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

It has recently been discovered that LaMg x Ta 1− x O 1+3 x N 2−3 x 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 Mg 2+ for Ta 5+ and O 2− for N 3− , 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 x Ta 1− x O 1+2 x N 2−2 x ( x ≥ 0.5). In LaM x Ta 1− x O 1+(5− n ) x N 2−(5− n ) x (M n + = Mg 2+ , Sc 3+ , and Zr 4+ ) solid solutions, increasing the M n + ( n = 2 and 3) content tuned the bandgap position, which facilitated water oxidation as well as overall water splitting.