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565
LITERATURE CITED
rearranging a t a different rate than the solution species is not directly observed optically in the single-potential step experiment. Adsorption would affect the data only if the rearrangement products desorb into solution and spectrally contribute. This is unimportant in the azobenzene case, since the products d o not interfere spectrally a t the wavelength selected for monitoring. I n the double-potential step experiment, adsorption effects would also be observable only indirectly if hydrazobenzene and azobenzene adsorb differently a t the two potentials involved. T h e extent of adsorption of these species on gold electrodes has not been reported. However, calculations indicate that greater than monolayer coverages would be necessary to affect the optical response by a measurable amount.
A. T. Hubbard and F. C. Anson in "Electroanalytical Chemistry", A. J. Bard, Ed., Vol. 4, Marcel Dekker, New York, 1970, p 129. C. N. Reilley, Rev. Pure Appl. Chem., 18, 137 (1968). R. W. Murray, W. R. Heineman, and G. W. O'Dom, Anal. Chem., 39, 1666 (1967). W. R. Heineman, B. J. Norris, and J. F. Goelz, Anal. Clem., 47, 79 (1975). H. R. Thrsk and J. A. Harrison, "A Gude to the Study of Electrode Kinetics", Academic Press, New York, 1972. S. W. Feldberg in "Electroanalytical Chemistry", A. J. Bard, Ed., Vol. 3, Marcel Dekker, New York, 1969, p 199. N. Wincgrad and T. Kuwana in "Electroanalytical Chemistry", A . J. Bard, Ed., Vol. 7, Marcel Dekker, New York. 1974. W. R. Heineman, J. N. Burnett, and R. W. Murray, Anal. Chem., 40. 1970 (1968). P. T. Kissinger and C. N. Reilley, Anal. Chem., 42, 12 (1970). J. L. Owens and G. Dryhurst. J . Electroanal. Chem., 80, 171 (1977). R. L. McCreery, Anal. Chem., 49, 206 (1977). T. M. Kenyhercz, A. M. Yacynych, and H. B. Mark, Jr.. Anal. Lett., 9. 203 (1976). T. M. Kenyhercz and H. B. Mark, Jr., J . Electrochem. SOC.,123. 1656 (1976). E. Itabashi, T. M. Kenyhercz, and H. B.Mark, Jr., private communication. E. Nygard, Ark. Kemi. 20, 163 (1963). W. M. Schwarz and I. Shain, J . Phys. Chem., 69, 30 (1965). R. P. Van Duyne, T. H. Ridgway, and C. N. Reiliey, J . Electroanal. Chem., 34, 283 (1972). B. McDuffie, L. E. Anderson, and C. N. Reilley, Anal Chem.. 38, 883 (1966). D. M. Oglesby, J. D. Johnson, and C. N. Reilley, Anal. Chem., 38, 385 (1966). R. H. Wopschaii and I . Shain, Anal. Chem., 39, 1535 (1967). W. M. Schwarz and I. Shain, J . Phys. Chem., 70, 845 (1966). M. L. Meyer, T. P. DeAngelis, and W . R . Heineman. Anal. Ghem.. 49, 602 (1977). R. B.Carlin, R. G. Nelb, and R. C. Odioso, J. Am. Chem. Soc.. 73, 1002 (1951). Mark S. Denton, Ph.D. Thesis, University of Cincinnati, Cincinnati, Ohio, 1977. D. A. Blackadder and C. Hinshelwood, J . Chem. Soc., 2898 (1957).
CONCLUSIONS Thin-layer spectroelectrochemistry is a viable technique for measuring rate constants of reactions involving electrogenerated species when optical monitoring of the reaction is possible. T h e time domain of the technique as implemented in this paper restricts its applicability to slower reactions. As such, it complements the much faster spectroelectrochemical techniques involving semi-infinite diffusion conditions where the time domain is ca. 10 ws to 60 s (7). The thin-layer method is relatively easy to implement: no solving or simulation of diffusion equations is necessary and no background corrections are required as in most electrochemical techniques where current or charge is the monitored response. ACKNOWLEDGMENT T h e authors acknowledge J. F. Goelz for suggesting the benzidine rearrangement as a model chemical system and T. H. Ridgway for helpful discussions.
RECEIVED for review October 23, 1978. Accepted January 2 , 1979. Partial support of this research was provided by the National Science Foundation.
Metallized-Plastic Optically Transparent Electrodes R. Cieslinski and N. R. Armstrong" Department of Chemistry, University of Arizona, Tucson, Arizona
8572 1
Polyester sheets covered with thin films of metal or metal oxide can be used as optically transparent electrodes (MPOTE). These new electrodes have low sheet resistivities and high transparency to visible wavelength light-better than most optically transparent electrodes. The flexibility of these metallized polyester films may facilitate their use in unusual environments or cell geometries. Preliminary electrochemical, optical, and surface analytical data are presented for gold and indium/tin oxide MPOTEs. The gold MPOTE, when covered with a thin antireflection coating of titanium oxide, exhibited unusual voltammetric behavior indicative of a porous, semipassive electrode surface.
Films of metal or metal oxides of 1000-10000 A thickness have been used as electrodes for several years (1-7). These materials were first developed because of their optical transparency to visible wavelength light, and the field of spectroelectrochemistry was developed as a result of their widespread use. Metal or metal oxide films can be formed 0003-2700/79/0351-0565$01 OO/O
by evaporation, sputtering, or chemical vapor deposition of the conductive material onto substrates such as glass or quartz, or other metals, if optical transparencv is not desired. Thin film electrodes also enjoy the advantage of reproducible surface properties ( 4 , 7 ) . Several electrodes can be made simultaneously which will have identical surface chemical composition and morphology. The use of thin film electrodes has been restricted to cases where rigid substrates could be employed and were difficult to fabricate into irregular or very small electrode configurations. The expense and difficulty of preparation of thin film electrodes with good optical and surface properties has been a problem (2-.1, 8 ) . Recently, polymer sheets, covered with thin films of metal or metal oxide (metallized-plastic optically transparent electrodes, MPOTE), have become commercially a\ ailable (9). Proprietary sputtering processes have resulted in the production of some polyester-polymw sheets covered with less than 1000 A of noble metals such as gold or metal oxides like indium/ tin oxide. The sheet resistivities of these materials are exceptional for their thickness (down to 10 Q/sq. for the gold films; down to 15-200 Q/sq. for the indiumitin oxide C 1979 American Chemical Society
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films), and their transparency t o visible wavelength light appears t o be as good or better than previously reported for most optically transparent electrodes (OTEs) (1-8). T h e substrate material is very flexible with no apparent degradation of electrode properties under mechanical deformation. This fact may lead t o a variety of experiments with OTEs in unusual geometries which could not have been previously attempted. T h e cost of these new materials is also quite low in comparison with previous thin film electrodes ($1.50/ft2 VS. $60-100/ft2). We report here the first of a series of experiments designed t o exploit the unique properties of these new thin film electrodes. Preliminary electrochemical, optical, and surface analysis data are presented to demonstrate the utility of these materials and to foster their further application. A metal oxide coating applied t o certain of the gold MPOTEs t o improve their optical transparency led t o unusual electrochemical behavior. Our preliminary characterizations of this new metal/metal oxide electrode material are also detailed herein. EXPERIMENTAL The basic cell design used for electrochemical experiments has been described previously ( I ) . This design allows sandwich-like positioning of the metallized plastic film electrode to a Teflon body with a cell volume of approximately 10 mL. Geometric area of the electrode was 0.5 cm2. A potentiostat of conventional design was used for all studies. Differential capacitance measurements were made by modulating the electrode potential with a 400 Hz, 10-20 mV sine wave and measuring the quadrature component of the potentiostat response with a Princeton Applied Research, 126, lock-in amplifier ( I , 7). Metallized plastic optically transparent electrodes (MPOTE) were obtained from Sierracin/Sylmar (Sylmar,Calif.) under trade names Intrex-G (gold) and Intrex-K (indium-tin oxide, ITO) films. The electrodes were thoroughly cleaned in an ultrasonic cleaner using successive washings of detergent, ethanol, and distilled water. All aqueous solutions were prepared from water which had been distilled three times from alkaline KMnO,. All buffers were prepared from potassium hydrogen phthalate, potassium dihydrogen phosphate, or sodium bicarbonate and either nitric acid or sodium hydroxide. High purity, nonaqueous solvents were obtained from Burdick and Jackson laboratories (Muskegon, Mich.) and were passed through activated alumina just prior to use. In all nonaqueous studies, tetraethyl ammonium perchlorate (TEAP) was used as the supporting electrolyte which had previously been recrystallized from ethanol and dried under vacuum. Reagent grade ferrocene (Eastman Kodak), bis(hydroxymethy1)ferrocene (Strem), and potassium ferricyanide (Mallinckrodt) were recrystallized twice from suitable solvents: methyl viologen (l,l'-dimethyl-4,4'-bipyridium) (Aldrich) was used as obtained. XPS spectra were obtained on a GCA-McPherson ESCA 36 spectrometer using a magnesium anode operating at 300 W of power. Vacuum was maintained at ca. Torr. Preliminary XPS spectra were also obtained by A. E%'. C. Lin (Ohio State University) on a Physical Electronics 548 ESCA/Auger Spectrometer. SEM analysis was carried out using an ETEC-Autoscan instrument. Small amounts of carbon were vapor-deposited on the electrodes prior to analysis to prevent image-charging effects. RESULTS AND DISCUSSION
Gold M P O T E s . Two types of gold M P O T E were investigated. One available form had a thin (ca. 300 A) gold coating over the polyester substrate (sheet resistance ca. 6 R/sq.). T h e second and more readily available form had an additional titanium oxide coating over the gold film of ca. 1200 A thickness ( I O ) . The visible spectra of the gold and titanium oxide-coated gold MPOTEs compared favorably with previous electrodes (2). T h e gold M P O T E had an absorbance minimum of A = 0.30 a t A,, = 550 nm. T h e addition of the titanium oxide coating improves the optical transparency (AjM = 0.20). b u t a t the expense of certain electrochemical parameters discussed below. Preliminary XPS data indicated
4 y
2
-I ~
. 60 s), indicative of adsorption or depletion of the electroactive species. We have observed this phenomenon in the voltammetric response of other viologens which may lead to electrochromic display applications of the M P O T E (13). It is clear that the indium/tin oxide films behave nearly as well as similar films of greater thickness. This property may lead to new experiments where a thicker semiconductor film would be optically or electronically undesirable ( 2 ) . CONCLUSIONS The metallized plastic optically transparent electrode represents an interesting new development in thin film electrode technology. The low cost and excellent optical properties of the MPOTE alone are unique. The ease of manipulation of small and/or distorted electrode geometries with the metallized polyester films may facilitate their use in unusual environments. Thin-layer electrochemical cells, with the electrode bonded to the inner wall of a narrow glass tube, are easily manufactured (13). The electrodes can be inserted in any cavity to 0.003 inch with a width constrained only by the skill of the experimenter in cutting the plastic sheet. We are also exploring the possibility that chemical functionalities from the polyester substrate could be used for covalent attachment sites for purposes of electrode chemical modification (13). The titanium oxide coating on the gold MPOTE may also lend itself to such an application. Multilayered electrochemical solar cells are also a possible application of these new metallized plastic films (14). ACKNOWLEDGMENT The authors would like to acknowledge the gift of the metallized plastic films from John Fenn, Sierracin/Sylmar. LITERATURE CITED (1) N. R. Armstrong, A. W. C. Lin, M. Fujihira, and T. Kuwana, Anal. Chem., 48, 741 (1976). (2) N. Winograd and T. Kuwana, in "Advances in ElectroanalyticalChemistry", A. J. Bard, Ed., Marcel Dekker, New York, 1974, Vol. 7. (3) H. A. Laitinen, C. A. Vincent, and T. M. Bednarski, J . Hectrochem. Soc., 115, 1024 (1968). (4) R. K. Quinn, N. R. Armstrong, and N. E. Vanderborgh, J . Vac. Sci. Techno/., 12, 160 (1975). (5) C. Y. Li and G. S. Wilson, Anal. Chem., 45, 2370 (1973). (6) F. Mollers and R. Memming, Ber. Bunseges Phys. Chem., 77, 879 (1973). (7) N. R. Armstrong and R . K. Quinn, Surf. Sci., 67, 451 (1977). (8) C. A. Vincent, J . Electrochem. Soc., 119, 515 (1972). (9) W. P. Townsend. in "Handbook of Adhesives", 2nd ed., I . Skeist, Ed., Van Nostrand-Reinhold Co., New York, 1976, pp 836-854. (10) John Fenn, Sierracin/Sylmar Corporation, private communication. (11) C. Sayers and N. R. Armstrong, Surf. Sci., 77, 301 (1978). (12) D. T. Clark, in "Characterization of Metal and Polymer Surfaces", L-H. Lee, Ed., Academic Press, New York, 1977, Voi. 2. (13) R . Cieslinski and N. R. Armstrong, unpublished results. (14) R. Shepard and N. R. Armstrong, unpublished results.
RECEILTD for review October 6, 1978. Accepted December 27, 1978. Research supported by a grant from the National Science Foundation, CHE 77-14683.
