Interfacial Charge-Transfer Transitions in BaTiO3 Nanoparticles Adsorbed with Catechol

Interfacial charge-transfer (ICT) transitions between wide band gap inorganic semiconductors such as titanium dioxide (TiO2) and organic materials are characteristic electronic transitions that enable the absorption of lower energy photons than the band gap of inorganic semiconductors and the HOMO-LUMO gap of organic compounds and also direct photocarrier injection to inorganic semiconductors. Despite these features, inorganic semiconductors that show ICT transitions are quite limited. Here we report ICT transitions in barium titanate (BaTiO3) nanoparticles with a pseudocubic structure using 1,2-benzenediol (catechol) that is a representative aromatic bidentate ligand to induce ICT transitions. BaTiO3 nanoparticles are instantaneously colored orange by the addition of the colorless solution of catechol, showing a broad absorption band with an onset at ca. 620 nm. The observed FT-IR spectrum indicates that catechol molecules chemisorb on BaTiO3 surfaces by chelate coordination with a surface Ti atom more predominantly than the bridge coordination with two surface Ti atoms. In the bridge and chelate BaTiO3-catechol complexes, the highest occupied molecular orbital of catechol is located above the valence band of BaTiO3 and coupled to Ti and O atoms electronically. The electronic couplings between catechol and BaTiO3 induce ICT excitations from catechol to BaTiO3 in the visible region, generating the broad ICT band.