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
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
复制标题
DOI：
10.1021/jp073511h
复制
发表时间：
2007-10-25
影响因子：
3.7
通讯作者：
Serpone, Nick
中科院分区：
文献类型：
--
作者：
Kuznetsov, Vyacheslav N.;Serpone, Nick
本文研究了TiO 2/聚合物组合物与各种聚合物的光致变色和色心在选定的照射波长从UV区域到近红外区域的光漂白。通过照射到在2.90 eV（427 nm，AB 1）、2.55 eV（486 nm，AB 2）和2.05 eV（604 nm，AB 3）处具有最大值的吸收带（AB）中来检查色心的光活化。观察到两种主要类型的光刺激吸光度变化：（i）吸光度增加和（ii）吸光度降低。后者是着色的TiO 2/聚合物组合物的光漂白的直接实验表现，其证明了这些TiO 2系统中色心的存在和光诱导的消失/破坏。光谱结果还表明，着色的TiO 2/聚合物组合物的光漂白源自TiO 2对光的固有吸收（在hv > 3.2eV处）以及色心在对应于其吸收光谱带的波长处对光的非固有吸收（即，HV <3.2eV）。因此，这些带在色心的光破坏中也是活跃的。光化学机制提出的光漂白过程涉及氧辅助湮没的氧空位。本文报道的明确的实验数据证实了早期的建议，即在可见光区的各种TiO 2系统的光吸收仅源于色心，而不是从原始TiO 2的带隙变窄。与色心不同的是，价带和导带不能被光破坏，有些人认为这与掺杂的可见光活性TiO（2）s的吸收边的红移有关，因为TiO 2的带隙明显变窄。通过简单的考虑，模拟了色心的竞争性光诱导形成和破坏，其结果与实验观察定性一致。
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.