Structures and energetics of H₂O adsorption on the Fe₃O₄ (111) surface

ater adsorption on the Fe_tet1-terminated and Fe_oct2-terminated surfaces of Fe₃O₄(111) has been calculated at the level of density functional theory (GGA/PBE). On the Fe_tet1-terminated surface at 1/5 monolayer (ML), the molecular adsorption mode with a hydrogen bond and the heterolytically dissociative mode show the highest stability, whereas the hydronium-ion-like structure OH₃⁺-OH becomes possible at 2/5 ML, followed by the hydrogen-bonded water aggregate. These results agree well with the available experimental observations. For Fe_oct2-terminated surface, the molecular water prefers to adsorb on the surface Fe_oct2 atom at 1/6 ML, whereas other adsorption modes become possible and may coexist at 1/3 ML. The Fe_tet1-terminated surface is more favorable than the Fe_oct2-terminated surface for water adsorption. The adsorption mechanism has been analyzed on the basis of the calculated local density of state.