parts. The electro-deposition has been done
based on five 6-month old electrode substrates. Two of them are found to be good and
the results are shown at the last part. Also a literature research on Lift-off method has
been done and another report will be handed in.
Next, Let's see the basic elements and my results for the electro-deposition:
1）
Electro-deposition Solution
Basically, most of the solutions commonly used today are based on the original
Yamanaka solution. The original Yamanaka solution is prepared as followed [1]: (a)
Dissolve Iridium chloride hydrate (IrCl4. H2O: 0.15 g) in 100ml of water by magnetic
stirring for 30 minutes. (b) Add 1ml of aqueous peroxide solution (H2O2: 30 wt.%)
and stir for 10 minutes. (c) Add Oxalic acid ((COOH)2. 2H2O: 0.5g) and stir for 10
minutes. (d) Add Anhydrous potassium carbonate to adjust the solution's pH to 10.5 (e)
Stand the resulting solution for at least two days to stabilize it.
The composition of the solution should be adjusted each time corresponding to
different situations. For example, the solution that Marzouk used composed of 75 mg
of IrCl4. H2O in 50 mL distilled water, 0.5 mL of 30% hydrogen peroxide, 365 mg of
potassium oxalate hydrate, and another small portions of anhydrous potassium
carbonate to raise the pH of the solution to 10.5. [2] Meanwhile, some other
researchers modified the original Yamanaka solution by using K3IrCl6 or IrCl3, [3, 4],
but their solution didn't offer apparent advantages.
2）
Electrode Substrate
The basic process for fabricating electrode substrate is illustrated below. Lift-off
method is used to make clear windows on the platinum, while cover the other parts
with SiOx. The titanium is used to improve the surface adhesion. The reason for the
deposition of SiOx layer is that the platinum layer cannot adhere well with the PDMS
when the electrode is put into the micro-fluid system. The Lift-Off Method is also
well explained in the 'Lift-off Method' report.
1. Clean glass substrate
glass
Clean glass substrate
Step 1
Pt (90 nm)
2. Apply
Electrodeposition
(Ti—10 nm, Pt—90 nm)
Ti (10 nm)
Step 2
PR
Pt
3. Apply Photoresist
glass
Ti
Step 3
UV light
Photomask
4. Expose to UV light
PR
Pt
Ti
glass
Step 4
PR
5) Chemical development of PR
Pt
glass
PR
6) Ion Etching
glass
Step 6
7) Chemical Development
glass
Step 7
LOL-2000
8) Apply Lift-off Photoresists
(For example, LOL-2000
on the bottom, and S-1813
on the top)
S-1813
glass
Step 8
Mask
9) Align mask, Expose to UV
light and Develop, notice that the
undercut profile is formed
After development
glass
Step 9
10) Cover some parts with
Al foil, and e-beam
deposition of SiOx on the
other parts
Cover some some of
the non-active part with Al foil
Al foil
Deposite SiOx
glass
Step 10
11) Apply Lift-off Process
And the electrode substrate is done
This is the active part
of the electrode, small
features are involved
SiOx
glass
Step 11
3）
Sensor Fabrication Procedure
A CHI Instruments: Model 660A electrochemical Workstation is used to control the
potential at the surface of the working electrode relative to the reference electrode
potential. The workstation is directly controlled by the computer and all experimental
data is collected by the computer.
Three electrodes are used during the electrodeposition process. They are working,
reference and auxiliary electrodes respectively. The reference electrode is a reactive
electrode and is non-polarizable.[5] We use a silver/ silver chloride electrode which is
a silver wire suspended in a saturated aqueous solution of potassium chloride and
silver chloride. When negative charge is applied to the electrode, the silver ions from
AgCl are reduced to silver metal; conversely, when a negative charge is removed
from the electrode, silver metal is oxidized to produce silver ions. The potential of the
working electrode can be maintained at the desired absolute potential by constant
comparison with the stable reference electrode. The amount of current through the
reference electrode needs to be limited to avoid a shift in its potential, which would
disrupt the stability of the system.[5] We use a platinum wire for our auxiliary
electrode. The auxiliary electrode is used to carry most of the current from the
working electrode. This electrode is usually an inert polarizable metal with a much
greater surface area than the working electrode, so as not to limit the current. [5]
The prepared electrode substrate is our working electrode. Except for the active
working area, all the other platinum areas are covered by PDMS. And the electrode
substrate on the glass is clung with another long plastic substrate on which there are
several metal channels. The platinum area is connected with the metal channels
through wires and make sure those channels are not connected to each other.
According to the literature, not all materials make suitable electrodes for all
electrochemical experiments. [5] Carbon and relatively inert precious metals, such as
platinum, mercury, and gold, are commonly used as working electrodes. Different
electrode surfaces do not have the same surface roughness, catalytic activity, potential
range, and affinity, which can have a considerable effect on the quality of the
observed signal. Platinum is one of the most commonly used electrode surfaces
because it catalyzes many reactions and does not suffer from oxide fouling.
At the beginning of the experiment, all of three electrodes are put into the vias in the
'electrode holder'. The reference electrode is put in the smallest via. Make connections
from the worksation to the electrodes as following: green to the working electrode (Pt
disk), red to the auxiliary electrode (Pt wire), and white to the reference electrode
(Ag|AgCl). The height of the electrodes is adjusted to make sure the bottom of the
electrodes are on the same level and the active area of the working electrode is under
the solution. The auxiliary electrode can be bended and move it as close as possible to
the working electrode, but not touch it.
Then open the workstation, open the software- ch660a.exe in the computer. Select
Chronoamperometry, and click OK. Choose Parameters under the Setup menu and
set the chronoamperometry parameters as follows:
Init E [V]= 0
High E [V] = 0.59 / 0.58
Low E [V] = 0
Initial Scan Polarity=Positive
Number of Steps = 1
Pulse Width [Sec] = 300/600
Sample Interval [Sec]= 0.5
Quiet Time [Sec]= 2
Sensitivity [A/V] = 1.e–007
Click OK. And Run the experiment by clicking on the play button. When the run is
finished, click the plot button, the Current vs. time figure will be on the screen.
One thing need to mention: there are two voltammetric methods which are commonly
used in electro-deposition. One is Chronoamperometry and another is cyclic
voltammetry. The first one is to observe the change in current with time after making
a step change in potential, and can be used to look at the diffusion of an electro-active
species.[5] While the cyclic voltammetry is to get the current response of a solution
over a range of potential. The peaks of a cyclic voltammogram can tell us a lot about
the identity and reactivity of the species in solution. By changing the potential of the
working electrode at a constant rate, we can scan over a large potential range while
measuring the resulting current, and determine the potentials where electrode
reactions occur for this solution. More explanations of the cyclic voltammogram can
be found in Harris' book: 'Quantitative Chemical Analysis'.[6] A lot of paper use
cyclic voltammogram during their electrodeposition procedure.[2,7,8] But for our
experiment, we just used Chronoamperometry method. I think if possible the cyclic
voltammetry method may be tried next time.
4）
Test pH sensitivity
This procedure is used to test the sensitivity of the electrode after the
electrodeposition. Three pH buffer solutions (pH=6, 7, 8) are used in a specific order.
Here several old electrode (probably 6 month old) are used, and only two are found to
be good. The results are shown in the next section.
The same instrument-Model 660A electrochemical Workstation is used. And this time
only two electrodes are needed. One is our working electrode, the other is the
reference electrode which is the same silver/ silver chloride electrode.
After open the software in the computer, select Open Circuit Potential-Time, and
click OK. Choose Parameters under the Setup menu and set the parameters as
follows:
Runtime [Sec]=630 /315/1260
Sample Interval [Sec]= 0.1
High E [V] = 0.5
Low E [V] = 0.1
Click OK. And Run the experiment by clicking on the play button. During the
procedure, every 30s/ 15s /60s, click the pause button, change the pH buffer solutions
according to a specific order. Potential vs. Time figures are got at the end of the
experiment.
5）
Results and Discussions
a) Electrodeposition
The following figure is the Current vs. Time plot got from Chronoamperometry
method:
(1) One process with parameters of high potential of 0.59V and pulse width of
300s.
figure 1. Current vs. Time for high E=0.59 and Pulse width=300
The electrode fabricated after this procedure is examined under the microscope:
figure 2. Electrode fabricated after the procedure (high E=0.59 and Pulse width=300)
From this one, we can already see the blue deposits of AEIROF.
Later, the electrode is put back to the electrode holder, and the same procedure is
taken for another 300s. The following figure is the current vs. time figure,
figure 3. Current vs. Time for another 300s
And under the microscope, the electrode image is illustrated as following:
figure 4. Electrode fabricated after another 300s
From the upper image, we can see the dark green deposit film is formed. The
dard greenish-blue layer shows the stable characteristic of IrO2.
(2) Another electrode is fabricated using similar electrodeposition process, but
with parameters of high E=0.58V and pulse width=600s.
figure 5. Current vs. Time
(high E=0.58 and Pulse width=600)
The electrodes under microscope for these two process are illustrated as
following:
figure 6. Electrode fabricated (high E=0.58 and Pulse width=600)
Figure7. Larger image
The blue-purple deposit film is formed. The nature of the stabilization process of
the electrodeposition solution that involves the blue color formation is not
exactly known and could be attributed to the formation of an iridium-oxalate
complex.[1, 2]
b) Test pH sensitivity
Five old electrodes are tested, and only #1 and #5 electrodes show stable
characters. For each of the electrode, we use three pH (6, 7, 8) buffer solution,
and change the solution every 15 / 30 / 60 second. The order of the switching is:
6,7,8,6,7,8,7,6,8,7,6,8,6,8,6,7,6,7,6,8,6. The results are:
The following is the Potential vs. Time plot for #1 electrode, and the solutions
are changed every 30 seconds. We can see that for pH=6 buffer solution, the
potentials basically are on the same level, the same for pH=7 and 8 solutions.
And
this
shows
the
stable
of
the
pH
electrode.
figure 8. Potential vs. Time for electrode #1 with time interval=30s
At the end of each time interval, the potential is recorded and the average value
are calculated for pH=6,7 and 8 solutions respectively. And we plot the average
potential against the pH value, we get the following figure:
EMF vs. pH (630s_30s_1)
350
y = -73.425x + 759.23
300
EMF, mV
2
R = 0.996
250
Series1
Linear (Series1)
200
150
5.5
6
6.5
7
pH
7.5
8
8.5
figure 9. EMF vs. pH for electrode #1, time interval=30s
The error bars in the figure are the deviations for the potential. We can see the
electrode shows linear responses in series of buffer solutions in the pH range
from 6 to 8. After making the linear fit, we get the slope is -73.425, which is
usually called the super-Nernstian slope.
Later the time interval for 15 seconds and 60 seconds are tried, the mean
potentials are calculated, and the results are shown as follows:
figure 10. Potential vs. Time for electrode #1 with time interval=15s
EMF vs. pH (315s_15s_1)
400
y = -65.625x + 759.78
2
R = 0.9985
EMF, mV
350
Series1
300
Linear (Series1)
250
200
5.5
6
6.5
7
7.5
8
pH
figure 11. EMF vs. pH for electrode #1 with time interval=15s
figure 12. Potential vs. Time for electrode #1 with time interval=60s
8.5
EMF vs. pH (1260s_60s_1)
400
y = -70.475x + 763.33
EMF, mV
350
2
R = 0.9969
300
250
Series1
Linear (Series1)
200
150
5.5
6
6.5
7
pH
7.5
8
8.5
figure 13. EMF vs. pH for electrode #1 with time interval=60s
For different time intervals, we can see the slopes are different, showing that for
the pH response of the electrode will change according to different time intervals.
Meanwhile, noticing that the error bar is changing for different time intervals, we
calculate the average values of the error bar for each of the time interval and plot
them. From the average, the super-Nernstian slope for this electrode is
-69.525

3.9 mV/pH which is a little bit lower than the generally known super-
Nernstian slope (-70--90 mV/pH) for iridium oxide pH sensors. The reason for
this is probably because the electrode is really old and they are not conserved in
the pH=7 buffer solution.
Error vs. Time Interval (#1)
14
y = 84.453x
12
-0.7577
2
R = 0.9614
Error
10
Series1
8
Power (Series1)
6
4
2
0
0
10
20
30
40
50
60
70
Time Interval
figure 14. Error vs. Time Interval for electrode #1
The figure 14 shows that the larger the time interval, the less the error. From 15s
to 30s, the error is dropping down quickly, but for time intervals larger than 30s,
the error dropping slowly. For shorter time interval, the potential measured may
has a large deviation from the actual value. The larger the time interval, the more
stable potential will be measured.
The same procedure is done for electrode #5, and the results are got:
EMF vs. pH (315s_15s_5)
140
y = -54.825x + 432.29
120
R2 = 0.9995
EMF, mV
100
80
Series1
Linear (Series1)
60
40
20
0
-20 5
6
7
8
-40
pH
figure 15. EMF vs. pH for electrode #5 with time interval=15s
9
EMF vs. pH (630s_30s_5)
130
y = -54.95x + 434.6
2
R = 0.996
EMF, mV
110
90
Series1
Linear (Series1)
70
50
30
10
-10 5.5
6.5
pH
7.5
8.5
figure 16. EMF vs. pH for electrode #5 with time interval=30s
EMF vs. pH (1260s_60s_5)
EMF, mV
140
120
y = -55.925x + 460.11
100
R2 = 0.9976
80
Series1
Linear (Series1)
60
40
20
0
5.5
6
6.5
7
pH
7.5
figure 17. EMF vs. pH for electrode #5 with time interval=60s
8
8.5
Error vs. Time Interval
20
y = 1166.8x-1.5425
R2 = 0.9964
Error
15
Series1
Power (Series1)
10
5
0
10
20
30
40
50
60
70
Time Interval
figure 18. Error vs. Time Interval for electrode #5
The super-Nernstian slope for this electrode is -55.375

0.55 mV/pH. And the
error is also decreasing for larger time intervals.
Until now, we finish the electrodeposition part, and we see both of the electrode
show stable characters. The super-Nernstian slope is not included in the
generally known range for Iridium Oxide pH electrode, the reason for that are:
the electrodes are really old and they are conserved in the air rather than
conserved in the pH=7 buffer.
Reference:
[1]. K. Yamanaka, 'Anodically Electrodeposited Iridium Oxide Films (AEIROF) from
Alkaline Solutions for Electrochromic Display Devices', Jpn. J. Appl. Phys. 1989,
28, 632-637
[2] Sayed A. M. Marzouk, ' Improved Electrodeposited Iridium Oxide pH Sensor
Fabricated on Etched Titanium Substrates', Anal. Chem. 2003, 75, 1258-1266
[3]. M.A. Petit, V. Pichon, 'Anodic electrodeposition of iridium oxide films', J.
Electroanal. Chem. 1998, 444, 247-252.
[4]. J.M. Zhang, C.J. Lin, Z. D. Feng, Z.W. Tian, 'Mechanistic studies of
electrodeposition for bioceramic coatings of caldium phosphates by an in situ
pH-microsensor technique', J. Electroanal. Chem. 1998, 452, 235-240.
[5].http://www.cm.utexas.edu/academic/courses/Fall2003/CH456/Shear/NewCV.pdf
[6]. D. C. Harris, Quantitative Chemical Analysis (6th ed., W. H. Freeman, NY,
2003)