光催化的氟效应

Effects of fluorine on photocatalysis

半导体光催化因有望可持续地解决日益严峻的环境与能源问题而得到国内外学者的广泛关注.但是,以TiO2为代表的半导体光催化材料,存在光响应范围窄和光生载流子容易复合的问题,导致其光催化效率不高.为了提高半导体光催化效率,科学家们采取了许多策略对本征半导体光催化剂进行修饰改性,如表面敏化、贵金属沉积、元素掺杂和半导体复合等,以拓展光吸收范围和促进光生载流子分离.近来,高能面TiO2纳米晶的报道为高性能半导体光催化材料的设计提供了新的思路.在所有对TiO2进行修饰改性的元素里面,氟因其独特的性能而对TiO2光催化产生了深远影响:(1)(在酸性溶液里面)氟离子与TiO2强烈的配位作用(化学吸附)会改变TiO2光催化材料表面的化学结构,生成氟化钛(≡Ti-F),进而影响污染物在催化剂表面的吸附(反应式(1));≡Ti-OH+H++F-→≡Ti-F+H2O(1)(2)吸附在TiO2表面的氟离子,很难被空穴氧化成氟自由基(EθF·/F-=3.6 V vsNHE),只能将溶剂水氧化成羟基自由基(·OH).与本征TiO2空穴氧化产生的表面吸附态羟基自由基(·OHbounded)不同(反应式(2)),表面氟修饰后空穴氧化溶剂水产生的羟基自由基(反应式(3)),是可以脱离催化剂表面(在溶液中自由扩散的),也就是游离态羟基自由基(·OHfree).≡Ti-OH+h+→≡Ti····OH (·OHbounded)(2)≡Ti-F+H2O+h+→≡Ti-F+·OHfree+H+(3)本文主要从以下几个方面综述了氟效应对半导体光催化的影响:(1)TiO2光催化的表面氟效应,包括游离自由基效应、表面电子结构效应和电子清除剂效应;(2)TiO2光催化的氟掺杂效应,包括氟离子掺杂、氟离子与非金属离子共掺杂、以及氟离子与金属离子共掺杂;(3)TiO2的氟离子形貌控制效应,包括空心结构TiO2、高能面TiO2和介晶TiO2.此外,我们也将氟效应拓展到其它非TiO2半导体上,包括Bi2WO4, BiPO4, Fe2O3, SrTiO3和g-C3N4.本文还总结了氟效应在半导体光催化领域的应用,包括(1)化学品的光催化选择性氧化;(2)污染物的光催化选择性降解;(3)光催化分解水产氢;(4)光催化还原二氧化碳;(5)制备高热稳定性TiO2.最后总结了氟效应的优缺点,并对氟效应的前景进行了展望.现在仪器表征技术(特别是原位表征技术)的快速发展,为半导体光催化氟效应的深入研究提供了新的武器.将氟效应与其它方式的耦合,如表面等离子体效应、晶体缺陷和单原子催化,来进一步提高半导体光催化性能,是未来氟效应研究的发展方向.

Tailoring the microstructure of pristine TiO2 is essential to narrow its band gap and prolong the charge lifetime. In particular, strategies involving fluorine have been used successfully to tune the surface chemistry, electronic structure, and morphology of TiO2 photocatalysts to improve their photocatalytic activity based on the strong complexation between fluoride ions and TiO2 and the high electronegativity of fluorine. In this review, we summarize the strategies involving fluorine to establish highly efficient TiO2 photocatalytic systems or fabricate highly efficient TiO2 photocatalysts. The main fluorine effects(i.e. the effects of fluorine on photocatalysis) include the following four aspects:(1) Surface effects of fluoride on TiO2 photocatalysis,(2) effects of fluorine doping on TiO2 photocatalysis,(3) fluoride-mediated tailoring of the morphology of TiO2 photocatalysts, and(4) the effects of fluorine on non-TiO2 photocatalysis. Additionally, the unique applications of these fluorine effects in photocatalysis, including selective degradation of pollutants, selective oxidation of chemicals, water-splitting to produce H2, reduction of CO2 to produce solar fuels, and improvement of the thermostability of TiO2 photocatalysts, are reviewed.