Photocatalysis has been widely applied in various areas, such as solar cells, water splitting, and pollutant degradation. Therefore, the photochemical mechanisms and basic principles of photocatalysis, especially TiO2 photocatalysis, have been extensively investigated by various surface science methods in the last decade, aiming to provide important information for TiO2 photocatalysis under real environmental conditions. Recent progress that provides fundamental insights into TiO2 photocatalysis at a molecular level is highlighted. Insights into the structures of TiO2 and the basic principles of TiO2 photocatalysis are discussed first, which provides the basic concepts of TiO2 photocatalysis. Following this, details of the photochemistry of three important molecules (oxygen, water, methanol) on the model TiO2 surfaces are presented, in an attempt to unravel the relationship between charge/energy transfer and bond breaking/forming in TiO2 photocatalysis. Lastly, challenges and opportunities of the mechanistic studies of TiO2 photocatalysis at the molecular level are discussed briefly, as well as possible photocatalysis models.

ABSTRACT:We have investigated the photocatalysis of partially deuterated methanol (CD 3 OH) and H 2 O on TiO 2 (110) at 400 nm using a newly developed photocatalysis apparatus in combination with theoretical calculations. Photocatalyzed products, CD 2 O on Ti 5c sites, and H and D atoms on bridge-bonded oxygen (BBO) sites from CD 3 OH have been clearly detected, while no evidence of H 2 O photocatalysis was found. The experimental results show that dissociation of CD 3 OH on TiO 2 (110) occurs in a stepwise manner in which the O−H dissociation proceeds first and is then followed by C−D dissociation. Theoretical calculations indicate that the high reverse barrier to C−D recombination and the facile desorption of CD 2 O make photocatalytic methanol dissociation on TiO 2 (110) proceed efficiently. Theoretical results also reveal that the reverse reactions, i.e, O−H recombination after H 2 O photocatalytic dissociation on TiO 2 (110), may occur easily, thus inhibiting efficient photocatalytic water splitting. ■ INTRODUCTIONTitanium dioxide has been extensively investigated as a catalyst or photocatalyst, 1−11 particularly in applications involving photodegradation of organic molecules and water splitting, 5,12,13 which have important implications in environmental remediation and clean energy. Pure TiO 2 is apparently not photocatalytically active for splitting water to produce hydrogen, 14 but the addition of methanol to water can dramatically enhance the photocatalytic activity for hydrogen production. 15 Therefore, understanding the key differences between the photocatalytic chemistry of methanol and water on a model TiO 2 surface at the molecular level may provide valuable insight into the dynamics of photocatalysis that would enhance efforts for developing new and efficient photocatalysts for water splitting.Theoretical and experimental studies often focus on TiO 2 (110) as a model surface, 6,16 with the methanol/ TiO 2 (110) system serving as a model for photocatalysis on TiO 2 . 17−20 Henderson and co-workers 18 conducted a temperature-programmed desorption (TPD) study of CH 3 OH on TiO 2 (110) and concluded that the majority of the CH 3 OH molecules are adsorbed in molecular form. This conclusion is consistent with a scanning tunneling microscopy study by Dohnalek et al. 10 that showed that methanol molecules are adsorbed molecularly on the Ti 5c sites and are dissociated only at bridge-bonded oxygen (BBO) vacancy sites. The photocatalysis of CH 3 OH on TiO 2 (110) was investigated in a twophoton photoemission (2PPE) experiment, which inferred the presence of an excited electronic state on the surface. 20,21 Zhou et al. attributed this surface state to a photocatalytic dissociated state of methanol using a time-dependent 2PPE (TD-2PPE) technique. 22 They also used a combination of photoexcitation with STM and found that 400 nm light could induce dissociation of methanol on the surface, and they assigned the dissociated state as methoxy (CH 3 O) on a Ti 5c site and a hydrogen atom on a BBO site. Shen and Hend...

Clean hydrogen production is highly desirable for future energy needs, making the understanding of molecular-level phenomena underlying photocatalytic hydrogen production both fundamentally and practically important. Water splitting on pure TiO 2 is inefficient, however, adding sacrificial methanol could significantly enhance the photocatalyzed H 2 production. Therefore, understanding the photochemistry of methanol on TiO 2 at the molecular level could provide important insights to its photocatalytic activity. Here, we report the first clear evidence of photocatalyzed splitting of methanol on TiO 2 derived from time-dependent two-photon photoemission (TD-2PPE) results in combination with scanning tunneling microscopy (STM). STM tip induced molecular manipulation before and after UV light irradiation clearly reveals photocatalytic bond cleavage, which occurs only at Ti 4+ surface sites. TD-2PPE reveals that the kinetics of methanol photodissociation is clearly not of single exponential, an important characteristic of this intrinsically heterogeneous photoreaction.

Photocatalytic hydrogen production and pollutant degradation provided both great opportunities and challenges in the field of sustainable energy and environmental science. Over the past few decades, we have witnessed fast growing interest and efforts in developing new photocatalysts, improving catalytic efficiency and exploring the reaction mechanism at the atomic and molecular levels. Owing to its relatively high efficiency, nontoxicity, low cost and high stability, TiO2 becomes one of the most extensively investigated metal oxides in semiconductor photocatalysis. Fundamental studies on well characterized single crystals using ultrahigh vacuum based surface science techniques could provide key microscopic insight into the underlying mechanism of photocatalysis. In this review, we have summarized recent progress in the photocatalytic chemistry of hydrogen, water, oxygen, carbon monoxide, alcohols, aldehydes, ketones and carboxylic acids on TiO2 surfaces. We focused this review mainly on the rutile TiO2(110) surface, but some results on the rutile TiO2(011), anatase TiO2(101) and (001) surfaces are also discussed. These studies provided fundamental insights into surface photocatalysis as well as stimulated new investigations in this exciting field. At the end of this review, we have discussed how these studies can help us to develop new photocatalysis models.