J. Phys. Chem. 1995,99, 6267-6269
6267
Molecular Surface Structure of a Low-Temperature Ice Ih(0001) Crystal N. Materer, U. Starke? A. Barbieri, M. A. Van Hove,* and G. A. Somorjai Materials Sciences Division, Lawrence Berkeley Laboratory, University of 'California, and Department of Chemistry, University of California, Berkeley, California 94720
G.-J. Kroes Theoretical Chemistry, Department of Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands
C. Minot Laboratoire de Chimie Organique Thkorique, URA 506, BGt. F, 4 Place Jussieu, Universitk Pierre et Marie Curie, F-75230 Paris, France Received: December 8, 1994; In Final Form: March 6, 1995@'
An ice film with thickness greater than 10 A was crystallized on a clean Pt( 1 11) surface. Its external surface structure was investigated at 90 K by dynarnical low-energy electron diffraction (LEED), followed by molecular dynamics simulations and ab initio quantum chemical calculations. The results favor the common hexagonal ice Ih structure over other forms of ice, with (0001) termination. A full-bilayer termination is found, but with much enhanced amplitudes of motion of the 0 atoms in the outermost layer of H20 molecules, even at 90 K, so that these molecules were undetected experimentally by LEED.
The surface properties of ice, especially those affecting mechanical behavior such as friction,' have been subjects of investigations for over a century. The existence of a liquid surface layer on ice between 240 and 273 K has been proposed and then ~ b s e r v e d . ~It. ~has been suggested more recently that ice surfaces catalyze chemical reactions in polar stratospheric clouds (around 180-205 K), especially when acidified by nitric oxide hydrate^.^.^ One of the suspected surface reactions is the conversion of atmospheric hydrochloric acid and other chlorine species to chlorine species that, when photolyzed, catalytically destroy ozone. Since such surface processes depend fundamentally on the molecular scale structure, we address in this paper the structural arrangement of water molecules at the ice surface. Surprisingly, this is largely unknown on the atomic scale, although molecular dynamics simulations6 suggest configurations to be proven by experiments. We have performed that first experimental analysis of the molecular surface structure of crystalline ice, by applying the well-established surface crystallographic technique of lowenergy electron diffraction (LEED) at low temperatures (90 K). In addition, we have repeated at a temperature of 90 K the molecular dynamics simulations previously done for T = 190250 K,6 and we have performed new ab initio quantum chemical calculations to investigate the ice surface structure at 0 K. A more complete account than possible here will be presented el~ewhere.~ Many previous studies have investigated the structure and other physical properties of adsorbed water molecules on singlecrystal surfaces.* The most detailed structural study of water adsorption was conducted on Ru(0001) by LEED.9 The water molecules form a single ordered bilayer of ice in which the oxygen positions were determined. The two different types of
* To whom correspondence should be addressed (e-mail:
[email protected]). ' Current address: Lehrstuhl f i r Festkorperphysik, Universitat ErlangenNiimberg, Staudstr. 7, D-9 1058 Erlangen, Germany. Abstract published in Advance ACS Absrrucrs, April 1, 1995. @
Figure 1. Sixfold symmetric LEED pattern, of the ordered ice film at 50 eV, obtained using the digital LEED detector and picoamp beam current.
oxygen atoms forming the bilayer structure (outer and inner half of the bilayer) were found to be nearly coplanar. In this collapsed arrangement, the outermost molecules rest directly on the metal surface almost as close as do the deeper molecules. However, a single bilayer bonded to a metal surface cannot be taken to represent the surface structure of bulk ice. Rather than study the surface of bulk single crystals of ice, we have opted to grow ultrathin films of ice on a flat solid substrate. As detailed elsewhere,I0 the ice film was obtained by adsorbing water vapor on Pt(ll1) at 140 K, resulting in a 6-fold symmetric LEED pattern (cf. Figure 1) with broad spots that indicate disorder. The absence of Pt substrate spots implies an ordered film of thickness greater than the probe depth of a
0022-3654/95/2099-6267$09.00/00 1995 American Chemical Society
6268 J. Phys. Chem., Vol. 99, No. 17, 1995
Letters
Ice Ih(0001) full-bilayer termination
half-bilayer termination
layer 1
no layer 1
layer 2
layer 2 2.74
layer 3 layer 4
Figure 2. Perspective grazing view of the ice Ih (0001) surface, showing two ideal terminations on top: full-bilayer termination at left (layer 1 is present) and half-bilayer termination at right (layer 1 is absent). Molecules in layer 1 are found to have enhanced vibrational amplitudes, making them invisible in LEED. Middle-size atoms are oxygens, with some hydrogens included as small atoms with assumed bulklike positions and randomness to form H20 molecules (covalent bonds are drawn medium thick and hydrogen bonds medium thin). Large spheres represent complete H20 molecules, emphasizing their tetrahedral bulklike bonding arrangement.
LEED electron. In the energy range of our experiment, we estimate that this probe depth is at least 10 A. The spot positions correspond to a lattice constant parallel to the surface equal to that of bulk ice within a few percent. LEED intensity data were collected with a digital LEED detector after cooling the crystal to 90 K. Electron beam damage was made negligible by maintaining a picoamp beam current that deposits only about electrons per surface unit cell during a whole experiment. The LEED calculations needed to extract structural information from the diffraction data utilized the automated tensor LEED method.’ We varied the rms uniaxial vibrational amplitudes away from the bulk-ice value of 0.13 A,12,13 separately in the outermost layer and jointly in the deeper layers. We modeled the ice surface as a semi-infinite ice sample. The LEED pattern only fits the two simplest among the many different known bulk ice phases:12J4the unreconstructed (0001) surface of hexagonal ice Ih (the naturally-occurring stable form) and the unreconstructed (111) surface of cubic ice IC (a metastable phase which can be obtained by condensation at about 140 K*). Both phases consist of bilayers of tetrahedrally bonded H20 molecules connected by hydrogen bonds; the oxygen planes are spaced by about 2.76 8, between bilayers and 0.92 A within bilayers at 273 K. The bilayers have the ...ABABAB... stacking sequence in ice Ih (cf. Figure 2) and the ...ABCABC... stacking sequence in ice IC, so that the distinction between Ih and IC only appears after two bilayers, Le., deeper than 7 A under the surface. Each oxygen is surrounded by four other hydrogen-bonded oxygens about 2.76 A away at 273 K (2.74 A at 90 K). H is not centered between two 0 atoms in the hydrogen bond but stays off-center by about 0.38 A in a largely random fashion. For both ice Ih and ice IC,our LEED calculations varied the lateral lattice constant and several spacings between the oxygen layers. We investigated the two terminations shown in Figure 2: the “full-bilayer” termination (in which each outermost 0 atom in layer 1 is hydrogen-bonded to three neighbors, three 0 atoms in layer 2) and the “half-bilayer” termination (in which each outermost 0 atom in layer 2 is only hydrogen-bonded to one neighbor, an 0 atom in layer 3, layer 1 being now absent). Hydrogen inclusion gave no meaningful improvement in the LEED calculations.
LEED systematically favors the Ih form over the ICform, if only slightly since the two forms differ only beyond 7 A below the surface. The top two spacings between 0 layers were found to be bulklike within our error bar of 0.05 A. But two inequivalent model structures give an almost equally good fit: a full-bilayfr termination with much enhanced vibrations of at least 0.25 A in the outermost layer (at left in Figure 2) and a half-bilayer termination with bulklike vibrations (at right in Figure 2). These models are in effect equivalent from the LEED perspective, because the Debye- Waller factor makes the strongly vibrating outermost molecules of the full-bilayer (Le., layer 1 at left of Figure 2) invisible. Similarly, enhanced vibrational amplitudes of the outermost molecules of the halfbilayer model, again from the LEED perspective, are equivalent to the full-bilayer model due to the Debye-Waller factor. Varying the amplitude decay rate into the substrate clearly favors a rapid decay within about one bilayer. To distinguish between the full- and half-bilayer terminations, we turned to molecular dynamics and ab initio theories. The molecular dynamics simulations performed for the new temperature of 90 K were done in essentially the same way as before.6 We used 12 moving layers of H20 molecules (six bilayers) held in place by four rigid layers below. The results show stability of the Ih(000 1) surface with bilayer termination and bulklike interlayer spacings. The top bilayer has much higher translational and rotational order than found previously at T = 190 K. While the previous calculations found a much enhanced rotational mobility of the top layer molecules (NMR correlation times of the order of 100 ps), no such enhanced rotational mobility was found for 90 K. On the other hand, for the 0 atoms large amplitudes of motion perpendicular to the surface were found in the top layers, the ‘;fnsamplitudes being 0.24, 0.20, 0.19, 0.16, 0.16, and 0.15 A from top to sixth monolayer (Le., through three bilayers). These values are lower estimates,’because the finite time of the computer simulation may not allow H-bond-breaking events to occur that would increase disorder at the surface and increase the rms amplitudes, while zero-point motion is neglected. The simulations indicate a more gradual decay of amplitudes into the substrate than LEED. Our result should be contrasted with that for a single bilayer grown on Ru(OOO~).~ There, the full-bilayer is almost coplanar,
Letters such that the outermost molecules are close to the underlying metal, probably bonding directly to the metal. This would prevent a large vibrational amplitude of the water molecules perpendicular to the Ru surface, thus keeping them detectable in LEED. The observed stability of the full-bilayer termination compared to the half-bilayer termination is equal to about one H bond using bond-counting arguments. The effect of this is readily shown by our ab initio quantum chemical calculations, which are based on Hartree-Fock theory applied to 2D periodic slabs of ice. The calculated energy difference between the fulland the half-bilayer terminated slabs is about 14 kcdmol, which is approximately double the H-bond energy of about 7-8 k c d mol. If one remembers that the slab computations have two surfaces, this value is in agreement with our expectations from bond-counting arguments. It is also found that the arrangement of molecular orientations contributes noticeably to the total energy through dipole-dipole interactions, but not to the extent of changing our conclusions. Calculations of the potential profile for individually displaced molecular layers also imply enhanced surface vibrations. In conclusion, our results clearly imply that the ice (0001) surface at 90 K is terminated by strongly vibrating, or otherwise disordered, molecules in the top half of the full-bilayer termination (layer l at the left in Figure 2). These molecules are not detected by LEED but are shown by Hartree-Fock computations to by needed in order to energetically stabilize the structure by maximizing the number of hydrogen bonds. Molecular dynamics simulations also support our conclusions. Thus, the liquid film existing on the surface of ice down to about 240 K solidifies only up to a point: large vibrational amplitudes continue to exist down to at least 90 K. Given the bulk zeropoint uniaxial rms vibration amplitude of 0.09 compared with 0.13 8, at 90 K, these large vibrational amplitudes should continue to exist down to 0 K.
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Acknowledgment. This work was supported in part by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy, under Contract DE-AC03-76SF00098. U.S. gratefully acknowledges financial support by the Deutsche Forschungsgemeinschaft (DFG). G.J.K. acknowledges support by the Koninklijke Nederlandse Academie van Wetenschappen (KNAW). References and Notes (1) Dowben, F. P.; Tabor, D. The Friction and Lubrication of Solids; Clarendon Press: Oxford, 1964; Part II, Chapter VIE. (2) Elbaum, M.; Schick, M. Phys. Rev. Lett. 1991,66, 1713. (3) Elbaum, M.; Lipson, S. G.; Dash, J. G. J. Cryst. Growth 1993, 129, 491. (4) Solomon, S. Nature 1990, 347, 347. ( 5 ) Graham, J. D.; Roberts, J. T. J. Phys. Chem. 1994, 98, 5974. (6) Kroes, G.-J. Surf. Sci. 1992, 275, 365. (7) Materer, N.; Starke, U.; Barbieri, A.; Van Hove, M. A.; Somorjai, G. A.; Kroes, G.-J.; Minot, C. To be published. (8) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1989, 7, 211. (9) Held, G.; Menzel, D. In The Structure of Surfaces IK Xie, X. D., Tong, S. Y., Van Hove, M. A,, Eds.;Springer: Berlin, 1994; p 213. S u ~ . Sci., in press. (10) Starke, U.; Materer, N.; Barbieri, A,; Doll, R.; Heinz, K.; Van Hove, M. A.; Somojai, G. A. Surf. Sci. 1993,286087,432. (11) Van Hove, M. A.; Moritz, W.; Over, H.; Rous, P. J.; Wander, A.; Barbieri, A,; Materer, N.; Starke, U.; Somorjai, G. A. Surf. Sci. Rep. 1993, 19, 191. (12) Eisenberg, D. S.; Kauzmann, W. The Structure and Properties of Water; Oxford University Press: New York, 1969. (13) Rask, H. J.; Trevino, S. F.; Gault, J. D.; Logan, K. W. J . Chem. Phys. 1972, 56, 3217. (14) Whalley, E. In The Hydrogen Bond, Schuster, P., Zundel, G., Sandorfy, C., Eds.; North-Holland: Amsterdam, 1976; p 1425. JP943257Q
