Theoretical Study On The Optical Properties Of Doped Anatase TiO₂

Titanium oxide semiconductors have attracted increasingly attention in recent years due to its many advantages including of non-toxic, stable, strong oxidizing character and excellent photoelectron-chemical effect. However, TiO₂ can only be activated by the ultraviolet irradiation due to its relatively large band gap about 3.2eV for the anatase phase. Ultraviolet in the nature light is about 5%, which is difficult to develop photochemical materials and dye sensitized solar cells of TiO₂. The effective use of visible light to expand TiO₂ scope of application and improve the TiO₂ absorption in the visible spectrum, so that its visible absorption edge occur red shift to enhance the TiO₂ photocatalytic ability in the visible region, which is one of the main research for TiO₂. In this thesis, according to some hot and difficult questions of TiO₂ photocatalytic research, computer simulations have been made to predict the electronic structures and optical properties for doped anatase TiO₂ with the different non-metallic and transition metal elements. The first-principles density functional calculation was chose to investigate the electronic structures and optical properties of non-metallic C/N/F-doped, transition metal Cr/Cu/Zn/V/Sc-doped and C-Cu co-doped anatase TiO₂. In chapter one, the categories, progress in research, research meaning and optical modified methods on photocatalytic performance of TiO₂ were reviewed. The works on this topic and research meaning are introduced.In chapter two, the first-principles density functional theory and calculation methods are descussed. Density functional theory with local density approximation and generalized gradient approximation was explained. Numerical methods for density functional theory such as Linear Combination of Atomic Orbitals and Pseudopotential methods were illustrated. The common packages for the first-principles, such as Gaussian, Material Studio, VASP, WIEN, PWSCF, SIESTA, ADF, ABINIT, CPMD and Octopus, were given a brief introduction. Some operational techniques and features for CASTEP program were described in detail.In chapter three, the energy band structures, density of states, electronic density and optical properties were calculated for non-metallic C/N/F-doped anatase TiO₂, respectively. The results show that, C/N in the anatase TiO₂ is acceptor impurity. It is clear that forbidden band is narrowing due to the formation of impurity energy levels by hybridized with O 2p states, Ti 3d states, and C/N 2p states, which lead to red-shift of the absorption edges toward visible-light region. F atom in the anatase TiO₂ is donor impurity. F doped anatase TiO₂ has lower energy levels because of strong electronegativity, which cause the valence band to shift toward lower energy levels, the optical band gap to narrow and the absorption edges to slightly blue-shift toward ultraviolet region. C doped anatase TiO₂ has better improved effects in the non-metallic C/N/F elements.In chapter four, the energy band structures, density of states, electronic density and optical properties were calculated for the Cr/Cu/Zn/V/Sc-doped anatase TiO₂, respectively. The results show that Cr in the anatase TiO₂ is a donor impurity. It is clear that optical band gap is narrowing due to the formation of impurity energy levels by hybridized with O 2p states, Ti 3d states, and Cr 2p states leading to red-shift of the absorption edges toward visible-light region. Cu in the anatase TiO₂ is an acceptor impurity. The holes originated from O 2p states. The optical band gap is narrowing due to the formation of the impurity energy levels from the O 2p hole states, which result in red-shift of the absorption edges toward visible-light region. Zn atom in the anatase TiO₂ is acceptor impurity. The optical band gap between O 2p states and Ti 3d states is basically the same in the both Zn-doped and pure anatase TiO₂. As a result optical absorption edges are essentially unchanged. V atom in the anatase TiO₂ is donor impurity. It is clear that optical band gap is narrowing due to the formation of the impurity energy levels from the V 3d states leading to red-shift of the absorption edges toward the visible-light region. Sc atom in the anatase TiO₂ is acceptor impurity. The optical band gap is essentially unchanged, which is for that reason absorption edges of Sc-doped TiO₂ don't red-shift toward the visible-light region. Cu-doped TiO₂ has better improved optical effects in five doping metallic elements.In chapter five, the electronic structures and the optical properties were calculated for metallic C-Cu-codoped anatase TiO₂. The results show that both of C and Cu elements in the anatase TiO₂ are donor impurity. Three impurity levels originating from C 2p states appear in the forbidden band. An impurity level from O 2p states appear in the band gap. The narrowing optical band gap is obviously observed due to the formation of impurity energy levels, which lead to red-shift of the absorption edges toward visible-light region. Therefore, C-Cu-doped anatase TiO₂ has better improved optical effects than theirs single doped.