Water adsorption on O (2×2 )/Ru (0001): STM experiments and first-principles calculations

We present a combined theoretical and experimental study of water adsorption on Ru(0001) precovered with 0.25 ML (monolayer) of oxygen forming a (2×2) structure. Several structures were analyzed by means of density functional theory calculations for which scanning tunneling microscope (STM) simulations were performed and compared with experimental data. Up to 0.25 ML, the molecules bind to the exposed Ru atoms of the 2×2unit cell via the lone pair orbitals. The molecular plane is almost parallel to the surface with its H atoms pointing toward the chemisorbed O atoms of the 2×2 unit cell forming hydrogen bonds. The existence of these additional hydrogen bonds increases the adsorption energy of the water molecule to approximately 616meV, which is ∼220meV more stable than on the clean Ru(0001) surface with a similar configuration. The binding energy shows only a weak dependence on water coverage, with a shallow minimum for a row structure at 0.125 ML. This is consistent with the STM experiments that show a tendency of the molecules to form linear rows at intermediate coverage. Our calculations also suggest the possible formation of water dimers near 0.25 ML.This work has been supported by the Basque Departamento de Educación, the UPV/EHU Grant No. 9/UPV 00206.215-13639/2001, the Spanish Ministerio de Educación y Ciencia Grant No. FIS2004-06490-C3-00, the European Network of Excellence FP6-NoE NANOQUANTA Grant No. 500198-2, and the projects NANOMATERIALES and NANOTRON funded by the Basque Departamento de Industria, Comercio y Turismo within the ETORTEK program, and the Departamento para la Innovación y la Sociedad del Conocimiento from the Diputación Foral de Guipúzcoa. The experimental work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. A.M. was financed by the Marie Curie Outgoing International Foundation, Project No. 514412.Peer reviewe

X-ray photoemission analysis of clean and carbon monoxide-chemisorbed platinum(111) stepped surfaces using a curved crystal

This work is licensed under a Creative Commons Attribution 4.0 International License.-- et al.Surface chemistry and catalysis studies could significantly gain from the systematic variation of surface active sites, tested under the very same conditions. Curved crystals are excellent platforms to perform such systematics, which may in turn allow to better resolve fundamental properties and reveal new phenomena. This is demonstrated here for the carbon monoxide/platinum system. We curve a platinum crystal around the high-symmetry (111) direction and carry out photoemission scans on top. This renders the spatial core-level imaging of carbon monoxide adsorbed on a 'tunable' vicinal surface, allowing a straightforward visualization of the rich chemisorption phenomenology at steps and terraces. Through such photoemission images we probe a characteristic elastic strain variation at stepped surfaces, and unveil subtle stress-release effects on clean and covered vicinal surfaces. These results offer the prospect of applying the curved surface approach to rationally investigate the chemical activity of surfaces under real pressure conditions.We acknowledge financial support from the Spanish Ministry of Economy (Grants MAT2013-46593-C6-4-P and MAT2013-46593-C6-2-P ), Basque Government (Grants IT621-13 and IT756-13). A.L.W. acknowledges support from the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-SC0012704. AXBR acknowledges support from the Basque Departamento de Educación and the UPV/EHU through the Zabalduz program. AXBR, PCS and DSP acknowledge the Deutsche Forschungsgemeinschaft through the Sonderforschungsbereich 1083.Peer Reviewe

Water-induced surface reconstruction of oxygen (2×1) covered Ru(0001)

10 páginas, 14 figuras, 1 tabla.-- PACS number(s): 68.43.Bc, 68.37.Ef.-- et al.Low-temperature scanning tunneling microscopy and density-functional theory (DFT) were used to study the adsorption of water on a Ru(0001) surface covered with half monolayer of oxygen. The oxygen atoms occupy hcp sites in an ordered structure with (2×1) periodicity. DFT predicts that water is weakly bound to the unmodified surface, 86 meV compared to the ∼200 meV water-water H bond. Instead, we found that water adsorption causes a shift of half of the oxygen atoms from hcp sites to fcc sites, creating a honeycomb structure where water molecules bind strongly to the exposed Ru atoms. The energy cost of reconstructing the oxygen overlayer, around 230 meV per displaced oxygen atom, is more than compensated by the larger adsorption energy of water on the newly exposed Ru atoms. Water forms hydrogen bonds with the fcc O atoms in a (4×2) superstructure due to alternating orientations of the molecules. Heating to 185 K results in the complete desorption of the water layer, leaving behind the oxygen-honeycomb structure, which is metastable relative to the original (2×1). This stable structure is not recovered until after heating to temperatures close to 260 K.This work was supported by the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering of the U.S. DOE under Contract No. DE-AC02-05CH11231. The theoretical work was supported by the Basque Department of Education, UPV/EHU (Grant No. IT-366-07), the Spanish Ministerio de Ciencia e Innovación (Grant No. FIS2007-66711-C02-00), and the ETORTEK program funded by the Basque Departamento de Industria and the Diputación Foral de Guipúzcoa.Peer reviewe

SAM-like arrangement of thiolated graphene nanoribbons: decoupling the edge state from the metal substrate

Density functional theory calculations have been used to analyze the electronic and magnetic properties of ultrathin zigzag graphene nanoribbons (ZGNRs) with different edge saturations. We have compared a symmetric hydrogen saturation of both edges with an asymmetric saturation in which one of the edges is saturated with sulphur atoms or thiol groups, while the other one is kept hydrogen saturated. The adsorption of such partially thiolated ZGNRs on Au(111) has also been explored. We have considered vertical and tilted adsorption configurations of the ribbons, reminiscent of those found for thiolated organic molecules in self-assembled monolayers (SAM) on gold substrates. We have found that saturation with sulphur atoms or thiol groups removes the corresponding edge state from the Fermi energy and kills the accompanying spin polarization. However, this effect is so local that the electronic and magnetic properties of the mono-hydrogenated edge (H-edge) remain unaffected. Thus, the system develops a spin moment mainly localized at the H-edge. This property is not modified when the partially thiolated ribbon is attached to the gold substrate, and is quite independent of the width of the ribbon. Therefore, the upright adsorption of partially thiolated ZGNRs can be an effective way to decouple the spin-polarized channel provided by the H-edge from an underlying metal substrate. These observations might open a novel route to build spin-filter devices using ZGNRs on gold substrates. This journal is © 2013 the Owner Societies.We acknowledge support from Basque Departamento de Educación, UPV/EHU (Grant No. IT-366-07), the Spanish Ministerio de Educación y Ciencia (Grant No. FIS2010-19609-C02-00), and the ETORTEK research program funded by the Basque Departamento de Industria and the Diputación Foral de Gipuzkoa. PCS acknowledges support by the Diputación Foral de Gipuzkoa and to IKERBASQUE, Basque Foundation for Science (48011, Bilbao, Spain).Peer Reviewe

On the structure of the first hydration layer on NaCl(100): Role of hydrogen bonding

The authors have investigated the structure and energetics of the first hydration layer on NaCl(100) by means of density functional calculations. They have analyzed in detail the role of the hydrogen bond between the adsorbed molecules for the determination of the most favorable structures. They have shown that, using the water dimers as basic building blocks, very stable structures can be constructed. They discuss here two important examples: (i) a model with (1×1) periodicity at 2 ML coverage, and (ii) icelike bilayers with a c(4×2)unit cell at 1.5 ML. Both structures present high adsorption energies per water molecule of ∼570meV, in comparison to the 350meV adsorption energy obtained for the previously studied (1×1) structures composed of weakly interacting monomers. Based on these findings, they propose an interpretation for the experimental observations of Toennies et al. [J. Chem. Phys. 120, 11347 (2004)], who found a transition of the periodicity of the first hydration layer on NaCl(100) from (1×1) to c(4×2) upon electron irradiation. According to the model, the transition would be driven by the partial desorption of (1×1) bilayer structures corresponding to a local coverage of 2 ML and the further rearrangement of the remaining water molecules to form a quasihexagonal structure with c(4×2) periodicity at coverage close to 1.5 ML.This work has been supported by the Basque Departamento de Educación, the UPV/EHU (Grant No. 9/UPV 00206.215-13639/2001), the Spanish Ministerio de Educación y Ciencia (Grant No. FIS2004-06490-C3-00), the European Network of Excellence FP6-NoE “NANOQUANTA” (Grant No. 500198-2), and the projects “NANOMATERIALES” and “NANOTRON” funded by the Basque Departamento de Industria, Comercio y Turismo within the ETORTEK programme and the Departamento para la Innovación y la Sociedad del Conocimiento from the Diputación Foral de Guipúzcoa.Peer reviewe

Diffusion of H2O on a NaCl(100)

Resumen del póster presentado a la 15th International Conference on Vibrations at Surfaces, celebrada en Donsotia-San Sebastián (España) del 22 al 26 de junio de 2015.The motion of D2O monomers is investigated on a NaCl(100) bilayer on Ag(111) between 42.3 and 52.3 K by scanning tunneling microscopy and density functional theory. In previous work, we did investigate the diffusion mechanism of H2O on NaCl(100) at low coverage. For that we determined the energy barriers associated with different hopping mechanisms, classified as translations and reorientations of the water molecule. The combination of these hopping mechanisms leads to net movement of the molecule along the surface with relatively low energetic cost, compared with bare parallel translation of the water molecule. In the recent work the mechanism of the motion is identified by comparison of the experimental results to theoretical calculations. Via low temperature adsorption site determination in connection with density functional theory, we reveal an influence of the metallic support onto the intermediate state of the diffusive motion.Peer reviewe

Substrate-induced cooperative effects in water adsorption from density functional calculations

9 páginas, 6 figuras, 2 tablas.-- PACS number(S): 33.15.Fm, 68.43.-h, 33.15.KrDensity functional theory calculations are used to investigate the role of substrate-induced cooperative effects on the adsorption of water on a partially oxidized transition-metal surface, O(2×2)/Ru(0001). Focusing particularly on the dimer configuration, we analyze the different contributions to its binding energy. A significant reinforcement of the intermolecular hydrogen bond (H bond), also supported by the observed frequency shifts of the vibration modes, is attributed to the polarization of the donor molecule when bonded to the Ru atoms in the substrate. This result is further confirmed by our calculations for a water dimer interacting with a small Ru cluster, which clearly show that the observed effect does not depend critically on fine structural details and/or the presence of coadsorbates. Interestingly, the cooperative reinforcement of the H bond is suppressed when the acceptor molecule, instead of the donor, is bonded to the surface. This simple observation can be used to rationalize the relative stability of different condensed structures of water on metallic substrates.We acknowledge support from Basque Departamento de Educación, UPV/EHU (Grant No. IT-366-07), the Spanish Ministerio Innovación y Ciencia (Grant No. FIS2007-66711- C02-00), and the ETORTEK research program funded by the Basque Departamento de Industria and the Diputación Foral de Guipúzcoa. M.V.F.-S. acknowledges support from DOE under Grant No. DE-FG02-09ER16052.Peer reviewe

NO adsorption on Cu(110) and O(2 × 1)/Cu(110) surfaces from density functional theory calculations

In a recent study [M. Feng, et al., ACS Nano, 2011, 5, 8877], it was shown that CO molecules adsorbed on the quasi-one-dimensional O(2 × 1)/Cu(110) surface reconstruction tend to form highly-ordered single-molecule-wide rows along the direction perpendicular to the Cu–O chains. This stems from the peculiar tilted adsorption configuration of CO on this substrate, which gives rise to short-range attractive dipole–dipole interactions. Motivated by this observation, here we study the adsorption of nitric oxide (NO) on O(2 × 1)/Cu(110) and Cu(110) using density functional theory, with the aim of elucidating whether a similar behaviour can be expected for this molecule. We first study NO adsorption on a clean Cu(110) surface, where the role of short-range attractions between molecules has already been pointed out by the observation of the formation of NO dimers by scanning tunnelling microscopy [A. Shiotari, et al., Phys. Rev. Lett., 2011, 106, 156104]. On the clean Cu(110), the formation of dimers along the [1[1 with combining macron]0] direction is favourable, in agreement with published experimental results. However, the formation of extended NO rows is found to be unstable. Regarding the O(2 × 1)/Cu(110) substrate, we observe that NO molecules adsorb in between the Cu–O chains, causing a substantial disruption of the surface structure. Although individual molecules can be tilted with negligible energetic cost along the direction of the Cu–O chains, the interaction among neighbouring molecules was found to be repulsive along all directions and, consequently, the formation of dimers unfavourable.The authors acknowledge support from the Spanish Ministerio de Economa y Competitividad (MINECO) (Grant No. MAT2013-46593-C6-2-P), the Basque Departamento de Educación and the UPV/EHU (Grant No. IT-756-13), and the Deutsche Forschungsgemeinschaft through the Sonderforschungsbereich 1083. AXBR acknowledges support from the Basque Departamento de Educación and the UPV/EHU through a Zabalduz grant.Peer reviewe

Hybridization between Cu-O chain and Cu(110) surface states in the O(2×1)/Cu(110) surface from first principles

The O(2×1)/Cu(110) surface reconstruction of the Cu(110) surface is induced by 0.5 ML of oxygen adsorption and is formed by Cu-O chains running along the [001] direction. Here, we show that hybridization between surface states of the Cu(110) substrate and one-dimensional states of the Cu-O chains is crucial in understanding the electronic structure of this surface. Specifically, the interaction between one occupied antibonding band of the Cu-O chain with O(py) character (y-axis taken along the Cu-O chain direction) and the partially occupied surface state at the Y point of the clean Cu(110) surface with Cu(py) character causes major changes in the electronic structure close to the Fermi energy (EF). This surface state decays very slowly into the bulk and a thick slab is needed to properly describe it, which might explain why the importance of this hybridization has not been recognized so far. In our calculations we obtain two hybrid bands: (i) a fully occupied band that strongly hybridizes with the bulk Cu sp states nearby EF, becoming a very broad resonance, thus explaining why it is not observed in photoemission experiments; (ii) an empty band that acquires surface state character, including its dispersion close to the zone boundary at the Y point. This splitting induces a partial population of the py antibonding band that is necessary to reconcile the calculated charge transfer from the Cu(110) substrate to the Cu-O chain (∼0.5 electrons/f.u.) with the apparently fully occupied band structure of the adsorbed Cu-O chain (consistent with 1 electron transferred per formula unit). © 2013 IOP Publishing Ltd.We acknowledge useful discussions withMFeng and H Petek and support from the Basque Departamento de Educación, UPV/EHU (Grant No. IT-366-07), the Spanish Ministerio de Educación y Ciencia (Grant No. FIS2010-19609-C02-00), and the ETORTEK research program funded by the Basque Departamento de Industria and the Diputación Foral de Gipuzkoa. PCS acknowledges support from the Diputación Foral de Gipuzkoa and from IKERBASQUE, Basque Foundation for Science.Peer Reviewe

Water adsorption and diffusion on NaCl(100)

At low coverage and temperature the water−surface interaction determines the adsorption geometry of the water molecule on the NaCl(100) surface. However, at room temperature the molecules are also able to move on the surface and form islands where the water molecules are held together by hydrogen bonds. As a step toward the description of such complex phenomenology, in this work we have used density functional theory calculations to study the most favorable adsorption geometry of an isolated water molecule and the energy barriers associated with different hopping mechanisms between equivalent adsorption configurations on this surface. We propose different hopping processes that can be classified as translations, if the molecule moves from one adsorption site to the adjacent one, or reorientations, if the molecule only changes its orientation on the surface and remains in the same adsorption site. The straightforward parallel translation of the water molecule along the surface exhibits the highest barrier. All other processes, either translations or reorientations, involve the rotation of the water molecule around certain axes and present much smaller barriers (at least 50% smaller). To obtain a net movement of the molecule along the surface it is always necessary to combine one of these translational and reorientational processes. Such combinations provide favorable and plausible pathways for the diffusion of the water molecule on the NaCl(100) substrate.This work has been supported by the Basque Departamento de Educación, the UPV/EHU (Grant No. 9/UPV 00206.215-13639/2001), the Spanish Ministerio de Educación y Ciencia (Grant No. FIS2004-06490-C3-00), the European Network of Excellence FP6-NoE “NANOQUANTA” (Grant No. 500198-2), and the projects “NANOMATERIALES” and “NANOTRON” funded by the Basque Departamento de Industria, Comercio y Turismo within the ETORTEK program and the Departamento para la Innovación y la Sociedad del Conocimiento from the Diputación Foral de Guipúzcoa.Peer reviewe