Abstract

The first water layer on hexagonal single crystal surfaces has a surprisingly complex structure, releasing the strain caused by a mismatch of the ice crystal structure and the substrate lattice by forming rotated hexagons, pentagons and heptagons of molecules in addition to strongly bound hexagonal rings commensurate with the substrate. In a vacuum environment, the water monolayer does not expose any dangling hydrogen bonds and all water molecules adsorb either flat-lying or with a hydrogen atom pointing towards the surface. The growth of the entropically favourable proton-disordered ice, however, requires flipping some of the molecules in the first layer to expose dangling hydrogen bonds. Using scanning tunnelling microscopy (STM) we studied this transition from the first layer to water multilayers on Pt(111) and Ru(0001). We observed that a second water layer initially forms an amorphous structure when grown on the crystalline monolayer containing pentagons, hexagons and heptagons of water molecules. To facilitate the growth of ice in a bulk-like hexagonal arrangement, the first wetting layer needs to rearrange into a hexagonal structure commensurate with the surface. Complementary STM measurements confirmed the facile re-orientation of certain molecules in the water monolayer on Pt(111) upon adsorption of ammonia (NH3) molecules. We found that NH3 binds preferentially to H2O molecules that are slightly elevated from the surface and weakly bound to the metal. Ammonia molecules thus detect locations in the wetting layer where a water molecule can change its orientation relatively easily to flip up a hydrogen atom.