Water and ammonia on Cu{110}: comparative structure and bonding

Water and ammonia are arguably the two most important inorganic molecular species in the modern world, and their interaction with metal surfaces is key to unlocking their further potential in a number of spheres. In this comparative study, conducted on the Cu{110} substrate, we present results from first-principles density functional theory that highlight the similarities and differences between these chemical cousins. We find that ammonia is less likely than water to undergo thermally induced partial dissociation, although we nevertheless identify the most likely product of electron-stimulated or defect-induced dissociation to be a surface amino species. We predict that ammonia, like water, will adopt a bilayer structure at high coverage, but that unlike water the net intermolecular interaction will be repulsive, despite the formation of a weak hydrogen-bonded network. Furthermore, we suggest that coadsorption of water and ammonia can give rise to an intimately mixed overlayer in which ammonia molecules are bound directly to the surface whilst water molecules are attached only via hydrogen bonds from below.