<p indent=0mm>Solar H<sub>2</sub>O splitting to H<sub>2</sub> and O<sub>2</sub> 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 H<sub>2</sub>O splitting into H<sub>2</sub> and O<sub>2</sub> on Pt and TiO<sub>2</sub> electrodes was reported by Fujishima and Honda in 1972, the photochemistry of H<sub>2</sub>O on TiO<sub>2</sub> 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 H<sub>2</sub>O splitting reaction on TiO<sub>2</sub> surfaces is still lacking. Thus, it is of great value to get an insightful understanding of the mechanisms of H<sub>2</sub>O photochemistry on TiO<sub>2</sub> surfaces at a molecular level. In this review, we highlight the recent progress that provides fundamental insight into H<sub>2</sub>O photochemistry on the TiO<sub>2</sub> model surfaces using surface science techniques. First of all, the structures of TiO<sub>2</sub> single crystal surfaces and the adsorption behaviors of H<sub>2</sub>O on these surfaces are discussed, which will affect the photochemistry of H<sub>2</sub>O on the surfaces. Nearly no H<sub>2</sub>O dissociation occurs at the regular Ti<sup>4+</sup> sites of the TiO<sub>2</sub> surfaces. Next, the details of H<sub>2</sub>O photochemistry on the TiO<sub>2</sub> surfaces are presented, and the roles of surface structures, intermolecular hydrogen bond and photon energy in H<sub>2</sub>O photochemistry are discussed. The mechanisms of H<sub>2</sub>O photochemistry on different TiO<sub>2</sub> surfaces are similar, which occurs via the ejection of OH radical into vacuum, leaving behind the remaining H atom on the adjacent O<sub>b</sub> site. Hydrogen bond is believed to play a double-edged sword role in H<sub>2</sub>O dissociation: The single hydrogen bond between H<sub>2</sub>O molecules can promote H<sub>2</sub>O dissociation efficiently, but one-dimensional hydrogen bonding chain hinders the dissociation of H<sub>2</sub>O significantly. Moreover, the arrangement of H<sub>2</sub>O changes dramatically on different TiO<sub>2</sub> surfaces, which can further affect the dissociation probability of H<sub>2</sub>O. In addition, the dissociation probability of H<sub>2</sub>O on rutile TiO<sub>2</sub>(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 TiO<sub>2</sub> 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 H<sub>2</sub>O photochemistry on TiO<sub>2</sub>. 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 TiO<sub>2</sub> photocatalysis at a molecular level are discussed briefly, which may guide the development of TiO<sub>2</sub> photocatalysis in the future.
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Apr 27, 2021
Younesse Ait Elmachkouri + 8
Open Access
