First Principles Study Of Nano-TiO₂ And Their Derivatives Photocatalyst Modification

Titanium dioxide (TiO₂) has received much attention as a promising material for photocatalysis, which is relatively wide in band gap, chemically stable, strongly reductive and oxidative, non-poisonous, environmentally friendly, et al. In the photocatalystic reaction process of TiO₂, the photo-generated electrons and holes have strong reductions and oxidations, respectively. However, the wide band gap requiring ultraviolet irradiation limits the practical application; so much effort has been made to develop second-generation photocatalysts in recent several years. Photocatalystic active and efficiency of TiO₂ is impacted by several factors. And the focus on improving the photocatalystic power of TiO₂ has taken four points into consideration: (1) Broaden the visible light absorption region; (2) Reduce the electrons and holes recombination ratio and enhancing the photo-quantum efficiency; (3) Adjust the conduction band level and valence band level to improve the reducing and oxidizing power of the surface; (4) Obtain higher active surface area. According to above the need of requirements, the new methods to improve the photocatalystic properties are considered and to design the highly photocatalystic active materials are performed by first principles calculations in this dissertation. The conclusions have outlined as follows:Firstly, seven known TiO₂ polymorphs crystal structure are established and then the structural optimizations are performed to obtain the equilibrium structural parameters, such as, total energy, enthalpy, elastic constants and energy band structure, et al. and further analyze the relative stability, elastic properties and electrical properties. On the basis of enthalpy versus pressure data obtained from our GGA calculations for high-pressure forms, it is demonstrated the phase transition pressure and the expected sequence of the phase transition of TiO₂. Furthermore, based on the third-Murnaghan equation and Voigt-Reuss-Hill theory, respectively, the elastic properties are predicted, which are in good agreement with the experimental results at lower pressure. In addition, the energy band structure shows that fluorite-type TiO₂ has a narrow band gap and high electrical conductivity for potential photocatalytic applications under visible light.Secondly, the geometry structure and stability of intrinsic point defects for TiO₂ are studied by first principles calculations. The results show the defect types and defect concentrations are related to the nonequilibrium growth condition. In general, under the O-rich condition, Tii4+ would form spontaneously, and under the Ti-rich condition, Tii4+ and VO2+ easily appear in Schottky defects. To obtain highly effective photocatalysts, the incorporation of impurities in TiO₂ has been made to modify the energy band structure. It is found that the formation energy of N- or C- doped TiO₂ is high, resulting in the lower p-type dopant concentration. In order to solve the p-type doping bottleneck, codoped TiO₂ with nonmetals and transition metals is studied. We find codoping have great influence on the original geometry, namely, TM/NM defect pairs tend to bind to each other. In addition, the energy band structures of codoped-TiO₂ show that N/V and C/Cr codoping is superior to other codoped systems, due to the passivated donor-acceptor codoping on anatase TiO₂ photocatalysts. Our results may help understanding synergistic effect of codoping approach for improving photoelectrochemical activity of anatase TiO₂.Thirdly, the intrinsic defect structure of clean anatase TiO₂(101) and TiO₂ surface modified noble metal are extensively studied by first principles calculations. The effects of point defects on clean anatase TiO₂(101) are firstly analyzed and the defect formation energy is calculated. The results show that under the O-rich condition, the VTi1 and Oi formation are favorable. However, under the Ti-rich condition, although the point defects of Tii2 and VO1 easily appear, the formation energy of the Tii2 is lower than one of the VO1. And O atom would self-diffusion from surface to bulk. Secondly, the models of the Pt and Au absorbed above TiO₂(101) and vacancy of different atom sites in TiO₂(101) are established and then optimized. The absorption interaction between noble atoms and the surface is small, resulting in a slight effect on electronic structure. Pt and Au is favor of the Ti vacancies under O-rich condition and the O vacancies under Ti-rich condition. These indicate that the appearance of suface vacancies induces Pt and Au wet the surface, and 5d dopant energy levels appear in the band gap. Contrary to Au atoms, Pt atoms tend to diffuse form surface to bulk.Lastly, the structure stability, the electronic structure and optical properties of TM-doped titanate nanotubes (TNTs) and lepidocrocite two-dimensional TiO₂ nanosheet have been investigated by first principles calculations. At first, the layered TNTs structure of Fe, Co or Ru atoms intercalation into the interlayer region are established and then optimized. The results show that doped atoms bond with the surrounding O atoms to form a kind of solid solution. It is found that Fe,Co or Ru intercalation reduces the band gap of TNTs and introduces new energy levels in the bang gap, extending the absorption edge of the doped TNTs well into the visible light region. Moreover, the valence bands of the doped TNTs move toward to the low energy, making the holes more oxidative. The absorption spectrums of the doped TNTs show that Ru-doped TNTs has a stronger absorptive capacity in the visible light region. Secondly, the structures of lepidocrocite two-dimensional TiO₂ nanosheet are established and optimized. The results show that they are much stable and no surface states in band gap. And the calculated band gap is 0.59 eV broader than anatase TiO₂. Furthermore, the dielectric function and absorption spectra shows highly anisotropic characteristics of optical properties. Additionally, the formation energies of Ti and O vacancy with different charges are calculatied. It is found that under Ti-rich condition, the Ti vacancy in charge state -4 has lower formation energy than other defect types. And the charged defects induce several deep surface levels, which is easy to become the recombination center of electron-hole pairs.In summary, the physical and chemical properties of TiO₂ and their derivatives are studies by first principles calculations. The phase transition, elastic properties and electrical properties are predicted. In addition, the formation energy of non-metalic and metallic doped TiO₂ is calculated, and then synergistic effect of codoping approach is discussed for improving photoelectrochemical activity of anatase TiO₂. Furthermore, the surface structures of TiO₂(101) modified by noble metal are optimized and analyzed. Finally, the structure stability and the electronic structure of TM-doped TNTs and lepidocrocite two-dimensional TiO₂ nanosheet have been investigated.