Influence of Reduced Cu Surface States on the Photoelectrochemical Properties of CuBi2O4

The beneficial effect of surface photoreduction on the photoresponse of copper bismuthate is reported for the first time. A detailed photoemission spectroscopy study (PES) reveals that irradiation of CuBi2O4 with h nu >= 2.7 eV in an inert atmosphere leads to the formation of reduced surface Cu. Surface states associated with reduced Cu species induce a 0.35 eV downward band bending, which improves the charge carrier transport properties of this material as judged by a measured increase of the characteristic surface photovoltage (SPV) from 0.07 to 0.27 V. In connection with this PES study, improvements of up to 30% in the photoresponse are observed for CuBi2O4 photocathodes that have been subjected to a visible light irradiation (100 mW/cm(2) white LED) in argon prior to photoelectrochemical performance testing using H2O2 as an electron scavenger. The stability of reduced surface states and associated SPV under relevant reaction conditions has been further studied by near-ambient pressure PES. Results indicate that reduced surface states remain stable in the presence of Ar and methanol, but reoxidation of surface Cu occurs in the presence of oxygen, which decreases the measured SPV. Hence, this work establishes a direct relationship between the presence of reduced Cu at the surface and SPV of CuBi2O4, having important implications on its photovoltaic properties. A similar downward band bending is observed at the interface between CuBi2O4 and a Cu thin film deposited by physical vapor deposition, which further highlights the importance of the Cu/CuBi2O4 buried interface in photoelectrodes.