Self-assembled heteroepitaxial AuNPs/SrTiO₃: influence of AuNPs size on SrTiO₃ band gap tuning for visible light-driven photocatalyst

Self-assembled heteroepitaxial offers tremendous opportunity to tailor optical and charge transport properties in noble metal-semiconductor interface. Here, we incorporated gold nanoparticles (AuNPs) onto the {001} facets of semiconductor strontium titanate, SrTiO₃ (STO), by means of heteroepitaxial approach to investigate the band gap tuning and its effect of photoresponse. We demonstrate that the Fermi energy level of the system can be tuned by controlling the AuNPs size. X-ray photoelectron spectroscopy (XPS) shows that the energy difference between Sr_3d and Au_4f core levels measured in the AuNPs/STO (100) heterojunction increases from 47.90 to 49.26 eV with decreasing AuNPs size from 65 to 16 nm, respectively. Hence, the Fermi energy level was shifted toward the conductive band of STO (100), and the system charge transfer efficiency was improved. It was also found that smaller AuNPs sizes exhibited a higher photoactivity as the result of the band gap narrowing effect. Photoactivity was improved by broadening the catalyst absorption spectrum to the visible light region. This study provides a basic understanding of the photoelectrochemistry of metal-semiconductor heterostructure for visible light-energy conversion.