Anal. Chem. 1092, 64, 2688-2692
2888
Characterization of a Gold Minigrid Cell for Fourier Transform Infrared Spectroelectrochemistry: Experimental vs Digitally Simulated Response Philip B. Graham+and David J. Curran' Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003
A cell capable of recordingFourler transform Infrared (FTIR) spectra of the reglon near a gold mlnlgrld electrode surface durlng electrochemlcal experlments was developed and characterlzed udng the fertVferrocyanlde couple as a test system. The I R cell was of the total Internal reflectlontype. The absorbance-thne response to potentlalstep and potentlal sweep experlments was Investbated and the experlmentally determlned response compared to a dlgltal slmulatlon of the response at dMerent dlstances from the electrode surface. Thk allowed the dlstance from the electrode surface to the polnt of observatlonto be estlmated. Spectra were obtalned durlng potentlal sweep experiments at sweep rates up to 200 mV/s.
INTRODUCTION The use of infrared spectroscopy to obtain spectra of electrochemically generated species was first described more than 20 years ago.' One of the disadvantages of the IR region in comparison to the UV region is the inherently lower extinction coefficients and consequential need for higher concentrations of species in order to obtain a similar signalto-noise ratio. Infrared monitoring of electrochemical processes imposes restraints on the cell design in terms of materials, path length, and solvent systems. Most IR spectroelectrochemicalcell designscan be grouped into the following five categories: (i) Cells based on internal reflection elements that also function as electrodes.'-3 A substantial compromise between optical and electrode properties is required. (ii) Internal reflection cells witha thin, optically transparent electrode film deposited on the surface of the reflection element.4-7 With this design there is an inherent compromise between the conductivity of the electrode film (usually carbon or a vapor-deposited metal) and it's optical transparency. (iii)Cells with a gold minigrid working electrodesandwiched between two IR transparent windows,8-14 normally sodium + Present address: Eli Lilly & Co., P.O. Box 685, Lafayette, IN 47902.
(1) Marks, H. B.; Pons, B. S. Anal. Chem. 1966 38 (l),119. (2) Yaniger, S. I.; Vidrine, D. W. Appl. Spectrosc. 1986,40 (2), 174180. (3) Tallant, D. R.; Evans, D. H. Anal. Chem. 1969, 41 (6), 835-8. (4) Mattaon, J. S.; Smith, C. A. Anal. Chem. 1975, 47 (7), 1122-25. (5) Bancroft, E. E.; Sidwell, J. S.; Blount, H. N. Anal. Chem. 1981,53, 1390-94. (6) Genies, E. M.; Lapowski, M. J . Electroanal. Chem. Interfacial Electrochem. 1987,220,67-82. (7) Gottesfield, S.;Ariel, M. J . Electroanal. Chem. Interfacial Electrochem. 1972,34, 327-344. (8) Heineman, W. R.;Burnett, J. N.; Murray, R. W. Anal. Chem. 1968, 40 (13), 1974. (9) Bullock, J. P.; Boyd, D. C.; Mann, K. R. Inorg. Chem. 1987, 26, 3084-86. (10) Enger, S. K.; Weaver, M. J.; Walton, R. A. Znorg. Chim. Acta 1987,129, Ll-L3. (11) Heath, G. A,; Yellowlees, L. J.; Braterman, P. S.J . Chem. Soc., Chem. Commun. 1981, 6, 287.
chloride. Construction is often difficult because of the fragile nature of the salt windows and the difficulty of obtaining a good seal. Response times are very long; typically on the order of 30 s. (iv) Barrel-plunger type cells with a platinum disk working electrode were developed by and applied extensively.1c24 Advantages of this design are the capability to "tune" the solution thickness depending on the nature of the solvent and analyte and to separately observe surface and solution species by selective polarization. (v) A high-conversionflow-through electrode immediately prior to a conventional IR ceL26 This method uses a flowing stream to carry the electrogeneratedspeciesfrom the electrode to the IR cell. In this case a trade-off between conversion efficiency and residence time in the electrode is necessary. An older cell design by Laser and Ariel26 is apparently unique in the literature and allows the favorable electrochemical properties of the gold minigrid electrode to be combined with an internal reflection germanium crystal. The more recent introduction and widespread use of Fourier transform instrumentation has led to a number of publication~+-'~ dealing with IR spectroelectrochemistry. These all involvethin-layer cell designs for applicationsto a number of organic, inorganic, or organometallic systems. One of the inherent drawbacks of electrochemical studies is the nonspecific nature of the data obtained. Current-time responses, for instance, can only by inference provide information on the reactants and products of electrochemical transformations. Reactions following electron transfer are even less easily studied. It is therefore desirable to have a means of directly observing reactions at or near the electrode surface on a time scale that allows useful information to be obtained. The cell described here is an attempt to meet this (12) Coombe, V. T.; Heath, G. A.; MacKenzie, A. J.; Yellowlees, L. J. Inorg. Chem. 1984, 23 (21), 3423. (13) Brisdon, B. J.; Enger, S. K.; Weaver, M. J.; Walton, R. A. Inorg. Chem. 1987,26, 3340-44. (14) Kadish, K. M.; Mu, X. H.; Lin, X. Q.Electroanalysis 1989, 1, 35-41. .. (15) Pons,S.;Davidson,T.;Bewick, A. J.Electroanal. Chem.Interfaciul Electrochem. 1982, 140, 211-16. (16) Pons, S.:Davidson, T.; Bewick, A. J . Am. Chem. SOC.1983, 105. 180245. (17) Pons, S.; Datta, M.; McAleer, J. F.; Hinman, A. S. J . Electroanal. Chem. Interfacial Electrochem. 1984, 160, 369-76. (18) Foley, J. K.; Pons, S. Anal. Chem. 1985,57 (8), 945A-956A. (19) Foley, J. K.; Pons, S.; Smith, J. J. Langmuir 1985, 1, 697-701. (20) Korzeniewski, C.; Pons, S. J. Vac. Sci. Technol. B 1985, 3 (5), 1421-24. (21) Daschbach, J.; Heisler, D.; Pons, S. Appl. Spectrosc. 1986,40 (4), 489-91. (22) Bkhoo, S.;Foley, J. K.; Korenziewski, C.; Pons, S. J . Electroanal. Chem. Interfacial Electrochem. 1987, 233, 223. (23) Bewick, A.; Pons, S.;Kunmatau, S.; Russell, J. W. J . Electroanal. Chem. Interfacial Electrochem. 1984, 160, 47. (24) Bewick, A.; Pons, S. Advances in Infrared and Raman Spectroscopy; Wiley-Hayden: London, U.K., 1985. (25) Clark, B. R.; Evans, D. H. J . Electroanal. Chem. Interfacial Electrochem. 1976, 69, 181. (26) Laser,D.;Ariel, M. J . Electroanal. Chem.InterfacialElectrochem. 1973, 41, 381. ~~
0003-2700/92/0364-2688$03.00/0 0 1992 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 64, NO. 22, Platinum Wire Counter Electrodes\
f
AgiAgCl Reference Electrode Fiow Outlet
.-
Minigrid Contact I\Goid
+Flow
lniet
Minigrid O-ring
Figure 1. Vertical cross section of cell.
objective and to overcome some of the difficulties associated with other spectroelectrochemical cell designs. The ferro/ ferricyanide couple in aqueous media is used as a test system with a gold minigrid electrode on a ZnSe prism in a FTIR spectrometer.
EXPERIMENTAL SECTION Construction of the Infrared Cell. The main body of the cell was constructed of Teflon and was modeled after a commercially available stainless steel FTIR cell: PLC-19M,Harrick Scientific Co., Ossining, NY. It was machined to fit the base plate of Harrick nine reflection prism liquid cell. The prism was 45 mm long and 9 mm square. The body was secured to the base plate by means of four hexagonal-key screws. Four standard flow line fittings were drilled in the body of the cell so flow lines, reference electrodes, and the contact to the gold minigrid could be secured. The base of the zinc selenide prism (Harrick Scientific) was sealed with an O-ring between the Teflon body and base plate. The top cover was secured with eight cross-head screws and sealed by means of an O-ring (Viton). This material is unsuitable for use with some organic solvents such as THF, in which case Kalrez (Du Pont) O-rings could be used. The manufacturer (Buckbee-Mears)suppliesthe gold minigrid as a sheet packed between two polycarbonate sheets. Leaving this assembly intact, a strip of gold minigrid (7.0 cm X 0.7 cm) was cut out using an Exacto knife. The gold minigrid could be easilypositioned on the prism surfacewhen moistenedwith either water or acetone. Acetone was preferred as it would quickly evaporateleavingthe minigrid in place. A Teflonclip, constructed locally (9 mm X 11mm), was slid from the bottom of the prism over the gold minigrid by squeezing its oversize sides. Then a folded 0.1-mm-diameter gold wire was placed between the minigrid and Teflon clip. A small Teflon wedge on one of the unused prism faces secured this clip in place. The gold wire was threaded through one of the lower flow line fittings for later contact. The prism with minigrid in place was lowered into the Teflon body and the smaller lower O-ring placed around the bottom of the prism. The base plate was then secured and the seal checked for leaks. The minigrid contact wire was threaded through a 0.3-mm-i.d. Teflon flow line inside a male flow line fitting. A small piece of 0.8-mm4.d. tygon tubing sealed the flow line to the copper contact wire. The remaining lower threaded fitting was used for the flow inlet to the cell. The two upper threaded fittings secured the silver/silver chloride reference electrodes which were prepared by coating the silver wires in aqua regia for 10min until a coating of silver chloride was visible. The silver wires had previously been partially coated with heat shrink Teflon and force-fit into a standard male flow line connection (1/4-28, Ranin Instrument Co. Inc., Woburn MA). The two reference electrodes were shorted together, as were the two counter electrodes. Vertical cross-section views of the assembled cell are given in Figure 1to show adjacent sides of the prism. After securing the top plate and exit flow line fitting the cell was checked for leaks by pumping water through the cell at about 5 mL/min. Leaking fittings were reflanged or repaired with Teflon tape when necessary.
NOVEMBER 15, 1992 2680
Data Collection. Sequential collection of spectra can be achieved with the Mattson Cygnus 100FTIR spectrometer, either by using the software for gas chromatography-FTIR or by collecting data directly into the spectrometer's on-board buffer. The latter was used when high rates of data acquisition were required. Data collection time and the interval between spectra were determined by monitoring of the mirror position using an oscilloscope. Operation of the Cell. The cell,onceassembled,was mounted on adjustable rails in the spectrometer. Electrical contacts and flow line connections were made through holes drilled in the cell compartment cover. The holes were light-proofed using appropriate rubber grommets. The MCT or TGS detector output was optimized by adjusting the cell position while displayingthe realtime interferogram. When using the MCT detector the iris was adjusted to an indicated value of 0% which, with a wire gauze attenuator placed over the source outlet to the sample compartment, gave a peak-to-peak output voltage of around 13 V. This is close to the maximum acceptable value before saturation of the detector occurs. For initial studies, the TGS detector was used; however, the poor sensitivity and long responsetime (maximummirror velocity = 0.63 cm s-l)limited its application to bulk electrolysisand very slow (2 mV s-l) potential sweeps. The MCT detector could be very simplyinstalled in the spectrometerallowing mirror velocities up to 2.53 cm 8-1. A Masterflex pump (Cole-Parmer) was used to introduce ferricyanide solutions into the cell without the need to break the spectrometer purge. Between runs the cell was emptied and then refilled with fresh solution. A Princeton Applied Research Model 174 potentiostat and Fisher Recordall Model 4520 strip chart recorder were used to conduct the electrochemical part of the experiment. Digital Simulation of Response. A program to digitally simulate the concentration-time behavior at different distances from the surface of a planar electrode waa written in turboPascal and could be executed on an IBM or IBM-compatible personal computer. The program listing is available from the authors. The program is based on a model of diffusion between "boxes" of s~lution.~'Essentially the solution is divided into segments of size X extending in a perpendicular direction away from the electrode surface. The concentration at the surface, in the first segment, can be calculated from the electrode potential using the Nernst equation. Variousprocedures can be selected to vary the surface concentration to mimic a particular electrochemical technique. The program selects a value for the variable T, the time interval between successive calculations of the concentration-distance profile. An important assumption of this model is that diffusionduring the time interval Tcan onlyoccur between neighboring segments, which means the model diffusion coefficient (DM),as defined in eq 1below, must have a value less than 0.5.
DOis the diffusioncoefficientfor ferricyanide. In this program Tis calculated based on a value of 0.45 for DM. A result of this restriction is that forX = 1pm, Tmust be approximately0.7 ms. Anothervaluethat is set is "maxdist" which represents the number of segments that are needed to completely encompass the diffusion layer. This is calculated according to eq 2
maxdist = (4D,t)'/'/X (2) Since a very large number of calculations are performed and much data is produced, only a fraction of that data is saved in the output file. A two-dimensionalarray with three rows is used to calculate successiveconcentration-distance profiles. The f i s t two rows store the present and most recent profiles, while the third can be used for storing the derivative data (if needed) before writing to an output file. (27) Kissinger, P. T.; Heineman, W. R. Laboratory Techniques in Electroanalytical Chemistry; Marcell Dekker: New York, 1984;Chapter 16.
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ANALYTICAL CHEMISTRY, VOL. 64,
NO. 22, NOVEMBER 15, 1992
Table I. Diffusion Profile Averaging Times for Different Grid Sizes no. of wires/in.
hole size* (rm)
wire sizen (rm)
333
66 18 7.6
10 7.6 5.0
lo00 2000
diffusion averaging* time (s) 0.91 0.065 0.012
08
I
2000 Ipl
+*
+
+
co
m
0
c?
0
0
0.4
Buckbee-Mears information sheet. Calculated using eq 3.
RESULTS AND DISCUSSION Potential Step Experiments. The dimensions of the three grid sizes investigated are shown in Table I. The diffusion profile averaging time is the time necessary for the developingdiffusionlayers from neighboringwires to coalesce. At this point the grid behaves as a planar electrode of somewhat diminished surface area. For the 1000 and 2000 lines (wires) per inch grids this process is rapid on the time sacle of these experiments and during potential step or potential sweep experiments the electrode essentially behaves in a planar manner. If the electrode to prism distance is 15 pm (a typical value for the 2000 lines per inch grid (lpi)) then the time (t) for the diffusion layer thickness (6)to extend 15 pm will be approximately 150ms based on eq 3. By this time,
01 0
Time (sec) Figure 2. Absorbance response to potentlal step for three grM sires. 1.0
I
0.8 -
0.6 -
0.2
y
(3) 01
however, significant changes in concentration of reactant and product will have occurred. The much larger holes of the 333 lpi grid suggest that even if perfect contact between the grid and the prism was achieved a considerable amount of the region of observation would be at least 20 pm from the electrode surface, thereby significantly reducing the speed of the response. As will be discussed later, zero clearance is not achieved in practice so that the distance involved is even greater. An assumption of the digital simulation model is that the wall of the cell is effectively at an infinite distance from the electrode surface. However it was possible that the behavior of the absorbance-time response might correspond to a thinlayer situation, as exists between the prism and electrode. A digital simulation of the response to a potential step in a thin-layer cavity was conducted. The results show rapid achievement (under 2 s) of maximum absorbance, at distances less than 30 pm. The experimental data is not in good agreement with the thin-layer model, but is well represented by the semiinfinite linear diffusion case. This is due to access to the bulk of solution by the layer of solution between the prism and grid, through what is essentiallya porous electrode. The electrodeis only 3-5 pm thick. By comparisonthe average distance to the walls of the cell from the electrode is on the order of 3000 pm. The chief effect of reducing the grid size is to reduce the averagedistance of the point of observation from the electrode surface. This in turn leads to a faster absorbance response to a potential step into the diffusion limited region. Figure 2 shows the normalized absorbance response for three grid sizes during the 20 s immediately following a potential step into a region where reduction of ferricyanide is diffusion limited. The absorbance reached after a set time is characteristic of the distance from the electrode to the point of observation. This is illustrated in Figure 3 which shows the calculated absorbance-time response, at different distances from the electrode surfaces, for a potential step to a diffusion limiting potential. These curves are obtained from eq 4 below (28) Boas, M. L. Mathematical Methods in the Physical Sciences; John Wiley & Sons: New York, 1983.
20
10
0
I
I
I
/
,
,
,
I
,
10
,
r
20
I
/
/
I 30
Time (sec) Figure 3. Simulated response at 10, 20,30, 40, and 50 pm from the electrode surface after a potential step.
where x is the distance away from the electrode surface, t is the time since the potential step, and cob is the bulk concentration of product. The error function in eq 4 was approximated using a McLaurin series28and the necessary number of iterations performed to achieve convergence. ~ , ( x , t )= ~,berf[x/2(~,,t)'/~1
(4)
To confirm that the digital simulation program was performing satisfactorily, it was used to generate concentration-time response curves for a simple potential step experiment. These curves were indistinguishablefrom those produced using eq 4. The digital simulation data was calculated to four significant figures, and comparison with data from eq 4 (calculated to six significant figures) indicated no differencesbetween the two data seta. This is an important test as algorithims to describe the concentration-distancetime profile in more sophisticated electrochemical experiments are exceedingly complex.27 Since the response curve is characteristic of the distance from the electrode to the point of observation, it is possible to estimate how close the minigrid is to the prism surface by means of curve fitting. To obtain a normalized concentration (or absorbance)-time curve it is necessary to know the maximum possible absorbance (A&. This is the absorbance if all of the reactant is converted into product in the region of observation. A, can be determined readily with the ferrocyanide ( Y C N = 2038cm-')/ferricyanide ( Y C N = 2116cm-1) couple as both ions are stable. If the reduction of 5 mM ferricyanide solution is under study, the cell is mounted in the spectrometer and filled with the product, a 5 mM solution of ferrocyanide. The absorbance at 2038 cm-1 is measured, and by correcting the absorbance for the area of the prism not covered by the grid, A, can be calculated (eq 5). The correction, shown below, is achieved by multiplying the
ANALYTICAL CHEMISTRY, VOL. 64, NO. 22, NOVEMBER 15, 1992 h
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Time (sec) Distance from Electrode (microns) Flguro 4. Ratio of absorbance at 2 and 0.5 8 after a potential step, as a function of dlstance from the electrode surface.
A,
= [A(soln)l(electrode area)/(prism area)
(5)
absorbance of the ferrocyanide solution by the proportion of the prism surface covered by the minigrid. The area of the minigrid is 4.9 cm2while the area of the prism exposed to the solution is 6.75 cm2. This value of A, can also be obtained during experiments on a long time scale. A, should be determined for each installation of the grid and for each time the cell is aligned in the spectrometer. For a 5 mM solution of ferrocyanide the value of A, (calculated from the absorbance at 2038 cm-l) was found to vary between 0.012 and 0.015 absorbance units, depending on cell alignment during installation. Another way in which as estimate of the prism-minigrid distance can be made is to calculatethe ratio of the absorbance 2 s after the potential step to the absorbance after just 0.5 s. At the surface this ratio would be unity as the surface concentration of product is a t its maximum value for all times after the step. This ratio increases as the point of observation is moved further from the electrode surface (Figure 4). The principle advantage of this approach is that it is not necessary to know A, in advance. Figure 5 shows data points for a potential step from +350 to 0 mV, a t a 2000 lpi grid, using 5 mM ferricyanide and monitoring the formation of ferrocyanide a t 2038 cm-l. Spectra were collected at 80-ma intervals. The solid line represents the predicted response a t 15 pm from the electrode surface. The ratio of the absorbance at 2 s to the absorbance a t 0.5sis 0.0089/0.0069 = 1.3. From Figure 4 a prism-minigrid distance of 13 pm is predicted. The prism-minigrid distances obtained from curve fitting and the absorbance ratio method, for 1000 and 2000 lpi grids, are shown in Table XI. The digital simulation of the response curve for a potantial step experiment provides a convenient method to estimate the electrode prism distance in the cell. As was noted earlier the grid could be positioned on the prism using acetone or water. The presence of the fluid lubricates the grid and allows it to be readily positioned. Because a layer of fluid must exist between the grid and prism during operation, zero clearance between the grid and prism is not likely to be achieved. Linear Potential Sweep. Figure 6 shows the digital simulation of the change of concentration (and absorbance) during a 20 mV s-l linear potential sweep for a reversible system a t the electrode surface and a t selected distances from the surface. As the point of observation is moved away from the electrode surface the slope of the rising part of the curve decreasesas does the absorbanceachieved at a given potential. The degree to which this distortion occurs depends on the
Flgure 5. Potential step at a 2000 Ipl grM compared to a simulated response at 15 pm from the electrode surface.
Table 11. Estimation of Electrode-Prism Distance no. of wires/in.
observation distance from curve fitting hm)
observation distance from absorbance ratioa (pm)
90 26 15
22 13
333
1000 2000
o.8hy
Determined from Figure 4. Data not available for 333 wires/in. grid. 1.o
0.8 0.6
3
0.6
E
sa
0.4
I
\ \I/\\
0.2 0 -200
-1 00
100
0
200
E - E112 Flgure 8. Digital simulation of 20 mVls potential sweep at 0, 8, 16, 24, 32, and 50 pm from the electrode surface.
--
10
5
8
8 0
s6 0
zx
4
ui
sa 2 0 0
2
4
6
8
10
Time (sec) Flgure 7. Potential sweep data at a 2000 Ipi grid compared to digital simulation at 15 clm from the electrode surface.
potantial sweep rate as well as the distance from the electrode surface. The data points in Figure 7 show the experimentally obtained response to a 50,100,and 200 mV 5-1 sweep. The solid lines are the simulated response a t a distance of 15 pm
2692
ANALYTICAL CHEMISTRY, VOL. 64, NO. 22, NOVEMBER 15, 1992
l'O
m I '
L
0
0
0 0
100
200
200
100
300
c
300
Potential (mV) vs Ag/AgCI
Potential (mV) vs Ag/AgCI Flgulv 8. Normalized absorbance-potential and current-potential data for a 50 mV/s sweep. from the electrode surface. This distance was determined from a potential step experiment conducted immediately afterwards. Figure 8 shows the absorbance and current response to a 50 mV 8-1 potential sweep. This figure shows that the absorbance does not level out until past the peak current. This is to be expected since the normalized surface concentration of product at this potential is 0.75. Ninety-five percent conversion to product at the surface is not obtained until approximately 75 mV past the peak potential. Cyclic Potential Sweep. The absorption-potential response for a 2 mV/s cyclic sweep at a 333 lpi minigrid (Figure 9a) and a ZOO0 lpi grid (Figure 9b) show significantly different responses. This is expected as the concentrations observed for the 2000 lpi grid are close to the surface value at a distance of 15 pm, particularly during a slow potential sweep experiment. In other words, electrolysis is essentially complete in the region of observation. For the 333 lpi grid the prismelectrode distance was about 90 pm so the absorbances do not reflect the surface concentrations well. The forward sweep data from Figure 9b were used to produce an optical analogZeof the Nernst plot with a slope of 64 mV/decade, which is close to the expected value of 59 mV/decade for a reversible one-electrontransfer. In this case the ratio of the concentration of oxidized to reduced species at the electrode surface is determined from the observed absorbance (A) and the maximum absorbance (A"). The response of the cell to potential sweep experiments depends on the prism electrode distance and the sweep velocity. At slow sweep rates and small prism-electrode distances the observed concentrations are a good indication of the electrode surface concentration. Increasing the sweep rate or prism-electrode distance results in a shallower absorbance-potential profile.
CONCLUSIONS By placing a gold-minigrid electrode on the surface of an infrared internal reflection element a cell can be constructed
1
2000 Ipi
3
1
I
D
O
4 L L
2
0 0
100
0 00
200
300
Potential (mV) v s Ag/AgCI Flgure S. Cyclic sweep at 2 mV/s: (top) 333 Ipl grid, (bottom)2000
Ipi grid.
that allows observation of the region of solution close to the electrode surface. This cell design combines the advantages of a low-resistance electrode with the favorable optical properties of zinc selenide. Relatively high (200 mV/s) scan rates were achievable due to the low electrode and cell resistance. Vapor-deposited gold film electrodes are not capable of this perfwplance and cannot be easily replaced in the event of fouling. This design allows easy replacement of the electrode. The absorbance-time response is very dependent on the distance from the electrode to the point of observation. This relationship was investigated using digital simulation of the absorbance response and by exppFimental observation. The shorter this distance the closer the absorbance response mimics the electrode surface concentration. When the ZOO0 lines per inch grid was used it behaved as though the distance from the electrode to the surface of the infrared crystal was around 15 pm. RECEIVED for review April 23, 1992. Accepted August 13, 1992. Registry No. Gold, 7440-57-5.
