pH-induced hydrothermal synthesis of Bi2WO6 nanoplates with controlled crystal facets for switching bifunctional photocatalytic water oxidation/reduction activity

Bifunctional photocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have attracted growing interest to understand the mechanisms governing different evolution reactions, and the bifunctional activity of a single type of crystalline photocatalyst has gained especial attention. We herein report the high photocatalytic OER and HER activities of Bi2WO6 nanoplates (BWO NPs) which are synthesized by a simple hydrothermal method, and the switchable OER and HER performances controlled by the pH value of the precursor solvent. In the pH range from 4 to 9, the thickness of BWO NPs along the [0 0 1] direction exhibits interesting dependence on the pH value, which decreases as the pH value increases. Correspondingly, the BWO NPs obtained at the pH value of 7 (BWO-7) show the highest photocatalytic OER activity, while the BWO NPs synthesized at the pH value of 9 (BWO-9) exhibit the highest photoactivity towards HER. The electronic band structure analysis indicates that the highest photocatalytic OER activity is related to the band alignment of the valence band maximum of Bi2WO6, which determines the efficient separation of photogenerated electrons and holes as well as the fast charge transfer kinetics. The crystal facet evolution resulting from thickness reduction promotes the exposure of {0 0 1} facets for HER and decreases the exposure of {1 0 0} and {0 1 0} facets for OER. This work provides new insights into the combined effects of crystal facets and electronic band structures on photocatalysis.