First Principles Study On Improving Photocatalytic Activity Of TiO₂

Recently,problems of energy and ecological environment seriously restrict the sustainable development of social economy,therefore how to govern environmental pollution and find renewable energy have attracted wide attention of the researchers.Among them,the sun which is a kind of renewable energy can not only provide hydrogen by catalyzing water decomposition on semiconducting materials,but also provide methods of solving the problem of ecological environment by light catalysis to decompose organic pollutants.Photocatalytic technology of semiconductors is used in the process of hydrogen production and environmental pollution improvement,and titania?TiO₂?is one of the photocatalytic semiconductor materials extensively studied by science and technology workers.TiO₂ has many advantages such as low price,stable chemical performance in solution,good photocatalytic performance and so on.However,the band gap of TiO₂ is so wide that it is only sensible to ultraviolet photons,greatly reducing the utilization of solar energy.In order to improve the photocatalytic performance of TiO₂,this dissertation uses the first principles method to study the photocatalytic performance of the rutile TiO₂?110?surface absorbed by halogen,and to interpret the better photocatalytic performance of the rutile and anatase TiO₂ composite structure through the band alignment technique.The whole thesis is organized as follow:In the first chapter,the research background is introduced.Firstly,the photocatalytic principle,TiO₂ crystal and band structures are present.Then the application of TiO₂ to water decomposition,environmental pollution control,anti-mist and other fields are summarized briefly.Finally,several common methods for modifying the surface of TiO₂,such as the metal,nonmetal,inorganic modification are outlined.Inorganic modification is selected for the study.In the second chapter,the theory and calculation methods about the study are introduced,including the density functional theory?DFT?and DFT+U,and VASP software package.In the third chapter,the effect of halogenation of the TiO₂?110?surface on the photocatalytic performance is studied.The stability of halogen at the surface is understood by means of adsorption energy.The relationship between the photocatalytic performance of F-TiO₂ and the solution pH is revealed by comparing the density of states and band edge positions,and the appropriate conditions for best photocatalytic effect of F-TiO₂ are obtained.Finally,the instability of Br-,I-TiO₂ structures and their better photocatalytic effect due to band tails are explained.In the fourth chapter,the photocatalytic performances of the rutile and anatase TiO₂ composite structure are studied.According to our computation,the band edge positions between the two solids depend on their distance d.When d is less than 5 A,a coupled band alignment is shown.The photogenerated electrons?holes?aggregate in the conduction?valence?band of anatase?rutile?,and the photocatalytic efficiency of the TiO₂ composite phase is increased due to spatial separation and migration of photogenerated electron-hole pairs.When d is greater than 5 A,the results are consistent with the observations of the EIA method,which is bad for photocatalytic reaction.The fifth chapter is devoted to the conclusion and perspective.