Abstract
The application of titanate nanosheets chemically modified by organic ligands to a photocatalyst was examined. Titanate nanosheets have very wide band gap, because of quantum size effect. Ultraviolet light with very short wavelength is required for photocatalysis of titanate nanosheets. In order to circumvent this problem, titanate nanosheets were modified by organic ligands, and the charge transfer from the organic ligands to titanate nanosheets was utilized for photocatalysis. Titanate nanosheets were synthesized by bottom-up process in aqueous solution. Catechol, hydrogen peroxide (H2O2), and 2,5-dihydroxyterephthalic acid (DHTP) were used as an organic ligand. Upon adding organic ligands to titanate nanosheet sols, the ligands were adsorbed preferentially on the edge of nanosheets. The adsorption of ligands enabled the absorption of light with a long wavelength. In the case using catechol and H2O2, the charge transfer from the Highest Occupied Molecular Orbital (HOMO) level of ligands to the conduction band of nanosheets occurred by photo-irradiation. On the other hand, the adsorbed DHTP caused photo-sensitization, that is, the charge transfer from HOMO through Lowest Unoccupied Molecular Orbital of ligands to the conduction band of nanosheets. Furthermore, the photocatalysis of the titanate nanosheets modified by H2O2 or DHTP was investigated by utilizing the reduction of [Ag(NH3)2]+ to metallic Ag nanoparticles. The titanate nanosheets modified by H2O2 did not exhibit photocatalytic reactions, while in the case using DHTP, the formation of Ag nanoparticles by photocatalysis was observed. Probably, this difference was attributed to the rate of the recombination of electrons and holes formed by photo-irradiation, which was likely dependent on the mechanism of the charge transfer caused by photo-irradiation.
