First principle study of electron and band structure of BixBa1-xTiO3

Abstract

Some perovskite structured catalysts have narrower forbidden band widths than pure TiO2, and they have been widely used in a number of photo-catalytic reactions. The ions in the perovskite may be replaced by other ions while maintaining the structure unchanged for its tailorable character. BiTiO can form into the typical perovskite composite oxide BiTiO3 under specific preparation conditions. The regulation of the energy gap of the perovskite BaTiO3 can be realized by substituting Bi for Ba to form the BixBa1-xTiO3 perovskite structure to improve its photo-catalytic activity. But the improvement mechanism and the electron and band structures of BixBa1-xTiO3 are still not very clear. In this study, we exhibit a detailed theoretical investigation to predict the electronic structure, band gap and optical absorption properties of BixBa1-xTiO3 structures based on the first-principles plane-wave ultrasoft pseudopotential method. The exchange and correlation interactions are modeled using the generalized gradient approximation and the Perdew-Burke-Ernzerhof exchange-correlation functional. The cutoff kinetic energy of the electron wave function is 340 eV, and the k-point sampling sets 333 division of the reciprocal unit cell based on the Monkhorst-Pack scheme. In the geometrical optimization, all forces on atoms are converged into less than 110-5 eV/atom, the maximum ionic displacement is within 0.001 and the total stress tensor decreases to the order of 0.05 GPa. The DFT calculation results reveal that the symmetry and binding energy decline in the BixBa1-xTiO3 structure, and the bond lengths of BaO and TiO decrease a little after Ba has been substituted by Bi atom, except for the structure of Bi0.5Ba0.5TiO3. The photo-catalysts of BixBa1-xTiO3 are direct band gap semiconductors, and the substitution Bi can regulate the band gaps of BixBa1-xTiO3. The band gaps become wider from x=0.125 to x=0.750 with the carrier concentration decreasing, and then decreases with the higher carrier concentration increasing when x=0.875. It is predicted that the band width of Bi-based perovskite will be much lower than that of Ba-based perovskite. In the case of the density of states we reveal that the top of the valence band is hybrided by O-2p and Bi-6s and the bottom of the conduction band state is mainly constituted by the Ti-3d state. The electron transport properties and carrier types are mainly determined by Ti-3d, O-2p state and Ba-5p electronic states in BaTiO3 and Ti-3d, O-2p, Bi-6s and Bi-6p electronic states in BixBa1-xTiO3 respectively. The absorption spectra indicate that the ultraviolet absorption performance can be improved in BixBa1-xTiO3 system, which may effectively improve the photo-catalytic activity of BaTiO3.