Linkage Dependence of Interfacial Charge-Transfer Transitions in ZnO: Carboxylate versus Sulfur Linker

Interfacial charge-transfer transitions (ICTTs) between organic compounds and inorganic semiconductors have recently attracted much attention due to the unique features of a wide range of visible light absorption with colorless organic molecules and direct interfacial charge separation for their potential applications in photoenergy conversions and chemical sensing. As the research on ICTT has almost been limited to titanium oxide semiconductors such as TiO₂, the exploration of ICTT in other inorganic semiconductors is a high-priority issue. Recently, we demonstrated that ICTT is strongly induced by chemisorption of aromatic thiols on ZnO nanoparticles via the sulfur atom. Here, we report on ICTT in ZnO nanoparticles adsorbed with benzoic acid derivatives and the linkage dependence of ICTT in ZnO. We observed ICTT bands in the visible region upon adsorption of 4-(dimethylamino)benzoic acid (4-DMABA) and 3,4-dimethoxybenzoic acid (3,4-DMOBA) on ZnO nanoparticles via the carboxylate group. Notably, the ICTT absorption intensities are about 1 order of magnitude lower than those in the ZnO surface complexes with aromatic thiol compounds. Time-dependence density functional theory (TD-DFT) calculations well reproduce the linkage dependence of ICTT. This characteristic linkage dependence of ICTT in ZnO is attributed to the difference in the valence orbital of bridging atoms. The sulfur bridging atom with the larger 3p valence orbitals gives rise to strong electronic couplings between ZnO and adsorbates for ICTT, in contrast to very weak electronic couplings via the smaller 2p valence orbitals of the oxygen bridging atoms in the carboxylate linkage. Our research reveals the important linkage dependence of ICTT in ZnO and elucidates the mechanism.