Photoinduced coloration and photobleaching of titanium dioxide in TiO2/Polymer compositions upon UV- and visible-light excitation of color centers' absorption bands:: Direct experimental evidence negating band-gap narrowing in Anion-/Cation-Doped TiO2s

The present article examines the photocoloration of TiO2/Polymer compositions with various polymers and the photobleaching of color centers at selected irradiation wavelengths from the UV region to the near infrared region. Photoactivation of color centers was examined by irradiating into the 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). Two principal types of photostimulated absorbance changes were observed: (i) increases in absorbance and (ii) decreases in absorbance. The latter is a direct experimental manifestation of the photobleaching of colored TiO2/Polymer compositions, which demonstrates the presence and photoinduced disappearance/destruction of color centers in these TiO2 systems. The spectral results also demonstrate that photobleaching of colored TiO2/polymer compositions originates from intrinsic absorption of light by the TiO2 (at hv > 3.2 eV) and also from extrinsic absorption of light by the color centers at wavelengths corresponding to their absorption spectral bands (i.e., at hv < 3.2 eV). These bands, therefore, are also active in the photodestruction of the color centers. A photochemical mechanism is proposed for the photobleaching process involving oxygen-assisted annihilation of oxygen vacancies. The unambiguous experimental data reported herein confirm an earlier proposal that the absorption of light by the various TiO2 Systems in the visible region originates only from color centers and not from a narrowing of the band gap of pristine TiO2. Unlike the color centers, the valence and conduction bands, which some have suggested as being involved in the observed red shifts of the absorption edges in doped visible-light-active TiO(2)s because of an apparent narrowing of the band gap of TiO2, cannot be photodestroyed. The competitive photoinduced formation and destruction of color centers were modeled by simple considerations, the results of which are in qualitative agreement with experimental observations.