Spectroelectrochemistry Using Transparent Electrodes - ACS

Chapter 30

Spectroelectrochemistry Using Transparent Electrodes A n Anecdotal History of the Early Years

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William R. Heineman and William B. Jensen Department of Chemistry, University of Cincinnati, Cincinnati, O H 45221-0172 Spectroelectrochemical methods involve the in situ coupling of a spectroscopic technique with an electrochemical technique and may be subdivided into those methods which focus on the spectroscopic characterization of the electrode surface and those which focus on the spectroscopic characterization of the electrogenerated solution species. This paper will provide an anecdotal account of the early history of techniques in the second category based largely on the senior author's personal recollections and on interviews with several of the key participants in the development of optically transparent electrodes. Strictly speaking, the term spectroelectrochemistry subsumes any technique which involves the in situ coupling of a spectroscopic technique with an electrochemical technique. However, from both a practical and historical standpoint, spectroelectrochemical methods may be further subdivided into those which focus on the spectroscopic characterization of the electrode surface and those which focus on the spectroscopic characterization of the electrogenerated solution species. Techniques in the first category are much older than those in the second and can be traced back to the pioneering work of L. Tronstad and his students in the late 1920's and early 1930's (14). Most of the work in this area was done in Europe and Great Britain and involved measurement of the changes in the polarization of light reflected from the surfaces of metallic electrodes — a procedure later given the name of ellipsometry ( 5 , 6 ) . In contrast, techniques in the second category are of much more recent origin, were largely developed by research groups in the United States, and are essentially coupled to the discovery and development of various kinds of optically transparent electrodes (OTE) during the 1960's. Though important advances were also made in the 1960's in the area of electrode surface characterization (7), this account will focus almost exclusively on the methods in the second category. In

0097-6156/89/0390-0442$06.00/0 © 1989 American Chemical Society

In Electrochemistry, Past and Present; Stock, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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addition, rather than attempt what would, in all probability, turn out to be a premature historical assessment of the importance of these relatively recent events, we will instead provide a more limited anecdotal account of their early development based largely on the senior author's personal recollections and on interviews with several of the key participants. Transmission Spectroelectrochemistry The origins of modern transmission spectroelectrochemistry appear to date from a conversation held in the late 1950's between Ralph Adams, who was at the time a young assistant professor at the University of Kansas and just beginning his classic researches on the mechanisms of the oxidation of organic compounds at solid electrodes (8), and Ted Kuwana, who was Adam's first graduate student. As Kuwana later recalled it, Adams, while observing the production of an intense yellow color in the solution near the platinum anode during the coulometric oxidation of o-tolidine, had wishfully mused that ". . . it would be nice to have a 'seethrough' electrode to spectrally identify the colored species being formed . . . (9)". This statement, to the best of the authors' knowledge, was the first to introduce the concept of an optically transparent electrode as a means of making in situ spectroscopic measurements of electrode processes. It also addressed the important "raison d'etre' for spectroelectrochemical techniques in general — namely — the fact that it is often quite difficult to deduce the detailed mechanism of a complicated electrode process unambiguously from the results of electrochemical measurements alone. Consequently, another physical probe acting simultaneously with an electrochemical perturbation is needed to give additional information about the electrogenerated species being formed. Adams' prophetic statement did not go unheeded by Kuwana (Figure 1). While still a graduate student, he attempted his first spectroelectrochemical experiment by passing a light beam parallel to the surface of a platinum electrode. Inability to focus sufficient beam intensity at the electrode surface stymied the experiment. [It was not until ca. 20 years later that this experiment was performed successfully with a laser (10.) ] Several years later and now as an assistant professor at the University of California at Riverside, Kuwana was visiting the laboratory of David Kearnes, a physical chemist who was studying photoconduction in organic crystals. Kuwana saw two wires emerging from one of the crystals and wondered how electrical contact was being made with the crystal. Closer inspection showed the crystal sandwiched between two pieces of a glass that was coated on one side with tin oxide, a semiconductor. The contact wires were glued to the tin oxide sides of each piece of glass. Since the tin oxide-coated glass was optically transparent, the organic crystal could be irradiated with a light beam through the glass and the resulting conductivity measured by means of the conducting coating of tin oxide. Kuwana returned to his office with samples of the "conducting glass". These samples became the first OTEs and were used for the first optical spectroelectrochemical characterizations of electrogenerated solution species reported in the literature. These were



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F i g u r e 1.

Ted Kuwana.




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performed by Kuwana, graduate student Keith Darlington, and under graduate student Don Leedy. From the many organic redox systems with which Kuwana was familiar from his Ph.D. research, he picked o-tolidine, a colorless compound . that undergoes a 2-electron oxidation to form an intensely yellow colored species — the very reaction that had sparked Adams' earlier comment:

The formation of the yellow species was monitored at 437 run during its electrogeneration by a constant anodic current (i.e., chronopotentiometry), and the results were reported in 1964 (11). The optical beam was passed through the OTE, at an angle perpendicular to the plane of the electrode, and the adjacent solution as shown in Figure 2A. This is now referred to as transmission spectroelectrochemistry. A drawing of the cell, which was reported later (12), is shown in Figure 3. The tin oxide glass working electrode and a piece of ordinary glass were clamped against the cylindrical cell body. After Kuwana moved to Case Western Reserve University, the electrochemical and optical characteristics of this OTE were studied in more detail by visiting professors Jerzy Strojek from Poland (12) and Tetsuo Osa from Japan (13) and the fundamental equation for chronoabsorptometry was derived

where A = absorbance, ε = molar absorptivity of the electrogenerated species being monitored optically, and C and D are the concentration and diffusion coefficients, respectively, for the species being electrolyzed. The first use of transmission spectroelectrochemistry to study an electrode mechanism was the study of o-tolidine oxidation at a pH where a semiquinone radical cation is formed (14). The first spectroelectrochemical working curves were calculated by Strojek and Kuwana in collaboration with Stephen Feldberg at Brookhaven, who developed the method of digital simula tion of electrode mechanisms (14,15). This work was carried on by postdoc Henry Blount and graduate student Nicholas Winograd (16). Internal Reflection Spectroelectrochemistry During the 1960s the North American Aviation Science Center in Thousand Oaks, California, was a hotbed of electrochemical research. Robert Osteryoung, Keith Oldham and Joseph Christie were involved in the development of chronocoulometry and pulse techniques. Wilford Hansen (Figure 4 ) , a physicist working in Osteryoung's group, had initiated research in the area of internal reflection spectroscopy (17,18), a new spectroscopic technique reported by Fahrenfort (19). Hansen was apparently the first to use multiple internal reflection spectroscopy in a "light pipe" to



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Figure 2. chemistry.

Various optical techniques used in spectroelectroA. Transmission. B. Internal reflection.

Figure 3. First cell for transmission spectroelectrochemistry. (Reproduced with permission from Ref. 12. Copyright 1968 Elsevier).





Figure 4.

Wilford Hansen.


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obtain visible spectra of liquids (17). Being in an electrochemistry group, he wanted to apply this new technique to electrochemistry. The optical beam would be directed into the back side of an OTE at an angle greater than the critical angle so that the beam would be totally reflected (Figure 2B). According to Hansen (20), "A key in the whole thing was my discovery that an enhanced evanescent wave could be arranged on the far side of a thin conducting film (metal or otherwise) placed on a prism or multiple reflection plate and operated in ATR mode with the conductor as an electrode. This was totally unexpected but was confirmed by my numerical calculations using Maxwell's equations for stratified media. We then proceeded to do spectroelectrochemistry more seriously. Skeptics kept trying to say that the effect was due to 'cracks in the surface where the solution seeped in,' etc. The work with Kuwana's o-tolidine silenced the critics." Hansen designed a spectroelectrochemistry cell for use with a tin oxide or gold film OTE, which he had discovered independent of Kuwana. However, Hansen needed a good chemical system with which to demonstrate the technique. Hansen and Osteryoung met with Ruwana at the Gordon Research Conference on Electrochemistry in Santa Barbara. Kuwana happened to have in his pocket a sample of o-tolidine, which he gave to Hansen (21). Experiments performed on o-tolidine at the Science Center were subsequently published as the first examples of multiple internal reflection spectroelectrochemistry (22,23). The cell is shown in Figure 5. Hansen continued to study phenomena associated with IRS (24-27) as did Kuwana (28,29). The first infrared spectroelectrochemistry was reported in 1966 by Harry Mark and Stan Pons (30). As a postdoc at Cal Tech with Fred Anson, Mark (Figure 6) had visited Kuwana at Riverside — a visit that sparked his interest in spectroelectrochemistry. He also visited the electrochemistry group at the Science Center where he discussed internal reflection spectroelectrochemistry with Hansen. Upon his arrival at the University of Michigan as an assistant professor, Mark decided to try to develop infrared internal reflection spectroelectrochemistry because of the structural information in an infrared spectrum. He was able to persuade Paul Wilks (Wilks Scientific Corp.) to lend him a double beam internal reflectance attachment. A Ge internal reflectance plate-electrode was made by Recticon Corp. from n-type semiconducting Ge, which was already known to function as a working electrode (31,32). Stan Pons, a first year graduate student, performed the experimental work in which the spectra of the reduction products of 8-quinolinol and tetramethylbenzidine free radical were chracterized. During this time they found out about an organic liquid complex of platinum (Engelhard Industries, Inc.) that could be painted on a glass substrate and reduced at sufficiently high temperatures to an optically transparent film of platinum. Pons, together with graduate student James Mattson and undergraduate Leon Winstrom, showed that these metal films on glass could function as an OTE for internal reflection spectroscopy (33). The chronopotentiometric oxidation of o-tolidine to the yellow product, which by now had become the standard for testing new spectroelectrochemical techniques, was monitored. Absorbance changes much



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Figure 5. Cell for internal reflection spectroelectrochemistry. Reproduced from Ref. 24. Copyright 1967 American Chemical Society.)


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Figure 6. Harry B. Mark, Jr. Mark's attire is not part of his technique for infrared spectroelectrochemistry, but rather reflects his interest at the time in driving race cars. (USAC Publicity Photo circa 1965. Only photo available from that era.)




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greater than those at a tin oxide OTE were observed. This apparent anomaly resulted in an argument with the reviewers, who were skeptical of the results (34). Subsequently, a student of Mark's, Arnold Prostak, spent a year with Hansen at the Science Center. Their work explained some of the anomalies of internal reflectance spectra (25,27). An Aside During this period, Fred Anson (California Institute of Technology) had founded the San Clemente Surfing and Discussion Society as a means of stimulating informal discussion among electrochemists. The society's name proved to be an impediment to some scientists receiving permission and/or funds to attend the discussions. Consequently, Mark, who was the Society's secretary and editor of its newsletter, held a contest among the membership for a new name. The winner was to receive a platinum-coated bottle of Cutty Sark. Ralph Adams was declared the winner for the name Western Electroanalytical Theoretical Society (i.e., WETS, which is the forerunner of the Society for Electroanalytical Chemistry, SEAC). Mark tried coating numerous empty Scotch bottles by the liquid Pt process. During these experiments, the Assistant Department Head at Michigan was giving some visiting dignitaries a tour of the Department. Mark was standing at a lab bench with about 20 bottles lined up in front of him facing the door, when the door was opened by the Assistant Head with the words "And here we have our laboratory for electrochemistry." He then saw Mark and his bottles and closed the door immediately without a word to anyone. Adams was subsequently presented the trophy at Robert Osteryoung's house in San Clemente at the next meeting of the Society. During this meeting the idea of a Gordon Research Conference on Electrochemistry was approved. Incidentally, the trophy presented to Adams was silver plated. Mark decided that the liquid platinum gave a surface that was inappropriately rough to be a fitting trophy. He claims that the idea of using the liquid platinum for an OTE came before the experiments with the liquor bottles (34). Optically Transparent Thin Layer Electrodes In 1967 the first OTE based on light passing through holes in an electrode and the first thin-layer spectroelectrochemical experiment were reported (35). The discovery of this OTE was fortuitous. John Ashley, a graduate student with Charles Reilley, had seen an electroformed mesh (i.e., minigrid) advertised by Buckbee-Mears, Inc. He mentioned this to Larry Anderson (36), a postdoc with Charles Reilley, who sent for a free sample. The minigrid was produced in various dimensions ranging from 5 to 2000 wires/inch from different metals — Au, Cu, Ni, Ag. Anderson showed the sample around the "Wigwam", which was the Murray-Reilley laboratory for electrochemistry. Royce Murray (Figure 7) took interest in the material in relationship to his studies on the inhibition of electrode processes by adsorbed films (37). One model system for this behavior involved islands of adsorbed material that completely inhibited the electrode reaction and regions in between with no adsorption and at which the electrode process was unaffected. The minigrid could serve as an electrode




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Figure 7.

Royce Murray.





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for this model — the holes simulating regions of adsorption inhibition and the wires simulating bare electrode. A mathematical model based on this geometry could be constucted and the electrochemical behavior of the minigrid electrode characterized and compared with that for systems in which adsorption inhibition occurred. Consequently, Murray ordered pieces of 100 and 1000 wire/inch gold minigrid. In the meantime, the classic article by Hansen, Kuwana and Osteryoung (23) appeared and stimulated interest in spectroelectrochemistry. Having noticed that the minigrid was transparent to light, Murray realized that this material would function as an OTE. This idea was mentioned to a postdoc, George O'Dom, who had done undergraduate research with Kuwana at Riverside, and a graduate student, William Heineman, both of whom expressed interest in the project. Heineman was given the task of designing and constructing a suitable cell. At this time the Reilley group was heavily involved in the development of thin layer electrochemistry. This probably sparked the idea of thin-layer spectroelectrochemistry. The first optically transparent thin layer electrode (OTTLE) was a piece of minigrid sandwiched between two glass microscope slides that were separated by spacers consisting of layers of Tygon paint along the periphery (Figure 8) (38). The first cell didn't work because it was so thin that air bubbles couldn't be dislodged from around the minigrid. Extra layers of Tygon paint were used to build up the spacer thickness for subsequent cells and the minigrid was moved to the bottom of the cell to minimize iR drop. Kuwana'8 now classic o-tolidine system was used to characterize the cell. During the next year, George O'Dom left the group and Heineman worked with another postdoc, Nick Burnett, to construct an OTTLE from an infrared cell with salt plates for the optical windows (39). Multiple minigrids were stacked between spacers to increase the optical pathlength of the cell while retaining thin-layer diffusional distances. This cell was used to demonstrate infrared thin-layer spectroelectrochemistry with ninhydrin. This was chosen for its three carboxyl groups, the reduction of which would be easily observable in the infrared. Another thin-layer cell was constructed with quartz plates in order to illustrate applicability in the UV (40). The summer after the gold minigrid thin layer work was initiated in Murray's group, two students of Reilley's, Attila Yildiz and Peter Kissinger, learned about a vapor-deposition facility in the Physics Department. Preliminary experiments showed that voltammetric measurements could be made on Pt films vapor deposited on glass. Subsequently, the first thin layer cell with an OTE vapor-deposited on a quartz slide was demonstrated (41). Spectra of oxidized rubrene were recorded and fluorescence spectra recorded of regenerated rubrene. Kissinger subequently designed thin-layer cells whereby specular reflectance measurements could be made in the UV-visible and infrared regions (42). Electron Spin Resonance Spectroelectrochemistry During the development of OTE-based spectroelectrochemistry, a totally different type of spectroelectrochemistry, which was also useful for the structural identification of electrogenerated solu-


Figure (38).

8.

First

optically

transparent

thin

layer

electrode


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tion species, was in the making based on a coupling of electrochemistry with electron spin resonance spectroscopy. Although this technique does not involve the use of an OTE and thus, strictly speaking, lies outside the limits of our historical survey, it has, nevertheless, proven effective in the study of the electrogeneration of transient radical species in solution and consequently deserves some mention. The first electrochemical experiment in an ESR spectrometer was reported in 1957 by A.A. Galkin, I.L. Shamfarov and A.V. Stephanishiva in Russia (43). One year later D.E.G. Austen, P.H. Given, D.J.E. Ingram, and M.E. Peover reported the formation of aromatic radical ions by electrochemical reduction in non-aqueous solvents (44). The samples were frozen prior to introduction into the spectrometer. A.H. Maki and D.H. Geske (Harvard University) are generally credited (45) with introducing the real potentialities of ESR spectroelectrochemistry (46). They carried out electrochemical generation at a mercury pool electrode in the microwave cavity and produced a number of radical ions in non-aqueous media. The technique was subsequently extended to aqueous solutions by R.N. Adams, L.H. Piette and P. Ludwig at the University of Kansas (47).

Conclusions As noted at the beginning of our survey, spectroelectrochemistry is of fairly recent origin and is still a developing area of research. Consequently, we have had to severely restrict our subject matter in terms of both breadth of coverage and time period. For surveys of current developments, the reader is referred to recent review articles (48,49). Literature Cited 1. Tronstad, L. Z. Phys Chem. (Leipzig) 1929, A142, 241. 2. Tronstad, L. In Optische Untersuchungen zur Frage der Passivitat des Eisens und Stahls; Kgl. Norske Videnskab Selsk. Skr., 1931, 1. 3. Tronstad, L. Trans. Faraday Soc. 1933, 29, 502. 4. Winterbottom, A.B. In Optical Studies of Metal Surfaces, Kgl. Norske Videnskab. Selskabs Skrifter, 1, F. Bruns Bokhandel, Trondheim, 1955. 5. Reddy, A.K.N. ; Genshaw, M.; Bockris, J.O'M. J. Electroanal. Chem. 1964, 8, 406-407. 6. Delahay, P. and Tobias, C.W. In Advances in Electrochemistry and Electrochemical Engineering; Muller, R.H., Ed.; Wiley: New York, 1973; Vol. 9, p. 168. 7. Optical Studies of Adsorbed Layers at Interfaces, Symposia of the Faraday Society, No. 4, The Faraday Society, London, 1970. 8. Adams, R.N. Electrochemistry at Solid Electrodes; Marcel Dekker: New York, 1969. 9. Interview with T. Kuwana, April 1988. 10. Pruiksma, R.; McCreery, R.L. Anal. Chem. 1979, 51, 2253-2257. 11. Kuwana, T.; Darlington, R.K.; Leedy, D.W. Anal. Chem. 1964, 36, 2023-2025. 12. Strojek, J.W.; Kuwana, T. J. Electroanal. Chem. 1968, 16, 471-483.


456 13. 14. 15.


16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.

32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46.

ELECTROCHEMISTRY, PAST A N D PRESENT Osa, T. and Kuwana, T. J. Electroanal. Chem. 1969, 22, 389406. Kuwana, T.; Strojek, J.W. Discussions Faraday Soc., 1968, 45, 134-144. Strojek, J.W.; Kuwana, T.; Feldberg, S.W. J. Am. Chem. Soc. 1968, 90, 1353-1355. Winograd, N.; Blount, H.N.; Kuwana, T. J. Phys. Chem. 1969, 73, 3456-3462. Hansen, W.N. Anal. Chem. 1963, 35, 765-766. Hansen, W.N. Spectrochim. Acta 1965, 21, 815-833. Fahrenfort, J. Spectrochim. Acta 1961, 17, 698-709. Hansen, W.N. Private communication, May 10, 1988. Interview with W.N. Hansen, April 1988. Hansen, W.N.; Osteryoung, R.A.; Kuwana, T. J. Am. Chem. Soc. 1966, 88, 1062-1063. Hansen, W.N.; Kuwana, T.; Osteryoung, R.A. Anal. Chem. 1966, 38, 1810-1821. Horton, J.A.; Hansen, W.N. Anal. Chem. 1967, 39, 1097-1100. Hansen, W.N.; Prostak, A. Phys. Rev., 1968, 174, 500-503. Hansen, W.N. J. Optical Soc. Am. 1968, 58, 380-390. Prostak, A.; Mark, Jr., H.B.; Hansen, W.N. J. Phys. Chem. 1968, 72, 2576-2582. Srinivasan, V.S.; Kuwana, T. J. Phys. Chem. 1968, 72, 11441148. Winograd, N.; Kuwana, T. J. Electroanal. Chem. 1969, 23, 333342. Mark, Jr., H.B.; Pons, B.S. Anal. Chem. 1966, 38, 119-121. Gerischer, H. In Advances in Electrochemistry and Electrochemical Engineering; Delahay, P., Ed.; Interscience: New York, 1961; Vol. I, pp. 139-232. Holmes, P.S. Electrochemistry of Semi-Conductors; Academic Press: New York, 1962. Pons, B.S.; Mattson, J.S.; Winstrom, L.O.; Mark, Jr., H.B. Anal. Chem. 1967, 39, 685-688. Interview with H.B. Mark, Jr. May 10, 1988. Murray, R.W.; Heineman, W.R.; O'Dom, G.W. Anal. Chem. 1967, 39, 1666-1668. Interview with L.B. Anderson, April 1988. Interview with R.W. Murray, April 1988. Heineman, W.R. Research Notebook 2, University of North Carolina, Feb. 9, 1967, p. 38. Heineman, W.R.; Burnett, J.N.; Murray R.W. Anal. Chem., 1968, 40, 1974-1978. Heineman, W.R.; Burnett, J.N.; Murray, R.W. Anal. Chem. 1968, 40, 1970-1973. Yildiz, A.; Kissinger, P.T.; Reilley, C.N. Anal. Chem. 1968, 40, 1018-1024. Kissinger, P.T.; Reilley, C.N. Anal. Chem. 1970, 42, 12-15. Galkin, A.A.; Shamfarov, I.L.; Stefanishina, A.V. J. Exptl Theoret. Phys. (U.S.S.R.) 1957, 32, 1581. Austen, D.E.G.; Given, P.H.; Ingram, D.J.E.; Peover, M.E. Nature 1958, 182, 1784. Adams, R.N. J. Electroanal. Chem. 1964, 8, 151-162. Maki, A.H.; Geske, D.H. J. Chemical Phys. 1960, 33, 825-832.


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P i e t t e , L.H.; Ludwig, P . ; Adams, R.N. J. Am. Chem. Soc. 1960, 83, 2671; 1962, 84, 4212. McCreery, R.L. In Physical Methods in Chemistry; R o s s i t e r , B., Ed.; Wiley: New York, 1987; Vol 2 . Heineman, W.R.; Hawkridge, F.M.; Blount, H.N. In Electroanalytical Chemistry; Bard, A . J . , Ed.; Dekker: New York, 1984; Vol. 13, pp. 1-113.


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Spectroelectrochemistry Using Transparent Electrodes - ACS

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