Adsorbed water-molecule hexagons with unexpected rotations in islands on Ru(0001) and Pd(111)

High-resolution scanning tunneling microscopy (STM) reveals that the first layer of water on Ru(0001) and also on Pd(111) consists of hexagonal molecular domains of two types, rotated by 30 degrees relative to one another. Pentagon and heptagon clusters bridge the two types of hexagons. One of the orientations is in registry with the substrate. Its molecules lie flat and their O atoms form strong bonds to the metal atoms lying directly below. In the other domain the molecules have dangling H bonds. They are weakly bound to the substrate and lie correspondingly higher. This bonding motif, though nonperiodic, is of similar nature to the periodic wetting structure recently reported on Pt(111), and very different from the conventional "ice-like" bilayer. First-principles density functional theory (DFT) simulations of the STM images support these conclusions.