The adsorption of water on clean and oxygen predosed Rh(111): Surface templating via (1x1)-O/Rh(111) induces formation of a novel high-density interfacial ice structure

Water adsorbed on clean Rh(111) forms an ordered structure with a (root 3x root 3)R30 degrees diffraction pattern. This is facilitated by the close ma tch of surface lattice constants for Rh(111) and the (0001) face of hexagon al ice, I-h. The preadsorption of small quantities of disordered oxygen imp roves the long-range ordering of the water overlayer. When a well-ordered h alf-monolayer of oxygen is grown on the Rh(111) prior to H2O exposure, ther e is no evidence of any long-range ordering of the water. However, when H2O is adsorbed on a (1x1)-O/Rh(111) surface, where there is a well-ordered mo nolayer of adsorbed oxygen, the adsorbed H2O forms a new high-density struc ture exhibiting a (1x1) diffraction pattern. The adsorbed H2O structure is epitaxial with respect to the underlying oxygen and rhodium. This structure persists for many layers of adsorbed water. On the clean Rh(111) surface, water molecules are adsorbed through the oxygen lone pair orbital. When the surface is fully covered with oxygen, the first layer of water can hydroge n bond to the surface, i.e., they likely adsorb with one or both of the hyd rogen atoms pointing toward the surface. This creates a template for a nove l structure that forms at low pressure, producing a high-density crystallin e form of interfacial ice. This discovery suggests that other molecules, es pecially those that hydrogen bond, may form new structures on metals covere d with a high-density oxygen overlayer, with associated consequences for in terfacial chemistry. (C) 2000 American Institute of Physics. [S0021-9606(00 )70118-8].