The Stability, Electronic Structure, And Optical Property Of TiO₂ Polymorphs

It’s very useful to know eletronic and optical properties of materials for searching the possible photocatalyst under visible light. So calculations of ground-state properties and excited-state properties (electronic band structure and optical absorption spectrum) have been major goals of our work. In this thesis, the band structures of 9 TiO2 polymorphs were calculated by DFT-GGA. GW method was adopted to calculate the band energies at high-symmetry k-points in the first Brillouin zone. The excited-state properties of 9 TiO2 polymorphs were calculated by solving two-body Bethe-Salpeter Equation.The thesis is dividied into six chapters. In the first chapter, experiments on different TiO2 polymorphs were summarized. The principle of photocatalysis, the aims and mean-ing of our research were introduced briefly. In chapter two, we elaborated the defination of the pseudopotential, the density functional theory and the many-body perturbation the-ory (GW approximation, Bethe-Salpeter Equation). In chapter three, effective pseudopo-tentials of Ti and O elements were produced and we introduced some basic parameters used in our calculations. In chapter four, structure parameters and enthalpies of 9 TiO2 polymorphs were calculated by DFT-GGA and DFT-GGA+D+U. In chapter five, the band structures, DOS, and optical absorption spectrum of 9 TiO2 polymorphs were carried out by using the many-body perturbation theory.The main achievements of this work are listed as follows:1. Phonon density of states calculation shows that a new TiO2 polymorph with tridymite structure is mechanically stable. Band structure calculation shows that its bandgap is widder than the other 8 structured TiO2. This tridymite-structured TiO2 has very low static dielectric constant.2. After applying DFT-GGA+D+U, we got correct structure parameters of 9 TiO2 poly-morphs (especially for the value of c in the anatase phase) and the correct enthalpy order of rutile, brookite, anatase phase. The relatively stability of 9 TiO2 poly-morphs from 0 GPa to 70 GPa were discussed.3. The quasiparticle band structures of 9 TiO2 polymorphs were calculated with GW approximation. The differences between direct band gap energies and indirect band gap energies are very small for rutile and columbite TiO2, indicating a quasi-direct band gap character. The band gap energies of baddeleyite TiO2 are close to that of anatase TiO2. The band gap energies of cotunnite TiO2 are close to that of rutile TiO2.4. Optical properties of 9 TiO2 polymorphs were calculated by solving two-body Bethe-Salpeter Equation. We have shown that TiO2 polymorphs with pyrite, and fluorite structures have optical transitions in the region of visible light. By the way, badde-leyite have optical transitions along the b direction in the region of visible light. In the region of ultra-visible light, anatase has the best absorption coefficient among 9 polymorphs.