表面相结结构在促进光催化电荷分离中的应用

二十世纪八十年代以来,特别是近十年,光催化研究在利用可再生能源太阳能的道路上飞速发展。越来越多的研究表明,相结结构的构筑是有效提高半导体光催化剂性能的重要策略。其中, TiO2作为重要的模型光催化剂,其相关研究成果呈现出指数增长的趋势。本综述围绕TiO2模型光催化剂,主要介绍TiO2表面相结的研究成果,包括TiO2表面相的表征、锐钛矿：金红石TiO2相结用于光催化产氢研究、TiO2相结在光催化中作用的最新认识等。在表征方面,通过表面灵敏的紫外拉曼光谱研究了TiO2相变过程中表面相结构的变化,结合可见拉曼以及XRD表征揭示了TiO2独特的相变过程,即相变始于锐钛矿粒子的界面处,小粒子逐渐团聚为大粒子,致其相变从大粒子体相开始最终扩展到整个粒子。使用CO, CO2探针红外光谱,根据锐钛矿和金红石表面吸附物种的差异,进一步证实了锐钛矿：金红石表面相结结构,为紫外拉曼光谱的表面表征特性提供坚实证据。同时,利用发光光谱观察到锐钛矿晶相的可见发光带和金红石晶相的近红外发光带,并基于此给出了TiO2材料表面相结结构的荧光表征新方法。此外荧光光谱还提供了锐钛矿、金红石相中载流子动力学信息,揭示了束缚态在光催化中的作用。在光催化应用方面,观察到混相结构TiO2较单独锐钛矿及金红石相具有更高的光催化产氢活性,通过在较大金红石颗粒上担载纳米锐钛矿粒子,证明了相结结构在提高光催化活性中的核心作用,并首次提出了锐钛矿：金红石表面异相结结构概念,推断其对电荷分离的促进作用是最终提高反应活性的原因。之后将此概念应用到改善商品TiO2(P25)光催化活性中,通过可控热处理精细调控P25的表面相结构,在光催化重整生物质衍生物产氢实验中,成功将P25光催化产氢活性提高3?5倍。之后发展了新的TiO2表面控制方法,通过加入Na2SO4等相变控制剂,延缓了TiO2从锐钛矿向金红石的相变过程,在较高温度下实现TiO2相结结构的调控,最终可将P25光催化重整甲醇制氢的活性提高6倍,同时通过高分辨电镜清晰观察到锐钛矿：金红石相结的原子层生长接触。在相结作用机理方面,多种时间分辨光谱技术以及理论计算被用作探索锐钛矿：金红石相结处的电子转移机理。通过时间分辨红外光谱对TiO2表面相结结构作用的研究,特别是利用锐钛矿、金红石不同的瞬态吸收光谱特征,证明了锐钛矿：金红石相结处的载流子转移过程,存在锐钛矿向金红石的电子转移过程。模型光催化剂TiO2相结的研究成果,加深了对光催化机理的认识,促进新型高效光催化体系的设计合成。

Charge separation promoted by phase junctions in photocatalysts

Since the 1980s, photocatalysis research has expanded at an unexpected rate. Fabrication of phase junctions has proved to be an effective method to enhance photocatalytic performance. As a model photocatalyst, titanium dioxide (TiO2) has been extensively studied. This feature article mainly reviews the study on TiO2 surface phase junctions, including the characterization of the surface phases of TiO2, the use of anatase:rutile TiO2 phase junctions in photocatalytic hydrogen produc-tion, and the current understanding of how TiO2 phase junctions work in photocatalysis. The surface structure of TiO2 can be well characterized by ultraviolet (UV) Raman spectroscopy, unlike X-ray diffraction and visible Raman spectroscopy. Based on these results, the mechanism of phase trans-formation processes of TiO2 was clearly identified. The infrared (IR) spectra of probe molecules CO and CO2 on TiO2 further characterized the surface structure of TiO2, strongly supporting the UV Raman results. Furthermore, the typical visible emission of anatase and near-infrared emission of the rutile phase of TiO2 make photoluminescence (PL) a suitable technique to characterize the sur-face phase structure of TiO2. PL can also provide information about the carrier dynamics of TiO2 in photocatalysis. Because of the surface phase junction formed between anatase and rutile, the mixed-phase structure of TiO2 exhibits a superior H2 production activity to those of pure anatase or rutile phase. The activity of Degussa P25 TiO2 can be further increased by three to five times by tuning the phase structure through thermal treatment. Moreover, the phase transformation of TiO2 from anatase to rutile can be controlled by surface modification with Na2SO4, resulting in catalysts with activity six times higher than that of P25. High-resolution transmission electron microscopy provided a clear phase-junction image of TiO2, which showed atomic contact at the interface of the phase junction. The mechanism of phase junctions improving photocatalytic performance was in-vestigated by time-resolved spectroscopic techniques. The charge transfer process across the ana-tase: rutile phase junction was confirmed by the results of time-resolved IR measurements, and electron transfer from anatase to rutile phases is proposed to occur in mixed-phase TiO2. These studies on the phase junctions of TiO2 improve our understanding of photocatalysis and may inspire new ideas for the design of promising photocatalytic systems.