水在二氧化钛表面的光反应机理研究进展

Advances in mechanisms of water photodissociation on TiO surfaces

利用太阳能光解水生成氢气一直是光催化领域的研究热点之一.在过去的几十年,利用二氧化钛来实现光催化分解水产氢被广泛研究.但是,由于体系的复杂性,水在二氧化钛表面的光解离反应机制至今仍有很多基本问题尚未得到解释.因此,从微观层面上理解水在二氧化钛表面的光解离机制对于能源化学和光催化而言均具有重要意义.本文系统综述了水在二氧化钛单晶表面光解离机理的最新研究进展,分析了影响光催化水解离效率的因素(如表面结构、分子间氢键、光子能量等),并对光催化模型进行了开放性讨论,指出了电荷/能量转移以及电荷载体与吸附质的相互作用在水光解离过程中扮演的关键作用,希望能为开发更高效的光催化剂提供线索.

Solar H2O splitting to H2 and O2 is one of the most interesting ways to achieve clean and renewable energy and remains one of holy grails in catalysis. Since the pioneer work about photoelectrochemical H2O splitting into H2 and O2 on Pt and TiO2 electrodes was reported by Fujishima and Honda in 1972, the photochemistry of H2O on TiO2 surfaces has been extensively investigated by various techniques during the past decades because of its potential applications in energy chemistry.However, due to its complexity, an unambiguous understanding of elementary processes that underlie the H2O splitting reaction on TiO2 surfaces is still lacking. Thus, it is of great value to get an insightful understanding of the mechanisms of H2O photochemistry on TiO2 surfaces at a molecular level. In this review, we highlight the recent progress that provides fundamental insight into H2O photochemistry on the TiO2 model surfaces using surface science techniques. First of all, the structures of TiO2 single crystal surfaces and the adsorption behaviors of H2O on these surfaces are discussed, which will affect the photochemistry of H2O on the surfaces. Nearly no H2O dissociation occurs at the regular Ti4+sites of the TiO2 surfaces. Next, the details of H2O photochemistry on the TiO2 surfaces are presented, and the roles of surface structures,intermolecular hydrogen bond and photon energy in H2O photochemistry are discussed. The mechanisms of H2O photochemistry on different TiO2 surfaces are similar, which occurs via the ejection of OH radical into vacuum, leaving behind the remaining H atom on the adjacent Obsite. Hydrogen bond is believed to play a double-edged sword role in H2O dissociation: The single hydrogen bond between H2O molecules can promote H2O dissociation efficiently, but onedimensional hydrogen bonding chain hinders the dissociation of H2O significantly. Moreover, the arrangement of H2O changes dramatically on different TiO2 surfaces, which can further affect the dissociation probability of H2O. In addition,the dissociation probability of H2O on rutile TiO2(110) is found to be highly dependent on photon energy, which raises doubt about the widely accepted photocatalysis model in which the excess energy of charge carriers is nearly useless for photocatalysis. Therefore, a sophisticated photocatalysis model that can perfectly incorporate the effect of charge carrier energy and the interaction between adsorbates(or intermediates) and TiO2 surfaces should be developed. Afterwards, an open discussion on a new photocatalysis model is presented, which highlights the energy of charge carriers and the interaction between charge carriers and adsorbates in H2O photochemistry on TiO2. This will deepen our understandings of fundamental processes in photocatalysis and provide clues for the development of more efficient photocatalysis. In the end,the challenges and opportunities of the mechanistic studies of TiO2 photocatalysis at a molecular level are discussed briefly,which may guide the development of TiO2 photocatalysis in the future.