Nowadays, development of energy conversion from natural sources is essential issue for construction of sustainable society and photocatalytic water oxidation using sun light is one of desirable processes to achieve it. For effective use of sun light, it is necessary to utilize visible light, since the solar provides over 50 % of visible light, but only 5 % of UV light in its whole energy. (Oxy)nitrides are promising candidates for visible-light response photocatalysts. BaNbO₂N is one of oxynitrides as photocatalyst, which belongs to cubic-type perovskite. BaNbO₂N exhibits superior visible-light absorption up to ca. 740 nm (band gap energy: ca. 1.7 eV) compared to other (oxy)nitrides: For example, the absorption edges are 700 nm for SrNbO₂N, 650 nm for LaTaO₂N and BaTaO₂N, and 600 nm for LaTiO₂N. Previous reports about BaNbO₂N indicated that the synthetic conditions affect to the photocatalytic performance, while there is still plenty of space to further improve the efficiency. The photocatalytic performance is known to depend on a variety of crystallographic characteristics. Specific surface area, crystal habits, qualities of crystallinity, and deficiencies are the representative factors. To improve the photocatalytic performance of BaNbO₂N in terms of crystallographic viewpoints, a precise understanding of the contribution of each factor to the performance would be inevitable. Herein, we prepared BaNbO₂N particles with different crystallographic characteristics by controlling morphology of precursor crystals as sacrificial templates grown in a molten flux. The samples’ particle characteristics and their photocatalytic water oxidation abilities were examined to discuss the contribution of crystallographic factors to the photocatalytic performances.

BaNbO₂N particles were prepared through the following 2-step procedure. At first, Ba₅Nb₄O₁₅ crystals were grown in molten BaCl₂ flux at a solute concentration of 5 mol% at designated temperatures for 10 h. In all temperature conditions, single phases of Ba₅Nb₄O₁₅ were obtained as primary particles and their sizes increased from 0.2 to 50 μm, as holding temperature increased. Subsequently, they were heated in NH₃ atmosphere at 950 ºC for 10 h for nitridation. The each obtained product was identified to single-phase BaNbO₂N. Looking at their morphology, segregations to porous secondary particles were found, while maintaining their initial morphologies in the secondary particles. The particle characteristics were investigated in terms of specific surface area, crystallinity, Nb states on the surface, anion deficiency, and band gap energy. Finally, we examined their photocatalytic performances by measuring photo-irradiated O₂ evolution from water. It was found that O₂ evolution rates differs between samples up to 8 times, depending on the precursor particle size and nitriding time. After comparing crystallographic characteristics of each BaNbO₂N particle, we propose competitive effects of crystallinity and anion deficiency to dominantly determine the photocatalytic performances. The detail discussion about the effects should be introduced in terms of quantitative evaluations of the crystallographic factors in the presentation.