Band engineering of perovskite-type transition metal oxynitrides for photocatalytic overall water splitting

Abstract

It has recently been discovered that LaMg_xTa_1−xO_1+3xN_2−3x solid solutions act as photocatalysts for water splitting under a wide range of visible wavelengths. In the present study, a detailed characterization of the crystal structure, optical properties, and electronic band structure of these photocatalysts was performed. It was found that increasing the Mg content decreased the N content in the solid solution as a result of co-substitution of Mg²⁺ for Ta⁵⁺ and O²⁻ for N³⁻, and enabled fine tuning of the bandgap energy and position. The bandgap increased due to a shift in the valence band maximum towards the positive electrode potential, rather than a change in the conduction band minimum. This facilitated water oxidation, and thus overall water splitting. On the basis of this finding, overall water splitting was also achieved for another series of solid solutions, LaSc_xTa_1−xO_1+2xN_2−2x (x ≥ 0.5).