Papers

AVS1998 Session SS2-FrM: Water and Ice Interfaces

Friday, November 6, 1998 8:20 AM in Room 309

Friday Morning

Start	Invited?	Item
8:20 AM		SS2-FrM-1 Dielectric Response and Ionization of Water Adlayers in High Electric Fields: Calculations and Experiments T.D. Pinkerton, D.L. Scovell, V. Medvedev, E.M. Stuve (University of Washington) Water/metal interfaces typically support high surface electric fields on the order of 1-3 V/Å (100-300 MV/cm) - fields strong enough to make or break chemical bonds. While the surface electric field depends upon electrode potential and the nature of both electrode and electrolyte, the response of the electric field to these parameters and its subsequent influence in electrochemical processes remain unknown. Controlled high surface electric fields can be obtained on sharp field emitter tips in vacuum through potentials of 1-5 kV applied to tips of radius 100-1000 Å. The response of both thin and thick water layers adsorbed on these tips thereby allows the influence of electric field to be probed directly. Theoretical and experimental results show the transition of the location of the field from the water/vacuum interface for thin water layers to the metal/water interface for water layers above 500 Å thickness. Ionization at the water/vacuum interface represents UHV adsorbed water, whereas ionization at the metal/water interface represents an electrochemical response. The field required for onset of ionization increases linearly with adlayer thickness, in agreement with calculations. With increasing field strength water ionizes to form H₃O⁺, OH^-, and H₂O⁺. These results demonstrate the electrochemical response of water as a function of field and have implications in basic electrochemistry, nanolithography, and the design of tips for field emitter arrays.
8:40 AM		SS2-FrM-2 Crystallization Kinetics of Amorphous Solid Water: The Effect of Underlying Substrate Z. Dohnálek, G.A. Kimmel, K.P. Stevenson, R.S. Smith, B.D. Kay (Pacific Northwest National Laboratory) Thermally induced crystallization of ultra-thin amorphous solid water (ASW) films grown from a directed H₂O(g) beam is investigated. The temperature programmed desorption (TPD) spectra of N₂(g) physisorbed on ASW and crystalline ice (CI) surfaces exhibit striking differences in their lineshapes. The spectroscopic sensitivity of the N₂(g) TPD provides a unique opportunity to study the ASW crystallization with high sensitivity at temperatures well below H₂O desorption. The ASW crystallization kinetics are followed isothermally on Pt(111) and CI substrates as a function of temperature and ASW film thickness. The crystallization kinetics are strongly substrate dependent. On Pt(111), the data is consistent with the 3-dimensional crystallization model with the nucleation occurring in the bulk of the ASW film as shown previously.¹ In contrast, on the CI substrate we observe that the CI surface serves as a 2-dimensional nucleation center and the crystallization proceeds from the ASW/CI interface towards the ASW surface. The activation barrier of the crystallization process on the CI substrate is significantly lowered as compared to the Pt(111) due to the presence of the crystalline template. ¹Pacific Northwest National Laboratory is operated for the Department of Energy by Battelle under Contract DE-AC06-76RLO 1830. ¹ R. S. Smith, C. Huang, E. K. L. Wong, and B. D. Kay, Surf. Sci. 367 (1996) L13.
9:00 AM		SS2-FrM-3 A Study of Amorphous Solid Water (ASW) Morphology using N₂ Gas Adsorption and Thermal Desorption K.P. Stevenson, Z. Dohnálek, G.A. Kimmel, R.S. Smith, B.D. Kay (Pacific Northwest National Laboratory) We have probed the morphology of amorphous solid water (ASW) thin films grown below 140 K using N₂ gas adsorption and temperature programmed desorption (TPD). Surprisingly, we find that ASW films grown with increasing angles of incidence from a directed vapor source show 20-100 fold increase in the uptake of N₂ gas at 26 K. The integrated N₂ TPD signals obtained from subsequent thermal desorption experiments reflect significant increases in the available surface area and porosity of ASW films with increasing incident angle. In comparison, ASW films grown by ambient H₂O backfilling of the experimental apparatus show integrated N₂ signals most comparable with ASW films grown at oblique angles of incidence. This observation has important implications for those studying ASW in laboratory settings, since the angle of incidence as an experimental control variable has not been widely appreciated in the preparation of ASW thin films. Further, we have investigated the affects of growth temperature, film thickness, and annealing on ASW morphology. The available ASW film surface area is observed to decrease with increasing growth temperature until 80 K where it becomes relatively constant thereafter. For a given incident angle, a roughly linear increase with increasing film thickness is observed for ASW films grown at 22 K. ASW films annealed above 120 K show a significant collapse of the micropore structure consistent with previously published accounts. These results have important implications for understanding the chemical and physical properties of ASW found in astrophysical media such as comets, planetary satellites, and interstellar grains. ¹Pacific Northwest National Laboratory is operated for the Department of Energy by Battelle under Contract DE-AC06-76RLO 1830.
9:20 AM		SS2-FrM-4 Origin of Non-Zero-Order H₂O Desorption Kinetics from Crystalline Ice Multilayers on Ru(001) F.E. Livingston, J.A. Smith, S.M. George (University of Colorado, Boulder) Recent studies have reported non-zero-order kinetics for H₂O desorption from crystalline ice multilayers on Ru(001). To understand the origin of the non-zero-order kinetics, D₂O desorption from ultrathin crystalline D₂O ice multilayers on Ru(001) was measured using a combination of laser-induced thermal desorption (LITD) spatial probing and isothermal desorption flux analysis. The ice multilayers were grown on Ru(001) using either backfill D₂O vapor deposition or multichannel capillary array dosing methods. The ice multilayers grown via backfill vapor deposition were smooth and highly uniform. The LITD and isothermal desorption flux studies demonstrated that D₂O desorption from these uniform ice multilayers exactly followed zero-order kinetics. Slight deviations from zero-order desorption kinetics were observed only at low D₂O coverages of ≤5 BL D₂O. In contrast, the ice films prepared using capillary array dosing were spatially non-uniform and exhibited a decreasing multilayer coverage versus distance from the center of the substrate. This initial non-uniform D₂O coverage distribution had a dramatic impact on the isothermal desorption flux measurements and produced non-zero-order desorption kinetics. The deviations from zero-order kinetics were directly related to changes in the ice film surface area as the non-uniform initial multilayer coverage is completely desorbed at various positions on the Ru(001) substrate at different times. The previous reports of non-zero-order kinetics on Ru(001) are assigned to a non-uniform initial D₂O multilayer coverage distribution.
9:40 AM		SS2-FrM-5 Low Energy Dynamics through the Verwey Transition : Water Adsorbed on Fe₃O₄(100)/MgO(100) C.J. Hirschmugl, M. Takasaki, M. Collins (University of Wisconsin, Milwaukee); C.H.F. Peden, S.A. Chambers (Pacific Northwest National Laboratory) The far- and mid-infrared broadband absorptions and discrete vibrations have been studied for water adsorbed on an epitaxial 2000 Å Fe₃O₄(100) film using infrared synchrotron radiation. Water on Fe₃O₄ represents an ideal example both practically and fundamentally, as Fe₃O₄ is a prominent subsurface mineral (magnetite), and the material exhibits an interesting electronic transition. Specifically, the substrate undergoes a metal-semiconductor (Verwey) transition at 120 K (ten-thousand-fold change in conductivity from approximately 10² [ohm-cm]^-1 to approximately .01 [ohm-cm]^-1), while water adsorbed on the surface is stable in vacuum until above 350 K. The frequency-dependent conductivity in normal-incidence reflectance measurements follows a Drude behavior above the transition, and is non-Drude below the transition. Hence, the infrared response of this system, both above and below the transition, provides a comparison of the water-substrate interaction in the metallic and semiconducting states. The present infrared studies employ synchrotron radiation, which can extend traditional IRAS measurements to below 400 cm^-1 with noise levels of approximately .01% attainable in 100 seconds measuring time. In addition, these measurements are complemented by TPD and concurrent resistivity measurements. Notably, three distinct cation adsorption sites are available on the reconstructed Fe₃O₄(100) surface: a tetrahedrally bonded Fe²⁺; a tetrahedrally bonded Fe³⁺; and an octahedrally bonded Fe³⁺. Molecularly adsorbed water is shown to sequentially fill these sites. In addition, adsorbed multilayers of water reveal large anti-absorption resonances in the infrared spectra for the molecular vibrations and the substrate phonons.
10:00 AM		SS2-FrM-6 Surface Restructuring of Magnesium Oxide at the Interface with Water J. Jupille (CNRS, France); P. Nael, D. Abriou (Laboratoire CNRS/Saint-Gobain, France); H. Arribart (Saint-Gobain Recherche, France) Interactions of water with oxide surfaces are among the most common chemical reactions since they can be observed at any time in the surrounding medium. Nevertheless, despite their apparent simplicity, they often lack microscopic description. A well known example is the adsorption of water on magnesium oxide. Cleaved MgO samples, which show mostly flat (100) terraces, undergo dramatic rearrangements upon exposures to surrounding atmosphere. Their surfaces are becoming very rough, so that flat (100) terraces are no longer visible. These changes in morphology are usually suspected to arise from the reaction with water molecules present in the ambient air. However, according to theory, water is not expected to dissociate on the dense (100) faces of MgO, but only on sites of lower coordination numbers. To demonstrate that water adsorption induces a restructuring of the MgO(100) surfaces, the behaviour of cleaved MgO surfaces which have been brought in contact with water have been examined by atomic force microscopy. It has been observed that pits are formed along <110> directions. Moreover, rearrangements along similar directions have been seen by exposing cleaved MgO surfaces either to the ambient air or to moist nitrogen. Consistently with calculations relative to hydroxylated MgO surfaces, the orientation of the pits suggests that the driving force for the restructuring is the change in surface energy upon water adsorption.
10:20 AM		SS2-FrM-7 Interaction of Water and Dimethyl Sulfoxide with Gold Surfaces A.A. Gewirth, N. Ikemiya, S.K. Si (University of Illinois, Urbana) We discuss recent results examining the initial stages of water and dimethyl sulfoxide adsorbtion on Au surfaces. We show that water first adsorbs on Au at low temperature as a planar, amorphous, monolayer-high film. The subsequent growth of water clusters occurs atop this film, but not on the bare metal surface. The growth and structure of dimethyl sulfoxide on Au surfaces is surface-structure dependent with more highly corrugated faces providing localized electron density yielding ordered arrays. Results obtained at room temperature from bulk liquid dimethyl sulfoxide are in close correspondance with low temperature UHV measurements.
10:40 AM		SS2-FrM-8 Coadsorption of Water and Hydrogen on Pt(110) P. Blowers, N. Chen, R. Masel (University of Illinois, Urbana) In previous papers, various investigators have discovered a new species when hydrogen and water coadsorb on various faces of platinum. However, the exact nature of the species is still in dispute. In this paper we combine high quality HREELS spectra with abinitio calculations to try to identify the species. The EELS spectrum of coadsorbed water an hydrogen shows peaks at 1180 and 3340 cm-1 which are not seen with water or hydrogen alone. The peaks shift with deuteration in a way expected for compound formation between water and hydrogen. Abinitio methods at the MP2/6-31G* level are used to identify the species. We have not found any (H2O)nH clusters, radical species or negative ions with vibrations near 1180 cm-1 However, cationic species e.g. [H3O]+, [H5O2]+, [H7O3]+ give vibrations in the right range. A detailed comparison of theory and experiment suggests that a number of hydrated ions form during the coadsorption process, with [H5O2]+, and [H7O3]+ being the most important.
11:00 AM		SS2-FrM-9 Equilibrium Water Structures at Well-Defined Organic Surfaces D.L. Allara, T.J. Boland (Pennsylvania State University) We report combined in-situ infrared vibrational spectroscopic and coverage measurements, from submonolayer to near condensation, of equilibrium water adsorption at well-defined organic surfaces synthesized by molecular self-assembly. The adsorption isotherm data were used to develop thermodyamic parameters of the adsorbed films while the IR data give information on the water structures. The IR spectra were interpreted quantitatively by using modeling based on electromagnetic theory and the known optical function spectra of various phases of water. The results reveal a range of water structures from ice-like to clathrate-like and liquid-like depending on the water chemical potential, the chemical functionality of the surface and on the presence of dilute inorganic salts dissolved in the near water films. In the case of hydrophobic methyl-terminated surfaces, the water adopts a liquid-like structure from submonolayer to near-condensation coverages in surprising contradiction to the traditional view of ordered water at hydrophobic surfaces. In contrast, for polar surfaces such as CO2H, the water adopts a strongly H-bonded structure up to the condensation point. The incorporation of dissolved salts at the interfaces serves to broaden the range of water structures observed.
11:20 AM		SS2-FrM-10 Effect of Boron on the Surface Chemistry of Single Crystal Ni₃(Al,Ti) B. Zhou, J. Wang, Y.W. Chung (Northwestern University) Previous work demonstrated that water dissociates into atomic hydrogen on Ni₃(Al,Ti)(100). There is clear evidence that this dissociation reaction results in the reduced ductility of many polycrystalline aluminum-based alloys in a moist environment. Ductility measurements further show that boron addition to these alloys increases ductility. Therefore, it is reasonable to expect that boron may affect the production of atomic hydrogen from water vapor dissociation and its surface mobility. To explore the effect of boron, we first dosed the surface of clean Ni₃(Al,Ti)(100) with controlled amounts of boron, using a specially designed low-energy boron ion source, followed by low-temperature exposure to D₂O and temperature-programmed desorption. The state of the surface was determined by x-ray photoemission. These studies show that addition of boron reduces the production of atomic hydrogen. The significance of this observation in explaining the beneficial effect of boron will be discussed.
11:40 AM		SS2-FrM-11 Interactions of 50-2500 eV Electrons with Ice C.D. Wilson, C.A. Dukes, R.A. Baragiola (University of Virginia) We investigate the interactions of 50-2500 eV electrons with vapor deposited ice by means of Electron Energy Loss Spectroscopy (EELS), Auger Electron Spectroscopy (AES), and Secondary Electron Emission (SEE). The EELS data is used to measure low lying excitations and to ascertain the production of radicals in the near surface region. This is done by observing the appearance of energy loss features in the band gap region as a function of irradiation fluence. We will discuss electrostatic charging and chemical alterations during irradiation and relate the results to astronomical problems.