The First-principle Study Of Nitrogen And Chalcogen Elements Codoped TiO₂

In recent years, the problems from environmental pollution and energy crisis haveseriously restrained the development of economy and society. TiO₂, as the most excellentphotocatalyst, has attracted much attention because of the best properties such as long-termstability against chemical corrosion, nontoxicity and low cost. However, the wide band gap ofTiO₂about （Eg～3.0-3.2eV） and the low availability of solar energy limit its application.In this paper, the electronic properties of nitrogen and chalcogen elements codoped rutileand anatase TiO₂were systematically calculated by VASP using the first-principle plane-waveultrasoft pseudopotential methods which were based on the density functional theory （DFT）.In the codoped systems, N element substituted O atom and chalcogen elements replaced by Tiatom.2×2×2supercell models of TiO₂were established and then optimised to interpret theimpacts on photocatalytic activity from optimized geometries, substitution energy, bandstructure, total density of states and Bader charge analysis.（1）From optimized geometries, it can be seen that lattice parameters and volumes havechanged as the different atomic radius of doped atoms, which eventually brought out thelattice distortions and volume expansions.（2）According to the results of substitution energy, for system of N and chalcogenelements codoped rutile TiO₂, Te and N/Te had the biggest substitution energy while N hadthe smallest energy. For the incorporations between nitrogen and chalcogen elements inanatase TiO₂, the effects of substituting Ti atoms is not as good as substituting oxygen atoms.It is because the differences of atomic interaction force stemmed from the variable atomicradius of doped atoms.（3）Based on analysis of the band structure and total density of states （DOS）, it is displayed in the figures that the band gap has been narrowed by the introduction of impuritystates which are composed of strongly hybridized orbitals of N2p, S3p, Se4p and Te5p intotal band structure. The conduction band is mainly dominated by Ti3d orbital. When Tiatoms are substituted chalcogen elements, the S3p （Se4p or Te5p）-N2p-Ti3d mixed statesare formed. Meanwhile, duo to the spin polarization of the localized bands and the extendedstates in the band edge from the codoped systems, the photocatalytic activity can also beimproved obviously.（4）From Bader charge analysis, when oxygen atoms are substituted by nitrogen andchalcogen elements, the electrons transfer from nitrogen to chalcogen elements, whileelectrons transfer from chalcogen elements to nitrogen when oxygen atoms are replaced bynitrogen and Ti atoms are substituted by chalcogen elements.In the nitrogen and chalcogen elements codoped rutile and anatase TiO₂systems, fromcodoped method, it is easier for electrons to transmit between valance band （conduction band）and impurity states by decreasing the recombination rate of electron-hole pairs. Therefore, animproved photocatalytic activity in the visible region has been achieved.