Visible light absorption by various titanium dioxide specimens.

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.