Direct Observation of Interfacial Charge Transfer between Rutile TiO2 and Ultrathin CuOx Film by Visible-Light Illumination and Its Application for Efficient Photocatalysis

A well-defined interface between CuOx film and TiO2 is constructed as a model for elucidating the interfacial charge transfer (IFCT) process that occurs during photocatalysis. Spectroscopic analysis revealed strong blue light absorption at 2.7eV in this film system. Based on photoemission spectroscopy and the UV-visible absorption analysis, the band offset of CuOx and TiO2 was investigated, and the observed blue absorption at 2.7eV could be explained by an IFCT process, in which electrons in the valence band of TiO2 were excited into the conduction band of the ultrathin CuOx film. Furthermore, the IFCT transition was found to strongly depend on the crystal face of TiO2, as the IFCT absorption was significantly increased in the ultrathin CuOx film coated over the rutile TiO2 (110) substrate. Probe reduction and oxidation reactions were also used to investigate the charge transport process and determine photocatalytic reaction sites, which were located around a few nanometers-scale region from the edge of the ultrathin CuOx and TiO2. The present findings provide a powerful method for the development of efficient visible-light sensitive photocatalysts based on the IFCT concept.