Anal. Chem. 1005, 05, 3720-3725
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Graphite-Coated Metal Mesh Optically Transparent Electrodes Matthias Kummer and Jon R. Kirchhoff' Department of Chemistry, University of Toledo, Toledo, Ohio 43606
INTRODUCTION
Another approach is the w e of carbon materialsto fabricate OTEs. Deposition of a thin carbon layer on optically transparent insulating substrates has been used to produce a carbon f i i 0TE.l3-l6The t h i n - f i method generallyforces researchers to accept a compromise between f i i thickness and optical transparency. Films must be sufficiently thick to provide low electrical resistance for reasonable electrochemical properties, yet maintain sufficient optical transparency for spectroscopic observations. Carbon-based OTEs have also used continuous carbon materials such as carbon cloth or reticulated vitreous carbon (RVC). Carbon cloth electrodes have been easily incorporated into OTTLE cella, but the optical transparency is estimated to be only 5% .17 The RVC electrode, which has a potential range of 1.2 to -1.0 V versus SCE at pH 7,3 has been used in many spectroelectrochemical applications with transmission,1az3 EXAFS," and luminescencez5spectroscopy. Sorrels and Dewald have conducted an extensivestudy on RVC OTTLE cella in which the effects of electrode porosity and thickness on the optical and electrochemical characteristics were eval~ated.~3 Their results demonstrate the range of optical and electrochemical properties that are possible for spectroelectrochemical measurements with RVC as the OTE material. As an alternative, we have investigated the preparation of a new carbon f i i OTE (CFOTE) by the deposition of thin films of carbon onto metal mesh substrates. Carbon f i h were produced by either spray coatingcolloidal graphite onto a metal mesh at room temperature or by pyrolysis of acetone on a resistively heated metal mesh. The metal mesh CFOTEa provide higher optical transparenciesthan other carbon-based OTEs reported in the literature, as well as a continuous conducting network to reduce electrode resistance. The CFOTE can easily be incorporated into OTTLE cella to give an electrode with the electrochemical properties of a carbon surface. This paper describes the preparation of CFOTEs and their characterization by scanning electron microscopy, electrochemistry, and spectroelectrochemistry.
Carbon has been extensively used as an electrode material for a range of electroanalytical applications. This popularity can be traced to the versatility and availability of many types of carbon, which can be easily fabricated into electrodes.13 Carbon materials also provide renewable and reproducible surfaces, low chemical reactivity, and awide potential window for electrochemical measurements in aqueous and nonaqueous solutions. In contrast, electrochemical measurements at negative potentials in aqueous solutions are complicated at solid metal electrodes by the low hydrogen overpotential. For example, the negative potential limit at a platinum electrode, E(-)hit (V vs SCE), is approximated by the relationship, E(-)Emit = (-0.OE191)pH.~Electrodes made from mercury have the best E(-)bit,5 but the liquid state and the hazards of mercury can limit certain applications. Therefore, for many chemical systems or experimental conditions, electrodes made from carbon rather than mercury or metals have been used as the electrode of choice. Optically transparent electrodes (OTEs) have been developed for spectroelectrochemical rnea~urements.~J Typical OTEs are thin f i i s of conductive materials such as carbon, gold, platinum, or metal oxides deposited onto glass, quartz, or plastic substrates. Continuous materials such as metal meshes or conductive foams of vitreous carbon or various metals have also been used. The most widely used materials are the metal meshes due to their commercial availability and ease of use in the optically transparent thin-layer electrode (O'ITLE) cell design.8 However, metal OTEs suffer from similar limitations in aqueous solution as described above for solid electrodes. As a result, attempts have been made to develop OTEs with a wide negative potential range. One approach that has been used to prepare OTEs with extended negative potential ranges is the electrodeposition of a thin film of mercury onto a platinum film OTE,BJo a nickel mesh," and a gold mesh.12 The mercury f i i electrodes exhibit an extended negative potential range by 300-600 mV; however, mercury droplet formation instead of formation of a continuous thin film promoted amalgamation of the platinum and nickel substrates.Sl1 Gold mesh as the electrode substrate has been found to produce quality thin mercury films.12
Apparatus. Electrochemical measurements were conducted with a Bioandytical Systems, Inc. (BAS)lOOB electrochemical d y z e r , which was interfad to a Gateway 386 PC and a Houston
(1) Kinoshita, K. Carbon: Electrochemical and Physicochemical Properties; Wiley: New York, 1988. (2) McCreery, R. L. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1991; Vol. 17, pp 221-374. (3) Dryhurst, G.; McAllister, D. L. In Laboratory Techniques in Electroanalytical Chemistry; Kissinger, P. T., Heineman, W. R., Eds; Marcel Dekker: New York, 1984; Chapter 10. (4) Meites, L. Polarographic Techniques; Wiley: New York, 1965; p 431. (5) Sawyer, D. T.;Roberts, J. L., Jr. Experimental Electrochemistry for Chemists; Wiley: New York, 1974; p 64. (6) Kuwana, T.;Winograd, N. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1974; Vol. 7, pp 1-78. (7) Heineman, W. R.; Hawkridge, F. M.; Blount, H. N. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, pp 1-113. (8)DeAngelis, T. P.; Heineman, W. R. J. Chem. Educ. 1976,53,594597. (9) Heineman, W. R.; Kuwana, T. Anal. Chem. 1971,43,1075-1078. (10) Heineman, W. R.; Kuwana, T. Anal. Chem. 1972,44,1972-1978. (11) Heineman, W. R.; DeAngelis, T. P.; Goeltz, J. F. Anal. Chem. 1975,47,1364-1369. (12) Meyer, M. L.; DeAngelis, T. P.; Heineman, W. R. Anal. Chem. 1977,49,602-606.
(13) Mattson, J. S.; Smith, C. A. Anal. Chem. 1976,47, 1122-1125. (14) D e b e l i s , T. P.;Hurst, R. W.; Yacynych, A. M.; Mark, H. B., Jr.; Heineman, W. R.; Matteon, J. S.Anal. Chem. 1977,49,1396-1398. (15) Anjo, D. M.; Brown, S.;Wang, L. Anal. Chem. 1993,66,317-319. (16) Elder, R. C.; Lunte, C. E.; Rahman, A. F. M. M.; Kirchhoff, J. R.; Dewald, H. D.; Heineman, W. R. J. Electroanal. Chem. 1988,240,361364. (17) Nishihara, C. Denki Kagaku 1987,55,469-470. (18) Norvell, V. E.; Mamantov, G. Anal. Chem. 1977,49,1470-1472. (19) Ward, E. H.; Hueeey, C. L. Anal. Chem. 1987,69,213-217. (20) Tyagi, S. K.; Dryhurst, G. J.Electroanal. Chem. 1987,223,119141. (21)Zamponi, S.; Dimarino, M.; Marassi, R.; Czerwinski, A. J. Electroanal. Chem. 1988,248, 341-348. (22) Zamponi, S.; Czerwinski, A.; Marassi, R. J. Electroanal. Chem. 1989,266,37-46. (23) Sorrels, J. W.; Dewald, H. D. Anal. Chem. 1990,62,1640-1643. (24) Dewald, H. D.; Watkins, J. W., III; Elder, R. C.; Heineman, W. R. Anal. Chem. 1986,58,2968-2975. (25) Lee, Y. F.; Kirchhoff, J. R. Anal. Chem. 1993,65, 3430-3434. (26) Standards in Absorption Spectrometry; Burgees, C., Knowles, A., W.;Chapman and Halk London, 1981; Vol. 1, p 55. (27) The molar absorptivitywaa determinedby abmrpbon spectroecopy in a 1.00-cmcell.
0003-2700/93/0365-3720$04.00/0
EXPERIMENTAL SECTION
CP 1993 American Chemlcal Society
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(a) Flgure 1. Sketch of the vapor deposition apparatus: (a) A, Scrubbing tower, B, vaporizer; C, empty 3 4 vessel; D, gas Inlet; E, gas outlet; F, electrical connections; G, metal mesh electrode and holder; (b) top view of glass frame, connecting wires, and metal mesh electrode.
Instruments DMP-40 digital plotter. Scanning electron microscopy was carried out on a JOEL JSM-6100scanning microscope. Absorption spectra were recorded with a Varian Cary 5E UVvis-NIRspectrophotometer. The cell compartment was modified to accommodate the electrical leads for spectroelectrochemical experiments. Voltages and currents in the Pyrolysis experiments were measured with a Keithley 197Amultimeter. Temperatures of the glowing grids were measured with an 8630 Series optical pyrometer from Leeds & Northrup Co. Unless otherwise stated, all potentials were measured versus a Ag/AgCl (3 M NaC1) reference electrode (BAS, MF2020). Materials. All solutions were prepared from distilled and deionized water, which was purified to a resistivity of at least 18 Ma ctn by a Barnstead Organicpure water system. Solutions of 0.1 M Has04 were prepared by diluting sulfuric acid that was doubly distilled from Vycor (GFS Chemicals). The supporting electrolytes were of reagent-grade quality and used without further purification. No significant electroactiveimpurities were observed in either the solvent or supporting electrolytes. Potassium ferricyanide (Fisher Scientific),methyl viologen dichloride hydrate (Aldrich), and o-tolidine dihydrochloride hydrate (Aldrich) were used as received. All other chemicals were of reagent-grade quality. Acetone was stored over 4A molecular sieves. Dag 40 graphite spray was purchased from Acheson Chemical (Scheemda, Holland). DAG 40 is a suspension of colloidal graphite and a polymeric binder in butyl acetate. Preparationof Spray-CoatedElectrodes. A 1-X 1-cmpiece of 100 wires per in. gold mesh (Buckbee-Mears, St. Paul, MN) was cleaned with acetone and dried, and the edges were glued to a plastic frame with epoxy resin (Bordens). From a distance of 20-25 cm, an acetone-saturated argon stream was focused on the mesh with a nozzle; this prevented clogging of the mesh holes during spray coating. The mesh was coated on both sides by applying short bursts of carbon spray. The coated electrode was then dried at 80 "C for 20 min to evaporate the solvent vehicle. The coating process was repeated two more times to ensure complete surface coverage. A copper wire was attached to the coated mesh with conducting silver epoxy (Epo-tek H20E, Epoxy TechnologyInc.), and the edgeswere insulatad with varnish (SPC Technology). Preparation of P rolytic Graphite-Coated Electrodes. Pyrolytic graphite- (P -) coated electrodes were prepared by the vapor deposition of graphite onto resistively heated metal mesh substrates. The apparatus for vapor deposition consistad of three main components a scrubbing tower, a vaporizer,and a reaction chamber (Figure 1). Acetone served as the carbon precursor, and argon was used as the carrier gas. Argon was first passed through the scrubbing tower to remove residual 0 2 and HzO and
d
then into a bubbling tower, which served as the vaporizer for the acetone. An empty container with an approximate volume of 3 L was inserted between the vaporizer and reaction vessel and was used to compensate for possiblepressure fluctuationscaused by bubbling in the vaporizer. A 500-mL Erlenmeyer flask fitted with a rubber stopper served as the reaction vessel. The rubber stopper was equipped with a gas inlet and outlet. T w o wires with alligator clamps affixed to the inside ends of the wires were sealed into the stopper to provide internal electrical contact. Argon purging of the system was possible with two three-way valves, which provided a bypass around the vaporizer. A flow meter was used to adjust the carrier gas flow rate to 0.5 L/min for pyrolysis. A Variac was used as the power murce for all mesh sizes. For the 1- X 1-cm and 3- X 1.5-cm meshes another transformer was insertedbetween the variac and reactionchamber in order to allow for fiier control of the power setting. The electrical power transformed was obtained by measuring the current and voltage over the mesh. The circuit was set up to exclude the voltage drop over the ammeter from the voltage measurement. Electrodes for bulk solution measurements were made from 1-X 1-cm pieces of nickel mesh, while electrodes for thin-layer spectroelectrochemical measurements were made from 6 X 1.1cm and 3- X 1.5-cm pieces of mesh. 1-X 1-cm and 3- X 1.5-cm meshes were prepared for the pyrolysischamber in the following manner. Two opposite edges of the mesh were attached with conducting epoxy to small rectangular glass frames. Different sized frames were made for each electrode size. Two 6-cm pieces of 1-mm-diametercopper wire were used for connectionsbetween the mesh and the alligator clips. One end (ca. 1cm)of each wire was bent at a right angle and attached to the two 1-cm edges of the mesh with conducting epoxy. This allowed the frame and electrode to be positioned parallel to the bottom of the reaction chamber with the wire connections extending vertically to the alligator clips. For the 6-cm-long pieces, a frame was not used. The mesh was attached to the bent ends of the wire with conducting epoxy and folded in the middle. A small piece of quartz was placed in the center of the folded mesh to pull the mesh downward. This design minimized mechanical strain and dietortion of the metal framework caused by thermal expansion of the mesh. Some distortion nevertheless occurred at the side edges during pyrolysis. These curved edges were removed to prevent cracks prior to construction of a thin-layer cell. Pyrolysis conditions, i.e., power levels and pyrolysis times, were determined for each mesh size. As expected, the rate of pyrolysis increases with grid temperature but is limited by the melting point of 1453 "Cfor nickel. The duration and temperature of the pyrolysisrepresent a compromisebetween achieving
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complete surface coverage and obtaining a durable and flexible mesh with high optical transparency. Aflexible meshminimizen thepotential for filmdegradation hycrackingduringconstruction of a thin-layer cell. The results described herein refer to coated meshes prepared by the following three methods. Variations in pyrolysis conditions will yield electrodes with different film thicknesses and optical transparencies. (i)Method A 1- X I-cm Mesh. Typicalpyrolysisconditions foral- X 1-cmmeshwereapowerof lft20Wappliedfor35min. The onset of pyrolysis is marked by some smoke evolution and condensation of tarlike producta on the flask walls. The mesh was removed from the reaction chamber after pyrolysis and cleaned in boiling methylene chloride for ca. 2 h.
(ii) Method B 6- X 1.1-cm Mesh. At the beginnii of the pyrolysis on the 6- X 1.1-cm mesh, a power of approximately 40-45 W was applied for 5 min. This resulted in the formation of some smoke. The initialsmokeevolution ismost prohablythe result of decomposition of the conducting glue since the atmm sphere in the flask gradually cleared. When the atmosphere in the reaction chamber cleared, the power was increased to 58-60 W. Immediatesmokeevolutionwasobsewed, and condensation of tarlike products on the flask indicated the initial stages of pyrolysis. The pyrolysis was continued for 16 min. The mesh was then removed from the reaction chamber and cleaned in boiling methylene chloride for 2 h. The approximate ranges of current and voltage for a typical pyrolysis were 2.6-3.7 A and 12.3-17.2 V, respectively. (iii)MethodC 3-x 1.5-cmMesh. Analtemtivepreparation method for electrodes for spectroelectroehemistry used smaller metal meshes with dimensions of 3 X 1.5 cm. The shorter mesh can be mounted on a glaes frame similar to the frame used for the 1- X 1-cm pieces of mesh. A lower pyrolysis power of approximately 33 W with a longer pyrolysis time of ca. 3 h produces coated meshes that are more flexible and easier to fabricate into electrodes. Because of the longer pyrolysis time, the resistance of the substrate decreases with the deposition of graphite and leads to a drop in the substrate temperature below the value necessary for pyrolysis. Therefore, the power must be slightly increased during the experiment to counteract this process. The final power was approximately 37 W. The resumption of pyrolysis can be recognized by smoke evolution. The coated mesh was cleaned as descrihed for methods A and B. The approximate ranges of current and voltage for a typical pyrolysis were 3.3-4.2 A and 1-10 V. respectively. Procedures. PG electrodes for hulk solution measurementa were constructed from a mesh mated by method A or a 1-x 1-cm sectionofameshcoatedhymethodB. Acopperwirewasattached to the PG mesh with conducting epoxy, and the edges were insulated with varnish. OTTLE cella were constructed with the CFOTEs from methods B or C according to the design of DeAngelisand Heineman! Either preparationmethod produced PG electrodessuitable for spectrcdectrochemical measuremente, the electrodes from method C were larger. The top and bottom edgesof thecoatedmeshwereinsulatedwithvarnishpriorto the cell assembly. An aspirator was used to fill the thin layer cell and to remove gas bubbles. The optical path length was determined to be 0.0244.025 em by measuring the difference in thickness of the individual microscope slides and the thickness of the OTTLE cell with a caliper. Measurement of the path length with a 2.50 mM KL30, standard solution in 0.05 M Nar HPO," (e = 4615 M-' cm-I at 373 nm=) yielded the same result. Cell volumesweredetermined by controlledpotential coulometry of a standard ferricyanidesolution! Values of approximately 21 pL (method B)and 35 pL (method C) were obtained for O'lTLES made with the CFOTE. The voltammetric performance of each electrode was evaluated with solutions of methyl viologen in 0.1 M Hd30,. AU solutions were purged with argon for 5 min prior to the electrochemical experiments. The temperature at which pyrolysis starts with a perceptible rate was measured at the power setting that was used for the preparation of each electrode size. This was accomplished by coating a mesh with pyrolytic graphite in order to obtain an object more similar to a black body. The sample was inserted into a clean Erlenmeyer flask and the system purged with argon at0,5L/minfor5min. Thepowerwas tumedtoitsinitialvalue, andthetemperatureoftheglowing meshwasmeasured with the optical pyrometer. The temperature was determined to be approximately 1070-1080, lffiC-llGfJ, and 98(tlMx) O C for methods A, B,and C, respectively. These values were corrected for the influence of the flask wall.
RESULTS AND DISCUSSION SEM Invwtigation of the electrode^. SEM photographs ofthreemicromeshelectrodes arecomparedin Figure 2. Figure2 (top)showstheshinysideofanunmodifiednickel mesh; a gold micromesh electrode is similar."J* The middle and bottom panels of Figure 2 depict the surfaces of carbon
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ElVOLT) D
Flguro 4. Thin-layer cycllc voltammogram of 5 mM methyl vlologen In 0.5 M Na2S04 and 0.1 M H&O4 at the PQ-mated nickel mesh electrode in the OTTLE cell. Effective electrode &e: 1.8 X 0.6 cm from method B. Scan rate: 5 mV/s.
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film micromesh electrodes that have been prepared by spray coating a gold mesh and pyrolytic deposition on a nickel mesh, respectively. The spray-coatedelectrode has a dull and black character, while the pyrolytic graphite-coated electrode has a shiny, graphite-like appearance. To a f i s t approximation, either coating procedure yields complete surface coverage of themetalmeshframework. Thiswasconfiiedbyanabsence of scattering from bare metal spota when thecoated electrodea were imaged in the back scatteringmode. The main difference between the two t@s of coated electrodes is the surface roughness. Spray coating yields a relatively rough surface with a random buildup of carbon at the intersection of the metal wires. In comparison, the growth of the carbon f i by pyrolytic deposition gives a uniform carbon layer. A distinct feature of the PG surface is the regular array of closely packed carbon spheres. The lower pyrolyais temperature used inmethodCreaultainsmallercarbonparticleaandanincrease in surfaceroughnessrelative to method A (Figure 2 (bottom)). Electrodes prepared by method B look similar to those prepared by method A. Similar surface features have been observed for the pyrolysis of ethylene on thermally decomposed nickel% or vapor-deposited PG from a commercial source.29 The modified electrode surface area is governed by the extent of growth of the carbon film. Film growth for the PG electrodes is easily reproduced to within 1-2 Fm by careful control of the precursor flow rate and the deposition time. The thickness of the PG film is estimated to be 13-15 Mm (methods A and B)and 7-10 Fm (method C) from scanning electron micrographs. Electrochemical Characteristics of the Carbon Film OTEs. Figure 3 shows a comparison of the cyclic voltammograms of a 1 mM solution of methyl viologen in 0.1 M HzS04 at (A)an uncoated nickel mesh electrode, (B)a spraycoated gold mesh, and (C) a PG-coated nickel mesh. As expected, the background current for the unmodified nickel mesh grows rapidly in the negative potential region due to the reduction of hydrogen ion and obscures the electrochemistry of methyl viologen; the cyclic voltammogram at a gold mesh electrode is similar. Consistent with previous measurements for uncoated metal mesh OTEs,12 the negative limit in acidic aqueous solution is ca. -0.300V. In contrast,
both the carbon spray-coated electrode and the PG electrode are characterized by an extended negative potential range such that the two one-electron reduction waves of methyl viologen can be clearly detected under semi-infiiite diffusion conditions. The peak potentials are observed at -0.697 and -0.926 V for the spray-coated electrode and at -0.696 and -0.910 V for the PG electrode. These values are in good agreement with those reported for methyl viologen at a conventional disk electrode in aqueous solution.3o Figure 3D shows the background cyclic voltammogram in the negative potential region at a PG-coated nickel mesh in 0.1 M HzSO4. A comparison of Figure 3A and D clearly shows the extension of E ( - ) w t for the PG electrode in 0.1 M HzSOd. The extension of the negative potential window is estimated to be 600 mV. This value is considered an approximation since the electrochemical surface area of the PG electrode is not easily ddtermined such that a criterion for the potential limit based on current density can be established. A scan initiateda t +O.O V in the positive directionyields an oxidation wave at +0.690 V and a coupled reduction at +0.290 V. The reduction wave is only observed after the oxidation wave is traversed and is presumably a surface-containedspecies. Long and frequent exposure to potentials beyond +0.650 V yields an increase in the overall background response. Therefore, the potential window for voltammetry at the PG-CFOTE under these specific conditione ranges from +0.5 to ca. -1.0 V. Upon electrolysis at -0.850 V for longer than 3 min in 0.1 M HzSO~, HZgas evolution is observed and limita longer timescale electrochemicalexperiments, vide infra. At pH 7.0,the carbon film also provides benefita for electrochemical measurements in the negative potential region. The negative potential limit is extended to about -1.3 V with a substantial reduction in the overall background current; this also represents a shift of at least 600 mV compared to a bare nickel mesh electrode. Gas evolution is not observed even after electrolysis at -0.900 V for 10 min. Spray-coated electrodes have a similar potential range and electrochemicalproperties as the PG-CFOTE under our experimental conditions. The negative potential limit for the spray-coated meshes is comparable to that observed for metal disk electrodes that have been spray coated with gra~hite.3~ Metal mesh electrodes prepared by pyrolytic deposition are found to yield highly reproducible electrodes and electrochemical characteristics. The spray-coating approach, however, is much less reproducible. Electrodes prepared by the same spray-coatingprocedure have variable background currenta. The background current was measured at -0.500 V for four spray-coated electrodes and was found to vary
(28) Saraceno, R. A.; Engstrom, C. E.; Rose, M.; Ewinp, A. G. Anal. Chem. 1989,61,66C-666. (29)Evans, J. F.; Kuwana, T. Anal. Chem. 1977,49,1632-1636.
(30) Kaifer, A. E.; Bard, A. J. J . Phys. Chem. 1985,89,4878-4880. Christian, G. D. (31) Kauffmann, J.-M.; Laudet, A; Patriarche, G.-J.; Talonta 1982,29,1077-1082.
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F l g u . 3. Cy& voltammograms of 1 mM methyl vblogen in 0.1 M H&O4 at 1- X l c m mesh electrodes under SemHnfinke dmudon conditkns: (A) unmodified nickel mesh; (B) spraycoetd goid mesh; (C) FQamted nickel mesh from method A; and (0)background cyck vdtemmcgram at W coated nlckel mesh from method B In 0.1 M HnSO4. Scan rate: 20 mV/s.
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E
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Figuro 6. Visible absorption spectra of 1 mM methyl vklogen (A) and the methyl v k k g e n radical cation (B) in 0.1 M HzS04. The radical catbn was generated after stepping the potentla1at the CFOTE in the OllLE cell to -0.807 V. Electrode preparation: method B.
from approximately4.5to 45PA. For the electrodes with the higher background current, it was difficult to resolve the reduction peaks of methyl viologen from the background. The reason for this behavior is not completely known but is presumably related to the random nature of the spray-coating process which can lead to a porous f i b . This can be inferred by comparison of the middle and bottom panels of Figure 2. Another reason could be aging of the spray coating. Cracks in the spray carbon film were observed over time by SEM investigations. In contrast, the PG fiis are found to adhere quite well to the mesh substrate to give a mechanically stable electrode. Neither the cleaning procedure nor hydrogen evolution at extreme negative potentials appears to disrupt the coating. Therefore, the PG-coated electrodes are found to be more reliable and were used for further investigations and applications of the CFOTE.
SpectroelectrochemicalMeasurementsat theCFOTE. Figure 4 shows the thin-layer voltammogram of 5mMmethyl at the PG-CFOTE viologen in 0.5 M NazSO4 and 0.1M in the OTTLE cell. The first reduction wave of methyl viologen is observed a t -0.750 V. The potential shift of approximately 50mV in the OTTLEcell relative to the smaller mesh electrodes is most probably a result of the uncompensated resistance inherent to the OTTLE cell configuration. The second reduction wave is shifted past the negative potential limit of the CFOTE. The potential window for voltammetry in the OTTLE cell is similar to the smaller electrodes. Spectroeiectrochemical measurements can be performed in the presence of large background currents since the optical response is not affected by the magnitude of the current. This is possible provided gas bubbles do not form and interfere with the spectral response. Figure 6 shows the visible spectra for methyl viologen (spectrum A) and the singly reduced methyl viologen radical cation (spectrum B) generated at the PG-CFOTE in the OTTLE cell configuration. The absorption spectrum of the radical cation was obtained under equilibrium conditions following a potential step to -0.807 V and exhibits the characteristic maxima at 395 and 601 nm.30 Gas evolution was not observed at -0.807 V during generation of spectrum B in Figure 5. However, on the longer time scale of a spectropotentiostatic experiment,H2 formationwas observed for extended electrolysis times at potentials above -0.800 V. Thus, the determination of E"' and n values for methyl viologen was not possible with these solution conditions. It was also found that long exposure of the PG electrode to the
4 350
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Figuo6. Spectra recorded durlng the absorptkn spectropotentkatatic experiment of 0.97 mM o-tolidine In 0.5 M acetlc ackl and 1 M at the PQ-CFOTE In the OllLE cell. Electrode preparation: me4W B. Applled potentlabin V vs Ag/AgCl are as folbws: (A) +0.799; (B) +0.679; (C) +0.651; (D) +0.639; (E) +0.627; (F) 4-0.817; (Q) +0.600; (H) 0.301.
+
methyl viologen radical cation increased the background current. It is believed thisresults from formationof polymeric f i i of methyl viologen analogous to those observed by Hawkridgeet. aL3*Therefore,o-tolidinewas used to evaluate the performance of the PG-CFOTEsfor spectropotentiostatic measurements. Figure 6 shows absorption spectra of 0-tolidine as a function of potential. The absorption maximum for o-tolidine is observed at 438nm. The equilibration time was determined to be 3.5 min by chronoabsorptometry. A Nernst plot of the data at 438 nm is linear and gives Eo' of +0.645 V with n equal to 1.99. These results are in good agreement with reported literature values.8 An important feature of the PG-CFOTE is the relatively small reduction in the optical transparency by the deposition of the carbon fii. The transmittance of the PG-CFOTE was determined to be 70% for electrodes prepared by method B and 75% for electrodes prepared by method C. This represents only a 7-12 % reduction in optical transparency relative to an unmodified 100 wires per inch mesh OTE.7 In comparison, the PG-CFOTE is 20-25'3 more transparent than RVC-OTEa prepared with various combinations of RVC porosity and thickness23and has a transparency at least 409% greater than carbon film OTEs prepared by vacuum pyrolysis of 3,4,9,10-perylenetetracarboxylic dianhydride.'s
CONCLUSIONS Pyrolysis of acetone on resistively heated metal mesh electrode materials provides a feasible method for the preparation of graphite-coated OTEs with high optical transparenciesand low electricalresistances. The PG coating extends the useful potential window of the metal mesh electrodes by approximately600-700mV under semi-infiiite or r e s t r i d diffusion conditions and allows for electrochemical characterizationof speciea in the negative potential region. Carbon f i iOTEs have also been prepared by a simple spraycoating procedure. The spray-coated OTEs exhibit similar electrochemical and optical properties to the PG-CFOTE, but the spray-coatingprocess yields electrodes with a higher variability in the electrochemical response than electrodes made by the pyrolytic depositionof graphite. Adsorption by (32) Hawkridge, F. M.;Landrum, H. L.; Salmon, R.T.J. Am. Chem. SOC.1977, 99, 3154-3158.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 24, DECEMBER 15, 1993
the chemical species under study is the main limitation for the reuse of the PG-CFOTE, since polishing procedures are not practical for the mesh electrodes. Electrodes large enough for incorporationinto OTTLE cells have been developed by the pyrolysis technique and the performance characterized by the spectral properties of methyl viologen and o-tolidine. OTTLE cells prepared with the CFOTE maintain the favorable characteristics of small sample volumes, rapid electrolysis, and ease of construction but provide enhanced performance for experimental conditions that require a carbon-basedelectrode. In addition, the CFOTE has the greatest optical transparency reported for a carbon-based OTE.
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ACKNOWLEDGMENT Financial support from the University of Toledo for a Summer Faculty ResearchFellowship and the College of Arts and Sciences for funding and operation of the Arts and Sciences Instrumentation Center is gratefully acknowledged. We wish to thank Pannee Burckel for technical assistance in the operation of the SEM, Dr.J. G. Edwards for use of the optical pyrometer, and Drs.William R. Heineman, Dean M. Giolando, and John L. Martin, Jr., for helpful discussions. RECEIVEDfor review June 1, 1993. Accepted August 30, 1993.
