Photooxidation of Halides and Water on n-Silicon Protected with Silicide Layers

The electrochemical and photoelectrochemical behavior of n-type silicon electrodes coated with noble metal silicides and RuO, in aqueous solutions containing various redox couples was investigated. Iridium silicide coated n-Si electrodes, n-Si( Ir), and Ru02-modified n-Si(1r) electrodes can photogenerate 12, Brz, and C12 with high stability and efficiencies (>5%) . The photooxidation of water on these electrodes is also feasible. Interest in the stabilization of small band gap n-type semiconductors against photocorrosion in photoelectrochemical (PEC) cells is based on the goal of photogenerating highly oxidizing species (e.g., Br,, 02, Cl,, etc.) on these semiconductor electrodes. Because of its wide use and availability, small band gap n-Si has been of special interest. While naked n-Si is easily photooxidized in aqueous solutions to produce an insulating SiO, layer, stable n-Si photoelectrodes can be achieved by coating the photoanode with a thin gold or platinum layer overcoated with a thick polypyrrole film1jSk or derivatizing the n-Si surface before coating with a polypyrrole fi1m.l' However, the photogeneration of highly oxidizing species, such as Br2, CI,, or 02, on these electrodes cannot be accomplished because of the instability of polypyrrole in the potential range where the oxidation of Br-, C1-, or H 2 0 takes place., We have recently demonstrated3 that Pt silicide coated n-Si electrodes can be employed to fabricate fairly stable PEC (1) (a) Nakato, Y.; Ohnishi, T.; Tsubomura, H. Chem. Lett. 1975, 19, 883. (b) Nakato, Y.; Abe, K.; Tsubomura, H. Eer. Eumenges. Phys. Chem. 1976, 80, 1002. (c) Kohl, P. A.; Frank, S. N.; Bard, A. J. J. Electrochem. SOC. 1977, 124, 225. (d) Tomkiewicz, M.; Woodall, J. Ibid., 1436. (e) Noufi, R.; Tench, D.; Warren, L. Ibid. 1980, 127, 2709, and references therein. (f) Noufi, R.; Frank, A. J.; Nozik, A. J. J . Am Chem. SOC. 1981, 103, 1849. (8) Bocarsly, A. B.; Walton, E. G.; Wrighton, M. S . Ibid. 1980, 102, 3390. (h) Bolts, J. M.; Bocarsly, A. B.; Palazzotto, M. C.; Walton, E. G.; Lewis, N. S.; Wrighton, M. S. Ibid. 1979, 101, 1378. (i) Bolts, J.; Wrighton, M. S. Ibid. 1978, 100, 5257. 6) Fan, F.-R. F.; Wheeler, B. L.; Bard, A. J.; Noufi, R. J . Electrochem. SOC. 1981, 128, 2042. (k) Skotheim, T.; Lundstrom, I.; Prejza, J. Ibid. 1625. (I) Simon, R. A.; Ricco, J.; Wrighton, M. S. J . Am. Chem. SOC. 1982, 104, 2031. (m) DuBow, J.; Hodes, G.; Rajeshwar, K., submitted for publication. (2) (a) Kanazawa, K. K.; Diaz, A. F.; Gill, W. D.; Grant, P. M.; Street, G. B.; Gardini, G. P. Synth. Met. 1979/1980, I , 329. (b) Bull, R.; Fan, F.-R. F.; Bard, A. J. J . Electrochem. SOC. 1982, 129, 1009. (3) Fan, F.-R. F.; Hope, G.; Bard, A. J., J . Electrochem. SOC. 1982, 129, 1647. cells with aqueous solutions containing various redox couples, e g , Fe2+/3+, 1-/13-, Fe(CN),3-/4-. There is a wealth of information concerning thin-film-semiconductor reactions and the nature of semiconductor/metal interfaces, especially metal thin films on ~ i l i c o n . ~ We report here a study of the electrochemical (EC) and PEC behavior of platinum and iridium silicide coated n-Si electrodes and suitably modified forms of these electrodes in aqueous solutions containing halide ions. We demonstrate that iridium silicide coated n-Si electrodes, denoted n-Si(Ir), and Ru0,-modified n-Si(Ir) electrodes (which are similar to the Ru0,-modified I T 0 coated n-Si electrodes described recently") can photogenerate I,, Br2, and C1, without noticeable decomposition with efficiencies above 5%. The photooxidation of water is also described. Experimental Section Silicides were prepared by procedures similar to those reported prev i ~ u s l y . ~ ~ ~ ~ n-Si single crystals (0.4-0.6 ohm cm) donated by Texas Instruments were cleaned ultrasonically in trichloroethylene, acetone, and methanol. Immediately prior to vacuum deposition of metal, the n-Si crystals were etched twice with 48% HF solution for 10-20 min. The etched crystals were then rinsed thoroughly with distilled water and methanol and dried under vacuum. The metal films were deposited by flash evaporation of a known amount of metal on the n-Si crystals a t a pressure of 10" torr. The thickness of the metal film deposited on silicon wafers was estimated from the optical density of a metal film deposited (4). (a) Tu, K. N.; Mayer, J. W. In 'Thin Films-Interdiffusion and Reactions", Poate, J. M., Tu, K. N., Mayer, J. W., Eds.; Wiley: New York, 1978; p 359. (b) Lepselter, M. P.; Andrews, J. M. In "Ohmic Contacts to Semiconductors", Schwartz, B. Ed.; The Electrochemical Society: New York, 1969. (c) Sze, S. M. "Physics of Semiconductor Devices"; Wiley-Interscience: New York, 1969; p 363. 0002-7863/83/ 1505-0220$01.50/0