The formation of surface point defects in the initial stage of TiO2 reduction (x up to 1012/cm2) was studied by mass spectrometry and ultra-violet photoelectron spectroscopy. Heating to 720K or UV illumination in ultrahigh vacuum creates surface color centers in TiO2 with an energy spectrum extending from the Fermi level to the valence band top. The continuous photoelectron spectrum exhibited a peak at 2.7eV, which varied in a manner correlated with the behavior of the optical absorption bands at 2.55 and 2.81eV assigned to O vacancies in the TiO2 crystal structure. The interaction of the surface point defects with molecular O was studied and a special form of the photo-adsorbed O (1.37eV) was found. It was shown that surface color centers may serve as centers of TiO2 sensitization in the visible spectral range.

Point Defects as the Centers of Titanium Dioxide Sensitization in the Visible Spectral Range. Lisachenko, A.A., Mikhaĭlov, R.V.: Technical Physics Letters, 2005, 31[1], 21-4