Surface Modification of Colloidal TiO2 Nanoparticles with Bidentate Benzene Derivatives

Surface modification of nanocrystalline TiO2 particles (45 Å) with bidentate benzene derivatives, that is, 2-hydroxybenzoic acid, 2,5-dihydroxybenzoic acid, 2,3-dihydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and catechol was found to alter optical properties of nanoparticles. The formation of the inner-sphere charge transfer (CT) complexes results in a red shift of the semiconductor absorption compared to unmodified nanocrystallites. The binding structures were investigated by using FTIR spectroscopy. The investigated ligands have the optimal geometry for chelating surface Ti atoms, resulting in ring coordination complexes (catecholate or salycilate type of binuclear bidentate binding), thus restoring six-coordinated octahedral geometry of surface Ti atoms. From the Benesi−Hildebrand plot, the stability constants at pH 2 of the order 103 M−1 have been determined.