Anal. Chem. 1994,66, 3528-3532
Reactivation and Stability of Pyrolytic Carbon Film Electrodes on Macor Substrates Jill K. Clark, William A. Schilling, Charles A. Wljayawardhana, and Paula R. Melaragno’ Ebaugh Laboratories, Denison University, Granville, Ohio 43023
The stability of pyrolytic carbon film electrodesbased on Macor substrates was evaluated. Electrode activity, estimated via cyclic voltammetry of ferricyanide, was virtually unchanged after 120 min in solution (kO = 0.018 cm/s). The response decayed by less than 40% after 20 h in solution. Electrodes were deactivated by air exposure but could be restored to good levels of performance via electrode anodization (to kO = 0.005 cm/s). Other reactivation methods were evaluated. Heat treatment in an Ar atmosphereand an acetone wash were nearly as effective as anodization but were less reliable. Polishing was relatively ineffective. Pyrolytic carbon films (PCFs) are potentially quite useful electrode surfaces. Rates of electron transfer equivalent to or even better than those obtained at conventionally polished glassy carbon (GC) electrode can beobtained without electrode pretreatment.’+ Electrode capacitance (and thus the level of residual current) is ordinarily quite low, on the order of 10 fiF/cm2.’** PCF electrodes based on Macor substrates are among those which display good charge transfer characteristics, although the capacitance is somewhat larger and more variable than that reported for other substrate^.^ And unlike the quartz and GC substrates typically employed for film work, Macor can be machined using ordinary high-speed tools. Therefore, PCF electrodes based on this substrate should be uniquely well-suited for construction of electrodes of unusual size and shape. However, routine use of this type of electrode required that we address two additional concerns. The first issue was electrode activity over long periods of time in solution. Ideally, the initial response would be maintained over several h, which would facilitate process monitoring and detector applications. In this paper, we demonstrate that for Macor-based films, performance is maintained for longer than 21 h. The second issue was that of film reactivation. Although the PCF can be completely removed from the substrate by heating in air and can then be replaced by fresh film, this is a time-consuming procedure. We evaluated other convenient methods of electrode reactivation, including electrochemical anodization, heating in an inert atmosphere, an acetone “wash”, and, finally, polishing. Electrochemical anodization was found to be the most effective of these methods, restoring films to activity levels comparable to that reported for conventionally polished GC e l e ~ t r o d e . ~ (1) Beilby, A.; Carlsson, A. J. Electroonal. Chem. 1988, 248, 283-304. (2) Eriksson, A,; Norekrans, A.; Carlsson, J. J . Elecrroanal. Chem. 1992, 324, 29 1-305. (3) McFadden, C.; Melaragno, P.; Davis, J. Anal. Chem. 1990, 62, 742-746. (4) McFadden, C.; Russell, L.; Melaragno, P.; Davis, J. Anal. Chem. 1992, 64, 1521-1527.
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Analytlcal Chemistty, Vol. 66,No. 20, October 15, 1994
EXPERIMENTAL SECTION Chemicals. All chemicals were reagent grade. Nanopure water was used in preparation of solutions. Ferricyanide solutions were 0.5-1 .O mM in 1.O M KCl. Macor was obtained from AstroMet Associates Inc. (Cincinnati, OH) in rods in. in diameter which werecut to desired lengths using ordinary high-speed tools and a water soluble coolant. Natural gas was taken from the common laboratory line and used without further purification. The composition was 95.24% methane, 2.43% ethane, 0.50% propane, 0.10% isobutane, 0.10% n-butane, 0.04% isopentane, 0.03% n-pentane, 0.14% hexanes and higher order hydrocarbons, 0.45% Nz, 0.96% CO2 and 0.10% tert-butanethiol. Argon was of 99.4% purity. Pyrolysis. All films were fabricated at temperatures of 926 “ C for a time of 3 h. The procedure was the same as described earlier4except for the following two changes. First, gas flow rates were controlled via a two tube proportioner (Matheson, Model 7392T). Second, pyrolysis was intentionally performed in a “dirty” furnace liner, i.e., a liner which contained residual film left over from previous pyrolyses. Films made in the presence of residual film were much more reliable than those made in clean quartz liners. Reliability of performance was gauged by the range of hE, observed for the ferri-/ferrocyanide redox couple. For clean liners, the range was 65-104 mV ( N = 10). For “dirty” liners the range was 64-77 mV ( N = 23). Electrochemistry. Two coats of black Dennison marker were used to insulate the sides of the Macor-based film such that only the bottom planar disk was exposed to solution. Use of this typeof insulation in aqueous solution has been described previously.6 A flat current base line was obtained when the entire film surface was coated (including the bottom planar disk) in the presence of ferricyanide. This verified that the insulation was sufficient. Care was taken to minimize film exposure to air prior to analysis. The minimum exposure was fixed by the time required to insulate the sides of the electrode, 2-6 min. No significant differences in performance were observed over this time range. Significant loss of performance was observed when films were exposed to air for longer than 6 min. A Cypress Systems Model CS1090 electrochemical analyzer was used for cyclic voltammetry. The reference was a BAS Ag/AgCl, 3 M NaCl electrode, and a Pt wire served as the auxiliary. A scan rate of 0.100 V/s was used throughout this work. The system software was used to determine k”/ D ‘v from the separation between the anodic and cathodic (5) McCreery, R. L. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker, Inc.: New York, 1991; Vol. 17, pp 221-374. (6) Sternitzke, K.; McCreery, R. Anal. Chem. 1990, 62, 1339-1344.
0003-2700/94/0388-3528$04.50/0
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peaks as described by Nichol~on.~ The diffusion coefficient used for ferricyanide was 0.677 X cm2/sa8 All electrochemical measurements were made at room temperature. Electrode Treatment. Electrochemical treatment (ECT) consisted of a 90 s anodization at potentials in the range of 1.0-1.8 V followed by cathodization for ca. 30 s at -0.100 V. The solvent was 1 M KNO3. Heat treatment consisted of heating the films in the tube furnace at ca. 600 "C for 2 h under an argon atmosphere. Polishing was done by hand, using 0.05 pM alumina (Beuler) on glass covered with a polishing cloth. In a few cases, the polishing cloth was not used. Polishing was ordinarily followed by ultrasonication in or copious rinsing with nanopure water.
RESULTS/DI SCUSS ION Electrodes were evaluated by comparing AE, obtained via cyclic voltammetry for the ferri-/ferrocyanide redox couple. This parameter can be related to k",the standard rate constant which governs the heterogeneous electron transfer, after the method of Nichol~on.~ Although this particular redox couple may not be the best choice for evaluating ease of electron t r a n ~ f e r it, ~has been the most widely used and continues to serve as a useful means of comparing different electrodes. Film Stability. The stability of fresh films in solution was excellent. As can be seen in Figure 1, over 120 min of continuous exposure to ferricyanide solution, AE, for the ferri-/ferrocyanide redox couple varied over a range of only 10 mV. We observed slightly more variation for a conventionally polished GC electrode, and these results are also shown in Figure 1. As a point of comparison, very active GC electrodes (k" ca. 0.2 cm/s) are also quite stable in solution, with less than 10% decrease in k" over 200 min in Nanopure electrolyte.1° It is important to note, in this case at least, that the PCF electrode exhibited a faster rate of electron transfer than the GC electrode at all times. (7) Nicholson, R. Anal. Chem. 1965, 37, 1351. (8) ANia, A. J.; Bazan, J. C.; Carrozza, J. S. W . Electrochim. Acta. 1968, 13,
81-90.
(9) Kneten, K.; McCreery, R. Anal. Chem. 1992,64, 2518-2524. (10) Poon, M.; McCreery, R. Anal. Chem. 1986, 58, 2745-2750.
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After 21 h in solution, AE, did increase but only slightly. The maximum value observed (at 20 h) was 80 mV, which corresponds to k" of 0.01 1 cm/s. This compares quite well to the initial value for this particular PCF electrode (0.018 cm/s). Semiintegral analysis of the CVs showed no evidence of ferricyanide adsorption.' Films based on Macor were much more stable than those described by Erikkson et. al., who report that k" for films on prepolished GC substrates decreased to 15% of the original value in just 300 min of continuous exposure, with much of that occurring in the first 100 min.2 These authors attribute the dramatic change to a surface rearrangement process which increases the proportionof basal to edge plane carbon. Perhaps in the case of Macor substrate, the film adheres more tightly to the rougher, machined surface, preventing such a rearrangement. Films were much less stable with respect to air exposure and in most cases were deactivated overnight. Deactivation was indicated by two voltammetric features: an increase in AE, of 20 mV or more and a marked decrease in peak current. In Figure 2, AE, for several PCF electrodes is tracked for up to 5 days of air exposure. All five electrodes exhibit AE, of C79 mV (k" ca. initially. After 48 h of air exposure, AE, had increased in all cases to more than 88 mV, corresponding to a k" on the order of cm/s. The rate of deactivation is not reproducible, even when the electrodes exhibit essentially the same initial activity. For example, note 0 and at 0 and 2 days. The change in appearance of the CV itself upon deactivation can be seen in Figure 3, by comparison of the initial CV (solid line) and that obtained after prolonged air exposure (dashdot line). The decrease in i, and i,combined with the increase in AE, results in an extended, less well defined voltammetric wave. Reactivation. The question of reactivation has not been systematically addressed for carbon films on any substrate. However, "preconditioning" via electrode
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(11) Bowling, R.; McCreery, R. Anal. Chem. 1988,60, 605-608.
Analytical Chemistty, Voi. 66, No. 20, October 15, 7994
3529
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potential cycling,13 or an organic solvent "washn l 4 has been used to improve the initial performance of some films. In this work, several methods of reactivating "aged" films were investigated. These included electrochemical treatment (ECT), heat treatment in an inert atmosphere (HT), polishing, and an acetone wash. The first three of these methods are commonly used to reactivate GC surface^,^ so they were obvious choices here. The acetone effect was actually observed by accident during the course of this work. However, in retrospect, this should not have been a surprise, since Lundstrom routinely washed with an organic solvent (CH2Cl2/ethanol) prior to analysis, ostensibly to activate the surface.14 In general, films were considered to be "aged" and thus good candidates for reactivation when AE, for the ferri-/ ferrocyanide couple was 200 mV or greater. By chance, the electrodes which underwent heat treatment were an exception to this rule, exhibiting an average AE, prior to reactivation of 143 mV. Without exception, all electrodes chosen for treatment exhibited extended, poorly defined voltammetric waves characteristic of deactivated electrodes. Table 1 contains average values of AE,obtained after each reactivation method along with the relative standard deviation (RSD). Also given is k o corresponding to each average Up. Values obtained for fresh films are given here for comparison. These are similar to those obtained previ~usly.~ (12) Rojo, A,; Rosenstratten, A,; Anjo, D.Anal. Chem. 1986, 58, 2988-2991. (13) Blaedel, W.; Mabbott, G. Anal. Chem. 1978, 50, 933-936. (14) Lundstrom, K. Anal. Chim. Acta 1983, 146, 97-108.
3530
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Analytical Chemistry, Vol. 66,No. 20, October 15, 1994
Figure 4. Residualcurrentat fresh PCF, initial(-) and second potential scan (-)and after 6 mln of air exposure (- - -). Solvent was 1 M KCI.
Electrochemical Treatment (ECT). This method was the most effective and reliable means of restoring electrode activity, as can be seen by inspection of the data in Table 1. ECT yielded AEp values closest to those for fresh films, with the smallest RSD. The original activity was not totally regained, asko was reduced by a factor of 3.5. However, ko wasidentical to the value most often quoted for conventionally polished GC.5 In other words, reactivated films were comparable to conventionally polished GC, at least for the ferri-/ ferrocyanide redox couple. The degree of reactivation depended upon the anodization voltage. Voltages in the range of 1.O-1.8 V were investigated. At 1.0 V or less, essentially no activation occurred. At anodization voltages of 1.7 V or greater, the increase in background current was prohibitively large. The best compromise between activation and increased background current was obtained for anodization voltages of 1.65 V. The extent of reactivation is evident in Figure 3, where the CV for the deactivated electrode (dash-dot line) is shown with that obtained after anodization at 1.5 (dotted line) and 1.65 V (dashed line). Kinetically, the result after 1.5 V was a little sluggish in comparison to that obtained when the film was fresh (the solid line in Figure 3). The increase in background was slight, except at potentials negative of -300 mV. After anodization at 1.65 V, the same film showed improved kinetics but increased background over the entire potential range. The background peak in the vicinity of -300 mV evident in Figure 3 after anodization was characteristic of electrochemically treated films and was frequently observed for fresh films as well. In the case of treated films, it can be attributed to reduction of a surface oxide produced during the anodization step. It was not present prior to ECT, as can be seen from Figure 3. And indeed, the formation of such oxides during anodization is well documented for other carbon materials like GC and HOPG.S In the case of fresh films, the surface oxide would seem to be the product of air oxidation. This is demonstrated by the series of CVs in Figure 4. This particular film exhibited an unusually pronounced cathodic peak at -3 10 mV after just 3 min of air exposure (solid line). The peak height decreased upon the second potential scan (dotted line)
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but increased again upon several min more exposure to air (dashed line). It is important to emphasize that no correlation between film activity and the magnitude of the cathodic peak in the background was observed. Films reactivated via ECT exhibited good stability in solution over at least 300 min, as can be seen from the data included in Figure 1. In addition, after ECT, films were less sensitive to air effects. For example, AE, increased by only about 10 mV for one film after 3 days storage in air. After an additional anodization, virtually no decline in performance was observed after 5 days of air exposure. The same effect has been noted for GC in solutionis and was attributed to the formation of an "oxide" layer which presumably affords some protection against adsorption of organic materials. The same effect may be important here. Films could be reactivated via ECT several times but with a concomitant increase in current background. One film was reactivated five times. Each time, ko returned to a value of 0.005 cm/s, but the background gradually increased until the peaks were nearly obscured. We found that initial rates of electron transfer were not improved by ECT. This was surprising because it contradicted an earlier report' in which rates of electron transfer as fast as 0.1 cm/s were observed after anodization at 1.65 V. However, these authors also note that the PCF film appeared to be at least partially removed by anodization. Therefore, their result is probably due to activation of the underlying prepolished GC substrate. No film loss was observed for Macor-based films, even upon multiple anodization. Again, this may reflect better adherence between the film and the relatively rough Macor surface. Other Treatment Methods. Heat treatment (HT) in an argon atmosphere was less effective and reliable than ECT, as can be seen from the average AEp and RSD given in Table 1. In addition, residual current increased significantly upon HT, as shown in Figure 5 . The cathodic peak in the vicinity of -300 mV was quite pronounced relative to that observed when the film was fresh and after the film was deactivated (15) Wang, J.; Tuzhi, P.Anal. G e m . 1986, 58, 1787-1790.
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by air exposure. Finally, some film loss was visible after the H T process. These last two points are consistent with film oxidation,which would occur during H T if trace 02was present in the system. An acetone wash was a surprisingly effective means of restoring film activity. This can be seen from AEpand RSD values given in Table 1. A significant advantage of this reactivation method was that residual current levels were not significantly increased upon the wash, either prior to the ferricyanide reduction peak or in the vicinity of -300 mV. This can be seen by inspection of the CVs in Figure 6. Surface cleansing would seem to be the obvious explanation for acetone activation. And, as noted earlier, an organic wash solvent has been used prior to initial analysis for this express purpose.14 However, this mechanism is inconsistent with two experimental observations. First, the acetone wash was effective only once. When films were allowed to degrade after an initial acetone treatment and were then retreated, the results were quite unimpressive, with average AEp of 327 mV (RSD of 36%, N = 4). Second, an acetone wash deactivated fresh films. For two fresh films, AEp values of 626 and 804 mV were obtained, values well outside the ordinary range of results. These results indicate that more than simple surface cleansing is involved. Polishing was the least effective and reliable reactivation method (Table 1). Polishing without a polishing cloth gave similarly poor results. In addition, without the cloth, some film was removed upon polishing. Residual current levels increased only slightly with respect to the inactive film.
CONCLUSIONS Macor-based PCF electrodes are viable alternatives to GC for routine analysis and should be particularly useful in applications requiring an electrode of unique size and shape. These electrodes display initial rates of electron transfer which are quite good, and these are maintained over long periods of solution exposure. Once films become inactive, they can be successfully reactivated via ECT. ECT is a convenient in situ method which does not require mechanical or optical access Analytical Chemistry, Vol. 66, No. 20, October 15, 1994
3531
to the surface like polishing or laser ablation, which is an important consideration for some geometries, particularly flowthrough types. One disadvantage of using Macor-based PCF electrodes is that residual current levels are somewhat higher than those observed at comparable electrode materials. As previously reported? residual currents on the order of 1 pA are typically obtained at a l / 8 in. diameter disk. This translates into a capacitance of 126 pF/cm2, which is 3-6 times that generally observed at GC and as much as 15 times larger than that reported for PCFs on prepolished GC substrates. This may be due, in part, to the relatively rough machined surface of the Macor substrate. However, preliminary results obtained in our laboratory indicate that the purity of the pyrolysis gas may be a more important factor. And, indeed, there is significant room for improvement here, as the natural gas used in this work to prepare films contained significant amounts of other components (see Experimental Section). It may be possible to decrease film capacitance through use of higher purity pyrolysis gases. Surface oxides are produced by ECT as well as air oxidation. Although this is typical of carbon materials in general, this (16) Fagan, D.; Hu, I.; Kuwana, T. Anal. Chem. 1985, 57, 2759-2163. (17) Rice, R.; Pontikos, N.;McCreery, R.J. Am. Chem. Soc. 1990, 112,46174622.
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Ana&ticalChemistry, Vol. 66, No. 20, October 15, 1994
poses a problem for routine analysis. Reduction of the surface oxide produces a peak in the region of -300 mV which can be quite pronounced and thus limits utility in this region. Since ECT does not improve initial rates of electron transfer, it is tempting to conclude that 0.02 cm/s represents an upper limit on the rate constant for ferri-/ferrocyanide at PCF electrodes. Results in this range have been measured independently by three different groups14 on both GC and Macor substrates. However, vacuum heat treatment160r laser ablation,'O the two most effective methods for activating GC, might yield faster rates. The first of these would afford rigorous exclusion of 0 2 during treatment, which was probably not provided by the experimental arrangement employed in this work. The advantage of the second method is that it is an in situ method which does not produce surface oxides.''
ACKNOWLEDGMENT This work was supported by a William and Flora Hewlett Foundation Award of Research Corporation and by the Denison University Research Fund and the Anderson Endowment Program of Denison University. Received for review March 10, 1994. Accepted July 4, 1994.' Abstract published in Advance ACS Absfracts, September 1, 1994.
