10076
J. Phys. Chem. 1996, 100, 10076-10082
Adsorbate-Induced Partial Ordering of the Irregular Surface and Subsurface of Crystalline Ice L. Delzeit, M. S. Devlin, Brad Rowland, and J. P. Devlin* Department of Chemistry, Oklahoma State UniVersity, Stillwater, Oklahoma 74078
V. Buch* Department of Physical Chemistry and the Fritz Haber Research Center, The Hebrew UniVersity, Jerusalem 91904, Israel ReceiVed: February 19, 1996; In Final Form: April 8, 1996X
In our recent studies, evidence was presented that a relaxed ice nanocrystal surface is disordered, in contrast to the nanocrystal interior. In this study it is argued, based on spectroscopic evidence, that disorder extends to several surface layers. Difference spectra between large and small D2O nanocrystals are used to obtain the infrared OD stretch absorption of the disordered subsurface layers. Moreover it is shown that certain select adsorbates at the ice surface induce a significant ordering of the ice subsurface as indicated by the conversion of one-third or more of the subsurface spectrum to that of interior ice. For ice nanocrystals, the new “interior” ice can be greater than 10% of the total amount of ice. Since the ice subsurface apparently consists of approximately three bilayers, this suggests that the influence of these adsorbates on the surface bilayer is reflected in the “ordering” of one or more subsurface bilayers. Examples of adsorbates that have this influence are the bifunctional molecules acetylene and H2S, either of which can act effectively as both proton donor and proton acceptor, while more weakly bonded small molecule adsorbates, such as N2 and CO, do not noticeably influence the subsurface structure. It is suggested that, by engaging in significant H-bonding with the unsaturated surface groups of the top bilayer (dangling-H and dangling-O molecules), the bifunctional adsorbates reverse the restructuring of the outer layer of ice that occurs with an increase of the number of H-bonds of the surface water molecules. This, in turn, reduces the distortion of the ice surface and the displacement of molecules within the ice subsurface layers that accompanies the restructuring. The new data and interpretation give strong support to the view that the equilibrium ice surface has a high degree of structural disorder.
I. Introduction The nature of the surface of ice is of critical importance to several areas of science, including atmospheric and interstellar chemistry and meteorology. Despite many years of ice research, the knowledge of the molecular structure of the ice surface is much more limited than that of the ice interior. Recently, we developed an experimental technique to prepare stable deposits of ice nanocrystals of a very large surface area; the deposits proved to be useful in spectroscopic investigations of ice surface properties and of ice-adsorbate interactions.1-3 In parallel, simulations were carried out of ice surface structure and spectroscopy2-3 and of adsorbate bonding and spectroscopy.1-2 Comparison of experimental results to simulations strongly suggested that the ice nanocrystal surfaces are noncrystalline, in contrast to the nanocrystal interior. For example, an infrared absorption feature at 2580 cm-1 was assigned to the asymmetric stretch of 4-coordinated D2O molecules with a strongly distorted hydrogen bond coordination shell; our studies suggested abundant presence of such molecules in the surface layer. In another study,1 CF4 adsorbate spectroscopy was shown to be a uniquely sensitive probe of the extent of disorder of the underlying surface. The experimental spectral patterns could be reproduced by simulations of CF4 uptake and spectroscopy on disordered ice surface models, while spectral patterns calculated for crystalline or nearly crystalline structures were in qualitative disagreement with experiment. X
Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(96)00497-2 CCC: $12.00
Evidence suggesting surface disorder of ice can also be found in studies of other authors. Several years ago Kroes4 carried out molecular dynamics simulations of the (0001) surface of hexagonal ice at 190 K and showed that the surface relaxes with respect to the crystalline structure, while increasing the hydrogen bond coordination at the expense of the lateral order. (A similar result was obtained by us for the closely related (111) surface of cubic ice,3 see below.) A recent LEED study5 of the surface of an ice film (most likely (0001) of hexagonal ice) indicated disorder in the topmost ice layer; the authors proposed dynamical disorder due to large-amplitude molecular vibrations, but the possibility of static disorder was not ruled out. Torchet et al.6 and Huang and Bartell7 reported electron diffraction patterns of ice clusters containing thousands of molecules; the clusters were shown to be crystalline (cubic) but included a disordered component, possibly originating from disordered cluster surfaces. Finally, Torchet et al.6 found that smaller H2O clusters containing several hundred water molecules were amorphous, in contrast to, e.g., CO2 clusters that retained cubic crystalline bulk structure down to an average size of several tens of molecules. These results support the notion that the crystalline ice structure is faVorable for the ice interior of small particles but not for the ice surface. If the ice surface is in fact disordered, the question is why. The low-pressure crystalline forms of ice include the usual hexagonal ice (ice Ih) and cubic ice (ice Ic); the latter appears to form preferentially during fast freezing of small droplets7 and is the most likely structure of the interior part of ice © 1996 American Chemical Society
Partial Ordering of Crystalline Ice Surfaces
J. Phys. Chem., Vol. 100, No. 24, 1996 10077
Figure 2. Simulated vertical distribution of O atoms in models S1 and S3 at 83 K (in the simulations, the bottom two layers of a sixbilayer slab were frozen at crystalline positions).
Figure 1. Topmost bilayer (160 molecules) of a model crystalline cubic ice surface (111) and of the S1 and S3 disordered surface models at 83 K.1,11 The S3 model yielded the best agreement with experimental data described in ref 1.
nanocrystals considered in this study. The two crystalline forms are closely related.8 Both correspond to a tetrahedral network of hydrogen bonds, and both contain H2O layers composed of hexagonal rings; the two forms differ in the stacking arrangement of the layers. While the arrangement of the O atoms is periodic, both crystalline forms are proton-disordered, i.e. the orientations of the water molecules are random within the constraint of four hydrogen bonds per molecule.8 The almost perfect tetrahedral network of hydrogen bonds of ice Ih and Ic allows for fully saturated hydrogen bond coordination of each interior molecule in an optimal geometrical arrangement. However, formation of a surface is associated with appearance of numerous surface molecules with dangling atoms, i.e. with O and H (or D) atoms with missing hydrogen bond partners . Figure 1 shows a top bilayer of a (111) surface of cubic ice; fully half of the water molecules of the bilayer (every second horizontal row) are 3-coordinated, with a dangling O or H atom. As noted above, simulations3,4 suggest that presence of dangling atoms has a destabilizing effect on the crystalline surface structure and that relaxation occurs toward a nonperiodic surface structure with a larger number of H-bonds, at the expense of the surface periodicity.3,4 The equilibrium surface structure corresponds to a minimum in the free energy.9 Thus ice surface relaxation could be driven to disorder by the energy or the entropy contributions to the
free energy, or both. Formation of additional (albeit strained) H-bonds on the surface during the relaxation is expected to lower the energy. In addition, enhanced freedom of motion of surface molecules may allow them to probe a large number of disordered configurations, while increasing the entropy. It is hard however to obtain in a simulation a true equilibrated surface structure (or distribution of structures) characteristic of some temperature; this is because feasible simulation times are much shorter than the time scales available for structural relaxation in nature. To advance the understanding of the ice surface structure, we then used an ad hoc procedure in which a number of ice surface models of varying extent of disorder were constructed and used to calculate observables, and their validity was assessed by comparison to experiment. The models were generated by simulated heating of a crystalline slab of several ice bilayers to some temperature Tmax (in the range 200∼300 K), followed by recooling to the experimental temperature (in the range 25-83 K); see refs 1-3 for details. Two of these models, corresponding to Tmax ) 197 and 250 K are shown in Figure 1; the notation S1 and S3 was adopted from ref 1. The sequence of structures shown in Figure 1 from top to bottom is characterized by an increasing extent of disorder, reflected by an increasingly broader distribution of H2O ring sizes. The number of 4-coordinated surface molecules increases in this sequence, while the number of 3-coordinated molecules decreases.10 The potential energy of the S1 model is slightly lower than that of a parent crystalline surface,3 which shows that the energy lowering can in fact be obtained as a result of loss of crystalline order. However model S1 was found in ref 1 to be totally inadequate for reproducing experimental data on CF4 adsorbate uptake and spectroscopy on nanocrystal surfaces. Agreement was attained using more disordered surface models such as S3, whose energy is higher than that of S1.1,10 One should emphasize that this comparison was to experimental data for annealed and apparently thermally equilibrated nanocrystal surfaces.1,9 The above results suggest that both energy and entropy drive toward surface disorder and concurrent increase in the number of surface hydrogen bonds. As seen in Figure 2, the surface disorder in model S3 propagates about three bilayers into the interior. In the present study, the existence of this disordered subsurface is demonstrated experimentally. Moreover the effect of adsorbates on the ice surface structure is investigated. Briefly, it appears that bifunctional hydrogen bonding adsorbates such as H2S and acetylene (which can serve both as proton donor and proton acceptor) induce significant ordering in the ice subsurface. The reason could be a decrease in the freedom of motion of water molecules, and/or relief of surface strain as a
10078 J. Phys. Chem., Vol. 100, No. 24, 1996 result of absorbate molecules taking over the strained hydrogen bonds formed by the surface water molecules. To establish that disorder/strain energy of the ice surface is reduced by particular adsorbate molecules, a reliable probe of the surface/subsurface order at the molecular level is required. A useful surface probe, previously applied to crystalline nanoparticles including ice, has already been noted; the pattern assumed by an infrared T-L doublet band system of monolayer and submonolayer amounts of adsorbed CF4 as compared to the computed patterns of the spectra of CF4 adsorbate molecules on model ice surfaces and for model free-standing CF4 layers. These patterns are remarkably sensitive to both the relative smoothness and regularity of the CF4 layer, so are a source of information on the outer bilayer of the ice surface. However, no specific experimental probe of the ice subsurface structure has been available. Here, we will describe a procedure based on the experimental spectra that is informative of the vibrational modes and structure of the subsurface bilayers of cubic ice. Subsequently, evidence will be developed which indicates that monolayers of either of the adsorbates, acetylene or H2S, induce a significant restructuring of the subsurface layers toward that of the interior ice. Finally, evidence will be noted that indicates that the ordering of the ice subsurface is reversible, with the full irregularity of the structures of the ice surface and subsurface restored when the adsorbate is removed. This reversibility supports the view that the equilibrium form of the bare ice surface at cryogenic temperatures is quite irregular. II. Experimental Section The Fourier-transform infrared (FT-IR) spectra that are reported and analyzed were obtained for networks of D2O ice nanocrystals assembled on the ZnS end windows of a static cluster cell and for near monolayer quantities of the molecules acetylene, H2S, and CF4 adsorbed on the surfaces of the ice nanocrystals. The details of the preparation of a network of nanocrystals of cubic ice have been presented elsewhere.3 The ice particles average ∼20 nm in diameter and can be formed in deposits of a thickness optimized for the particular experimental objective. For particles formed at 70 K and used in the 80140 K temperature range, a few percent of the water molecules are at the surface so high-quality spectra are obtained of the ice surface-molecule modes as well as of modes of adsorbate molecules. By annealing at temperatures above 120 K, larger nanocrystals, with less surface area, can be prepared via vapor transfer from smaller nanocrystals (Ostwald ripening), without a significant change in the amount of ice within the network of particles of a deposit. Because of a need to distinguish absorption features of surface and subsurface molecules from each other and from the bands of the interior ice vibrational modes, extensive use has been made of spectral subtraction. In particular, the identification of the modes of the water molecules at the ice surface has been facilitated by subtraction of spectra of the larger ice nanocrystals from those of smaller ice particles that necessarily have a greater fraction of surface molecules;3 a procedure that permits the minimization of the interior D2O-ice mode absorption and clearly reveals the modes of the surface D2O molecules in the range 2500-2750 cm-1. A related procedure, that reveals the general appearance of the spectrum of the subsurface water molecules and, therefore, allows recognition of the influence of adsorbates on the subsurface layers, will be described in detail below. The adsorbates have been studied under equilibrium conditions that favor near monolayer coating of the particles. Temperatures for the adsorption experiments have been chosen
Delzeit et al. so that the adsorbate vapor pressure is minimal (
