First-principles Study On Transition Metal Oxides With Surficial Oxygen Deficiency

For the transition metal oxides, O vacancy defect is one of the most important defects, which produce deep levels to capture the carrier, thereby affecting the performance of the semiconductor device. In addition, due to the existence of vacancies, a large number of carriers have been formed. At the same time, the interaction between defects caused some other effect on the properties of the material. When solid materials participate in chemical reactions, only the surface could be contacted with other molecule. Many properties of transition metal oxides might be depended on the surface structure, such as the different crystal, grain arrangement at the surfaces and different surface lattice defects. Oxygen vacancies on the surface as photo generated electron trapping centers, and the forbidden band of material would generate defect levels after the introduction of oxygen vacancies. When the surface of the material introduced an appropriate concentration of oxygen vacancies, the electrical properties and optical properties of the material could be improved significantly.In this paper, we used the first-principles density functional theory method to calculate the electron structure and optical properties of Ti O2, Zn O and Ni O surface with oxygen vacancies and without oxygen vacancies systems, with the help of Materials Studio(MS) package CASTEP module. The electron-electron interaction associated with the exchange of energy correction was the generalized by PBEsol gradient approximation method. We mainly studied the effects of oxygen vacancies on the band gap, density of states and optical/electronic properties of these systems. The conclusions as follow:Firstly, the presence of oxygen vacancies in anatase crystals unfavorable improve its photocatalytic properties, but when the oxygen vacancies exist on the surface, the band gap of Ti O2 can be reduced and the optical absorption edge shifted to the long-wavelength range, which explained the photocatalytic performance of Ti O2 have been improved.Secondly, the concentration of oxygen vacancy on the Zn O(10-10) surface effect the structure stability and the electrical properties of the Zn O surfaces system. Within the concentration range which used in our article, the higher the concentration of oxygen vacancies, the stable of surface structure better. The visible light absorption coefficient of Zn O surface system also increased with the concentration of surface oxygen vacancies, results the photocatalytic properties of Zn O surface improved.For the last one, in the Ni O(001) surface system, the formation energy of oxygen vacancy was 8.39 e V, which higher than that of Ti O2 and Zn O systems. After the introduction of oxygen vacancies, the conduction band moved to the low energy range and there were defect states in the band gap. As a result, the band gap of Ni O reduced significantly. In addition, the occupy range of both O 2p states in valence band and the Ni 4s states in conduction band were increased, which in favor of the movement of electrons. The analysis of electronic charge showed that the elelctropositivity of Ni atoms reduced while the elelctronegativity of O atoms increased. The work function had been reduced due to the oxygen vacancies.In short, the introduction of oxygen vacancies in the surface will change the electronic structure of material, thus affecting their electrical and optical properties. For some transition metal oxide,the introduction of oxygen vacancies causes the Fermi level moved into the conduction band, which would improve the conductivity. Additionally, the formation defect energy level reduced the distance of electrons to conduction band, thereby improving the performance of material.