Flux synthesis of single crystal bismuth vanadate (BiVO4) nanowires and their visible light driven photocatalytic water oxidation properties†

Abstract

Bismuth vanadate (BiVO₄) is a well-known visible light active photocatalyst for the oxygen evolution reaction (OER). In this study, single-crystal BiVO₄ nanowires were synthesized for the first time. The nanowires are obtained by recrystallization of BiVO₄ microparticles from a NaVO₃ flux in the 550–700 °C temperature range. They exhibit an average thickness of 433.4 ± 110.6 nm and lengths exceeding 20 μm. X-ray diffraction and electron microscopy confirm that the nanowires are single crystals of the monoclinic scheelite structure type with the [010] crystal direction oriented along the principal wire axis. The nanowires have an optical band gap of 2.41 eV and generate a negative surface photovoltage signal under band gap illumination, confirming their n-type character. A nanowire suspension in aqueous ferric nitrate solution generates oxygen under visible light (390 mW cm⁻²) at a rate of 28.75 μmol h⁻¹ and with an apparent quantum efficiency of 0.44% at 405 nm. The relatively low photocatalytic activity of the nanowires can be explained by the absence of a facet-induced charge separation mechanism. Indeed, photolabeling experiments with silver (+) and manganese (2+) ions demonstrate that both photoholes and electrons are extracted along the cylindrical nanowire surface. The fiber morphology makes the BiVO₄ nanowires uniquely suited for the construction of membranes for solar energy conversion and photocatalysis.