This article presents a systematic analysis of the absorption spectral features of various titanium dioxide specimens, whether doped or undoped, in the visible spectral domain reported extensively in the literature; also it briefly examines the origins of such bands in what have become known as visible-light-active TiO 2 photocatalysts or second-generation photocatalysts. We have deduced that at energies of h ν less than 2.0 eV (λ > ∼600 nm) the three spectral features occurring in the near-infrared and infrared regions originate from Ti n + -related ( n = 3, 2) color centers, whereas the three absorption bands seen in the visible spectral region AB1 (2.75−2.95 eV), AB2 (2.50−2.55 eV), and AB3 (2.00−2.30 eV) are associated with oxygen vacancies ( F -type color centers) on the basis of recently demonstrated experimental observations. The question also discussed is why reduction of TiO 2 that accompanies the process of TiO 2 doping results only in the formation of the absorption bands AB1, AB2, and in some cases AB3; that is, the absorption features of Ti-related centers are totally suppressed. Recent studies that have demonstrated the reduction of TiO 2 during the doping process were examined to argue on the dominant role of F -type color centers in the visible-light-activity of TiO 2 photocatalysts.
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Кузнецов et al. (2009) studied this question.