Evaluation of an Improved Thin-Layer Electrode AttilP Yildiz,’ Peter T. Kissinger, and Charles N. Reilley Department of Chemistry, University of North Carolina, Chapel Hill,N . C. 27514
Electrodes based on deposited-metal thin films are described and evaluated for thin-layer electrochemistry. The application of such electrodes for controlledpotential and controlled-current techniques is demonstrated for several model systems. Ultraviolet, visible, and infrared transmission experiments are reported, and a unique fluorescence application is described. Minimum edge-effect, spectroscopic utility, ease and rapidity of construction, and minimal cost are seen as the principal advantages over previous designs.
A PRINCIPAL HINDRANCE to the general application of thinlayer electrochemistry (TLE) has been the development of an inexpensive, easily constructed electrode that minimizes edge effects. In our experience, previously used micrometer-based designs are deficient in one or more of these requirements (1-3). A smdwich-type thin-layer cell employing glass blocks and a separate platinum foil was an early thin-layer configuration (4). The use of glass plates with an electrode deposited on one of the plates was another approach; however, difficulty in securing a workable electrode surface precluded practical development (5). Recently, a glass-slide electrode utilizing a gold “minigrid” was developed in this laboratory (6). This design shows promise for spectroelectrochemical work but has the disadvantages of the near-anodic limit of gold, and restriction to a single working electrode. While grid electrodes can be considered as continuous surfaces from the diffusional point of view for slow experiments, they have obvious disadvantages for surface-effect studies and have less utility for reflectance experiments. This paper reports the use of electrodes based on vapor-deposited metal films. Three procedures are available for applying a metal film to glass: reduction from solution, cathode sputtering, and thermal evaporation. The use of “platinum paint” as an electrode surface has been reported (7). Semiconducting metal oxide electrodes deposited on glass have been particularly valuable where light transmittance is desirable (8). All of these surfaces could be employed in the configuration described herein ; however, the present work has been limited to platinum and gold films deposited by the thermal evaporation technique, This technique affords purer and more uniform surfaces than can be achieved by solution-reduction methods (9). Thin films and their preparation are discussed 1 Predoctoral fellow, on leave of absence from Ataturk University, Erzurum, Turkey. ~~
(1) A. T. Hubbard and F. C. Anson, ANAL.CHEM., 36, 723 (1964). (2) D. M. Oglesby, S. H. Omang, and C. N. Reilley, Ibid.,37, 1312 (1965). (3) J. E. McClure and D. L. Maricle, Ibid.,39, 236 (1967). (4) C. R. Christensen and F. C. Anson, Ibid.,35,205 (1963). (5) C. N. Reilley and D. M. Oglesby, unpublished work, 1964. (6) R. W. Murray, W. R. Heineman, and G . W. O’Dom, ANAL. CHEM.,39, 1666 (1967). (7) P. S. Pons, J. S. Mathson, L. 0. Winstrom, and H. B. Mark, Jr., Ibid.,p 687. (8) J. W. Strojek and T. Kuwana, 153rd National Meeting, American Chemical Society, Miami, Fla., April 1967, No. 122. (9) L. Holland, “Vacuum Deposition of Thin Films,” Wiley, New York, 1955.
1018
ANALYTICAL CHEMIC’~.~Y
in several recent texts (10-12), and the optical propertie of thin films have been reviewed by Heavens (13). EXPERIMENTAL
The basic electrode element is shown in Figure 1, a, and the complete assembly is given in Figure 1, b. A Consolidated Vacuum Corp. metal evaporator was used to deposit platinum and gold films on the microscope slides. The slides were precleaned with hot detergent solution and reagent 2-propanol. In addition, they were cleaned in the vacuum chamber by ionization of a nitrogen atmosphere at 50 to 100 microns with a 600-volt discharge. Bombardment of the surface with ions functions to expurgate adsorbed gasses and organics. This cleaning routine is essential for a solutionstable metal-to-glass bond. The electrode area was masked out with household aluminum foil; thus, the geometry could easily be varied. The thickness of the film can be adjusted so that uniform, low resistance, and, if desired, optically transparent electrodes are easily obtained. All of the films used here had a resistance of about 10 ohms cm-l with 20% transmittance. Considerations in the design of the electrode area (Figure 1, a) include the following: a must be small (ca. 1 mm) t o minimize the iR drop between the bulk solution and the electrode surface; b may be increased for maximum sensitivity and to minimize the relative edge effect (dependent on edgeto-area ratio) at the sacrifice of a minimal iR effect (see below). The cell (Figure 1 , b) is constructed by laying 2-mm strips of Teflon film down the slide-on one edge and 0.5 cm from the other edge. A second slide (either a barrier or another electrode) is offset 0.5 cm from the first. The two are clamped together with three Hoffmann clamps and the edges spotted with epoxy cement. After 30 minutes under an infrared heat lamp, the clamps are removed and the gluing is completed. A certain amount of care is necessary to prevent cement from completely sealing the ends of the cell by capillary action. Twin-electrode TLE requires that the second plate be identical to the first, so that contact to each working electrode can be made as indicated in Figure 1, b. When it is necessary to eliminate the small edge effect, the reactant-getter element shown in Figure 1, c, is used as the second slide. The use of Teflon film permits the I value to be predetermined to within at least 2%. The cell thicknesses were measured spectrophotometrically using standard solutions of 9,lO-diphenylanthracene in benzene. Calculations were made from both the 357- and 375-nm bands. A variety of film thicknesses (3.2 to 125 microns) are available (Dilectrix, Inc.). The use of multiple layers of film increases the number of possible thicknesses. Many cells can be completed in a 1-hour period. (10) American Society for Metals, Metals Park, Ohio, “Thin Films,” 1963. (11) C. F. Powell, J. H. Oxley, and J. M. Blocher, Eds., “Vapor Deposition,” Electrochemical Society, Wiley, New York, 1966. (12) J. C. Anderson, “Use of Thin Films in Physical Investigations,” Academic Press, New York, 1966. (13) 0. S. Heavens, “Optical Properties of Thin Solid Films,” Academic Press, New York, 1955.
t o p view
i (uA)
-I
7
(W,)contact
cw2)
Ion
T
b
L
acti;e
a
F
area'
s cup b
jt200
C
Figure 1. Thin-layer assembly Typical electrode element (electrode side up) b. Complete thin-layer assembly. Reference salt bridge centered behind cell not shown. An alternate twin-electrode contact point indicated by Wz c. Reactant-getter electrode element (glass side up) a.
The entire assembly was placed in a nitrogen-filled glove bag (Instruments for Research and Industry) for nonoptical experiments. The cells used in these experiments (Figures 2 to 6) showed a resistance of ca. 5 kiloohms to a 10-KHz (20 mV peak to peak) signal from the working electrode to the bulk solution with 0.01M KC1 in the thin layer. Twin-electrode cells gave a resistance of ca. 10 ohms face to face using this same standard solution. These two values represent effective cell constants of ca. 7 and 0.01 cm-', respectively. Electrodes for the optical transmittance experiments were constructed from 1 x 2 inch quartz slides using a and b values of 1 and 0.5 inch, respectively. The large a value prevents the exterior solution from interfering with absorbance measurements. The cell assembly was mounted on an L-shaped stand dimensioned for the Cary Model 14 spectrophotometer and drilled so that the light beam passed only through the working electrode area. The cavity of the Cary 14 was covered with light polyethylene and flushed with nitrogen saturated with the supporting medium. Each sample was introduced to the cup from a 0.036-inch polyvinyl tube leading from a previously described deaeration bottle (2). The reference beam of the instrument was passed through an identical empty cell to compensate for the absorbance of the thin film. Visual observation of fluorescence from the thin-layer volume was made by use of a portable ultraviolet lamp (Mineralight UV-11). Fluorescence measurements were made with an Aminco Spectrophosphorimeter. A brass jig positioned the cell in the cylindrical cavity in such a way that it bisected the 90" angle between the exciting and receiving monochromators. The slide bearing the platinum film faced the incident beam, so that fluorescence was transmitted through the masked barrier slide. This configuration gives maximum advantage to the emission intensity with minimum interference from the exciting radiation. All chemicals were reagent grade, with the exception of polarographic-grade tetraethylammonium perchlorate (TEAP) and tetrabutylammonium perchlorate (TBAP) (Southwestern Analytical
1
Figure 2. Cyclic voltammetry of 5 mM Cu(I1) perchlorate ' solution Supporting medium. 0.25M (C2H&NClOain CHXN Platinum electrode with a = 1 mm, b = 9 mm, and d microns Scan rates, mV/sec, indicated
=
77
Chemicals), chromatographic quality acetonitrile and benzene (Matheson Coleman and Bell), spectroquality dimethylformamide (Matheson Coleman and Bell), and practical grade ferrocene. Rubrene and perylene were obtained from K & K Laboratories, Inc. The operational-amplifier circuitry was similar to that previously described (2). RESULTS AND DISCUSSION The only serious difficulty with TLE cells of the present design is the iR gradient across the electrode face and between the electrode and the bulk solution. This potential gradient is minimized by restricting the electrode area (see above), increasing /, and increasing the supporting electrolyte concentration. The use of a properly placed internal reference and/or auxiliary electrode can lessen (but cannot possibly eliminate) such gradients. Such an alteration in design would sacrifice simplicity without a warranted improvement in response. These gradients will affect the shape of an electrochemical response ; however, with the exception of chronopotentiometry, in no way do they alter the application of Faraday's law to quantitative determinations by TLE. This quantitative simplicity is, indeed, the tour de force of thinlayer us. ordinary electroanalytical methods. With careful attention to the aforementioned parameters, the iR effects can be made inconsequential to quantitative analysis. The edge effect-i. e., the diffusion of active species into the thin-layer volume during an experiment-is minimized in this cell because the solution extending on either side of the electrode region is in a thin layer and because the electrode edgeto-area ratio is minimal (typically 0.01). An initial evaluation of the present design was undertaken using millimolar solutions of cupric perchlorate in acetoniVOL. 40, NO. 7, JUNE 1968
1019
Table I. Comparison of Techniquesa Milli- No. of coulombs6 data rel. Calcd. Technique (av.) points std. dev. V, p l c 8.89 4.28 2 ... Voltammetry 5 k1.3 9.02 4.36 Chronocoulometry 1 ... 9.01 Scan coulometry 4.35 8.43 6 k1.2 Chronopotentiometryd 4.07 * All experiments 5 m M Cu(II), 0.25M in tetraethylammonium perchlorate-acetonitrile. b Corrected for background except for chronopotentiometry. Measured volume from A and 1 is 8.8 pl. d Corrected for P/3D for 7 values from 50 to 80 sec. 0
3wh
t (sed Figure 3. Chronoamperometry of Cu(I1) in 0.25M TEAP/CHaCN
I = 77.4 microns. Potential stepped from 1.2 to 0.2 us. SCE
.
trile with 0.25M tetraethylammonium perchlorate as the supporting electrolyte, This system was chosen because it is relatively reversible and affords a test of the cell under adverse nonaqueous conditions. In addition, the avoidance of deposition reactions for our initial experiments on platinum could be realized by working only with the Cu(II)/(Cu(I) couple. This system has not been previously studied by TLE. All solutions were carefully deaerated with nitrogen before each experiment. Although copper(I1) is reduced to copper (I) more anodically than oxygen, the presence of oxygen causes the homogeneous regeneration of copper(I1). Six techniques were studied : (1) cyclic voltammetry, (2) chronoamperometry, (3) potential-step coulometry, (4) potentialscan coulometry, ( 5 ) chronopotentiometry, and (6) steadystate voltammetry. Cyclic Voltammetry. Voltammograms of copper(I1) at different scan rates are shown in Figure 2. As has been pointed out (14), slow sweep rates where homogeneous diffusional mixing across the thin layer can be effected, give the best response definition. In the present design there is a vertical inhomogeneity in concentration caused by the iR drop across (top to bottom) the electrode face. This causes the cathodic and anodic peaks to separate by an amount proportional to sweep rate. Under ideal conditions of negligible iR drop and complete electrochemical reversibility, the anodic and cathodic peak potentials would be identical (14). Thus, the most qualitatively pleasing results are obtained at slow sweep rates with a geometry affording minimum resistance. Graphical integration of the response curves, corrected for background, gave the number of coulombs consumed, in good agreement with other techniques (Table I). Chronoamperometry and Step Coulometry. The disparity of a chronoamperometry curve from ideal behavior is shown in Figure 3. This irregularity results from the fact that the bottom of the electrode surface is initially closest to the applied potential; thus, it is at this point where the electron transfer is initiated. As the reactant in this region is exhausted, the local current density decreases, and the potential moves up the electrode face in a continuous process until all of the reactant has been electrolyzed. The result is an attenuated slope for the chronoamperometric response until this limit is reached. A visual demonstration of the potential gradient across the electrode face is witnessed when the electrochemical reactant or product is colored (or fluorescent, see below). When the potential is stepped to initiate the reaction, product begins to form at the bottom of the electrode face and a front of prod(14) A. T. Hubbard and F. C. Anson, ANAL.CHEM.,38,58 (1966). 1020
0
ANALYTICAL CHEMISTRY
1. t m M 2. 4 m M 3. 10 mM
uct moves up the electrode surface until exhaustion of reactant is completed at the upper electrode edge. In effect, the active area of the electrode moves upward during the time of the experiment. Typical step coulometric curves are given in Figure 4. The minimal edge effect is indicated by the flatness of the plateau. Thus, unambiguous values of Q can be easily obtained. The vertical iR gradient causes the ‘‘rise time” to increase without affecting the plateau value. This rise time can be shortened with a given cell by stepping the potential in excess of that required to complete the reaction, so that at least this minimum potential will be applied across the entire electrode face during the time of the experiment. Such a step requires the absence of any species which can react at the excessive stepping potential. The background contribution can easily be measured in step coulometry and subtracted from the total response. In potential-step coulometry, the time between the application of the initial potential and the step can be important because of background reactions at the initial potential. Therefore, it is often necessary to utilize a precision timing switch to control this time for both background and total response measurements. Values of Q compare well with values calculated from the concentration and measured electrode volume (within 2.5 The quantitative usefulness of step coulometry in a thin layer of solution was demonstrated (Table 11) by determining the cell volume, V , with a standard copper solution and subsequently measuring the concentration of “unknown” copper solutions and determining n-values for ferrocene and ferricene using Equation 1.
z).
Q
= nFVC
(11
The precision compares very favorably with that obtained by McClure and Maricle using a dip-type micrometer cell (3). Potential Scan Coulometry. The principle of integrating the current response from a linear ramp potential excitation of a thin-layer cell has been reported (15). The accuracy of the earlier work was seriously hampered by uncertainties in the thin-layer volume and by large edge effects. The present design circumvents both of these difficulties. The definition and flatness of response are typified by Figure 5 . The value (15) D. M. Oglesby, L. B. Anderson, B. McDuffie, and C. N. Reilley, ANAL.CHEM.,37, 1317 (1965).
54v
3-
0
21-
trl tr2 I I 20 40 ' 6 0 '80 100. 120 t(sec1 Figure 4. Step coulometric curves for copper(I1) 1
I
I
+IO
I
I
+0.4
+0.2
Sweep rate, 3.33 mV/sec System as per Figure 2
of Q obtained from the wave height matches that from other techniques. Slower scan rates are advantageous for the reasons given above for voltammetry. Coulometric analysis was reviewed by Bard in 1966 (16). The difficulties associated with thin-layer electrolysis at that time are no longer extant, and a more optimistic view of the method is now justified. Indeed, the conceptual and experimental simplicity of TLE makes it potentially the most useful of all coulometric techniques (17). Chronopotentiometry. Chronopotentiometry was the first electroanalytical technique to be applied to thin layers of solution (4). The advantages of TL-chronopotentiometry are the sharpness of the transition time and the mathematical simplicity. Using the present cell design, excellent chronopotentiograms are obtained when iR effects are minimized and the applied currents are low. When the iR effect is significant, the waves become more poorly defined, and ir drops below the theoretically anticipated value. Since relatively large I values may be utilized, the second term in Equation 2 is likely to become important and knowledge of the diffusion coefficient (2)
is requisite. Unfortunately, larger / values are accompanied by significant diffusion across the edges, increasing r. An optimum I value should minimize both iR and edge effects. Another problem with this technique is the difficulty of compensating for background effects. With these inherent difficulties, we cannot unqualifiedly recommend chronopoten(16) A. J. Bard, ANAL.CHEM., 38, 88R (1966), (17) W. H. Reinmuth, Ibid., p 270R.
+0.6 E vs. SCE
Figure 5. Potential scan coulometry of copper(I1)
1. Stepping from 1.2 to 0.0 volt us. SCE 2. Stepping from 1.2 to 0.2 volt System as per Figure 2
r = nFVCo/i - P / 3 D
+0.8
tiometry in the thin-layer cells described here. The edge and iR effects in chronopotentiometry are visually elusive compared to other techniques. Steady-State (Twin-Electrode) Voltammetry. The application of steady-state methods is a unique feature of TLE impossible to achieve under ordinary electroanalytical conditions. The principles and potential applications have been described (18-22). The present design offers the advantage of an easily measured and reproducible / value. In addition, nonparallelism and nonplanarity are not significant (18). The iR effect is not important once the steady-state condition has been achieved, because the current flows between the two working electrodes and not through the bulk solution. The diffusion coefficient of Cu(I1) in CH,CN was determined from the steady-state current where
(3) and Qcis obtained from the integrated cathodic current (18); an analogous expression permits calculation of D for copper(1) (Table 111). Using D c " ( I I )it, was possible to evaluate the contribution of P/3 D to the chronopotentiometric transition times. (18) L. B. Anderson and C. N. Reilley, J . Electroanal. Chem., 10, 295 (1965). (19) L. B. Anderson and C. N. Reilley, Ibid.,p 538. (20) B. McDuffie, L. B. Anderson, and C. N. Reilley, ANAL. CHEM., 38, 883 (1966). (21) L. B. Anderson, B. McDuffie, and C. N. Reilley, J. Electroanal. Chem., 12, 477 (1966). (22) B. McDuffie and C. N. Reilley, ANAL.CHEM., 38, 1881 (1966).
Table 11. Step Coulometry Determinations Av. value Species Concn. mM Millicoulombs," av. determined Cu(I1) 4 (std) 6.06 V = 15.70~1 10 (unk) 15.17 C = 10.0mM 2 (unk) 3.09 C = 2.04mM Ferriceneb 5 6.18 n = 1.07 Ferroceneb 5 6.03 n = 1.04 Ferriceneh 5 6.15 n = 1.06 a All experiments 5 m M Cu(II), 0.25M in tetraethylammonium perchlorate-acetonitrile. V = 12.03 pl b 2.6z. c In 0.25M TEAP/DMF.
No. of
data points 7
7
rel. std. dev. 3~1.6 jz2.6
7 7 2
b5.3 b3.0
4
i6:5
VOL. 40, NO. 7, JUNE 1968
1021
100
200
300
400
t (sec) Figure 7. Elimination of edge effect in step coulometry with reactant-getter electrode Figure 6 . Voltammetry of Pb(1I) nitrate a. On Pt b. On Au TFL.E's 2 mM Pbm) in 0.5M KCl
Scan rate i mv~sec
Table 111. Diffusion Coefficients for Cu(1) and Cu(II).sb No. of rel. D X los data points std. dev. Cu(I1) 0.739 I 4.5 CU(U 0.504 4 4.1 All experiments 5 mM Cu(II), 0.25M in tetraethylammonium perchlorate-acetonitrile. * At 26 "C ambient temperature.
A unique feature of steady-state voltammetry is that the background currents can be greatly attenuated. The potential ramp is applied at one electrode, and the current is recorded at the other, constant potential electrode, This fact, along with the steep concentration profiles obtainable in thin layers of solution, makes the twin-electrode technique particularly sensitive. With a cell of 77.4 microns, concentrations of copper as low as 5 X 10-5 M could easily be determined. A much greater sensitivity can be achieved at smaller I values and in aqueous solutions where the background contribution is less (18). Surface Effects. A very high degree of uniformity and a desired geometry are features which should prove especially useful in studies of surface effects that include adsorption, electrode history effects, and the mechanism of metal deposition and stripping reactions. The deposition and stripping of metal layers on solid electrodes have been extensively studied by Schmidt and Gygax (23), who used an elaborate thinlayer configuration to examine anomalous behavior in the voltammetry of these processes. The behavior of lead on platinum and gold thin-film electrodes was briefly investigated to demonstrate the utility of the present design for such studies. Solutions of 5mM lead nitrate in 0.5M potassium chloride were studied at platinum and gold electrodes by cyclic voltam(23) E. Schmidt and H. R. Gygax, J. Electroanal. Chem., 13, 318
(1967) and references therein.
1022
ANALYTICAL CHEMISTRY
I
= 150
us.
SCE
microns. Potential stepped from +1.2 to +0.2 1. Without reactant getter 2.
With reactant getter System as per Figure 2
metry (Figure 6). Even at very slow sweep rates (0.5 mV per second), we were unable to resolve the undervoltage peaks previously observed. The stripping peaks attributed by Schmidt and Gygax to alloy formation (AupPband AuPbz) were observed, though poorly resolved. Similar peaks were not observed with a platinum electrode. The deposition and stripping of lead on the gold surface were found to be much more reversible (Ep' = -0.64 volt, Epa = -0.45) than on platinum (Ep' = -0.52, Epa = +0.50). The differences in behavior exhibited by thin and semi-infinite metal electrodes may provide information as to the importance of alloy formation and the distance into the electrode surface that contributes to the electrode-solution interphase. Electron microscopy of the film surface before and after deposition can give information about the surface structure of alloys and specifically adsorbed layers (12). Electrochemical Elimination of Edge Diffusion. The possibility of abrogating the edge effect by scavenging active species from the solution surrounding the active thin-layer volume element has been suggested (15). Thin-film thinlayer electrodes (TFTLE) have consummated this suggestion. A triple electrode cell was employed using a single working electrode (from which current is read) masked by two similar electrode bands on either side of, but insulated from, the working electrode. These two reactant-getter electrodes (Figure 1, c) are maintained at the same potential as the working electrode and thus prevent lateral diffusion of additional product into the active thin-layer region (shadowed by the working electrode). This is well demonstrated by step coulometry of Cu(I1) in a 150-micron cell both with and without the reactant getters (Figure 7). Thus, perhaps the greatest problem with previous thin-layer cell designs ( 2 ) has been circumvented. This configuration permits the use of cells with 1 values far greater than the 100 microns previously considered to be a practical upper limit for TLE. These larger 1 values afford a lower iR effect; however, they increase the time required for homogeneous diffusional mixing.
1.0
\I\
4
0.6
!I \
0.5 A 0.4
0.3 I \
300
I
I
400 WAVELENGTH
I
1
I
500 (nm)
Figure 8. Transmittance characteristics of thin-layer cell components 1. Uncoated microscope slide 2. Pt film on glass 3. Au film on glass 4. Pt film on quartz - - All spectra us. air Transmission Spectroscopy. The utility of thin-layer electrochemistry for transmittance studies has been discussed (18) and demonstrated (6). Thin-film electrodes are particularly suited to this application. The transmittance properties of various cell components are shown in Figure 5. A platinum film deposited on quartz behaves as a neutral density filter throughout the UV-VIS region (200 to 700 mp). The oxidation of the aromatic hydrocarbon rubrene is of considerable interest because of the electrochemiluminescent activity of this compound. Although a detailed mechanism for this oxidation remains elusive, the absorption spectrum of the oxidation product(s) has now been determined (Figure 9). That the reaction is chemically reversible was demonstrated by stepping the potential between 0.0 and f1.1 volts every 5 minutes. The rubrene spectrum was obtained at 0.0 volt and the product spectrum at f l . 1 volts. This cycle was repeated three times with the same solution in the thinlayer volume; the three sets of spectra were identical. The application of infrared spectroscopy to thin-layer transmittance studies is limited only by the availability of a suitable substrate. Preliminary experiments with platinum films deposited on sodium chloride plates indicate that such an application is feasible ; however, film instability makes these data unreliable. Other substrates and procedures are currently being investigated. Fluorescence Spectroscopy. The combined transmittance and reflectance properties of platinum thin films are advantageous for fluorescence experiments involving electrochemical reactants or products. Preliminary experiments have been undertaken with 2 m M solutions of rubrene in acetonitrilebenzene with 0.2.44 tetrabutylammonium perchlorate 40 as the supporting electrolyte. Stepping the potential to +l.O us. SCE oxidizes the aromatic hydrocarbon to a nonfluorescent state. The nature of this state has not been elucidated; however, it is definitely not the radical cation, as evidenced by its lack of the characteristic green color. This species, possibly a dimer, can be converted back to rubrene. Fluorescence (visually observed) of the original hydrocarbon in the thin layer decays as the oxidation proceeds. Subsequently stepping the potential to f0.6 volt reduces the product back to rubrene, as shown by an increase in fluorescence with time.
0.2 0.1 400 500 WAVELENGTH (nm) Figure 9. Spectral study of rubrene oxidation Product spectrum observed after application of +1.1 volts 17s. SCE for 5 min. Deaerated solution of 1 mM rubrene, 0.1M TEAP, 25% benzene in CHICN. Quartz cell with I = 77 microns, 4 in Figure 8
300
The measurement of fluorescence is possible for very low concentrations of material ; thus, a new method for following electrochemical reaction mechanisms at very low concentration levels is suggested. For example, the fluorescence spectrum of 5 X 10-6M perylene in a 77-micron thin-layer cell was easily monitored with a commercial fluorometer. This concentration is several orders of magnitude lower than that detectable by UV-VIS absorption spectroscopy of aromatic hydrocarbons in a thin-layer cell of identical /-value. OTHER APPLICATIONS FOR THIN-FILM ELECTRODES Many potential applications for TFTL electrodes can be envisioned. The feasibility of the following ideas is under active investigation. The dual properties of transmittance and reflectance (24) suggest their use in a wide variety of spectroelectroanalytical experiments. Usable electrodes with transmittance from 0 to 70% can easily be obtained. In addition to the advantages of transmitting light perpendicular to the thin layer (18), the feasibility of multiple specular reflections within the lateral dimension of the (twin-electrode) thin layer should provide greatly enhanced sensitivity because of the enormous increase in effective path length. A flow-through electrochemical cell for the study of adsorption by radioactive monitoring has been reported (25). Relatively thin layers (ca. 300 microns) of solution were employed in order to enhance the signal level attributable to the adsorbed layer us. the activity of the solution species. This suggests another potential application of the present electrode design. A flow-through cell could be applied to the detection of effluents from micro-liquid chromatography (adsorption or ion exchange) utilizing a single cell containing successively (24) 0. S. Heavens, "Optical Properties of Thin Solid Films," Academic Press, New York, 1955, p 162. (25) K. Schwabe and W. Schwenke, Electrochim. Acta, 9, 1003 (1964). VOL. 40, NO. 7 JUNE 1968
1023
both the chromatographic column and the thin-layer electrode between two glass plates. Such a system could easily separate and detect submilligram quantities of material. In addition, a flow-through cell containing two thin workingelectrode bands in succession could be constructed as a sensitive analog to the ring-disk electrode. Product produced at the first would be electroanalytically determined at the second. Since a wide variety of metal surfaces can be thermally deposited, it would be possible to use such surfaces to generate metal cations for a coulometric titration (or for other purposes) within a twin-electrode cell. In addition, the second electrode in a twin-electrode configuration could be incorporated as a reference electrode-e.g., Ag/AgCl. The determination of the oxygen content of both aqueous and nonaqueous solvents utilizing this cell configuration should have immediate practical applications. The independence of thin-layer coulometry from diffusion control (and thus diffusion coefficients) adds the element of temperature independence to such analyses. Combinations of minigrid and thin-metal-film electrodes for use in bulk solution electrochemistry have come to mind. One basic configuration would be a glass plate incorporating the metal film as one working electrode ( W , ) and a minigrid ( W Z )situated (using epoxy and Teflon spacers) in the neighborhood of 100 microns from the film. Using this design, the advantages of thin-layer steady-state methods (18-22) could be brought to bulk solution electrochemistry. A steady-state concentration profile (and thus current) could be established between the grid and metal film for a species present in the bulk solution. The magnitude of the steady-
state current would be directly related to this bulk solution concentration. The electrochemical generation of free radicals for electron spin resonance studies has been reviewed by Adams (26) and Poole (27). The incorporation of a TFT’LE into a cell for this purpose would have several advantages. The exhaustive nature of thin-layer electrolysis would preclude the line broadening attributed to parent-radical exchange. Shortlived species could be rapidly regenerated to a steady-state concentration level in such a design without the necessity of the inefficient flow systems now used. The thin geometry would mitigate the dielectric loss associated with the supporting medium. ACKNOWLEDGMENT
The authors thank E. N. Mitchell, J. Zunes, and P. Smejtek of the University of North Carolina Physics Department for technical advice and loan of the vacuum deposition apparatus. Helpful discussions with R. W. Murray and W. R. Heineman are acknowledged. RECEIVED for review December 1, 1967. Accepted March 20, 1968. Research supported by the Directorate of Chemical Sciences, Air Force Office of Scientific Research, Grant AF-AFOSR-584-66 and by the Advanced Research Projects Agency. (26) R. N. Adams, J . Electroanal. Chem., 8, 151 (1964). (27) C. D. Poole, “Experimental Techniques in Electron Spin Resonance,” Interscience Publishers, Wiley, New York, 1967, p 620.
Pulse Response of an Electrode Reaction Keith B. Oldham Science Center of the North American RockweN Corporation, Thousand Oaks, Calif. 91360 DERIVATION IN TERMS OF CONCENTRATION PROFILE
General expressions are derived for the faradaic current which flows following the imposition of a constant potential on a planar electrode at which an electrochemical reaction can occur. A less exact treatment of the dropping electrode case is included. No assumption is made about the conditions prior to the pulse. The response is related both to the concentration profiles at the instant of pulse application and to the electrical conditions existing at that instant. The results obtained apply to reversible or irreversible electrode reactions. The implications of the theory to the analysis of nonuniform solutions are discussed and a new method is proposed for the determination of kinetic parameters.
may occur. At the instant t = 7 the potential pulse is imposed. The third and final period embraces all times thereafter, but prime concern will be for current behavior in the period r < t < r T where T is small in comparison with r . The initial absence of an electrode reaction may arise from a variety of causes, the simplest being the electrical isolation
SEVERAL ELECTROANALYTICAL methods-square-wave polarography (I), pulse polarography (2,3)-employ the concept of a potential-pulse, as do a number of techniques-potentialstep method (4,voltage-step method (5),pulse-polarographic method (6, 7)-for the measurement of electrode kinetic parameters. By a potential-pulse is meant the sudden imposition of a constant potential to an electrode. The response to this pulse, that is the current passing at times soon after pulse application, reflects the conditions existing at the instant of application. This article seeks to establish the response for any arbitrary condition of concentration or of electrical parameters immediately prior to the pulse.
(1) G. C. Barker, R. L. Faircloth, and A. W. Gardner, Report C/R 1786, Atomic Energy Research Establishment, Harwell, England, 1955. (2) G. C. Barker and A. W. Gardner, Z . Anal. Chem., 173, 78 (1960). (3) E. P. Parry and R. A. Osteryoung, ANAL.CHEM.,37, 1634 (1965). (4) . . H. Gerischer and W. Vielstich, 2.Physik. Chem., (Frankfurt am Main) 3, 16 (1955). (.5.) W. Vielstich and P. Delahav, _ .J . Amer. Chem. Soc.,. 79,. 1874 (1957). (6) . _J. H. Christie. E. P. Parry, .. and R. A. Osteryoung, . - Electrochim. Acta, 11, 1525 (1966). (7) K. B. Oldham and E. P. Parry, ANAL.CHEM.,40,65 (1968).
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ANALYTICAL CHEMISTRY
Three distinct periods of time will be considered. Initially, for t < 0, there is no electrode reaction whatsoever. A period 0 < t q r then ensues in which the electrode reaction Ox(so1n)
+ ne-(M)
Rd(so1n)
(1)
+
