Elucidating the electronic structure of CuWO4 thin films for enhanced photoelectrochemical water splitting

Abstract

CuWO₄ is an n-type oxide semiconductor with a bandgap of 2.2 eV which exhibits great potential for photoelectrochemical (PEC) conversion of solar energy into chemical fuels. However, the photocurrent achieved so far is limited to ∼0.3 mA cm⁻² at +1.23 V vs. reversible hydrogen electrode (RHE). Possible limiting factors include slow surface reaction kinetics, poor charge carrier mobility and/or presence of surface defect states. A detailed understanding of the fundamental electronic structure and its correlation with PEC activity is of significant importance for devising strategies for further improvements. In this work, we have synthesized CuWO₄ thin films showing a record photocurrent density of 0.50 mA cm⁻² at +1.23 V vs. RHE. Importantly, we have used a synergistic combination of photoemission spectroscopy, X-ray absorption spectroscopy and density functional theory (DFT) to unravel the electronic structure of CuWO₄. Our results show that the valence band (VB) consists of strongly hybridized states of O 2p⁶ and Cu 3d⁹, while the bottom of the conduction band (CB) is primarily composed of unoccupied Cu 3d states. The localized nature of the Cu 3d state leads to the low charge carrier mobility and the localization of the photo-excited electrons to the CB. The combined experimental and theoretical results also indicate that CuWO₄ is better described as having a direct but d–d forbidden optical bandgap, leading to a low absorption coefficient for visible light. Furthermore, the implication of the electronic structure on its PEC characteristics and strategies for further improvements by adding Co₃O₄ as a co-catalyst or surface layer to increase the interfacial band bending to facilitate photo-carriers transport, are discussed.