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...

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) have emerged as attractive platforms in next-generation nanoelectronics and optoelectronics for reducing device sizes down to a 10 nm scale. To achieve this, the controlled synthesis of wafer-scale single-crystal TMDs with high crystallinity has been a continuous pursuit. However, previous efforts to epitaxially grow TMD films on insulating substrates (e.g., mica and sapphire) failed to eliminate the evolution of antiparallel domains and twin boundaries, leading to the formation of polycrystalline films. Herein, we report the epitaxial growth of wafer-scale single-crystal MoS2 monolayers on vicinal Au(111) thin films, as obtained by melting and resolidifying commercial Au foils. The unidirectional alignment and seamless stitching of the MoS2 domains were comprehensively demonstrated using atomic- to centimeter-scale characterization techniques. By utilizing onsite scanning tunneling microscope characterizations combined with first-principles calculations, it was revealed that the nucleation of MoS2 monolayer is dominantly guided by the steps on Au(111), which leads to highly oriented growth of MoS2 along the ⟨110⟩ step edges. This work, thereby, makes a significant step toward the practical applications of MoS2 monolayers and the large-scale integration of 2D electronics.

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.

Reaction resonances, or transiently stabilized transition-state structures, have proven highly challenging to capture experimentally. Here, we used the highly sensitive H atom Rydberg tagging time-of-flight method to conduct a crossed molecular beam scattering study of the F + H2 --> HF + H reaction with full quantum-state resolution. Pronounced forward-scattered HF products in the v' = 2 vibrational state were clearly observed at a collision energy of 0.52 kcal/mol; this was attributed to both the ground and the first excited Feshbach resonances trapped in the peculiar HF(v' = 3)-H' vibrationally adiabatic potential, with substantial enhancement by constructive interference between the two resonances.

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.

Using x-ray absorption and resonant inelastic x-ray scattering, charge dynamics at and near the Fe L edges is investigated in Fe pnictide materials, and contrasted to that measured in other Fe compounds. It is shown that the XAS and RIXS spectra for 122 and 1111 Fe pnictides are each qualitatively similar to Fe metal. Cluster diagonalization, multiplet, and density-functional calculations show that Coulomb correlations are much smaller than in the cuprates, highlighting the role of Fe metallicity and strong covalency in these materials. Best agreement with experiment is obtained using Hubbard parameters U 2eV and J ≈ 0.8eV.

Previous observations of methyl formate (HCOOCH 3 ) during the photo-oxidation of methanol (CH 3 OH) on TiO 2 catalysts suggested that photocatalysis on TiO 2 could be used to build up complex molecules from a single precursor. We have investigated the mechanism of HCOOCH 3 formation by irradiating a CH 3 OH-adsorbed TiO 2 (110) surface with 400 nm light at low surface temperatures. Through the detection of volatile products after irradiation by temperature programmed desorption, we have found, as previously reported [Phillips et al. J. Am. Chem. Soc. 2013, 135, 574−577] that HCOOCH 3 is formed by the cross-coupling reaction of CH 3 O and CH 2 O, which are products of the first and second dissociation steps, respectively, in the stepwise photocatalysis of CH 3 OH on TiO 2 (110). Unlike the previous study, we have observed the photocatalytic production of HCOOCH 3 without preoxidation of the surface, and we have concluded that the final reaction step to produce HCOOCH 3 (i.e., the cross-coupling reaction of CH 2 O with CH 3 O) does not involve a transient HCO intermediate.

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