Electronic properties and effective masses of hexagonal Bi₉O_7.5S₆: First-principles calculations

The electronic band structure, density of states, bond character and effective masses of hexagonal Bi₉O_7.5S₆ crystal have been systematically investigated by applying first-principles calculations based on the density functional theory. Approving agreement between experimental and computational results for lattice parameters shows that the GGA-PBE method is suitable. Bi₉O_7.5S₆ is an indirect band gap semiconductor with the value of 1.383eV using the GGA-PBE functional. The detailed analysis of Mulliken populations in Bi₉O_7.5S₆ demonstrates the covalent nature orders for Bi₁–S₁, Bi₂–O₁(1), Bi₂–O₁(3) and Bi₂–O₂ bond are Bi₁–S₁>Bi₂–O₁(3)>Bi₂–O₂>Bi₂–O₁(1) on account of bond population values. More interestingly, the electron effective masses are larger in the interlayer direction than in the intralayer one, while the hole effective masses are lower in the interlayer direction than in the intralayer one. The photoinduced carriers for Bi₉O_7.5S₆ have the high mobility that can be inferred from the effective masses of electron and hole in comparation with Bi₂O₃ and Bi₂S₃. The Bi₉O_7.5S₆ is a strong photo-oxidation catalyst but the reduction power is limited in aqueous solutions.