Heterogeneous ice nucleation has a key role in fields as diverse as atmospheric chemistry and biology. Ice nucleation on metal surfaces affords an opportunity to watch this process unfold at the molecular scale on a well-defined, planar interface. A common feature of structural models for such films is that they are built from hexagonal arrangements of molecules. Here we show, through a combination of scanning tunnelling microscopy, infrared spectroscopy and density-functional theory, that about 1-nm-wide ice chains that nucleate on Cu(110) are not built from hexagons, but instead are built from a face-sharing arrangement of water pentagons. The pentagon structure is favoured over others because it maximizes the water-metal bonding while maintaining a strong hydrogen-bonding network. It reveals an unanticipated structural adaptability of water-ice films, demonstrating that the presence of the substrate can be sufficient to favour non-hexagonal structural units.

Establishing the nanoscale details of organized amino acid assemblies at surfaces is a major challenge in the field of organic-inorganic interfaces. Here, we show that the dense (4 x 2) overlayer of the amino acid, (S)-proline on a Cu(110) surface can be explored at the single-molecule level by scanning tunneling microscopy (STM), reflection absorption infrared spectroscopy (RAIRS), and periodic density functional theory (DFT) calculations. The combination of experiment and theory, allied with the unique structural rigidity of proline, enables the individual conformers and adsorption footprints adopted within the organized assembly to be determined. Periodic DFT calculations find two energetically favorable molecular conformations, projecting mirror-image chiral adsorption footprints at the surface. These two forms can be experimentally distinguished since the positioning of the amino group within the pyrrolidine ring leads each chiral footprint and associated conformer to adopt very different ring orientations, producing distinct contrasts in the STM images. DFT modeling shows that the two conformers can generate eight possible (4 x 2) overlayers with a variety of adsorption footprint arrangements. STM images simulated for each structural model enables a direct comparison to be made with the experiment and narrows the (4 x 2) overlayer to one specific structural model in which the juxtaposition of molecules leads to the formation of one-dimensional hydrogen bonded prolate chains directed along the [110] direction.

Understanding the composition and stability of mixed water-hydroxyl layers is a key step in describing wetting and how surfaces respond to redox processes. Here we show that, instead of forming a complete hydrogen bonding network, structures containing an excess of water over hydroxyl are stabilized on Cu (110) by forming a distorted hexagonal network of water-hydroxyl trimers containing Bjerrum defects. This arrangement maximizes the number of strong bonds formed by water donation to OH and provides uncoordinated OH groups able to hydrogen bond multilayer water and nucleate growth. DOI: 10.1103/PhysRevLett.106.046103 PACS numbers: 68.43.Bc, 68.43.Fg, 71.15.Mb On many wet oxide, semiconductor, and metal surfaces the first contact layer is not comprised of pure water but is instead a mixture of water and hydroxyl molecules, often caused by spontaneous dissociation of water. Although formation of water-hydroxyl wetting layers has been intensively investigated on well-defined metal surfaces [1], molecular-level understanding of this important class of overlayer is still far from complete and little is known about their local hydrogen bonding structure [2]. In particular, while it is established that hydroxyl coadsorption plays an important role in stabilizing water on metals [3][4][5], there is not yet a clear picture of the H bonding motifs adopted on different metals, nor how this changes the properties of the interface. For example, hydroxyl coadsorption can dramatically change the wetting behavior [6], while tuning the stability of adsorbed hydroxyl is key to optimizing the activity of surfaces for the oxygen reduction reaction [7]. Developing a detailed understanding of the coverage, bonding motifs and stability of hydroxyl underpins attempts to model electrochemical activity [8] and to develop a molecular picture of wetting [2][3][4][5]9].The cð2 Â 2Þ H 2 O-OH phase formed on the open Cu (110) surface is one of the most widely studied model systems and affords an excellent opportunity to understand the structure and properties of this important class of overlayer. Water forms a number of unusual structures on this surface, including 1D chains of interlocking pentagons [10,11], an intact 2D network at higher coverage [10,12] and several partially dissociated structures [13][14][15], but the structure most commonly studied is the cð2 Â 2Þ overlayer. Although it was originally believed that this corresponded to an intact water bilayer, more recent studies have shown that this is not the case and that it is instead comprised of a mixture of water and hydroxyl [12,13]. Whereas stoichiometric structures containing equal amounts of H 2 O and hydroxyl have been observed on several metal surfaces, having each of the H atoms involved in 1 H bond and no uncoordinated OH groups [4], adsorption on Cu(110) [6] and Ru(0001) [5] results in a mixed phase containing an excess of water over hydroxyl. The excess of water is puzzling, since it provides too many OH bonds to form a complete H bonding network and may be in viola...

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