Interfacial Charge-Transfer Transitions in TiO2 Nanoparticles Adsorbed with 4-Mercaptobenzenoic Acid: Carboxy versus Thiol Anchor and Adsorbate-to-TiO2 versus TiO2-to-Adsorbate Charge Transfer

Abstract

Interfacial charge-transfer transitions (ICTTs) in surface coordination complexes of organic compounds with inorganic semiconductors enable the absorption of visible light with colorless organic molecules and direct photoinduced charge separation for applications in photoenergy conversions and biosensing. The surface complex of 4-mercaptobenzenoic acid (4-MBA) with TiO2 nanoparticles has attracted much attention for the surface-enhanced Raman scattering (SERS). However, the adsorption and electronic structures and optical properties of the TiO2–4-MBA surface complex remain to be clarified. Here, we examine the TiO2–4-MBA surface complex in detail experimentally and computationally. Upon adsorption of 4-MBA, TiO2 nanoparticles show a broad absorption band in the visible region. The IR vibrational analysis clearly indicates that 4-MBA adsorbs on TiO2 via the carboxylate group, contrary to the reported thiol linkage. Ionization potential measurements and time-dependent density functional theory (TD-DFT) calculations reveal that the absorption band is attributed to ICTT from the highest occupied molecular orbital (HOMO) of the adsorbed 4-MBA to the conduction band of TiO2, contrary to the reported TiO2-to-adsorbate photoinduced charge-transfer scheme. Our research clarifies the predominant adsorption mode of 4-MBA via the carboxylate linkage and organic-to-inorganic ICTT and suggests that the reported SERS originates from the ICTT.