First Principles Calculation On Electronic Structure And Optical Properties Of Anatase TiO₂ Doped Sixth Cycle Transition Elements

The semiconductor material TiO₂ has many advantages,such as stable performance,strong photocatalytic ability,no toxicity,easy preparation,high efficiency and is inexpensive.It is a very good photocatalyst and widely used in the treatment of the environment,Which anatase advantage is more prominent,and thus by the people's attention.However,pure TiO₂ has large band gap and only responds to ultraviolet light less than 384 nm wavelength.Therefore,extending the range of TiO₂ response to visible light is beneficial to the enhancement of photocatalytic activity of TiO₂.In this paper,anatase TiO₂ was used as the intrinsic body,whose doping elements are the sixth period transition elements?X=Hf,Ta,W?,is calcuted.And the suitable doping concentration of Hf,Ta and W for anatase TiO₂ was found by calculation and analysis.After fixing the chose appropriate mix ratio of X element,S would be co-doped in the systems.The cell volume,density and other properties of the pure and co-doped systems are computed and comparatively analyzed.Conclusions are drawn as following.?1?When X elements are doped in anatase TiO₂,the lattice parameters are changed,and the lattice distortion of the doped TiO₂ makes the volume of the doped system larger with the increase of X doping concentration.When S is doped with anatase TiO₂,the lattice distortion occurs due to the ionic nature of the Ti-S bond is stronger than the Ti-O bond before doping.After Transition metals X?X=Hf,Ta,W?and S are co-doped with anatase phase TiO₂,the lattice parameters change,the lattice becomes further distorted,and the volume is increased.?2?With the addition of Hf,Ta and W doping concentration,relative to the pure anatase TiO₂,the conduction band is continuously moving down,and the valence band top position does not change much with the Ta-doped system relative to pure anatase TiO₂,the valence band of the other two doped system moves down gradually.With the increase of doping concentration of Hf or W doped system,the degree of conduction band down is greater than the valence band,which leads to the narrowing of the forbidden band width of the co-doped system,and effectively enhance the photocatalytic activity of TiO₂.When the doping concentration of Hf,Ta and W is 0.125,the doping amount is ideal,which is suitable to be used as co-doped substrate.The transition metal X?X = Hf,Ta,W?single admixture system were compared with pure anatase TiO₂,and the reflection coefficient decreases in the visible region.The same concentration Hf,Ta,W transition element doped,the static refractive index increases in turn.The conduction band bottom of the doped system moves down after the doping of S with the TiO₂,and the top of the valence band does not change much,so the band gap narrows,relative to pure anatase TiO₂,the electronic state moves toward low energy,which enhances the photocatalytic activity of TiO₂.At the same time,the absorption spectrum of S doped system has a red shift,and the photocatalytic activity has been greatly improved..?3?With the co-doping of Hf,Ta,W and S,the conduction band is continuously moving down,and the valence band top position does not change much with the Ti_0.875Hf_0.125O_1.875S_0.125 system relative to pure anatase TiO₂,the valence band of the other two co-doped systems gradually shifted down,resulting in impurity energy level,while the conduction band down a large degree,greatly reducing the gap width,effectively enhance the photocatalytic activity of TiO₂.After the transition metal X?X=Hf,Ta,W?and S were mixed,the conduction band is determined by Ti-3d,X-5d and O-2p,the O-2p state,Ti-3d state,X-5d state and S-2p state are hybridized to affect the valence band,and the electron state moves toward the low energy direction compared with the pure anatase TiO₂ and single dopant,Among them,Ta,W and S co-doped system,the electrical conductivity of TiO₂ is enhanced.After the co-doping of the transition element with the S element,the red shift of the absorption spectrum of the system is more obvious,which increases the response range to visible light,and the reflection coefficient of the co-doped system decreases in the visible region.The static dielectric constant and static refractive index of Hf,Ta,W and S co-doped systems increase in succession(Where Ti_0.875Hf_0.125O_1.875S_0.125 system static dielectric constant and static refractive index are less than pure anatase TiO₂).The results show that the transition element and the S element are single-doped and co-doped to change the electronic structure of TiO₂,narrow band gap,so that the absorption band edge moves toward the low energy region,and the absorption peak appears.It is suggested that the single element of the transition element and the co-doping with the S element can effectively improve the response of TiO₂ to visible light.In this paper,the reason of redshift is explained from the mechanism of electron transition,which provides the direction for the researchers to improve the photocatalytic efficiency of TiO₂ by doping.