A DFT Study On The Visible-Light Photocatalytic Activity Of Doped TiO₂

Among all kinds of photocatalysts,titanium dioxide?TiO₂?is considered as one of the most promising environmental-friendly materials,which is due to its excellent properties,such as high oxidizing power,long-term stability,nontoxic,and low cost.However,its wide intrinsic forbidden band gap only allows TiO₂ to absorb the ultraviolet light;On the contrary,it cannot respond to the visible light,which approximately accounts for 43% of solar energy.To effectively improve the utilization of the visible light and enhance the photocatalytic activity of TiO₂,this thesis systematically studied electronic band structures and optical properties of Bi/N,S/Ce,Si/S codoped TiO₂ and Bi or Ce doped TiO₂ with oxygen vacancy?OV?by a Density Functional Theory?DFT?.The main work and primary conclusions are presented as the follows:?1?The crystal structure,electronic and optical properties of Bi/N codoped anatase TiO₂ and rutile TiO₂ are systematically investigated.The calculated results suggest that the outstandingly enhanced visible-light absorption as well as excellent oxidizing and reducting power are both appeared in the Bi/N codoped anatase and rutile TiO₂.In the case of Bi/N codoped anatase TiO₂,its optical absorption intensity is significantly higher than that of pure,single Bi or N doped anatase TiO₂ either in the visible region or in the ultraviolet light region.Similarly,the optical absorption intensity of Bi/N codoped rutile TiO₂ is also stronger than that of pure,Bi or N-monodoped rutile TiO₂ in the wavelength coverage of 400⁴71.6nm.Furthermore,it is found that the Bi/N codoped rutile TiO₂ possesses excellent properties of visible-light absorption intensity compared with Bi/N codoped anatase TiO₂ for the same codoping concentration.?2?The electronic structure and optical propties of Bi doped anatase TiO₂ with OV are also researched.As OV being introduced in Bi doped anatase TiO₂ system,it is found that the band gap of TiO₂ is narrowed substantially.However,the band gap of Bi/OV-codoped TiO₂ is still larger than that of pure anatase TiO₂,which result to apparent blue shift of its optical spectrum.On the other hand,the emergence of shollow donor level located below the conduction band mimimum can be observed as well.Moreover,the Fermi energy level is pinned at the bottom of the conduction band.Therefore,under the visible light irradiation,the optical intensity of Bi/OV-codoped anatase TiO₂ in the wavelength coverage above 460 nm is remarkably strong in comparison to pure and Bi-monondoped TiO₂.At the same time,the conduction band edge position of Bi/OV-codoping system shows marvellous reduction power.?3?The influence of Bi doping concentrations on the electronic and optical properties of rutile Ti1-x Bix O2 have been studied.The calculated results indicate although Bi doping in rutile TiO₂ induces an increasing band gap;but in the visible-light range of 400 to 600 nm,Ti1-x Bix O2 displays a high optical intensity relative to pure rutile TiO₂.With the decrease of the Bi doping concentration,the band gap of rutile TiO₂ considerably becomes small,and the overlap between the impurity level and the energy level of conduction band gradually changes into weakness and even separation from each other.As a result,this impurity level turns into an isolated intermediate energy level.In addition,the visible-light absorption of Ti1-x Bix O2 shows a significant decreasing trend with the decresing of the doping concentration of Bi,whereas its absorption intensity under the irradiation wavelength more than 514.9nm will enhance as the lower doping level of Bi,which can ascribe to the effects of gap states emerged in the forbidden band.?4?Two kinds of S/Ce-codoped anatase TiO₂ are constructed by using various supercell models.One is an anion-cation codoping model;the other is cation–cation codoping model.The calculated results indicate that these two kinds of S/Ce-codoped TiO₂ exhibit higher absorption compared with pure TiO₂,and their band edge positons reveal that these codoping systems can keep excellent oxidizing and reducing power.As for the anion-cation codoping configuration,its optical absorption in visible-light region exhibits higher absorption with the increasing of S/Ce codoping concentration.In order to investigate the synergetic effect of OV and Ce,we further explore preliminarily the electronic and optical properties of the Ce-doped TiO₂ with OV.The band gap of Ce/OV codoping system is discovered to be prominently narrowed,thus,an enhancement of visible-light absorption emerges in the case of the codoping of Ce with OV.?5?The crystal structures,defect formation energies,electronic and optical properties of Si/S codoped anatase TiO₂ are investigated.The optical spectrum indicates that the visible-light absorption intensity of Si/S codoping is higher than that of pure,single Si or S doped TiO₂.Moreover,with the increasing of S doping level,the overlap between the impurity levels and valence band maximum in the band gap of Si/S-codoped anatase TiO₂ become to be more close,which result in a narrower bang gap and a higher visible-light absorption.Especially,the band edge positions of Si/S-codoped TiO₂ with an appropriate codoping concentration of Si and S can profitable for water splitting.