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Anodic growth of CdS thin films: an in situ Raman spectroelectrochemical study
417
J. Electroanal. Chem., 310 (1991) 417-422 Elsevier !kquoia S.A., Lausanne
JEC 01554 Short communication
Anodic growth of CdS thin films: an in situ Raman spectroelectrochemical study D. Ham, Y. Son, K.K. Mishra and K. Rajeshwar * Department of Chemistry, Box 19065, The University of Texas at Arlington, Arlington, TX 760~9-0065 (USA) (Rweivcd 28 January 1991; in revised form 11 March 1991)
INTRODUCTION
There has been much recent interest in the fabrication of Group II-VI compound semiconductor thin films by anodic and cathodic electrodeposition methods [l-9]. These studies have been, in part, motivate by the use of these materials in solar and other optoelectronic device applications. A major problem with electrodeposition, however, is contamination of the target material with “impurity” phases; this is a particular difficulty with compound semiconductors (e.g., Te is usually deposited along with CeTe, cf. ref. 10). Unfortunately, monitoring of electrochemical parameters (e.g. charge, current, potential) alone, during the deposition process, provides only limited info~ation content in terms of molecular details. It is advantageous to combine an in situ spectroscopic probe with electrochemistry in such cases because of the extreme sensitivity and molecular selectivity of the former. We had used this strategy previously via cyclic photovolt~met~ to study electrodeposition mechanisms within the Group II-VI system 171. We now wish to report how Raman spectroscopy could be an especially useful probe for monitoring the chemistry associated with a semiconductor electrodeposition sequence. We have observed routinely via surface analysis, high oxygen levels (up to - 13 atom %) in Group II-VI se~#nductor tbin films synthesized by the anodic route (cf. ref. 6). Analysis of Auger peak shapes and XRS binding energies, however, revealed the 0 to be not associated with the chalcogen component within the film. Further, depth profiles revealed this 0 to be uniformly distributed throughout the thin film, thus ruling out surface oxides as the origin. The present Raman spectro* To whom correspondence should be addressed. 0022-0728/91/$O3.50
0 1991 - Elsevier Sequoia S.A.
418
electr~he~st~ study on the CdS system has clearly identified the 0 to arise from the intermediate formation of hydrated cadmium oxide and the resultant incomplete conversion of this to the corresponding chalcogenide. Indeed, a mechanistic scheme comprising intermediate hydrous oxide (“complex oxide/hydroxide”) formation had been invoked for CdS by other authors [9] albeit without direct experimental evidence. There has been increasing use of the Raman technique in spectroelectrochemical situations; much of this work has been recently reviewed [l&12]. Metallic substrates have been largely utilized in these studies although there are sporadic instances of the use of semiconductor electrodes also (cf. ref. 13). More directly related to this study, however, is the use of Raman spectr~l~tr~he~st~ for the examination of metal (Zn, Cu, Fe and Ni) corrosion in alkaline and chalcogenide media [14]. To our knowledge, the corrosion of Cd in these media has not been addressed hitherto via Raman spectroscopy. EXPERIMENTAL
The Raman spectroscopy system utilized a Spex 1680 double monochromator, a Lexel Model 95 Ar+ ion laser and a photomultiplier operated in the photon-counting mode (Products for Research Inc.). A personal computer and commercial software were utilized for data acquisition. The 488 nm laser line was used at back scattering geometry for excitation at power levels ranging from 10 mW to 50 mW. An instrumental slit width of 1 mm was used to obtain a large scattered image onto the spectrometer. The spec~~l~tr~he~c~ cell was custom-bolt; its details will be given elsewhere 1151. The three-electrode single-compartment cell used a Cd rod anode (Johnson-Matthey), a saturated calomel reference, and a Pt wire counterelectrode. An EG&G Princeton Applied Research Model 273 electrochemistry system was used for potential control and current measurements. A Bio-Rad Digilab Model FTS-40E instrument was used for Fourier transform infrared spectroscopy. Measurements in the mid-infrared region (4000400 cm-‘) were performed in the reflectance mode. Standard samples of CdS single crystals (Cleveland Crystals), hydrated cadmium oxide (henceforth designated as CdO/CdOH) in powder form (Johnson-Matthey), and electrolyte chemicals were used as received. RJZXJLTS AND
DISCUSSION
The Pourbaix (E-pH) diagram for the Cd-S-H,0 system shows a large stability zone for CdS [16]. It also predicts that the corrosion of Cd in alkaline media leads to hydroxides and hydrous oxides. This has been studied in detail by previous authors [17]. Saidman et al. (91 postulate two sequences for CHS formation in alkaline media - one electrochemical and the other chemical - both of which invoke CdO/CdOH as an intermediate. Figures 1 and 2 contain sequences of Raman spectra for Cd anodes potentiostatted at +0.35 V in Na,S + 1 M NaOH electrolytes. This poten-
419
1
0
I
200
I
400
I
600
1
600
Raman Shift /cm-1 Fig. 1. In situ Raman spectra of a Cd anode held at +0.3.5 V in 1 M NaOH electrolyte containing variable sulfide ion concentrations: (a) 0.01 M Na,S, (b) 0.1 M Na,S, (c) 1 M Na,S. The spectra were acquired after 20 min anodization time in each case.
tial is located just past the CdO/CdOH anodie wave in the passive voltammetric regime [18J. Figure I contains data with the sulfide concentration as the variable (20 min. caption time), and the spectra in Fig. 2 correspond to variable deposition times at 0.1 M sulfide conflation. Three peaks at 318 cm-‘, 492 cm-l and 622 cm-’ require discussion in these spectra. The 492 cm-’ feature is prominent at low sulfide concentration and during the early stages of anodic thin film growth. The question of whether the species responsible for the Raman signals reflect insoluble surface layers or originate from solution species which are a&orbed on the electrode surface is an interesting one. Saidman et al. [9J postulate competitive adsorption of OH- and sulfide ions for Cd sites followed by chemical pr~ipitation of CdS. We believe, however, that we are probing a relatively advanced stage of thin film growth here, and therefore the species responsible for the Raman signals are true surface layers. Independent evidence accrues from the photoresponse data (diagnostic of CdS) gathered on similarly prepared interphases. Returning to the spectral features in Figs. 1 and 2, the 318 cm-’ and 622 cm-i peaks become stronger with higher sulfide concentration (e.g., 0.1 M, Fig. 1) or long deposition times (e.g., 180 min, Fig. 2). To assign the chemical species responsible for the peaks at 318 cm-‘, 492 cm-’ and 622 cm-’ in Figs. 1 and 2, the “solid-state” Raman spectra of standard samples of CdS and CdO/CdOH were acquired. These are shown in Fig. 3a and 3b respectively. The assignment of the 318 cm-’ and 622 cm-’ signals to CdS, and the 492 cm-’ feature to CdO/C!dOH is
(4
(4
I
0
I
200
I
400
I
606
I
660
Raman Shift /cm ml Fig. 2. In situ Raman spectra of a Cd anode held at + 0.35 V in 1 M NaOH + 0.1 M Na,S electrolyte for variable duration: (a) 2 min, (b) 20 min, (c) 180 min.
clear and unambiguous. Our data for CdS are in good agreement with reported work [19] which interpretes these signals to arise from longitudinal phonon modes. The 492 cm-’ peak of the CdO/CdOH standard is broad reflecting the amorphous nature of the powdered material. The hydrous nature of this oxide sample is confirmed by the 3605 cm-’ feature in the FT-IR spectrum (not shown). Concomitantly, incompletely converted CdS anodic thin films (corresponding, for example, to the situations in Fig. 2a or 2b) manifested FT-IR signatures at 3605 cm-‘, 960 cm-’ and 933 cm-‘, which have been reported earlier for /3- and y-Cd(OH)2 (cf. ref. 17). The possibility of the CdO/CdOH signal arising from photocorrosion of the CdS thin film during the Raman measurement (as pointed out by a reviewer) was checked carefully. Thus measurements on a CdS single crystal under conditions mimicking those employed in Figs. 1 and 2 revealed the absence of the 492 cm-’ band. Further, identical trends as in Figs. 1 and 2 were observed when the 514 nm laser line was used for excitation although the peak intensities were significantly attenuated. The latter signals the importance of resonance enhancement effects in the Raman scattering signals observed at the anodic film/electrolyte interphase.
421
622 (a)
316
I
I
I
I
I
I
I
I
100
200
300
400
500
600
700
Raman
Shift /cm -1
Fig. 3. Solid-state Raman spectra of standard samples of CdS single crystal (a) and hydrated cadmium oxide powder (b).
We view the above data as furnishing direct experimental evidence for the electrodeposition sequence postulated earlier by Saidman et al. [9]. In particular, the fast kinetics associated with CdO/CdOH formation and the subsequent dissolution of these surface species to yield CdS via a chemical precipitation route are both consistent with the trends seen in Figs. 1 and 2. It is important to emphasize that an in situ examination of the photoresponse of the anode surface under identical conditions does not reveal unusual features other than a significant tailing of the optical absorption of CdS beyond the - 500 nm band-edge. Clearly, while contamination of the CdS with oxide could well explain this artifact, the molecular specificity of this information remains ambiguous. The present Raman spectroelectrochemical data also provide a rational explanation for the high 0 levels seen earlier in our anodic films. In this vein, the Raman probe could prove to be a valuable aid in optimizing and ensuring the compositional purity of anodic films. Further application of the in situ Raman spectroelectrochemistry technique to the study of semiconductor and conducting polymer thin films [15] is continuing in this laboratory.
422 ACKNOWLEDGEMENTS
This research was supported, in part, by the National Science Foundation (Grant MSM 86-17850) and the Texas Higher Education Coordinating Board (Advanced Technology Program). The Raman instrumentation was purchased via funds from a Defense Advanced Research Projects Agency/~~versity Research Initiative contract monitored by the Office of Naval Research. REFERENCES 1 B. Miller and A. Heller, Nature (London), 262 (1976) 680; B. Miller, S. Menexes and A. Heller, J. Electroaual. Chem., 94 (1978) 85; M. Skyllas-Kazacos and 3. M&r, J. EIectrochem. Sec., 127 (1980) 2378; 127 (1980) 869; M. Skyllas-Kaxacos, J. EIectroanaI. Chem., 148 (1983) 233; J.M. Rosamiha and B. Miller, J. Electroanal. Chem., 215 (1986) 249. 2 L.M. Peter, EIectrochim. Acta, 23 (1978) 165; L.M. Peter, J. Electroar& Chem., 98 (1979) 49; M.I. Da Silva Pereira and L.M. Peter, ibid., 140 (1982) 103. 3 F.A. Kroger, J. Electrochem. Sot., 125 (1978) 2028; M.P.R. Panicker, M. &raster and F.A. Kriiger, ibid., 125 (1978) 566. 4 R.N. Bhattacharya, K. Rajeahwar and R.N. Not&, J. EIectrochem. Sot., 131 (1984) 939; R,N. Bhattacharya and K. Rajeshwar, ibid., 131 (1984) 2032; R.N. Bhattacharya, K. Rajeshwar and R.N. No&i, ibid., 132 (1985) 732; R.N. Bhattacharya and K. Rajeshwar, J. Appl. Phys., 58 (1985) 3590; E. Mori, C.K. Baker, JR. Reynolds and K. Rajeshwar, J. Electroanal. Chem., 252 (1988) 441; E. Mori and K. Rajeshwar, ibid., 258 (1989) 415; E. MO& K.K. Mishra and K. Rajeshwar, J. Electrochem. sot., 137 (1990) 1100. 5 J.P. Szabo and M. Cocivera, Can. J. Chem., 66 (1988) 1065; A. Darkowski and M. Cocivera, J. Electrochem. Sot., 132 (1985) 2768; B.W. Sanders and M. Cocivera, ibid., 134 (1987) 1075; J. Szabo and M. Cocivera, ibid., 133 (1986) 1247. 6 D. Ham, K.K. Mishra, A. Weiss and K. Rajeshwar, Chem. Mater., 1 (1989) 619. 7 K.K. Mishra and K. Rajeshwar, J. Electroanal. Chem., 273 (1989) 169, and references therein. 8 V.I. Birss and LE. Kee, J. Electrochem. Sot., 131 (1986) 2097. 9 S.B. Saidman, J.R. Vilche and A.J. Arvia, Electrochim. Acta, 32 (1987) 1153, and references therein. 10 W.-Y. Lin, K.K. Mishra, E. Mori and K. Rajeshwar, Anal. Chem., 62 (1990) 821, and references therein. 11 R.L. McCreery and R.T. Packard, Anal. Chem., 61(13) (1989) 775A. 12 R.E. Hester, in R.G. Compton and A. Hammett (Eds.), Comprehensive Chemical Kinetics, Vol. 29, Elsevier, Amsterdam, 1989, Ch. 2. 13 For example, B.H. Loo, J. Electroar&. Chem., 136 (1982) 209; R.P. Van Dupe and J.P. Haushatter, J. Phys. Chem., 88 (1984) 2446; Q. Feng and T.M. Cotton, ibid., 90 (1986) 983; K. Metcalfe and R.E. Hester, 3. Chem. Sot., Chem. Commun., (1983) 133; R. Rosetti and LE. Brus, J. Phys. Chem., 90 {1986) 558. 14 CA. Melendres and S. XII, J. Electrochem. Sot., 131 (1984) 2239: C.A. Melendres, W. Paden, B. Tani and W. WaIczak, ibid., 134 (1987) 762; C.A. Meleudres, N. Camihone III and T. Tipton, Electrochim. Acta, 34 (1990) 281; R. Grubbs and C.A. Melendres, Electrochemical Society Meeting, San Diego, CA, 1986, Abstract No. 30; A. Hugot-LeGoff, S. Joiret, 3. Saidani and R. Wiart, J. EIectroanaI. Chem., 263 (1989) 127. 15 Y. Son and K. Rajeshwar, to be pubhshed. 16 S-M. Park and ME. Barber, J. ElectroanaI. Chem., 99 (1979) 67. 17 For example, G.T. Burstein, J. Electrochem. Sot., 130 (1983) 2133; R. Barnard, J. Appl. Electrochem., 11 (1981) 217. 18 D. Ham, K.K. Mishra and K. Rajeahwar, J. Electrochem. Sot., 138 (1991) 100. 19 B. Tell, T.C. Damen and S.P.S. Porto, Phys. Rev., 144 (1966) 771.
J. Electroanal. Chem., 310 (1991) 417-422 Elsevier !kquoia S.A., Lausanne
JEC 01554 Short communication
Anodic growth of CdS thin films: an in situ Raman spectroelectrochemical study D. Ham, Y. Son, K.K. Mishra and K. Rajeshwar * Department of Chemistry, Box 19065, The University of Texas at Arlington, Arlington, TX 760~9-0065 (USA) (Rweivcd 28 January 1991; in revised form 11 March 1991)
INTRODUCTION
There has been much recent interest in the fabrication of Group II-VI compound semiconductor thin films by anodic and cathodic electrodeposition methods [l-9]. These studies have been, in part, motivate by the use of these materials in solar and other optoelectronic device applications. A major problem with electrodeposition, however, is contamination of the target material with “impurity” phases; this is a particular difficulty with compound semiconductors (e.g., Te is usually deposited along with CeTe, cf. ref. 10). Unfortunately, monitoring of electrochemical parameters (e.g. charge, current, potential) alone, during the deposition process, provides only limited info~ation content in terms of molecular details. It is advantageous to combine an in situ spectroscopic probe with electrochemistry in such cases because of the extreme sensitivity and molecular selectivity of the former. We had used this strategy previously via cyclic photovolt~met~ to study electrodeposition mechanisms within the Group II-VI system 171. We now wish to report how Raman spectroscopy could be an especially useful probe for monitoring the chemistry associated with a semiconductor electrodeposition sequence. We have observed routinely via surface analysis, high oxygen levels (up to - 13 atom %) in Group II-VI se~#nductor tbin films synthesized by the anodic route (cf. ref. 6). Analysis of Auger peak shapes and XRS binding energies, however, revealed the 0 to be not associated with the chalcogen component within the film. Further, depth profiles revealed this 0 to be uniformly distributed throughout the thin film, thus ruling out surface oxides as the origin. The present Raman spectro* To whom correspondence should be addressed. 0022-0728/91/$O3.50
0 1991 - Elsevier Sequoia S.A.
418
electr~he~st~ study on the CdS system has clearly identified the 0 to arise from the intermediate formation of hydrated cadmium oxide and the resultant incomplete conversion of this to the corresponding chalcogenide. Indeed, a mechanistic scheme comprising intermediate hydrous oxide (“complex oxide/hydroxide”) formation had been invoked for CdS by other authors [9] albeit without direct experimental evidence. There has been increasing use of the Raman technique in spectroelectrochemical situations; much of this work has been recently reviewed [l&12]. Metallic substrates have been largely utilized in these studies although there are sporadic instances of the use of semiconductor electrodes also (cf. ref. 13). More directly related to this study, however, is the use of Raman spectr~l~tr~he~st~ for the examination of metal (Zn, Cu, Fe and Ni) corrosion in alkaline and chalcogenide media [14]. To our knowledge, the corrosion of Cd in these media has not been addressed hitherto via Raman spectroscopy. EXPERIMENTAL
The Raman spectroscopy system utilized a Spex 1680 double monochromator, a Lexel Model 95 Ar+ ion laser and a photomultiplier operated in the photon-counting mode (Products for Research Inc.). A personal computer and commercial software were utilized for data acquisition. The 488 nm laser line was used at back scattering geometry for excitation at power levels ranging from 10 mW to 50 mW. An instrumental slit width of 1 mm was used to obtain a large scattered image onto the spectrometer. The spec~~l~tr~he~c~ cell was custom-bolt; its details will be given elsewhere 1151. The three-electrode single-compartment cell used a Cd rod anode (Johnson-Matthey), a saturated calomel reference, and a Pt wire counterelectrode. An EG&G Princeton Applied Research Model 273 electrochemistry system was used for potential control and current measurements. A Bio-Rad Digilab Model FTS-40E instrument was used for Fourier transform infrared spectroscopy. Measurements in the mid-infrared region (4000400 cm-‘) were performed in the reflectance mode. Standard samples of CdS single crystals (Cleveland Crystals), hydrated cadmium oxide (henceforth designated as CdO/CdOH) in powder form (Johnson-Matthey), and electrolyte chemicals were used as received. RJZXJLTS AND
DISCUSSION
The Pourbaix (E-pH) diagram for the Cd-S-H,0 system shows a large stability zone for CdS [16]. It also predicts that the corrosion of Cd in alkaline media leads to hydroxides and hydrous oxides. This has been studied in detail by previous authors [17]. Saidman et al. (91 postulate two sequences for CHS formation in alkaline media - one electrochemical and the other chemical - both of which invoke CdO/CdOH as an intermediate. Figures 1 and 2 contain sequences of Raman spectra for Cd anodes potentiostatted at +0.35 V in Na,S + 1 M NaOH electrolytes. This poten-
419
1
0
I
200
I
400
I
600
1
600
Raman Shift /cm-1 Fig. 1. In situ Raman spectra of a Cd anode held at +0.3.5 V in 1 M NaOH electrolyte containing variable sulfide ion concentrations: (a) 0.01 M Na,S, (b) 0.1 M Na,S, (c) 1 M Na,S. The spectra were acquired after 20 min anodization time in each case.
tial is located just past the CdO/CdOH anodie wave in the passive voltammetric regime [18J. Figure I contains data with the sulfide concentration as the variable (20 min. caption time), and the spectra in Fig. 2 correspond to variable deposition times at 0.1 M sulfide conflation. Three peaks at 318 cm-‘, 492 cm-l and 622 cm-’ require discussion in these spectra. The 492 cm-’ feature is prominent at low sulfide concentration and during the early stages of anodic thin film growth. The question of whether the species responsible for the Raman signals reflect insoluble surface layers or originate from solution species which are a&orbed on the electrode surface is an interesting one. Saidman et al. [9J postulate competitive adsorption of OH- and sulfide ions for Cd sites followed by chemical pr~ipitation of CdS. We believe, however, that we are probing a relatively advanced stage of thin film growth here, and therefore the species responsible for the Raman signals are true surface layers. Independent evidence accrues from the photoresponse data (diagnostic of CdS) gathered on similarly prepared interphases. Returning to the spectral features in Figs. 1 and 2, the 318 cm-’ and 622 cm-i peaks become stronger with higher sulfide concentration (e.g., 0.1 M, Fig. 1) or long deposition times (e.g., 180 min, Fig. 2). To assign the chemical species responsible for the peaks at 318 cm-‘, 492 cm-’ and 622 cm-’ in Figs. 1 and 2, the “solid-state” Raman spectra of standard samples of CdS and CdO/CdOH were acquired. These are shown in Fig. 3a and 3b respectively. The assignment of the 318 cm-’ and 622 cm-’ signals to CdS, and the 492 cm-’ feature to CdO/C!dOH is
(4
(4
I
0
I
200
I
400
I
606
I
660
Raman Shift /cm ml Fig. 2. In situ Raman spectra of a Cd anode held at + 0.35 V in 1 M NaOH + 0.1 M Na,S electrolyte for variable duration: (a) 2 min, (b) 20 min, (c) 180 min.
clear and unambiguous. Our data for CdS are in good agreement with reported work [19] which interpretes these signals to arise from longitudinal phonon modes. The 492 cm-’ peak of the CdO/CdOH standard is broad reflecting the amorphous nature of the powdered material. The hydrous nature of this oxide sample is confirmed by the 3605 cm-’ feature in the FT-IR spectrum (not shown). Concomitantly, incompletely converted CdS anodic thin films (corresponding, for example, to the situations in Fig. 2a or 2b) manifested FT-IR signatures at 3605 cm-‘, 960 cm-’ and 933 cm-‘, which have been reported earlier for /3- and y-Cd(OH)2 (cf. ref. 17). The possibility of the CdO/CdOH signal arising from photocorrosion of the CdS thin film during the Raman measurement (as pointed out by a reviewer) was checked carefully. Thus measurements on a CdS single crystal under conditions mimicking those employed in Figs. 1 and 2 revealed the absence of the 492 cm-’ band. Further, identical trends as in Figs. 1 and 2 were observed when the 514 nm laser line was used for excitation although the peak intensities were significantly attenuated. The latter signals the importance of resonance enhancement effects in the Raman scattering signals observed at the anodic film/electrolyte interphase.
421
622 (a)
316
I
I
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100
200
300
400
500
600
700
Raman
Shift /cm -1
Fig. 3. Solid-state Raman spectra of standard samples of CdS single crystal (a) and hydrated cadmium oxide powder (b).
We view the above data as furnishing direct experimental evidence for the electrodeposition sequence postulated earlier by Saidman et al. [9]. In particular, the fast kinetics associated with CdO/CdOH formation and the subsequent dissolution of these surface species to yield CdS via a chemical precipitation route are both consistent with the trends seen in Figs. 1 and 2. It is important to emphasize that an in situ examination of the photoresponse of the anode surface under identical conditions does not reveal unusual features other than a significant tailing of the optical absorption of CdS beyond the - 500 nm band-edge. Clearly, while contamination of the CdS with oxide could well explain this artifact, the molecular specificity of this information remains ambiguous. The present Raman spectroelectrochemical data also provide a rational explanation for the high 0 levels seen earlier in our anodic films. In this vein, the Raman probe could prove to be a valuable aid in optimizing and ensuring the compositional purity of anodic films. Further application of the in situ Raman spectroelectrochemistry technique to the study of semiconductor and conducting polymer thin films [15] is continuing in this laboratory.
422 ACKNOWLEDGEMENTS
This research was supported, in part, by the National Science Foundation (Grant MSM 86-17850) and the Texas Higher Education Coordinating Board (Advanced Technology Program). The Raman instrumentation was purchased via funds from a Defense Advanced Research Projects Agency/~~versity Research Initiative contract monitored by the Office of Naval Research. REFERENCES 1 B. Miller and A. Heller, Nature (London), 262 (1976) 680; B. Miller, S. Menexes and A. Heller, J. Electroaual. Chem., 94 (1978) 85; M. Skyllas-Kazacos and 3. M&r, J. EIectrochem. Sec., 127 (1980) 2378; 127 (1980) 869; M. Skyllas-Kaxacos, J. EIectroanaI. Chem., 148 (1983) 233; J.M. Rosamiha and B. Miller, J. Electroanal. Chem., 215 (1986) 249. 2 L.M. Peter, EIectrochim. Acta, 23 (1978) 165; L.M. Peter, J. Electroar& Chem., 98 (1979) 49; M.I. Da Silva Pereira and L.M. Peter, ibid., 140 (1982) 103. 3 F.A. Kroger, J. Electrochem. Sot., 125 (1978) 2028; M.P.R. Panicker, M. &raster and F.A. Kriiger, ibid., 125 (1978) 566. 4 R.N. Bhattacharya, K. Rajeahwar and R.N. Not&, J. EIectrochem. Sot., 131 (1984) 939; R,N. Bhattacharya and K. Rajeshwar, ibid., 131 (1984) 2032; R.N. Bhattacharya, K. Rajeshwar and R.N. No&i, ibid., 132 (1985) 732; R.N. Bhattacharya and K. Rajeshwar, J. Appl. Phys., 58 (1985) 3590; E. Mori, C.K. Baker, JR. Reynolds and K. Rajeshwar, J. Electroanal. Chem., 252 (1988) 441; E. Mori and K. Rajeshwar, ibid., 258 (1989) 415; E. MO& K.K. Mishra and K. Rajeshwar, J. Electrochem. sot., 137 (1990) 1100. 5 J.P. Szabo and M. Cocivera, Can. J. Chem., 66 (1988) 1065; A. Darkowski and M. Cocivera, J. Electrochem. Sot., 132 (1985) 2768; B.W. Sanders and M. Cocivera, ibid., 134 (1987) 1075; J. Szabo and M. Cocivera, ibid., 133 (1986) 1247. 6 D. Ham, K.K. Mishra, A. Weiss and K. Rajeshwar, Chem. Mater., 1 (1989) 619. 7 K.K. Mishra and K. Rajeshwar, J. Electroanal. Chem., 273 (1989) 169, and references therein. 8 V.I. Birss and LE. Kee, J. Electrochem. Sot., 131 (1986) 2097. 9 S.B. Saidman, J.R. Vilche and A.J. Arvia, Electrochim. Acta, 32 (1987) 1153, and references therein. 10 W.-Y. Lin, K.K. Mishra, E. Mori and K. Rajeshwar, Anal. Chem., 62 (1990) 821, and references therein. 11 R.L. McCreery and R.T. Packard, Anal. Chem., 61(13) (1989) 775A. 12 R.E. Hester, in R.G. Compton and A. Hammett (Eds.), Comprehensive Chemical Kinetics, Vol. 29, Elsevier, Amsterdam, 1989, Ch. 2. 13 For example, B.H. Loo, J. Electroar&. Chem., 136 (1982) 209; R.P. Van Dupe and J.P. Haushatter, J. Phys. Chem., 88 (1984) 2446; Q. Feng and T.M. Cotton, ibid., 90 (1986) 983; K. Metcalfe and R.E. Hester, 3. Chem. Sot., Chem. Commun., (1983) 133; R. Rosetti and LE. Brus, J. Phys. Chem., 90 {1986) 558. 14 CA. Melendres and S. XII, J. Electrochem. Sot., 131 (1984) 2239: C.A. Melendres, W. Paden, B. Tani and W. WaIczak, ibid., 134 (1987) 762; C.A. Meleudres, N. Camihone III and T. Tipton, Electrochim. Acta, 34 (1990) 281; R. Grubbs and C.A. Melendres, Electrochemical Society Meeting, San Diego, CA, 1986, Abstract No. 30; A. Hugot-LeGoff, S. Joiret, 3. Saidani and R. Wiart, J. EIectroanaI. Chem., 263 (1989) 127. 15 Y. Son and K. Rajeshwar, to be pubhshed. 16 S-M. Park and ME. Barber, J. ElectroanaI. Chem., 99 (1979) 67. 17 For example, G.T. Burstein, J. Electrochem. Sot., 130 (1983) 2133; R. Barnard, J. Appl. Electrochem., 11 (1981) 217. 18 D. Ham, K.K. Mishra and K. Rajeahwar, J. Electrochem. Sot., 138 (1991) 100. 19 B. Tell, T.C. Damen and S.P.S. Porto, Phys. Rev., 144 (1966) 771.