Growth of intact water ice on Ru(0001) between 140 and 160 K: Experiment and density-functional theory calculations

We report low-energy electron diffraction (LEED) and reflection absorption IR spectroscopy (RAIRS) results for water adsorption on Ru(0001) at temperatures between 140 and 160 K, where water forms intact hydrogen bonded structures on the surface. We find that H2O and D2O adsorption show identical behavior, with no evidence for a structural isotope effect. At low coverage LEED shows a diffuse (root 3 x root 3)R30 degrees pattern, which becomes sharp and intense only as the coverage reaches 0.6 to 0.67 monolayer. The LEED pattern becomes broadened and diffuse as the surface saturates with a coverage of 0.76 monolayer. In RAIRS the low-frequency bands associated with the out of plane libration of hydrogen bonded water appear at low coverage, with the water stretch and scissors bands appearing as broad bands only as the coverage is increased. Water adsorbs flat on Ru(0001) at low coverage, forming small clusters which buckle to create an extended, hydrogen bonded structure only as the adlayer is completed. The free OH(OD) stretch band appears only as the monolayer approaches completion, indicating flat or H-down adsorption up to 0.67 monolayer with H-up water appearing as the root 3 structure compresses. Density functional calculations at low coverage find that water forms stable clusters with water adsorbed flat on the surface. Calculations for a complete 0.67 monolayer structure find water adsorbed near the Ru atop site in a hydrogen bonded honeycomb network, containing chains of "flat" lying water, linked by upright chains bonded "H down" in the hexagonal, hydrogen bonded superstructure. This structure is similar to 20% more stable than the conventional ice bilayer structure and is expected to wet the Ru(0001) surface. We propose a model in which disordered, short chains of "flat" and H-down water are imbedded in a honeycomb network of hydrogen bonded water, which imposes long-range order on the adlayer but allows substantial local disorder, and discuss the agreement with existing experimental results.