Surface structure and water adsorption on Fe₃O₄(111): Spin-density functional theory and on-site Coulomb interactions

The surface structure of magnetite Fe₃O₄(111) in contact with oxygen and water is investigated using spin-density functional theory plus on-site Coulomb interactions. The present results unravel apparent contradictions in the experimental data regarding the equilibrium stoichiometry of the bare surface termination. Both for 298 and 1200 K, the equilibrium structure is terminated by 1/4 monolayer (ML) of iron (Fe) on top of a full oxygen layer, consistent with an earlier low-energy electron diffraction analysis. Nonetheless, the calculated negative slope of the surface energies vs oxygen partial pressure shows that a 1/2 ML Fe termination would become stable under oxygen-poor conditions at high temperatures, in agreement to interpretation of scanning tunneling microscopy experiments. Initial water adsorption is dissociative and saturates when all Fe sites are occupied by OH groups, while the H atoms bind to surface oxygen. Further, water bridges the OH and H groups resulting in a unique type of H-bonded molecular water with its oxygen forming a hydronium-ion-like structure OH₃⁺-OH. This water structure is different from the water dimeric structures found as yet on oxide and metal surfaces for partially dissociated (H₂O-OH-H) overlayers.