A DFT Study:Structures And Catalytic Performance Of TiO₂Surfaces

As a versatile metal oxide, titanium dioxide (TiO2) has been extensively studied and widely used in many application fields, such as materials science, catalysis, environmental protection, energy development, biological science and medical treatment, etc. In particular, TiO2is universally applied in heterogeneous catalysis and photocatalysis, mainly because of its mature manufacturing process and suitability for almost all the experimental characterization and analysis methods. A great deal of effort has been devoted to the catalytic performance of TiO2over the past few decades. However, the surface structure of catalysts largely determines their properties in promoting catalytic process and therefore a comprehensive understanding of relationship between structure and the related reactivity is definitely necessary. In this dissertation, density functional theory (DFT) studies of structure and activity of T1O2catalyst are performed as below:Step-edge structures of rutile TiO2(101):rutile TiO2(110) surface is considered as the most thermodynamically stable surface of rutile. Defective rutile TiO2(110) surface always presents higher activity than the defect-free surface, particullay for step-edge structure. In this dissertation, we performed a DFT study and scanning tunneling microscope (STM) simulations of several low index directions of step edge on rutile TiO2(110) surface. According to comparing formation energy of different step-edge structures on rutile TiO2(110) surface, we determined the2X1reconstructed step edge along<110> orientation is the most stable. Simultaneously, the simulated STM image of this step-edge structure is completely consistent with the experimental STM result. It should be mentioned that the new step-edge model on rutile TiO2(110) surface is extremely important for a deeper understanding of structures and reactivities of stepped structures.Adsorption of H2O molecule on rutile TiO2(011) surface:rutile TiO2(011) surface is studied with more interests due to its considerably high performance in many catalytic reactions. Some resent studies proposed a new model of2×1reconstructed rutile TiO2(011) surface as "brookite(001)-like" with a lower surface energy. In this dissertation, we performed a DFT study and STM simulation of H2O adsorption on the reconstructed surface. It is suggested that weak molecular adsorption is predicted on perfect2×1reconstructed surface, however, stronger dissociated adsorption on O-defected surface foming hydroxyl group. Water molecules are aggregated into clusters as molecular-dissociated adsorption as the number of adsorbed water increases. On the "brookite(001)-like" surface, water cluster growth is observed along one dimension in simulated STM images and experimental STM data. This is mainly because of the corrugated geometry on the surface. The ID water clusters in nanosized channels on2×1reconstructed rutile TiO2(011) surface is important in different fields, such as, nanofluidics, enzyme catalysis, and biosensors.Comparative study of structures and reactivities of anatase and brookite TiC>2surfaces: Anatase TiO2(101) and brookite TiO2(210) surfaces are the two major surfaces exposed at anatase and brookite crystals, which have the similar structures but obviously different reactivities. Both two surfaces have the same structural building blocks, but the interatomic distances are slightly shorter and the blocks are arranged in a different way. Highly active junction sites are generated between rows of blocks on brookite TiO2(210) surface, which can greatly promote molecular adsorption and catalytic reaction on the surface. In this disertation, we performed a comparative DFT study of probe molecules (H2O and HCOOH) and gold clusters adsorption at two surfaces. The results shown as follows:(1) For H2O molecule, stronger molecular and dissociated adsorption are both confirmed at brookite TiO2(210) than anatase TiO2(101). For HCOOH molecule, only molecular adsorption exists at anatase TiO2(101), while HCOOH dissociates at brookite TiO2(210). Moreover, a rather stable dissociated structure (bidentate configuration) is found to occur at junction site.(2) For Au nanoparticles, they prefer to nucleate and grow at O vacancies of defected surfaces. Extended2D Au-film structures are favored at anatase TiO2(101), while dispersion of3D Au clusters is favored at brookite TiO2(210). Furthermore, CO oxidation reaction over Au6-8/brookite catalyst is also studied. The mobility of Au atoms in Au clusters and their capacity to tune conformations can significantly facilitate the adsorption and reaction processes during CO oxidation. The unique surface structure makes brookite TiO2(210) a promising candidate in catalysis and photocatalysis.B-N synergistic effect in co-doped anatase TiO2:boron and nitrogen co-doped TiO2has attracted much attention and been widely studied. However, the synergistic effect between the two dopants is still controversial. In this disertation, we performed a DFT study of geometric and electric properties of boron and nitrogen co-doped TiO2(anatase TiO2(101) and (001) surfaces). Combining with experimental results, newly forming structures are different according to the doping order of B and N dopants. The B-N synergistic effect is shown as Ti-B-N-Ti and Ti-B-N-O structures at anatase surfaces, which can both improved photocatalytic activity of TiO2. Furthermore, the B-N synergistic effect can largely decrease the bandgap of (101) surface, which is not obvious at (001) surface. Heterostructures and electronic properties of mixed-phase TiO2:mixed-phase TiO2has been reported to exhibit improved photocatalytic activities than single-phase TiO2. In this disertation, we performed a systematic DFT study of geometric and electronic properties of two-phase TiO2by joinig two single-phase TiO2slabs. Our results indicated that electronic properties of mixed-phase TiO2are affected by several factors:the relative positions of HOMO and LUMO states of the two composites; the lattice match between the two slabs of the compositon and how well the two slabs interact with each other in the interfacial region. In particular, when the HOMO and LUMO levels of one of the two single-phase TiO2slabs are higher than the corresponding ones of the other, the composite may have native electronic structures with phase separated HOMO-LUMO states. Such electronic properties may provide useful information to guide the design and preparation of highly active semiconductor materials with mixed phases.