Structures, electron density and characterization of novel photocatalysts, (BaTaO₂N)_1-x(SrWO₂N)_x solid solutions

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, (BaTaO₂N)_1-x(SrWO₂N)_x, with a cubic Pm3[combining macron]m perovskite-type structure (x = 0.01 and 0.02) have been prepared by heating oxide precursors under an ammonia flow. These (BaTaO₂N)_1-x(SrWO₂N)_x catalysts exhibited photocatalytic water oxidation activity under visible light irradiation. The crystal structure, electron-density distribution, and optical properties of (BaTaO₂N)_1-x(SrWO₂N)_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 (BaTaO₂N)_1-x(SrWO₂N)_x decreased linearly with increasing SrWO₂N content x. The minimum electron density (MED) at the (Ta,W)-(O,N) bond, determined by the MEM analysis of (BaTaO₂N)_1-x(SrWO₂N)_x, increased with x, as supported by DFT-based calculations. These results indicate the formation of (BaTaO₂N)_1-x(SrWO₂N)_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 (BaTaO₂N)_1-x(SrWO₂N)_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 (BaTaO₂N)_1-x(SrWO₂N)_x contains W⁵⁺ cations with a [Xe] 4f¹⁴ 5d¹ electron configuration and exhibits a more n-type semiconducting character compared with BaTaO₂N, which could improve the photocatalytic water oxidation activity under visible-light irradiation.