NATURE OF COLOR-CENTERS IN REDUCED TITANIUM-DIOXIDE

The nature of color centers in reduced TiO2 was studied by optical dif fuse-reflectance spectroscopy in the 3.5-0.5 eV range, thermal-desorpt ion (TD) mass analysis, and temperature-programmed annealing (TPA). Th e absorption in the 3.0-0.5 eV range with a maximum at similar to 1.24 eV observed in an H-2 or CO atmosphere at T>700 K disappeared in the presence of O-2 at 300 K and P > 10(-3) Pa, and the band at 2.8 eV rem ained in the spectrum. If no O-2 was admitted, this band was stable up on heating the sample to 600 K. After exposure to O-2, color centers w ere annealed at 300-550 K. The profile of temperature-programmed annea ling (d Delta rho(lambda)/dT) and TD spectra of chemisorbed oxygen wer e in close agreement and had a common maximum at 400 K. The analysis o f the decay mechanism of the O-2 form (with a peak at 400 K in the TD spectrum) allowed one to assign the band at 2.8 eV to oxygen vacancies in the surface and/or to subsurface oxide layers.