Chuanyao Zhou

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.

Localized Excitation of Ti3+ Ions in the Photoabsorption and Photocatalytic Activity of Reduced Rutile TiO2

In reduced TiO2, electronic transitions originating from the Ti(3+)-induced states in the band gap are known to contribute to the photoabsorption, being in fact responsible for the materials blue color, but the excited states accessed by these transitions have not been characterized in detail. In this work we investigate the excited state electronic structure of the prototypical rutile TiO2(110) surface using two-photon photoemission spectroscopy (2PPE) and density functional theory (DFT) calculations. Using 2PPE, an excited resonant state derived from Ti(3+) species is identified at 2.5 ± 0.2 eV above the Fermi level (EF) on both the reduced and hydroxylated surfaces. DFT calculations reveal that this excited state is closely related to the gap state at ∼1.0 eV below EF, as they both result from the Jahn-Teller induced splitting of the 3d orbitals of Ti(3+) ions in reduced TiO2. Localized excitation of Ti(3+) ions via 3d → 3d transitions from the gap state to this empty resonant state significantly increases the TiO2 photoabsorption and extends the absorbance to the visible region, consistent with the observed enhancement of the visible light induced photocatalytic activity of TiO2 through Ti(3+) self-doping. Our work reveals the physical origin of the Ti(3+) related photoabsorption and visible light photocatalytic activity in prototypical TiO2 and also paves the way for the investigation of the electronic structure and photoabsorption of other metal oxides.

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.

Coverage Dependence of Methanol Dissociation on TiO2(110).

Although the photochemistry of methanol on TiO2(110) has been widely investigated as a prototypical model of the photocatalytic reaction of organic molecules, the most fundamental question of the adsorption state of methanol on TiO2(110) is still unclear. We have investigated the adsorption of methanol on TiO2(110) using sum frequency generation vibrational spectroscopy (SFG-VS) and density functional theory (DFT) calculations. The SFG results indicate the dissociation of methanol is highly dependent on the coverage. The DFT calculations suggest that the methanol prefers the partially dissociated structure at low coverage, whereas the second layer methanol, which is hydrogen-bonded to the bridge-bonded oxygen site, largely blocks the dissociation of the first layer methanol. Our results not only resolves a long-standing debate regarding the adsorption state of methanol on TiO2(110) but also provides a detailed insight into the adsorption structure and sheds light on the photochemistry on this surface at the molecular level.

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.

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.

Macroscopic Wires from Fluorophore-Quencher Dyads with Long-Lived Blue Emission

We report the formation of macroscopic wires up to centimeters in length from a series of structurally flexible, covalently tethered small-molecular fluorophore-quencher dyads (FQDs, average MW = 425 Da), comprised of carbazole, melatonin, and cyanobenzoate moieties. These FQDs are nonemissive in organic solutions but become moderately to highly luminescent (ΦF = 0.037-0.39) upon formation of wires with emission maxima in the blue region (446-483 nm). The blue photoluminescence (PL) is ascribed to a combination of singlet charge transfer, localized triplet state, and possibly delayed fluorescence emissions with intrinsic luminescence lifetimes ranging from 0.228 to 21333 μs, based on luminescence, transient absorption measurements, X-ray diffraction, and calculations.

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.