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

Sabine Maier, Ingeborg Stass, Toshiyuki Mitsui, Peter J. Feibelman, Konrad Thürmer, and Miquel Salmeron

Phys. Rev. B 85, 155434 – Published 16 April 2012

Abstract

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 $30^{\circ}$ 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.

Received 30 September 2011

DOI:https://doi.org/10.1103/PhysRevB.85.155434

Authors & Affiliations

Sabine Maier^1,*, Ingeborg Stass^1,2, Toshiyuki Mitsui^1,†, Peter J. Feibelman³, Konrad Thürmer⁴, and Miquel Salmeron^1,5,‡

¹Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
²Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany
³Sandia National Laboratories, Albuquerque, New Mexico 87185-1415, USA
⁴Sandia National Laboratories, Livermore, California 94550, USA
⁵Materials Science and Engineering Department, University of California, Berkeley, California 94720, USA

^*Present address: Department of Physics, University of Erlangen-Nürnberg, 91058 Erlangen, Germany.
^†Present address: Department of Physics and Mathematics, Aoyama Gakuin University, Kanagawa, 252-5258 Japan.
^‡mbsalmeron@lbl.gov

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Vol. 85, Iss. 15 — 15 April 2012

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Images

Figure 1
Models of water adsorption on closed-packed metal surfaces. (a) Schematic illustration of water molecules with their planes parallel (flat) and normal to the surface, with either one H pointing up into the vacuum (H-up) or down toward the surface (H-down). (b) The cluster of flat-lying (green) molecules in the center of the cluster cannot be extended with more flat-lying molecules. Accordingly, the periphery of the rosette consists of twelve (red) dangling-H water molecules ( $\sim$ vertical), in this case with their dangling H atoms lying between their O atoms and the underlying metal.Reuse & Permissions
Figure 2
High-resolution STM images of one-molecule-thick water structures formed on (a) Ru(0001) and (b) Pd(111). These structures are obtained after water adsorption at 50 K on Ru and 40 K on Pd followed by heating to 110 K on Ru and 80 K on Pd. H $_{2}$ O was used on Ru(0001) and D $_{2}$ O on Pd(111). The images reveal domains of hexagons with two distinct height levels, labeled with High and Low. The close-packed crystallographic directions of the substrates are indicated by the arrow at the bottom. Imaging parameters: (a) 7 K, 57 pA, $-$ 38 mV and (b) 80 K, 156 pA, $-$ 124 mV.Reuse & Permissions
Figure 3
(a) Schematic illustration with clusters composed of $0^{\circ}$ and $30^{\circ}$ rotated hexagons on a close-packed surface. The lower part (yellow) shows an experimental atomically resolved image of Ru(0001) (Ru atoms $=$ bright spots) before water adsorption. (b) and (c) High-resolution STM images of one-molecule-thick water clusters. (b) D $_{2}$ O clusters adsorbed on Pd(111) at 40 K and annealed at 80 K. (c) H $_{2}$ O clusters adsorbed on Ru(0001) at 50 K and annealed at 110 K. The images reveal domains of hexagons with two distinct height levels. The lower-lying domains (blue) are marked by gray hexagons, while the higher ones (red) are marked by black hexagons. The lower-lying hexagonal structure is in registry with the surface lattice. Unexpectedly however, the higher-lying structures are rotated by $30^{\circ}$ . The close-packed substrate directions in the three images are the same. Imaging parameters: (b) 80 K, 156 pA, $-$ 124 mV and (c) 7 K, 25 pA, $-$ 72 mV.Reuse & Permissions
Figure 4
STM image (4.2 nm $\times$ 3.4 nm) showing pentagonal and heptagonal defects (marked by white lines) in the first water layer on Ru(0001), which bridge hexagonal rings in registry with the substrate (blue), and hexagonal rings rotated $30^{\circ}$ relative to the metal lattice (green). Water was adsorbed at 140 K and subsequently imaged at 78 K. The inset shows a pentagonal ring of water molecules (H $_{2}$ O adsorption at 50 K followed by heating at 110 K and imaging at 7 K). STM image parameters: 11 pA, 175 mV, inset: 86 pA, $-$ 11 mV.Reuse & Permissions
Figure 5
Series of topography images of D $_{2}$ O clusters on Pd(111) capturing the rearrangement of water molecules on the surface. The images, recorded at a sample temperature of around 100 K, show notable rearrangement of the molecules as the size and shape of both low- and high-lying water domains changes from frame to frame. Note that in all images the higher-lying water molecules are located toward the center of the cluster surrounded by low-lying molecules. The brighter spots on the edge of the islands (160–180 pm above the surface) correspond to molecules H bonded to the periphery. They interact only weakly with the substrate metal atoms.[12] The time stamp in the upper left corner refers to the time passed since the first image was recorded. Water was adsorbed at 100 K and annealed approximately 1700 s at this temperature before recording the first image shown in this figure.Reuse & Permissions
Figure 6
Comparison of experimental and calculated STM images of water adsorbed on Ru(0001). (a) High-resolution STM image showing water hexamers rotated by $0^{\circ}$ and $30^{\circ}$ relative to the surface lattice and connected by pentagons and heptagons. (b) Structural model optimized by DFT calculations. The brown-colored oxygen atoms are located between 2.3 and 2.5 Å above the Ru atoms in the first layer, while the height of the yellow ones is 4.4 Å, and that of the orange ones is close to 3.5 Å. (c) Calculated STM image based on the DFT optimized structure shown in (b). STM image parameters: $-$ 137 mV and 145 pA. Water was deposited at 50 K and annealed to 130 K prior to imaging at 50 K.Reuse & Permissions

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