Surface modification of anatase nanoparticles with fused ring catecholate type ligands: a combined DFT and experimental study of optical properties

Surface modification of nanocrystalline TiO2 particles (45 angstrom) with catecholate-type ligands consisting of an extended aromatic ring system, i.e., 2,3-dihydroxynaphthalene and anthrarobin, was found to alter the optical properties of the nanoparticles in a similar way to modification with catechol. The formation of inner-sphere charge-transfer (CT) complexes results in a red shift of the semiconductor absorption compared to unmodified nanocrystallites and the reduction of the band gap upon the increase of the electron delocalization on the inclusion of additional rings. The binding structures were investigated by FTIR spectroscopy. The investigated ligands have the optimal geometry for binding to surface Ti atoms, resulting in ring coordination complexes of catecholate type (binuclear bidentate binding-bridging) thus restoring the six-coordinated octahedral geometry of surface Ti atoms. From the Benesi-Hildebrand plot, stability constants in methanol/water = 90/10 solutions at pH 2 of the order 10(3) M-1 have been determined. Quantum chemical calculations on model systems using density functional theory (DFT) were performed to obtain vibrational frequencies of charge transfer complexes, and the calculated values were compared with the experimental data.