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results in improved kinetics for H2 evolution from the p-type semiconductor. Kinetics for H2 evolution was previously identified as a major problem at p-semiconductor aqueous electrolyte interfaces (1-3). Surface modification of electrodes has recently been an active area of research, and examples of electrocatalysts are emerging (4). To produce an efficient photosensitive interface from which to evolve H2 we take advantage of the light absorption and charge separating properties of a semiconductor (5-8). But additionally we employ surface modification to achieve good H2 evolution kinetics. A p-type semiconductor should serve as a photocathode, because the minority carrier is the electron, e-. As indicated in Fig. 1, the photoexcited electron becomes available at the interface as a reducing equivalent with reducing power no greater than the bottom of the conduction band at the interface ECB. Efficient solar-driven electrolysis requires: (i) use of a photocathode having a small Eg, 1-2 eV; (ii) optimization of the product of Ev and photocurrent; and (iii) a durable interface. Experiments show that Ev can be significant for small Eg photocathodes, but the kinetics for H2 evolution are so poor that little if any efficiency can be realized for p-type Si (Eg =

The value of EV is generally clcse to EB at high illumination intensity.The finding of a constant E B fo,' n-CdTe, independent of Eredo x or s, leads to the conclusion that n-CdTe is Fermi level pinned. 2 ' 5 We take the term "Fermi level pinned" to refer to a semiconductor that is measured to have a constant EB, independent of the contacting medium for a wide range of Eredox or 0. The origin of a constant EB can be attributed to surface states situated between the top of the valence band, EVB, and the bottom of the conduction band, ECB. The density and distribution of surface states can control the value of EB as a function of E redox or 0.5,6 When the region between ECB and EVB is free of surface states, EB of an n-type semiconductor is expected to vary with Eredox according to equation (1)7for E redox situated between EVB and ECB whereEFB is the electrochemical potential of the semiconductor, Ef, when there is no band bending. When Ef = EFB there is no accumulation or depletion of the majority charge carrier at the surface and E B = 0. But even for surface-state-free semiconductors, equation (1) Pyrex tube and all surfaces but the exposed front surface of CdTe were sealed with ordinary epoxy. The exposed surface of the CdTe was the (111)face. Just prior to use all electrodes were etched and cleaned as described below.Etching Procedures. Two different types of etchants were used, oxidizing and reducing. An oxidizing etch was one of the following: (i) 4 g K 2 Cr 2 0 7 , 10 ml concentrated HNO 3 , and 20 ml H 2 0; 12 (ii) 1/1/l by volume saturated solution of K 2 Cr20 7 , concentrated HCl, and concentrated HNO 3;13 (11) 3/2/1 by volume solution of 48% HF, 30% H 2 0 2 , and H 2 0 and then briefly rinsed with concentrated HCl;14 (iv) 3/2/1/0.01 by volume of concentrated HNO 3 , concentrated H 2 SO 4 , glacial aceic acid, and concentrated HCl; (v) 2/1/l by volume solution of 48% HF, concentrated HNO 3 , and glacial' acetic acid. 15 For an oxidizing etch the CdTe electrode was immersed in the etchant for Abbreviations for redox couples are MV 2+ E N,N'-dimethyl-4,4'-bipyridinium; gun was used to produce a 2 keV Ar ion beam for sputtering. The pressure was maintained at 3 x -10 torr in the main vacuun, chamber and 1.5 x 10 -4 torr of Ar in the ionization chamber, while sputtering.X-ray photoelectron spectra (XPS) were obtained ona Physical Electronics model 548 spectrometer with a magnesium anode. The broad scans (0-1000 eV)were recorded with a pass energy of 100 eV and the narrow scans with a pass energy of 25 eV. The peak energies at the Cd and Te peaks were referenced to the Cls binding energy (284.6 eV) to correct for charging. Samples were mounted as above and sputtering was oune with a 5 keV Ar+ beam, after introducing Ar into the vacuum chamber to bring the pressure to -7 x 10 torr.Elements detected by Auger and XPS were identified by reference to data previously reported using these techniques. 17 ' 18. . Results a.Determination of EV vs. E redox for n-CdTe. Scheme I illustrdte', the interface enerqetLi...

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Electrodes can be functionalized with (7V,7V '-bis(3-(trimethoxysilyl)propyl)-4,4'-bipyridinium| dibromide (I), yielding a surface-confined, electroactive polymer, (PQ2+'2Br")". The anions are labile and can be replaced readily by a number of anions. By a combination of Auger and electrochemical techniques we have studied the incorporation of the anions ptoluenesulfonate, Cl", Br", I", C104", SCN', S04* 2", Fe(CN)64", RuíCN)*4", Co(CN)63", Mo(CN)g4", IrCl62", and PtCl62" into (PQ2+)". Generally, the transition metal complex anions are more firmly bound and less labile than the other anions. The ordering of monoanions is p-toluenesulfonate at Cl' < Br at C104" su SCN" < I". The S042' anion competes well with C104" and not as well with . For the monoanions and S042" the selectivity for binding spans a range of approximately an order of magnitude. For example, a solution having 0.09 M Cl" and 0.01