A novel aqueous process for preparation of crystal form-controlled and highly crystalline BiVO₄ powder from layered vanadates at room temperature and its photocatalytic and photophysical properties

BiVO₄ photocatalysis for O₂ evolution, which work under visible light irradiation, were prepared by an aqueous process. The BiVO₄ photocatalysts were obtained by the reaction of layered potassium vanadate powder (KV₃O₈ and K₃V₅O₁₄) with Bi(NO₃)₃ for 3 days in aqueous media at room temperature. Highly crystalline monoclinic and tetragonal BiVO₄ were selectively synthesized by changing the ratio of vanadium to bismuth in the starting materials. X-ray diffraction and scanning electron microscopy measurements showed that the monoclinic BiVO₄ was formed via a tetragonal BiVO₄ intermediate. Tetragonal BiVO₄ with a 2.9 eV band gap mainly possessed an ultraviolet absorption band while monoclinic BiVO₄ with a 2.4 eV band gap had a characteristic visible light absorption band in addition to the UV band. The UV bands observed in the tetragonal and monoclinic BiVO₄ were assigned to the band transition from O(2p) to V(3d) whereas the Visible light absorption was due to the transition from a valence band formed by Bi(6s) or a hybrid orbital of Bi(6s) and O(2p) to a conduction band of V(3d). The photocatalytic activity for O₂ evolution from an aqueous silver nitrate solution under UV irradiation (300 < λ < 380 nm) on the tetragonal BiVO₄ was comparable to that on the monoclinic BiVO₄. The monoclinic BiVO₄ also showed the high photocatalytic activity for the O₂ evolution under visible light irradiation (λ > 420 nm). When the monoclinic BiVO₄ was calcined at 700-800 K the activity was increased. The activity of this monoclinic BiVO₄ was much higher than that of BiVO₄ prepared by a conventional solid-state reaction. The quantum yield at 450 nm for the O₂ evolution on the monoclinic BiVO₄ was 9%.