Electronic Structures Of Titanium Dioxide Studied By Many-Body Green's Function Theory

With the continuous consumption of fossil energy,energy and environmental issues are becoming more and more complicated and difficult to solve,and have become important challenges for human beings.New renewable energy source is favored by people,and its development and utilization is urgent.Solar energy is a kind of renewable,clean and pollution-free energy with abundant reserves.And it has been widely studied.However,utilization of solar energy is still very limited.It is particularly important to find suitable photocatalysts of high performance.Titanium dioxide(TiO2),zinc oxide(ZnO),ferric oxide(Fe2O3)and zinc sulfide(ZnS)are some common photocatalysts.Among them,titanium dioxide is a widely used photocatalyst,which has abundant content and high chemical stability.TiO2 has three crystalline forms,including rutile(R type)which is the most stable one thermodynamically,anatase(A type)which has high photocatalytic activity and brookite(B type)which has been less studied yet.Experimentally,the optical band gap of R is 3.0 eV,and it has a direct band gap;the absorption edge of A is 3.2 eV,and it has an indirect band gap;the direct band gap of B ranges from 3.2 eV to 3.4 eV.The inherent wide band gap of Ti02 makes it less efficient for solar energy conversion since it can only absorb high-energy ultraviolet light.Titanium dioxide cannot be utilized for absorption of infrared and visible light which account for most of sunlight.This phenomenon prevails in many photocatalytic materials,such as g-C3N4.In the past decades,scientists have made great efforts to reduce the band gap of Ti02,thereby enhancing its utilization of light energy.Frequently used strategies include introducing defects,doping atoms and reconstructing surfaces.They all produce new states in the band gap.In recent years,titanium dioxide has been developed rapidly not only in the field of photocatalysis,but also in both dye-sensitized solar cells and environmental cleaning.Studying the basic physical and chemical properties of defects and dopants in titanium dioxide can promote the research and utilization of titanium dioxide in photocatalysis,making more efficient use of solar energy.Intrinsic defects are very important since they determine the conductivity and optical properties of titanium dioxide.The most common methods to introduce defects into TiO2 experimentally are thermal annealing and ion sputtering.The three crystalline forms of TiO2 have their own specific defect levels.Experimental measurements,e.g.by UPS,PES and STM,have demonstrated the deep defect level is 0.7?1.0 eV,1 eV and 0.4 eV below the conduction band minimum for R,A and B types respectively.Meanwhile,shallow defect levels were also observed in rutile and anatase.As we all know,DFT is inaccurate for strong correlation systems containing localized d-and f-orbitals.It is necessary to add empirical parameter U in the DFT calculation,and the magnitude of U will also affect the calculation result.However,the GW method can calculate the electronic properties of semiconductors without the need of empirical parameters.So far,a large number of literatures have reported the successful application of this method in solids.GW is one of the most accurate and advanced method for calculating electronic structures of semiconductors and insulators.In this paper,we use the GW method,which is based on the first principle many-body Green's function theory,to study the charge distribution and band structure of defects and dopants in titanium dioxide,intending to provide a theoretical basis for further understanding the optical and electrical properties of the of TiO2.The main research contents and conclusions of this thesis are as follows:1.Using the GW method,the charge distributions and energies of the defect states of the anatase TiO2,including those of surface oxygen vacancy,subsurface oxygen vacancy,surface hydroxyl groups,subsurface Ti interstitial and bulk oxygen vacancy,were studied.Our main purpose is to find out what kind of defect is responsible for the deep level of the anatase TiO2(101)surface as deteced in the experiment.At the same time,we carry out STM simulations of the surface oxygen vacancy and subsurface Ti interstitial,comparing them with experimental STM images.The results show that the hydroxyl groups and subsurface oxygen vacancy produce shallow energy levels;the subsurface Ti interstitial can produce both shallow and deep levels,but Ti interstitial are more stable in anatase bulk.Additionally,simulated STM images of Ti interstitial do not match the images in experiment.The dda bond formed in the surface oxygen vacancy is more stable on the surface,and our results indicate that the deep level measured in experimental might be induced by the dda bond.The simulated STM images of surface oxygen vacancy is in agreement with the STM images measured in experiment,which further proves that the Ti-Ti dd? bond causes the deep level in the anatase(101)surface.By comparing the band gap of the perfect surface and that of the defected surface,it is found that defects reduce the conduction band edge greatly to the experimental value.Our results also show that the polaron produces a very shallow energy level,which can help to understand why the Fermi level of anatase is close to the conduction band minimum.In addition,our results explain the coexistence of shallow and deep defect states in anatase.2.The GW method is used to study the band structure and charge distribution of bulk titanium dioxide,both rutile and anatase,doped by carbon.The results show that there is no band-gap state for carbon cation,while there exists band-gap states for carbon anion.For carbon anion,both rutile and anatase have two occupied band-gap states and one unoccupied band-gap state.The results show that the red shift of the absorption spectrum is related to the generation of band gap states.