Crystal transformation of 2D tungstic acid H₂WO₄ to WO₃ for enhanced photocatalytic water oxidation

Abstract

New photocatalytic materials for stable reduction and/or oxidization of water by harvesting a wider range of visible light are indispensable to achieve high practical efficiency in artificial photosynthesis. In this work, we prepared 2D WO₃·H₂O and WO₃ nanosheets by a one-pot hydrothermal method and sequent calcination, focusing on the effects of crystal transformation on band structure and photocatalytic performance for photocatalytic water oxidation in the presence of electron acceptors (Ag⁺) under simulated solar light irradiation. The as-prepared WO₃ nanosheets exhibit enhanced rate of photocatalytic water oxidation, which is 6.3 and 3.6 times higher than that of WO₃·H₂O nanosheets and commercial WO₃, respectively. It is demonstrated that the releasing of water molecules in the crystal phase of tungstic acid results in transformation of the crystal phase from orthorhombic WO₃·H₂O to monoclinic WO₃, significantly improving the activity of photocatalytic water oxidation in the presence of Ag⁺ because the shift-up of conduction band of WO₃ matches well with the electrode potential of Ag⁺/Ag(s), leading to efficient separation of photoinduced electrons and holes in pure WO₃ nanosheets.