Anal. Chem. 1983, 55,2219-2222
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Thin-Layer Electrochemical Cell for Long Optical Path Length Observation of Solution Species Jerzy Zak, Marc D. Porter, and Theodore Kuwana*
Department of Chemistry, The Ohio State University, Columbus, Ohio 43210
A long path length thln-layer cell whlch allows spectroelectrochemlcal studles wlth opaque conductor materials has been developed. Thls cell provldes high optical sensltlvlty, small electrolysls volume, and short electrolysls tlme. It Is compatible with commerclally avallable lnstrumentatlon. The deslgn allows easy replacement of electrodes and reproducible InJectlonof test solutlons. The optical and electrochemical responses of the cell, uslng ferrlherrocyanlde as the test species, are descrlbed for cycllc voltammetry, cyclic voltabsorptometry, and steady-state potential step measurements. The cell also appears to possess, by virtue of Its long optlcal path length and large electrode surface area to electrolyslr volume ratlo, the requlslte sensltlvlty for concurrent optical (for typlcal UV-VIS chromaphores) and electrochemlcal quantltatlon of species adsorbed on the electrode surface at monolayer levels.
Spectroelectrochemistry (SEC) is a valuable tool for investigating various aspects of heterogeneous and/or homogeneous electron transfer processes (1-4). However, the utility of this method for the study of weak chromophores or changes in solution concentration due to adsorption of species onto the electrode surface is limited, in several instances, by the short optical path lengths (typically 100-200 pm) of conventional SEC cell designs. Much of the existing methodology also requires that the electrode be optically transparent (OTE), thereby limiting the range of conductive materials which can be used. There have been several reports that describe SEC methods and cell designs to increase the sensitivity of the optical measurements. Glancing incidence external reflection as well as external and internal multiple reflection techniques have been employed to increase the optical path length (2,5-7). Another approach to increase the path length is to irradiate the electroactive region with the optical axis parallel and, in some cases, adjacent to the electrode/solution interface. In this mode, the path length is equal to the length of the electrode and the optical properties of the electrode do not govern the accessible spectral region. Solution mixing has been utilized to improve the electrolysis time for a small electrode surface area/cell volume ratio (8) and also to couple a cell to a remote 1cm path length (9). Fiber optic probes have been used to bring the light into a long path length thin-layer cell; however, the transmissive characteristics of the fibers govern the available spectral region (10). A “pseudo” thin-layer cell, with an equilibration time of ca. 30-40 min, which ailows both absorption and fluorescence measurements, has been fabricated (11). Holey electrodes, fabricated by drilling a small diameter hole (typically 500 pm) in an opaque conductor material to produce a long path length thin-layer SEC, have also been developed (12). In this paper, the fabrication and preliminary evaluation of a long path length (1.62 cm) thin-layer (solution layer thickness 100 pm) cell (LPTLC) in which the optical axis is parallel and adjacent to the electrode/solution interface is
described. The unique feature of this design is that the adsorption of monolayer coverages of solution species on an electrode surface can be quantitated both electrochemically (13-16) and optically due to the combination of the large electrode surface area to electrolysis volume ratio and the long optical path length, The LPTLC also has a small electrolysis volume (11.3 pL) and a short electrolysis time (25 s for 99% complete electrolysis) and is compatible with commercially available instrumentation. The design also allows facile interchange between conductor materials for possible chemical modification studies and reproducible injection of the test solution into the thin-layer cavity. The optical and electrochemical responses of this cell, using ferri/ferrocyanide as the test species, are reported for cyclic voltammetry (CV), cyclic voltabsorptometry (CVA), and steady-state potential step measurements. The experimental aspects for the optical quantitation of adsorbed monolayers with this cell are proposed and discussed.
EXPERIMENTAL SECTION Cell and Electrode Fabrication. A simplified operational diagram of the LPTLC is shown in Figure 1. The incoming light passes through the cell cavity, which is defined by the wall of the cell, the two thickness spacers, the quartz windows, and the working electrode (WE). The solution layer thickness and, hence, equilibration time are controlled by the thickness of the spacers. The cell is fabricated from Plexiglas and contains compartments for a Pt auxiliary electrode (AE) and a Ag/AgCl (saturated KC1) reference electrode (RE). Both the AE and RE are connected to the cell with polycarbonate Luer fittings (Value Plastics, Loveland, CO). Thin channels (ca. 1 mm in diameter) provide solution contact between the RE, AE, and WE. A distance of 6 mm between the Teflon spacers was chosen to allow most of the incident light (beam height 7 mm) to pass through the cell. The optical path length, which corresponds to the length of this cavity, is 1.62 cm. In the results to be described, the thickness of the spacers was 100 pm. This thickness ensured rapid equilibration of the test species with the applied electrochemical perturbation while allowing adequate light throughput for the spectral measurement. The solution inlet channel has been omitted from Figure 1for clarity. A detailed diagram of the LPTLC and the mounting accessory for its placement in the spectrometer is given in Figure 2. Figure 2A is a side view of the cell with the optical axis perpendicular to the plane of the page. Figure 2B is a top view of the cell to show the interior of the cell cavity. Important features in this cell design are the spacer material and the method of sealing the WE to the cell and windows. The spacer is fabricated from two different materials. A Teflon spacer, 8a, is used to define the thickness of the cell. The softer silicone rubber gasket (RTV, General Electric Co., Waterford, NY) adequately seals the electrode to the cell. The silicone gasket is made slightly thicker than the Teflon portion of the spacer and is compressed to the thickness of the Teflon when the WE is clamped to the cell with the thumbscrew. With this configuration, interchange between electrodes is accomplished by simply loosening and retightening the thumbscrew. The WEs were cut from glassy carbon (grade 20, Tokai Carbon Co., Tokyo, Japan) to dimensions slightly greater than 1 cm by 1.62 cm. The electrode was then precisely ground so that the edges were parallel to one another and perpendicular to the electrode surface. This ensured proper seating of the electrode between
0003-2700/83/0355-2218$01.50/0 0 1983 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 14, DECEMBER 1983
a WE b
Figure 1. Simplified schematic of the long path length thin-layer spectroelectrochemical cell: (a) cell body, (b) thickness spacers, (c) quartz windows, (d) solution contact channels, (WE) working electrode, (RE) reference electrode, (AE) auxiliary electrode.
B
A 4
11
5
i
-l-
\, 2
/-
I
0.4
I
0.2
I
I
0.0
P O T E N T I A L , V O L T vsAgiAgCl
Figure 2. (A) Detailed schematic of the long path length thin-layer spectroelectrochemiclcell and spectrometer placement accessory. (B) View of the interior of the cell cavity: (1) base plate, (2) cell mount, (3) thumbscrew and mount, (4) cell, (5) auxiliary electrode chamber, (6) pressure plate, (7) working electrode, @a) Teflon thickness spacer, (8b) silicone rubber seal, (9) quartz window, (10) reference electrode chamber, (11) solution inlet chamber.
the windows. The edges of the electrode were coated with a thin bead of silicone rubber for a good seal to the windows and for insulation from solution contact. The glassy carbon electrodes were prepared according to published procedure (17,181 with a fial polishing step using 1Km a-alumina (Buehler Ltd., Evanston, IL) and were then placed in an ultrasonic bath containing doubly distilled water for about 1 min just prior to use. Solution was injected into the cell via a 5-mL syringe connected to a three-way polyethylene valve. Degassing was accomplished by sealing a valve into the top of the syringe and passing nitrogen through the remaining channel of the three-way valve or by a vacuum degas/filling procedure (19). The resistance between the WE and AE was 1.3 k0 for 0.5 M KCl electrolyte, as measured with a high-frequency impedance bridge. The base plate allows easy placement and alignment of the LPTLC within the spectrometer. The cell is held in the mount by a pressure fitting and setscrew. Instrumentation,andReagents. Spectral measurements were made with a DMS 90 UV-VIS spectrometer (Varian Instrument Co., Palo Alto, CA). A conventional three-electrode potentiostat w a used ~ for all electrochemical experiments. Data were displayed on an x-y recorder (Model 2000, Houston Instruments Co., Bellaire, TX). The potential of the WE was measured vs. a Ag/AgCl (saturated KC1) RE. Solutions were prepared daily from reagent grade K,Fe(CN)6 (Fisher Scientific,Inc., Chicago, IL) in 0.5 M KC1. Doubly distilled water was used for all preparations.
RESULTS AND DISCUSSION Cyclic Voltammetry and Voltabsorptometry. The electrochemical and spectral responses of the LPTLC are shown in the CV current-potential (i-E) curves in Figure 3a and the CVA absorbance-potential (A-E) curves in Figure 3b. The sweep rates are 2 and 1mV for a solution of 4.0 X lo4 M Fe(CN)6%in 0.5 M KCl. The wavelength for optical monitoring of the change in Fe(CN)63-concentration was 420 nm. The shapes of the i-E curves are characteristic for an exhaustive electrolysis in a thin-layer cell (20-22). The separations between the cathodic and anodic peak current potentials, E, and Epa,are ca. 15 and 10 mV for the 2 and l mV
Flgure 3. Cyclic voltammetric current-potential (a) and cyclic voltabsorptometric absorbance-potential (b) responses using 0.4 mM ferricyanide in 0.5 M KCI: scan rates (-) 2 mV s-’,(---) 1 mV s-’.
s-’ sweeps, respectively. The cathodic and anodic peak cur-
rents, i,, and i,,, are both equal to 6.9 pA in the 2 mV s-l sweep. For the 1 mV s-l sweep, i,, is 3.5 pA and i,, is 3.4 p A . The integrated charges under each of the i-E waves for the anodic and cathodic scans, Q, and Q,, are equal. In an ideal thin-layer cell, the peak potentials, E,, and Epa, for CV i-E waves coincide and correspond to the formal redox potential, EO’. The Q, and Q, values should be identical (20-22). A zero peak separation indicates that the confined solution layer maintains equilibrium with the applied potential at the given sweep rate, with a negligible iR drop. Equal values of Q, and Q, indicate retention of the solution species within the electrolysis volume. As given above, the anodic and cathodic peak separations were very small and Q, and &, were equal. These results indicate that the LPTLC behaves similarly to an ideal thin-layer electrochemical cell. The A-E responses in Figure 3b also indicate behavior characteristic of a confined thin layer of solution. The absorbance values for the cathodic potential sweeps remain relatively constant a t Eapplvalues from the initial potential of 0.575 V to ca. 0.4 V. As Eo‘ is approached, the absorbance decreases due to the reduction of Fe(CN):-. The absorbance falls to zero a t ca. 0.1 V, inferring complete reduction of Fe(CN)63-to Fe(CN):- within the electrolysis volume. The shape of A-E curve during the reverse sweep (oxidation of Fe(CN),&) is similar to the cathodic wave with the separation at the inflection points of the cathodic and anodic sweeps the same as for the CV peak current potential difference. The absorbance returns to its initial value at the end of the anodic sweep, indicating retention of the electroactive material within the restricted volume. The 1 mV s-l sweep shows a smaller potantial separation at the inflection point of the cathodic and anodic curves due to the longer time frame of the experiment. It should be noted that the magnitude of the absorbance change for this LPTLC is ca. 80-160 times greater than those of conventional optically transparent thin-layer electrochemical cells where the path length may be on the order of 100-200 pm. Equilibrium Redox Studies. An application of the LPTLC is the evaluation of absorbance data taken during
ANALYTICAL CHEMISTRY, VOL. 55, NO. 14,DECEMBER 1983
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remaining in electrolysis volume after adsorption. The feasibility of using the LPTLC for deriving such information can be demonstrated by the following calculation. The molecular parameters chosen for this calculation are considered to be typical for UV-VIS chromophores. The optical path length and electrode surface area (1.0 cm2)of the LPTLC were also used in the computation. If the minimal change in absorbance, &4-, that can be measured is assumed to be 0.002 absorbance unit (at a signal to noise ratio of 2), a corresponding change in solution concentration, ACmk, can be calculated via Beer’s law as
300
400 WAVELENGTH, n m
500
Figure 4. Absorption spectra for 0.4mM ferricyanide in 0.5 M KCI at several applied potentials: (a) 0.000 V, (b) 0.100 V, (c) 0.200 V, (d) 0.225 V, (e) 0.250 V, (f) 0.275 V, (g) 0.300V, (h) 0.450V, (i) 0.600 V vs. Ag/AgCI (saturated KCI).
steady-state potential step experiments (23). Such studies are useful for the determination of the stoichiometry and Ea’ of redox bicomponents (24,25). A long optical path length cell that maintains a small volume is advantageous for studies of redox species with low molar absorption coefficients. In this experiment the ratio of [Fe(CN)64-]/[Fe(CN)63-]is controlled by the value of Eappk Assuming Nernstian equilibrium is attained within the cell cavity, the absorbance values measured at various applied potentials allow the determination of Eo’and n for the redox couple. The equilibration time between each change in EaPp1 was ca. 60 s (diffusion limited potential step experiments indicated that 99% electrolysis was achieved within 25 9). Spectra corresponding to several values of Eappl are shown in Figure 4. The spectra at an Eappl at 0.600 and 0.450 V are nearly identical, since at these values, the ratio of [Fe(CN)6P]/[Fe(CN)6”] is essentially zero. From a Nernst plot (Eappl vs. log ([Red]/[Ox])), a n value of 1.03 is calculated from the slope. A value for EO’, taken from the y intercept, is 0.248 V. The reproducibility of these measurements was better than f 3 % for three separate trials. The Eo’compares favorably with the 0.250 V and 0.252 V values from the CV data (calculated as the average of the E,, and E,, for the 2 and 1 mV s-l scan rate i-E curves, respectively) and agrees with previous investigations (11,12,23,26). The correlation coefficient for the plot is 0.999. Solution Injection Reproducibility. The reproducibility of successive sample introductions of test solution into the cell cavity was examined by evaluating the charge required for the exhaustive electrolysis of 4.0 X M Fe(CN)63-in 0.5 M KC1 in a multiple injection experiment. The average charge (corrected for background charging) for six separate injections was 437 f 11pC which gave a computed cell volume of 11.3 f 0.3 pL. Description of the Optical Quantitation of Adsorbed Monolayers of Solution Phase Species onto an Electrode Surface. It has recently been shown that information for predicting the probable orientation of an adsorbed solution phase species on an electrode surface at monolayer coverages can be inferred from electrochemical data using a thin-layer cell with a large electrode surface area to electrolysis volume ratio (13-16). This method takes advantage of the high sensitivity and precision of electrochemical measurements. The LPTLC can provide cross-correlation of optical and electrochemical data for such quantitation and orientation information via measurement of the electroactive chromophore
where q is assumed to be 1000 M-’ cm-l and 1 is the 1.62-cm path length of the LPTLC. The calculation gives a value of 1.2 X lo4 M for ACmin. Therefore, a change or loss of 1.2 X lo4 M from a cell volume of 11.3 pL corresponds to 1.3 X mol of electroactive chromophore. This limiting detectable loss, in terms of adsorbed species on an electrode surface, I‘-, is given by
where Vceuis the electrolysis volume of the cell and A W E is the area of the working electrode. If a molecule is absorbed on an electrode so as to have a surface area of 60 A2 molecule-’, then a monolayer coverage in the closest packing arrangement would be 2.8 X mol cm-2. Thus, with an electrode area of 1.0 cm2 it is possible to quantitate the adsorption of electroactive chromophores on an electrode surface with a sensitivity ca. 20 times that required for the given monolayer coverage. Application of such optical data, as has been done electrochemically (13-16), then allows the prediction of the probable orientation of the adsorbed species on an electrode surface.
CONCLUSIONS A long path length thin-layer cell has been described for which the spectral and electrochemical responses exhibit nearly ideal thin-layer behavior. It appears that experiments using the LPTLC can provide both optical and electrochemical data regarding the quantitation and, hence, the prediction of the probable orientations of adsorbed solution phase species. The cross correlation of electrochemical and spectral data for such predictions are currently in progess. Complementary measurements for verification of the molecular orientation of adsorbed layers, using polarized FT-IR specular reflectance spectrometry, are under way. It also seems feasible that this LPTLC will be useful for quantitation of surface Raman (both resonance and surface enhanced) and FT-IR specular reflectance measurements. ACKNOWLEDGMENT We thank Dale H. Karweik for valuable discussions concerning this work. LITERATURE CITED (1) Kuwana, Theodore Ber. Bunsenges. ’ Phys. Chem. 1973, 7 7 , 050-871. (2) Kuwana, Theodore: Winogard, Nicholas I n “Electroanalytlcal Chemistry”; Bard, Allen J., Ed.; Marcel Dekker: New York, 1974; Voi. 7. (3) Kuwana, Theodore: Heineman, William R. Acc. Chem. Res. 1976, 9 , 241-240. (4) Heineman, Wllllam R. Anal. Chem. 1977, 5 0 , 390A-402A. (5) McCreery, Richard L.; Prulksma, Richard: Fagan, J. Robert Anal. Chem. 1979, 51, 749-752. (6) Baumgartner, C. E.: Marks, G. T.; Aikens, D.A.; Richtol, H. H. Anal. Chem. 1980, 52, 267-270. (7) Davis, James E.; Winograd, Nicholas Anal. Chem. 1972, 4 4 , 2152-2156.
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(8) Rubinson, Kenneth L.; Mark, Harry B., Jr. Anal. Chem. 1082, 54,
1204-1206. (9) Anderson, James L. Anal. Chem. 1970, 51,2312-2315. (10) Brewster, Jeffery D.; Anderson, James L. Anal. Chem. 1082, 54, 2560-2566. Slmone, Michael J.; Heineman, William R.; Kreishman, George P. Anal. Chem. 1982, 54,2382-2384. Porter, Marc D.;Kuwana, Theodore, submitted for publication in Anal. Chem . Sorlaga, Manuel P.; Hubbard, Arthur T. J. Am. Chem. SOC. 1082, 104,2735-2742. Soriaga, Manuel P.; Hubbard, Arthur T. J. Am. Chem. SOC. 1982, IO4 I 2742-2747. Soriaga, Manuel P.; Hubbard, Arthur T. J. Am. Chem. SOC. 1082. 104,3937-3945. Sorlaga, Manuel P.; Wilson, Peggy H.; Hubbard Arthur T.; Benton, Cllfford S. J. Electroanal. Chem. 1982, 142,317-336. Miller, Charles W.; Karwelk, Dale H.; Kuwana, Theodore Anal. Chem. 1081, 53, 2319-2323. Zak, Jerzy; Kuwana, Theodore J . Am. Chem. SOC. 1082, 104, 55 14-55 15. Hawkridge, Fred W.; Kuwana, Theodore Anal. Chem. 1073, 45, 1021-1027,
(20) Hubbard, Arthur T.; Anson, Fred C. I n "Electroanalytlcal Chemistry"; Bard, Allen J., Ed.; Marcel Dekker: New York, 1970; Voi. 4. (21) Hubbard Arthur T. CRC Crit. Rev. Anal. Chem. 1073, 3 ,201-242. (22) Bard, Allen J.; Faulkner, Larry R. "Electrochemical Methods: Fundumentals and Applications"; Wlley: New York, 1980; Chapter IO. (23) DeAngells, Thomas P.; Helneman, Wllllam R. J. Chem. €doc. 1076, 53,594-597. (24) Anderson, James L.; Kuwana, Theodore; Hartzell, Charles R. Biochemistry 1076, 15,3847-3855. (25) Rohrback, D. F.; Deutsch, E.; Helneman, W. R.; Pasternack, R. F. Inorg. Chem. 1077, 16,2650-2652. (26) Kolthoff, I. M.; Tomslek, W. J. J. Phys. Chem. 1035, 39,945-954.
RECEIVED for review July 22, 1983. Accepted September 12, 1983. The support of the National Science Foundation (Grant No. 8110013),The Air Force Office of Scientific Research, and The Ohio State University Materials Research Laboratory is appreciated* M*D.P.gratefully acknowledges the support of an OSU Presidential Fellowship.
Square-Wave HydrodynamicalIy Modulated Voltammetry for Study of Anodic Electrocatalysis Deborah S. Austin and Dennis C. Johnson*
Ames Laboratory-U.S.D.O.E.
and Department of Chemistry, Iowa State University, Ames, Iowa 50011
Theodore G . Hines and Edward T. Berti
Pine Instrument Company, Grove City, Pennsylvania 16127
A response, dependent solely upon the mass transport coupled component of the total current, Is obtained by alternating the rotational velocity of a rotating disk electrode about a nonzero mean value and cuinputlng the difference of the total currents obtained for the two velocities. Hence, relatively small transport-controlled currents can be observed wlthout interference from large, simultaneous, surface-controlled reactions, e.g., the formation of surface oxides and the evoiutlon of 0,. As a r e a , the useful anodic potentlal lhlt Is extended ca. 350 mV beyond the practical ilmlt for conventional voltammetry. An important application of the title technique Is for the study of the eiectrocataiysis of anodlc reactlons by surface oxldes at noble-metal electrodes. Results are shown for the oxidation of I- at a Pt electrode In 0.5 M H2S0,. I n addition to the transport-ilmlted production of I, In 0.5 mM I(E,,, N 0.48 V vs. SCE), the oxidatlon to IOs-Is observed to occur at a transport-limlted rate (E,,, N 1.45 V vs. SCE) simultaneously with 0, evolution.
The analytical utility of hydrodynamically modulated voltammetry based on application of sine-wave and squarewave modulations about a nonzero, average rotational velocity for rotating disk electrodes was examined by Miller, Bellavance, and Bruckenstein (1). Subsequently, Miller, Bruckenstein, and co-workers have focused their efforts on the sinusoidal version of hydrodynamically modulated voltammetry (SHMV) (2-7). Major emphasis by these workers has been placed upon the theoretical basis of the technique, the determination of heterogeneous kinetic parameters for quasi-reversible and irreversible systems, the determination of diffusion coefficients, and applications to trace analysis.
Blaedel and co-workers (8, 9) investigated the square-wave modulated technique which they referred to as "pulsed rotation voltammetry". They also emphasized the application to trace analysis, and the determination of reaction rate constants and transfer coefficients. In addition, they recognized the advantages of automating the square-wave technique through use of small computers (9). The theory and application of hydrodynamically modulated techniques have been reviewed extensively by Wang (10). Our interest in modulated voltammetry has been primarily in the application of computer-controlled, square-wave, hydrodynamically modulated voltammetry (QHMV) for the characterization of surfacecatalyzed anodic reactions at noble-metal electrodes. Numerous anodic reactions of proven or potential electrosynthetic importance in aqueous solutions occur at oxidecovered "inert" electrodes with simultaneous evolution of 02, and the electrocatalytic involvement of surface oxygen has been implicated. Yet, the voltammetric study of such processes is not convenient by conventional electroanalytical techniques, particularly when the faradaic current resulting from the reaction of interest is only a small component of the total current. Both SHMV and QHMV have been demonstrated to successfully extract the transport-limited faradaic signal from a total current dominated by the surface-controlled processes of double-layer charging and formation of surface oxides. The large extension of the positive potential limit for modulated voltammetry beyond the useful limit for conventional voltammetry has become an important advantage of the modulated techniques. Our choice of QHMV for the study of anodic electrocatalysis was based on the commercial availability and ease of operation of the computerized square-wave technique. Conway et al. have studied extensively the formation of surface oxides on Pt in acidic solutions (11-13). They con-
0 1983 American Chemical Soclety 0003-2700/83/0355-2222$01.50/0
