Structure And Optical Properties Study Of Rare Earth Elements Doped Titanium Dioxide

Titanium dioxide(Ti O2), an important wide-band-gap semiconductor, has received much attention in a large variety of functional applications including solar cells and photocatalysis due to its wide set of interesting properties, such as excellent optic-electronic characteristics, high light-conversion efficiency, strong oxidation power, nontoxic, etc. However, many of the potential applications are seriously limited by the properties of the Ti O2 material itself, among which the leading factors are the wide energy gap(~3.2 e V) and anatase phase metastability. Doping with various metallic or nonmetallic ions has been considered as one of the effective methods in designing and synthesizing of Ti O2 with improved performances. The rare earth(RE) elements with unique electronic configurations have been paid more attention in the field of semiconductor because of their wide range of optical,electronic, magnetic and photocatalytic characteristics. Recently, a series of lanthanide ions(La, Ce, Pr, Eu, Dy, etc.) doping Ti O2 have been extensively studied both experimentally and theoretically, and considerable success has also been achieved in different fields ranging from photocatalysis, solar cells and gas sensors.Moreover, numerous studies indicate that both photocatalytic activity and phase stabilization of Ti O2 can be significantly enhanced by doping with RE elements.The present work in this thesis is focused on the effect of RE(Tm and Nd)doping on the electronic and optical properties of both anatase and rutile Ti O2. All the band structures, charge densities and optical properties of RE(Tm and Nd) doping Ti O2 systems have been theoretically investigated by means of the plane-wave density functional theory method as implemented in the first-principle computer software package CASTEP, where the exchange and correlation contributions are calculated with the Perdew-Burke-Ernzerhof generalized gradient approximation(GGA-PBE)functional. The main results are given as follows:Firstly, three different Tm-doping configurations are obtained when one Tm atom is introduced at the regular Ti lattice site in different supercells of both anatase and rutile Ti O2, which are corresponding to the doping levels of 1.39 at%, 2.08 at% and4.17 at%, respectively. Both the electronic structures and optical absorption properties have been theoretically investigated. Based on the analysis of band structures and density of state, it indicates that the band gap is clearly narrowed owing to the introduce of Tm 4f orbital. In this work, the Tm doping narrows the band gap to amaximum value of about 0.24 e V in anatase Ti O2 at a doping level of 0.0139,however, a maximum band gap narrowing of 0.13 e V is obtained in rutile Ti O2 at a doping level of 0.0417. Furthermore, the optical absorption spectra do reveal that a significant red shift results from all Tm-doping anatase phases.Secondly, three different Nd-doping configurations are obtained when one Nd atom is introduced at the regular Ti lattice site in different supercells of both anatase and rutile Ti O2, which are corresponding to the doping levels of 1.39at%, 2.08at% and4.17at%, respectively. Both the electronic structures and optical absorption properties have been theoretically investigated. Based on the analysis of band structure density of state, it indicates that the band gap is clearly narrowed owing to the introduced Nd-4f orbital. In this work, the Nd doping narrows the band gap to a maximum value of about 0.91 e V in anatase Ti O2 at a doping level of 0.0417, and a maximum band gap narrowing of 1.06 e V is obtained in rutile Ti O2 at a doping level of 0.0417 as well.However, the optical absorption spectra has no obvious changes in the case of Nddoping rutile Ti O2.