ANALYTICAL CHEMISTRY, VOL. 50, NO. 7 , JUNE 1978
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Pyrolytic Carbon Film Electrode W. J. Blaedel' and G. A. Mabbott Deparfment of Chemistry, University of Wisconsin, Madison, Wisconsin 53706
by 5 min a t -0.3 V improved the reproducibility and reversibility of the electrode response in ferricyanide solution. Blaedel and Jenkins also observed that an electrode pretreatment consisting of cycling between an oxidizing and a reducing potential enhanced the oxidation of NADH a t glassy carbon (12). In this paper we describe the fabrication and use of a carbon film electrode t h a t can be deposited easily on quartz. The reduction of ferricyanide and the oxidation of NADH at the carbon film are compared with these reactions a t glassy carbon. With proper pretreatment and use, the carbon film electrode yields well-defined current-voltage curves.
The fabricatlon and electrochemical characterization of a carbon film electrode is presented. The film is easily deposited on a quartz surface using simple glassblowing equipment. An electrochemical pretreatment procedure of f4 V at 70 Hz enhances the electrode response and yields well-defined current-voltage curves. Current-voltage data for the reduction of ferricyanide and for the Oxidation of NADH were used to compare the carbon film electrode with glassy carbon.
Workers have been using carbon electrodes for electroanalytical measurements for about 25 years (1-3). Carbon electrodes are particularly useful for studying anodic processes in cases where the use of platinum or gold electrodes is complicated by t h e adsorption of organic materials (3)or by extensive surface oxidation as in molten salt solutions ( 4 ) . Several different forms of carbon suitable for electrochemical analysis are readily available. Glass-like or vitreous carbon (glassy carbon) has been popular because of its low porosity and relatively reproducible performance. T h e commercial fabrication processes start with various resins but all seem to include a high temperature pyrolysis step under a controlled atmosphere ( 5 ) . The resulting material is thought t o consist of layers of tangled ribbons similar to graphite in structure but without the extensive, oriented sheets t h a t graphite forms (6). Pure graphite has had limited analytical use as an electrode material because of its high porosity, which leads to high residual or background currents. Adams reviewed work with wax-impregnated and carbon paste electrodes which minimize this problem (7). Beilby and co-workers compared the performance of the wax-impregnated graphite electrode with a pyrolytic carbon film electrode produced by depositing carbon in a methane-nitrogen mixture passing over a ceramic rod (8). For deposition of the film, the rod was maintained at 1025 OC in a rotating combustion tube inside a furnace for 24 h. Commercially prepared pyrolytic graphite is deposited in a similar manner on graphite substrates (9). From chronopotentiometric curves in ferricyanide-ferrocyanide mixtures, Beilby showed that the electron transfer reaction was more reversible a t the pyrolytic carbon film than a t a n impregnated graphite surface, but not as reversible as a t a platinum electrode. More recently, reports of other carbon film electrodes have appeared. Mattson and co-workers have done spectroelectrochemical studies using optically transparent carbon films on germanium and quartz (10, 1 1 ) . Their electrodes were prepared commercially by an electron-beam method. Procedures for preparing the carbon surface for an experiment vary. Many workers have been satisfied with the performance of glassy carbon after a simple polishing. Polishing is not recommended for pyrolytic graphite surfaces because the material is formed in layers whose basal surfaces have different properties from those of the edges (9). Grinding exposes varying amounts of the layer edges causing poor reproducibility. Acid-dichromate cleaning solution followed by a mild electrochemical pretreatment has been used for cleaning glassy carbon and pyrolytic carbon. Beilby found that polarizing the film electrode for 15 min a t +1.5 V followed 0003-2700/78/0350-0933$01 OO/O
THEORY Rate constants for the reduction of ferricyanide were calculated as a basis for comparing electrode materials and different pretreatment methods. The approach used here is similar to that used by Jordan and co-workers (13,141. The equations below were derived by Blaedel and Schieffer (15) to eliminate the need to represent the hydrodynamic properties of convective electrode systems mathematically. (It can be demonstrated that this is equivalent to t h e method used by Randles (16)for calculating rate constants.) Here transport coefficients, TOand TR, are determined empirically from the limiting currents, i, and i,, in the cathodic and anodic regions of the current-potential curve. T h e potential for which the net current is zero for an equimolar mixture of the oxidized and reduced forms was assumed to be a measure of the formal potential, EO'.
i, = n F A T o Cob and i, = nFA TRCRb
(1)
where
Cob and C R b are the bulk concentrations of the oxidized and reduced forms. k j and kb' are the forward and backward heterogeneous rate constants for the electron exchange reaction. All other terms have their usual electrochemical significance. Values of k j are calculated for various applied potentials and measured currents. Based on the ButlerVolmer equation, a plot of In k ; vs. E - E"' yields a straight line whose intercept (In k"') contains the formal rate constant k"' and whose slope (-unF/RT) is proportional to the transfer coefficient, u. EXPERIMENTAL Procedure for Depositing Carbon Film. Clean quartz rods of -0.08 and -0.19 in. diameter were heated to a white glow in a natural gas/oxygen flame. The quartz was then quickly placed in another stream of natural gas that was oxygen-free. This was accomplished by passing the gas through a quartz tube of -0.5-in. i.d. and inserting the rod an inch or two down into the open end. A second torch was used to heat the working end of the tube so that the rod inside continued to glow with a red-orange color for a few minutes. This procedure produced a very shiny black coating. Best results were obtained when the rod was passed directly into the gas stream from the hot flame without cooling in the transfer. (In this respect, large diameter quartz rod was easier to work with than small diameter material.). The coated
C
1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978 ,ROTATED
SHAFT,
STAINLESS STEEL
I20
H E R C U R V DOOL
c z w CAWON FILM ON Q
b
0
0
Figure 1. Carbon film rotated rod (a) and disk (b) electrodes
end was then cut off from the stock piece and the fresh end of the stock quartz was fire-polished before depositing another coating. It was noted that transparent films (with much higher resistances) could be made. Neither the optical nor electrochemical properties of these thinner films were investigated. Apparatus. Small diameter electrodes served in early investigations. Coated rods about 1inch long were held in a rotated shaft as illustrated in Figure la. A short sleeve of Tygon tubing was used to provide a water-tight seal when the Plexiglas parts were threaded together. The film at the very tip of the electrode was sometimes coated with epoxy to restrict the working area to the cylindrical portion. Electrical contact was made through a stainless steel hypodermic tube dipping into a mercury pool that covered the inside end of the coated rod. Disk electrodes were made from the larger diameter rods by press fitting them into Teflon (Figure Ib). The experiments with glassy carbon and platinum involved rotated disk electrodes epoxy-sealed in Plexiglas similar to those described in an earlier paper (12). A laboratory-built rotated electrode system using an offset drive and a rotation counting system (12) were used to control the electrode speed. All potentials are referenced to a Ag/AgCl electrode in 0.1 M KC1 solution, separated from the working solution by an agar-filled coarse glass frit. Water from a thermostated bath (25.0 i 0.3 “C) was pumped through a jacketed cell in which all of the measurements were made. Pretreatment. Electrochemical pretreatment of the electrode surface was performed either by cycling manually between dc voltage levels applied through a Sargent XV polarograph or by a simple square wave generator. The square wave was produced by adjusting the output of a NE555 clock circuit to give high and low states of equal duration. This output was fed into an operational amplifier to vary the amplitude and to make the pretreatment signal symmetric about a voltage of zero. Following the pretreatment the electrode was held at the potential for the start of the scan (-30-60 s) until most of the transient current decayed. Generation of Current-Voltage Data. The voltage ramp of the polarograph was modified to permit precise recording of current-voltage curves. Deviations from linearity in the voltage ramp and inadequate precision in reading the applied voltage from the polarograph chart caused large errors in calculating rate constants from some of the steeply rising current-voltage curves. To eliminate these errors, the voltage ramp generated by the polarograph was applied to the summing operational amplifier in a laboratory-built potentiostat. By using a gain of about for the ramp voltage, a slower, more precise voltage sweep was obtained for recording the rising portion of the polarographic curve. Both the initial and the final applied potentials were read to h0.05 mV with a Leeds and Northrup potentiometer. Intermediate values were interpolated from the chart. A Keithley Model 414s picoammeter was used to monitor the current and a Houston Omniscribe recorder followed the signal at a chart speed of 2 in./min. Since this was a two-electrode system, the potential in all calculations was corrected for ZR drop through the solution. This procedure gave a voltage sweep that was known to about 1 mV a t all points along the curve. The pulsed rotation voltammetry (PRV) data were obtained by stepping the potential manually and recording the current at 300 and 1200 rpm. For the ac-treated electrodes, a pretreatment step preceded each voltage step.
-60
.~
~
00
04
a2
00 APPLIED
-02
- 0 4
POTEhirlAL
-06
-08
.
-io
(YOII~)
Figure 2. Scanning voitammograms at a carbon film electrode. 20 pM each of K,Fe(CN), and K,Fe(CN), in 0.1 M KCI at 1000 rpm. (a) Untreated carbon film. (b) Pretreated at k1.35 V, 2 min at each level, 5 cycles. (c) Pretreated at f4 V, -70 Hz for 4 min. (d) Background, pretreated at f4 V, -70 Hz, -10 s
Reagents. All solutions were prepared from analytical grade chemicals without further purification. NADH from Sigma Chemical Co. was stored over a desiccant at 6 “C. Fresh analyte solutions were prepared daily using deionized, doubly distilled water (the second distillation being from an alkaline permanganate solution). Solutions were deaerated with filtered nitrogen during the experiments. Equimolar solutions of potassium ferricyanide (20 pM) and potassium ferrocyanide (20 wM)in 0.1 M KC1 were used for the determination of rate constants.
RESULTS AND DISCUSSION A microscope with a n eyepiece micrometer was used to measure the thickness of the deposit. The film was estimated to be about 1 I m thick. The resistivity of the carbon film was estimated by plotting t h e resistance measured along a coated rod between two mercury contacts as a function of distance. A resistance of 32.5 R/mm2 was interpolated from the straight line graph a t a linear distance equal to the circumference of the rod. E f f e c t of P r e t r e a t m e n t o n M e a s u r e m e n t s i n F e r r i CyanideFerrocyanide Systems. The untreated carbon film gave very poorly defined current-voltage curves for the reduction of ferricyanide. Figure 2 is a series of current-voltage curves obtained in ferricyanide-ferrocyanide systems after various electrode pretreatments. All curves were recorded with the same electrode under identical conditions except for the pretreatment.. Increasingly “reversible” behavior was obtained for increasingly intensive pretreatment. Pretreatment procedures in which the potential was cycled between various Hz to 1000 anodic and cathodic levels a t rates from 4 X Hz were tried. An ac pretreatment of =t4 V a t 70 Hz for 4 min yielded t h e steepest current-voltage curves. Some complications accompany the ac pretreatment. Successive scans on blank solution interspersed by 4-min ac pretreatments gave nonreproducible background currentvoltage curves, each curve exhibiting higher cathodic currents than its predecessor. Also, long ac pretreatments eventually strip the carbon film from the quartz surface. It was found t h a t limiting the ac pretreatment to 10 s between scans following a 4-min ac pretreatment a t the beginning of a day gave reproducible backgrounds and steep current-potential curves for ferricyanide-ferrocyanide systems after about four scans. With this pretreatment procedure, a carbon film disk electrode was used for over 60 scans without any sign of film deterioration.
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
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Table I. Heterogeneous Electron Transfer Rate Constants and Transfer Coefficients for the Reduction of Ferricyanide
Electrode Carbon film Glassy carbon Glassy carbon Platinum
Pretreatment ac4
dc ac4 dcC
k o t cm/s
E"',V +0.136 -0.139 -0.134 +0.130
(iO.OO1)d (rO.001) (rO.001) (i0.002)
1.58 1.12 1.22 2.63
01
(i0.29) X ( t 0 . 1 7 ) x 10-3 (rO.ll) x (r0.24) x
0.465 0.385 0.685 0.378
(~0.08) (~0.06) (r0.04) (r0.07)
i.1.35 V, 2 min at each level for 5 cycles. i 0 . 5 V, 2 min at each level for 5 cycles. The i 4 V, 70 Hz, 10-15 s. voltammograms were recorded in equimolar ferricyanide-ferrocyanide solutions, nominally 20 pM (200 pM for platinum). Parenthetical values represent the standard deviation.
The carbon film electrode press fitted into Teflon easily withstands soaking in concentrated acid dichromate solution (at room temperature for 10 min). This chemical pretreatment also enhances the shape of current-potential curves in a manner comparable to the ac electrochemical treatment, and may be used as an alternative to electrochemical pretreatment. It should be noted, however, that hot acid dichromate dissolves the film. The effect of pretreatment on the response of glassy carbon was also studied. In this case an optimization experiment using a simplex triangle method (17) was performed for 1-min pretreatment periods. That combination of voltage levels and cycling frequency that produced the most anodic half-wave potential for the reduction of ferricyanide was considered to be the most favorable. The electrode was polished between tests to eliminate cumulative effects. It was concluded that f4 V a t 60-80 Hz was the optimal pretreatment procedure. although more extreme voltage levels were not tried. Table I lists the formal heterogeneous rate constants and transfer coefficients calculated for the reduction of ferricyanide a t carbon film, glassy carbon, and platinum electrodes. The ac-treated carbon film appears to be comparable to the dctreated glassy carbon electrode. It should be noted that the ac-pretreatment of glassy carbon also enhances the electron transfer rate by an order of magnitude. Pretreating platinum in the same manner produced very steep voltammograms. However, the current attributed to the reduction of platinum oxides varied on successive background scans, indicating that for platinum this pretreatment procedure was not reproducing the same surface before each scan. T h e current response of the carbon film electrode on the transport-limited plateau (-0.3 V) was a linear function of ferricyanide concentration over a range from 1.0 to 350 FM. In this experiment the electrode was pretreated only a t the beginning of the experiment. M e a s u r e m e n t s i n NAD Systems. To demonstrate the utility of the carbon film electrode for anodic studies, current-voltage data were recorded for the oxidation of NADH by pulsed rotation voltammetry (PRV). Details of PRV have been presented by Blaedel and Engstrom (18). Briefly, the technique involves recording the difference in the current response of a RDE a t fixed applied potentials for two different rotation speeds. The current difference depends only on the convectively transported electroactive materials in solution. PRV permits the observation of current-voltage data for submicromolar levels of electroactive materials-a sensitivity comparable to that of steady-state voltammetry (19), but without the need to wait for transient currents to decay. The shape of a PRV curve appears to be rounded somewhat and shifted toward higher over-potentials compared to a curve obtained by conventional scanning techniques. This is merely a characteristic of the PRV method and is predicted by theory (18). For this reason, PRV half-wave potentials should not be compared directly with those obtained in conventional voltammograms. However, Blaedel and Engstrom demon-
o
-
03
04
0 2 APPLIED
0 8
06 'QTENT
A.
lioffs,
Figure 3. PRV of NADH oxidation at a carbon film RDE, pH 7.25
phosphate buffer, at 300 and 1200 rpm Table 11. PRV Half-Wave Potentials for the Oxidation of NADH4
Electrode Carbon film Glassy carbon Glassy carbon
Pretreatment ac dc ac
E , / * .v i 0.40
-0.29
+0.18
10.2 pM NADH in pH 7.3, 0.05 M phosphate
buffer, strated that it is possible t o calculate rate constants for heterogeneous electron transfer from PRV curves. The values obtained were shown to be similar to those calculated for a model system using other techniques. Figure 3 is a PRV current-voltage curve for the oxidation of 10.2 pM NADH a t a carbon film rotating disk electrode. The oxidation reaction proceeds more easily on a conventionally pretreated (dc) glassy carbon electrode than on the carbon film as indicated by the less anodic half-wave potential in Table 11. The performance of the glassy carbon is also enhanced by the ac-treatment. The ac-treated electrodes were given a 10-15 s treatment at each new potential. In the potential region near the rising portion of the curve, the difference current signal decreased linearly with time. Pretreating the electrode and stepping back to the same applied potential reproduced the same pattern-a large Pi initially, followed by a linear decay in the current difference. I t was hypothesized that the decay in the signal was caused by a potential-dependent absorption of a reaction product. (Earlier workers reported a similar current loss attributed to surface fouling or a loss of effective surface area (12).) T o correct for this decay, the observed difference current was found by extrapolating a current-time plot back to the moment when the new potential was applied. The signal difference currents were also corrected for the transport-
936
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
W.J. Wheeler and C. M. Amling in developing the carbon
dependent current recorded in the blank buffer solution. Similar behavior was observed with both ac-treated glassy carbon and carbon film electrodes. Pretreatment appears to renew the electroactive surface and the PRV technique permits separation of the transport-dependent current signal from background current transients. The procedure appears t o be a workable approach to overcoming a surface fouling problem. The oxidation of NADH appears to have a significantly greater over-potential on the carbon film, but the very broad, well-defined plateau indicates considerable analytical promise for the carbon film electrode.
coating process.
LITERATURE CITED L. B. Rogers and S. S. Lord, Jr., Pittsburgh Conference on Analytical Chemistry, March 1952. H. E. Zittel and F. J. Miller, Anal. Chem., 37, 200 (1965). R. E. Panzer and P. J. Elving, J . Electrochem. Soc., 119, 864 (1972). H. A. Laitinen and D. R. Rhodes, J. Electrochem. Soc., 109, 413 (1962). T. Noda, M. Inagaki, and S. Yamada, J . Non-Crystalline Solids. I,285 (1969). G. M. Jenkins, K. Kawarnura, and L. L. Ban, Proc. R . Soc., London(A), 327, 501. (1972). R. N. Adams, "Electrochemistry at Sola Electrodes", Marcel Dekker, New York, N.Y., 1969. A. L. Beilby, W. Brooks, Jr., and G. L. Lawrence, Anal. Chem., 36, 22 (1964). R. E. Panzer and P. J. Elving, Electrochim. Acta, 20, 635 (1975). J. S . Mattson and C. A. Smith, Anal. Chem., 47, 1122 (1975). T. P. DeAngelis, R. W. Hurst, A. M. Yacynych, H. B. Mark, W R. Heineman, and J. S. Mattson. Anal. Chem., 49, 1395 (1977). W. J. Blaedel and R. A. Jenkins, Anal. Chem., 47, 1337 (1975). J. Jordan, Anal. Chem., 27, 1708 (1955). J. Jordan and R. A. Javick, Electrochim. Acta, 6, 23 (1962). W. J. Blaedel and G. W. Schieffer, J . Electroanal. Chem., 80, 259 (1977). J. E. B. Randles, Can. J . Chem., 37, 238, (1959). S . N. Deming and S. L. Morgan, Anal. Chem.. 45, 278A (1973). W. J . Blaedel and R. C. Engstrom, Anal. Chem., 5 0 , 476 (1978). W. J. Blaedel and R. A. Jenkins, Anal. Chem., 46, 1952 (1974).
C0NCLUS I ON S An adherent, shiny, black carbon film can be formed on quartz surfaces simply, inexpensively, and in a variety of shapes. As yet, no attempt has been made to study the structure of the film,but electrochemical experiments indicate t h a t it compares moderately well with glassy carbon as an electrode material. It appears to be a practical tool for electrochemical studies. Well-defined, reproducible current-voltage curves can be obtained by conditioning the surface with an ac-treatment of 1 4 V a t -70 Hz before the current measurement.
RECEILTD for review December 27, 1977. Accepted March 22, 1978. This work has been supported in part by a grant (No. CHE76-15128) from the National Science Foundation.
ACKNOWLEDGMENT The authors appreciate the assistance of glass technicians
Exchange Kinetics at Potassium-Selective Liquid Membrane Electrodes Karl Cammann' Department of Chemistry, University of Chicago, Chicago, Illinois 60637
The ion exchange between a valinomycin-containing organic phase and aqueous solutions was studied. Estimates of apparent exchange current densities with potassium salt solutions as well as with solutions containing various interfering ions demonstrate a high correlation of this exchange current density and the corresponding Nernstian behavior.
This study continues the work described earlier in this journal ( I ) . T h e similarity of ion-transfer reactions a t ionselective membranes with redox reactions a t metal electrodes was demonstrated. In the theoretical treatment of ion-selective electrodes, one among other assumptions is that of a thermodynamic equilibrium a t the interfaces (2-5). Deviations from an ideal Nernstian behavior may then be explained by additional diffusion potentials inside the ion-selective membrane. Another explanation would be a deviation from the equilibrium situation at the interface of interest. The importance of t h e ion-exchange rate constants, their potential dependences, and the resulting linearized charge transfer resistances has already been pointed out by Buck (6) in the case of solid Present address, University of Munich, Institut fur Mineralogie und Petrographie, Theresienstr. 41, 8 Munchen 2, West Germany. On leave at the University of Chicago during 1976. 0003-2700/78/0350-0936$01.00/0
electrodes and by Gavach (7-9) in the case of liquid ionexchanger electrodes. Buck (10) has also given an experimental criterion for cases where the effect of surface rate shows up. Jaenicke and Haase (11) already determined the cationand anion-exchange current density at silver halide electrodes. They found deviations from the equilibrium a t the interface if the silver salt was solvated by complexing agents even in the case of exchange current densities in the order of 0.1 A/cm2. Tadros and Lyklema (12) demonstrated several aspects of nonequilibrium behavior a t glass electrodes. The determination of exchange current densities of different ions a t the same ion-selective membrane is therefore relevant to understanding whether thermodynamic equilibrium is achieved or not. The exact experimental determination of the exchange current density a t membranes with high bulk resistances however is difficult. According to the previous article ( I ) and Equation 79 of (13), the charge transfer resistance which is assumed t o be very low, is in series to bulk resistance Rhf. Lev et al. (14) determined the sum of the two charge transfer resistances a t a membrane by varying the membrane thickness and thereby 2Mand extrapolation. This study uses the concentration dependency of one charge transfer resistance to extrapolate to a term which is assumed to be constant in Equation 79, a t least over short time intervals. This technique, however, works only if the charge transfer resistance is comparable to the bulk resistance at least a t low concentrations of the potential determining ion. C 1978 American Chemical Society
