Interfacial Charge-Transfer Transitions in Rutile TiO₂ Nanoparticles Functionalized with Benzoic Acid Derivatives

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. Accordingly, ICTTs provide new mechanisms of visible-light absorption and efficient charge separation for colorless organic and inorganic substances to perform photoenergy conversions. TiO2 is the most well-studied semiconductor for ICTTs. However, the research is limited to the anatase crystalline phase. The rutile phase is more thermally stable and useful for versatile applications. Nevertheless, ICTTs in rutile TiO2 have not yet been reported. Here, we demonstrate ICTTs in rutile TiO2 for the first time by using benzoic acid (BA) derivatives as organic ligands. BA derivatives are adsorbed on rutile TiO2 nanoparticles via deprotonation of the carboxy group. In the rutile TiO2 complexes, ICTTs occur from the highest occupied molecular orbital (HOMO) of the adsorbed benzoate derivatives to the conduction band of the rutile TiO2 nanoparticles. The introduction of electron-donating groups to BA systematically red-shifts the absorption onset via the elevation of HOMO depending on the electron-donating ability. Compared with similar anatase TiO2 complexes, the ICTT band is blue-shifted in the rutile TiO2 complexes. This result reveals that the conduction band of the rutile phase is higher in energy (ca. 0.3 eV) than that of the anatase one.