| Titanium dioxide (TiO2) is a model system for surface properties of transition metal oxides. Chemical surface properties play a critical role in photocatalysis for which TiO2is also the prototypical material. A detailed discussion of the surface chemistry of TiO2was presented, covering important work from the surface structure, chemisorption, photocatalysis, as well as synthesis methods. We also introduced the basic concept of density functional theory (DFT) and reviewed its further application in describing van der Waals interactions and theoretical modelling of scanning tunnelling microscopy.Dinitrotoluene (2,4-DNT) is a decomposition product of TNT with a higher vapor pressure and thus has been used for detecting explosives. In particular, solid-state gas sensors based on TiO2have shown promising properties for sensing DNT. However, the adsorption of DNT on TiO2has not been understood well at the molecular level before. Based on the scanning tunneling microscopy and X-ray photoemission spectroscopy findings, we investigated the adsorption of2,4-DNT on TiO2(110) by density functional theory simulation. We found that DNT chemisorbs molecularly intact in a bridge bidentate configuration with two oxygen atoms of one nitro group binding to two neighboring five-fold coordinated titanium atoms at the surface. At saturation coverage, half a monolayer with an ordered2×1structure was observed. The DFT calculations corrected by London dispersion force showed that the intermolecular interactions were the determining factor for the formation of ordered adsorption structures under both low and high DNT coverages.The prototype transition metal oxide TiO2has been extensively studied due to its importance in photocatalysis and heterogeneous catalysis. However, most of these studies were focused on the (110) surface of the majority rutile polymorph. Here, we investigate the rutile TiO2(011) surface, which is the second most abundant surface orientation for an equilibrium rutile crystal and thus is critical for understanding, for example, the photocatalytic properties of TiO2powder catalysts. Unlike the (110) face, which often exhibits a bulk truncation, the (011) surface exhibits a2×1reconstruction at the vacuum interface. This reconstruction lowers its surface free energy by reducing the number of dangling bonds and thus also reduces its chemical reactivity, which would make this surface fairly inert. However, the density functional theory calculations presented in this paper demonstrated that the surface can restructure to increase interaction with molecular adsorbates. Interestingly, the adsorbate induced surface instability was strongly anisotropic, which result in the formation of directional adsorbate clusters and thus such adsorbate induced surface restructuring may also be an approach for structuring at the close to atomic scale.In the application of photochemical splitting of water over TiO2electrodes in particular, and in other chemical and environmental applications as well, the formation, stability, and reaction of hydroxyls at the surface are of paramount importance to describing surface processes. We employed density functional theory simulation for the hydrogen adsorption and reaction on the rutile TiO2(011)-2×1by a combination of high-resolution STM. Hydroxyl formation on the reconstructed surface is weak, and hydroxyls have only been observed on one of the three different surface oxygen sites. Recombination of hydrogen and desorption of H2was prevented by a large kinetic barrier. Instead, hydrogen was removed from the surface at elevated temperature by diffusion into the bulk. Our studies are also compared to previous studies on the rutile TiO2(110) surface where different thermal and photoinduced processes have been reported. These differences were explained by three competing reaction pathways:H2recombination, water formation by lattice oxygen abstraction, bulk diffusion. The dependence of the reaction on the hydrogen adsorption energies as well as on kinetic diffusion and reaction barriers and pathways can explain the observed differences between these two surface orientations.Owing to their scientific and technological importance, inorganic single crystals with highly reactive surfaces have long been studied. Conventionally, anatase TiO2crystals are dominated by the thermodynamically stable (101) facets (94percent, according to the Wulff construction) and a minority of (001) facets. Unfortunately, the surfaces with high reactivity usually diminish rapidly during the crystal growth process as a result of the minimization of surface energy. Thus, increasing the percentage of known highly reactive surfaces or creating new favorable surfaces is highly desirable. An efficient scheme to synthesize anatase TiO2with high index (105) facets have been devised for the first time, and this well-faceted surface may have promising potential applications owing to the unique atomic configuration. It is well known that high-index facets exhibit a much higher density of unsaturated stepped atoms, ledges, and kinks, which usually serve as active sites for breaking chemical bonds. We focus on three possible terminations of TiO2(105) surface and show that the interaction with water leads to an inversion of the stabilities of these terminations. This indicates that surface structures determined in vacuo or at different water coverages are not generally representative of those occurring in the aqueous environments typical of most photocatalytic applications of TiO2. Furthermore, ion doping has been widely used to modify the electronic structure of a semiconductor photocatalyst. Here, we successfully synthesized Sn doped single crystalline anatase TiO2particles dominated with (105) facets by a gas phase oxidation process. The photoluminescence emission spectra measurements revealed that the small amount of doped Sn in TiO2could suppress the recombination of photogenerated electron-hole pairs. Thus, the Sn doped TiO2showed a significantly enhanced photocatalytic hydrogen evolution performance, with its hydrogen generation rate being4.5times higher than that of pure TiO2. First-principle simulation was performed thoroughly to analysis their geometry configuration and electronic structure. The results suggested that the doped Sn at the edge exhibit higher adsorption energy toward H, which could promote the H2generation from the splitting of water. |
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