Study On Doping And Photocatalytic Properties Of Anatase TiO₂

From the beginning of the 20th century, the global environment is deteriorating increasing and the natural resources are depleting. The key to realize the strategy of sustainable development is to effectively control and curb environmental pollution as well as develop new fuels.TiO₂ is a new kind of semi-conduction material and photocatalytic material. As an ideal photocatalyst, TiO₂ not only can be used to purify the environment but also is a type of material for solar cell people pay much attention to, just due to its advantages such as nontoxic, excellent chemical stability, low-cost and high oxidation. However, because of its wide band gap(3.2eV), TiO₂ can only be irradiated under UV(λ≦387nm)that accounts for a very small fraction (～5%) in the spectrum on earth, so its solar energy utilization is very low .The aim people have been working hard for is to make the absorption band shift to the red light and realize its visible spectral response by adapting the energy gap of TiO₂ through doping. Thus, the researchers focus on the study of TiO₂ doping.In this paper, through adopting the first-principles method based on density functional theory(DFT) and using the computer program VASP, we investigate the effects of TiO₂ doped by N,V,Cr,Cu,Co,Ni and the influence of oxygen vacancy on electronic structure and conductive properties of anatase TiO₂ doped by transition metal elements. The conclusions are :1. Anatase TiO₂ without defect is a large band-gap semi-conductive material. By analyzing the band diagram and the density of states diagram, we know that the valence band(VB) of anatase TiO₂ is made up by the mixture of the electrons of Ti atom on the 3d orbit and the electrons of O atom on the 2p orbit. Meanwhile, the diagrams also show that the electrons on the p orbit of O atom and the electrons on the d orbit of Ti atom make a larger contribution to the energy band near the Fermi level.2. The impurity level is formed in the band gap of doped TiO₂ In the study of the electronic structure on N-doping anatase TiO₂ we find that the half-full impurity level is produced near the top of the VB just because Ti atoms and N atoms at the valence band area happens a kind of strong correlated interaction. Therefore, the electronic transition happens between the impurity level and VB, at the same time the Fermi level moves to the high energy area. As a result, the band gap will narrow. In the studies of anatase TiO₂ doped by transition metal (V,Cr,Cu,Co,Ni), we find that, to some extent, the positions of VB and CB of the doping system move to the low energy area comparing with pure TiO₂,which will enhance its redox capacity. And the impurity state is formed mainly by the electrons of transition metal atom on the 3d orbit.3. O vacancy can improve conductive properties of anatase TiO₂ doped by V, Cr.