Is the band gap of pristine TiO2 narrowed by anion- and cation-doping of titanium dioxide in second-generation photocatalysts?

Second-generation TiO2-D-x(x) photocatalysts doped with either anions (N, C, and S mostly) or cations have recently been shown to have their absorption edge red-shifted to lower energies (longer wavelengths), thus enhancing photonic efficiencies of photoassisted surface redox reactions. Some of the studies have proposed that this red-shift is caused by a narrowing of the band gap of pristine TiO2 (e.g., anatase, E-bg = 3.2 eV; absorption edge ca. 387 nm), while others have suggested the appearance of intragap localized states of the dopants. By contrast, a recent study by Kuznetsov and Serpone (J. Phys. Chem. B, in press) has proposed that the commonality in all these doped titanias rests with formation of oxygen vacancies and the advent of color centers (e.g., F, F+, F++, and Ti3+) that absorb the visible light radiation. This article reexamines the various claims and argues that the red-shift of the absorption edge is in fact due to formation of the color centers, and that while band gap narrowing is not an unknown occurrence in semiconductor physics it does necessitate heavy doping of the metal oxide semiconductor, thereby producing materials that may have completely different chemical compositions from that of TiO2 with totally different band gap electronic structures.