In situ Monitoring Copper-based Oxide/Water Interfacial Reactions by X-ray Photoelectron Spectroscopy*

Current research in material sciences is continuously trying to enhance the efficiency of photoelectrochemical (PEC) cells using various semiconductor photoelectrodes for water splitting. Such photoelectrodes should have good stability in aqueous media with suitable band-edge and band-gap energies to match both the potential for water oxidation-reduction reactions and the solar spectrum, respectively. Copper-based oxide semiconductors are candidates showing a great promise to fulfill these criteria, but little is known about the interfacial properties of these compounds with H₂O under operational conditions. Therefore, knowledge of interfacial reactions on their surface under realistic conditions is essential to improve our understanding of water-splitting mechanisms, as well as to increase the stability and efficacy of PEC devices. To perform chemical characterization of the interfaces under reaction conditions, we used an ambient pressure X-ray photoelectron spectroscopy. In this study, thin films of CuFeO₂ and CuFe_1−xGa_xO₂ composites were exposure to H₂O. Water interactions with the Cu-based oxide surface and the electronic structures of the surface atoms were evaluated from the Cu 2p, Fe 2p, C 1s and O 1s photoemission spectra to identify surface species newly formed.