Preparation of fine particles of sheelite-monoclinic phase BiVO4via an aqueous chelating method for efficient photocatalytic oxygen evolution under visible-light irradiation

Abstract

In this paper, we introduce a new synthesis method to prepare fine particles of BiVO₄ with a scheelite-monoclinic (s-m) phase, which is known as the most favorable crystal phase for photocatalytic water oxidation (O₂ evolution) under visible light irradiation, based on a coordination chemistry approach in water. Stable aqueous solutions that contain both Bi³⁺ and V⁵⁺ complexes were prepared by simply mixing two aqueous solutions in which each cation was stabilized with an appropriate chelating agent. The use of chelating agents (glycolic acid (gly), L(+)-tartaric acid (tart), citric acid (cit), or ethylenediamine tetraacetic acid (edta)) was effective to form stable V⁵⁺ complexes from NH₄VO₃. On the other hand, only the use of two equivalents of edta with Bi(NO₃)₃·5H₂O was effective to stabilize the Bi³⁺ complex in water, while the use of other ligands resulted in precipitations. Evaporation of the aqueous solution containing the stable Bi³⁺ and V⁵⁺ complexes and subsequent calcination in air at 500 °C yielded s-m BiVO₄ particles smaller than 300 nm, which were much smaller than BiVO₄ particles prepared via conventional solid-state reactions (1–10 μm). In particular, the BiVO₄ particles that were prepared with the tart ligand for V⁵⁺ stabilization possessed the smallest size (∼80 nm) and exhibited the highest photocatalytic activity for O₂ evolution from an aqueous solution containing an electron acceptor (Ag⁺ or Fe³⁺) under visible-light irradiation. These results strongly suggested that the tart ligand effectively suppresses particle growth during the crystallization process and thereby affords small BiVO₄ particles with high crystallinity, both of which are necessary to achieve highly efficient photocatalysis.