School of Physical and Chemical Sciences, Queensland University of Technology, Brisbane, Qld 4001, Australia.
J Am Chem Soc. 2009 Dec 16;131(49):17885-93. doi: 10.1021/ja906774k.
A new efficient photocatalyst structure, a shell of anatase nanocrystals on the fibril core of a single TiO(2)(B) crystal, was obtained via two consecutive partial phase transition processes. In the first stage of the process, titanate nanofibers reacted with dilute acid solution under moderate hydrothermal conditions, yielding the anatase nanocrystals on the fiber. In the subsequent heating process, the fibril core of titanate was converted into a TiO(2)(B) single crystal while the anatase crystals in the shell remained unchanged. The anatase nanocrystals do not attach to the TiO(2)(B) core randomly but coherently with a close crystallographic registry to the core to form a stable phase interface. For instance, (001) planes in anatase and (100) planes of TiO(2)(B) join together to form a stable interface. Such a unique structure has several features that enhance the photocatalytic activity of these fibers. First, the differences in the band edges of the two phases promote migration of the photogenerated holes from anatase shell to the TiO(2)(B) core. Second, the well-matched phase interfaces allow photogenerated electrons and holes to readily migrate across the interfaces because the holes migrate much faster than excited electrons, more holes than electrons migrate to TiO(2)(B) and this reduces the recombination of the photogenerated charges in anatase shell. Third, the surface of the anatase shell has both a strong ability to regenerate surface hydroxyl groups and adsorb O(2), the oxidant of the reaction, to yield reactive hydroxyl radicals (OH(.)) through reaction between photogenerated holes and surface hydroxyl groups. The adsorbed O(2) molecules can capture the excited electrons on the surface, forming reactive O(2)(-) species. The more reactive species generated on the external surface, the higher the photocatalytic activity will be, and generation of the reactive species also contributes to reducing recombination of the photogenerated charges. Indeed, the mixed-phase nanofibers exhibited superior photocatalytic activity for degradation of sulforhodamine B under UV light to the nanofibers of either pure phase alone or mechanical mixtures of the pure phase nanofibers with a similar phase composition. Finally, the nanofibril morphology has an additional advantage that they can be separated readily after reaction for reuse by sedimentation. This is very important because the high cost for separating the catalyst nanocrystals has seriously impeded the applications of TiO(2) photocatalysts on an industrial scale.
一种新的高效光催化剂结构，即锐钛矿纳米晶的壳层包裹在 TiO(2)(B) 单晶体的纤维状核上，通过两个连续的部分相转变过程得到。在该过程的第一阶段，钛酸盐纳米纤维在适度的水热条件下与稀酸溶液反应，在纤维上生成锐钛矿纳米晶。在随后的加热过程中，钛酸盐纤维状核转化为 TiO(2)(B) 单晶体，而壳层中的锐钛矿晶体保持不变。锐钛矿纳米晶不是随机地附着在 TiO(2)(B)核上，而是与核具有紧密的晶体学配准，形成稳定的相界面。例如，锐钛矿的（001）面和 TiO(2)(B)的（100）面结合在一起形成稳定的界面。这种独特的结构具有几个增强这些纤维光催化活性的特征。首先，两个相的能带边缘的差异促进光生空穴从锐钛矿壳层迁移到 TiO(2)(B)核。其次，良好匹配的相界面使得光生电子和空穴可以很容易地在界面迁移，因为空穴迁移速度比激发电子快得多，更多的空穴迁移到 TiO(2)(B)，这减少了锐钛矿壳层中光生电荷的复合。第三，锐钛矿壳层的表面具有很强的再生表面羟基的能力，并通过光生空穴与表面羟基之间的反应吸附反应中的氧化剂 O(2)，生成反应性羟基自由基（OH(.))。吸附的 O(2)分子可以捕获表面上的激发电子，形成反应性 O(2)(-)物种。外部表面上生成的反应性物种越多，光催化活性就越高，生成反应性物种也有助于减少光生电荷的复合。事实上，混合相纳米纤维在紫外光下对磺基罗丹明 B 的降解表现出比单独的纯相纳米纤维或具有相似相组成的纯相纳米纤维机械混合物更高的光催化活性。最后，纳米纤维形态具有另外一个优点，即在反应后可以通过沉降很容易地分离出来以备再利用。这非常重要，因为分离催化剂纳米晶的高成本严重阻碍了 TiO(2)光催化剂在工业规模上的应用。
