Structures, electron density and characterization of novel photocatalysts, (BaTaO2N)1−x(SrWO2N)x solid solutionsElectronic supplementary information (ESI) available: (A) Sample photos, (B) photocatalytic activity measurement details, (C) Rietveld analysis results of synchrotron X-ray diffraction data and compositional dependence of lattice parameter, (D) band gap estimation, and (E) photocatalytic activity. See DOI: 10.1039/c7dt02873c

• Main Authors: Hibino, Keisuke, Yashima, Masatomo, Oshima, Takayoshi, Fujii, Kotaro, Maeda, Kazuhiko
• Format: Journal Article
• Language: English
• Published: 07.11.2017
• DOI: 10.1039/c7dt02873c
• ISSN: 1477-9226

Tungsten-modified barium tantalum oxynitride is a new visible-light photocatalyst for water oxidation. In the present work, novel barium tantalum strontium tungsten oxynitride solid solutions, (BaTaO 2 N) 1− x (SrWO 2 N) x , with a cubic Pm 3&cmb.macr; m perovskite-type structure ( x = 0.01 and 0.02) have been prepared by heating oxide precursors under an ammonia flow. These (BaTaO 2 N) 1− x (SrWO 2 N) x catalysts exhibited photocatalytic water oxidation activity under visible light irradiation. The crystal structure, electron-density distribution, and optical properties of (BaTaO 2 N) 1− x (SrWO 2 N) x ( x = 0, 0.01, and 0.02) have been studied using synchrotron X-ray powder diffraction, Rietveld analysis, the maximum-entropy method (MEM), and UV-Vis reflectance measurements. The lattice parameters of (BaTaO 2 N) 1− x (SrWO 2 N) x decreased linearly with increasing SrWO 2 N content x . The minimum electron density ( MED ) at the (Ta,W)-(O,N) bond, determined by the MEM analysis of (BaTaO 2 N) 1− x (SrWO 2 N) x , increased with x , as supported by DFT-based calculations. These results indicate the formation of (BaTaO 2 N) 1− x (SrWO 2 N) x solid solutions and enhanced covalent bonding due to the stronger W-N bond. The MED of the (Ta,W)-(O,N) bond was higher than that of (Ba,Sr)-(O,N), indicating that the (Ta,W)-(O,N) bond is more covalent. The presence of nitrogen in (BaTaO 2 N) 1− x (SrWO 2 N) x was confirmed by the occupancy factor refined using neutron diffraction data and by the weight gain observed by thermogravimetric analysis in air. UV-Vis reflectance spectra and DFT calculations indicated that (BaTaO 2 N) 1− x (SrWO 2 N) x contains W 5+ cations with a [Xe] 4f 14 5d 1 electron configuration and exhibits a more n-type semiconducting character compared with BaTaO 2 N, which could improve the photocatalytic water oxidation activity under visible-light irradiation. W 5+ doping in BaTaO 2 N enhances (Ta,W)-(O,N) covalent bonding and can improve the photocatalytic activity.