【作者】 陈斌；

【导师】 丁开宁；

【作者基本信息】 福州大学 ， 物理化学， 2014， 硕士

【摘要】 本论文采用密度泛函理论,系统地研究了BiVO4三种晶相(单斜相、四方白钨矿相和四方锆石矿相)和BiMO4 (M=V, Nb, Ta)的几何结构、偶极矩、电子结构、带边位置、有效电子质量和吸收光谱等性质。对Mo掺杂和F掺杂的单斜BiVO4体系,我们详细地研究了掺杂前后体相电子结构和表面吸附能力等性质的差异。本论文取得的主要成果如下：1、系统地研究了BiVO4三种晶相的电子结构、带边位置和吸附光谱等性质。研究表明,单斜相BiVO4的载流子有效质量最小,而且在[010]方向上的偶极矩将有利于其载流子的分离。虽然单斜BiVO4是间接半导体,但它具有直接光学跃迁点,而且其能隙较小,这决定了它在可见光区的吸收要强于另外两种晶相。我们的理论研究很好地解释了单斜相BiVO4具有高活性的原因。2、对比研究了BiMO4(M=V,Nb,Ta)三种化合物的的电子结构、带边位置和吸收光谱等性质。研究发现,BiNbO4和BiTaO4是直接半导体,且其带隙明显比BiVO4的大,这导致了它们对可见光的吸收明显弱于BiVO4。此外,BiVO4还拥有优越的载流子迁移率和沿[010]方向的载流子分离能力。因此,我们预测BiVO4的活性最强。3、系统研究了Mo掺杂对BiVO4体相和表面性质的影响。在体相,Mo更易掺杂到V位,而且掺杂V位能够促进载流子分离。对于表面,Mo更易掺杂到表层Bi位,而且会在Bi位引入表面氧缺陷,同时增大Bi位的表面酸性及对H2O的吸附能。综上所述,我们的理论研究很好地解释了Mo掺杂能够提高BiVO4的光催化活性的实验现象。4、系统研究了F掺杂对BiVO4体相和表面性质的影响。结果表明,无论是在体相还是在表面,F均易掺杂到O位,而且不会引起明显弛豫。电子结构计算表明,F掺杂在体相和表面均能在导带底引入自旋极化的能态,不仅能降低其带隙,还能促进载流子分离。H2O在掺杂表面的吸附研究表明,表面F掺杂能明显增强表面与H2O之间的氢键作用,从而提高其对H2O的吸附能力。我们的理论研究很好地解释了F掺杂能够提高BiVO4的光催化活性的实验现象。更多还原

【Abstract】 In this paper, the geometric structures, dipole moments, electronic structures, band edge positions and optical properties of the three crystalline phases (monoclinic, scheelite-tetragonal and zircon-tetragonal) of BiVO4 and BiMO4 (M=V, Nb, Ta) were investigated by means of density functional theory calculation. And for the Mo or F doped monoclinic BiV04, we have thoroughly studied the variations of electronic structures and surface adsorptive capacities between the pure and doped monoclinic BiVO4. The main achivements of this paper are listed as follow:1. The electronic structures, band edge positions and optical properties of the three crystalline phases of BiVO4 were investigated by means of density functional theory. Our results indicate that the effective mass of carriers for monoclinic BiV04 were examined to be lightest, which implies that it has superior mobility of carriers. Meanwhile, the dipole moment along the [010] direction of monoclinic BiVO4 should contribute to its separation of carriers. Although monoclinic BiVO4 was predicted to be indirect semiconductor, it still has two direct optical transition points and their energy gaps are relatively small, which indicates that monoclinic BiVO4 should have more intensive absorption of visible light. Based on the above analysis, we have successfully explained why the monoclinic BiVO4 show the best photocatalytic activity among the three phases.2. The electronic structures, band edge positions and optical properties of the three compounds of BiMO4 (M=V, Nb, Ta) were investigated to predict their different photocatalytic activities. The results indicate that BiNbO4 and BiTaO4 show direct band gaps which are obviously larger than the indirect band gap of BiVO4. The narrow band gap of BiVO4 should result in its better visible light absorption. By comparing the relative ratio of effective mass, it is found that BiVO4 has not only the superior mobility of carriers but also excellent separation of photoexcited electron-hole pairs in the [010] direction. Because of the narrow band gap, superior mobility and separation of carriers and preeminent absorption in visible light, it is predicted that BiVO4 should have better photocatalytic activity as compared to BiNbO4 and BiTaO4.3. The variations of the bulk and surface properties of monoclinic BiVO4 introduced by the Mo dopant have been investigated by means of density functional theory. For the bulk phase, Mo atoms prefer to substitute the V atoms, which can effectively accelerate the separation of carriers. For the surfaces, Mo atoms prefer to substitute the Bi atoms at the outmost layer, and the Mo doping on the surface will result in the surface oxygen vacancies and the surface acidity of Bi sites may be enhanced, which is confirmed to improve the adsorption of water molecules. Our results demonstrate that the enhanced photocatalytic activity of Mo-doped monoclinic BiVO4 is derived from the facilitated separation of photoinduced carriers and improved adsorption capacity.4. The variations of the bulk and surface properties of monoclinic BiV04 introduced by the F dopant have been investigated by means of density functional theory. The results show that F atoms should be easy to substitute O atoms both in the bulk and on the surface. And both the bulk cells and surface cells were not severely distorted. The analysis of electronic structure shows that the F dopant should introduce spin-polarized energy states below the bottom of conduction band, which would contribute to the narrowing of band gaps and separating photoinduced carriers. The simulation of H2O adsorption on F doped surfaces indicates that the doping could obviously enhance the hydrogen bond between the H2O molecule and F-doped surfaces, which should remarkably improve the adsorptive capacity of monoclinic BiVO4 surfaces. The above improvements introduced by the F dopant should be the origin of the enhanced photocatalytic activity of F-doped BiVO4.更多还原