c(2×2)Water-Hydroxyl Layer on Cu(110): A Wetting Layer Stabilized by Bjerrum Defects

Abstract

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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“…However, the imposed order within the adlayer dictates that relaxation occurs over long timescales. Similar hydrogen-bonding defects have been observed experimentally under ultrahigh vacuum conditions at low temperatures on Pd(111) (23) and in water-hydroxyl films on Cu(110) (24).…”

The Many Faces of Heterogeneous Ice Nucleation: Interplay Between Surface Morphology and Hydrophobicity

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“…2k). Interestingly, the two inner bridging water molecules always pair up but avoid facing each other because of the repulsive force between the H atoms, forming a Bjerrum D-type defect (a crystallographic defect specific to ice, where there are two protons between the nearestneighbour pair of oxygen atoms) 12,29 . The Bjerrum D-type defect has two orthogonal orientations, which can be switched reversibly by the STM tip (Supplementary Note 2 and Supplementary Fig.…”

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