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

Pepa Cabrera-Sanfelix, M. V. Fernández-Serra, A. Arnau, and D. Sánchez-Portal

Phys. Rev. B 82, 125432 – Published 16 September 2010

Abstract

Density 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, $O (2 \times 2) / Ru (0001)$ . 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.

Received 18 June 2010

DOI:https://doi.org/10.1103/PhysRevB.82.125432

Authors & Affiliations

Pepa Cabrera-Sanfelix¹, M. V. Fernández-Serra², A. Arnau^1,3,4, and D. Sánchez-Portal^1,3

¹Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, San Sebastian 20018, Spain
²Physics and Astronomy, State University of New York, Stony Brook, New York 11794-3800, USA
³Centro de Física de Materiales, Materials Physics Center (MPC), CSIC–UPV/EHU, Paseo Manuel de Lardizabal 5, San Sebastián 20018, Spain
⁴Departamento de Física de Materiales, Facultad de Química, UPV/EHU, Apdo. 1072, San Sebastián 20080, Spain

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Vol. 82, Iss. 12 — 15 September 2010

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Images

Figure 1
(Color online) Calculated water configuration at 0.5 ML coverage on the $O (2 \times 2) / Ru (0001)$ surface. The structure is formed by dimers in which one of the molecules has its oxygen $2.26 Å$ above a Ru top site and it is hydrogen bonded to one of the surface oxygen atoms ( $2.12 Å$ bond length) and to the adjacent water molecule (very short H bond of $1.67 Å$ ). The second molecule adsorbs $3.61 Å$ above the Ru topmost layer and forms a H bond to the substrate oxygen atom right below ( $1.73 Å$ bond length).Reuse & Permissions
Figure 2
(Color online) Isosurface of the calculated charge density for a water dimer: (a) free-standing water dimer $(ρ_{dimer})$ ; (b) adsorbed water dimer on the $O (2 \times 2) / Ru (0001)$ surface. Notice that in this second case we have subtracted the charge density corresponding to the surface, i.e., $ρ_{dimer} = ρ_{dimer / surface} - ρ_{surface}$ . In both cases, the value of the density used to plot the surfaces is 0.02 a.u.Reuse & Permissions
Figure 3
(Color online) Calculated induced charge density for a water dimer: (a) free-standing dimer $(ρ_{ind} = ρ_{dimer} - ρ_{monomers})$ ; (b) adsorbed water dimer on the $O (2 \times 2) / Ru (0001)$ surface $(ρ_{ind} = ρ_{dimer / surface} - ρ_{surface} - ρ_{monomers})$ . Light (pink) surfaces correspond to electron accumulations while dark (blue) correspond to electron depletions. Surfaces are plotted for the isovalues $\pm 0.003 a .u .$ Reuse & Permissions
Figure 4
(Color online) VDOS of the relaxed free-standing water dimer (left panel) and of the adsorbed dimer on the $O (2 \times 2) / Ru (0001)$ surface (right panel). Only the contributions to the VDOS from those H atoms involved in the H bonds of the adsorbed dimer have been included in both panels.Reuse & Permissions
Figure 5
(Color online) [(a)–(d)] Relaxed structures of a water dimer interacting with a small Ru cluster. The distance between the cluster apex and the oxygen atom of the donor molecule is kept constant during the relaxation. Case (d) corresponds to the optimum Ru-O separation. All distances are given in $Å$ . Panel (e) shows the energetics of the system. $E_{A − B}^{i n t}$ is the interaction energy of the fragments A ( $cluster + donor$ molecule) and B (acceptor molecule) defined in panel (a). The changes in $E_{A − B}^{i n t}$ $(Δ E_{A − B}^{i n t})$ give a direct estimation of the H-bond strength variation with the Ru-O distance. $E_{binding}$ represents the total binding energy of the system respect to their components, i.e., the isolated cluster and two isolated water molecules.Reuse & Permissions
Figure 6
(Color online) Panels (a)–(d): calculated induced charge density for the systems presented in Fig. 5. The induced charge is calculated as $ρ_{induced} = ρ_{dimer / cluster} - ρ_{donor} - ρ_{acceptor} - ρ_{cluster}$ , where $ρ_{dimer / cluster}$ is the electron density of the combined system, $ρ_{donor}$ and $ρ_{acceptor}$ are the densities of the isolated water molecules, and $ρ_{cluster}$ that of the isolated Ru cluster. Light (pink) surface correspond to the isovalue $+ 0.004 a .u .$ (electron accumulation) and the dark (blue) surface correspond to isovalue $- 0.004 a .u .$ (electron depletion). Panel (e) shows the relaxed configuration when the dimer is interacting with the cluster through the acceptor molecule, along with the corresponding induced charge $ρ_{induced}$ . All distances are given in $Å$ .Reuse & Permissions

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