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 (CuOx)/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 (MnOx) 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.
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