Phase Transition of Thin Water Films on Graphite Probed by

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Phase Transition of Thin Water Films on Graphite Probed by Interactions with LiCl Additives Ryutaro Souda* International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ABSTRACT: To gain insight into the glass−liquid transition and crystallization kinetics of water, thin films of amorphous solid water (ASW) were deposited in different ways onto highly oriented pyrolytic graphite. Their interactions with LiCl additives were investigated as a function of temperature using time-of-flight secondary ion mass spectrometry. The uptake behavior of LiCl in ASW deposited at 100 K was found to be significantly different from that deposited at temperatures below 90 K. The LiCl is incorporated in the latter at ca. 160 K immediately before crystallization because supercooled water is created. However, the LiCl uptake is suppressed strongly during crystallization of the former at ca. 150 K, suggesting that the evolution of supercooled water and its spontaneous nucleation are circumvented. The interfacial structure of water in direct contact with graphite plays a role in this behavior, as evidenced by the fact that a monolayer of n-hexane at the substrate interface recovers the LiCl uptake when the ASW film is deposited at 120 K. Probably, distinct liquid water, designated as low density liquid, crystallizes before the occurrence of supecooled water under the presence of specifically oriented water species (i.e., nuclei) at the substrate interface. A liquidlike phase that integrates the LiCl additives also evolves at the free surface or grain boundaries of cubic ice Ic at T > 150 K because of premelting. and crystallization kinetics of thin water films. Nevertheless, very little is known about the origins of these behaviors. In the framework of polyamorphism, high-pressure experiments have shown at least two distinct phases of glassy water:6−8 ASW is classifiable as low-density amorphous ice (LDA) and is expected to have a corresponding liquid form, designated as low density liquid (LDL).8 Distinct from normal water (or high-density liquid; HDL), LDL is a tetrahedrally structured liquid with strong, highly directional hydrogen bonds. Reportedly, anomalous behaviors of water in the supercooled region originate from the existence of a second critical point (Tsc = 220 K, Psc = 100 MPa),8 where a liquid− liquid (L−L) phase transition line between HDL and LDL terminates. This hypothesis has been validated only slightly by experimentation because bulk water crystallizes in the relevant temperature range. Homogeneous nucleation of water at 235 K can be avoided using porous media; Tg of supercooled water in nanoconfined geometry has been determined as 180−190 K,9−11 which is a higher temperature than that found using ASW. The discrepancy occurs because the latter corresponds to Tg of LDL. It is also expected that Tg of supercooled water in nanoconfined geometry is affected strongly by interaction with pore walls. Consequently, phase behaviors of the liquidlike water formed in the deeply supercooled region, as well as the influence of the substrate interface, must be understood to

1. INTRODUCTION Water in nanoconfined geometry is ubiquitous in nature. Elucidation of its properties is important in many research fields such as biology, astrophysics, and environmental science. The unique physical and chemical properties of water, which results from highly directional hydrogen bonds, can be modified by nanoconfinement because of the influences of the substrate and the free surface. When water molecules are deposited onto transition metal substrates at cryogenic temperatures, an ice-like bilayer1 and a nearly flat overlayer2 can be formed, where water binds to the substrate via an oxygen lone-pair orbital (the former) and metal−oxygen and metal−hydrogen bonds (the latter). Because of the interfacial stability, thin amorphous solid water (ASW) films deposited on such hydrophilic substrates tend to behave similarly to a bulk material in terms of the glass−liquid transition and crystallization, as we have demonstrated by experiments using time-of-flight secondary ion mass spectrometry (TOFSIMS).3−5 In fact, ASW formed on Ni(111) becomes diffusive at Tg = 136 K.3 Then the film morphology changes abruptly during crystallization at Tc = 160 K.4 In contrast, ASW films on hydrophobic substrates agglomerate at temperatures (ca. 100− 120 K) lower than Tg because of surface diffusivity and interfacial instability.5 Unique features are observed when ASW is deposited onto the substrate of highly oriented pyrolytic graphite (HOPG).4,5 The ASW film fundamentally wets the HOPG substrate, but the water molecules tend to reorient at the substrate interface upon heating. The film finally dewets during crystallization at 145−150 K.4,5 Apparently, the substrate interface has a long-range effect on the glass transition © XXXX American Chemical Society

Received: March 16, 2017 Revised: April 25, 2017

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DOI: 10.1021/acs.jpcc.7b02481 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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made at a ramping speed of 5 K min−1. For cleaning, the HOPG substrate was heated to 1200 K by electron bombardment from behind. Water (H216O and H218O) and n-hexane were purified via several freeze−pump−thaw cycles. The ASW films were formed on HOPG at different substrate temperatures by backfilling the UHV chamber with the water gas admitted through high-precision variable leak valves. LiCl was deposited onto the ASW film surface from a Ta boat placed in front of the sample surface via thermal evaporation. The cleanliness of the HOPG substrate and of the thin films deposited on it was checked based on the TOF-SIMS spectra. The coverage of water (LiCl) was determined from the evolution curves of sputtered ion intensities as a function of exposure (deposition time). The exposure of the water molecules at ca. 2 langmuirs (1 langmuir = 1 × 10−6 Torr s) was required to form a monolayer (ML). In general, SIMS is sensitive to the surface because slow secondary ions formed in deeper layers tend to be neutralized. However, such is not the case for already ionized species such as LiCl. The escape depths of the secondary ions depend on their respective masses and ionic sizes. The ion intensities decay almost exponentially with water coverage; the Li+ (Cl−) intensity becomes approximately 0.6 (0.2) times as large as the initial intensity after deposition of 1 ML of H216O molecules (not shown). Because of this behavior, intensities not only of Li+ but also of Cl− were measured to evaluate the depth of the incorporated LiCl species from the film surface.

elucidate the glass transition and crystallization kinetics of water. The phase change of water can be explored based on interactions with solute species because their uptake is probably determined by the microscopic hydrogen-bond structures of water molecules. In fact, nonpolar molecules are known to be entrapped in the ASW film interior;12−15 they are released at around 160 K because the water cage collapses during crystallization.12 In contrast, supercooled water is expected to dissolve electrolyte additives. Small amounts of LiCl, LiBr, and LiI species are incorporated into the film interior at around Tc, as demonstrated using ASW formed on the Ni(111) substrate.16 Moreover, a concentrated LiCl solution can be formed transiently at around 160−165 K when ASW is deposited on a solid LiCl film.16 These results strongly suggest that supercooled water can be formed immediately before crystallization. Nevertheless, no direct evidence for the presence of bulk supercooled water has been obtained to date in the deeply supercooled region using other experimental techniques, such as infrared absorption,17−19 differential scanning calorimetry,20−23 and dielectric spectroscopy.24 In this respect, a debate also exists related to the precise assignment of water’s Tg to either 136 or 160 K.20−25 The former corresponds to Tg of LDL, whereas the latter agrees with Tc (or the L−L phase transition temperature) of water determined using TOF-SIMS, suggesting that supercooled water is so short-lived that it cannot be identified spectroscopically. Apparently, more experimental evidence must be accumulated to establish this presumption. This study investigated how the glass transition and crystallization kinetics of the ASW films deposited onto HOPG are influenced by the film thickness, deposition temperature, and interfacial structure. To this end, interactions of small amounts of LiCl additives with the ASW films deposited onto clean and n-hexaneadsorbed HOPG substrates were examined using TOF-SIMS as a function of the substrate temperature. Attention was devoted specifically to the occurrence of supercooled water and its roles in the crystallization kinetics of thin water films on HOPG.

3. EXPERIMENTAL RESULTS A liquidlike phase formed upon heating is identifiable based on the self-diffusivity of water molecules or uptake of adspecies. Figure 1a presents TOF-SIMS intensities from a 100 ML H216O film deposited at 70 K, on which 1 ML of H218O was adsorbed at 100 K. The H318O+ intensity decreases gradually with increasing temperature because the adspecies is incorporated in the ASW film interior. The ion intensities decay at around 180 K because of the evaporation of water molecules. A fundamentally identical result is obtained when the 100 ML thick film deposited at 100 K is used, as presented in Figure 1b. These results indicate that both water films deposited at 70 and 100 K are glassy. The molecules become mobile at around water’s Tg of 136 K. The H318O+ intensity decreases gradually because surface diffusivity commences at 100−120 K,5 leading to the uptake of adspecies in the subsurface region. For comparison, the crystalline H216O film (100 ML; prepared by annealing the ASW film up to 162 K) was examined. The results are portrayed in Figure 1c. In contrast to the glassy ASW films, the diffusivity of 1 ML of H218O adspecies at Tg is quenched. Because grains are formed in this case, the film is not uniform. No cations are emitted from the clean HOPG patches, but C+ is emitted when a (sub)monolayer of water is present on it. The H218O adspecies on the grains, as well as the HOPG patches, tend to be incorporated in the former at temperatures higher than 150 K, as revealed from the simultaneous decay of C+ and H318O+ intensities. This behavior indicates that a liquidlike phase occurs even after crystallization. Figure 2 displays temperature-programmed TOF-SIMS intensities of typical cations sputtered from LiCl adsorbed H216O films that are formed on HOPG in a different manner. The ASW films (100 ML) were deposited at 90 K (a) and 100 K (b); the crystalline ice film was prepared by postannealing the latter to 162 K (c). The LiCl additives of 0.1−0.2 ML were deposited on them at 100 K. At this coverage, the LiCl is

2. EXPERIMENT The TOF-SIMS experiment was performed in an ultrahighvacuum (UHV) chamber with base pressure of 2 × 10−10 Torr, equipped with an electron-impact-type ion source and a linear flight tube for TOF-SIMS, a differentially pumped quadrupole mass analyzer for temperature-programmed desorption (TPD), and an interlock system for sample transfer. The TOF-SIMS spectra were taken with incidence of a pulsed He+ beam (2.0 keV) onto a sample surface, which was floated with a bias voltage of ±500 V. Positive or negative secondary ions were extracted into the field-free TOF tube through a grounded stainless steel mesh placed ca. 4 mm above the sample surface. They were detected together with backscattered He0 atoms using a microchannel plate. The primary beam current (∼10−12 A) was monitored through the backscattered He0 intensity; it was used to normalize the secondary ion intensities when the He+ beam was fluctuated. On a sapphire plate for electric insulation, the sample holder was mounted on a Cu rod that extended from a closed cycle helium refrigerator cooled to 20 K. The sample temperature was monitored using Au(Fe) chromel thermocouples attached to the Cu rod close to the sample position and was controlled using a digital temperature controller and a cartridge heater. The temperature-programmed TOF-SIMS measurement was B

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scattered without agglomeration, as evidenced by the presence of Li+(H2O) and the absence of Li+(LiCl) in the TOF-SIMS spectra (not shown). In Figure 2a, the Li+ intensity decays steeply at 160 K and then recovers after evaporation of the water molecules at 180 K. This result implies that highly diffusive molecules are formed during crystallization at 160 K. However, LiCl is not incorporated by the liquidlike water formed at Tg = 136 K. Consequently, the uptake behavior differs significantly between the H218O and LiCl adspecies. It is particularly interesting that LiCl tends to remain on the surface of the film deposited at 100 K (Figure 2b). This film must be amorphous, as evidenced by the occurrence of self-diffusivity at Tg (see Figure 1b), but the experimental result rather resembles that obtained using the crystalline ice, as presented in Figure 2c. Figure 3 displays experimentally obtained results of anion TOF-SIMS. The intensities of Cl−, H−, and O− ions are shown

Figure 1. Temperature-programmed TOF-SIMS intensities of typical cations sputtered from water on the HOPG substrate. The H218O molecules (1 ML) were adsorbed at 100 K on the ASW films formed by deposition of 100 ML of H216O at 70 K (a) and 100 K (b) and on a crystalline ice film formed by annealing the latter to 162 K (c). The temperature was ramped at a rate of 5 K min−1.

Figure 3. Same as in Figure 2, but intensities of typical anions are displayed.

using films prepared in the same manner as that in Figure 2. On the ASW film deposited at 90 K (a), the Cl− intensity drops at 160 K because of the LiCl uptake, whereas the H− and O− ions increase in intensity because of water crystallization. The uptake of LiCl is suppressed on the ASW film deposited at 100 K (b). Instead, gradual decay occurs in the Cl− intensity at T > 150 K. The increase in the H− and O− intensities during water crystallization is not identified in this case. The gradual decrease in the Cl− intensity is also observed using the crystalline film, as depicted in Figure 3c. The decay of the Cl− intensity after crystallization is identifiable even for the ASW film deposited at 90 K (Figure 3a) as an additional dip at 170 K following the steep decay during crystallization at 160 K. Consequently, the uptake rate of LiCl is sensitively dependent on the deposition temperature of the ASW film on HOPG, but liquidlike water formed at Tg = 136 K does not play a role in this behavior.

Figure 2. Temperature-programmed TOF-SIMS intensities of typical cations sputtered from 100 ML ASW films deposited onto HOPG at 70 K (a) and 100 K (b) and on the crystalline ice film (c), obtained after adsorption of LiCl additives (0.1−0.2 ML) on the film surface at 100 K.

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K) followed by deposition of the 50 ML ASW on it at 100 K (70 K); LiCl was adsorbed onto them at 100 K. Crystallization occurs at lower temperatures when ASW is first deposited on HOPG at 100 K, as revealed from a jump of the O− intensity. On this film surface, the uptake of LiCl during crystallization is suppressed considerably. The result resembles that shown in Figure 3b, except for the jump in the O− intensity. The experimentally obtained results shown by solid lines are fundamentally identical to those obtained using the 100 ML ASW film deposited on HOPG at 90 K (Figure 3a). Consequently, the uptake behavior of LiCl during crystallization is determined by the interfacial interaction of the water molecules with HOPG rather than the microscopic hydrogenbond structure (e.g., “porosity”) of the ASW film surface in direct contact with the LiCl adspecies. As described already, the water mobility can be enhanced on the free surface of ASW. Therefore, LiCl might be encapsulated on the ASW film surface by diffusive water molecules without uptake in the film interior. This possibility is explored using thinner ASW films. Experimental results of negative TOF-SIMS obtained for (a) 10, (b) 25, and (c) 50 ML ASW films deposited at 70 K are displayed in Figure 6. The intensity of O−

To shed light on the effects of the substrate interface on the water crystallization and LiCl uptake, experiments were performed using a “contaminated” HOPG substrate. A monolayer of n-hexane was adsorbed onto the HOPG substrate at 70 K; then 100 ML of the water molecules was deposited on it. Figure 4 shows typical TOF-SIMS intensities of (a) cations

Figure 4. Temperature-programmed TOF-SIMS intensities of typical cations (a) and anions (b) sputtered from LiCl (0.1−0.2 ML) adsorbed ASW films (100 ML). The water molecules were deposited on the n-hexane (1 ML; 70 K) adsorbed HOPG substrate at 120 K.

and (b) anions obtained using the ASW film deposited at 120 K. The uptake of LiCl adsorbed onto the film surface occurs steeply at ca. 160 K in both measurements because of the phase transition of water. Consequently, the water multilayer film deposited at temperatures below 120 K is amorphous in nature. Experimental results characteristic of the normal ASW film are obtained by the presence of the interfacial n-hexane molecules. The anomalous behavior like crystalline ice (i.e., absence of the abrupt LiCl uptake at 160 K) observed for films deposited on the clean HOPG substrate at 100 K is probably associated with the interfacial structure of the water molecules. The deposition temperature effects are explored further using differently tailored binary ASW films formed on HOPG. Experimentally obtained results are displayed in Figure 5. The solid (broken) lines show intensities of the O− and Cl− ions from the 50 ML ASW film deposited onto HOPG at 70 K (100

Figure 6. Temperature-programmed TOF-SIMS intensities of typical anions sputtered from ASW films with thickness of (a) 10, (b) 25, and (c) 50 ML formed on HOPG at 70 K; LiCl (0.1−0.2 ML) was adsorbed onto them at 100 K.

increases at ca. 160 K, irrespective of the film thickness because of the water phase transition. The opening of the HOPG patches (i.e., film dewetting) is also identified from the C2− ion ejection. Therefore, the mobile water molecules are created during crystallization at ca. 160 K for all films examined here. However, the LiCl adspecies is not hindered by the 10 ML thick water molecules. The result in Figure 5b rather resembles that in Figures 3b and 3c, suggesting that LiCl tends to be incorporated in the crystalline ice. The steep decay of the Cl− ion during crystallization is observable for a considerably thick

Figure 5. Temperature-programmed TOF-SIMS intensities of Cl− and O− ions sputtered from LiCl (0.1−0.2 ML) adsorbed binary ASW films that were prepared in a different manner. The solid (broken) lines exhibit results obtained using the film formed by adsorption of 50 ML water onto HOPG at 70 K (100 K), followed by deposition of 50 ML of water on it at 100 K (70 K). D

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The Journal of Physical Chemistry C (50 ML) ASW film. These results show that the decay of the Cl− ion intensities at 160 K is not ascribed to hindrance of the LiCl adspecies by mobile water molecules formed on the film surface. The uptake of LiCl is likely to occur as a result of hydration in the film interior. The hydration of electrolytes requires a thicker water film because many water molecules are rearranged to form hydration shells.

transition is likely to occur immediately before water crystallization. Because of the instability of supercooled water, the dilute LiCl solution undergoes phase separation into crystalline ice and more concentrated solutions immediately. In fact, the latter is formed as a metastable state when water is deposited onto the solid LiCl film.16 Consequently, the experimentally obtained results support the occurrence of supercooled water via the L−L phase transition, although it can be inferred only from interactions with electrolytes. The LiCl uptake is inhibited when the ASW film is deposited onto HOPG at 100 K. From the context presented above, this behavior is expected to occur when LDL crystallizes directly without experiencing the L−L transition. If this is the case, not only is Tc lowered, but also the crystalline ice morphology differs from those formed via spontaneous nucleation of supercooled water. The latter manifests itself in the lack of characteristic jumps of the H− and O− intensities during crystallization in Figure 3. However, this does not necessarily mean that the film morphology is unchanged during LDL crystallization. It is likely that LDL crystallizes rather gradually in comparison with the spontaneous nucleation of supercooled water, resulting in different morphology of the crystalline ice. To date, the crystallization kinetics has been investigated using thinner ASW films (10 ML) deposited onto HOPG at 100 K based on combined TPD and TOF-SIMS measurements,4,29 where film dewetting is identified after crystallization at ca. 150 K. The route of water crystallization on HOPG is likely to be determined by the interfacial water structure, as evidenced by the effects of n-hexane at the substrate interface. The water monolayer immediately after deposition is oriented randomly, but they are expected to form a hydrogen-bonded hexagonal ring on the clean HOPG substrate upon heating.30 The TOFSIMS measurement also revealed that the H+ intensity from the first monolayer of ASW is reduced relative to the H3O+ intensity with increasing substrate temperature because the unpaired OH group of water forming the ring is directed preferentially toward the HOPG substrate,4,5 thereby reducing the interfacial energy. This structure is consistent with theoretical predictions.30−32 The TOF-SIMS evolution curves also revealed that the aligned water molecules tend to form 3D clusters in the multilayer regime when deposited at 100−120 K because of the water surface mobility on HOPG.4 Such a specific water species formed at the substrate interface is likely to provide nuclei for the LDL crystallization at 145−150 K, thereby bypassing spontaneous nucleation of supercooled water at ca. 160 K. Results in Figure 5 also reveal that LDL crystallization occurs at ca. 155 K when the ASW film deposited at 100 K is capped with the additional film deposited at 70 K, as evidenced by absence of the abrupt LiCl uptake. The nucleation initiated at the substrate interface propagates across the interface of the differently tailored ASW film before spontaneous nucleation occurs in it, thereby ensuring the long-range effect of the substrate interface on the water phase transition. The liquidlike phase coexists with the crystalline ice at T > 150 K, as revealed from the uptake not only of H218O (Figure 1c) but also of LiCl (Figure 3c). This result is consistent with observations based on transmission electron microscopy (TEM).18 The crystalline ice resembles liquid droplets because their morphology changes. These phenomena are explicable in terms of premelting of crystallites. The decrease of Li+ during uptake of LiCl is rather weak in comparison with Cl− because

4. DISCUSSION Self-diffusivity of water occurs at T > Tg; then water crystallizes in droplets of fluidized liquid, thereby resulting in the film morphology change.3,4 Based on measurements of dewetting temperatures, Tc is assigned as ca. 160 K using hydrophilic substrates such as Ni(111) and OH-terminated SiO2,5 whereas the dewetting temperature of ASW deposited onto HOPG tends to decrease to ca. 145−150 K when water is deposited at 100 K.4,5 The specificity of the water crystallization kinetics on HOPG might be attributed to hydrophobicity of the substrate. However, the situation is not so simple because the presence of hydrophobic species (e.g., n-hexane) at the substrate interface quenches this behavior. The increase in the TOF-SIMS H− and O− intensities at Tc is associated with film morphology changes because it is commonly observed during crystallization of pure ASW films deposited onto the hydrophilic substrates as well as the n-hexane adsorbed HOPG substrate. The absence of this behavior in Figure 3b implies that the morphology of the crystalline ice on the clean HOPG substrate differs from that on the other substrates. The most notable observation in this study is that the water crystallization kinetics, or the crystalline film morphology, on the clean HOPG substrate is changeable by increasing the water deposition temperature from 90 to 100 K. The LiCl additives with coverage of as small as 0.1−0.2 ML are not expected to influence the crystallization kinetics of the 100 ML thick ASW films. Rather, information about the phase transition of water is obtainable from interactions with such additives. The occurrence of liquidlike water at Tg is evidenced not only by the uptake of H218O from the free surface but also by surface segregation of methanol3 and acetonitrile26 deposited at the substrate interface. In contrast, the water− LiCl interaction appears to be weak at Tg < T < Tc despite the fact that liquid water forms both dilute and concentrated solutions with LiCl at room temperature. This result is explainable: diffusive water formed at Tg is LDL. The local hydrogen-bond structure of LDL resembles that of crystalline ice rather than normal water.27 Therefore, LDL formed at Tg does not dissolve the LiCl additives. In this respect, molecular dynamics (MD) simulations revealed the presence of two minimum energy geometries for clusters consisting of the alkali halide (NaCl) and water:28 one is a solvent-separated ion pair; the other is a contact ion pair. The water structure in the latter geometry is less disrupted by the solute species, thereby maintaining almost intact hydrogen bonds: Na+ is embedded in the water cluster whereas Cl− occupies the surface of water clusters because of the higher polarizability of anions. Probably, the contact ion pair is realized during the interaction with LDL, so that LiCl tends to remain on the film surface as a surfactant. The dissolution of LiCl in the film interior during crystallization results from the formation of the separated ion pair, which is expected to be realized in normal (supercooled) water with distorted hydrogen bonds, as depicted by the MD simulations.28 The film thickness dependency observed in Figure 6 strongly suggests that the LiCl uptake is characteristic of the bulk phenomenon (i.e., hydration). Therefore, the L−L phase E

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the former can be emitted from deeper layers, indicating that the melting layer is limited to the shallow regions or boundaries of crystal grains in contrast to the supercooled water formed during the L−L transition. No detailed information about this liquid is obtained, but the uptake of LiCl implies that the local hydrogen bond structure is more distorted in comparison with LDL. Probably, premelting is associated with the metastable nature of cubic ice Ic. The liquidlike phase (quasi-liquid) might occur during or before the phase transition of ice Ic into stable hexagonal ice Ih. This phase transition is not accessible in this study because of the evaporation of ice Ic. Consequently, some distinct liquid might be formed transiently during the first-order phase transition, as demonstrated not only for the L−L transition (i.e., crystallization into ice Ic) but also for premelting of ice Ic (phase transition to ice Ih).

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5. CONCLUSION The glass−liquid transition and crystallization of water are explored in terms of the polyamorphism and interfacial effects using HOPG as a substrate. The occurrence of supercooled water is probed by interactions with LiCl additives. The crystallization kinetics of ASW is found to be sensitively dependent on the deposition temperature of the water molecules. LiCl remains on the film surface without hydration during the glass−liquid transition because a distinct water, LDL, is formed at Tg = 136 K. The surfactant activity of LiCl may result from the icelike local structure of LDL and the formation of the contact ion pair. On the ASW film deposited at temperatures below 90 K, the LiCl additives are incorporated in the film interior at 160 K immediately before crystallization occurs because LDL transforms into supercooled water. In contrast, the uptake of LiCl during crystallization tends to be quenched when the ASW film is deposited on HOPG at 100 K. This behavior is explainable as that water crystallizes directly from LDL without experiencing the L−L phase transition. The LDL crystallization is likely to be induced by some nuclei. Probably, specifically oriented water species formed at the substrate interface play a role, as demonstrated by the fact that a monolayer of n-hexane formed on HOPG quenches this phenomenon. The LDL crystallization commences at ca. 150 K from the substrate interface. For that reason, the film dewetting might be less pronounced than that during the spontaneous nucleation of supercooled water in the bulk. The liquidlike water occurs even after crystallization into ice Ic, but it is identifiable only at the surface or grain boundaries because of premelting.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ryutaro Souda: 0000-0003-4769-9034 Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS This research was partly funded by the Japan Society for the Promotion of Science through a Grant-in-Aid for Scientific Research (C), No. 22540339. F

DOI: 10.1021/acs.jpcc.7b02481 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.7b02481 J. Phys. Chem. C XXXX, XXX, XXX−XXX