Matthew
Forster
a,
Rasmita
Raval
a,
Javier
Carrasco
b,
Angelos
Michaelides
c and
Andrew
Hodgson
*a
aSurface Science Research Centre and Department of Chemistry, University of Liverpool, Oxford Street, Liverpool, UK L69 3BX. E-mail: ahodgson@liverpool.ac.uk
bInstituto de Catálisis y Petroleoquímica, CSIC, Marie Curie 2, E-28049, Madrid, Spain
cLondon Centre for Nanotechnology and Department of Chemistry, University College London, London, UK WC1E 6BT
First published on 25th August 2011
Hydroxyl is a key reaction intermediate in many surface catalyzed redox reactions, yet establishing the phase diagram for water/hydroxyl adsorption on metal surfaces remains a considerable challenge for interfacial chemistry. While the structures formed on close packed metal surfaces have been discussed widely, the phase diagram on more reactive, open metal surfaces, is complex and the H-bonding structures are largely unknown. Based on scanning tunnelling microscopy and density functional theory calculations, we report the phase diagram for water/hydroxyl on Cu(110), providing a complete molecular description of the complex hydrogen bonding structures formed. Three distinct phases are observed as the temperature is decreased and the water/hydroxyl ratio increased: pure OH dimers, extended 1H2O:1OH chains, aligned along the close-packed Cu rows, and finally a distorted 2D hexagonal c(2 × 2) 2H2O:1OH network. None of these phases obey the conventional ‘ice rules’, instead their structures can be understood based on weak H donation by hydroxyl, which favours H-bonding structures dominated by water donation to hydroxyl, and competition between hydroxyl adsorption sites. Hydroxyl binds in the Cu bridge site in the 1D chain structures, but is displaced to the atop site in the 2D network in order to accommodate water in its preferred atop binding geometry. The adsorption site and stability of hydroxyl can therefore be tuned simply by changing the surface temperature and water content, giving a new insight as to how the open metal template influences the water/hydroxyl structures formed and the activity of hydroxyl.
In contrast to the close packed faces, the f.c.c. (110) metal surfaces offer the opportunity to explore how a different surface symmetry influences the wetting behavior. Moreover, these surfaces have an enhanced reactivity compared to close packed surfaces and are active in a number of important catalytic reactions involving water, for example in the low temperature water gas shift reaction.1,2,19 Since hydroxyl is a key intermediate in many of these reactions, understanding its thermodynamic and kinetic behaviour is of direct practical relevance, yet we have only a very limited picture of the water/hydroxyl phases formed on open f.c.c. (110) faces.13 The Cu(110) surface has received the most study, but the structure and composition of the water/hydroxyl phases remains the source of debate.6,20–26Water adsorbs in its molecular form at low temperature, desorbing intact above 160 K.6,21–25Low energy electron diffraction (LEED) studies reported a c(2 × 2) structure, which was initially attributed to a distorted water bilayer,24–26 but recent work shows this structure is formed by electron induced dissociation of water.6,22,23Scanning tunnelling microscopy (STM) images reveal a dramatic change in structure with coverage,22water initially forming 1D chains along the [001] direction, then 2D islands and finally completion of the (7 × 8) layer. A comparison of STM images and vibrational spectra with density functional theory (DFT) structure calculations identifies the 1D chain structure as a face sharing arrangement of water pentagons.6 This structure is favored because the Cu(110) template is too compact to accommodate a water hexagon, whereas the pentamer chain structure allows two thirds of the water to bond in its favored site, in a flat geometry atop Cu, optimizing the water–metal bonding whilst still maintaining a strong hydrogen bonding interaction. It is now believed that the wetting structure formed on other surfaces, such as Pt(111),14,27 may contain H bonded water rings of different size, revealing a surprising flexibility in the structure adopted by water at different surfaces.
The behavior of the mixed water/hydroxyl phases on Cu(110) is less well established, with debate over both their composition and structure. At high water coverage an extended c(2 × 2) mixed water/hydroxyl phase is formed below 180 K. Recent STM results find this phase contains at least two water molecules per hydroxyl within a distorted hexagonal network,17 consistent with the composition found in high pressure X-ray photoelectron spectroscopy (XPS) measurements.28 DFT calculations indicate that the excess water is stabilized by the formation of D type Bjerrum defects within the hydrogen bonding network. These defects have two H atoms between an adjacent pair of oxygen atoms, in this case two hydroxyl molecules being H bonded such that their H atoms lie just off the hydroxyl–hydroxyl axis. This structure breaks the well known Bernal–Fowler–Pauling (BFP) ice rules, sacrificing a complete H bonding network in order to maximize the number of strong bonds formed by water donation to hydroxyl.17 In addition to the extended c(2 × 2) structure, low dimensional structures have been reported following water dissociation or reaction with adsorbed O atoms. Kumagai et al. investigated the stability of small water clusters29 and the tunnelling barrier in water,30water/hydroxyl31 and hydroxyl dimers32 at low temperature by STM, while Lee et al.33 recently reported chains of water/hydroxyl form parallel to [10] and tentatively attributed these to a 2H2O:1OH structure.
In this study we combine high resolution STM images and state of the art DFT structure calculations to investigate the stability and structure of water/hydroxyl phases with different composition on Cu(110). Three different water/hydroxyl phases form as the surface temperature is decreased and the water content increased: a pure hydroxyl phase consisting of OH dimers, aligned along the [001] direction, two related 1H2O:1OH chain structures and finally 2D islands of the c(2 × 2) 2H2O:1OH phase.17 Excellent agreement is found between simulations of the minimum energy structures and experimental STM images, allowing us to obtain a detailed picture of the structures formed. Instead of obeying the Bernal–Fowler–Pauling ice rules and forming three H bonds, two as an acceptor and one as a donor, hydroxyl coadsorbed with water acts as an acceptor but not as a donor, maximizing the number of strong H bonds formed by water molecules donating to hydroxyl, rather than the overall number of H bonds. Hydroxyl in the 1H2O:1OH chain structures accepts a single H bond from a water chain, whereas increasing the water content to form the c(2 × 2) 2H2O:1OH phase creates D type Bjerrum defects,17 with each hydroxyl accepting H bonds from two water molecules. Associated with the change in structure from a 1D chain to 2D network, hydroxyl moves from its optimum short bridge site into an atop geometry, allowing water to bond in the favored atop adsorption geometry and form the defective 2D c(2 × 2) network. The rich range of adsorption behaviour found on Cu(110) is qualitatively different to that found on the close packed Pt(111)18,34,35 and Pd(111)36,37 surfaces, where hydroxyl adsorbs flat atop the metal atom, forming a stoichiometric 2D 1H2O:1OH phase. Our results indicate that simple structural models, based on the BFP ice rules and optimizing the H bonding coordination,5,38 are not adequate to describe the structure of water/hydroxyl phases on open, reactive metal surfaces; instead the water/hydroxyl structures formed optimize both the water and hydroxyl binding sites and the number of strong H bonds formed. This behavior has analogies to the over-coordination of hydroxide by H-donation in bulk water,39 where it may be energetically favorable to leave the hydroxyl proton uncoordinated at some sites.40 Other metal surfaces, such as Ru(0001)41,42 and Ag(110),43,44 also form non-stoichiometric or low dimensional structures, with a reduced H bonding coordination, and further studies will be necessary to explore how generally the ideas developed here for hydroxyl on Cu(110) can be transferred to other metal surfaces.
In order to explore possible structures associated with the different phases, DFT calculations were performed for a wide variety of pure water and water–hydroxyl overlayers. The calculations were made using the VASP code46,47 and employed the standard PBE functional48 and an accurate version of the non-local van der Waals density functional of Dionet al.,49 referred to as “optB88-vdW”.50 Atomic geometries and simulated STM images shown correspond to the PBE functional (in this respect the optB88-vdW results do not differ to any great extent). In addition, the relative stability of the different structures considered was not altered by inclusion of dispersion forces. The metal slabs were 4 layers thick, separated by 14 Å, with the two bottom layers fixed in their bulk PBE optimal position, aCu = 3.627 Å. Core electrons were treated with the projector augmented-wave method,51 whilst valence electrons were expanded in a plane wave basis with 600 eV cut-off energy. Monkhorst–Pack52 k-point meshes of at least 12 × 12 × 1 per (1 × 1) surface unit cell were employed, as was a dipole correction along the direction perpendicular to the surface. All adsorption energies (including those for pure OH adsorption systems) are referenced to H2O as a gas phase water molecule, with dissociated H atoms adsorbed in a separate slab. STM images have been simulated using the Tersoff–Hamann approach53 at 3.7 Å from the metal surface and V = −200 meV
Fig. 1 TPD traces for water from an O (0.1 ML) pre-covered surface following adsorption of 0.05–0.4 ML water at 140 K. The 185 K peak is assigned to decomposition of the c(2 × 2) 2H2O:1OH structure, with peaks at 220 K and 260 K from decomposition of the H2O:OH [10] chain and OH dimer structures respectively, as marked. Heating rate 1 K s−1. |
Fig. 2 STM images showing (a) partial reaction of an O(2 × 1) island (continuous bright chains in centre of image) with water at 240 K to form hydroxyl dimers, lying perpendicular to the close packed [10] rows (135 × 140 Å2, V(t) = −190 mV, I(t) = −0.26 nA), (b) high resolution image showing 3 adjacent hydroxyl dimers formed by complete reaction at 230 K (22 × 20 Å2, V(t) = −190 mV, I(t) = −0.36 nA). |
Density functional calculations for OH adsorption on Cu(110) find that single hydroxyl groups adsorb preferentially at bridge sites along Cu close packed rows,55 the next most stable adsorption site being the bridge between two close packed rows, about 360 meV less stable. The isolated hydroxyl points along the [001] azimuth with a tilt angle of about 60° with respect to the Cu(110) surface normal (Fig. 3). Flipping the H orientation has a barrier of 0.17 eV, consistent with the recent DFT results reported by Kumagai et al.32 who measured the rate at low temperature by STM. Coalescence of hydroxyls to form an (OH)2 dimer is favoured by ca. 214 meV, one hydroxyl group rotating towards the surface to form the hydrogen bond. As a result of this stabilization, hydroxyls are invariably found as dimers at the temperatures studied here. Simulated STM images show two bright features, slightly closer than the Cu spacing, surrounded by a dark region, similar to the images observed experimentally (Fig. 2). Arranging hydroxyls as a continuous chain along the [001] direction results in only a small energy gain, less than 10 meV (Table 1). The separation of adjacent Cu rows (3.6 Å) is too large to allow OH to form a continuous hydrogen bonded chain, instead hydroxyls remain as discrete dimers, with an O–O separation of 2.8 Å. The small cohesive energy between (OH)2 dimers is consistent with the experimental observation of a few short dimer chains along with isolated dimers, as shown in Fig. 2.
Fig. 3 Calculated binding geometry and simulated STM images for (a) an isolated OH group, (b) an OH dimer and (c) an array of OH forming a dimer chain along [001]. |
PBE | optB88-vdW | |
---|---|---|
Isolated OH | 428 | 645 |
OH dimer | 535 | 766 |
OH dimer chain | 542 | 770 |
Fig. 4 STM images of co-existing [10] chain structures formed at 200 K. (a) (255 × 88 Å2), (b) “zig-zag” (Z) type chain with 2aCu repeat and regular branched spacing (41 × 21 Å2), (c) “pinched” (P) type chain with irregular branches and a 4aCu repeat. |
Based on the periodicity of these chains, and their approximate composition, we investigated the stability of different possible [10] chain structures using DFT. From our previous analysis of the c(2 × 2) 2H2O:1OH phase,17 we expect hydroxyl, which is a poor H donor but good acceptor,18,28,56,57 to accept an H from water but not to act as a donor. Based on this idea, we examined chains containing a zig-zag water backbone, with each water bonded to hydroxyl and either a 2aCu or 4aCu repeat along [10]. Geometry optimization of the resulting chains gave the structures shown in Fig. 5a and 5b, with binding energies of 690 and 682 meV for chains with a two unit (Z) and four unit (P) period respectively. Despite the different arrangement of water along the chain, both structures have very similar adsorption sites and bond lengths for water and hydroxyl. Hydroxyl is adsorbed in the short bridge site, close to the surface (dCu–O = 2.02 Å), with the water slightly further from the surface (dCu–O = 2.14 Å), adsorbed roughly flat, displaced ca. 0.7 Å from the atop Cu site. This structure allows both hydroxyl and water to adopt adsorption geometries very similar to those found for the isolated, non-hydrogen bonded species. In both types of chain, the H bond formed by water donation to hydroxyl is short (dO–O = 2.63 Å) and that between the water molecules slightly longer, 2.77 to 2.86 Å. The offset between the bridge adsorption site of the hydroxyl and the atop site of the water causes the branches to point slightly along the [10] direction, rather than perpendicular to the chain. STM simulations for the Z structure, Fig. 5a, reproduce the alignment of the branches up or down the chain that is observed experimentally in Fig. 4b (see also Fig. S2 and S3†). For P type chains (Fig. 5b) the offset in water and hydroxyl binding site along [10] results in the characteristic alternation between open and partially closed rings shown in Fig. 4c. The offset between the hydroxyl and water species along [10] is quite distinct from the situation for the isolated OH/H2O pairs formed by manipulation at low temperature by Kumagai et al.,31 where the dimer forms a symmetric unit with both species adsorbed in the bridge site, aligned directly along the [001] direction. In that case the proton is believed to be symmetrically distributed between the hydroxyl groups, reducing the zero point vibrational energy sufficiently to stabilise this symmetric structure over the asymmetric form. In the thermodynamically stable OH/H2O chains reported here, creation of the water–water hydrogen bonding chain increases the average coordination number sufficiently to make water adsorb flat in its atop site and forgo the proton delocalisation seen in an isolated OH/H2O pair.
Fig. 5 Calculated structures (left) and simulated STM images (right) for the two most stable chain structures containing 1H2O:1OH. Each chain contains a central water backbone with H bonds donated to hydroxyl groups arranged with either (a) a 2aCu or (b) a 4aCu repeat along [10]. These structures are referred to as “Z” (zig-zag) and “P” (pinch) respectively in the text. The solid line shows the (4 × 5) unit cell employed and binding energies are given in Table 2. |
Structure | H2O/OH | PBE | optB88-vdW |
---|---|---|---|
5(a) | 1 | 690 | 895 |
5(b) | 1 | 682 | 890 |
6(a) | 3 | 659 | 835 |
6(b) | 2 | 624 | 808 |
6(c) | 3 | 609 | 791 |
6(d) | 1 | 576 | 763 |
6(e) | 2 | 575 | 752 |
6(f) | 2 | 505 | 686 |
6(g) | 1 | 470 | 669 |
6(h) | 1 | 434 | 622 |
To confirm if the structures shown in Fig. 5 are indeed the most stable [10] chain structures obtained from calculations, we calculated minimum energy structures, binding energies and STM simulations for a number of chain structures with different compositions and H bonding arrangements (see Fig. 6 and Table 2). None of the other structures we tried had a stability comparable to the 1H2O:1OH chains shown in Fig. 5a and 5b, nor was able to satisfactorily explain the STM images. Alternative possible chain structures having 1H2O:1OH composition found experimentally are shown in Fig. 6g and 6h. These structures have an adsorption energy at least 200 meV/OH lower than the structures shown in Fig. 5a and 5b and do not resemble the observed STM images. Displacing the hydroxyl groups to into the atop site (Fig. 6d) decreases the adsorption energy by 114 meV/OH. Amongst the other structures with different compositions investigated, the closest in terms of energy is a chain of hexagonal rings with a 3H2O:1OH composition, Fig. 6a, but this structure remains 20–30 meV/OH less stable than the 1H2O:1OH chains depicted in Fig. 5a and 5b and does not match the observed STM images. Our identification of the [10] chains as 1H2O:1OH structures is at odds with Lee et al.33 who also observed zig-zag chains in STM and tentatively suggested a 2H2O:1OH structure (Fig. 6b), similar to the Z type chain (Fig. 5a) but with half the hydroxyls missing. This assignment is not consistent with the composition obtained from TPD, see section 3.1, while the adsorption energy is some 80 meV smaller than the Z type chain. The STM images reported earlier show diffuse features along one side of the chain33 and we believe the structures most likely correspond to partially resolved images of the zigzag chains (Fig. 5a). Both the Z and P type chains have hydroxyl groups pointing at 42–43° to the surface normal along [001], consistent with the sharp H+ ESDIAD emission detected at 43° in this direction by Polak25 and by Lee et al.33
Fig. 6 Calculated structures (left) and simulated STM images (right) for other possible [10] chain structures arranged with either a 2aCu, 4aCu or 6aCu repeat along [10]. The unit cells are indicted by the solid lines and the binding energies and proportion of water to hydroxyl are given in Table 2. |
The H2O/OH chains are sometimes terminated by a complete ring, as shown on the right hand side of Fig. 4a. These rings are ca. 11 Å wide, roughly three times the spacing of the Cu close packed rows, with each side having a similar branched arrangement to the larger ring of the irregular P type chain. These ring type structures are also observed as defects formed during reaction of the chain structures to form c(2 × 2) islands, discussed in the next section. Based on these observations we believe that these features are small water rings with the P type water arrangement, stabilized by donation to hydroxyl in the same way as for the 1D chains. As the initial O coverage is increased the surface becomes covered in 1H2O:1OH chains that have no regular alignment between chains (see ESI†). This result is consistent with decoration of the chains by OH preventing any direct H bonding between adjacent chains.
Fig. 7 (a) STM image of the c(2 × 2) phase showing the termination of a small island (44 × 38 Å2, It = −0.3 nA, Vt = −190 mV. (b) Calculated structure of the c(2 × 2) phase showing a Bjerrum defect circled. The rectangle marks the unit cell employed in the calculation.17 |
Whereas intact 2D water clusters formed on close packed surfaces maximize the water H bonding coordination, forming a network of closed rings,16,58 the c(2 × 2) islands show an open edge structure. Where the structure is well resolved, e.g. in the lower right side of Fig. 7, the [10] edges have a 2aCu repeat, with the edge features displaced slightly relative to the neighbouring hexagon. This structure mimics exactly the Z type termination of 1H2O:1OH chains, shown in Fig. 4b and 5a, and allows us to assign the bright low coordinate edge features as hydroxyl groups. The offset between the edge hydroxyl features and the hexagonal network supports the assignment from DFT that the c(2 × 2) structure has O adsorbed atop Cu.17 This is an important distinction between the chain and 2D structures; in the 2D network the hydroxyl groups have been forced to adopt a less favorable atop binding site, allowing water to adsorb in its favored atop site at the expense of a reduced hydroxyl–metal interaction. In contrast the 1D chain structures have greater flexibility in the choice of water and hydroxyl adsorption site, allowing them to form a hydrogen bonding network with both species adsorbed at their favored sites.
The most obvious difference between Cu(110) and the close packed surfaces is, of course, the spacing and symmetry of the metal template, which plays a key role in determining both the local H bonding motif and the long range organization of the water and water/hydroxyl H bonding structures. DFT calculations find that a single water molecule binds in a flat geometry near the atop site on Cu(110),5,38,60,61 adopting a very similar adsorption geometry to that found on close packed metal surfaces such as Pt(111)62 and Ru(0001).12 Although the close packed faces offer the possibility of pseudomorphic ice growth, with water adopting the atop metal site to form a simple commensurate hexagonal water layer, experiments now suggest this is uncommon. A commensurate structure is found on Ru(0001),59 where the metal spacing is just 3% larger than the lateral spacing of water in bulk ice Ih(0001), but more complex unit cells are found on surfaces such as Pt(111)63 and Ni(111),64 where the metal spacing deviates further (+6% and −4.5% respectively) from the water ice repeat. The stability of large unit cell structures on Pt(111) appears to originate from the formation of small flat clusters of water, tightly bound atop Pt, embedded within a complex H-down water network.14 Templating the surface by forming a (√3 × √3)R30° surface alloy removes adjacent adsorption sites and destabilizes the chain type structures, forcing water into an H-down bilayer arrangement.65 The rectangular Cu(110) surface contains close packed rows of Cu atoms with aCu = 2.55 Å along [10], just 2% less than in bulk ice, but the match in the perpendicular direction is poor, with the Cu rows ca. 1 Å closer together along [001] than required to create an unstrained hexagonal ring of water bridging two Cu rows. Instead of forming such a strained structure, water forms 1D chains of face sharing pentamers, with 2/3 of the water bonded flat atop the Cu rows,6 maximising the O–Cu interaction while minimising the strain associated with the commensurate adsorption site. Although formation of a 2D network would increase the average H bond coordination from 8/3 to 3, the loss of the commensurate adsorption site reduces the average water–Cu interaction sufficiently that the 1D chains are favored. Increasing the coverage so that the chains approach within ca. 10–15 Å destabilizes the chains and a 2D structure forms,6 whose complex (7 × 8) H bonding network is not yet understood.23
The second key difference between Cu(110) and the close packed surfaces is the adsorption site adopted by hydroxyl. Whereas an isolated water molecule preferentially binds near the atop site on all metal faces examined, hydroxyl adopts a different site on the open Cu(110) and close packed metal faces. On Pt(111), hydroxyl slightly prefers the atop over the bridge adsorption site, by ca. 0.02 eV, with the OH axis pointing just 20° out of the surface plane.11 This common adsorption geometry for water and hydroxyl leads naturally to a planar 1H2O:1OH structure being formed on Pt(111) and Pd(111),36,66 in which each species bonds atop the metal with 3 H bonds in a commensurate (√3 × √3)R30° arrangement.35 The amount of water incorporated within this structure can be varied from ca.1H2O:2OH upwards, with no substantial change in the H bonding network. In contrast, on Cu(110) the hydroxyl potential is highly corrugated, with the short bridge site favored by 0.36 eV over the long bridge site and hydroxyl adsorbed upright, pointing at 60° from the surface along the [001] direction. The preference for a different adsorption site and orientation from that of water directly influence the structure of the water/hydroxyl phases seen on Cu(110), stabilising three very different phases depending on the water/hydroxyl ratio.
Both the 1D phases formed on Cu(110) contain hydroxyl adsorbed in the short bridge site, with a similar adsorption geometry to the isolated species. The pure hydroxyl phase, formed at high temperatures, contains dimers with one hydroxyl rotated towards the surface to form an H bond with dO–O = 2.8 Å. The spacing of the close packed Cu rows (3.6 Å) is too large to allow formation of H bonds between adjacent hydroxyl dimers, but hydroxyl prefers to retain the tilted bridge adsorption site, rather than adapting its adsorption site to increase the number of H bonds. A pure hydroxyl phase also forms on Pt(110)-(2 × 1),67Ni(110)68 and Ag(110),44 but little is known about the H bonding in these systems. Hydroxyl adsorbs in a tilted geometry on Ni(110) and Ag(110), similar to the geometry found on Cu(110). Since STM images of Ag(110) show hydroxyl forms chains parallel to the close packed rows,43 the orientation of hydroxyl along [001]69 appears to rule out H bonding stabilizing this structure. In contrast to these open f.c.c. (110) surfaces, close packed transition metal faces such as Pt(111)66 and Pd(111)36 do not form a pure hydroxyl phase, both surfaces requiring a water/O ratio of at least 2 in order for water to react to form a mixed water/hydroxyl structure. This difference can be directly correlated to the much greater exothermicity of water dissociation on Cu(110) compared to Pt(111) and Ru(0001), (0.77 eV38 compared to 0.05 eV11 and 0.38 eV70 respectively for monomer dissociation), driven by the stability of hydroxyl adsorbed in the bridge site.
Formation of stoichiometric 1H2O:1OH water/hydroxyl chains on Cu(110) allows both water and hydroxyl to adsorb in their optimum atop and bridge sites, forming strong H bonds by water donation to water and hydroxyl but giving an average coordination number of just 2. The calculated adsorption energy (PBE) of the 1D chain structures (690 and 682 meV) exceeds that of the best 2D 1H2O:1OH network by 20 to 30 meV, correctly predicting that the stoichiometric layer on Cu(110) forms 1D chains, rather than a c(2 × 2) network as proposed earlier.5,38,60,61 Formation of the 2D c(2 × 2) network occurs only once the water/hydroxyl ratio is increased and is associated with hydroxyl being displaced into the atop site to form Bjerrum defects. This structure reduces the average H bond coordination but allows water to retain the atop adsorption site while maximizing the number of strong H bonds formed by water donation to hydroxyl. This 2H2O:1OH structure is stabilized by around 45 meV compared to the best stoichiometric 1H2O:1OH c(2 × 2)network17 and is quite different to the ordered, 2D 1H2O:1OH networks formed on Pt(111)35,57 and Pd(111)36 which have all H atoms involved in hydrogen bonding. A direct consequence of the absence of uncoordinated OH groups is that the water/hydroxyl networks formed on Pt(111) and Pd(111) do not wet,36,71 whereas on Cu(110) Bjerrum defects in the c(2 × 2) network17 and uncoordinated hydroxyl in the chain structures are available to stabilize multilayer water nucleation (see for example the bright features above some of the chains in Fig. 4a).
Unlike other close packed faces studied so far, Ru(0001) also forms a water/hydroxyl structure containing an excess of water over hydroxyl,13 suggesting a parallel to Cu(110). STM images show narrow elongated stripes of a hexagonal network,41 whose edges are decorated by bright features, in an open arrangement that is superficially similar to the network found in the 1D chains and at the boundaries of the c(2 × 2) 2H2O:1OH network on Cu(110). Calculations indicate that Bjerrum D defects should also form in the Ru(0001) water/hydroxyl layer,72 analogous to the stabilization of the non-stoichiometric c(2 × 2) network on Cu(110),17 but experimental evidence for this is so far lacking. The location of hydroxyl on Ru(0001) was discussed by Tatarkhanov et al.41 who compared STM images with model DFT structures. Since water donor–hydroxyl acceptor bonding is more stable than water–water bonding by about 100 meV, it had been anticipated that hydroxyl would sit at the edge of the islands, as on Cu(110), but instead the most stable model structures had non-donor, single acceptor water molecules located at the periphery of islands and OH groups in the interior of the stripes. Reasons for this difference in behavior are not yet clear and further study is needed to understand the location of hydroxyl and the role of Bjerrum defects in the water/hydroxyl structure formed on Ru(0001).
Footnote |
† Electronic supplementary information (ESI) available: STM images showing high coverage chains, growth of the c(2 × 2) structure from 1D 1H2O:1OH and registry of these structures to the metal surface. See DOI: 10.1039/c1sc00355k |
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