On the Origin of the Spectral Bands in the Visible Absorption Spectra of Visible-Light-Active TiO2 Specimens Analysis and Assignments

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 TiO2 photocatalysts or second-generation photocatalysts. We have deduced that at energies of hν 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 TiO2 that accompanies the process of TiO2 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 TiO2 during the doping process were examined to argue on the dominant role of F -type color centers in the visible-light-activity of TiO2 photocatalysts.