A set of heat-induced and photoinduced absorption spectra of various compositions of Degussa P25 TiO2 and different polymers has been examined. The spectra are described as the sum of overlapping absorption bands (ABs) with maxima at 2.90 eV (427 nm, AB1), 2.55 eV (486 nm, AB2), and 2.05 eV (604 nm, AB3); the spectra correlate entirely with the experimentally observed absorption spectra after the reduction of TiO2. Absorption spectra of visible-light-active TiO2 photocatalysts reported recently in the literature have also been analyzed. Relatively narrow absorption spectra are very similar and independent of the method of photocatalyst preparation. The average absorption spectrum can be described reasonably well by the sum of the two absorption bands AB1 and AB2. It is argued that visible light activation of TiO2 specimens (anion-doped or otherwise) implicates defects associated with oxygen vacancies that give rise to color centers displaying these absorption bands and not to a narrowing of the original band gap of TiO2 (EBG approximately 3.2 eV, anatase) through mixing of dopant and oxygen states, as has been suggested recently in the literature.

This article briefly reviews some factors that have impacted heterogeneous photocatalysis with next generationTiO2photocatalysts, along with some issues of current debate in the fundamental understanding of the science that underpins the field. Preparative methods and some characteristics features of N-dopedTiO2are presented and described briefly. At variance are experimental results and interpretations of X-ray photoelectron spectra (XPS) with regard to assignments of N 1s binding energies in N-dopedTiO2systems. Relative to pristine nominally cleanTiO2with absorption edges at 3.2 eV (anatase) and 3.0 eV (rutile), N-dopedTiO2s display red-shifted absorption edges into the visible spectral region. Several workers have surmised that the(intrinsic) band gapofTiO2is narrowed by coupling dopant energy states with valence band (VB) states, an inference based on DFT computations. With similar DFT computations, others concluded that red-shifted absorption edges originate from the presence of localized intragap dopant states above the upper level of the VB band. Recent analyses of absorption spectral features in the visible region for a large number of dopedTiO2specimens, however, have suggested a common origin owing to the strong similarities of the absorption features, and this regardless of the preparative methods and the nature of the dopants. The next generation of (doped)TiO2photocatalysts should enhance overall process photoefficiencies (in some cases), since dopedTiO2s absorb a greater quantity of solar radiation. The fundamental science that underpins heterogeneous photocatalysis with the next generation of photocatalysts is a rich playing field ripe for further exploration.

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.