Direct imaging of visible-light-induced one-step charge separation at the chromium(iii) oxide–strontium titanate interface

Abstract

Efficient visible-light-active photocatalysts are of great interest for practical applications. First-row transition metal oxide nanoclusters have been loaded onto ultraviolet (UV)-driven semiconductors for several photocatalytic reactions under visible-light illumination. However, the excitation mechanisms, which initiate these reactions, are yet to be understood. Here, using an ultrathin Cr₂O₃ film that was coated on a SrTiO₃ substrate (a Cr₂O₃/SrTiO₃ thin-film system) as a well-defined photocatalyst model, we elucidated a unique interfacial charge transport pathway (i.e., reductant-to-band charge transfer (RBCT)) between these materials. The Kelvin probe force microscopy (KPFM) measurement on the Cr₂O₃ thin film showed a negative shift in the surface potential difference (SPD) during visible-light irradiation. Taking into account the SPD results of the control samples (Cr₂O₃/quartz (SiO₂), Cr₂O₃/alumina (Al₂O₃)/SrTiO₃, and copper oxide (CuO_x)/SrTiO₃), the negative shift was due to the one-step electron excitation from the valence band (VB) of the Cr₂O₃ thin film to the conduction band (CB) of the SrTiO₃ substrate, leaving holes in the thin film. The thickness dependence of the SPD signals of Cr₂O₃ suggested that the distribution of the holes was within 90 nm from the interface. Moreover, the atomic force microscopy (AFM) observation of the photo-deposited gold or manganese oxide (MnO_x) particles indicated that the reduction and oxidation reactions proceeded on the SrTiO₃ substrate and Cr₂O₃ thin film, respectively; both reactions took place laterally at distances of several hundred nanometres from the edge of the thin film. This study provides insights into the development of efficient visible-light-sensitive photocatalysts that are driven by the interfacial charge transfer.