Interfacial Charge-Transfer Transitions in Phenol-Functionalized SrTiO₃ for Highly Reducible Visible-Light Photocatalysts

Interfacial charge-transfer transitions (ICTTs) between organic compounds and inorganic semiconductors enable the absorption of visible light and direct charge separation regardless of the optical properties of the organic and inorganic components. These features provide new possibilities for colorless organic and inorganic substances to function as photoenergy conversion materials. Anatase TiO2 is the most well-studied semiconductor for ICTTs. However, the photoreducing power of anatase TiO2 is not enough for such photocatalytic reactions as reduction reactions of CO2 and H2O owing to the lower-energy conduction band. SrTiO3 is a useful semiconductor for such photocatalytic reactions due to the higher-energy conduction band than anatase TiO2. Nevertheless, the research of ICTTs in SrTiO3 is still very limited. Here, we report ICTTs between SrTiO3 nanoparticles and phenolic compounds. Phenol is adsorbed on SrTiO3 via the deprotonation of the hydroxy group. The SrTiO3–phenol complex shows ICTTs from the highest occupied molecular orbital (HOMO) of the phenolate adsorbate to the conduction band of SrTiO3 in the visible region. The absorption range can be expanded to longer wavelengths by chemical modification of phenol with electron-donating groups. The ICTT absorption onset in the SrTiO3 complexes is ca. 0.4 eV above that in analogous anatase TiO2 complexes, which indicates the direct electron injection into the higher-energy conduction band of SrTiO3. Our research provides important knowledge for the development of highly reducible visible-light photocatalysts.