Effect of Mg2+ substitution on the photocatalytic water splitting activity of LaMgxNb1−xO1+3xN2−3x

Abstract

Overall water splitting using a photocatalyst absorbing a wide range of visible light is an ideal means of producing renewable hydrogen, and Nb-based oxynitrides are potential candidates for this purpose due to their narrow band gaps. However, it is challenging to prepare such materials as efficient photocatalysts because Nb⁵⁺ species are readily reduced during synthesis, and the energy offsets between the band edges of such materials and the water redox potentials are small. In this study, a series of LaMg_xNb_1−xO_1+3xN_2−3x solid solutions (0.0 ≤ x ≤ 0.5) was synthesized in an attempt to modify the band structures and improve the water splitting activity of these compounds. The substitution of Mg²⁺ enlarged the band gap of LaNbON₂, shifting the conduction and valence band edges negatively and positively, respectively, while retaining visible light absorption up to 600 nm and beyond. Moreover, the concentrations of reduced Nb species at the oxynitride surfaces generated during nitridation were significantly decreased with increasing Mg²⁺ content. As a result, increasing the amount of Mg²⁺ in these materials enhanced their photocatalytic H₂ evolution activity. Moreover, the co-loading of RhCrO_y and CoO_z as cocatalysts for water reduction and oxidation, respectively, allowed LaMg_xNb_1−xO_1+3xN_2−3x (0.2 ≤ x ≤ 0.5) to simultaneously evolve gaseous H₂ and O₂ from water under visible light (λ > 420 nm). These observations demonstrate the effectiveness of this band engineering technique and the necessity for further refinements of material properties and surface modifications to realize visible-light-driven overall water splitting over Nb-based oxynitrides.