J. Phys. Chem. 1995,99, 6103-6109
6103
Overlayer Formation in the n-CdSe/[Fe(CN)6]4-'3- Photoelectrochemical System As Probed by Laser Raman Spectroscopy and Electrochemical Quartz Crystal Microgravimetry N. R. de Tacconi,* N. Myung, and K. Rajeshwar" Department of Chemistry and Biochemistry, The University of Texas at Arlington, Box 19065, Arlington, Texas 76019 Received: October 6, 1994; In Final Form: January 3, 1995@
Electrodeposited thin films of n-CdSe were anodically polarized under white light illumination in 0.01 M &[Fe(CN)6]/1 .O M KC1. The film mass increased parallel with anodic current flow as probed by combined linear sweep voltammetry/electrochemical quartz crystal microgravimetry measurements. This behavior contrasts with that observed for the parent 1.0 M KC1 electrolyte wherein mass loss is observed because of photoanodic corrosion of n-CdSe. The mass increase in the present case is attributable to overlayer formation. Laser Raman spectroscopy (LRS) identifies this overlayer to be &[CdFe(CN)6] by comparison with the cyanide stretching bands of authentic samples of this compound. The redox behavior of this overlayer was probed by cyclic voltammetry and in-situ LRS measurements on Cd films derivatized with the overlayer. The [CdFe(CN)6I2-/- redox could be monitored by both these techniques in 0.1 M NaN03, attesting to the ability of the overlayer at n-CdSe to shuttle charge between the semiconductor and the electrolyte. The kinetics however are rather slow, and incomplete oxidation of the overlayer is obtained even after -3 min anodic polarization in 0.1 M NaN03. A high surface area of the Cd layer facilitates stable overlayer formation as borne out by comparison of the behavior of electrodeposited Cd film on Au and a polished Cd rod. Finally, the ramifications of this trend in terms of overlayer stability in operating photoelectrochemical cells are discussed.
Introduction
Experimental Section
The n-cdSe/[Fe(CN),~]~-/~photoelectrochemical (PEC) system has been the topic of many studies in recent These studies mainly have revolved around the examination of various factors affecting cell efficiency and photoanode stability including solution additives such as nature of cation,4b,c electrolyte P H , ~and irradiation ~ a v e l e n g t h .An ~ ~ important factor in the photostability of the test system (and also in its counterpart, n-CdS/[Fe(CN)6]3-/4-) was found to be the formation of an interfacial l a ~ e r . ~This , ~ paper describes the characterization of this layer via laser Raman spectroscopy and electrochemical quartz crystal microgravimetry (EQCM). The efficacy of these techniques for similar experiments in this laboratory on other n-CdX-based PEC interfaces has been pre~ented.~ It is specifically shown below that: (a) The mass at the illuminated n-CdSe electrode surface increases when an EQCM scan is taken to positive potentials in [Fe(cN)6l4- redox electrolyte. This behavior contrasts with the EQCM data previously described'O by us for the illuminated n-CdSeKC1 interface wherein mass loss was observed. The mass increase in the present test system is consistent with overlayer formation. (b) Complementary Raman spectroscopy data on this overlayer after an anodic scan unambiguously identify it as Kz[CdFe(CN)6]. (c) In-situ Raman spectroscopy on a gold electrode derivatized with a cadmium hexacyanoferrate film affords insights into the [CdFe(cN)6]*-/- redox process. (d) The redox of the overlayer supports the role of the latter in mediating charge transfer between the underlying n-CdSe layer and the [Fe(CN)6]3-/4- redox electrolyte. (e) The morphology of the cadmium layer on the gold support is a key factor in the stability of the hexacyanoferrate overlayer subsequently formed on it.
The EQCM cell has been described elsewhere." A 5 MHz AT-cut quartz was used and had a calibrated mass sensitivity of 18 ng Hz-' cm-2. The geometric area of the working electrode was 0.7 cm2. Laser Raman spectroscopy and spectroelectrochemical measurements employed the 514.5 nm line of an Ar+ laser on a Spex Ramalog instrument equipped with a Model 1680 double monochromator and a cooled photomultiplier tube (Model R928) operated in the photon-counting mode. The experimental resolution was 5 cm-'. More details of the spectral measurements are given e l ~ e w h e r e . ~ ,A' ~ holographic edge filter (Physical Optics Corp.) was added for Rayleigh line rejection. The laser source was operated at a nominal output power of 20 mW. Scanning electron microscopy (SEM) was performed on a JEOL Model 6100 machine. The nominal electron beam voltage was 20 kV. n-CdSe film was electrodeposited either on the gold EQCM electrode surface or on a gold disk electrode (Bioanalytical System) using procedures described elsewhere.1° Briefly, the films were potentiostatically electrodeposited at -600 mV for 5 min from a 0.1 M Na2S04 electrolyte containing 0.5 M CdS04 and 5 mM Se02. The EQCM cell also had facilities for illuminating the n-CdSe film.1° A tungsten-halogen lamp was used as beforelo with an incident light intensity of ca. 30 mW/ cm2 (uncorrected for cell reflection and electrolyte absorption losses). The instrumentation for cyclic and linear sweep voltammetry was standard. Three-electrode cell geometry was used in all the cases. A pt spiral served as the counter electrode, and a Ag/AgCl/3 M KC1 was employed as reference; all potentials in the study are quoted with respect to this reference. Deionized
* To whom correspondence should be addressed. @Abstractpublished in Advance ACS Abstracts, March 15, 1995. 0022-3654/95/2099-6103$09.00/0
0 1995 American Chemical Society
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Figure 1. Combined anodic linear sweep voltammetry (curves 1 and 2) and electrochemical quartz crystal microgravimetry (curves 3 and 4) scans for a CdSe film electrodeposited at a Au surface. Scans 1 and 3 are for the electrode in the dark; the corresponding scans under white light illumination are contained in 2 and 4. Electrolyte: 0.01 M &[Fe(CN)6]/1.0 M KCl; potential scan rate: 5 mV/s.
(ultrapure N2 purge) and double-distilled water (Coming Megapure) was used in all cases. All chemicals were from commercial sources and were used as received. The polycrystalline Cd rod was obtained from Johnson-Matthey and was of 99.9995% purity. Authentic samples of Kz[CdFe(CN)6] and K[CdFe(CN)6] were prepared as per procedures prescribed by previous authors.8 The gold surface was derivatized with these compounds also via a procedure employed by previous authors for at'F electrode s ~ r f a c e . ~Briefly, J~ a layer of metallic cadmium was first electroplated on the Au surface by holding the latter at - 1.OV in 1.0 x M CdS04/0.05 M for periods ranging from 30 s to 2 min. After a distilled water rinse, the Cd-coated electrode was soaked in 0.01 M K3[Fe(CN)6] for times ranging from several minutes to 2 h to produce the [CdFe(CN).#-/- surface derivative. A similar procedure was adopted for derivatizing the surface of the Cd rod electrode except that its surface was fmt polished with alumina (Buehler) down to 0.05 pm. Results and Discussion A u / C ~ S ~ / F ~ ( C N )KCl ~ ~ - ,Interface. Figure 1 contains combined anodic linear sweep voltammetry (ALSV)/EQCM data for a CdSe film that was electrodeposited on the Au working electrode surface. The 1.O M KC1 electrolyte initially contained 0.01 M h[Fe(CN)6], and the scans are shown both in the dark and under white light illumination of the electrode surface. The voltammetry and EQCM scans (curves 1 and 3) are featureless in the dark as is to be expected for an n-type semiconductor/ electrolyte interface in reverse bias. That is, there is no significant minority carrier (hole) generation in the semiconductor at these potentials to sustain an oxidation current. Under illumination, an anodic current flows and attains a plateau at potentials around 0 V (curve 2, Figure 1). Concomitantly, the mass also increases systematically in the EQCM trace (curve 4). The data in Figure 1 are to be contrasted with the corresponding voltammetry/EQCM trends for the parent n-CdSe/l .O M KC1 electrolyte interface.1° These are shown in Figure 2 for comparison. Note that the EQCM trace (curve 4) under
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Figure 2. Data as in Figure 1 but for the 1.0 M KCl electrolyte instead. The insert is a charge (Q)-frequency (AA plot synthesized from coulometry-EQCM data for a scan from -0.6 to 0 V in 0.1 M KCl under illumination.
illumination shows a mass loss, corresponding to the photoanodic corrosion of n-CdSe: CdSe(s)
+ 2hf hv Cd2+(aq) + Se(s)
(1)
The elaboration of this process via combined voltammetry/ EQCM was presented earlier.l0 Briefly, the value (-2) of the electron stoichiometry that is computed from the slope of a plot of the charge Q vs the mass (or frequency) change (cf. insert, Figure 2) is consistent with that expected from reaction 1. In the presence of the reducing agent [Fe(CN)6I4- in the electrolyte, the photogenerated holes are captured by it in competition with those consumed via reaction 1. Thus, the mass increase seen in Figure 1 (curve 4) is to be attributed to the formation of a cadmium hexacyanoferrate overlayer at the n-CdSe surface! Overlayer Formation at the n-CdSe Surface. Previous authors have built upon observati~ns'~ on oxidatively unstable metal (e.g., Ni) electrodes in the presence of aqueous [Fe(CN)6l4to invoke overlayer formation as a source of corrosion stabilization in the n-CdS/[Fe(CN)6I4- system. Accordingly, reflectance Fourier transform infrared spectroscopic examination of such electrode surfaces revealed the presence of a 2064 cm-' band assignable to a bridging cyanide group in the surface derivative.8a This band coincided with the signature observed for an authentic compound of K2[CdnFen(CN)6].8a These studies were later extended by the same group to include the n-CdSe system? X-ray photoelectron spectroscopy data have been presented by these authors for polycrystalline n-CdSe pellets treated with the [Fe(cN),~l~-'~redox electrolytes as in Figure 1?b Aside from iron signals [which were assigned to the M,[CdFe(CN)6] overlayer, M = alkali metal], peaks attributable to Cd(OH)2 and SeO species were also observed signaling the importance of reaction 1 even in the presence of the Fe(CN),j4- hole capture agent in the electrolyte (see below). Figure 3 contains new laser Raman spectroscopy data on the cadmium hexacyanoferrate overlayers in this study in the cyanide stretching frequency range. Spectrum a in Figure 3 was obtained after the Au-supported n-CdSe film was subjected
Overlayer Formation in n-CdSe/[Fe(CN)6]4-/3-
J. Phys. Chem., Vol. 99, No. 16, 1995 6105 LASER ON
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Figure 3. Raman spectra of (a) CdSe film after an ALSV scan under illumination as in Figure 1, (b) &[CdFe(CN)6], and (c) K[CdFe(CN)b]. TABLE 1: Raman Frequencies and Assignments of Cyanide Stretches for Hexacyano Compounds frequency, cm-1
CdSe/overlayer (Figure 3a) CdSe/overlayer (Figure 5)
21 16 2075
Adoverlayer (Figure 7a)
21 16 2075
2121 2083
Adoverlayer 21 16 2075 (Figure 7b-e) Adoverlayer (Figure 7 0
2170
2125 2086
2075 2062 2131
ex-situ Raman after white light irradiation in-situ Raman under 514.5 nmirradiation of CdSe/[CdFe(CN)6]'-/ [Fe(CN)64-(aq) in-situ Raman at 0.05 V prior to oxidation/ reduction under 514.5 nm irradiation in-situ Raman at 0.80 V when the overlayer is gradually oxidized in-situ Raman at 0.15 V after overlayer oxidation/reductionunder 514.4 nm irradiation chemically prepared following ref 13b 0.01 M aqueous solution 0.01 M aqueous solution
to an ALSV scan under white light illumination in 0.01 M &[Fe(CN)6]/1.0 M KCl. The film was thoroughly rinsed with distilled water and then transferred to the Raman spectrophotometer for analyses. Spectra b and c in Figure 3 correspond to authentic samples of K2[CdFe(CN)61 and K[CdFe(CN)61 prepared as described in the Experimental Section. Clearly, the overlayer on the n-CdSe surface corresponds to K2[CdFe(CN)6] under the conditions in Figure 1. The bands at 2075 and 21 16 cm-' are diagnostic of the reduced redox state of the overlayer (cf. Table 1). Data shedding light on the charge-mediating characteristics of this overlayer are presented later. In cadmium hexacyanoferrate complexes with Fe-CGNCd bridge structures, the Raman-active modes for the cyanide stretches, v1 (A,) and v3 (E,), are shifted to higher frequencies with respect to the parent [Fe(CN)6I3-l4- system (cf. Table 1). The band position depends on the oxidation state of the iron redox center being neatly separated when the iron is in the reduced (Fe+2) but almost overlapped in the oxidized state (Fef3). The higher the iron oxidation state, the stronger the C-Fe a-bonding and the higher the v(CN). Photocurrent-Time Behavior under 514.5 nm Laser Irradiation. For the CdSe[Fe(CN)6I4- system, it was previously reported4athat an initial irradiation with light of 1 < 550 nm led to an increase of the photocurrent stability. Transient photocurrent response of our electrochemically grown CdSe in 1 M KC1 f 0.01 M K2[Fe(CN)6] at -0.05 V during chopping
Figure 4. Photocurrent transients for the Au/CdSe film/Kz[cdFe(CN)s] electrode in 0.1 M KC1 4- 0.01 M &[Fe(CN)6] at -0.05 V. The 514.5 nm Ar+ laser line (20 mW output power, spot size 8 x cm2) was manually and repetitively interrupted. of the 514.5 nm laser Ar+ line was found to be reproducible and stable (Figure 4). The photocurrent transients are anodic and rapidly attain a plateau under light and in the dark, giving no evidence of current decay as would be expected under photocorrosion conditions. Under prolonged 5 14.5 nm direct laser irradiation, initial photocurrents of iph = 17 pA were found to decrease to 13 pA in 6-8 min. The iph-time profile subsequently was stable for long periods (1-2 h). In-Situ Spectroscopyunder Photocurrent Flow. For CdSga and CdTegb we have previously demonstrated that the use of suprabandgap laser excitation (Le., the Ar+ laser line exciting electron-hole pairs in the semiconductor films) can provide in-situ (and even real time) information on the interfacial photoelectrochemistry. We develop this idea further in this study with the CdSe film electrode with an added feature that the (probe) laser line also provides stable photocurrent flow at the semiconductor/electrolyte interface. Figure 5 contains in-situ Raman spectroscopy data on the n-CdSe/[Fe(CN)6I4- system. For comparison, the corresponding spectra of a 0.01 M [Fe(CN)6I4- solution (spectrum a) and of a solid layer formed on an anodized CdO.01 M [Fe(CN)6I4interface (spectrum b) are also included. Spectra c-f were obtained at -0.05 V after 8-10 min irradiation and for iph = ca. 10-14 pA. These scans were taken at different areas of the n-CdSe film in close (0.4-0.5 mm) proximity. These spectral data are averaged from three replicate sets, and the band intensities vary slightly during spectral averaging. The laser cm2 in these experiments. spot size was 8 x Spectrum c is primarily composed of the two peaks at v1 = 2094 cm-' and v3 = 2062 cm-', resembling spectrum a of [Fe(CN)6I4- aqueous species, but the valley between the two peaks is shallower and the band intensity ratio, Z(~l)/Z(v3), is higher than in the reference spectrum a, pointing to an extra contribution from bridge surface species. Spectra d-f are more complex than spectrum c, showing three overlapped peaks at 2062,2083-2085, and 2121 cm-', the 2062 cm-' band coming from the v3 frequency of [Fe(CN)6I4- anion and the other two bands originating from vg and v1 vibrations of the [CdFe(CN)6I2in the surface layer (compare with spectrum b). The in-situ spectra are complex due to the overlapping contribution from vibration modes of surface and solution species and due to the fact that the bands are broader. Several points are worthy of note. Even though the spectra were obtained under photocurrent conditions, no oxidized state in the surface layer was detected. Probable reasons include the following: (1) The holes were transferred directly to solution species, in the semiconductor area spot where spectrum c was taken. (2) The band intensity corresponding to the oxidized portion of the overlayer weakens and tends to disappear during spectral averaging due to the
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Figure 5. Reference Raman spectra: (a) 0.01 M [Fe(CN)6l4- aqueous solution in 0.1 M KCl; (b) overlayer grown on a Cd metal electrode by anodization using solution as in (a). Spectra c-f were obtained in situ for the AdCdSe fiim in contact with aqueous solution (a) on photocurrent flow and in different areas of the photoelectrode (refer to text).
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E N VI AglAgCl Figure 6. Cyclic voltammogram for a cadmium hexacyanoferrate film at gold in 0.1 M NaN03. Potential scan rate: 25 mVls.
transfer of holes from the overlayer to solution species. (3) The amount of layer converted to the oxidized state is a small part of the total overlayer, and its intensity is not enough to be detected by our Raman setup. Furthermore, the in-situ Raman bands of the overlayer are broader and slightly shifted to higher
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Figure 7. In-situ Raman spectra of the Ad[CdF’e(CN)s]*-/- electrode in 0.1 M NaN03 at (a) 0.05 V, (b-e) 0.80 V for increasing time of polarization culminating in -3 min for spectrum e, and (f) 0.15 V after a cathodic scan (25 mVls) from 0.80 V.
frequencies (5-7 cm-’) with respect to those obtained ex situ from the white light illuminated CdSe (cf. Figure 3a). The latter surface was found to coincide spectroscopically in terms of
Overlayer Formation in n-CdSe/[Fe(CN)6]4-B-
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Figure 8. Scanning electron micrographs of an electrodeposited Cd film at gold (a) and of a polished Cd rod (b).
positions and bandwidths with those obtained for authentic samples of &[CdFe(CN)s]. Changes in the overlayer microstructure produced by 514.5 nm irradiation when the semiconductor interface is in contact with [Fe(CN)6l4- aqueous solution could be claimed for these spectral changes. The data in Figure 5 clearly attest to the charge-mediating role of the cadmium hexacyanoferrateoverlayer. Mediation of hole transfer by this layer between the n-CdSe film and the electrolyte is predicated on its ability to shuttle between the
[CdnFen(CN)6]*-and [CdnFem(CN)6]-redox states. The strong cation dependence noted by previous authorssb was invoked as unambiguous support for the existence of this mediated charge transfer pathway. In other words, hole transport across this overlayer must be coupled with countercation transfer to preserve local electroneutrality. To explore this further, we electrosynthesized a Au/Cd/[CdFe(CN)6]*-/- interface as described in the Experimental Section. Our findings on this interface are detailed next.
6108 J. Phys. Chem., Vol. 99, No. 16, 1995
AulCdl[CdFe(CN)6l2-’-/O.1 M NaN03 Interface. Figure 6 contains a representative cyclic voltammogram for this interface. The trace shown was obtained after the derivatized Cd film was cycled between 0.05 and 0.80 V in 0.1 M NaNO3-a stable trace was usually obtained after 5-6 cycles. The redox charge for the particular example in Figure 6 corresponds to ca. 4.9 mC cm-2. The redox transformation of the cadmium hexacyanoferrate overlayer was monitored in situ by Raman spectroelectrochemical experiments. Figure 7 contains data in the 2000-2250 cm-’ spectral region as a function of electrode potential and time. Spectrum a in Figure 7 was obtained at 0.05 V. Note that this potential corresponds to the cathodic limit of the voltammogram in Figure 6; i.e., the film is in its reduced state. Interestingly enough, this spectrum resembles that presented earlier in Figure 3a. Spectra b-e in Figure 7 were obtained at 0.80 V while the surface layer was gradually oxidized. A photostationary condition was ultimately reached as in Figure 7e after -3 min. Clearly, the film is not completely oxidized even at 0.80 V. Spectrum f in Figure 7 was obtained at 0.15 V after a linear cathodic scan from 0.80 V. It shows an increase in the full width at half-height (fwhh) and a shift to higher wavenumbers that presumably was brought about by redox cycling of the hexacyanometallate layer under 5 14.5 nm irradiation. These spectral changes (compare spectra a and f) provide evidence for systematic physical changes in the overlayer microstructure. To probe morphological effects, a smooth (alumina-polished) Cd rod and an electrodeposited Cd film were compared in terms of their proclivity to bind with the overlayer. Figure 8 compares the SEM topography of these two surfaces. The electrodeposited Cd film is fine-grained and presents a higher surface dispersion (Figure 8a) relative to the Cd rod counterpart (Figure 8b). After derivatizing the latter with the overlayer, cyclic voltammetry was performed (as in Figure 6 ) in 0.1 M NaNO3. Contrasting with the stable behavior noted in Figure 6, the film on the Cd rod was only stable at potentials lower than -0.62 V. The film was dislodged from the Cd rod even during the first positive-going scan. The anodic oxidation (corrosion) of Cd appears to instigate the overlayer instability. The higher surface (and thus the lower local current density) at the electrodeposited polycrystalline Cd surface appears to favor surface stabilization and overlayer intedty. The results have important ramifications for photoelectrochemical (PEC) applications. Specifically, initial roughening of the photoelectrode surface because of incipient photocorrosion (reaction 1) may provide a fortuitous route to its subsequent stabilization. This is especially true for initially polished singlecrystal surfaces. An etch of the photoelectrode surface may have similar beneficial effects. A Brdmethanol etch indeed was observed to afford stable overlayer formation at n-CdS in the dark in the [Fe(CN)6]4-’3- redox electrolyte.8b The etch treatment undoubtedly generates the Cd2+ ions needed for the hexacyanoferrate precipitation, but it is also tempting to conclude that a simultaneous roughening of the electrode surface provides for stable overlayer formation. In retrospect, it is also perhaps not surprising that some degree of initial corrosion appears to be a prerequisite for stable PEC cell operation. As long as the photocorrosion does not continue (resulting in Seo layer buildup), a steady-state situation may result in the maintenance of a thin and permeable protective hexacyanofenrate overlayer at the CdX surface through which charge and ions can move relatively freely. In summary, new insights have been presented into the factors underlying overlayer formation and function in [Fe(CN)6]4-’3based CdX interfaces using the n-CdSe and Cd/[CdFe(CN)#-/-
de Tacconi et al. systems as models. The usefulness of PEC-QCM and laser Raman spectroscopy as real-time and in-situ monitoring tools for semiconductor/interfaces also is borne out by the data obtained in this study. In particular, Raman spectroscopy has been used for the f i s t time for characterizing overlayers at the n-CdX/[Fe(CN)a]3-/4- system. The use of this technique appears to have several advantages relative to other candidate spectroscopic tools that have been previously used such as FTIR. For example, the Raman technique is compatible with aqueous solvent media and glasdquartz optics. As demonstrated herein, the data can also be obtained in situ under conditions of photocurrent flow at the semiconductor/electrolyte interface. Such capability is important in that the spectroscopic information is obtained under conditions mimicking an operating photoelectrochemical cell. Admittedly, we have not yet optimized many of the variables in the redox characteristics of the cadmium hexacyanoferrate overlayer such as cation composition and the layer morphology. The importance of these two variables is suggested by the previous work cited earlier.4S8 However, it is unlikely that even under optimized conditions, mediated charge transport through this overlayer alone is sufficient to sustain the fluxes (approximately several mA/cm2) typical of efficient and operating photoelectrochemical devices based on similar interface^.^^^ Thus, the conclusion is inescapable that direct oxidation of electrolyte [Fe(CN)6I4- species by the photogenerated holes in n-CdSe must be simultaneously occurring. A similar conclusion was reached by previous authors.8b It must also be noted (as pointed out by a reviewer) that KOH was not added to the [Fe(CN)6I3-l4- redox electrolyte in this study. The use of a more alkaline electrolyte would have a beneficial influence on the redox kinetics (and on the magnitude of the observed photocurrent, cf. Figure 1) although the conclusions of this study in terms of the role of the overlayer are not likely to be altered. Finally, it must be noted (as also pointed out by the reviewer) that the present data pertain to a polycrystalline semiconductor surface with the attendant complications related to grain boundary effects, etc. The vast majority of the previous data on this PEC system were acquired on single-crystal CdSe. Again, judging from the present trends (especially those related to Figure 8), the hexacyanoferrate overlayer would be better stabilized at a polycrystalline surface. Our new finding in this regard corroborates the work of previous authors15 on the use of polycrystalline electrodes for stable PEC devices. Further efforts on this topic will be directed at a better understanding of these comparative aspects as well as on the details of how the overall charge transport is partitioned between the direct and mediated pathways for the test systems in this study and similar interfaces. Acknowledgment. This research was funded in part by a grant from the Department of Energy, Office of Basic Energy Sciences. References and Notes (I) (a) Noufi, R.; Tench, D.; Warren, L. F. J. Electrochem. Sac. 1980, 127, 2709. (b) Ibid. 1981, 128, 2363. ( 2 ) Frese, K. W., Jr. Appl. Phys. Lett. 1982, 40, 275. ( 3 ) Reichman, J.; Russak, M. A. J . Electrochem. SOC. 1984,131,796. (4) (a) Rubin, H. D.; Arent, D. J.; Bocarsly, A. B. J. Electrochem. SOC. 1985, 132, 523. (b) Arent, D. J.; Rubin, H. D.; Chen, Y.; Bocarsly, A. B. Ibid. 1992, 139, 2705. (c) Arent, D. J.; Hidalgo-Luangdilok, C.; Chen, J. K. M.; Bocarsly, A. B.; Woods, R. E. J. Electroanal. Chem. Interfacial Electrochem. 1992, 328, 295. (5) Marcu, V.; Strehblow, H. H. J. Electrochem. SOC. 1991, 138,758. ( 6 ) (a) Licht, S.; Peramunage, D. Nature 1990, 345, 330. (b) Licht, S.; Peramunage, D. Ibid. 1991, 354, 440. (c) J . Electrochem. SOC. 1992,
Overlayer Formation in n-cdSe/[Fe(CN)~]~-/~139, L23. (d) Ibid. 1992, 139, 1792. (e) Peramunage, D.; Licht, S . Sol. Energy 1994, 52, 197. (7) (a) Seshadri, G.; Chen, J. K. M.; Bocarsly, A. B. Nature 1991, 352, 508. (b) Bocarsly, A. B. J . Electrochem. SOC. 1992, 139, 1791. (8) (a) Rubin, H. D.; Humphrey, B. D.; Bocarsly, A. B. Nature 1984, 308, 339. (b) Rubin, H. D.; Arent, D. F.; Humphrey, B. D.; Bocarsly, A. B. J . Electrochem. SOC. 1987, 134, 93. (9) (a) de Tacconi, N. R.; Rajeshwar, K. J . Phys. Chem. 1993, 97, 6504. (b) de Tacconi, N. R.; Lema, R. 0.; Rajeshwar, K. Ibid. 1994, 98,
4104. (10) Myung, N.; Wei, C . ; de Tacconi, N. R.; Rajeshwar, K. J . Electroanal. Chem. Interfacial Electrochem. 1993, 359, 307. (11) Bose, C. S . C . ; Rajeshwar, K. J . Electroanal. Chem. Interfacial Electrochem. 1992, 333, 235.
J. Phys. Chem., Vol. 99, No. 16, 1995 6109 (12) de Tacconi, N. R.; Son, Y.; Rajeshwar, K. J . Phys. Chem. 1993, 97, 1042.
(13) (a) Luangdilok, C. H.; Bocarsly, A. B.; Woods, R. E. J . Phys. Chem. 1990, 94, 1918. (b) Luangdilok, C. H.; Arent, D. J.; Bocarsly, A. B. Lmgmuir 1992, 8, 650. (14) (a) Bocarsly, A. B.; Sinha, S . J . Electroanal. Chem. Interfacial Electrochem. 1982, 137, 157. (b) Sinha, S.;Humphrey, B. D.; Bocarsly, A. B. Znorg. Chem. 1984,23,203. (c) Humphrey, B. D.; Sinha, S.;Bocarsly, A. B. J . Phys. Chem. 1984, 88, 736. (15) (a) Hodes, G.; Manassen, J.; Cahen, D. J . Electrochem. Soc. 1981, 128, 2325. (b) Hodes, G. ; Manassen, J.; Cahen, D. Solar Energy Mater. 1981, 4 , 373.
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