Photoelectrochemical properties of crystalline perovskite lanthanum titanium oxynitride films under visible light

Thin films of the lanthanum titanium oxynitride perovskite (LaTiO _xN_y) 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 LaTiO_xN_y is found to change with pH of the reactant solution. Surface modification with colloidal IrO₂is also demonstrated to result in a marked increase in photoactivity, with the modified epitaxial LaTiO_xN_y film exhibiting a photocurrent density of ca. 70 μA cm^-2 at +1.0 V vs. Ag/AgCl at pH 4.5 in aqueous Na₂SO₄ solution under irradiation at visible wavelengths (λ > 420 nm).