Formation Manner of Lattice Defect in BaNbO2n Photocatalysts during Their Nitridation in Molten Flux
Authors: Yamada Tetsuya , Nagafusa Shinya , 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 fully understand what kind of factors exist and how much they contribute to the performance. Lattice defects, such as atomic vacancy and electric defect, should be one of dominant factors to determine the photocatalytic performance. It is known that they exhibit two kinds of aspects in photocatalytic reaction : The first one is recombination center of excited electron, and second one is modification of surficial electric states, which give complicated contributions to the performance. So, optimal design of lattice defects based on well understanding of their structure and role to photocatalytic performance is necessary to improve the photocatalytic performance of BaNbO2N up to theoretical value.

BaNbO2N is usually prepared under reduction condition, which leads reduction of niobium (V) and formation of lattice defects. Considering that thermodynamic parameters correlate to formation of lattice defect, we expected that inhibition of the lattice defects is possible if optimal reaction pathway is adopted. So, we set our final goal of this research to discover reaction process to control the lattice defects down to negligible order.

As a first step of our research, it is important to understand the origin of the lattice defects. In this presentation, we will report formation manner of the lattice defect in terms of their species, quantity, and location. Submicron, mono-dispersed BaNbO2N crystals with different nitridation degree were grown using flux method. Elemental, structural, and valence analyses were performed to determine chemical formula with lattice defects in each nitridation-state sample to describe chemical states of BaNbO2N in each nitridation degree. We also introduced mathematic statistics analyses to visualize correlation between lattice defect formations and their driving force, which should give hint to suggest optimal preparation pathway to non-lattice defect BaNbO2N.

Acknowledgement : This work was supported by JSPS Grant-in-Aid for Scientific Research (A) 25249089, Regional Innovation and Ecosystem Formation Program from the MEXT, and JST Program on Open Innovation Platform with Enterprises, Research Institute and Academia.