氮铁共掺锐钛矿相TiO_2电子结构和光学性质的第一性原理研究

First-principles study on electronic structure and optical properties of anatase TiO_2 codoped with nitrogen and iron

本文采用基于密度泛函理论的平面波超软赝势方法研究了N,Fe共掺杂TiO2的晶体结构、电子结构和光学性质.研究表明,N,Fe共掺杂TiO2的晶格体积、原子间的键长及原子的电荷量发生变化,导致晶体中产生八面体偶极矩,并因此光生电子-空穴对有效分离,提高TiO2的光催化活性;N,Fe共掺杂同时在导带底和价带顶形成了杂质能级,使TiO2的禁带宽度变窄,光吸收带边红移到可见光区,这些杂质能级可以降低光生载流子的复合概率,提高TiO2的光催化效率;与Fe掺杂TiO2的态密度相比,共掺杂位于价带顶的杂质能级的态密度峰明显增大,导致电子从杂质能级跃迁到导带的概率增加,使其对太阳能的利用率提高;在不考虑杂质能级的情况下,与纯TiO2相比,N,Fe共掺杂TiO2的带边位置只有微小变化,因此N,Fe共掺杂TiO2的强氧化还原能力得以保持.

The crystal structure,electronic structure and optical properties of nitrogen and iron codoped anatase TiO2 were studied by using the plane-wave ultrasoft pesudopotentials method based on density functional theory.The calculated results show that the octahedral dipole moments in nitrogen and iron codoped TiO2 increase due to the changes in lattice parameters,bond length and charge of atoms,which is very effective for the separation of photoexcited electron-hole pairs and the improvement of the photocatalytic activity of TiO2.Some impurity energy levels of codoped TiO2 are below the conduction band minimum,and others are above the valence band maximum.The distance between them is narrowed,which results in the redshift of the optical absorption edges to visible-light region.These impurity energy levels can reduce the recombination rate of photoexcited carriers and improve the photocatalytic efficiency of TiO2.Compared with that of Fe doped TiO2,for the codoped TiO2,the density of states peak of impurity energy levels above the valence band maximum increase apparently,which increases the electronic transition probability from the impurity energy levels to the conduction band,and improves the solar energy utilization.If the impurity level is not taken into account,compared with that of pure TiO2,the CB edge position and the VB edge position of codoped TiO2 is only slightly changed,it means that the strong redox capacity of codoping photocatalysts is still excellent.