| 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. |
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