Theoretical Study Of Anatase TiO₂ Surface

TiO₂ has attracted considerable attention in recent years, due to their unique photophysical properties and photocatalytic degradation activity of organic compounds. Anatase is a TiO₂ polymorph which is less stable than rutile, but more efficient than rutile for catalysis, photocatalysis. In these applications, surface properties are of major importance. In present work, the surface structure of anatase TiO₂ surface and the properties of anatase TiO₂ surface modified with sulfur have been studied in theory. This thesis has done the following work:1) First-principles calculations based on the plane-wave pseudopotential method have been studied the surface energy and structure of anatase TiO₂ (101) surface. The calculation result shows that anatase TiO₂ (101) crystal surface structure should be terminated by O_2c and Ti_5c. Slab thicknesses of at least 18 atom layers and vacuum widths of more 4A are sufficient to converge the surface energy to within 0.01 J/m². The calculated results show that the O₂c is an inward relaxation of 0.012A, and the Ti5c is outward relaxation of 0.155A?.2) The slab thickness has significant effect on the quality of band structure and density of states. A fine analysis of band structure and density of states of the TiO₂ (101) surface shows atomic relaxations results in a large transfer of surface charges from outmost layer to inner layer, and the surface bonds have a rehybridization, which make the ionization reduce and the covalence increase, which causes the surface bond shorten. Oxygen vacancies on the surface induce a defect state that pins the Fermi level just below the conduction-band minimum.3) Comparing anion with cation doping in anatase and rutile, we found different energy band structures and origins of photoactivity of S-doped TiO₂. Sulfur adsorption on the surface oxygen vacancies leads to the disappearance of this vacancy-related occupied state. The results show that the bonding of Sulfur to O atoms on the surface trends to form SO₂ molecules. Calculated surface formation energies suggest that the vacancies at the TiO₂ surface influence the adsorption of molecules, and sulfur adsorption on the surface titanium vacancies is most stable.