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

Abstract

BiVO 4 photocatalysts for O 2 evolution, which work under visible light irradiation, were prepared by an aqueous process. The BiVO 4 photocatalysts were obtained by the reaction of layered potassium vanadate powder (KV 3 O 8 and K 3 V 5 O 14 ) with Bi(NO 3 ) 3 for 3 days in aqueous media at room temperature. Highly crystalline monoclinic and tetragonal BiVO 4 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 BiVO 4 was formed via a tetragonal BiVO 4 intermediate. Tetragonal BiVO 4 with a 2.9 eV band gap mainly possessed an ultraviolet absorption band while monoclinic BiVO 4 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 BiVO 4 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 O 2 evolution from an aqueous silver nitrate solution under UV irradiation (300 < λ < 380 nm) on the tetragonal BiVO 4 was comparable to that on the monoclinic BiVO 4 . The monoclinic BiVO 4 also showed the high photocatalytic activity for the O 2 evolution under visible light irradiation (λ > 420 nm). When the monoclinic BiVO 4 was calcined at 700−800 K the activity was increased. The activity of this monoclinic BiVO 4 was much higher than that of BiVO 4 prepared by a conventional solid-state reaction. The quantum yield at 450 nm for the O 2 evolution on the monoclinic BiVO 4 was 9%.