low index surfaces*

We used density functional theory approach, with the inclusion of a semi-empirical dispersion potential to take into account van der Waals interactions, to investigate the water adsorption and dissociation on cobalt sulphides Co₉S₈ and Co₃S₄ (100) surfaces. We first determined the nanocrystal shape and selected representative surfaces to analyse. We then calculated water adsorption and dissociation energies, as well as hydrogen and oxygen adsorption energies, and we found that sulphur vacancies on Co₉S₈ (100) surface enhance the catalytic activity toward water dissociation by raising the energy level of un-hybridized Co3d states closer to the Fermi level. Sulphur vacancies, however, do not have a significant impact on the energetics of Co₃S₄ (100) surface.