Anal. Chem. 1993. 65, 317-319
917
TECHNICAL NOTES
Performance Evaluation of an Easily Prepared Optically Transparent Carbon Film Electrode Dennis M. Anjo,' Shelton Brown, and Linda Wang Department of Chemistry and Biochemistry, California State University, Long Beach, California 90840
INTRODUCTION We have developed a new and useful optically transparent carbon film electrode (OTCFE) and also designed an inexpensive cell assembly that works well on a standard optical bench. The carbon film is prepared on a quartz substrate by the vacuum pyrolysis of 3,4,9,10-perylenetetracarboxylic dianhydride; we have shown that this produces a conductive carbon film with properties similar to glassy carbon.' When thick, the film is a lustrous silvery metallic layer, but it can be plated in thin layers that are optically transparent. This paper presents the initial performance evaluation of the OTCFE employed under semiinfinite diffusion conditions. Spectroelectrochemistry is a valuable tool in the determination of electrode mechanisms and is also of some utility Carbon electrodes are used in analytical extensively in electroanalytical chemistry, and optically transparent electrodes are important in the study of the reaction mechanisms a t these carbon electrodes. A number of conductive carbons have been used in optical systems, and these techniques have been well r e v i e ~ e d . ~Optically ?~ transparent carbon f i ielectrodes have been used by Mattson and Smith' and also by DeAngelis, Hurst, Yacynych, Mark, Heineman, and Mattson.8 Carbon fiis are attractive because the film is deposited directly on the optical components; this places the electrode directly on the optical window and does not take up volume in the electrochemical cell. Presented below is the initial performance evaluation of the OTCFE using chronoabsorptometry under semiinfinite diffusion conditions. The chromophores chosen are those commonly used to evaluate spectroelectrochemical cells, and as indicated below the optical response of the electrode to a potential step follows well-established response parameters.
EXPERIMENTAL SECTION Reagents. The chemicalsemployed in these experiments are as follows: 3,3'-dimethoxybenzidinedihydrochloride (dianisidine, Pfaltz and Bauer), 3,3'-dimethylbenzidine dihydrochloride (otolidine,Aldrich), methyl viologen dichloride hydrate (Aldrich). The 3,3'-dimethoxybenzidine hydrochloride was recrystallized (1) Rojo, A.; Rosenstratten, A.; Anjo, D. Anal. Chem. 1986,58,29882991. (2) Kuwana, T.;Winograd, N. Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1974; Vol. 7, pp 1-78. (3) Kuwana, T.;Heineman, W. R. Acc. Chem. Res. 1976, 9, 241. (4)Heineman, W. R.; Hawkridge, F.; Blount, H. Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, pp 1-113. (5) McCreery, R. Physical Methods of Chemistry, 2nd ed.; Rossiter, B., Hamilton, J., Eds.; John Wiley and Sons: New York, 1986; Vol. 11, pp 591-661. (6) Spectroelectrochemistry, Theory and Practice; Gale, R. J., Ed.; Plenum Press: New York, 1988. (7) Mattaon, J.; Smith, C. Anal. Chem. 1975,47, 1122-1125. (8) DeAngelis, T.; Hurst, R.; Yacynych, A.; Mark, H.; Heineman, W.; Mattaon, J. A m l . Chem. 1977,49, 1395-1398. 0003-2700/93/0365-0317$04.00/0
from 1M hydrochloric acid. The other electroactive compounds were used as received. The supportingelectrolyteswere prepared from reagent-grade chemicals. All solutions were made in deionized water which was prepared from house-distilled water. All the solutions were deaerated by purging with high-purity nitrogen; this was done immediately following their preparation. The 3,4,9,l0-perylenetetracarboxylic dianhydride was from Aldrich. Apparatus. The single-beam spectroelectrochemicalinstrument was assembled from modular componente; the components were (a) an Oriel Photomax Lamp with either a Xe or a quartz halogen lamp; (b) an Oriel '/s-m monochromator; (c) a PTI chopper; (d) a fD.8 lens; (e) the electrode cell; (f') a UDT PIN10D photodiode reverse biased at 9 V; (g) a PAR 6204 lock-in amplifier; (h) a linear recorder. The potentiostat used was a PAR 362scanningpotentiostat, employinga Pt auxiliaryelectrode and an AgIAgC1quasi-reference electrode. The Ag/AgCl quasireference electrodewas a silver chloride coated silver wire inserted into the tube leading to the center of the electrode assembly. The silver chloride coating was accomplished using the procedure of Sawyer and Ft~berta.~ Routine spectra of the electrode and solutionswere recorded using a Shimadzu 260spectrophotometer. Routine pH measurements were made with an Orion 91-02 pH combination pH electrode using Orion 811 meter.
PROCEDURE The electrodes were prepared using the method of Rojo, Rosenstratten, and Anjo' using 3,4,9,10-perylenetetracarboxylic dianhydride as the precursor of the carbon coating. They were prepared in an evacuated quartz tube (0.05 mmHg) heated to 800 OC in a tube furnace. The '/s-in. quartz rod substrate was placed in the center of the tube, and the dianhydride was allowed to sublime into the heated region. The dianhydride vapor pyrolyzed on the surface of the quartz substrate producing a uniform lustrous mirror-like conductive coating. An optically thin coating could be prepared in approximately 1h; the rate of coating was dependent on the dianhydride sublimation, and the tube was removed at 15min intervals to monitor the progress. The tube was cooled under vacuum, and the film electrodes were then removed, washed in methylene chloride, and finally washed in an aqueous detergent solution. The resistance of each electrode tip was measured by contact with a mercury pool; the instrument used for resistance measurement was a Keithley 179Amultimeter. The tips had a resistance range of 54-730 s2 with a mean of 184 fl (n = 14). An attempt was made to measure the film thickness by measuring the diameter change of the rods after pyrolysis. The change in thickness was below the 1-rm resolution of our micrometer. The spectrochemical cell is shown in Figure 1. It is constructed in a Kontes Teflon cross for '/s-in. tubing. The (9) Sawyer, D.; Roberta,J. Experimental Electrochemistry for Chemists; John Wiley and Sons: New York, 1974.
0 1993 American Chemlcai Society
318
ANALYTICAL CHEMISTRY, VOL. 65, NO. 3, FEBRUARY 1, 1993
Table I. Potential Step Chmnoabsorptometry Results slopelcon observed, slopelcon predicted, compoundlwavelength,nm s112 L mol-' s112 L mol-' 63.1' 62.9-68.6 dimisidinel445 o-tolidine1431
( n = 20) 132' (n = 16)
methyl viologenl605
22b
solution conditions 0.973 mM, 0.1 M HzSO, 0.340 mM, 0.1 M H B O I
134
1.20 mM, p H 7 buffer,0.05 M phosphate, 0.1 M
31
KCI
( n = 16) 0
Potential step, 0.0 to +LOO V v8 AgIAgCl quasi-reference.b Potential step, 0.0 to 4.95 V
vs AglAgC1 quasi-reference.
Y
0.6-
em -
e5:
0.1-
0
4
0 2
1 .
o m Flgure 1. The spectroalectrochemicaicell assembly.
OTCFE is prepared on a 'I&. quartz rod, and this rod also acts as a light guide for the transmitted light. The flat end oftheelectrodeinthecenterofthecrossistheactiveelectrode. A second piece of hare quartz rod acts as a collector and light guide for incident light into the cell. The 'is-in. fitting with ferrulemakesasolvent tightfitforthe'is-in.quartze1ectrode and the quartz light guide leading from the cell. The 1-mm passage through the cross was drilled out to allow the insertion of the electrode and the light collector. The electrode was mounted flush with the drilled out Teflon to limit the active electrochemical surface to the flat end of the rod. Contact wasmade with the working electrodeusingastripofaluminum foil tightly wrapped around the rod extending from cross; an alligator clip then gripped the aluminum foil. The position of the collector was variable, and it was moved out to allow a 2-mm path of solution. The quasi-reference electrode was placed in the exit tubing helow the cell, and the end of the silver wire was approximately 2 mm from the working electrode. The solutions were passed through the cross using a syringe to draw liquid through the tubing from the beaker helow. The syringe and tubing allowed rapid changing of solutions and cell cleaning without disassembly. The syringe also allowed the efficient removal of air bubbles in the cell assembly.
,
3m
.
,
un
.
,
,
5m
I
.
B M ' 7 m
Wavelength (nm) spectrum of a typical carbon film electrode. Absorbance measured in air. F ~ I O 2. The opUcal
QY", i
1'5
i
2'5
3
3'5
4
is=
A
Chronoabsorptomebyof dianisidine. Dianiskiine (0.973 mM) M AglAgCl quasi-reference. Absorbance monitored at 445 nm.
Qurea.
In 0.1 M sulfuric acki. Potentlal step. 0.0 to +1.00 V
The optical spectrum of a typical pyrolytic OTCFE is shown on Figure 2. Except for noise and optical interference the spectrum is essentially featureless from 700 to 190 nm; the absence of spectral features is similar to the reticulated vitreous carbon electrode of Norvell and Mamantov" but different from the carbon film electrode of DeAngelis et a1.8 which had a broad peak in the ultraviolet. The value of the ahsorbancewasdependent on the periodof carbon deposition; the film transmittance varied from 1to 32 %T. We have recorded the UV-vis spectrum of the dianisidine oxidation product; the spectrum has a maximum at 445 nm and a shoulder at 515 nm. We chose to use the peak at 445 nm for the subsequent chronoabsorptometry because of its
high molar absorptivity; McCreery and co-workersllhave used dianisidine absorbance at 515 nm which corresponds to the laser emission employed in their experiments. Chronoahsorptometry was carried out with o-tolidine, dianisidine, and methyl viologen using the cylindrical electrodes in the cross cell. The result of a typical experiment using dianisidine is shown in Figure 3. Experimental results of the three groups of experiments are shown in Table I. Both o-tolidineand dianisidine gave diffusion controlled plots that were linear with t ' I z , butthemethylviologenresultsdisplayed curvature after approximately 4 s into the experiment. Cyclic voltammetry indicated poor resolution between the first and second reduction peaks of methyl viologen in the aqueous system presented and also with studies done in acetonitrile. Clearly, the potential necessary for a diffusioncontrolled response exceeds the potential for the second reduction step; numerous electrochemical activation methods in acid, neutral, and base electrolyte were unable to produce a reversible response with methyl viologen. The ohmic loss due to the resistance of the electrode may contribute as much as 10 mV to the overpotential, hut other sources of overpotential must also be considered to explain the lack of reversihility. The lackof reversihility was alsoohservedwith
(10) Norvell, V.; Mamantov, G . And. Chem. 1977,49, 147&1472.
(11)Robinson, R.; MeCreery, R. Anal. Chem. 1981.53, 997-1001.
RESULTS
ANALYTICAL CHEMISTRY, VOL. 65, NO. 3, FEBRUARY 1, 1993
thick, low-resistance, carbon films prepared by a longer pyrolysis. Table I shows both the observed and predicted slope for the chronoabsorptometryexperiments. The predicted slopes for o-tolidine and methyl viologen were based on published data of the molar absorptivity and diffusion coefficients.2J2 Because we used the more sensitive 445-nm peak with dianisidine,we estimated the 445-nm molar absorptivity using the published molar absorptivity of McCreery and co-workers a t 515 nm." By ratioing the absorbance at the two wavelengths we arrive a t a molar absorptivity of s = 2.90 X lo4a t 445 nm. The diffusion coefficients used for dianisidine were those of Adams;13the range of slopes predicted for dianisidine reflect the wide range of diffusion coefficienta reported.
CONCLUSION This evaluation of an OTCFE indicates that the electrode performs in a normal manner under semiinfinite diffusion conditions. As with most carbon electrodes, this electrode performs best under anodic conditions, and the reduction of (12) Bancroft, E.;Sidwell, J.; Blount, H. Anal. Chem. 1981,53,13901394. (13) Adams, R. N. Electrochemistry at Solid Electrodes; Marcel Dekker: New York, 1969.
S1Q
methyl viologen was not completely successful. The electrode performs as well as previous carbon film electrodes with the advantage of a flatter background absorbance. Preparation of the electrode does not require exotic vapor deposition equipment, and it can be made with a minimal investment. As with any film electrode this electrode is delicate and has a relatively short lifetime before the surface is depleted by repeated activation procedures. The unique electrode assembly presented in this note can be used in flowingsolution, and it can be rapidly cleaned and changed without a major disassembly. We are in the initial development stages in the modification of the cell to allow thin-layer spectroelectrochemistry.
ACKNOWLEDGMENT This work was supported by Public Health Service Grant 1R15NS27342-01from the National Institutes of Health and by the Minority Biomedical Research Support Program of the National Institutes of Health S06RRO8238-02. Linda Wang was a participant in the NSF Young Scholarsprogram, Grant RCD 8955516. Acknowledgment is made to the Office of University Research, California State University, Long Beach, for partial support of this research. RECEIVEDfor review June 17, 1992. Accepted October 22, 1992.
