Co3O4 (111) surfaces in contact with water: molecular dynamics study of the surface chemistry and structure at room temperature

Abstract

In this work, we have used ab initio molecular dynamics at room temperature to study the adsorption and dissociation of a thin water film on Co₃O₄ (111) surfaces, considering the O-rich and Co-rich terminations named as the A-type and B-type surface terminations, respectively. We investigate the occupation of active sites, the hydrogen bond network at the interface and the structural response of the surfaces to water adsorption. On both terminations, water adsorbs via a partial dissociative mode. The contact layer is populated by molecular water as well as OH groups and surface OH resulting from proton transfer to the surface. The B-termination is more reactive, with a higher degree of dissociation in the contact layer with water (46%). On the B-terminated surface, water barely adsorbs on the Co²⁺ sites and almost exclusively binds and dissociates on the Co³⁺ sites. The interaction with the surface consists mostly of Co³⁺–O_w bonds and proton transfer exclusively to the 3-fold unsaturated surface O_s1. Hydrogen bonds between water molecules in the aqueous film dominate the hydrogen bond network and no hydrogen bonds between water and the surface are observed. The A-terminated surface is less reactive. Water binds covalently on Co²⁺ sites, with a dissociation degree of 13%. Proton transfer occurs mostly on the 3-fold unsaturated surface oxygens O_s1. Besides, short-lived surface OH arising from proton transfer to 3-fold unsaturated surface oxygens O_s2 is observed. H-bonding to surface O_s1 and O_s2 constitutes 12.7% and 19.8% of the H-bond network, respectively, and the largest contribution is found among the water molecules (67.4%). On both surfaces, the coordination number of the active sites drives the relaxations of the outermost atom positions to the their bulk counterparts. The occupation of active sites on B-termination could reach up to 3 adsorbates per Co³⁺ leading to a binding motif in which the Co is octahedrally coordinated and which was observed experimentally.