GGA+U的方法研究Ag掺杂浓度对ZnO带隙和吸收光谱的影响

目前, 当Ag掺杂ZnO摩尔数为0.0208-0.0278的范围内, Ag掺杂对ZnO吸收光谱影响的实验研究均有文献报道, 但是, 有两种不同的实验结果, 掺杂体系吸收光谱红移或蓝移两种相悖的报道. 为了解决本问题, 本文采用自旋密度泛函理论(DFT)框架下的广义梯度近似(GGA+U)平面波赝势方法, 构建三种Zn_1-xAg_xO (x=0, x=0.0278, x=0.0417)模型, 分别对所有模型进行几何结构优化和能量计算. 结果表明, 与纯的ZnO布居值和Zn-O的键长相比, 掺杂体系布居值减小、Ag-O键长增加、共价键减弱、离子键增强. 当Ag掺杂ZnO摩尔数为0.0278-0.0417的范围内, Ag掺杂量越增加、O原子2p轨道、Zn原子的4s, 3d轨道电荷数不变、Ag原子的5s轨道电荷数越增加、Ag原子的4d轨道电荷数越减小、掺杂体系晶格常数越增加、体积越增加、总能量越增加、稳定性越下降、形成能越下降、掺杂越难、掺杂体系的带隙越变窄、吸收光谱红移越显著. 计算结果与实验结果相一致. 并且合理解释了存在的问题. 这对设计和制备Ag掺杂ZnO体系的光催化剂有一定的理论指导作用.

GGA+U study on the effects of Ag doping on the electronic structures and absorption spectra of ZnO

Nowadays, the studies on absorption spectra of Ag-doped ZnO have given two distinctly different experimental results, that is, the red shift or blue shift when the mole fraction of the impurity increases in a range from 0.0278 to 0.0417. To solve this contradiction, according to the first-principles plane-wave pseudopotential of the spin-polarized density functional theory (DFT), we set up three models for Zn_1-xAg_xO (x=0, 0.0278, 0.0417) to calculate the geometric structure and energy via the method of generalized gradient approximation (GGA+U). Calculated results indicate that compared with the Zn-O bond in pure ZnO system, the value of population decreases, and the bond length of Ag-O in Ag-doped ZnO system increases, this means covalent bond weakens and ionic bond strengthens. With the mole fraction of impurity increases in a range from 0.0278 to 0.0417, the orbital charges of O-2p, Zn-4s and Zn-3d keep unchanged, while the orbital charge of Ag-5s increases, and that of Ag-4d is reduced; the volume and total energy of the doped system increases, causing the system more unstable. Moreover, the formation energy of the doped system becomes lower, thereby making the doping difficult. Meanwhile, the band gap in the system narrows, and its absorption spectra exhibits a redshift. The calculated results are consistent with the experimental data, and can explain the phenomena reasonably. These results may be used in future design and preparation of new type photocatalyst from Ag-doped ZnO as a theoretical basis.