The Investigation Of Visible-light Photocatalytic Activity Of N-doped Anatase TiO 2 By First Principles

Along with the swift development of computer technology and the constant improvement of computational methods of quantum chemistry, material computational technology has become more and more important in modern physics. This is because it has been used to interpret the experimental phenomena, and the special important role is that it can be used to predict and develop the new functional materials. On the other hand, recent researches have reported that N-doped TiO₂ can remarkably improve the photocatalytic activity under visible light irradiation, and it has received extensive attention. However, the uniform unambiguous understanding on the mechanism of visible-light photocatalysis of N-doped TiO₂ is still lacking, and its electronic structural properties still need to be deeply investigated by modern computational simulation techniques. So the first principles calculations are employed to investigate some systems about N-doped TiO₂ in this paper. Based on the calculated results, we further discuss the reasons of the electronic structural change and the visible-light photocatalytic activity of all the doped systems. The main contents are presented as the followings:In the first chapter, some main basic information about TiO₂ is introduced. Titanium dioxide, as one of the most promising photocatalyst, has exceptional properties such as nontoxicity, low cost, and long-term stability against chemical corrosion; and has attracted worldwide research concerns. Firstly, we simply introduce the basic principle of photocatalyic oxidation and the major applications of the TiO₂. Secondly, some common methods used to improve the visible-light photoactivity of TiO₂ by doping are narrated. Then, we highlight the progresses and some unresolved problems in research on N-doped TiO₂. Finally, based on the present problems, we describe the purpose and meaning of our research on N-doped TiO₂。In the second chapter, we firstly give a brief review on the development of computational material science and quantum chemistry. Then, we focus on describing the theoretical basis of the density-function methods and several common exchange-correlation functional approaches in details. At last, we simply introduce the simulation package Castep used in this work.In the third chapter, we investigate the anatase TiO₂ doped with N by using spin-polarized plane-wave method based on density functional theory. The calculated results show that in comparision with pure TiO₂, the conduction band minimum is almost unchanged. However, its valence band maximum shifts to high energy by 0.272 eV, and the band-gap states composed of N 2p, O 2p and Ti 3d states are formed through the three states entering into the gap. The combinations of the three states make the band-gap states expanded and delocalized to some extent. The origin of the electronic structural changes for N-doped TiO₂ is revealed by the electron density difference and the population analysis. It is because that the N-doped TiO₂ supercell has one electron less than pure TiO₂ one and the comparatively stronger N-Ti covalent bonds are formed due to the effect of electronegative difference between nitrogen and oxygen. The lifted VBM and the wide band-gap states enhance the visible-light photocatalytic performance of N-doped TiO₂, providing a good interpretation for the experiment phenomena.In the fourth chapter, we systematically investigate the effect of oxygen vacancy on the electronic structure and the photocatalytic performance of N-doped TiO₂ by calculating three kinds of models. Considering the interactions between the oxygen vacancy and the N dopant, we employ two different methods to calculate the electronic structural properties of the three systems, respectively. The DFT+U and the standard DFT methods are used to study the systems with and without donor states, respectively. The calculated results of titania containing oxygen vacancy show that the oxygen vacancy introduces the deep donor states in band gap, and the distributions of the band-gap states have highly localized character in real space. The calculations of TiO₂ supercell including one oxygen vacancy next neighboring to a nitrogen indicate that there exist two localized density of state peaks in band gap. One peak just locates above the valence band maximum and the other one sites at about 1 eV above the valence band maximum. The obtained data by calculating the TiO₂ supercells with one oxygen vacancy next neighboring to two N, describe that the valence band maximum is shifted to high energy by 0.18 eV as the two NTi₃ units lie in different plane, and that there localizes one density of state peak just above the valence band maximum as the two NTi₃ units place in the same plane. According to the data, we further analyze the reason of electronic structural change for every system. At last, the visible-light photocatalytic activities of all systems are simply discussed.In the fifth chapter, the ultrasolft-pseudopotential spin-polarized plane-wave method is used to investigate the influence of H on the N-doped TiO₂. The calculated results show that the H bonding to the N in N-doped TiO₂ can improve the stability of systems, and hence increases the contents of N dopant. Under certain conditions, the TiO₂ containing N bonded with H is more stable than the TiO₂ doped with single N even if the N concentration in former system is twice as high as that in the latter one. In addition, the electronic structure calculations for all systems show that the valence band maximum of N/H-codoped TiO₂ slightly shifts to high energy and the band-gap states of TiO₂ doped with single N disappear. This means that with the same N contents, the single N-doped TiO₂ has advantage over the N/H-codoped TiO₂ in improvement of visible-light photocatalytic performance. However, for the TiO₂ supercell including one nitrogen as well as a N bonded to a H, the valence band maximum is upraised by about 0.54 eV, the acceptor states weakly mix with valence band, and the distribution of the electronic states near the Fermi level has highly delocalized nature compared to the single N-doped TiO₂. All these factors can promote the increase of visible-light photocatalytic activity. The reasons of electronic structural variations for all N-doped TiO₂ with H, are mainly attributed to the relatively higher electron energy in N-Ti bond.