Photoelectrochemical Properties of Crystalline Perovskite Lanthanum Titanium Oxynitride Films under Visible Light

Thin films of the lanthanum titanium oxynitride perovskite (LaTiOxNy) synthesized by reactive radio-frequency sputtering on conductive Nb-doped strontium titanate (Nb:SrTiO3) substrates are evaluated as photoelectrodes for water splitting reaction under visible light. The films are characterized by X-ray diffraction analysis, energy-dispersive spectroscopy, scanning electron and atomic force microscopy, and ultraviolet-visible spectroscopy. Thin films with polycrystalline, oriented, or epitaxial structures are obtained depending on the substrate temperature and plasma composition. The band-gap energies of the films thus prepared are in the range 2.05-2.35 eV. Photoelectrochemical measurements reveal that the photoactivity of these films increases with the quality of film crystallization. The flat-band position of LaTiOxNy is found to change with pH of the reactant solution. Surface modification with colloidal IrO2 is also demonstrated to result in a marked increase in photoactivity, with the modified epitaxial LaTiOxNy film exhibiting a photocurrent density of ca. 70 mu A cm(-2) at + 1.0 V vs. Ag/AgCl at pH 4.5 in aqueous Na2SO4 solution under irradiation at visible wavelengths (lambda > 420 nm).