Elucidation of the Ta(O,N)₆ Octahedral Arrangement in Flux-Grown BaTaO₂N Photocatalysts by Experimental and Computational Structural Modeling

BaTaO₂N (BTON) is a visible-light-responsive photocatalyst used for water splitting. The flux method, which involves the use of a molten salt, is an effective synthetic strategy for achieving a high photocatalytic activity. Fluxes with alkali metal cations strongly affect the photocatalytic activity of the BTON crystals. In particular, RbCl flux-grown BTON exhibits hydrogen evolution activity over several times higher than that grown in other chloride fluxes, such as NaCl, under visible-light irradiation. One factor of this difference is presumably owing to the change in the Ta(O,N)₆ octahedral arrangement in the BTON triggered by the doping of alkali metal ions into the crystal lattice. However, the precise Ta(O,N)₆ octahedral arrangement remains unclear. Herein, X-ray absorption spectroscopy, structural analysis by neutron diffraction, and computational structural modeling based on comprehensive structural energy predictions were performed for two types of BTONs. The results suggested that the configuration manners of Ta(O,N)₆ octahedral units strongly depend on the flux composition. Specifically, in NaCl-flux-grown BTON crystals, the number of N20 cis planes parallel to the (100), (010), and (001) crystal planes in a TaO₄N₂ unit is anisotropic, resulting in differences in their electron-hole conduction characteristics. The findings indicate that in addition to lattice defects, the interconnections of mixed-anion units such as Ta(O,N)₆ should be taken into account to improve the photocatalytic activity of BTONs and develop other mixed-anion compounds.