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