First-principles energy band calculation for CaBi2O4 with monoclinic structure

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Abstract

The electronic structure of CaBi2O4 is calculated by a GGA approach. The valence band maximum is approximately located at the Γ-point or the Y-point and the conduction band minimum at the V-point. This means that CaBi2O4 is an indirect energy gap material. The conduction band is composed of Bi 6p–O 2p interaction. On the other hand, the valence band can be divided into two energy regions ranging from −9.92 to −7.40 eV (lower valence band) and −4.69 to 0 eV (upper valence band). The former is mainly constructed from Bi 6s states interacting slightly with O 2s and 2p states, and the latter consists of O 2p states hybridizing with Bi 6s and 6p states. The states near the valence band maximum are strongly localized and the mobility of holes generated by band gap excitation is predicted to be fairly low.

Introduction

Photocatalysis of TiO2 with anatase structure has been widely studied from the viewpoint of the utilization of solar energy, because of its abundant raw materials, non-toxicity, and chemical stability. However, anatase TiO2 shows no photocatalytic activity in visible region because of its large band gap. The narrowing of the band gap of anatase was attempted by doping of impurities such as C, N, S and transition metals [1], [2]. For instance, Asahi et al. reported that N-doped anatase has a photocatalytic activity in visible region [1]. Nevertheless, it has been revealed that the doping of impurities is an ineffective technique for the development of visible-response photocatalysts. The reason for this is that light doping gives rise to the formation of localized states in the band gap, which are responsible for the recombination of electron–hole pairs. In the case of heavy doping, the solubility of impurities N or C atoms in oxides was not so high that impurity bands are formed in the band gap. Instead of impurity doping, a new material with a narrow band gap has been searched.

Recently, oxides containing Bi atoms have attracted considerable attention as new oxides in which the band gap energy may be controlled by the energy separation of Bi 6s and 6p orbitals [3]. If this qualitative model is correct, the band structure of Bi-related oxides is significantly different from charge-transfer type oxides such as TiO2, SnO2, and WO3 [1], [4], [5], in which the top of the valence band is mainly composed of O 2p orbitals. In fact, Tang et al. reported that CaBi2O4 with monoclinic structure has an efficient photocalatytic activity in visible region [6]. They interpreted a higher activity in terms of a narrow band gap of CaBi2O4 (3.08 eV) [7]. However, there are no reports on the electronic structure of CaBi2O4 with many atoms in the unit-cell and the complex crystal structure. In the present work, we perform first-principles band calculation of CaBi2O4 to determine the type of the optical transition, the characters of valence and conduction bands, and the band gap energy. Based on the band calculations, we discuss the reason why CaBi2O4 is an efficient photocatalyst in the visible region.

Section snippets

Calculation method

CaBi2O4 crystal has a space group C12/c1 (C62h) and unit-cell parameters: a = 1.66430(40) nm, b = 1.1580(4) nm, c = 1.3994(5) nm, β = 133.92(2)°and all atoms in CaBi2O4 occupy Wyckoff positions 8f [8]. There are symmetrically four independent Bi atoms (from Bi(1) to Bi(4)), two independent Ca atoms (Ca(1) and Ca(2)), and eight independent O atoms (from O(1) to O(8)) in the unit-cell. The coordination numbers of Bi atoms and Ca atoms are 4 and 7, respectively.

In this theoretical study, the energy band

Results and discussion

After the optimization by CASTEP, the residual stresses were almost zero and the unit-cell parameters were a = 1.6978 nm, b = 1.1424 nm, c = 1.4134 nm, and β = 132.838°. These optimized lattice parameters and the bond distance between Bi and O atoms for CaBi2O4 crystal are listed in Table 1, together with experimental values. Compared with experimental values, a- and c-axes slightly increased, and b-axis and β-value slightly decreased after the optimized calculation. These calculated lattice constants are

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