Geometric and electronic structures of three polymorphs of BiPO_4(m MBIP, n MBIP and HBIP) have been investigated by the first-principles calculations. The results show that PO_4 tetrahedron in n MBIP is distorted most, and m MBIP possesses minimum effective mass of carriers in three polymorphs of BiPO_4. Further, the leading role of inductive effect of dipole moment or effective mass of carries in the separation of electron-hole pairs is analyzed. Based on the fact that n MBIP has higher photocatalytic activity than m MBIP, it can be inferred that the inductive effect of dipole moment deriving from distorted PO_4 tetrahedron is the dominant factor affecting the separation efficiency of carries. The calculated results represent that n MBIP has more appropriate redox potential and narrower band gap than others. These findings may provide meaningful guidance for further understanding on the relationship between unique crystal structure and photocatalytic activity of BiPO_4.
The effect of relaxation on the energetics and electronic structure of clean Ag_3PO_4(111) surface has been studied, carried out using first-principles density functional theory(DFT) incorporating the GGACU formalism.After atomic relaxation of the Ag_3PO_4(111) surface, it is found that O atoms are exposed to the outermost surface,due to an inward displacement of more than 0.06 nm for the two threefold-coordinated Ag atoms and an outward displacement of about 0.004 nm for three O atoms in the sublayer. The atomic relaxations result in a large transfer of surface charges from the outermost layer to the inner layer, and the surface bonds have a rehybridization, which makes the covalence increase and thus causes the surface bonds to shorten. The calculated energy band structures and density of states of the Ag_3PO_4(111) surface present that the atomic relaxation narrows the valence band width0.15 e V and increases the band gap width 0.26 e V. Meantime, the two surface peaks for the unrelaxed structure disappear at the top of the valence band and the bottom of the conduction band after the relaxed structure, which induces the transformation from a metallic to a semi-conducting characteristic.
The effect of the distribution of organic cations CH_3NH_3^+(MA^+) on the stability,electronic structures and optical properties of CH3NH3 Pb I3 perovskite have been investigated using the plane-wave ultrasoft pseuudopotentials. Generalized gradient approximation and local density approximation are used to optimize the geometries of six models, which are different in the orientation of organic cations. The results show that model C is more stable than others, and the main contribution to the top of valence band is from I 5p states. In the bottom of conduction bands, the main components are Pb 6s states with an overlapping of I 5p states. When the orientation of organic group is transforming, the Pb I6 octahedra will distort and the band structure will alter with it, which affect the generation and migration of photon-generated carriers and optical properties.
