Substitution Effect of Ta⁵⁺ by Nb⁵⁺ on Photocatalytic, Photophysical, and Structural Properties of BiTa_1–xNb_xO₄(0 ≦ x≦ 1.0)

Substitution effects of Ta⁵⁺ by Nb⁵⁺ in BiTa_1−xNb_xO₄(0 ≦ x ≧ 1.0) on photocatalytic, photophysical, and structural properties were investigated. The powder x-ray diffraction and Rietveld structural refinement showed that the structure of BiTa_1−xNb_xO₄ at R = Nb/Bi = 0.0 and 0.5 is a triclinic system with space group P1. However, the structure at R = 0.2, 0.8, and 1.0 is an orthorhombic system with space group Pnna. Ultraviolet-visible diffuse reflectance spectroscopy measurement revealed that the band gap of orthorhombic samples is narrower than that of triclinic compounds. The H₂ evolution was obtained from an aqueous CH₃OH/H₂O solution and pure H₂O with BiTa_1−xNb_xO₄ under ultraviolet irradiation. The orthorhombic samples exhibit much higher activity than that of triclinic compounds. The orthorhombic compound at R = 0.2 showed the highest activity, which is higher than that of the well-known TiO₂ photocatalyst.