Photoinduced Coloration and Photobleaching of Titanium Dioxide in TiO2/Polymer Compositions upon UV- and Visible-Light Excitation of Color Centers' Absorption Bands: Direct Experimental Evi
详细信息 查看全文
- 作者：Vyacheslav N. Kuznetsov ; Nick Serpone
- 刊名：Journal of Physical Chemistry C
- 出版年：2007
- 出版时间：October 25, 2007
- 年：2007
- 卷：111
- 期：42
- 页码：15277 - 15288
- 全文大小：319K
- 年卷期：v.111,no.42(October 25, 2007)
- ISSN：1932-7455
文摘
The present article examines the photocoloration of TiO2/polymer compositions with various polymers andthe 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 principaltypes of photostimulated absorbance changes were observed: (i) increases in absorbance and (ii) decreasesin absorbance. The latter is a direct experimental manifestation of the photobleaching of colored TiO2/polymercompositions, which demonstrates the presence and photoinduced disappearance/destruction of color centersin these TiO2 systems. The spectral results also demonstrate that photobleaching of colored TiO2/polymercompositions originates from intrinsic absorption of light by the TiO2 (at h > 3.2 eV) and also from extrinsicabsorption of light by the color centers at wavelengths corresponding to their absorption spectral bands (i.e.,at h < 3.2 eV). These bands, therefore, are also active in the photodestruction of the color centers. Aphotochemical mechanism is proposed for the photobleaching process involving oxygen-assisted annihilationof oxygen vacancies. The unambiguous experimental data reported herein confirm an earlier proposal thatthe absorption of light by the various TiO2 systems in the visible region originates only from color centersand not from a narrowing of the band gap of pristine TiO2. Unlike the color centers, the valence and conductionbands, which some have suggested as being involved in the observed red shifts of the absorption edges indoped visible-light-active TiO2s because of an apparent narrowing of the band gap of TiO2, cannot bephotodestroyed. The competitive photoinduced formation and destruction of color centers were modeled bysimple considerations, the results of which are in qualitative agreement with experimental observations.