Influences of pulverization and annealing treatment on the photocatalytic activity of BiVO4 for oxygen evolution

Abstract

In this study, the photocatalytic activity of small particles of BiVO₄ for O₂ evolution using Fe³⁺ as an electron acceptor was investigated. Characterization by X-ray diffraction and electron microscopy revealed that small particles of BiVO₄ with 20–50 nm of sizes were obtained from micron-sized monoclinic scheelite-type BiVO₄ without changes in the crystal structure by ball milling treatment. The specific surface area increased from 0.7 to 22.6 m² g⁻¹ by the milling treatment. Although the milling treatment decreased the photocatalytic activity, the successive annealing treatment at 400 °C remarkably improved the activity of milled BiVO₄. As a result, the annealed small particles of BiVO₄ achieved a slightly higher external quantum yield than the pristine one, 22.6% versus 20.8%. In contrast, the activity of non-milled BiVO₄ decreased due to the annealing treatment. Transient absorption spectroscopy using femtosecond and nanosecond pump lasers clarified the influences of the ball milling and annealing treatment on the carrier dynamics of BiVO₄. The ball milling treatment accelerated recombination in picosecond and microsecond regions. For example, only 40% of photogenerated holes survived in milled BiVO₄ at 10 ps whereas 60% of holes remained in the non-milled sample. Annealing treatment at 400 °C suppressed the recombination in milled BiVO₄ but facilitated the recombination in the non-milled sample. The small BiVO₄ particles annealed at 400 °C, which exhibited high activity in sacrificial O₂ evolution, also showed high performance in overall water splitting by Z-scheme systems combined with Ru/SrTiO₃:Rh of a H₂-evolving photocatalyst. A small amount of annealed small BiVO₄ particles (5 mg) showed comparable activity in Z-scheme water splitting to the reaction with 30 mg of non-milled sample. In addition, in the Z-scheme water splitting based on interparticle electron transfer, the annealed small BiVO₄ allowed 17% higher activity than the non-milled sample.