Zhibo Ma

Stepwise Photocatalytic Dissociation of Methanol and Water on TiO2(110)

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.

Elementary photocatalytic chemistry on TiO2 surfaces

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.

Site-specific photocatalytic splitting of methanol on TiO2(110).

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 TiO2 is inefficient, however, adding sacrificial methanol could significantly enhance the photocatalyzed H2 production. Therefore, understanding the photochemistry of methanol on TiO2 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 TiO2 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 Ti4+ 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.

Effect of defects on photocatalytic dissociation of methanol on TiO2(110)

Photocatalytic dissociation of deuterated methanol (CD3OD) on both stoichiometric and reduced TiO2(110) surfaces was investigated using the time-dependent two-photon photoemission (2PPE) method, in order to understand the effect of defects on the kinetics of methanol dissociation on TiO2(110). By monitoring the time evolution of the photoinduced excited state on the methanol covered surface, the photocatalytic dissociation kinetics of methanol on the TiO2 surface were observed. The measured photodissociation rate on the reduced TiO2(110) surface is more than an order of magnitude faster than that on the stoichiometric surface. Since the reduced TiO2(110) surface has considerably more surface and subsurface defects than the stoichiometric surface, the experimental observation suggests that one or both of them could accelerate the photocatalysis process of methanol on the TiO2(110) surface in a significant way.

A surface femtosecond two-photon photoemission spectrometer for excited electron dynamics and time-dependent photochemical kinetics.

A surface femtosecond two-photon photoemission (2PPE) spectrometer devoted to the study of ultrafast excited electron dynamics and photochemical kinetics on metal and metal oxide surfaces has been constructed. Low energy photoelectrons are measured using a hemispherical electron energy analyzer with an imaging detector that allows us to detect the energy and the angular distributions of the photoelectrons simultaneously. A Mach–Zehnder interferometer was built for the time-resolved 2PPE (TR-2PPE) measurement to study ultrafast surface excited electron dynamics, which was demonstrated on the Cu(111) surface. A scheme for measuring time-dependent 2PPE (TD-2PPE) spectra has also been developed for studies of surface photochemistry. This technique has been applied to a preliminary study on the photochemical kinetics on ethanol/TiO2(110). We have also shown that the ultrafast dynamics of photoinduced surface excited resonances can be investigated in a reliable way by combining the TR-2PPE and TD-2PPE techniques.

Surface photochemistry probed by two-photon photoemission spectroscopy

Two-photon photoemission (2PPE) has been widely used in the study of electronic structure and dynamics of unoccupied electronic states on different types of surfaces and interfaces. Since 2PPE probes electronically excited states, it should be sensitive to surface excited electronic structure changes that accompany surface chemical reactions. Therefore, this method could potentially be used to study the kinetics and dynamics of surface chemical reactions as well as surface photocatalysis. In this article, we briefly review recent progress made in the study of surface photochemistry and photocatalysis using the time-dependent 2PPE (TD-2PPE) method. A few examples are given to demonstrate the application of this method in probing surface photochemistry and photocatalysis, particularly photocatalysis of methanol on TiO2 surfaces. Since many problems associated with surface photochemistry and surface photocatalysis are related to energy and environmental issues, the 2PPE technique could have important applications in the study of the fundamental problems in energy and environmental sciences.

Effect of the Hydrogen Bond in Photoinduced Water Dissociation: A Double-Edged Sword.

Photoinduced water dissociation on rutile-TiO2 was investigated using various methods. Experimental results reveal that the water dissociation occurs via transferring an H atom to a bridge bonded oxygen site and ejecting an OH radical to the gas phase during irradiation. The reaction is strongly suppressed as the water coverage increases. Further scanning tunneling microscopy study demonstrates that hydrogen bonds between water molecules have a dramatic effect on the reaction. Interestingly, a single hydrogen bond in water dimer enhances the water dissociation reaction, while one-dimensional hydrogen bonds in water chains inhibit the reaction. Density functional theory calculations indicate that the effect of hydrogen bonds on the OH dissociation energy is likely the origin of this remarkable behavior. The results suggest that avoiding a strong hydrogen bond network between water molecules is crucial for water splitting.

Kinetics and Dynamics of Photocatalyzed Dissociation of Ethanol on TiO2(110)

the kinetics and dynamics of photocatalyzed dissociation of ethanol on tio2(110) surface have been studied using the time-dependent and time-resolved femtosecond two-photon photoemission spectroscopy respectively, in order to unravel the photochemical properties of ethanol on this prototypical metal oxide surface. by monitoring the time evolution of the photoinduced excited state which is associated with the photocatalyzed dissociation of ethanol on ti-5c sites of tio2(110), the fractal-like kinetics of this surface photocatalytic reaction has been obtained. the measured photocatalytic dissociation rate on reduced tio2(110) is faster than that on the oxidized surface. this is attributed to the larger defect density on the reduced surface which lowers the reaction barrier of the photocatalytic reaction at least methodologically. possible reasons associated with the defect electrons for the acceleration have been discussed. by performing the interferometric two-pulse correlation on ethanol/tio2(110) interface, the ultrafast electron dynamics of the excited state has been measured. the analyzed lifetime (24 fs) of the excited state is similar to that on methanol/tio2(110). the appearance of the excited state provides a channel to mediate the electron transfer between the tio2 substrate and its environment. therefore studying its ultrafast electron dynamics may lead to the understanding of the microscopic mechanism of photocatalysis and photoelectrochemical energy conversion on tio2.

Fundamental Processes in Surface Photocatalysis on TiO 2

Due to the potential applications of TiO2 in photocatalytic hydrogen production and pollutant degradation, over the past few decades, we have witnessed the fast-growing interest and effort in developing TiO2-based photocatalysts, improving the efficiency, and exploring the reaction mechanism at the atomic and molecular level. Since surface science studies on single crystal surfaces under UHV conditions could provide fundamental insights into these important processes, both thermal chemistry and photo-chemistry on TiO2, especially on rutile TiO2(110) surface, have been extensively investigated with a variety of experimental and theoretical approaches. In this chapter, we start from the properties of TiO2 and then focus on charge transport and trapping and electron transfer dynamics. Next, we summarize recent progresses made in the study of elementary photocatalytic chemistry of oxygen and methanol on mainly rutile TiO2(110) along with some studies on rutile TiO2(011) and anatase TiO2(101) and (001). These studies have provided fundamental insights into surface photocatalysis as well as stimulated new investigations in this exciting area. At the end of this chapter, implications of these studies for the development of new photocatalysis models are also discussed.

Elementary Chemical Reactions in Surface Photocatalysis

Photocatalytic hydrogen evolution and organic degradation on oxide materials have been extensively investigated in the last two decades. Great efforts have been dedicated to the study of photocatalytic reaction mechanisms of a variety of molecules on TiO2 surfaces by using surface science methods under ultra-high vacuum (UHV) conditions, providing fundamental understanding of surface chemical reactions in photocatalysis. In this review, we summarize the recent progress in the study of photocatalysis of several important species (water, methanol, and aldehydes) on different TiO2 surfaces. The results of these studies have provided us deep insights into the elementary processes of surface photocatalysis and stimulated a new frontier of research in this area. Based on the results of these studies, a new dynamics-based photocatalysis model is also discussed.