J. Phys. Chem. 1996, 100, 4937-4943
4937
Photoreduction of Methylviologen Adsorbed on Silver Hannah Feilchenfeld,† George Chumanov, and Therese M. Cotton* Ames Laboratory and Department of Chemistry, Iowa State UniVersity, Ames, Iowa 50011 ReceiVed: August 8, 1995; In Final Form: December 27, 1995X
Methylviologen adsorbed on a roughened silver electrode is reduced to its cation radical upon irradiation with laser light at liquid nitrogen temperature. Surface-enhanced Raman scattering (SERS) spectra were obtained with different excitation wavelengths between 406 and 752 nm and compared to those obtained at room temperature in an electrochemical cell under potential control. From two-color experiments, in which one laser frequency was used to generate the radical and a second to excite the SERS spectra, it was determined that radical formation occurs mainly with excitation in the blue spectral region. A comparison of the SERS spectra of the dication and cation radical forms of methylviologen with their solution spectra suggests that the former interacts more strongly with the surface than the latter. The cation radical appears to be stable for several hours in liquid nitrogen but has a short lifetime at room temperature. Two mechanisms for the photoreduction are discussed: plasmon-assisted electron transfer from the metal to the methylviologen dication and formation of a resonance charge transfer complex. The current experimental data are insufficient to determine the particular role of these mechanisms.
Introduction The production of photocurrents by irradiation of a mercury electrode with UV light was first noted by Barker et al. approximately 30 years ago.1 Photons of sufficient energy were shown to induce optical transitions in the metal, thereby allowing the excited electrons to overcome the potential barrier at the interface. The ejection of electrons into an electrolyte solution and their subsequent hydration results in the measured photocurrent. The magnitude of the photocurrent was found to increase with the concentration of scavenger species present in the aqueous electrolyte. This observation may be rationalized by considering that at low concentrations the ejected photoelectrons can return more readily to the electrode before undergoing capture by the scavenger, resulting in a lower net current. In these experiments, the electrode potential and the wavelength of light were also found to be interrelated: lower energy radiation required a more negative electrode potential in order to reach the threshold for electron ejection. Since this initial observation, photoemission of electrons at metal electrodes has been extensively investigated. In the absence of scavengers, this process has a very low quantum yield. However, the photoyield can be enhanced by different strategies. Sass et al.2 showed that optical excitation of plasmon resonances in roughened silver electrodes resulted in a significant increase in the quantum yield at the photon energy corresponding to the surface plasmon frequency (3.6 eV). Other methods for enhancing the quantum yield include the formation of a surface dipole layer on the electrode surface, reducing the barrier for electron emission, or the formation of a chemical bond between an adsorbate and the electrode surface, which also decreases the work function of the metal.3 With respect to the latter, the chemical nature of the adsorbate was found to be very important. Molecules with lone-pair nitrogens provide the greatest effect on the enhancement of photoyields at the surface plasmon frequency.4 In the above studies,3,4 the relationship between the enhancement of the photoemission process and the surface-enhanced † Permanent address: Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel. X Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-4937$12.00/0
Raman scattering (SERS) effect was highlighted. The requirement for roughened surfaces or strong interaction between an adsorbate and the metal parallels the electromagnetic5 or chemical (charge transfer)6 mechanisms of SERS. The role of surface plasmons in the electromagnetic mechanism is clearly established in systems exhibiting long-range effects.7 Charge transfer effects are implicated by the dependence of the enhancement on the excitation frequency (different from that of the plasmon) and on the electrode potential.8 Plasmon excitation can also give rise to enhancement of other optical processes, including absorption, fluorescence, and photochemistry according to the electromagnetic theory.5,9 Experimental evidence in support of enhanced photochemistry can be found in studies of various types of molecules on silver surfaces.10-18 As in the photoemission studies cited above, the wavelength dependence of the photochemical reactions was often shown to track the plasmon resonance of silver. However, in the case of the photochemical reduction of p-nitrobenzoic acid, a charge transfer mechanism was proposed to rationalize the dependence of the photolysis rate as a function of photon flux, excitation energy, electrode potential, and the nature of the solvent.18 In the present study, the photochemical reduction of methylviologen adsorbed on a silver electrode has been investigated. The mechanism for the unexpected and irreversible photochemical reduction at liquid nitrogen temperature is discussed and may involve both surface plasmon excitation and coupling or charge transfer between methyl viologen and silver. Experimental Section Chemicals and Materials. Methylviologen dichloride hydrate (98%) was obtained from Aldrich and used without further purification. All solvents were HPLC grade. Substrate and Sample Preparation. The substrates for SERS and electrochemical measurements were thin film electrodes. They were prepared by vacuum evaporating successively a 15 nm thick layer of Cr, a 50 nm layer of Au, and a 2.5 µm layer of Ag on 30 by 3.5 mm glass slides. The electrodes were roughened by three oxidation-reduction cycles, stepping the electrode in 0.1 M Na2SO4 to a 0.55 V potential, allowing 25 mC/cm2 of charge to pass, and then stepping the © 1996 American Chemical Society
4938 J. Phys. Chem., Vol. 100, No. 12, 1996 potential to -0.60 V until the current reached a minimum. The roughened electrodes were either dipped into a 1 mM methylviologen solution for a few minutes and then placed into a liquid nitrogen Dewar, or immersed into a 1 mM methylviologen solution in aqueous 0.1 M KCl in an electrochemical cell at room temperature. In some cases, the electrode was rinsed with distilled water following adsorption of the methylviologen. For studies in which the temperature of the samples was changed from 77 K to room temperature the electrodes were sealed in helium filled glass tubes. In all of these manipulations, special care was taken to ensure the purity of the samples. Instrumentation. An Innova 200 Ar+ laser, an Innova 100 Kr+ laser, and a Rhodamine 6G Model 590 dye laser (all from Coherent, Inc.) were used as excitation sources for Raman measurements. The scattered light was collected in a backscattering geometry by an f/1.2 camera lens and focused onto the slit of a Spex 1807 triple spectrometer. A Princeton Applied Research Model 1420 photodiode array and OMA III or a Princeton Instruments CCD LN1152 were used for detection. The laser power at the sample was between 10 and 20 mW unless otherwise stated. The spectra were calibrated using indene. The spectral resolution was 5 cm-1. The electrochemical system consisted of a glass cell, the previously described Ag electrode, a Pt counterelectrode, and a saturated calomel electrode (SCE) as reference. An EG&G Princeton Applied Research Model 173 Potentiostat/Galvanostat was used for applying the external potentials. A home-built potentiostat with an integrator/comparator circuit was used to roughen the electrodes. All potentials are reported vs SCE. Results and Discussion Methylviologen is readily adsorbed from aqueous solution onto Ag surfaces. It is known to be an electron acceptor which can undergo the reactions:
MV2+ + e- f MV•+ MV•+ + e- f MV0 Each step of the reduction can be clearly observed by cyclic voltammetry.19 At smooth silver electrodes the dication is reduced to the cation radical near -0.6 V, and the cation radical in turn yields the neutral form around -1.1 V. With roughened silver the cyclic voltammogram is somewhat more complex; both the solution and adsorbed viologen reduction peaks can be detected.19 The reduction potential of the adsorbed methylviologen is more positive by about 100 mV. Figure 1a shows the SERS spectrum of methylviologen adsorbed on a roughened Ag film electrode at 77 K, obtained with 406.7 nm excitation. For comparison the normal Raman (NR) spectrum of a solid sample is shown in Figure 1c. Significant differences are observed between these spectra, including frequency shifts of the 1539, 1302, 1234, 1194, 839, and 660 cm-1 bands of the NR spectrum to 1531, 1357, 1249, 1207, 816, and 681 cm-1 in the SERS spectrum. A third spectrum (Figure 1b) was obtained by applying a potential of -0.6 V to a roughened Ag electrode immersed in the methylviologen solution for approximately 5 min, allowing formation of the cation radical on the electrode surface. The electrode was then removed from the cell while under potential control (-0.6 V), quickly disconnected from the potentiostat, and immediately dipped into liquid nitrogen. The resulting SERS spectrum of the cation radical is identical to that (Figure 1a) of methylviologen adsorbed on the roughened surface,
Feilchenfeld et al.
Figure 1. Raman spectra of methylviologen: (a) SERS of methylviologen adsorbed on roughened Ag, at 77 K; (b) SERS of electrochemically generated MV•+ adsorbed on roughened Ag, at 77 K; (c) NR of solid methylviologen, at room temperature. All measurements were performed with 406.7 nm excitation.
indicating that the latter spectrum must also be assigned to the cation radical. The reduction of the adsorbed methylviologen on an Ag electrode at 77 K is not easily explained, as it seems unlikely that the open circuit potential of the silver in liquid nitrogen is sufficiently negative to produce by itself the cation radical. This is supported by experimental results, obtained with red laser excitation, to be presented and discussed below. The possibility that the chloride counterion functions as an electron donor was explored because it is known that MVCl2 can be directly photoreduced with 337 nm light.20 Exchange of the chloride for other halides had no effect on the reaction. Also, if charge transfer from the chloride ion were responsible for the photoreduction, the process should occur at the other metals examined in this study. Potential Dependence of the SERS Spectra. The fact that adsorption of methylviologen on roughened silver shifts its reduction potential to a more positive value may account for the unexpected appearance of the cation radical on the electrode at 77 K. To check this point the SERS spectra of methylviologen, adsorbed on a roughened silver electrode, were measured in an electrochemical cell as a function of the applied potential, between -0.2 and -1.2 V. The 640 nm laser line was chosen as an excitation source for three reasons. First, resonance enhancement of the cation radical spectrum is weaker at this wavelength than with blue excitation in spite of the fact that the absorbance of the cation radical form (Figure 2) is only a factor of ca. 3 less than at 413 nm. This is apparent in much lower signal to noise ratio for spectra taken with 640 nm as compared to 413 nm excitation (see Figure 4). Undoubtedly, the lower enhancement is a result of the much broader, overlapping transitions in the red region. Thus, excitation at 640 nm allows simultaneous observation of both the cation radical and the dication spectra; the latter would be masked by
Photoreduction of Methylviologen Adsorbed on Silver
J. Phys. Chem., Vol. 100, No. 12, 1996 4939
Figure 2. UV-vis absorption spectrum of chemically generated MV•+. The full scale absorbance is (a) 4.0 and (b) 0.4.
Figure 4. Effect of excitation wavelength on the SERS of methylviologen adsorbed on roughened Ag. The spectra were recorded at 77 K under open circuit conditions.
Figure 3. Potential dependence of the SERS spectra of methylviologen adsorbed on roughened Ag. The electrode was placed in electrochemical cell containing 1 mM methylviologen in 0.1 M KCl at room temperature. The spectra were measured with 640 nm excitation at the following applied potentials: (a) -0.2 V; (b) -0.4 V; (c) -0.6 V; (d) -0.8 V; (e) -1.0 V; (f) -1.2 V.
the intense cation radical spectrum at blue excitation. Second, the weaker resonance enhancement allows selective observation of the cation radical on the electrode surface and minimizes contributions from the radical generated in the diffusion layer near the electrode. This selectivity arises from the enhancement for the surface adsorbed species due to the SERS effect. Third, the low energy of the exciting photons minimizes possible photochemistry. The results are shown in Figure 3. The spectrum observed at -0.2 V (Figure 3a) corresponds to the dication form of methylviologen. A comparison of this spectrum with our previously reported21 normal Raman spectrum of the dication in solution shows that a number of bands are shifted by 7-15 cm-1 to lower frequency on the surface. These shifts can be attributed to the interaction between the dication form of methylviologen and silver. At -0.4 V (Figure 3b), weak new bands appear at about 1530 and 1350 cm-1; these
bands increase in intensity at -0.6 V (Figure 3c) and completely dominate the spectra at -0.8 or -1.0 V (Figure 3, d and e). They disappear at -1.2 V (Figure 3f), when the methylviologen is in its neutral form and exhibits a completely different spectrum. The 1530 and 1350 cm-1 bands, observed only in the presence of the cation radical form, are characteristic of this species. These spectroscopic results confirm that methylviologen adsorbed on roughened Ag undergoes electrochemical reduction at a potential close to that determined by cyclic voltammetry.19 These data will allow identification of adsorbed species under open circuit conditions in our subsequent discussion. Excitation Wavelength Dependence of the SERS Spectra. The possibility that methylviologen is reduced by laser light during acquisition of the spectra was explored. If this is the case, the photoreduction might be expected to depend on the wavelength. The SERS spectra of methylviologen at liquid nitrogen temperature were recorded for a number of different excitation wavelengths between 400 and 800 nm (Figure 4). It should be noted that each spectrum was measured on a fresh electrode, so that no sample was exposed to more than one excitation wavelength. This is an important point, as will become obvious later. All samples were prepared under identical conditions to ensure reproducibility. As can clearly be seen, the Raman bands at 1530 and 1350 cm-1, previously assigned to the cation radical form, are relatively strong at shortwavelength excitation, decrease in intensity as the laser wavelength becomes longer, and practically disappear at 752.5 nm excitation. These results suggest that the cation radical is formed on the metal surface by a photoinduced process. However, the higher intensity of bands characteristic of the cation radical at the blue excitation wavelengths can also be attributed to the fact that the cation radical form exhibits strong absorption in the blue spectral range (Figure 2). Thus, the spectra observed at shorter wavelengths are not pure SERS spectra, but rather combinations of resonance Raman and SERS. With blue excitation the spectrum of the cation radical will be preferentially enhanced over that of the dication. In order to
4940 J. Phys. Chem., Vol. 100, No. 12, 1996
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Figure 5. Excitation wavelength dependence of the SERS spectra of the methylviologen dication. The spectra were recorded on a roughened Ag electrode in an electrochemical cell containing 1 mM methylviologen in 0.1 M KCl, at room temperature, with an applied potential of -0.2 V.
Figure 6. Excitation wavelength dependence of the SERS spectra of the methylviologen cation radical. The spectra were recorded on a roughened Ag electrode in an electrochemical cell containing 1 mM methylviologen in 0.1 M KCl, at room temperature, with an applied potential of -0.8 V.
evaluate the relative contributions of the dication and cation radical species to the different Raman spectra, SERS experiments were carried out in an electrochemical cell at room temperature at a series of excitation wavelengths. Figures 5 and 6 illustrate the series of spectra obtained at two different applied potentials, -0.2 and -0.8 V, corresponding to the dication and cation radical forms, respectively. For each potential the same sample was used throughout, excitation wavelengths being changed gradually from the red to the blue spectral region. The spectra of the dication (Figure 5, -0.2 V) measured with excitation in the blue-green spectral region (406.7, 413.1, 457.9, and 488.0 nm) are significantly different from those obtained at low temperature under open circuit conditions. The major differences include the presence of a broad band, consisting of several overlapping sharp features in the 1655-1500 cm-1 region. Also, the bands at 1355 or 1535 cm-1 are extremely weak or absent under these conditions. For 590 nm and longer wavelengths, these bands are much weaker, especially those in the 1600 cm-1 vicinity. The broad complex feature near 1600 cm-1 suggests some degradation of the methylviologen may be occurring at the higher energy excitations. Demethylation of methylviologen to 4,4′-bipyridine has been noted in the literature.6 The SERS spectra of the cation radical (at -0.8 V) as a function of excitation wavelength are shown in Figure 6. In contrast to those of the dication, the spectra of the cation radical are extremely sharp and highly resolved at all excitation wavelengths. As the wavelength is altered from the red to the violet the relative intensities of certain bands change. For instance, peaks near 1597, 1508, and 1026 cm-1 decrease in intensities as the excitation is changed from 752.5 to 457.9 nm; at shorter wavelengths these bands are undetectable. This behavior can be attributed to resonance effects in Raman scattering due to multiple electronic transitions in the cation radical (Figure 2).
In contrast to results obtained for the dication, the SERS spectrum of the cation radical is very close to its RR spectrum.21 No band shifts are observed within the accuracy of the measurements ((2 cm-1). This indicates that the interaction between the cation radical and the electrode is weaker than in the case of the dication. In summary, this study shows that frequencies of bands unique for the dication and cation radical species vary little, or not at all, with excitation wavelength. As expected, in the case of the cation radical the relative intensities of the various bands do change considerably with resonance conditions. These results provide the basis for the following reevaluation of the SERS spectra of methylviologen measured at 77 K. The spectrum obtained at 752.5 nm in liquid nitrogen (Figure 4) is identical to that of the dication. No photoreduction product (or extremely little) is present. It is also evident that, as the excitation moves toward higher energies, more of the cation radical is generated on the metal surface. The photoreduction at 77 K is thus shown to be strongly wavelength dependent. The intense spectra of the cation radical observed at higher energies are not due solely to preferential resonance enhancement of this species, but also represent a real increase in the cation radical concentration as a result of the photoreduction process. Further evidence for the photoreduction of methylviologen adsorbed at a roughened silver surface at liquid nitrogen temperature was obtained from “two-color” experiments. The SERS spectrum was first measured with 640 nm excitation. The electrode in the liquid nitrogen Dewar was then irradiated with the 406.7 or 413.1 nm laser line for varying time periods and the spectrum was again recorded with 640 nm excitation. Figure 7 presents the spectra before (Figure 7a) and after (Figure 7bd) irradiation of the sample. The bands at 1530 and 1350 cm-1 exhibit a very clear increase in intensity following only a few seconds’ exposure to the violet light. The bands become stronger with longer irradiation time and approach saturation
Photoreduction of Methylviologen Adsorbed on Silver
Figure 7. Two-color measurement. SERS of methylviologen adsorbed on roughened Ag, with 640 nm excitation, at 77 K: (a) before irradiation with 406.7 nm laser line; (b) after irradiation with 406.7 nm for 2-3 s; (c) after irradiation with 406.7 nm for 10 s; (d) after irradiation with 406.7 nm for 1 min.
within less than 1 min. The weak resonance for the cation radical at 640 nm excitation allows observation of both the oxidized and reduced species and their evolution with blue light irradiation time. It should be stated, however, that the spectrum of the methylviologen cation radical could not be detected with 640 nm excitation on a smooth silver electrode in liquid nitrogen, whereas a clear, albeit weak, Raman spectrum of cation radical can be observed on a smooth Ag electrode with 406.7 nm excitation. These facts suggest that roughness might not be important for cation radical formation but is essential for SERS and, hence, for detection of the cation radical at red excitation wavelengths. In a somewhat different experiment methylviologen adsorbed on roughened silver was sealed into a glass tube filled with He in order to eliminate contact of the sample with oxygen. The sample was immersed into liquid nitrogen and warmed to room temperature several times. During each cycle, SERS spectra were recorded at both temperatures (Figure 8). The bands at 1530 and 1350 cm-1, characteristic of the cation radical which is formed by irradiation with the 406.7 nm line at 77 K (Figure 8, b and c), disappeared when the sample was warmed to room temperature (Figure 8d). These bands reappeared when the sample was again cooled to liquid nitrogen temperature and irradiated with 406.7 nm light (Figure 8f). The cation radical thus seems to be rather short-lived at room temperature. Nonetheless, the presence of the cation radical could be detected for a short time after irradiation of the sample with the 406.7 nm line at room temperature (compare Figure 8, g and h). Powder Dependence of the SERS Spectra. In the previous experiments, even before irradiation with violet light, a small amount of cation radical was detected with red excitation. This observation suggests that methylviologen can be photoreduced by red light. The 647.5 nm line was used to investigate the effect of laser power on the cation radical formation.
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Figure 8. Effects of temperature and 406.7 nm irradiation on SERS of methylviologen adsorbed on roughened Ag. Spectra a-h were recorded successively with 640 nm excitation: (a) sample cooled to 77 K, before 406.7 nm irradiation; (b) sample at 77 K, after brief 406.7 nm irradiation; (c) sample at 77 K, after longer 406.7 nm irradiation; (d) sample warmed to room temperature, no further 406.7 nm irradiation; (e) sample cooled to 77 K, no further 406.7 nm irradiation; (f) sample at 77 K, after additional 406.7 nm irradiation; (g) sample warmed to room temperature, no further 406.7 nm irradiation; (h) sample after additional 406.7 nm irradiation at room temperature.
The data presented in Figure 9 show that photoreduction does occur at this wavelength. This effect is rather weak, as an increase in excitation power by a factor of 50 results in an increase of cation radical band by less than a factor of 6. The intensity of the 1651 cm-1 dication band was found to be directly proportional to the laser power, implying that the amount of photoreduced dication is negligible. This allows normalization of the spectra with respect to this band. The normalized intensity of the 1530 cm-1 increases slowly, approaching saturation at high powers (Figure 9, insert). Conclusions The experimental results show that methylviologen adsorbed on silver is photoreduced to the cation radical at liquid nitrogen temperature. An explanation for this phenomenon must take into account several important issues: (1) The photoreduction is metal dependent. No photoreduction was observed for methylviologen adsorbed on Pt, Al, or Au.22 Silver is thus directly involved in the electron transfer process. (2) The efficiency of the photoreduction is wavelength dependent. The yield of cation radical was found to increase with the energy of the exciting photons. (3) The methylviologen dication is colorless and not in resonance with the visible range wavelengths which induce its photoreduction. (4) The cation radical is stable in liquid nitrogen. Its spectrum persists for several hours, whereas it decays rapidly at room temperature. (5) The amount of cation radical formed with red excitation is power dependent. This dependence is nonlinear, appearing to reach a limiting concentration of cation radical. The role of silver in the photoreduction of methylviologen can be attributed to its rather unique optical properties. It exhibits strong transitions in the near-UV region, and its surface plasmon resonance is at 3.6 eV, only 0.5 eV below the threshold for electron photoemission.3 This threshold can further be reduced by adsorption of molecules or ions on the surface. It is
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Figure 9. Power dependence of the SERS spectrum of methylviologen adsorbed on roughened Ag. As the 647.5 nm exciting power is varied from 10 to 500 mW, the bands at 1533, 1354, and 1025 cm-1 increase and the bands at 1297 and 1191 cm-1 decrease in intensity (as indicated by the arrows on top of the bands). Insert: The ratio I1533/I1651 of the intensity of the 1533 cm-1 band relative to that of the 1651 cm-1 band (taken as standard).
likely that one or both of the mechanisms (electromagnetic and chemical) proposed to explain the SERS phenomena are also involved in the photoreduction of methylviologen. The metal and wavelength dependencies could be attributed to surface plasmon excitation in the silver, to the formation of a metaladsorbate charge transfer complex, or to a combination of both. Sun et al.18 proposed two possible models for the photoreduction of p-nitrobenzoic acid on silver: (1) charge transfer from the roughness features of the silver surface to the adsorbate; (2) excitation of electrons in a silver-molecule complex by direct absorption of light. The first mechanism requires optical excitation of surface plasmons as a step prior to the charge transfer. The second model suggests that light absorption produces an excited state of the surface complex, a metal to molecule charge transfer then occurring by vibronic coupling between the molecule and the metal. On the basis of their experimental results, the authors could not distinguish between these two mechanisms. The case of methylviologen is somewhat similar. The appearance of the cation radical form can be assigned to surface plasmon excitation followed by electron transfer to the adsorbate. Plasmon excitation provides a mechanism for transferring energy from photons to electrons. On the other hand, the existence of a complex on the surface cannot be excluded. Such a complex could lower the energy barrier for electron transfer. The surface roughness provides “active sites” for complex formation and is also essential for optical excitation of plasmons. Additional resonance due to the complex is not required to explain the wavelength dependence of cation radical formation in the blue spectral region, but could explain its appearance with red excitation. At this time, we do not have any direct evidence supporting the existence of a complex.
Feilchenfeld et al. The stability of the methylviologen cation radical at liquid nitrogen temperatures is difficult to explain. The question is why the back electron transfer from the cation radical to the silver surface does not occur to an appreciable extent at liquid nitrogen temperature. No decrease in radical signal was observed over a period of several hours. In addition, no difference in signal was observed whether or not the electrode was rinsed following adsorption of methylviologen prior to immersion into liquid nitrogen. Thus, the possible formation of multilayers of methylviologen does not appear to be responsible for this effect. To decrease the probability of back reaction, it is necessary to reduce the interaction between the metal and the molecule after the forward electron transfer. One possible explanation might be that at low temperature the position assumed by the molecule relative to the surface after electron capture is frozen in a state that minimizes interaction with the surface; at room temperature the system is more fluid. This is consistent with our experimental results, which suggest that the cation radical form of methylviologen interacts more weakly with the surface than the dication form. A detailed understanding of the mechanism of photoreduction will require additional experiments, including, for example, determination of the relationship between the yield of the photoproduct and the laser power. Time-resolved surface-enhanced Raman scattering (TRSERS) will also be utilized. The considerable potential of this technique for elucidating photoelectrochemical processes at silver was demonstrated recently by Zhang et al.23 These authors observed direct photoinduced charge transfer from adsorbed flavin mononucleotide (FMN) to a Ag electrode and observed two short-lived radical ion intermediates. Based on the results, new mechanisms were proposed for photoinduced charge transfer between FMN and Ag and for the photogalvanic effect at the FMN-modified electrode. Acknowledgment. Research at the Ames Laboratory was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. W-7405-Eng-82. The authors thank Mr. Junwei Zheng for his assistance with the electrochemical measurements. References and Notes (1) Barker, G. C.; Gardner, A. W.; Sammon, D. C. J. Electrochem. Soc. 1966, 113, 1182. (2) Sass, J. K.; Sen, R. K.; Meyer, E.; Gerischer, H. Surf. Sci. 1974, 44, 515. (3) Even, U.; Holcomb, K. A.; Snyder, C. W.; Antoniewicz, P. R.; Thompson, J. C. Surf. Sci. 1986, 165, L35. (4) Kim, C.-W.; Villagran, J. C.; Even, U.; Thompson, J. C. J. Chem. Phys. 1991, 94, 3974. (5) Funtikov, A. M.; Sigalaev, S. K.; Kazarinov, V. E. J. Electroanal. Chem. 1987, 228, 197. (6) Birke, R. L.; Lombardi, J. R. In Spectroelectrochemistry: Theory and Practice; Gale, R. J., Ed.; Plenum: New York, 1988; pp 263-348. (7) Cotton, T. M.; Uphaus, R. A.; Mobius, D. J. Phys. Chem. 1986, 90, 6071. (8) Otto, A.; Mrozek, I.; Akeman, W. J. Phys.: Condens. Matter 1992, 4, 1143. (9) Moskovits, M. ReV. Mod. Phys. 1985, 57, 783. (10) Nitzan, A.; Brus, L. E. J. Chem. Phys. 1981, 75, 2205. (11) Garoff, S.; Weitz, D. A.; Alvarez, M. S. Chem. Phys. Lett. 1982, 93, 283. Garoff, S.; Weitz, D. A.; Alvarez, M. S.; Gersten, J. I. J. Chem. Phys. 1984, 81, 5189. Gersten, J. I.; Nitzan, A. Surf. Sci. 1985, 158, 165. Das, P.; Metiu, H. J. Phys. Chem. 1985, 89, 4680. Leung, P. T.; George, T. F. J. Chem. Phys. 1986, 85, 4729. (12) Goncher, G. M.; Harris, C. B. J. Phys. Chem. 1982, 77, 3767. Whitmore, P. M.; Alivisatos, A. P.; Harris, C. B. Phys. ReV. Lett. 1983, 50, 1092. Chen, C. J.; Osgood, R. M. Appl. Phys. A 1983, 31, 171. Goncher, G. M.; Parsons, C. A.; Harris, C. B. J. Phys. Chem. 1984, 88, 4200.
Photoreduction of Methylviologen Adsorbed on Silver (13) Tabares, F.; Marsh, E. P.; Bach, G. A.; Cowin, J. P. J. Chem. Phys. 1987, 86, 738. Harrison, I.; Polanyi, J. C.; Young, P. A. J. Chem. Phys. 1988, 89, 1475; 1988, 89, 1498. (14) Costello, S. A.; Roop, B.; Liu, Z. M.; White, J. M. J. Phys. Chem. 1988, 92, 1019. Roop, B.; Costello, S. A.; Greenlief, C. M.; White, J. M. Chem. Phys. Lett. 1988, 143, 38. (15) Marsh, E. P.; Schneider, M. R.; Gilton, T. L.; Tabares, F. L.; Meier, W.; Cowin, J. P. Phys. ReV. Lett. 1988, 60, 2551. Marsh, E. P.; Tabares, F. L.; Schneider, M. R.; Gilton, T. L.; Meier, W.; Cowin, J. P. J. Chem. Phys. 1990, 92, 2004. (16) Marsh, E. P.; Gilton, T. L.; Meier, W.; Schneider, M. R.; Cowin, J. P. Phys. ReV. Lett. 1988, 61, 2725. (17) Chuang, T. J.; Domen, K. J. Vac. Sci. Technol. A 1987, 5, 473. Domen, K.; Chuang, T. J. Phys. ReV. Lett. 1987, 59, 1484.
J. Phys. Chem., Vol. 100, No. 12, 1996 4943 (18) Sun, S.; Birke, R. L.; Lombardi, J. R.; Leung, K. P.; Genack, A. Z. J. Phys. Chem. 1988, 92, 5965. (19) Feng, Q.; Yue, W.; Cotton, T. M. J. Phys. Chem. 1990, 94, 2082. (20) Ebbesen, T. W.; Levey, Gerrit, G.; Patterson, L. K. Nature (London) 1982, 298, 545. (21) Lu, T.; Cotton, T. M.; Hurst, J. K.; Thompson, D. H. P. J. Phys. Chem. 1988, 92, 6978. (22) Chumanov, G. Unpublished results. (23) Zhang, W.; Vivoni, A.; Lombardi, J. R.; Birke, R. L. J. Phys. Chem. 1995, 99, 12846.
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