空位缺陷对β-AgVO_3电子结构和光吸收性能的影响

基于密度泛函的第一性原理研究了Ag空位、O空位和Ag-O双空位对β-AgVO_3的电子结构及光学性质的影响.采用广义梯度近似平面波超软赝势GGA+U方法,对不同缺陷体系的形成能、能带结构、电子态密度、差分电荷密度和吸收光谱进行了计算和分析.通过比较不同Ag空位和O空位的形成能,确定了β-AgVO_3中主要形成Ag3空位和O1空位,并且Ag空位较O空位更容易形成.Ag3空位和O1空位的存在都使得β-AgVO_3带隙有一定程度的减小;Ag3空位使β-AgVO_3呈现p-型半导体性质,而O1空位和Ag3-O1双空位使β-AgVO_3呈现n-型半导体性质.Ag3和O1空位对晶体在可见光范围内的光吸收影响较小.

Electronic structure and optical absorption properties of β-AgVO3 with vacancy defects

Semiconductor photocatalysts have received much attention due to their applications of wastewater treatment and air purification. The monoclinic β-AgVO₃, which has narrow band gap (2.11 eV) and can respond to visible light, has been considered as one of the promising semiconductor photocatalysts. The vacancy defects always exist in β-AgVO₃ prepared under the conventional synthesis conditions and have important influences on the structure and properties of β-AgVO₃. Systematic theoretical study of the vacancy defects in β-AgVO₃ is still lacking. In this paper, using density functional theory plus U (DFT+U) approach, the Ag vacancy, O vacancy and Ag-O bivacancy in β-AgVO₃ are studied. The formation energy, band structure, differential charge density and optical absorption spectrum of β-AgVO₃ with vacancy defects are carefully investigated. When the U values are chosen as 6 eV and 2.7 eV for the Ag-4d and V-3d electrons respectively, the reasonable lattice parameters and band gap value can be obtained for β-AgVO₃. By comparing the formation energies of different Ag and O vacancies, we find that the dominating vacancy defects in β-AgVO₃ are Ag3 and O1 vacancies, and the formation of Ag vacancy is much easier than that of O vacancy. The analyses of the total and partial density of states indicate that the conduction band arises mainly from V-3d orbit, and the valence band is mainly composed of Ag-4d and O-2p states for β-AgVO₃. With Ag3 vacancy, O1 vacancy or Ag3-O1 bivacancy, the band gaps of β-AgVO₃ all decrease in different degrees. The Ag3 vacancy behaves as p-type donor, allowing the Fermi level to shift down to the valence band maximum. However, O1 vacancy and Ag3-O1 bivacancy both act as n-type donors, and the Fermi level shifts to the conduction band minimum. The change of the Fermi level for the vacancy defect systems also means that the charge transfer occurs among the atoms around the vacancy, which is analyzed by calculating the differential charge density. The Ag3 vacancy and O1 vacancy have little effects on the light absorption of β-AgVO₃ in the range of visible light, while O1 vacancy and Ag3-O1 bivacancy in β-AgVO₃ cause the obvious absorption of light in the near infrared region.