First Principles Study On Electronic Structure And Optical Properties Of TiO₂ Doped Rare Earth Elements

As a widely applied photocatalyst material, the TiO₂ semiconductor has many excellent properties,such as, stable chemical stability, non-toxic, low-cost, easy preparation, and intense ability of photocatalysis. TiO₂ could also degrade many kinds of organic pollutants, so it is increasingly emphasized by people. But due to the large band gap of TiO₂, it could only absorb the UV light, the UV light just occupied about 4 percent of visible light, so the utilization rate of visible light is seriously limited and influences the photocatalytic efficiency of TiO₂. However, a large number of studies show that the capability of light absorption can be improved because of the doping between TiO₂ and the rare earth, but the theoretical researches of the rare earth doping also have some deficiencies.Based on the above factors, in this article, it is selected two crystal structures of TiO₂, they are anatase and rutile. As the base of doping, we selected the element N and five kinds of rare earth elements, La, Ce,Nd, Eu, Gd. Using, by the first principles method, we used the CASTEP software to calculate the electronic structures and optical properties of intrinsic TiO₂ and single doped and co-doped types TiO₂. We hope that what we have done could offer theory direction for the practical application. The main contents and results are as follows:Firstly, in this article, we studied the crystal structure, electronic structures, and optical properties of pure anatase TiO₂, La single doping system and La-N co-doping system. The results showed that the forbidden band gap of the pure anatase TiO₂ system was 2.255 e V, the unit cell of La single doping system was discovered the lattice distortion, and the forbidden band gap of the system was decreasing. The absorption band edge of the doped systems while moving to the low energy region and it showed red shift.The unit cell of La-N co-doped system also was discovered the lattice distortion, but the degree of distortion was less than La single doped system. The crystal structure is relatively more stable, the forbidden band gap of the co-doped system was decreasing significantly. There was impurity level in the system, and the red shift phenomenon was more obvious. With the doping of two elements, the static dielectric constant and the index of refraction of the system had been changed.Secondly, in this paper, the single doping of Ce, Nd, Eu, Gd and the co-doping between Ce-N, Nd-N,Eu-N, Gd-N and the anatase TiO₂ were calculated, the results showed that single doped system also was discovered lattice distortion and the degree of distortion was determined by the radius of the atomic atoms.The forbidden band gap of the system was decreasing gradually and the system showed red shift. The degree of unit cell distortion in co-doped system was less than the single doping system, and the forbidden band gap of the system was decreasing, and significantly even disappeared.Thirdly, in this article, we calculated the crystal structure, electronic structures, and optical properties of pure rutile TiO₂, La single doped system and La-N co-doped system. The calculated results showed that the forbidden band width of rutile TiO₂ system was 1.955 eV and less than it was in anatase TiO₂ system, the absorption coefficient was increasing at 2.1eV. The unit cell volume of La single doped system expanded,the forbidden band gap was decreasing slightly and the system showed red shift. The unit cell volume of La-N co-doped system also expanded, the forbidden band gap was obviously decreasing, there was impurity level in the system and it also appeared in the absorption spectrometer. The degree of red shift was stronger than it was in La single doped system. The static dielectric constant and the index of refraction are much larger than they were in La single doped system.Finally, the unit cell distortion of the remaining four rare earth elements in single doped system and co-doped system were as same as the anatase TiO₂ doped system. The forbidden band gap of Ce single doped system had increased slightly and there was no red shift in the system. The forbidden band gaps ofthe other elements in the doped system were greatly decreasing and there was red shift in the system. With the codoped of rare earth elements and N element, the forbidden band gap of Ce-N co-doped system was decreasing, the forbidden band gap of the other co-doped systems disappeared, the systems emerged the energy level degeneracy and the degree of red shift was better than it was in single doped system.Compared with pure rutile TiO₂, the static dielectric constant and the index of refraction of the doped system had been changed and the energy loss peak moved to the low energy region.