Crystallization Kinetics of Thin Amorphous Water Films on Surfaces

mobility in the film as demonstrated experimentally by following O2 release during crystallization ..... whole surface; III, thin film limit incomplet...
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Langmuir 2003, 19, 265-274

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Crystallization Kinetics of Thin Amorphous Water Films on Surfaces Patrik Lo¨fgren,†,‡ Peter Ahlstro¨m,†,§ Jukka Lausma,†,| Bengt Kasemo,† and Dinko Chakarov*,† Department of Applied Physics, Chalmers University of Technology and Go¨ teborg University, SE-41296 Go¨ teborg, Sweden, Manne Siegbahn Laboratory, Frescativ. 24, SE-104 05 Stockholm, Sweden, School of Engineering, University College of Borås, SE-501 70 Borås, Sweden, and SP Swedish National Testing and Research Institute, Box 857, SE-501 15 Borås, Sweden Received March 5, 2002. In Final Form: October 2, 2002 The paper presents experimental investigation of the crystallization kinetics of thin (from several and up to 100 monolayers, ML) amorphous ice films grown in a vacuum on clean and adsorbate modified Pt(111) and graphite (0001) surfaces. The crystallization kinetics was followed by the associated decrease of the desorption rate by isothermal desorption mass spectroscopy. The process is strongly substrate dependent for thin films (below 20-30 ML), gradually becoming substrate independent in 30-100 ML range, and approaching the bulk ice characteristics for thicker films. For thin films, the enthalpy of vaporization is 52 kJ/mol for the amorphous and 54 kJ/mol for the crystalline films. The activation energy for crystallization was estimated to be 75 kJ/mol. The crystallization is accompanied by an enhanced mobility in the film as demonstrated experimentally by following O2 release during crystallization of ice films on O2 precovered Pt(111).

1. Introduction Few substances are so extensively studied as water, reflecting its prominent importance. Both liquid and solid water (ice) possess many intriguing properties. It has often been stated that life depends on the anomalous properties of water. A recent example where ice has received much attention is in atmospheric chemistry and in particular its role in catalyzing chemical reactions in arctic stratospheric clouds. The latter reaction, which is one of the steps in the network of chemical reactions responsible for the depletion of the ozone layer, is suggested to be sensitive to the molecular structure of the ice surface.1,2 An area where thin ice/water films are of both scientific and practical interest central is in biologyshydration shellsssurrounding proteins, other biomolecules, biomembranes, and so forth.3-5 Although water/ice has been studied extensively, many properties and processes are still far from well understood. A case in point is the properties of vapor-deposited water films at temperatures below ∼200 K where many of the earlier results scatter substantially regarding, for example, the reported structural phases, phase-transition temperatures, and sublimation rates.6,7 The large spread in such data can probably partly be explained by the fact that the properties of vapor-deposited ice are very sensitive to the preparation conditions including the type of surface * Corresponding author. E-mail: [email protected]. † Chalmers University of Technology and Go ¨ teborg University. ‡ Manne Siegbahn Laboratory. § University College of Borås. | SP Swedish National Testing and Research Institute. (1) Schaff, J. E.; Roberts, J. T. J. Phys. Chem. 1994, 98, 6900. (2) Schaff, J. E.; Roberts, J. T. J. Phys. Chem. 1996, 100, 14151. (3) Kasemo, B. Curr. Opin. Solid State Mater. Sci. 1998, 3, 451. (4) Kasemo, B.; Gold, J. Adv. Dental. Res. 1999, 13, 8. (5) Ratner, B. D. Mol. Recognit. 1996, 9, 617. (6) Hobbs, P. V. Ice Physics; Clarendon: Oxford, 1974. Petrenko, V.; Whitworth, R. Physics of Ice; Oxford University Press, 1999. (7) Sack, N. J.; Baragiola, R. A. Phys. Rev. B: Condens. Matter 1993, 48, 9973.

used for condensation, nucleation and growth conditions, gas-phase purity, and so forth and also to the conditions and methods by which various properties are studied. The general picture from the earlier reported results, dominated by X-ray and electron diffraction techniques6 and different IR spectroscopic methods8 together with theoretical studies such as molecular dynamics simulations,9 is that ice grown at substrate temperatures below ∼130 K forms a more or less amorphous or vitreous solid or a frozen liquid.6,10-12 Frequently, a glass transition to a liquid at about 120-140 K is reported; the measured glass-transition temperatures vary over a wide range.13 It is disputed whether this liquid is supercooled “normal liquid water” or another unique liquid state.14-16 In the following, we will for simplicity denote these noncrystalline films “amorphous films” or water films. Upon heating, the amorphous water structurally transforms, irreversibly, to a metastable crystalline form, cubic ice (Ic), a process accompanied by a decrease in the vapor pressure. The reported transition temperature for this transformation ranges from 120 to 160 K and the reported vapor pressure change, due to the crystallization, differs in some cases by almost 2 orders of magnitude.7 Increasing the temperature even further (∼200 K) leads to a second-phase transition in which the cubic ice is transformed to the stable hexagonal ice (Ih), the so far only found naturally occurring stable form of ice.6 (8) Engquist, I. In Microscopic Wetting; Linko¨ping University: Linko¨ping, 1996; p 166. (9) Essmann, U.; Geiger, A. J. Chem. Phys. 1995, 103, 4678. (10) Jenniskens, P.; Blake, D. F. Science 1994, 265, 753. (11) Johari, G. P.; Hallbrucker, A.; Mayer, E. J. Chem. Phys. 1990, 92, 6742. (12) Johari, G. P.; Hallbrucker, A.; Mayer, E. J. Chem. Phys. 1991, 95, 2955. (13) Jenniskens, P.; Banham, S. F.; Blake, D. F.; et al., J. Chem. Phys. 1997, 107, 1232. (14) Sciortini, F.; Poole, P. H.; Essman, U.; et al. Phys. Rev. E 1997, 55, 727. (15) Stanley, H. E.; Buldyrev, S. V.; Canpolat, M.; et al., Physica D 1999, 133, 453. (16) Bellisent-Funel, M. C. Europhys. Lett. 1998, 42, 161.

10.1021/la020218u CCC: $25.00 © 2003 American Chemical Society Published on Web 12/12/2002

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Besides an unsatisfactory spread in the reported data, there are also several aspects of the vapor-deposited water films that have not been investigated in detail at all. There were, for example, until recently few studies of the kinetics and energetic of the phase transition from the amorphous frozen liquid state to the crystalline state, and the role of the substrate for the properties of the films were lacking, until two parallel independent reports by Smith et al.17 and our group.18 A majority of the earlier studies focused on either very low coverages (ca. 1 monolayer), where the substrate properties are known to strongly influence the adsorbed water,19 or very high coverages (∼µm), with little or no substrate control, since the substrates were assumed to have no or very little influence on the vapor-deposited water films.6 The topic of the present study bridges the strongly substrate-dependent monolayer regime and the true bulk regime. At mesoscopic film thickness, the substrate can still have an effect on the vapor-deposited water films, but the properties start to be successively more bulklike as the thickness increases. Several interesting questions can be posed about the mesoscopic regime: At what thickness are pure bulk properties established? How does the latter thickness depend on the substrate and on deposition conditions? To what extent are thermodynamically stable phases and morphologies reached during growth or postannealing, and to what extent are the resulting structures kinetically stabilized? What is the kinetics and energies of phase transformations from unstable to stable phases? Where in space do the new phases nucleate and grow upon annealingsat the interface, in the bulk, or at the surface of the ice film? Another interesting question is how the resulting ice film morphologies correlate with the macroscopic wetting-nonwetting properties, directly related to the concepts of hydrophilicity-hydrophobicity. The first detailed study of the desorption and phasetransition kinetics in mesoscopic film thickness regime were the above-mentioned studies by us17 and Smith et al.18 Recently, Livingstone et al. 20 have performed similar studies for ice films on Ru(001). They explained the deviations from zero-order desorption kinetics of H2O from crystalline ice multilayers on Ru(001) by changes in the ice film surface area. A different set of studies addresses the structure of ice. Even usually powerful techniques to reveal the atomic structure of the surface layers such as scanning tunneling microscope (STM) and low-energy electron diffraction (LEED) are hampered in studies of ice because of the high mobility of water molecules. Nevertheless, using STM it was shown21 that at 140 K H2O molecules form an ordered bilayer on the Pt(111) surface with a structure resembling a (0001) bilayer of hexagonal ice (Ih). Vapor-deposited ice on same surface at 125-140 K has been studied by LEED 22 and by helium atom diffraction.23 These experiments show that the (0001) surface has a “full-bilayer termination”. The experiments also concluded that there is greatly enhanced vibrational motion of the molecules on the ice/ vacuum interface, which is consistent with the LEED (17) Smith, R. S.; Huang, C.; Wong, E. K. L.; et al., Surf. Sci. Lett. 1996, 367, L13. (18) Lo¨fgren, P.; Ahlstro¨m, P.; Chakarov, D.; et al., Surf. Sci. Lett. 1996, 367, L19. (19) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211. (20) Livingston, F. E.; Smith, J. A.; George, S. M. Surf. Sci. 1999, 423, 145. (21) Morgenstern, M.; Ilr, J. M.; Michely, T.; et al., Z. Phys. Chem. 1997, 198, 43. (22) Materer, N.; Starke, U.; Barbieri, A.; et al., Surf. Sci. 1997, 381, 190. (23) Braun, J.; Glebov, A.; Graham, A. P.; et al., Phys. Rev. Lett. 1998, 80, 2638.

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observations. The formation of similar crystalline multilayers on Ru(001) has been described by Brown and George.24 Using optical sum-frequency vibrational spectroscopy, Su et al.25 have shown that ice multilayers deposited on Pt(111) are at least partially ordered ferroelectrically perpendicular to the surface. Fletcher26 has speculated about the possibility of ordered arrangements of the dangling bonds. From the works by Devlin and Bush27 it appears, however, that a free ice surface with dangling bonds is unstable and that molecules are displaced in ways, which partially eliminate the dangling bonds, but this reconstruction is disordered. We have in the present work studied water films with coverages ranging from approximately two to hundreds of monolayers on different substrates. To obtain reproducible conditions, the water films are grown on singlecrystal substrates under UHV conditions and with controlled doses. Preliminary findings about mesoscopic water films using this approach were reported in a Letter.18 Here, we focus on the kinetics and energetics of the structural phase transition from amorphous to cubic ice and how it is influenced by the substrate. In contrast to earlier findings, we observe that the desorption kinetics is influenced by the substrate for water films as thick as 100 ML (monolayers) and even beyond that for very hydrophobic surfaces. 2. Experimental Section The experiments were performed in two different UHV systems described elsewhere.28,29 The Pt(111) experiments were carried out in a home-built, stainless steel UHV chamber, turbo pumped down to a base pressure in the mid 10-10 mbar regime. In brief, this system is equipped with a quadrupole mass spectrometer (MS), a directed water vapor inlet (thermal “beam” at normal incidence), a gas inlet (O2, CO, octane), and evaporation sources for amino acids and alkali metals. All sources are reached for direct sight exposure by rotating the sample manipulator. An important feature of the system is that both the sample manipulator and the ion source of the MS are surrounded by a liquid nitrogen cooled dewar. Since this dewar acts as an efficient pump for condensable gases, for example, water, the background water pressure is very low (∼1 × 10-11 mbar) and it is possible to expose the sample to high water doses without any measurable pressure increase in the chamber. This arrangement is particularly important during ITDS; it eliminates the background H2O vapor increase during desorption and therefore very low desorption rates are measurable. (The MS detection of H2O desorbing from the sample is only by direct of sight, see below.) The sample positions are adjusted with a liquid nitrogen cooled sample manipulator, which is positioned in the center axis of the dewar. The Pt(111) crystal is mounted on a thin Ta foil, which is resistively heated, and the temperature is monitored by a chromel-alumel thermocouple attached to the edge of the sample. The sample temperature is controlled with PID-regulator. The sample was cleaned by cyclic heating in oxygen and in a vacuum. The experiments with the graphite substrate were performed in a separate UHV (base pressure e 3 × 10-10 mbar) chamber described in detail in ref 29. The analytical instruments in this system, employed in the present study, were quadrupole mass spectrometers (Balzers QMC311 and QMC511) and a highresolution electron energy loss spectrometer (Leybold ELS22). The sample was a 12 × 6 × 1 mm piece of highly oriented pyrolytic graphite (HOPG) grade XYA (Union Carbide Co), exposing the basal (0001) plane. It could be resistively heated to 1300 K and (24) Brown, D. E.; George, S. M. J. Phys. Chem. 1996, 100, 15460. (25) Su, X.; Leanos, L.; Shen, I. R.; et al., Phys. Rev. Lett. 1998, 80, 1533. (26) Fletcher, N. H. Philos. Mag. B 1992, 66, 109. (27) Devlin, J. P.; Buch, V. J. Phys. Chem. B 1997, 101, 6095. (28) Lo¨fgren, P.; Krozer, A.; Lausmaa, J.; et al. Surf. Sci. 1997, 370, 277. (29) Chakarov, D. V.; O ¨ sterlund, L.; Kasemo, B. J. Chem. Phys. 1997, 106, 982.

Crystallization Kinetics of Amorphous Water Films subsequently cooled to 85 K (within ∼15 min) by circulating liquid nitrogen. For details on the graphite cleaning and submonolayer H2O on graphite, see ref 30. Pure water was obtained by repeated pumping and heating cycles on a container with deionized and degassed water. This was repeated until the major impurity content was less than 0.1% as measured by the MS. During deposition, the sample is positioned in front of the water doser, terminating ∼10 mm from the sample surface. The doser has an aperture area that ensures a homogeneous flow over most of the sample surface (see later discussion of this point). Typical deposition rates were 0.1 ML s-1. After deposition, the sample is moved to the MS position where it has direct sight to ionization source in the MS. The detection geometry in both systems is such that it discriminates contributions from the sample holder and background gases. After dosing, and when the sample has been positioned in front of the MS ionization source, the isothermal desorption experiments are performed by increasing the temperature linearly from the deposition temperature up to the desorption temperature where it is kept constant, normally until all water molecules have desorbed. The coverage and desorption rates were calibrated by using background exposures at a given water pressure and assuming sticking one. In addition, the water coverages on Pt(111) were calibrated by comparing the area under the saturated monolayer peak in a TDS experiment with earlier reported TDS studies on Pt(111).31,32 When coverages are given in units of water monolayer equivalents (ML), a uniform distribution is assumed (layer by layer growth). Because of a certain roughness of the water films, which is substrate dependent (see below), this number only gives the relative amount of deposited water, or average thickness, and should not be taken as an absolute film thickness over the whole surface.

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Figure 1. (a) Isothermal desorption rate vs time for an 80 ML ice film deposited on Pt(111) at 105 K. The inset displays, for comparison, a similar experiment but with a film of 10 ML of glycine. The latter is an example of the simple zero-order desorption kinetics, expected for a thick molecular solid. (b) The same data as in a plotted as desorption rate vs remaining amount of ice on the surface. The three regimes discussed in the text are I, the amorphous ice; II, crystalline ice covering the whole surface; III, thin film limit incomplete surface coverage.

3. Experimental Results Figure 1a displays an isothermal desorption spectrum for an ice film of 80 monolayers deposited at 100 K on Pt(111). For comparison, a film of glycine, deposited at the same temperature and 10 monolayers thick, is shown in the inset. (Glycine was chosen for this comparison because it exhibits the “perfect” zero-order desorption kinetics expected for simple molecular solid.) The noticeable feature in Figure 1a, reported earlier,17,18 is that zeroorder desorption kinetics is not observed; instead, the desorption rate initially declines slowly, and then the decline rate accelerates and eventually levels out at a constant value after ca. 1300 s, which then stays constant for about 3500 s. The desorption rate finally starts to decline again, until it eventually ceases as the ice film has completely sublimed. We call these three kinetic regimes region I, II, and III, respectively, as indicated in the figure. The explanation for this behavior was first given in the two parallel independent studies.17,18 In region I, two thermally driven, parallel processes occur in the originally amorphous ice film, formed during film condensation: (i) desorption from the film surface and (ii) crystallization of the film. Since these two processes have comparable activation energies (see below), they occur simultaneously. The initial high desorption rate is due to the higher vapor pressure of the amorphous phase, compared to the crystallized film. As time elapses, the crystallization process causes crystalline patches to appear on the ice surface. These patches of lower vapor pressure (higher average number of H-bonds) grow monotonically until they cover the entire ice surface (provided the film is still continuous over the whole surface until the crystallization is complete); at this point, the desorption rate levels out (30) Chakarov, D.; O ¨ sterlund, L.; Kasemo, B. Vacuum 1995, 46, 1109. (31) Fisher, G. B.; Gland, J. L. Surf. Sci. 1980, 94, 446. (32) Jo, S. K.; Kiss, J.; Polanco, J. A.; et al., Surf. Sci. 1991, 253, 233.

Figure 2. Thermal desorption spectra (heating rate 2 K/s) for two different coverages, ∼1 and 50 ML, of water on Pt(111). The three arrows indicate (a) the phase transition from amorphous to crystalline ice and (b) second and (c) first layer in the bilayer structure formed on Pt(111).

to a constant value (region II), where zero-order kinetics (constant desorption rate) is observed. Region II is observed only above a certain film thickness and is lost when the film is so thin that it after completion of the crystallization process no longer covers the whole surface (region III); “bare” patches are then appearing on the surface and the desorption rate declines because the total exposed ice area diminishes. On Pt(111) the “bare” patches are actually a 2D layer (bilayer) of strongly bound water to Pt19,31,32 that does not desorb at 146 K but only by raising the temperature to g170 K, as observed both by ITDS (not shown) and TDS. In the TDS traces in Figure 2, for different amounts of deposited ice the amorphous peak (region I) appears as a shoulder on the low-

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temperature side of the main multilayer peak (region II, III). The bilayer, which is not observed in the ITDS trace in Figure 1 because of too low temperature, is in the TDS trace appearing at 170 K. (Note the different origin of the low-temperature TPD peaks in platinum and graphite.) The ITDS trace for glycine shown in the inset of Figure 1a demonstrates that the complex kinetics shown is specific for H2O ice. With glycine, simple zero-order kinetics is observed from the beginning already for 10 monolayers (region II), until bare patches appear on the surface (region III). (For a detailed TDS study of glycine on Pt(111) see ref 28.) In Figure 1b, the same data as in Figure 1a are plotted as desorption rate versus remaining amount of ice on the surface (this eliminates time as an explicit parameter and has some advantages for some of the data analysis). This plot shows that the amounts of water desorbing in regions I, II, and III are, respectively, 18 ML (22%), 34 ML (43%), and 28 ML (35%) for this particular film thickness. For thicker films, it is essentially the amount in region II that grows (see below). For sufficiently thin films, region II disappears and there is a direct transition from region I to region III, as explained above. In the following, we will analyze the type of behavior seen in Figure 1a, b more closely with respect to the following aspects: How does the kinetics depend on the underlying substrate (3.1), on deposited film thickness (3.2), on deposition (3.3), and desorption temperature (3.4). The latter includes analysis of the energetic and activation energy for crystallization. We will also investigate some special effects, such as trapping and eventual release of O2 molecules at the ice-Pt interface (3.5). 3.1. Influence of the Substrate on the Desorption Kinetics. Figure 3 shows data similar to the ones in Figure 1a but for many different substrates. All films were deposited at approximately the same temperature (100 K) and deposition rate (∼ 0.1 ML s-1). It is obvious that the desorption kinetics, even qualitatively, is strongly dependent on the substrate on which the ice film has been grown. 3.1.1. The “Amorphous Peak” (Region I). For all substrates, an initial peak (region I; the amorphous ice peak) is observed. This shows that amorphous growth of an ice film is a general phenomenon on all (or almost all) substrates at j 100 K. (Stevenson et al.33 have shown that even lower temperatures make the film increasingly more porous, while at ∼100 K the film is nonporous.) The shape of the amorphous peak on Pt is distinctly different from that on, for example, graphite and octanecovered Pt. In the latter two cases, the desorption rate declines much faster, that is, the crystallization rate appears to be faster, which we connect with the different initial condensation conditions on the different substrates and maybe also with different interfacial crystallization conditions as discussed further below. 3.1.2. Region II and Its Relation to the Wetting Properties of the Substrate. For the film thickness shown in Figure 3, only Pt(111) exhibits region II, representing desorption with a constant desorption rate from a crystallized film, covering the whole surface. In other words, for the other two substrates, region I is directly followed by region III, where the fraction of bare surface is successively increasing, causing a declining desorption rate. This is a consequence of the different wetting properties of the different substrates. Pt(111) is strongly hydrophilic and forms a strongly bound bilayer closest to the surface,19,32 which offers additional hydrogen bonding for the third ice layer, and so forth. Therefore, we expect a continuous ice film to form at very low film thickness on Pt(111). This film obviously grows amorphous

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Figure 3. Isothermal desorption of water deposited on different substrates: (a) clean Pt(111), (b) graphite(0001), and (c) Pt(111) with a multilayer of octane. The initial water coverages and the sublimation temperatures are indicated in each plot. The top panel (a) defines τ, which is later used to plot Figure 6. Quantitatively, we define τ as the time from the onset of the ITDS trace to point determined by the intersection of two lines forming the tangents of the desorption trace as it approaches the transition between regions I and II (or III) from the left and the right, respectively.

at 100 K despite the relatively good lattice match between Pt(111) and hexagonal ice (the Pt lattice constant is 6% larger than that for ice19); by annealing thin ice layers on Pt(111), it is possible to make crystalline layers (see Starke et al.34 and below). The important point in the present context is that the ice films wet platinum, and therefore a continuous film is established for thin films. Region II is then observed, provided the film is thick enough to allow the crystallization to be completed before the film is depleted of water molecules. The situation is quite different with graphite (Figure 3b), which is fairly hydrophobic,30 and even more so for ice grown on top of a very hydrophobic condensed film of (33) Stevenson, K. P.; Kimmel, G. A.; Donha´lek, Z.; et al., Science 1999, 283, 1505. (34) Starke, U.; Materer, N.; Barbieri, A.; et al., Surf. Sci. 1993, 287/288, 432.

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Figure 4. Isothermal desorption experiments, like the ones in Figure 1, for a Pt(111) substrate with two different multilayer coverages of octane. The ice coverages in this case are much higher compared to Figures 2 and 4.

Figure 5. Isothermal desorption rates vs time for different initial water coverages on Pt(111). The initial coverages in ML are 10, 19, 29, 44, and 61 and the sublimation temperature is 148 K.

octane on Pt(111) (Figure 3c). In the two latter cases, water does not wet the surface and tends to form 3D droplets, which grow successively in size with increasing deposition. Direct evidence of this large difference in wetting/H2O bonding is observed for octane precovered surface, where the initial sticking coefficient for H2O is ,1 at 100 K.35 Octane and graphite thus require much larger deposits than Pt before the film coalesces to a uniform ice film. Specifically, we find that 115 ML (average thickness) of ice on the octane film is still far from enough to establish the uniform film regime (II) after crystallization (Figure 3c). However, if a sufficiently thick film is grown on octane, region II is eventually reached as shown in Figure 4 for a 365 ML ice film. However, even for this relatively thick film the amorphous peak shape is distinctly different from that on Pt(111). Obviously, the different condensation and growth kinetics on the different surfaces cause a “memory effect” well beyond 300 ML average thickness on a strongly hydrophobic surface (see section 3.2 below). Qualitatively, the condensation kinetics on Pt(111), graphite and octane, can be described as follows. On Pt(111) low or zero contact angle droplets coalesce already for very thin layers, creating a film with relatively low roughness, while on graphite and octane high contact angle droplets are formed, which coalesce only at much larger average thickness and with much larger roughness. For the latter reason, even relatively thick films lack region II on hydrophobic surfaces, and when region II is eventually established, the desorption kinetics in region I during crystallization is different compared to Pt, because there are still (thin) regions in the film which crystallize faster than thick regions. An additional difference between hydrophobic and hydrophilic surfaces is that the nucleation density is lower in the former case at the present temperature because of weak H2O monomer-surface bonding.35 On the other hand, a crystalline-like boundary layer is often found in liquid water at hydrophobic surfaces. The layer can form a template for growing ice crystals. 3.1.3. Influence of Prechemisorbed Overlayers on Pt(111) on Ice Film Desorption Kinetics. To explore if different chemisorption layers influence the measured desorption kinetics and the wetting properties, ice films were grown on Pt(111) with saturated chemisorption layers of O atoms (∼1/4 monolayer), O2 molecules (∼1/2 monolayer),36 and CO molecules (slightly more than ∼1/2 monolayer37). H2O adsorption does not displace these

monolayers, as controlled by TDS after evaporation of the ice film. Only for the CO monolayer is there a small but significant deviation from the clean Pt(111) results; for equal ice film thickness (80 ML) the ice film grown on CO/Pt(111) showed a measurable slope in region II, signaling that the wetting on CO/Pt(111) is slightly lower than on clean Pt(111). A possible cause for this is that CO prevents the bilayer of H2O from forming on Pt and that there is therefore a (likely) weaker H2O bonding to the O atoms in the CO molecules, compared to the direct H2O bonding to Pt. Chemisorbed O and O2 obviously does not affect the wetability significantly. For O atoms, this is not surprising; they should offer strong H bonding for adsorbed H2O molecules. We have also observed that Cs18 and Na38 deposited on graphite converts this surface to a hydrophilic one, on which water wets nearly as well as on clean Pt(111). 3.1.4. A New Method for Wetting Measurements. On the basis of the discussion above, we suggested in ref 18 that desorption traces of the kind shown in Figures 2, 4, and 5 offer a simple way of measuring the (relative) wetting properties under UHV conditions. Since weaker wetting requires thicker films to establish region II, as discussed above, one can simply measure the minimum amount of water that must be deposited on a surface to establish region II. The larger this value is, the weaker is the wetting. The amount of deposited water is easily obtained as the area under the desorption trace. Alternatively, one can deposit the same amount of water on different surfaces, and take the slope in region II, just after the crystallization is completed, as a measure of the wetting. The latter is less accurate, however, because of the different shapes of the amorphous peak on different surfaces. Using the data collected so far and applying the above method to rank the wetting properties, we have found the following ranking (most wetting surfaces first): Pt(111), O-Pt(111), O2-Pt(111) > CO/Pt > Cs/graphite > Na/graphite> graphite > octane/Pt. 3.2. Dependence of the Desorption Kinetics on Ice Film Thickness. 3.2.1 Regions I and II. The dependence of the desorption/crystallization kinetics on ice film thickness was studied in detail for Pt(111) and graphite (0001). Figure 5 shows similar desorption traces as in Figure 1a but for films with different thickness on platinum. There are two prominent features to note in

(35) Lindroth, T.; Zhdanov, V. P.; Kasemo, B. to be published, 2002. (36) Gland, J. L. Surf. Sci. 1980, 93, 487.

(37) Steininger, H.; Lehwald, S.; Ibach, H. Surf. Sci. 1982, 123, 264. (38) Gleeson, M.; Kasemo, B.; Chakarov, D. submitted to Surf. Sci.

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Figure 6. A plot of τ, the time required to complete the phase transition vs water film thickness. (+) and (b) denotes results from clean Pt(111) and clean graphite surfaces, respectively. τ and θ are for each substrate normalized to the substratedependent parameters τ(∞) and θ*. (τ(∞)Pt ) 1350 s and τ(∞)Gr ) 300 s); (θ*Pt ) 24 ML and θ*Gr ) 12 ML).

these data. The most important is that the amorphous peak has a width that increases with increasing thickness for the thinner films. To quantify this dependence, we have measured τ, defined as the time required completing the phase transition (see Figure 3a) for different initial coverages. Figure 6 shows plots of τ versus the initial coverage θ for desorption at 145 K for ice deposited on clean Pt and graphite. The figure shows that the time to complete the phase transition attains a constant value for sufficiently thick films but monotonically decreases as the films become thinner.18,17 The other notable feature in Figure 5 is that region II disappears for the thinnest films, as discussed and explained in the previous paragraph. 3.2.2. Region III. This regime is the least interesting in the present context. It represents the desorption kinetics of a heterogeneous film only partly covering the substrate. It is therefore more complex to analyze. The desorption rate declines monotonically because a successively smaller fraction of the film covers the surface as evaporation proceeds. The detailed kinetics in this region depends on the film roughness, which in turn increases from hydrophilic to hydrophobic surfaces as discussed earlier. In the spectra shown here, there may also be some effects (in region III) of nonuniform film thickness after deposition, as explored in detail by Livingston et al.20 In later experiments, we have taken precautions to obtain uniform films over the whole sample. This results in a faster decline of the desorption rate versus time in region III. This effect of nonuniform versus uniform deposition is shown in Figure 7. The two ITDS traces were taken with equal deposited amounts of ice, but in one case the peripheral parts of the sample had a declining thickness gradient. The net effect is an artifact in the shape of the trace toward the late end of region II and in region III. For a uniform film, region II extends over a broader range, and the tail in region III is much steeper, as shown in detail by Livingston et al.20 Region I and the early region II, which are our prime interest here, are unaffected. The difference between uniform and nonuniform deposited films thus does not appear in region I and II where the whole film covers the surface anyway but becomes obvious in region III where the slope becomes shallower with decreasing uniformity. Nonuniform deposition can delay the appearance of region II, that is, require larger

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Figure 7. Isothermal desorption spectra of H2O from clean Pt(111) illustrating the effect of nonhomogeneous surface coverage. The curve labeled 34 ML was made with an arrangement ascertaining uniform thickness. For the 27 ML curve, there was a declining thickness at the sample edges.

Figure 8. Isothermal desorption trace from a water film grown at 105 K on 50 ML thick crystalline ice (see the text).

film thickness than if a perfectly uniform film is deposited. See also ref 39. 3.2.3. Ice Deposition on a Crystallized Film. The desorption traces for ice on Pt show that the fairly good lattice match between Pt(111) and the hexagonal planes of crystalline ice is not enough to promote epitaxial growth at 100 K. Starke et al.34 have, however, shown that single crystalline ice films can be grown on Pt(111) by careful deposition plus annealing, as evidenced by the resulting LEED pattern. The question then arises whether the noncrystalline growth on Pt(111) at 100 K is a purely kinetic effect, that is, due to insufficient mobility of the water molecules, or is due to the lattice mismatch (or both). For this reason, the experiment shown in Figure 8 was performed: after deposition of a 65 ML film at 100 K, an isothermal desorption trace was recorded, but it was this time interrupted when ∼16 ML (25% of the total amount) had desorbed, by cooling the substrate after the amorphous peak had been completed and when the system had advanced a bit into the plateau region (region II). Then, a new film of 13 ML thickness was deposited at 100 K, and a new desorption trace was performed. The latter, shown in Figure 8, has almost no amorphous peak. Only a very weak peak is seen at the very beginning of the (39) Ahlstro¨m, P.; Lo¨fgren, P.; J. Lausmaa et al., submitted to J. Non. Cryst. Solids.

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Figure 10. Isothermal desorption experiments at different sublimation temperatures on clean Pt(111) surface. The initial coverage for all measurements is approximately 80 ML, deposited at 105 K.

Figure 9. Relative fraction of water molecules desorbing from the amorphous phase as a function of deposition temperature for (a) Pt and (b) graphite.

trace. This shows that the film grown on crystalline ice either grows crystalline at 100 K or crystallizes much more rapidly on the latter surface by fast interfacial nucleation and growth during the desorption trace. New data by Dohna´lek et al.40 indicate that the latter is the case, contrary to our interpretation in ref 18. 3.3. Influence of Deposition Temperature and of Preannealing of the Ice Film on the Desorption Kinetics. The deposited films always show an amorphous peak when deposited at 100 K on all substrates except crystallized ice (very thick films were not explored for the latter case). All films, independent of substrate, crystallize during the desorption trace at T g 144 K. Consequently, the substrate temperature during deposition must play an important role for the desorption and crystallization kinetics. We explored this by depositing the same amount of ice at different temperatures (all well below the desorption temperature) on platinum and graphite. The measured quantity that best reflects changes in the film properties is the amount of water desorbing in the amorphous peak. These results are shown in Figure 9. The films grown on Pt clearly have a larger “inertia” or activation energy (for quantification see below) toward crystallization, compared to the films grown on graphite. (40) Dohna´lek, Z.; Siolli, R. L.; Kimmel, G. A.; et al., J. Chem. Phys. 1999, 110, 5489.

The absolute film thickness was constant for all films on the same substrate, but much thinner on graphite. Despite this, it is clear that the ice films on Pt need significantly higher temperatures to transform to the crystalline phase; for Pt the amorphous peak disappears completely only when the ice films are grown at T ) 130 K. On graphite, the corresponding temperature is 120 K. This larger “inertia” for crystallization on Pt(111) will be further illustrated in the next paragraph. It reflects a “memory” of the initial condensation and growth kinetics on the substrate surface that extends over many monolayers. Another factor of interest is film annealing prior to desorption. Films deposited on Pt at 100 K and annealed to different temperatures prior to desorption show that the desorption traces are unaffected by annealing up to 140 K for up to 30 min. Comparing this observation with the influence of substrate temperature during film growth (Figure 9), discussed above, we can draw the conclusion that the activation energy for crystallization of an already deposited amorphous film is much larger than the activation energy for crystallization during the film growth. This intuitively sound result reflects that bulk molecules in the amorphous phase are less mobile, that is, much more “locked” into their amorphous (low-coordinated) arrangement, than surface molecules, which easier anneal into a high-coordinated structure as demonstrated by the HREEL spectra (Figure 1). 3.4. Temperature Influence and Energetics of Desorption Kinetics and Crystallization. The most informative type of measurement with regard to the crystallization kinetics and energetics is to vary the isothermal desorption temperature. Since the desorption rate increases exponentially with temperature, the available T-range is quite limited; at temperatures below 130 K the desorption experiments take an unreasonably long time (many hours), while too high temperatures (>150 K) lead to too fast desorption to give reliable data. Thus, the accessible T range is ∼140-150 K. Desorption traces for different desorption temperatures are shown in Figure 10. The films were all grown on Pt(111) and with approximately the same thickness. (The thickness variation is given by the variation in area under the traces.) The two most important observations are (i) the rate increase with increasing temperature for the amorphous peak and for the plateau region. The dependence can be used for an Arrhenius type analysis of the desorption energetic for the two phases as in Figure 11. The corresponding apparent activation energies for de-

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Figure 11. Arrhenius plot of the desorption rate for the amorphous (open circles) and crystalline ice (filled circles). The desorption rates were obtained from similar recordings as in Figure 8 and are the mean value of several measurements.

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Figure 12. Arrhenius plot of the time, τ, required to complete the phase transition at different desorption temperatures, see Figure 6.

sorption are 52 ( 2 kJ mol-1 (0.53 eV) (amorphous phase) and 54 ( 2 kJ mol-1 (0.56 eV) (crystalline). Since the sticking coefficient for both phases is unity within experimental accuracy, these values can be interpreted as the corresponding cohesive energies. An Arrhenius plot of the ratio of the desorption rates from the two phases (not shown) gives that the desorption energy for the amorphous phase is approximately 2 ( 1 kJ mol-1 lower than for the crystalline phase. The cohesive energy for the crystalline phase can be compared with reported values for bulk ice and other TDS values reported in the literature that range from 42 kJ mol-1 to 54 kJ mol-1.7,19,41,42 (ii) Second, the relative amount of ice desorbing in the amorphous phase (region I), is increasing at the expense of the amount desorbing in the crystalline phase (regions II + III) as the desorption temperature is increased. From this qualitative observation, we conclude that the activation energy for crystallization (of thick films) is larger than the desorption (cohesive) energy. This statement is quantified in Figure 12, showing an Arrhenius type plot of the time to complete the phase transition, τ, versus inverse temperature. The obtained activation energy for crystallization is 75 kJ mol-1 (0.78 eV), comparable to the value 84 kJ mol-1 obtained by Smith et al.17 Similar analysis of data for graphite shows a much lower (apparent) activation energy for crystallization, consistent with the more rapid disappearance of the amorphous peak in this case. We associate this with the different growth and crystallization modes on Pt and graphite, causing a rougher film on graphite, where the thinnest regions may be dominated by interfacial crystallization (cf. the accompanying paper). Thus, the activation energy for graphite has a more complex origin than for Pt and is composed of both bulk and interface nucleation components (i.e., the true bulk limit was not reached for graphite). In contrast, the Pt data are obtained for sufficiently thick films that bulk nucleation and growth of the crystalline phase should dominate, and we regard this activation energy for crystallization representative for the amorphous ice in the bulk limit.

We conclude by noting that the amorphous phase is less stable by about 2 kJ mol-1 and can be thermally crystallized in the thick film (bulk) limit by activation energy of about 75 kJ mol-1. 3.5. Water Mobility during the Phase Transition. To obtain information about the film dynamics during the crystallization phase and the different ice structures evolving during the ice annealing and desorption process, we performed experiments with preadsorbing O2 (nondissociated), prior to the water film deposition. A similar experiment was reported by Smith et al.43 using CCl4. The basic idea with this experiment was the following. When adsorbed on the clean Pt(111), a major fraction (70%) of the O2 molecules desorb around 150 K upon heating. The remaining fraction dissociates to O atoms at the same temperature and desorbs only upon heating to 700 K, where O atoms recombine to O2 again. However, when ice is deposited on top of O2/Pt(111) at ∼100 K, the O2 monolayer may be trapped at the ice-Pt(111) interface, if the water film is molecularly dense and if there is a negligible O2 diffusion rate into ice, when the temperature is raised to a typical ITDS temperature (145155 K). If, on the other hand, the water film is not dense, or the O2 diffusion rate in ice is high, O2 may penetrate the water film even in region I and desorb from the ice surface. The question addressed in this experiment is thus the following: Will the O2 adlayer stay trapped at the ice-Pt(111) interface until region III is reached and openings for O2 desorption are created in the ice film, or will there be sufficient dynamics, diffusion, pores, or other effects allowing O2 desorption at a much earlier stage? A separate question of interest to the community studying the O2/Pt(111) desorption/dissociation dynamics is if the ice layer affects the desorption/ dissociation branching ratio, which is about 2.5 on the clean surface.36,44 The ITDS runs in Figure 13a-c, performed at 148 K, give the answer to the first question. The top diagram (Figure 13a) provides the reference desorption spectrum for a full monolayer of O2 on clean Pt(111) with no H2O coadsorbed. (It is not a complete ITD spectrum because

(41) Haynes, D. R.; Tro, N. J.; George, S. M. J. Phys. Chem. 1992, 96, 8502. (42) George, S. M.; Livingstone, F. E. Surf. Rev. Lett. 1997, 4, 771.

(43) Smith, R. S.; Huang, C.; Wong, E. K. L.; et al., Phys. Rev. Lett. 1997, 79, 909. (44) Lo¨fgren, P.; Kasemo, B. Catal. Lett. 1998, 53, 33.

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s in Figure 13b not shown), no additional O2 desorption is observed. This phenomenon is further elucidated by the same type of experiment as in Figure 13b, using a much lower ice layer thickness (20 ML, Figure 13c). Here, the O2 desorption peak is much less distinct but coincides with the amorphous peak. The new information in Figure 13c is that the O2 desorption has a long tail in region III demonstrating that molecular O2 is trapped at the water/ Pt interface after the crystallization transition and that this trapped O2 is successively released as bare area is exposed. There is an increase in O2 desorption at the very end of the trace. The branching ratios between desorption and dissociation of O2 were obtained by measuring the high-temperature O2 desorption peak at 700 K, caused by recombination of O atoms. The branching ratios were approximately 0.14 and 0.11 for Figures 13b and 14c, respectively, that is, about 20 times smaller than on the clean surface.36,44 The main conclusions of this experiment are similar to the ones by Smith et al.43 using interfacially trapped CCl4. The amorphous ice layer effectively traps O2 at the ice-Pt interface if the water film is thick enough (Figure 13b). At the crystallization transition, there is a high mobility in the water film, maybe a percolation-like behavior,43 opening pathways for O2 to diffuse through the film to its surface, where they desorb. As the crystallization is completed, the now crystalline film traps the remaining O2 at the interface (if the film covers the whole surface as in Figure 13b). Obviously, the lateral mobility of O2 under crystalline ice areas is very small, since essentially all the ice must be desorbed before the remaining O2 can desorb. The most important conclusion from these results is that there is a very high mobility in the water film accompanying the crystallization process. In addition, trapping of O2 at the ice-Pt(111) interface increases the dissociation probability by more than an order of magnitude compared to clean Pt(111) because of the “locking in” of the O2 molecules. 4. Summary Figure 13. Isothermal desorption experiments with Pt(111) surface saturated with molecular oxygen (∼0.5 ML), before an ice film is deposited. Isothermal desorption of (a) a saturated O2 layer without ice, (b) 90 layers of water on an O2 precovered Pt(111) surface, and (c) 20 layers of water on the O2 precovered Pt surface. Note the different time scales in the different spectra.

some O2 desorption starts already well below 148 K, during the finite time of rise of the T-ramp.) This produces a peak maximizing after 60 s, which already starts to decline when 148 K is reached. This serves as a reference peak for the analysis of Figure 14b, c. In Figure 13b, exactly the same O2 coverage was adsorbed and then about 90 ML of H2O was deposited on top. The subsequent ITDS run yields, within experimental errors, the same H2O desorption rate versus time as if no O2 had been preadsorbed. However, there is a clear delay of the O2 desorption; an O2 maximum is seen after about 450 s, and with a peak area of about 5% of that in Figure 13a. No O2 desorption is observed after 60 s as in Figure 13a. The O2 desorption from the ice-covered surface instead coincides with the rapidly declining H2O desorption rate at the amorphous to crystalline phase transformation, signaling considerable dynamics and mobility in the ice film at this occasion. The O2 peak ratio between Figure 13b and 14a is about 0.05. At the completion of the ITDS trace (beyond 7000

The majority of the experimental results were discussed and analyzed in connection with their presentation above. Here, we summarize the main picture that has emerged, add a few concluding remarks, and suggest the sequential paper39 for further discussion. In the narrow temperature range of a few tens of degrees around 100 K, H2O molecules arriving with thermal energy at a surface grow in dense, amorphous films on a variety of substrates. At these temperatures, condensation is irreversible with negligibly low evaporation. Although all films, independent of substrate (a possible exception is thin layers of crystalline ice), grow amorphous, they still exhibit substrate-dependent desorption kinetics up to and in some cases far beyond average thickness of ∼100 monolayers. This is attributed to a combination of two factors. First, the different wetting properties of different substrates affect the film properties, even if the temperature, deposition rate, and deposited amount are kept constant. On hydrophilic (wetting) substrates, with Pt(111) as our prototype system, essentially 2D islands nucleate and grow until they coalesce to a coherent relatively smooth film, covering the whole substrate at a very thin film thickness (a few monolayers). The deposited amount required to observe region II on such surfaces (plateau region with zero-order kinetics) is somewhat larger than the thickness to obtain a coherent film during deposition, because the whole film must be crystallized to

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observe region II, and during the crystallization a substantial amount of H2O desorbs. The thickness uniformity of such films makes them attain bulklike properties at much lower average film thickness than on hydrophobic surfaces. On more or very hydrophobic surfaces such as graphite and octane, respectively, nonwetting droplets are formed during film growth, and a much larger amount of ice must be deposited, compared to, for example, Pt, before the droplets coalesce to a coherent film. Even larger amounts are required to observe region II in ITDS traces (hundreds of ML for octane). Because of the large contact angle on such surfaces, a much larger roughness is expected compared to hydrophilic surfaces. This is amplified by the likely lower ice nucleation density on such surfaces. A consequence of this growth kinetics is that ice films on hydrophobic surfaces exhibit a “memory” effect of the underlying substrate, extending to hundreds of monolayers. Since the substrate-dependent desorption kinetics is strongly coupled to the surface wetting properties, it offers a new method to measure wetting under UHV-clean conditions: A simple measure of the wetting ability is the minimum amount of water required to establish region II. The smaller this amount, the better the wetting. (This

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type of wetting determination does not require film deposition in the amorphous film range.) When amorphous films, deposited at ∼100 K, are raised in temperature to J145 K, two simultaneous processes are initiated: evaporation from the surface and a phase transition from the amorphous phase to crystalline ice. On hydrophobic films, the crystallization kinetics becomes more complex in the mesoscopic thickness regime because of the larger roughness, originating from poor wetting. The crystallization process is associated with high mobility in the water film, as demonstrated with “trapped oxygen” experiments. From the experimental data, we find an enthalpy of activation of 75 kJ mol-1 for the crystallization of the amorphous substance. This is nearly twice as high as the enthalpy of desorption. This apparent discrepancy is, however, resolved when considering the fact that this value is the enthalpy for the formation of an ice cluster involving many water molecules. In our estimate,39 we get about 37 water molecules and the enthalpy of activation for the crystallization is thus only about 2 kJ mol-1, a value easily brought in agreement with the desorption enthalpies. LA020218U