Tungstic acids H2WO4 and H4WO5 as stable photocatalysts for water oxidation under visible light

Abstract

The development of new photocatalytic materials that can stably reduce and/or oxidize water by harvesting a wider range of visible light is indispensable to achieve high practical efficiency in artificial photosynthesis. Although we have recently found that tungstic acid monohydrate (H₂WO₄) can stably oxidize water to O₂ by harvesting a wider range of visible light (∼500 nm) than WO₃ (∼460 nm), the origin of its narrower bandgap has not been clarified. Herein, we studied H₂WO₄ in detail, along with the analogous tungstic acid dihydrate (H₄WO₅), especially focusing on their band structures and their photocatalytic activities for water oxidation in the presence of different electron acceptors (Ag⁺ and Fe³⁺). It was revealed that the conduction band minimum (CBM) of H₂WO₄ is considerably lower than that of conventional monoclinic WO₃, whereas both the band gap and level of H₄WO₅ are similar to those of WO₃. The lower CBM of H₂WO₄ can probably be explained by a more dispersed conduction band caused by greater orbital overlap between W-5d and O-2p orbitals than in monoclinic WO₃. Both tungstic acids stably generate O₂ from aqueous solutions containing Fe³⁺ corresponding to each of their photoabsorption properties (∼460 and ∼500 nm), while they showed negligible activity from sacrificial electron acceptor Ag⁺. Visible-light-driven Z-scheme water splitting proceeded using both tungstic acids as O₂-evolving photocatalysts.