Effects of V-Ion Doping on the Photoelectrochemical Properties of Epitaxial TiO2(110) Thin Films on Nb-Doped TiO2 (110) Single Crystals

V-doped TiO2(110) epitaxial films were grown on atomically flat Nb-doped TiO2 (110) single-crystal substrates by a pulsed laser deposition method, and their photoelectrochemical properties were investigated. In the photocurrent-potential curves measured in a 0.1 M HClO4 electrolyte for V-doped TiO2/Nb-doped TiO2 photoelectrodes, the photocurrent was almost proportional to the electrode potential, and there was a good linear relationship between the inverse of the photocurrent density obtained at +0.6 V versus Ag/AgCl KCl (sat.) and the V doping level. The potential at which the photocurrent appeared, that is, the flat-band potential, was about +0.33 V irrespective of the doping level, whereas it was about -0.25 V for the nondoped one. When a 0.01 M AgNO3 electrolyte was used, a sizable amount of Ag metal particles was deposited at +0.25 V on the electrode surfaces at V doping levels above 6 at %. The V-doping effects on these photoelectrochemical properties are discussed in terms of the in-gap states induced by the V impurities in TiO2 as well as their acting not only as a recombination center but also as a mediator of interfacial electron transfer.