Crystallographic Controls of Polycrystalline BaNbO2n Particles Using Flux Method and Their Photocatalytic Water Oxidation Ability
Authors: Yamada Tetsuya , Teshima Katsuya 
Published: February 2020

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.