Structural and electronic properties of BiOX (X = F, Cl, Br, I) considering Bi 5f states

Density functional theory (DFT) calculations have been performed to investigate the structural and electronic properties of BiOX (X = F, Cl, Br, I) photocatalysts, considering the Bi 5f states. Comparing with our previous results without Bi 5f states, the present work conveys similar gap features and comparable gap widths for the unrelaxed species, but the relaxation here causes opposite directions of atomic displacements and different trends for band and gap widths. The unrelaxed BiOF exhibits a direct band gap of 3.25 eV, while the other three species present the indirect gaps of 2.62, 2.16 and 1.51, respectively. Atomic relaxation expands the above gaps to 3.41, 2.69, 2.21 and 1.62 eV correspondingly. Analyses based on wavefunction isosurfaces and decomposed orbitals reveal that at the valence-band tops, O 2pz and F 2pz states prevail in BiOF, O 2py and X npy dominate in BiOCl and BiOBr, and the I 5px state governs in BiOI. The conduction-band bottoms are composed mainly of the Bi 6pz orbital.