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Anal. Chem. 1986, 58, 2988-2991
Characterization of a Conductive Carbon Film Electrode for Voltammetry Arthur Rojo, Andrew Rosenstratten, and Dennis Anjo* Department of Chemistry, California State University, Long Beach, California 90840 A conductive carbon fllm electrode prepared from the pyrolysis of 3,4,9,lO-perylenetetracarboxylk dlanhydrlde has been evaluated as a working electrode for voltammetry. Cycllc voltammetry was employed to characterize the electrode. The response parameters, A€, and I,, were monltored as a functlon of anodk pretreatment t h e In a series of solutions. I t was found that after 1-5 mln of anodk pretreatment the electrode response was optlmum. The anodk potential limit was found to be similar to other carbon electrodes. Electrodes that had excessive anodic pretreatment for 20 rnln or longer were found to sorb the catecholamines studled causlng prepeaks; excesslvely treated electrodes were also found to glve dtfferent catecholamfne oxldatlon products than the less treated electrodes. Electrodes treated between 1 and 10 mln at 1.8 V were found to be analytically useful. The electrode response with all the catecholamines was superlor to other electrode materials, but the response was inferior for the ferrocyanide ion.
Many materials are being used as carbon electrodes in voltammetry; these include the graphitelike turbostratic carbons, anisotropic glassy carbons, and the carbon paste electrode. Because carbon is the most versatile electrode in anodic voltammetry, improvements in behavior and an understanding of the electron transfer steps are of great interest. We present here the results of recent studies on the voltammetric behavior of a pyrolytic carbon film electrode that has unique physical and chemical properties. The effect of anodic pretreatment on the electrode response in a series of substances has been studied as have the potential limits of the electrode. The film is prepared by the surface pyrolysis of 3,4,9,10perylenetetracarboxylic dianhydride (PTDA) at a hot quartz substrate. This film was first prepared and physically characterized by Kaplan et al. (1) who found the following properties. The film’s conductivity, 250 0-l cm-’, is below graphite and similar to glassy carbon. X-ray analysis indicates that it lacks the extensively layered turbostratic structure typical of graphite and carbon fibers. Electron microscopy and Auger analysis indicate the film is composed of multiple macroscopic layers of a hydrogen-containing conductive carbon that does not react with oxygen upon exposure to the atmosphere. This film is quite different from most carbon films and conductive forms of carbon, although it most closely resembles glassy carbon. The original carbon film electrode prepared by Beilby et al. (2)and the subsequent electrodes studied by Blaedel and Mabbott (3) and by Lundstrom (4-7) were all prepared through a pyrolytic deposition of low molecular weight aliphatic gas sources. Mattson and co-workers (8)have prepared an optically transparent carbon film electrode from carbon atom sputtering. Films of these types have generally been shown to be graphitelike with lamellar turbostratic packing. Glassy carbon (9) and the carbon film investigated here ( I ) are both believed to be composed of locally organized ribbons of aromatic rings spanning the solid. This unique structure yields a carbon solid of amorphous structure with a high conductivity through the ribbons.
Because of the film’s similarities to glassy carbon (GC), it seems appropriate to compare the response of this carbon film electrode with previous studies carried out using glassy carbon. Numerous studies have been conducted on the treatment and behavior of glassy carbon. Although it has been shown to be superior to various forms of graphite, it is still not without problems. The behavior of any glassy carbon electrode is highly dependent on its history, which makes reproducibility difficult unless surface conditions are carefully controlled. Kuwana and co-workers (10, 11) have shown that this lack of reproducibility may be due to (1)variations in GC production, (2) differences in the pretreatment formation of surface oxides, and (3) differences in the layer of surface microparticles due to differing polishing methods. The carbon filmwe have studied has the anisotropic nonporous properties of glassy carbon, is reproducible from lot to lot, and does not require polishing prior to use. This work presents the results of the characterization of an electrode constructed from this pyrolytic carbon film. Blaedel and Mabbot were the first to systematically study the effects of electrochemical pretreatment of a carbon film electrode. Their method used square-wave treatments at various potentials and frequencies. We chose to use electrochemical pretreatment methods that separated the anodic and cathodic effects of pretreatment such as had been used by Engstrom (12),Engstrom and Strasser (13),and Cabaniss et al. (14). We specifically studied the effect of anodic and cathodic pretreatment time on the electrode response to a series of depolarizers. The compounds studied, catechol, ferrocyanide, and four catecholamines, have been previously used to characterize glassy carbon, carbon film, and other carbon electrodes. Also presented are some unexpected sorption and product variation results with the film electrodes after extensive anodic oxidation.
EXPERIMENTAL SECTION Apparatus. A Princeton Applied Research 114A electrochemical analyzer was used for all the experiments. The reference electrode was the saturated calomel electrode separated from the working cell with a NaN03 salt bridge. The counter electrode used was a 20 mm2platinum button. The film electrode contact was a 2-cm-wide band of aluminum foil wrapped tightly around the top of the electrode and attached to an alligator clip. Reagents. The solutions were all prepared with deionized water prepared with a Barnstead Bantam demineralizer with an S-27781 cartridge, which was fed distilled water. The sulfuric acid, KCl, NaN03,K2HP04,K4Fe(CNI6,methylene chloride, and catechol were all analytical reagent grade chemicals. The tris(hydroxymethy1)aminomethane was Sigma Trizma base. The Triton X-100 was from Rohm and Haas. The 3,4,9,10perylenetetracarboxylicdianhydride was from Aldrich. The epinephrine, norepinephrine, dopamine, and DOPA were all from Sigma. All reagents were used as received except the catechol, which was recrystallized from toluene, as it had discolored. Procedure. The films were prepared by use of the method of Kaplan et al. ( 1 ) with minor modification. The films were deposited in a 60-cm-longquartz tube with a 2.5 cm diameter and with a 29/42 vacuum fitting at one end. One-quarter gram of 3,4,9,10-perylenetetracarboxylicdianhydride was placed in the end of the tube and was covered with a 2-mm-thick layer of glass wool, The substrate, 3-mm-diameter quartz rod c u t to 8-cm lengths, was placed in the center of the tube, and a piece of copper
0003-2700/86/0358-2988$01.50/0 Q 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
wire twisted around the rod kept it up and allowed an even coating of film. The tube was evacuated to a pressure of 0.01 torr, and after degassing the tube was quickly thrust into a tube furnace at 850 "C.A chromel-alumel thermocouple was used to monitor the temperature. The end with the anhydride was allowed to extend out of the far end of the furnace to keep it from pyrolyzing prior to sublimation. The substrate was allowed to heat up to the oven temperature for 15 min. The quartz tube was then moved so that the anhydride plug was 6 mm from the oven opening. The tube was then moved 1mm every 15 min into the oven until the plug of anhydride was flush with the furnace exterior. After 3 h of sublimation, the tube was removed from the furnace for examination of the substrate. The plating of carbon on the tube interior was at the same rate as on the substrate, and when an opaque metallic layer had formed on the tube, the substrate was also coated. The tube was cooled, the vacuum broken, and the substrate removed. The substrates were covered with the carbon film, which appeared as a silver-colored metallic mirror. The electrodes were initially washed in 0.1 N NaOH to remove any residual anhydride and acid on the surface; the electrodes were then washed in boiling methylene chloride and in boiling distilled water. The cylindrical electrodes were then fit into a sheath of heat-shrinkable Teflon tubing (Voltrex TTS), which allowed only the circular bottom face to contact the solution. The tubing was shrunk by touching the exterior surface to a hot plate. For anodic pretreatment the electrode was placed in a solution of 0.1 M sulfuric acid and held at 1.8 V relative to the SCE for the times indicated in the text. The cathodic pretreatment was done at -1.0 V relative to SCE, and within 1 min the current had died down to a negligible value. The cyclic voltammetry scans were all made at 10 mV/s with a 15-s pause at the end of anodic scans prior to reversing. The solutions were not deaerated except when studying the cathodic potential limit. The supporting electrolyte for the catecholamines and for catechol was 0.1 M sulfuric acid. The supporting electrolyte for the ferrocyanide solutions was a 0.1 M phosphate buffer fixed at pH 7 with 0.1 M sodium nitrate. The background scans were made in the same supporting electrolyte used to prepare the corresponding sample solutions.
RESULTS AND DISCUSSION The bare areas of the quartz substrate were uniformly coated with a metallic conductive carbon film coating when they were removed from the oven. The copper wire coil that lifted the substrate off the reactor tube left a helical pattern of lighter coating. The conductivity of the coating varied from 0.8 to 6 Q-' mm-I depending upon the film thickness. The end surfaces of the cylindrical rods, which were used as the active electrodes, were well mirrored with the carbon film coating. The first attempts a t preparing the conductive film layer often led to failure due to two errors: (1) the PTDA was heated too high and much of it pyrolyzed directly instead of subliming, and (2) the PTDA was sublimed too quickly and instead of forming a conductive film a black powdery soot was deposited. Attempts at using spectroscopic graphite rods as substrates were abandoned early in the project. The film did not bridge or seal porosities in the graphite, and the coated rods had the same memory effects as untreated graphite rods. The following electrode characterization was carried out using cyclic voltammetry a t the rate of 10 mV/s. This relatively slow rate was chosen so that the results would be comparable with those of Lundstrom ( 4 , 5 ) ,who recently worked on an unrelated conductive pyrolytic carbon film prepared from natural gas. Without anodic pretreatment the response of the electrode was unpredictable and generally very irreversible with the compounds studied. A typical example of an untreated electrode is shown in Figure l a for catechol; Figure l b is a scan with a treated electrode in the same catechol solution. Without the pretreatment the film did not wet well with the aqueous solutions even after repeated washings with methylene chloride and with 1% aqueous Triton X-100. This is in contrast with Lundstrom's electrode
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2989
o
200 .
my
.
I
S
Io
min
20
10
Figure 2. A€, as a function of anodic treatment time for the electrode measured directly after anodic treatment: (0)1.0 X M K,Fe(CN),, (0)1.0 X M catechol, (A)1.0 X low3M DOPA, (0)1.0 X M norepinephrine, (+) 1.0 X M dopamine, and (X) 1.0 X M epinephrine.
1
5
10
mtn
10
30
Figure 3. i , as a function of anodic treatment time for the electrode measured directly after anodic treatment: (0)1.0 X M K,Fe(CN),, (0)1.0 X M catechol, (A)1.0 X M DOPA, (0) 1.0 X M norepinephrine, (+) 1.0 X M dopamine, and ( X ) 1.0 X M epinephrine.
( 4 ) , which operated a t the optimal level without anodic pretreatment. Kaplan et al. (I)found very little oxygen or other heteroatom groups on the surface of pyrolytic PDTA films using Auger analysis. It is clear that without the surface oxides wetting and electron transfer reactions are inhibited. The electrode pretreatment procedure was as follows: (1) The electrode was anodically treated for a fixed time a t 1.8 V (relative to SCE) in 0.1 M H2S04,and then the electrode response was determined in a series of aqueous solutions. The solutions were ferricyanide, catechol, DOPA, dopamine, epinephrine, and norepinephrine all a t 1.0 X M. The anodic-to-cathodic peak separation, AEp, is plotted against treatment time in Figure 2 as is the maximum current of the anodic peak, i,, in Figure 3. (2) The anodic treatment was followed by a cathodic treatment a t -1.0 V in 0.1 M H2S04 for approximately 1 min until the current had decayed to a hundredth of its original value. After this cathodic treatment, the voltammogram for each of the above solutions was measured and the response parameters, AEP and i,, were also
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14,DECEMBER 1986 200
18
Table I. Anodic Range of the Carbon Film Electrode
PH
potentiala
1 7 9
1.30 0.97 0.86
Potential is relative to the saturated calomel electrode. a
Figure 4. A€, as a function of anodic treatment time for the electrode measured directly after cathodic treatment: (0) 1.O X M K,FeM catechol, (A)1.0 X M DOPA, (0)1.0 1.0 X (CN),, (0) X M norepinephrine, (+) 1.0 X M dopamine, and ( X ) 1.0 X M epinephrine. 40
i
IT
I
._ - r---
5
IO
20
30
",I
Figure 5. i , as a function of anodic treatment time for the electrode measured directly after cathodic treatment: (0)1 .O X M K,FeM catechol, (A)1.0 X M DOPA, (0) 1.0 (CN),, (0) 1.0 X X loT3 M norepinephrine, (+) 1.0 X M dopamine, and (X) 1.0 X M epinephrine.
plotted as a function of anodic treatment time in Figures 4 and 5. It is clear from Figures 2 and 4 that AE, settled to a stable value at between 1 and 5 min of anodic pretreatment for all the compounds. The response then remained relatively flat with a slight increase in with some of the compounds as the treatment time increased. The point of stable a, corresponds to the point where the electrode became well wetted with the solution; the electrode also went from silver to a gold-copper color at this point. The color and wetability did not change with cathodic treatment. The AEp separation also did not undergo any significant change with cathodic pretreatment. The slight increase in Upseparation seen with catechol and DOPA with increasing anodic treatment may have been due to some kinetic overpotential caused by the increased thickness of the oxide layer on the electrode. At 20-30 min of anodic treatment the electrode surface underwent a color change from copper-gold to a variety of colors depending on the treatment time. The intensity and shade of color varied with tilting; we believe the colors were due to optical interference in the transparent oxide layer. After the first 5 min of treatment the anodic i, for the ferrocyanide did not significantly increase. The peak current for the catecholamines continued to increase during the entire anodic treatment time as shown in Figures 3 and 5. The steady increase in I , for the catecholamines when the i, had plateaued for ferrocyanide suggests a difference in effective electrode area for the two compounds. We feel the effective electrode area for the catecholamines increased continually with anodic treatment. This may be due to a difference in the mechanism of electron transfer to the electrode. Cabaniss et al. ( 1 4 ) have shown evidence that oxidation of some ruthenium complexes at the glassy carbon electrode proceeds by an inter-sphere hydrogen atom transfer. If this mechanism applies to catecholamine oxidation then the anodic treatment may be increasing the number of hydrogen atom acceptors on the electrode surface. Once anodically activated the cathodic treatment did not appear to change the i, values with any of the depolarizers.
a,
I
I
v
0
Figure 6. Response of the electrode pretreated anodically for 30 min showing sorbed dopamine: (a) response of an electrode dipped in 1 .O X M dopamine and then washed in pure supporting electrolyte for 10 min, and (b) response of the electrode immersed in the dopamine solution.
The cathodic treatment was carried out because Engstrom (12),Engstrom and Strasser (13),and Cabaniss et al. ( 1 4 ) all
reported that it improved the electrode response when working on anodically treated glassy carbon. There were no significant differences in the peak separation between anodically treated film electrodes and those receiving a subsequent cathodic treatment in this study. The only effect of the cathodic pretreatment was a 3- to &fold increase in residual current accompanied with excessive hysteresis. The potential limits were determined by using the definition of Adler, Fleet, and Kane (15);this definition sets the limit at the potential where the solvent breakdown current is equal M ferrocyanide soto half the peak current for a 1.0 X lution. The potential limits in solutions at three pH values were measured with an electrode that had been anodically pretreated for 30 min. The potential limits were measured in the following three solutions: (1)a pH 1solution of sulfuric acid, (2) a pH 7 phosphate buffer with 0.1 F NaN03, and ( 3 ) a pH 9 Tris buffer with 0.1 F NaN03. The anodic limits for the three solutions are shown in Table I. The cathodic scans from 0 to -1.0 V gave multiple broad peaks before solvent breakdown; these peaks made interpreting a cathodic limit impossible. The anodic limits compare well with those of Adler, Fleet, and Kane ( 1 5 ) for glassy carbon and pyrolytic carbon. During the pretreatment experiments it became apparent that with increasing anodic treatment time the catecholamines gave a prepeak with oxidation as is shown in Figure 6b and Figure 7. The prepeaks are seen as shoulders on the rising portion of the anodic peaks. These prepeaks indicate that both the catecholamines and quinone products were sorbing to the electrode surface and that the sorbed moieties underwent electron transfer with lower overpotential than the solution species. The catecholamines were quite tenaciously sorbed to the electrode surface as washing and soaking in pure supporting electrolyte did not diminish the sorbed peak. Figure 6a shows the voltammogram for an electrode that had
ANALYTlCAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
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Table 11. Response of the Electrode with Selected Compounds response: compd
U
ferrocyanide catechol DOPA dopamine epinephrine norepinephrine 1
I
P
138 109 105 53 70 72
mV
E,
EPIZ
317 564 547 483 514 514
216 503 488 436 464 469
.
V
0
Figure 7. Response of an electrode pretreated for 30 rnin anodically showing multiple products: (1) first cycle and (2) second cycle through the anodic potentials. The solution is 1.0 X M DOPA.
been dipped in 1.0 X M dopamine and then washed in supporting electrolyte for 10 min. The peak-to-peak separations of the sorbed catecholamines were all approximately 28 mV, which would be expected for a reversible two-electron couple. The sorbed catecholamine peaks showed no measurable reduction in height or area during the 10-min washing, but the peak height diminished slowly with repeated anodic scans. Both the anodic peak height and area were considerably greater than the cathodic peak for sorbed catecholamines. This was not a n artifact of the scan chosen for publication, but was true for all the sorbed catecholamines on the treated electrodes. We feel the oxidized quinone was probably less strongly sorbed to the oxide layer, and some of it may have diffused away while the potential was above the reduction potential. This would also explain why the anodic peak was reduced by repeated cycling. If both the oxidized and reduced moieties were strongly sorbed the height of the peaks would remain constant during repetitive cycling. A change in the catecholamine oxidation product composition also accompanied the anodic activation process. Initially only one oxidation and one reduction peak were observed with the catecholamines when the electrodes were first activated. With electrodes pretreated over 20 rnin in pH 1 solution, two distinct reduction peaks were observed after an initial anodic scan, and during subsequent anodic scans two oxidation peaks were seen. A typical example of these products is shown in M DOPA. Adams and co-workers (16) Figure 7 for 1.0 X have studied catecholamine oxidation a t the carbon paste electrode and found that a t pH 1 the quinone product was the only observable product and that upon reversal the catecholamine was regenerated. They also fond that a t above pH 3 the initial unprotonated fraction of quinone product was in equilibrium with a cyclic leucaminochrome, which was subsequently oxidized in solution by some of the remaining quinone. This led to a mixture of quinone and the cyclic aminochrome, which yielded two peaks during the cathodic scan. The reduced products, the catecholamine and leucaminochrome, produced two oxidation peaks with the second anodic scan. The multiple peaks observed with the carbon film electrode at pH 1 suggest that the extensively activated electrode has the ability to cyclize the quinone produds. The sorbed catecholamines on an activated electrode immersed in pure supporting electrolyte gave no indication of multiple peaks even after repetitive cycling. The sorption and variation in product yield both occurred when the electrode had been anodically pretreated for 20-30 min. After this period of treatment the electrode had a thick layer of surface oxide, which we believe caused the optical interference colors observed after activation. The thick layer of surface oxides precludes the use of an overtreated electrode as an analytical probe for catecholamines because of the
(I
All potentials relative t o t h e saturated calomel electrode.
sorption and multiple product peaks. On the other hand, this electrode with a thick layer of oxides is of interest in understanding the fundamental process of sorption at the electrode surface. The oxide layer may be thick enough to allow spectroscopy of sorbed molecules both by transmittance and reflectance, as the electrode may be prepared in an optically thin form. The electrode does have analytical utility when activated for a short period of time, from 1 to 5 min. Table I1 lists the AE,,E,, and E,/* values for the compounds tested using an electrode treated for a short period of time. The responses with catechol and the catecholamines are superior in AE, separation to Lundstrom’s work, the work of Cabaniss et al., and the work of Hepler et al. (17)with low-temperature isotropic carbon. The E, values and EPjzvalues determined with the carbon film electrode are also at potentials lower than those reported by Hepler et al. The AJ3, value for ferrocyanide is less reversible than the values reported for Lundstrom’s (4) carbon film electrode and the treated glassy carbon of Cabaniss et al. (22). Cathodic treatment does not seem to affect the faradaic response, but does degrade the residual current; a cathodic treatment is therefore not indicated for analytical use. Finally, because the material can be plated on flat surface substrates it would be an excellent choice as an electrode material for construction of thin-layer electrochemical cells.
LITERATURE CITED Kaplan, M. L.;Schmidt, D. H.; Chen, C-H; Walsch, W. M., Jr. Appl. Phys. Lett. 1980, 3 6 , 867-869. Beilby, A. L.; Brooks, W.; Laurence, G. L. Anal. Chem. 1964, 3 6 , 22-26. Blaedel, W. S.:Mabbott, G. A. Anal. Chem. 1978, 5 0 , 933-936. Lundstrom, K. Anal. Chim. Acta 1983, 146, 97-108. Lundstrom. K. Anal. Chim. Acta 1983, 146, 109-115. Gustavsson, I.; Lundstrom, K. Talanfa 1983, 30, 959-962. Urbaniczky, C.; Lundstrom, K. J. Nectroanal. Chem. 1983, 757, 22 1-23 1. DeAngelis, T. P.; Hurst, R. W.; Yacynych, A. M.; Mark, H. 8.;Heineman, W. R.: Mattson, J. S.Anal. Chem. 1977, 4 9 , 1395-1398. Jenkins, G. M.; Kawamura, K. Nature (London) 1971, 237, 175-176. Kazee, 8.;Weisshaar. D. E.; Kuwana, T. Anal. Chem. 1985, 5 7 , 2736-2739. Fagan, D. T.; Hu, I.-F.; Kuwana, T. Anal. Chem. 1985, 5 7 , 2759-2763. Engstrom, R. C. Anal. Chem. 1982, 5 4 , 2310-2314. Engstrom, R. C.; Strasser, V. A. Anal. Chem. 1984, 5 6 , 136-141. Cabaniss, G. E.; Diamantis, A. A.; Murphy, W. R.; Linton, R. W.; Meyer, T. J. J. Am. Chem. SOC. 1985, 107, 1845-1853. Adler, J. F.; Fleet, B.; Kane, P. 0. J. Nectroanal. Chem. 1971, 30, 427-43 1. Hawley, M. D.; Tatawawadi, S. V.; Piekarski, S.; Adams, R. N. J. Am. Chem. SOC. 1967, 8 9 , 447-450. Hepler, B. R.; Weber, S. G.: Purdy, W. C. Anal. Chlm. Acta 1978, 702,41-59.
RECEIVED for review April 14, 1986. Accepted July 29, 1986. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Part of this work was supported by a stipend from the Office of Research, California State University, Long Beach, CA. Part of this work was presented as paper A1 at the Pacific Conference on Chemistry and Spectroscopy, Oct 9, 1985, San Francisco, CA.
