10096
J. Phys. Chem. 1992,96, 10096-10098
experiments were carried out at pH 7, proton transfer from the solvent is unlikely. We thus tentatively interpret these results as stemming from a photoinduced molecule to metal charge transfer, forming a flavin radical cation. Such a species has been identified in the one-electron oxidation of a flavin triplet ~pecies.’~J’The energy needed for ionization is considerably below that for the liquid-phase ionization potential if the excited electron tunnels to the low-lying empty Fermi levels of the metal, and indeed we can measure a threshold in the visible region for this process as a function of Fermi level. On the other hand, UV irradiation at 253.7 nm has been required to produce flavin radical cations in solution.18 Thus, for an adsorbed molecule on a metal surface where the metal substrate and absorbate wave function overlap, an excitation to an electronic level above the Fermi level can lead to electron tunneling to the metal by resonant charge transfers6 It must be assumed that the reverse process is quenched by some relaxation mechanism, either solvent reorganization or possibly intramolecular enolization, leading to a possible candidate for photoproduct I. We are currently conducting further work to characterize these photoproducts and to determine the kinetics more precisely. To our knowledge, this is the first report of SERS observation of nanosecond time scale kinetics on an electrode surface. Acknowledgment. This work has been supported by the National Science Foundation (CHE-8711638 and CHE-9122257) with supplementary support from the PSC-BHE award program of the City University of New York (666367,667261, and 66875)
and the National Institutes of Health MBRS program (RR08 168).
Referencea and Notes (1) Birke, R. L.; Lombardi, J. R. In Spectr~lectrochemfst~; Theory und Pructice; Gale, R. J., Ed.;Plenum Press: New York, 1988; p 263. (2) Birke, R. L.; Lu, T.;Lombardi, J. R. In Techniquesfor the Charucterizution of Electrodes and Electrochemical Processes; Varma, R.. Selman, J. R., Eds.; John Wiley & Sons: New York, 1991; p 211. (3) Gao, P.; Gasztola, D.; Weaver, M. J. J . Phys. Chem. 1988,92,7122. (4) Shi, C.; Zhang, W.; Birke, R. L.; Gosser, Jr., D. K.; Lombardi, J. R. J . Phys. Chem. 199L95.6276. ( 5 ) Shi, C.; Zhang, W.; Birke, R. L.; Lombardi, J. R. J. Phys. Chem. 1990, 94, 4766. (6) Avouris, P.; Persson, B. N. J. J. Phys. Chem. 1984, 88, 837. (7) Voss, D. F.; Paddock, C. A.; Miles, R. B. Appl. Phys. h t r . 1982,41, 51. (8) Xu, J.; Birke, R. L.; Lombardi, J. R. J . Am. Chem. Soc. 1987, 109, 5645. (9) Furtak, T. E.; Macombcr, S. H. Chem. Phys. Lett. 1983, 95, 38. (10) Furtak, T. E.; Roy, D. Phys. Rev. Lett. 1983, 50, 1301. (11) Billman, J.; Otto, A. Surf Sci. 1984, 138, 1 . (12) Lombardi, J. R.; Birke, R. L.; Sanchez, L. A.; Bernard, I.; Sun, S. C. Chem. Phys. I r r t . 1984,104,240. (13) Lombardi, J. R.; Birke, R. L.; Lu, T.;Xu, J. J. Chem. Phys. 1986, 84, 4174. (14) Abe, M.; Kyogoku, Y. Spectrochim. Acta 1987, 43.4, 1027. (15) Shi, C.; Zhang, W.; Birke, R. L.;Lombardi, J. R. Proc. SHE-lnr. Soc. Opr. Eng. 1992, 1637, 41. (16) Heelis, P. F.; Parsons, B. J.; Thomas, B.; Phillip, G. 0.J. Chem. Soc., Chem. Commun. 1985,954. (17) Heelis, P. F.; Parsons, B. J.; Phillips, G. 0.;Swallow, A. J. J . Phys. Chem. 1986, 90,6833. (18) Getoff, N.; Solar, S.; McCormick, D. B. Science 1978, 201, 616.
Electron Transfer Dynamlcs at p-GaAs/Liquid Interfaces Y. Rosenwaks, B. R. Thacker, R. K. Ahrenkiel, and A. J. Nozik* National Renewable Energy Laboratory lformerly the Solar Energy Research Institute), Golden, Colorado 80401 (Received:September 9, 1992; In Final Form: October 6, 1992)
The rates of photoinduced electron transfer from sulfide-passivated p-GaAs to outer-sphere redox acceptors (ferricenium and cobalticinium) in acetonitrile have been measured using time-correlated single-photon-countingof photoluminescence e sfor electron transfer were found to be very fast, manifested by electron transfer velocities decay. The characteristic time d ranging from 2 X lo5to lo6 cm/s at 1 m M concentrations. These rates are 4-5 orders of magnitude faster than predicted by other workers.
A critical issue in the field of photoelectrochemistry based on semiconductor/liquid junctions is the rate at which photoinduced charge carriers can be transferred from the illuminated semiconductor to redox acceptors in the adjacent solution. This is vital not only for understanding the fundamental processes of charge transfer at semiconductor/liquid interfa- but also for clarification of the important issue of whether hot carrier injection’-’ into liquid solutions from illuminated semiconductor photoelectrodes can be an important process in photoelectrochemical (PEC) cells. Relatively few measurements have been reported of the chargetransfer rates from illuminated semiconductor electrodes to redox acceptors. All such previous experiments8-” have been conducted with n-type semiconductors; the reported time scales for hole transfer in these systems ranged from
