Interaction of water with a metal surface: Importance of van der Waals forces (original) (raw)
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The role of van der Waals forces in water adsorption on metals
The Journal of Chemical Physics, 2013
The interaction of water molecules with metal surfaces is typically weak and as a result van der Waals (vdW) forces can be expected to be of importance. Here we account for the systematic poor treatment of vdW forces in most popular density functional theory (DFT) exchange-correlation functionals by applying accurate non-local vdW density functionals. We have computed the adsorption of a variety of exemplar systems including water monomer adsorption on Al , Cu(111), Cu(110), Ru(0001), Rh(111), Pd(111), Ag(111), Pt(111), and unreconstructed Au(111), and small clusters (up to 6 waters) on Cu . We show that non-local correlations contribute substantially to the water-metal bond in all systems, whilst water-water bonding is much less affected by non-local correlations. Interestingly non-local correlations contribute more to the adsorption of water on the reactive transition metal substrates than they do on the noble metals. The relative stability, adsorption sites, and adsorption geometries of competing water adstructures rarely differ when comparing results obtained with semi-local functionals and the non-local vdW density functionals, which explains the previous success of semi-local functionals in characterizing adsorbed water structures on a number of metal surfaces.
(H2O) 3 on a virtual metal surface: the growth of the water bilayer
Chemical Physics Letters, 2002
We introduce a simple model to predict whether the first water bilayer on hexagonal metal surfaces grows two-or three-dimensionally and compare the predictions of the new model with experimental data. Water-water interaction energies were calculated with the classical interaction potential TIP4P and quantum chemically (RHF, RMP2, RMP4(SDTQ) and B3LYP with the DZP and aug-cc-pVDZ basis sets). The metal surface was replaced by an hexagonal mesh of auxiliary geometrical points (a virtual surface) and additional water-metal interaction energies were approximated with a Morse potential. Ó
In the light of recent intensity-voltage low energy electron diffraction LEED-IV experiments Surf. Sci. 316, 92 1994; Surf. Rev. Lett. 10, 487 2003, the electronic and geometric structure of a water bilayer adsorbed at the Ru0001 surface are investigated through first-principles total energy calculations, using periodic slab geometries and gradient-corrected density functional theory DFT. We consider five possible bilayer structures, all roughly consistent with the LEED-IV analysis three intact structures and two half-dissociated, and a water single layer at Ru0001. Adsorption energies and substrate-adsorbate geometry parameters are given and discussed in the light of the experiments. We also give a comparative analysis of the electron density redistribution and of the dipole moment change induced by water adsorption on the Ru0001 surface. In agreement with Feibelman Science 295, 99 2002, the half-dissociated structures are found to be more stable than the intact ones, and their adsorption geometries in better agreement with the LEED-IV data. However, the analysis shows that a half-dissociated structure induces a 0, which would be incompatible with the experimentally measured decrease of the work function following bilayer adsorption; the latter would be consistent, instead, with the 0 induced by the intact structures. It is the aim of this paper to compare various possible adsorption structures, most of them already considered previously , with one and the same method. For this purpose, thick slabs and restrictive computational parameters are chosen to generally address the accuracy and the limits of DFT in reproducing adsorption energies and bond lengths of water-metal interacting systems.
A molecular perspective of water at metal interfaces
Nature Materials, 2012
Water-solid interfaces are ubiquitous and of the utmost importance to industry, technology and many aspects of daily life. Despite countless studies from different areas of science, detailed molecular-level understanding of water-solid interfaces comes mainly from well-defined studies on flat metal surfaces. These studies have recently shown that a remarkably rich variety of structures form at the interface between water and seemingly simple flat metal surfaces. Here we discuss some of the most exciting examples of recent work in this area and the underlying physical insight and general concepts that emerge about how water binds to surfaces. A perspective on the outstanding problems, challenges, and open questions in the field is also provided.
2011
The structure of liquid water at ambient conditions is studied in ab initio molecular dynamics simulations using van der Waals (vdW) density-functional theory, i.e. using the new exchange-correlation functionals optPBE-vdW and vdW-DF2. Inclusion of the more isotropic vdW interactions counteracts highly directional hydrogen-bonds, which are enhanced by standard functionals. This brings about a softening of the microscopic structure of water, as seen from the broadening of angular distribution functions and, in particular, from the much lower and broader first peak in the oxygen-oxygen pair-correlation function (PCF), indicating loss of structure in the outer solvation shells. In combination with softer non-local correlation terms, as in the new parameterization of vdW-DF, inclusion of vdW interactions is shown to shift the balance of resulting structures from open tetrahedral to more close-packed. The resulting O-O PCF shows some resemblance with experiment for high-density water (A. K. Soper and M. A. Ricci, Phys. Rev. Lett., 84:2881, 2000), but not directly with experiment for ambient water. However, an O-O PCF consisting of a linear combination of 70% from vdW-DF2 and 30% from experiment on low-density liquid water reproduces near-quantitatively the experimental O-O PCF for ambient water, indicating consistency with a two-liquid model with fluctuations between high- and low-density regions.
Structure of Water at Charged Interfaces: A Molecular Dynamics Study
Langmuir, 2014
The properties of water molecules located close to an interface deviate significantly from those observed in the homogeneous bulk liquid. The length scale over which this structural perturbation persists (the so-called interfacial depth) is the object of extensive investigations. The situation is particularly complicated in the presence of surface charges that can induce long-range orientational ordering of water molecules, which in turn dictate diverse processes, such as mineral dissolution, heterogeneous catalysis, and membrane chemistry. To characterize the fundamental properties of interfacial water, we performed molecular dynamics (MD) simulations on alkali chloride solutions in the presence of two types of idealized charged surfaces: one with the charge density localized at discrete sites and the other with a homogeneously distributed charge density. We find that, in addition to a diffuse region where water orientation shows no layering, the interface region consists of a "compact layer" of solvent next to the surface that is not described in classical electric double layer theories. The depth of the diffuse solvent layer is sensitive to the type of charge distributions on the surface and the ionic strength. Simulations of the aqueous interface of a realistic model of negatively charged amorphous silica show that the water orientation and the distribution of ions strongly depend on the identity of the cations (Na + vs Cs + ) and are not well represented by a simplistic homogeneous charge distribution model. While the compact layer shows different solvent net orientation and depth for Na + vs Cs + , the depth (∼1 nm) of the diffuse layer of oriented waters is independent of the identity of the cation screening the charge. The details of interfacial water orientation revealed here go beyond the traditionally used double and triple layer models and provide a microscopic picture of the aqueous/mineral interface that complements recent surface specific experimental studies.
Structure, Bonding and Chemistry of Water and Hydroxyl on Transition Metal Surfaces
2006
The structure, bonding and chemistry of water and hydroxyl on certain well-defined metal single-crystal surfaces are presented in this thesis. Synchrotron based core level spectroscopies (x-ray photoelectron (XP)-and x-ray absorption (XA) spectroscopy) in combination with scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and density functional theory (DFT) calculations form the basis of the presented results. Taken together these techniques provide chemically quantitative, local electronic and geometric information. Conditions for the experimental investigations span the temperature range 35-520 K (-240 to 250 • C) and pressure range from ultra-high vacuum (UHV) [10 −11 Torr (∼10 −14 Atm)] to near ambient pressures [∼1 Torr (∼10 −3 Atm)]. With the sampled range of experimental conditions and techniques at hand we address the structure and bonding of water at metal surfaces along with activation barriers for water dissociation, structure and bonding in mixed water-hydroxyl phases and the fundamental importance of hydrogen (H-) bonding interactions on structure and kinetic barriers. Adsorption of water at the Pt(111), Ru(001) and Cu(110) surfaces at temperatures below 150 K under UHV conditions, i.e. below the temperature for significant ice sublimation rates, is found to proceed molecularly and no dissociation is observed. Complete 2-dimensional wetting layers can be formed on Pt(111), Ru(001) and Cu(110). At water adsorption temperatures above 150 K on Ru(001), it is found that previously reported isotope dependent features in thermal desorption spectra are due to qualitatively different surface chemistry for H 2 O and D 2 O. Whereas D 2 O desorbs molecularly intact, H 2 O dissociates in kinetic competition with the desorption channel above 150 K, the difference explained by the delicate change in energetics introduced by the approximately 0.1 eV lower zero point vibrational energy of the intramolecular O-H bond compared to O-D bond in the water isotopes. The molecularly intact water overlayer is found very sensitive to x-ray and electron induced damage and it is argued that this reconciles conflicting results in the literature over the, in essence, magnitude of the activation barrier for water dissociation on Ru(001). The structure of the mixed H 2 O:OH phases on the hexagonally close-packed Ru(001) and Pt(111) surfaces were studied and compared. On Ru(001) it consists of stripe-like structures 4 to 6 Ru lattice parameters wide where OH, in a non-donor configuration, decorates the edges of the stripes whereas the inner structure consists of intact water. The observed short-range order of the mixed H 2 O:OH stripes and the tendency of OH not to fully dissolve into the H 2 O-containing H-bond network on Ru(001) is radically different compared to the mixed H 2 O:OH phases observed on Pt(111). On Pt(111) two types of extended long-range order mixed H 2 O:OH H-bonding networks with 3×3 and (√ 3 × √ 3)R30 • symmetry were studied and found to be interrelated by geometric distortions originating from the asymmetric H-bond donor-acceptor properties of OH towards H 2 O. On the open Cu(110) surface the structure of the intact water monolayer is a mixed H-down and H-up structure in a 2:1 ratio. Similarly to the H 2 O/Ru(001)-system the molecularly intact water monolayer on Cu(110) start dissociating slightly above 150 K and is very sensitive to x-ray and electron induced damage. The studies on Cu(110) were extended to near ambient conditions utilizing in-situ XPS and compared to results on Cu(111). Whereas the Cu(111) surface remains adsorbate free, we find that the Cu(110) surface at room temperature up to about 430 K in the presence of only 1 Torr water holds significant amounts of water in a mixed H 2 O:OH layer. The differences are explained by the differing activation barriers for water dissociation, leading to the presence of OH groups on Cu(110) which lowers the desorption kinetics of water by orders of magnitude due to the formation of H 2 O-OH bonds of significant strength. By lowering the activation barrier for water dissociation on Cu(111) by pre-adsorbing atomic O, generating adsorbed OH, similar results to those on Cu(110) are obtained.
First-principles realization of a van der Waals–Maxwell theory for water
Chemical Physics Letters, 2001
We generalize the van der Waals±Maxwell description of the¯uid phase diagram to account for chemical speci®cities of polar molecular¯uids such as hydrogen bonding in water. The theory is based on the reference interaction site model (RISM) integral equation method in the partially linearized hypernetted chain (PLHNC) approximation. The predictions for the liquid±vapor coexistence of water are in qualitative agreement with molecular simulations. The theory can be extended to electrolyte as well as non-electrolyte solutions, and to ionic liquids.
Structural and dynamical aspects of water in contact with a hydrophobic surface
European Physical Journal E, 2010
By means of molecular dynamics simulations we study the structure and dynamics of water molecules in contact with a model hydrophobic surface: a planar graphene-like layer. The analysis of the distributions of a local structural index indicates that the water molecules proximal to the graphene layer are considerably more structured than the rest and, thus, than the bulk. This structuring effect is lost in a few angstroms and is basically independent of temperature for a range studied comprising parts of both the normal liquid and supercooled states (240K to 320K). In turn, such structured water molecules present a dynamics that is slower than the bulk, as a consequence of their improved interactions with their first neighbors.