Water adsorption on the Fe₃O₄(111) surface: dissociation and network formation

We monitored adsorption of water on a well-defined Fe₃O₄(111) film surface at different temperatures as a function of coverage using infrared reflection–absorption spectroscopy, temperature programmed desorption, and single crystal adsorption calorimetry. Additionally, density functional theory was employed using a Fe₃O₄(111)-(2 × 2) slab model to generate 15 energy minimum structures for various coverages. Corresponding vibrational properties of the adsorbed water species were also computed. The results show that water molecules readily dissociate on regular surface Fe_tet1–O ion pairs to form “monomers”, i.e., terminal Fe–OH and surface OH groups. Further water molecules adsorb on the hydroxyl covered surface non-dissociatively and form “dimers” and larger oligomers, which ultimately assemble into an ordered (2 × 2) hydrogen-bonded network structure with increasing coverage prior to the formation of a solid water film