Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session V41: Focus Session: The Role of Water in Energy Production and Utilization II |
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Sponsoring Units: DCP Chair: Angelos Michaelides, University College London Room: A115/117 |
Thursday, March 24, 2011 8:00AM - 8:36AM |
V41.00001: The structure of water/hydroxyl phases at metal interfaces Invited Speaker: On all but the least reactive metals, the first contact layer with water is a mixture of water and hydroxyl, often formed by spontaneous dissociation [1]. Understanding the composition and stability of these layers is a key step in describing both the wetting and the redox behavior of the surface. Here we discuss the wetting of Cu(110) and the formation of mixed water-hydroxyl layers by reaction with adsorbed O. This surface does not conform to the traditional hexagonal symmetry associated with an ice Ih, and the hydrogen bonding structure must accommodate to the surface symmetry. A number of unusual structures are seen, including 1D chains of interlocking pentagons [2], an intact 2D network with a (7x8) unit cell at higher coverage [3] and several partially dissociated structures, including both 1D and 2D phases [4]. The composition of these structures and hydrogen bonding arrangements will be discussed, highlighting the way changing the composition and relative metal-adsorbate and adsorbate-adsorbate interactions drives the structural rearrangement of these phases. \\[4pt] [1] A. Hodgson and S. Haq, Surf. Sci. Rep., 64(9), 381 (2009). \\[0pt] [2] J. Carrasco et al. Nat. Mat., 8, 427 (2009). \\[0pt] [3] T. Schiros et al., Chem. Phys. Lett., 429, 415 (2006). \\[0pt] [4] M. Forster et al., submitted. [Preview Abstract] |
Thursday, March 24, 2011 8:36AM - 8:48AM |
V41.00002: Quantum nature of the proton in water-hydroxyl overlayers on metal surfaces Angelos Michaelides, Xinzheng Li, Matthew Probert, Ali Alavi Using \textit{ab initio} path integral molecular dynamics we show that water-hydroxyl overlayers on transition metal surfaces exhibit surprisingly pronounced quantum nuclear effects. The metal substrates serve to reduce the classical proton transfer barriers within the overlayers and, in analogy to ice under high pressure, to shorten the corresponding intermolecular hydrogen bonds. Depending on the substrate and the intermolecular separations it imposes, the traditional distinction between covalent and hydrogen bonds is lost partially (e.g. on Pt(111) and Ru(0001)) or almost entirely (e.g. on Ni(111)). We suggest that these systems provide an excellent platform on which to systematically explore the magnitude of quantum nuclear effects in hydrogen bonds. [Preview Abstract] |
Thursday, March 24, 2011 8:48AM - 9:00AM |
V41.00003: Water adsorption on oxygen covered Ru(0001) surfaces Sabine Maier, Pepa Cabrera-Sanfelix, Ingeborg Stass, Daniel Sanchez-Portal, Andres Arnau, Miquel Salmeron We present a combined scanning tunneling microscopy (STM) and density functional theory (DFT) study of the adsorption of water on a Ru(0001) surface covered with half monolayer of oxygen. The adsorption of water causes a shift of half of the oxygen atoms in the O(2x1) structure from hcp sites to fcc sites, creating a honeycomb structure where water molecules bind strongly to the exposed Ru atoms [1]. The energy cost of reconstructing the oxygen overlayer is more than compensated by the larger adsorption energy of water on the newly exposed Ru atoms. STM images reveal a (4x2) superstructure due to alternating orientations of the water 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 (2x1). This stable structure is not recovered until after heating to temperatures close to 260K. \\[4pt] [1] S. Maier et al. Phys. Rev. B 82, 075421 (2010). [Preview Abstract] |
Thursday, March 24, 2011 9:00AM - 9:36AM |
V41.00004: Water monolayers on metals -- a new framework Invited Speaker: The first wetting layer on a solid embodies the boundary condition for water transport along its surface, is the template for ice nucleation, and governs aqueous surface chemistry. Today's talk is focused on the wetting of close-packed, precious metal surfaces, which are both relatively easily prepared, and susceptible to study by a host of surface science techniques. For decades, wetting layers on such surfaces have been thought to be ``ice-like'' -- strained into registry with the metal lattice, but otherwise like the layers that stack to form the naturally occurring crystal, ice Ih. Interpretations of STM images of periodic wetting layers on Pt(111) [1], of TPD [2], and of IR absorption spectra [3] contradict the ``ice-like'' picture, but submit to a common, physics-based and DFT-supported interpretation. It is that several ice-like hexagonal rings of H$_{2}$O molecules, per repeated cell, are replaced by pentagons and heptagons, allowing a compact subset of H$_{2}$O's, with planes parallel to the metal surface, to approach the metal exceptionally closely and to anchor the wetting layer strongly to it. This motif, amounting to formation of energetically favorable di-interstitial ``defects,'' appears to be general; similar molecular arrangements account for what we know experimentally (and, largely, could not previously explain) of water bonding to Ni, Ru, and Pd close-packed surfaces. \\[4pt] [1] S. Nie, P. J. Feibelman, N.C. Bartelt, and K. Th\"{u}rmer, Phys. Rev. Lett. ~\textbf{105}, 026102(2010). \\[0pt] [2] P. J. Feibelman, N.C. Bartelt, S. Nie, and K. Th\"{u}rmer. ~J. Chem. Phys. \textbf{133}, 154703(2010). \\[0pt] [3] P. J. Feibelman, G. A. Kimmel, B. D. Kay, N. Petrik, R. S. Smith and T. Zubkov, unpublished. [Preview Abstract] |
Thursday, March 24, 2011 9:36AM - 9:48AM |
V41.00005: First-principles simulation of water on graphene using different levels of theory P. Ganesh, P.R.C. Kent, De-en Jiang We show results from molecular-dynamics simulations of water confined between flat graphene sheets using different levels of theory. DFT simulations using PBE exchange-correlation show strong layering near the graphene sheet, with the hydrogens closer to the graphene surface. Correlations die off to bulk values after $\sim $10{\AA} from the surface. Inclusion of an empirical Grimme-type van der Waals potential has a small effect on the interfacial C-H distance but a seemingly large effect from the second coordination shell onwards from the surface. Existing reactive force-fields for water, e.g. ReaxFF, do not capture the structure of water on graphene accurately and require refitting to more closely reproduce the DFT results. Molecular dynamics results with available self-consistent vDW-DFx kernels and contrasts with existing classical water models will also be presented. [Preview Abstract] |
Thursday, March 24, 2011 9:48AM - 10:00AM |
V41.00006: How Water Meets Graphene Hua Zhou, Paul Fenter, Jake McDonough, Volker Presser, Yuri Gogotsi, Matthew Wander, Kevin Shuford The interactions of electrolyte fluids with solids control many complex interfacial processes encountered in electrochemical energy storage systems. In this talk, we will demonstrate how to develop a fundamental atomic-scale understanding of interfacial structures at the water-graphene interface, a model fluid-solid interface combination. We have performed systematic measurements of high resolution X-ray reflectivity from epitaxial graphene films in contact with electrolytes including deionized water and aqueous salt solutions. The electron density profiles and structural models from the fully analyzed data reveal the intrinsic interfacial structures. It is noted that the interfacial water structure above the first graphene layer exhibits remarkable differences with those of subsequent graphene layers. The latter one, resembling water on freestanding graphene, is well predicted by parallel computational simulations. Moreover, the pH of aqueous solutions was found to have a subtle influence on the interfacial water structure above the first graphene layer. This may well be an indication of the interfacial structural distortions that might exist in this layer, and which may play an important role in controlling the chemical activity of monolayer epitaxial graphene. [Preview Abstract] |
Thursday, March 24, 2011 10:00AM - 10:36AM |
V41.00007: The Dynamics of Supercooled Water Invited Speaker: We present an overview of recent experiments performed on transport properties of water in the deeply supercooled region, a temperature region of fundamental importance in the science of water. We report data of nuclear magnetic resonance, quasi-elastic neutron scattering, Fourier-transform infrared spectroscopy, and Raman spectroscopy, studying water confined in nano-meter-scale environments (nano-tubes and the protein hydration water) and in bulk solutions. When contained within small pores, water does not crystallise, and can be supercooled well below its homogeneous nucleation temperature Th. On this basis it is possible to carry out a careful analysis of the well known thermodynamical anomalies of water. Studying the temperature and pressure dependencies of water dynamics, we show that the liquid-liquid phase transition (LLPT) hypothesis represents a reliable model for describing liquid water. In this model, water in the liquid state is a mixture of two different local structures, characterised by different densities, namely the low density liquid (LDL) and the high-density liquid (HDL). The LLPT line should terminate at a special transition point: a low-T liquid-liquid critical point. In particular We discuss the following experimental findings on liquid water: (i) a crossover from non-Arrhenius behaviour at high T to Arrhenius behaviour at low T in transport parameters; (ii) a breakdown of the Stokes-Einstein relation; (iii) the existence of a Widom line, which is the locus of points corresponding to maximum correlation length in the p-T phase diagram and which ends in the liquid-liquid critical point; (iv) the direct observation of the LDL phase; (v) a minimum in the density at approximately 70K below the temperature of the density maximum. In our opinion these results represent the experimental proofs of the validity of the LLPT hypothesis. [Preview Abstract] |
Thursday, March 24, 2011 10:36AM - 10:48AM |
V41.00008: Structural, Dynamic, and Spectroscopic Properties of the SCC-DFTB Water Model Laura Kinnaman, Kathie Newman, Steven Corcelli The interactions of water and many interfaces are not understood at a mechanistic level. The accuracy of simulations of the system are limited by the accuracy of the water model used. Classical models such as SPC/E use empirically derived parameters to match their behavior to desired bulk water properties, but cannot participate in reactions that require the making or breaking of bonds. Ab initio quantum mechanical methods such as Car-Parrinello (CP) do allow water to dissociate, but are computational intractable for large systems. A potential middle ground is the self-consistent charge density-functional tight-binding method (SCC-DFTB), which has a smaller associated computational cost, and therefore can access larger systems than CP, while still allowing for the making and breaking of bonds. The DFTB+ implementation of SCC-DFTB allows for 2nd and 3rd order expansions of the density fluctuations in the energy and, in the 3rd order expansion, an optional damping correction factor. For each of these models we compare the structural, dynamic, and spectroscopic properties of bulk SCC-DFTB water to classical SPC/E and experimental results. [Preview Abstract] |
Thursday, March 24, 2011 10:48AM - 11:00AM |
V41.00009: Interfacial water on TiO$_2$ anatase (101) and (001) surfaces: First-principles study with TiO$_2$ slabs dipped in bulk water Yoshitaka Tateyama, Masato Sumita, Chunping Hu Density functional molecular dynamics simulations using supercells with ``bulk'' water between the TiO$_2$ anatase (101) and (001) surfaces were carried out to elucidate the behaviour of water molecules and hydrogen bond networks on the interfaces of representative photocatalysts. It is demonstrated that the adsorption manners (molecular or dissociative) of water molecules on the vacuum surfaces still hold in the presence of ``bulk'' water on the interfaces. We also showed explicit atomistic structures of strong and weak hydrogen bonds on the TiO$_2$/water interfaces, which had been proposed experimentally so far. We then suggested a two-layer model for the interfacial water on both surfaces investigated. Our results also give insights into the H$_2$O or OH adsorption coverage on the interfaces and their hydrophobicity- hydrophilicity, which is important to understand the photocatalytic reaction mechanisms microscopically. [Preview Abstract] |
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