Intact water adsorption on Co(0001) at 100 K: transition from ordered bilayer to amorphous ice structures

While cobalt metal is recognized as a versatile catalyst in various chemical reactions, such as Fischer-Tropsch synthesis, limited attention has been paid to understanding the detailed adsorptive interactions between water molecules and cobalt metal. In this study, we investigated the adsorption of water molecules on Co(0001) at 100 K using infrared reflection adsorption spectroscopy and low-energy electron diffraction. We experimentally revealed, for the first time, that D2O adsorbed intact on the Co(0001) surface forms hexamer islands with coexisting D-up and D-down geometries, in line with the "ice bilayer" model. Upon completion of the first adlayer, a partially ordered (root 3 x root 3)R30 degrees water bilayer structure is established, featuring coexisting D-up and D-down domains in a ratio of approximately 2 : 3. This results in a surface with mixed hydrophilic and hydrophobic regions. As a second adlayer grows, water molecules preferentially adsorb on the D-up domains before distributing onto the D-down domains. The adsorption of the second adlayer causes a partial disordering of the first water adlayer underneath, resulting in the transition from an ordered bilayer to disordered layer structures. Further increases in water coverage led to an amorphous ice structure.