ARTICLE pubs.acs.org/JPCC
Wetting of Intact and Partially Dissociated Water Layer on Ru(0001): a Density Functional Study Sabri Messaoudi,*,† Adnene Dhouib,†,‡ Manef Abderrabba,† and Christian Minot‡ † ‡
Unite de Recherche de Physico-Chimie Moleculaire, Universite de Carthage, Boite postale BP51, 2070 La Marsa, Tunisia Laboratoire de Chimie Theorique, Universite Pierre et Marie Curie (Paris VI), UMR CNRS 7616, Paris F-75005, France
bS Supporting Information ABSTRACT: Factors that influence the multilayered wetting of Ru surface are discussed by use of density functional theory calculations of adsorption of bilayers of water on Ru(0001), comparing addition of intact bilayers to a first one containing partially dissociated or intact water molecules. For the first bilayer on Ru(0001), we find that a partial dissociation of 3/8 of the water molecules is slightly more stable than the halfdissociation; this bilayer contains H-up water molecules pointing up to the vacuum, allowing further adsorption. We show that wetting of the first water layer on Ru(0001) is more favorable for the partially dissociated structures than for the intact one. For two intact water bilayers that are almost planar, there is no hydrogen available to adsorb a third bilayer, which agrees with the experimental results showing unstable ice films on adsorbed intact-water bilayer. We found that the most stable structures of two bilayers on Ru(0001) are those in which 3/8 of the water molecules from the first bilayer are dissociated. This structure has hydrogen atoms pointing to the top, suggesting its ability to adsorb a third layer. The structures with 1/4 partially dissociated first bilayer are close in energy, while structures with half-dissociated first layer are less stable.
1. INTRODUCTION Water adsorption on Ru(0001) was one of the first molecular adsorption systems investigated by surface science techniques17 and has become a test case for our understanding of water adsorption at metal Many studies have reported a √ surfaces. √ commensurate ( 3 3) R30° phase in low-energy electron diffraction (LEED)1,3 and the “bilayer” model was developed to describe this structure, based on the structure of ice I.1,3,6 810 Held and found that both H2O and D2O formed a √ Menzel √ diffuse ( 3 3) R30° LEED, and a LEED IV analysis of the √ √ ( 3 3) R30°-2D2O structure concluded that the structure is similar to a bilayer of bulk ice Ih, but the two O atoms are almost coplanar, the vertical distance being just 0.10 ( 0.02 Å compared to a buckling of about 1 Å in ice Ih. The stability of different water structures on Ru(0001) was examined by Feibelman11 using density functional theory (DFT). He calculated the binding energy of different structures containing 0.67 monolayer (ML) of water and compared these to the energy of bulk ice. The binding energies of intact √ sublimation √ ( 3 3) R30° ice bilayer structures were significantly inferior to the sublimation energy of bulk ice estimated from different generalized gradient approximation (GGA) calculations. When water is allowed to partially dissociate to form a mixed H/OH/ H2O structure, the binding energy is increased and depends slightly on the location of the H atoms. When H is adsorbed atop r 2011 American Chemical Society
Ru within the mixed OH/water structure, it is similar to the sublimation energy of ice, and when H is allowed to segregate to its favored hollow binding site on bare Ru, it is even more increased, becoming greater than the sublimation energy of ice. This is the most stable structure found for water on Ru(0001); the O atoms of water and OH are almost coplanar (dz = 0.05 Å), consistent with the minimal corrugation found experimentally by Held and Menzel.8 On this basis, the stable wetting layer for water on Ru(0001) was predicted to be a partially dissociated OH/H2O/H structure.11 √ √ The suggestion that the ( 3 3) R30° wetting layer formed on Ru(0001) is a partially dissociated structure stimulated several further calculations of water adsorption on Ru(0001).1216 All these confirmed that a partially dissociated layer is significantly more stable than intact water. Recently Tatarkhanov et al.17,18 also investigated this system by comparing X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and scanning tunneling microscopy (STM) images with DFT simulations of the mixed OH/ H2O domains. The XAS results find that the composition of this phase depends on preparation conditions, with OH accounting for Received: November 29, 2010 Revised: February 16, 2011 Published: March 15, 2011 5834
dx.doi.org/10.1021/jp1113416 | J. Phys. Chem. C 2011, 115, 5834–5840
The Journal of Physical Chemistry C
ARTICLE
Figure 2. Successive dissociations starting from one up-water intact bilayer (U): (a) two, (b) three, and (c) four dissociations. Figure 1. Intact one water bilayer on Ru(0001) (a) U, H-up water bilayer (b) Fd, flat and H-down water chains. The unit cell is indicated by black lines.
30% ( 10% of the coverage of the mixed OH/H2O structure on Ru(0001). STM images of this structure show long narrow stripes aligned perpendicular to the close-packed atomic rows of the Ru(0001) surface, with the rest of the surface covered by H. The internal structure of the stripes shows a honeycomb network of H-bonded water and hydroxyl species, When the geometry of a 0.67 ML water layer is optimized in larger unit cells,19 without imposition of any additional constraints on the local periodicity, water adopts the honeycomb hydrogen-bonded network with marked differences compared to the generic ice bilayer. In this structure, water adsorbed near the Ru atop forms flat-lying chains. Each flat water molecule has two neighbors with a similar orientation. Several recent studies have investigated the wetting and multilayer growth of D2O under conditions where the first layer is expected to remain intact.2022 At low temperatures, where water mobility is limited, continuous amorphous solid water films are formed. These films are unstable, roughening upon annealing to form multilayer ice crystallites and expose regions of water monolayer.20 The growth of water multilayers on Ru(0001) is complicated by the presence of either an intact or a partially dissociated OH/H2O/ H structure on the surface, depending on the isotope and the adsorption temperature, and it is expected that the detailed wetting behavior of Ru(0001) will depend on the first layer structure. Despite considerable study, relatively little is known about the growth of extended ice films on top of this layer on structures of water at Ru(0001). Here, we use first-principles density functional calculations to explore the factors that determine multilayer growth on different wetting layers on Ru(0001) by comparing a water bilayer adsorption on top of intact and partially dissociated water layer on Ru(0001). First, we start with a study of different mixed OH/H2O/H in a single first bilayer; second, we study the addition of a second bilayer to an intact water bilayer; and finally, we study the addition of a second bilayer to a mixed OH/H2O/H first bilayer.
2. METHODOLOGY √ √ Wetting of the Ru(0001) ( 3 3) R30° structure and model ice bilayers were explored by density functional theory calculations (using the PW91 exchangecorrelation functional)23,24 implemented in the Vienna ab initio simulation package (VASP) code25,26 to examine the structure and binding energy of two-dimensional (2D) structures on top of the OH/
Figure 3. (a) UaD = UD with no water molecule attached to Ru surface (all dRuO > 3.30 Å). (b) UbD = UD with one water molecule attached to Ru surface (1 dRuO < 2.50 Å). (c) UcD = UD with two water molecules attached to Ru surface (2 dRuO < 2.50 Å). Flat H2O of the first bilayer attached to Ru are marked in the top views by dashed black circles.
H2O/H and pure water structures. The PW91 functional performs for describing H-bonds27,28 and ice.29 With the water dimer taken as a test, differences with MP2 are only 0.38 kcal/ mol for dissociation energies and 0.03 Å for the intermolecular distance.30 The plane-wave cutoff energy was 396 eV. The surface was represented by a three-layer slab of Ru, with the experimental lattice constants. The vacuum gap varied from 20 to 12 Å depending on the number of water layers (0, 1, or 2) adsorbed on top of the Ru surface. The bottom two Ru layers were fixed in the bulk lattice positions and all other atoms were relaxed until no force component was larger than 0.03 eV/Å. All calculations √used a√ 3 3 1 MonkhorstPack k-point set31 with a (2 3 2 3)R30° unit cell. This supercell generates many structures introducing a picture of disorder. Hodgson and co-workers19,32 have shown that√the binding energies for water models on √ Ru(0001) in a 2 3 2 3 unit cell were unaffected by using thicker slabs, varying k-point sets, and including dipole corrections normal to the surface. Adsorption energies (Eads) are calculated from Eads ¼ EA þ ERu EA=Ru where EA, ERu, and EA/Ru are the total energies of the isolated adsorbate, the clean Ru(0001) surface, and the chemisorption system, respectively. In this definition, positive adsorption energies correspond to an exothermic adsorption process.
3. MODELS Many previous theoretical studies were interested in an intact (undissociated) and a half-dissociated first bilayer on Ru(0001). In order to study the wetting of different mixed OH/H2O/H 5835
dx.doi.org/10.1021/jp1113416 |J. Phys. Chem. C 2011, 115, 5834–5840
The Journal of Physical Chemistry C
Figure 4. (a) Fu(a)Fd = Fd bilayer on top of a Fu bilayer. (b) Fu(b)Fd = Fd bilayer on top of a Fu bilayer with one water molecule attached to the Ru surface (
