The Annealing Mechanisms Of The Oxygen Defects In TiO₂

Titanium dioxide （TiO₂） has attracted intensive attention because of its wideapplication backgrounds such as photoelectric conversion, photocatalyst, chemicalsensors and luminescence materials. For example, in the light of the sun, TiO₂canutilize the light energy to produce the electrical energy, to photodegrade water togenerate H2and to photocatalytic degrade organic pollutants. Although the perfect TiO₂can only use the sun radiation in the short wavelengths range, it has been found thatintroducing the oxygen vacancies into the crystal could expand the reactive scope to thevisible light region; In addition, when TiO₂is used as the oxygen sensors, the workingmechanism is closely related with the oxygen vacancies as well. So issues regarding tothe oxygen defects in the semiconductor oxide nanoparticles have become the researchhotspot. On the other hand, annealing treatment is such a process closely related to thethermal motion of the crystal’s defects, involving the generation and transformationamong these defects especially when the crystal itself is not according to thestoichiometric ratio as well as the variability of the component elements’ charge statesand the structure pleomorphism, etc. Whether the oxygen in the atmosphere incorporateor escape from the crystals,（that is to say, the oxidation or reduction reactions of thepoint defect inside the crystal）, is closely related to the annealing temperature and theoxygen partial pressure in the atmosphere showing very complicated physical andchemical processes. Meanwhile, researches on the annealing treatment at home andabroad are concentrated on the exploration of annealing technology and the analysis ofexperimental results is still in the qualitative analysis level lacking understanding of themicroscopic mechanism for the defect movements during the heat treatment process,and so people cannot fundamentally know the influence of annealing process on thephysical mechanisms for the crystal physicochemical properties. Therefore,theoretically investigating the microscopic mechanism in the crystal annealing processand finding out the microscopic dynamics mechanism through the simulationcalculation have important theoretical and practical significance in the point defectresearch not only for the TiO₂but also for other oxide crystals.The paper is composed of six chapters:1) Introduction, first introduce the basic properties of TiO₂and application, thenintroduce the current research hot spots and some unknown problems, and finallyput forward the aim of this paper and content. 2) Theoretical basis, introduce the density functional theory （DFT） and the plane wavepseudo-potential theory as well as the first-principle software: VASP.3) Discussion on the physical properties of the perfect crystal. Mostly, we investigatethe detailed atomic and electronic structures of the bulk and some low Miler Indexsurfaces.4) Calculation on the atomic and electronic structures of the TiO₂crystals containingthe oxygen vacancy, thus explaining the experimental discovery that its adsorptionspectrum can be expanded to500nm after annealing treatment in the reductionatmosphere, which is the origin of the visible light catalytic reaction. Also wediscuss the effect of the different annealing processing conditions on the oxygenvacancy.5) Study on the effect of the annealing treatment on some oxygen defects in severalmost stable surfaces. In addition, the effect of the sub-surface oxygen vacancy onthe oxygen adsorption is investigated. Then we also explain the superior propertiesof the oxygen rich titanium dioxide nanocrystal which is discovered recently: stableanatase phase in the high temperature with highly effective visible light catalyticefficiency.6) Conclusion of the thesis.