Density Functional Theory Explore Defect Titanium Dioxide Electronic Structure

With the improvement of people’s living condition,energy shortage and environmental pollution have become urgent crisis in nowadays.Photocatalysis is considered to be an effective measures to produce clean and renewable energy.Therefore,photocatalyst has been widely studied.Titanium dioxide（TiO₂）has been extensively used in the fields of photocatalysis,degradation of organic pollutants and artificial hydrogen production,due to its excellent photocatalytic performance.The main shortcomings of titanium dioxide are the wide band gap and the high photogenerated electron-hole recombination rate,which can reduce the photocatalytic activity of titanium dioxide.In this thesis,the first principles calculation based on Density Functional Theory（DFT）and Hubbard U correction are used to explore the influence of defective titanium dioxide electronic structure.Hydrogen production through water splitting by TiO₂ is very popular.In this thesis,DFT is used to calculate the adsorption energy of water（molecular and dissociated states）on rutile TiO₂（110）slab,with different O-Ti-O layers and different vacuum thicknesses.The relationship between adsorption energy and model structure is obtained by analyzing the adsorption energy of H₂O in different states on different TiO₂surfaces.The results shows that the adsorption energy is mainly related to the number of model layers.Once the four O-Ti-O layered slab is adopted,the molecular states on the adsorbed surface is relatively stable.When the slab is changed to five O-Ti-O layers,the dissociated states on the adsorbed surface is relatively stable.In practical applications,the introduction of surface defects is the most commonly used method to improve the photocatalytic activity of rutile TiO₂.Two common surface defects models of rutile TiO₂（110）surface,Oxygen vacancy and Ti interstitials were built according to previous research.The Density of States（DOS）of each defective states was calculated using DFT+U method.The above two kinds of surface defects result in the gap state between valence band and conduction band,which mainly comes from the 3d orbital of Ti³⁺ion in the surface defect systems.Many studies have proved that the gap state improves the photocatalytic activities of rutile TiO₂.To verify the influence of on-site correlation energy on the Hubbard band of rutile TiO₂,a series of on-site correlation values were added to the Ti-3d orbit of the rutile TiO₂（110）surface with hydroxyl defects.The DOS and band structure under each Hubbard U are calculated respectively.The DOS show that,the Coulomb repulsion potential between the low Hubbard band and the high Hubbard band increases with the increases of the U_Ti values.The gap state move towards the valence band maximum,while the other components move away from conduction band minimum.It is confirmed that the linear relationship between the Hubbard band and on-site correlation energy i.e.U values.Subsequently,the band structure of rutile TiO₂ primitive cells was calculated,and it was concluded that the band gap of rutile TiO₂primitive cells gradually increased with the increase of on-site correlation energy.Combined with the results of DOS above,the influence of the on-site correlation energy on Hubbard band was verified.The above conclusions show that setting a series of on-site correlation values on the Ti-3d orbit can affect the energy-level splitting of the Hubbard band.The findings help to further understand the electronic structure modification of rutile TiO₂.