First-principles determination of the absolute band-edge positions of BiOX (X = F, Cl, Br, I)

To determine the band-edge positions of bulk BiOX (X = F, Cl, Br, I) in energy with respect to the vacuum level, we align the potentials of the bulk with those of the interior of the unrelaxed (0 0 1) and (1 0 0) slabs, based on density functional theory (DFT) calculations. The valence-band tops determined via the (0 0 1) slabs are generally > 1 V more positive than those through the (1 0 0) ones, and the strong anisotropy can be interpreted in terms of the orientation differences of electron spreading and charge redistributions, which are correlated with the surface stabilities. Since electron spilling is inevitable on surfaces, evaluating the energy positions of the bulk is not straightforward even using unrelaxed slabs. Although not covering the realistic surface and environmental effects, the present results still confirm the strong oxidative capabilities of the valence band holes in the present species as observed experimentally.