Interfacial charge-transfer transitions in SnO₂ functionalized with benzoic acid derivatives

Interfacial charge-transfer transitions (ICTTs) between organic compounds and inorganic semiconductors have recently attracted increasing attention for their potential applications in solar energy conversions and chemical sensing due to the unique functions of visible-light absorption with colourless organic molecules and direct charge separation. However, inorganic semiconductors available for ICTT are quite limited to a few kinds of metal-oxide semiconductors (TiO₂, ZnO, etc.). Particularly, the exploration of ICTT in inorganic semiconductors with a lower-energy conduction band such as SnO₂ is an important issue for realizing a wide range of visible-light absorption for organic adsorbates with the deep highest occupied molecular orbital (HOMO) such as benzoic acid derivatives. Here, we report the first observation of ICTT in SnO₂. SnO₂ nanoparticles show a broad absorption band in the visible region by chemisorption of 4-dimethylaminobenzoic acid (4-DMABA) and 4-aminobenzoic acid (4-ABA)) via the carboxylate group. The wavelength range of the ICTT band significantly changes depending on the kind of substituent group. The ionization potential measurement and density functional theory (DFT) analysis reveal that the absorption band is attributed to ICTT from the HOMO of the adsorbed benzoic acid derivatives to the conduction band of SnO₂. In addition, we clarify the mechanism of ICTT in SnO₂ computationally. Our research opens up a way to the fundamental research on ICTT in SnO₂ and applications in solar energy conversions and chemical sensing.