[The surface states and the electron-hole pair recombination of TiO2 nanopowders]

The nanometer TiO2 was prepared by sol-gel method. The structure and composition of the nanometer TiO2 were analyzed in detail by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectrometry (FTIR). The UV-visible spectrum and fluorescence spectrum were studied too. The results show that the nanometer TiO2 contains better anatase crystallinity, and the average size of the nanometer TiO2 is about 10 nm. The surface states, Ti-OH and Ti-H2O, are formed on the surface of nanometer TiO2 in aqueous solution because of high absorption ability, whose energy levels are about 0.6 and 0. 54 eV above the valence band, respectively. The surface state Ti-H2O is destroyed after heat treatment at 500 DC for two hours, but the Ti-OH still exists, and the crystalline lattice presents some oxygen vacancies whose energy level is about 3.13 eV above valence band. For the nanometer TiO2 aqueous solution, the surface recombination of electron-hole pairs via Ti-OH and Ti-H2O is the main recombination manner, but for the samples after heat treatment, direct recombination from conduction band to valence band is the main recombination manner, due to the destroying of surface states, with the companion of indirect recombination via oxygen vacancy and surface recombination via Ti-OH.