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

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 LaMgxNb1-xO1+3xN2-3x 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 H-2 evolution activity. Moreover, the co-loading of RhCrOy and CoOz as cocatalysts for water reduction and oxidation, respectively, allowed LaMgxNb1-xO1+3xN2-3x (0.2 <= x <= 0.5) to simultaneously evolve gaseous H-2 and O-2 from water under visible light (lambda > 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.

Automated summary

In this study, LaMgxNb1-xO1+3xN2-3x solid solutions were synthesized to improve the water splitting activity of oxynitrides, where the substitution of Mg2+ enlarged the band gap and decreased the concentration of reduced Nb species, enhancing the photocatalytic H2 evolution activity and achieving simultaneous H2 and O2 production under visible light. This band engineering technique proves to be effective for visible-light-driven overall water splitting over Nb-based oxynitrides, highlighting the need for further refinements in material properties and surface modifications.