Interactions of LiI with Thin Methanol Films during Glass–Liquid

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Interactions of LiI with Thin Methanol Films during Glass−Liquid Transition and Premelting Ryutaro Souda* International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ABSTRACT: This study investigated interaction of thin methanol films with LiI at cryogenic temperatures using time-of-flight secondary ion mass spectrometry, temperature-programmed desorption, and reflection absorption infrared spectroscopy to elucidate properties of liquid-like phases formed during the glass−liquid transition and premelting of crystallites. The LiI additives are incorporated into the film interior from both the free surface and substrate interface at temperatures higher than methanol’s glass-transition temperature of 103 K. A dilute LiI solution is formed in supercooled methanol, as revealed from invariance of the OH stretching vibration frequencies relative to those of pure methanol. The uptake of LiI in supercooled methanol is quenched after crystallization at 120 K, where film morphology changes because of the grain growth. The incorporated LiI additives diffuse into deeper positions of the crystalline film at temperatures >140 K because premelting occurs at grain boundaries. When methanol is deposited onto a solid LiI film, a concentrated LiI solution is formed transiently, as evidenced by the blue shift of the OH stretching band. A multilayer methanol film as thick as 250 monolayers is converted to the LiI solution at 140 K, indicating that the premelting layer is equilibrated with methanol crystal grains. A thinner (10 monolayers) methanol film deposited on LiI produces a concentrated solution at 120 K, indicating that smaller grains premelt immediately after crystallization. only when the temperature approaches bulk Tm.21−25 The size dependence of Tm reduction has been investigated using free and supported clusters, and using nanoconfined materials with transmission electron microscopy26−28 and calorimetric methods.29−34 Those methods revealed that melting transitions of small systems are broad, although premelting is not identified except for specific clusters. To date, the properties of liquid-like phases formed in the deeply supercooled region have been explored using vapordeposited thin films of water and alcohols interacting with alkali halide additives.35−42 Alkali halides form dilute and concentrated solutions with liquid water and methanol at room temperature. Therefore, they are expected to be incorporated into thin films when a liquid-like phase occurs. The solubility is probably determined not only by diffusivity but also by the microscopic solvation structures of molecules. Reportedly, amorphous solid water (ASW) reflects properties of the respective liquid: Good agreement is obtained between experimentally obtained results37−39 and simulations.43 However, whether properties of the liquid-like phases formed at cryogenic temperatures are identical to those of the normal liquid at room temperature remains an open question in terms of polyamorphism.44 We have recently suggested, based on interactions between ASW and LiCl, that supercooled water forms at cryogenic temperatures via the liquid−liquid (L−L) phase transition of a distinct liquid.45 The L−L transition might

1. INTRODUCTION Solid−liquid transitions of thin films have attracted considerable attention because of their fundamental features and their importance for technological advancement. Deposition of gaseous molecules onto a cold substrate can form a glassy film. The molecule dynamics is nearly arrested at temperatures lower than its glass-transition temperature Tg. Supercooled liquid forms at temperatures higher than Tg because the translational molecular diffusion is activated. However, fragile liquids exhibit a strong non-Arrhenius behavior in viscosity because of spatial heterogeneity or decoupling of translational diffusivity and viscosity in the deeply supercooled region of T < 1.2Tg.1 Actually, Tg is lower in smaller systems confined in nanoporous media,2 although that reduction is much less than that of melting temperature Tm for simple molecules. The formation of 2D liquid on the free surface3 and a dead layer at the substrate interface4 has also been discussed along with their respective roles in the modification of thin film’s Tg. In this respect, polymer films have attracted much attention because greater reduction of Tg has been observed.5−8 Melting is another type of solid−liquid transition that plays an important role in nanoscale systems. Tm is depressed for nanoparticles because ensembles of clusters represent a mixture of solid and liquid phases.9−20 The solid−liquid phase change of nanoparticles is initiated by surface premelting because weakly bound surface species are less constrained in their thermal motion than those in the interior. The premelting of nanoparticles appears to be distinct from surface melting of macroscopic systems where the liquid layer becomes evident © 2017 American Chemical Society

Received: July 26, 2017 Published: July 27, 2017 17421

DOI: 10.1021/acs.jpcc.7b07389 J. Phys. Chem. C 2017, 121, 17421−17428

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The Journal of Physical Chemistry C

onto the methanol film surface by thermal evaporation from a Ta boat placed in front of the HOPG substrate. The coverage of methanol (LiI) was determined from the evolution curves of sputtered ion intensities as a function of exposure (deposition time). One monolayer (ML) of methanol was attained by exposure of ca. 2 langmuir (1 langmuir = 1 × 10−6 Torr s). The RAIR spectra were collected in a separate UHV chamber (base pressure of 3 × 10−10 Torr) using a spectrometer (FTS 40A; Bio-Rad Laboratories) with a liquid-nitrogen-cooled mercury cadmium telluride detector. The spectra were taken over the wavenumber range of 400−4000 cm−1 at 4 cm−1 resolution. A polycrystalline gold plate was used as a substrate. A LiI film was also deposited onto the substrate at room temperature by prolonged thermal evaporation from the LiI source placed in front of the surface. The substrate was cooled using liquid nitrogen. The methanol film was deposited on it at 90 K. The sample temperature was monitored using type K thermocouples. Spectra were taken at the same ramping speed used for TOF-SIMS and TPD.

not be identified straightforwardly using any spectroscopic method because supercooled water crystallizes immediately. The ethanolic solution of LiI is formed after crystallization when ethanol is deposited onto a solid LiI film. 41 Consequently, liquid-like phases can be formed before and after crystallization of thin molecular solid films. Nevertheless, little is known about their properties and relations to normal liquid at room temperature. Apparently, more experimental studies are necessary. As described in this paper, we investigate interactions of LiI with vapor-deposited methanol films from comparisons of experimental results between secondary ion mass spectrometry (TOF-SIMS), temperature-programmed desorption (TPD), and reflection absorption infrared spectroscopy (RAIRS) as a function of temperature. The properties of liquid-like phases formed during the glass−liquid transition and after crystallization are discussed based on the solvation ability of methanol against LiI.

2. EXPERIMENTAL SECTION For this study, TOF-SIMS and TPD experiments were performed in an ultrahigh vacuum (UHV) chamber with base pressure of Tg before film dewetting occurs at Tc = 120 K, indicating that intermixing with or surface segregation of LiI occurs in the liquid-like methanol. The methanol species bound to LiI tend to remain after evaporation of the physisorbed molecules at 160 K, as evidenced by the Li+(CH3OH) intensity. The Li+(LiI) ion increases in intensity at temperatures higher than 180 K because dried LiI patches are formed locally. The long tail of the Li+(CH3OH) intensity reveals that complete methanol detachment is not attained up to ca. 270 K. To gain more insight into the interaction between solid LiI and methanol films formed on it, temperature evolutions of the Li+(CH3OH) intensity are measured using methanol films with different thickness. Experimental results are summarized in Figure 4. The intensity increases at Tg = 103 K for thinner films, indicating that solid LiI dissolves in the liquid-like methanol and vice versa. The mixing appears to occur gradually over a wide temperature range, suggesting that the diffusivity of LiI in supercooled methanol is low. In contrast, the ion

Figure 2. Temperature-programmed TOF-SIMS intensities of cations from the 250 ML CH3OH film deposited onto HOPG at 70 K (a) and the crystalline methanol film formed by heating it to 125 K (b), obtained after adsorption of LiI additives (0.2 to 0.3 ML) on film surfaces at 70 K.

experimental results of temperature-programmed TOF-SIMS obtained for LiI (0.2 to 0.3 ML) adsorbed onto the glassy methanol film (250 ML) formed on HOPG. The LiI tends to be incorporated in the film interior at T > Tg. The uptake is quenched when methanol crystallizes at Tc = 120 K. However, the Li+ intensity decreases steeply again at T > 140 K, indicating that the additives are delivered to deeper sites of the crystalline methanol because a liquid-like state evolves. The experimental result for LiI deposited on a crystalline methanol film is also shown in Figure 2b. The mobility of methanol almost disappears at 103−120 K, but the Li+ intensity decreases gradually with increasing temperature. The LiI uptake is accelerated at T > 140 K. The Li+ intensity minimizes immediately before methanol evaporates at ca. 160 K. Consequently, the crystalline methanol film exhibits a liquidlike behavior in interactions with LiI additives as well, although the dip occurs at different temperatures between the initially glassy and crystalline films. The difference in LiI uptake between panels a and b of Figure 2 is ascribable to the difference in film morphology: The uptake rate of the LiI adspecies can be different locally for the crystalline (i.e., granular) film because its thickness in not uniform. The ions descended from methanol decay at 160 K because of film evaporation. No residues are observable on the HOPG surface, in contrast with the results obtained using Ni(111). This behavior is ascribable to the substrate effect: The energetic He0 atoms, which are backscattered more efficiently from the heavier Ni(111) substrate, play a role in the secondary ion

Figure 4. Temperature-programmed TOF-SIMS intensities of Li+(CH3OH) ions from the polycrystalline LiI film covered by methanol films at different thicknesses. 17423

DOI: 10.1021/acs.jpcc.7b07389 J. Phys. Chem. C 2017, 121, 17421−17428

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The Journal of Physical Chemistry C intensity increases steeply at ca. 140 K, even for the 250 ML methanol film. It is noteworthy that methanol is crystalline at this temperature. The result is consistent with the uptake behaviors of submonolayer LiI additives deposited on the glassy methanol film, as displayed in Figure 2a. Results show that a liquid-like state formed at 140 K exhibits higher diffusivity and uptake rate of LiI than that formed at Tg. The interaction between the solid LiI and methanol films was also investigated through the desorption kinetics. The TPD spectra of m/e = 32 species (corresponding to CH3OH) are displayed in Figure 5 using methanol films (50 ML) deposited

Figure 6. Temperature evolution of the OH stretching band for the CH3OH film (250 ML) deposited onto the polycrystalline Au substrate at 90 K.

Figure 5. TPD spectra of m/e = 32 species desorbed from methanol films (50 ML) deposited on the Ni(111) substrate and the polycrystalline LiI film.

on the solid LiI and Ni(111) substrates. The molecules evaporated from Ni(111) form a main peak at 150−160 K; a small peak from residues is also identifiable at 190 K. This result is consistent with the observation using TOF-SIMS in Figure 1, where the secondary ion intensities from residues are enhanced considerably because of the Ni substrate effect. For the solid LiI film, the main peak of physisorbed methanol at 150 K has lower intensity. Additional peaks are identifiable at 200, 230, and 245 K. Their peak positions and relative intensities depend on the initial film thickness of methanol (not shown). In any case, dominance of the high-temperature peaks indicates that methanol molecules in the multilayer regime tend to be bound tightly to LiI via the formation of complexes. Figure 6 shows RAIR spectra obtained using a methanol (250 ML) film deposited onto Au. A broad band of OH stretching vibration (3100−3400 cm−1) is observed after methanol deposition at 90 K. Two broad peaks centered at around 3290 and 3200 cm−1 are distinguishable. The IR absorption band is unchanged before and after the glass−liquid transition at 103 K until crystallization occurs at 120 K. At this temperature, the band narrows and the peak is shifted to 3300 cm−1. Figure 7 depicts the OH stretching band obtained using the 250 ML methanol film deposited on the solid LiI film as a function of temperature. Methanol crystallization occurs at 120 K, as revealed from narrowing of the peak. However, the peak is broadened again at ∼140 K with its leading edge shifted to the higher wavenumber side. At this temperature, the broad OH stretching band, which is characteristic of the amorphous phase, becomes dominant relative to the narrow peak of crystals. The hydrogen bond of liquid methanol is expected to be weakened by interactions with incorporated LiI relative to that of pure crystalline methanol, thereby causing the blue shift of the OH stretching vibration frequency. Consequently, a methanolic

Figure 7. Temperature evolution of the OH stretching band for the CH3OH film (250 ML) deposited onto the polycrystalline LiI film at 90 K.

solution of LiI is probably formed transiently in this temperature region. This temperature corresponds well to that for the abrupt uptake and mixing of LiI, shown, 17424

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created in the film interior without influence of the free surface. No dead layer is formed at the methanol−LiI interface. The film morphology changes after crystallization at Tc = 120 K because of grain growth. The crystalline methanol melts at Tm = 176 K. However, the normal liquid phase is not accessible in the present study because methanol evaporates up to ca. 160 K under the UHV condition. Despite this fact, the liquid-like methanol is identifiable after crystallization through interactions with LiI additives. The additives with coverage as small as 0.2 to 0.3 ML do not influence the crystallization or melting kinetics of the 250 ML thick methanol. They tend to be incorporated into the film interior at T > 140 K, although no liquid-like phase is distinguishable from crystals at this temperature in the IR absorption spectra of pure methanol deposited on Au. In this study, polycrystalline methanol films comprising nanometric-size grains are probably created because the supply of molecules is limited at Tc via viscous flow of supercooled liquid on the substrate. The occurrence of film dewetting and the presence of the Ni+(CH3OH) species in Figure 1 indicate that the crystal grains and monolayer patches of methanol coexist on the Ni(111) substrate. The liquid-like phase occurs after crystallization because of premelting. In general, a quasiliquid layer formed during premelting is expected to be correlated strongly with the underlying crystal grains. It can be regarded as a layer with large-amplitude vibrations at the free surface of crystallites.11 Molecular dynamics (MD) simulations14 have revealed that a few atoms popped out from the surface of free clusters. They migrate as “floaters” on the surface during premelting. The liquid-like shape fluctuations of free clusters are thought to be induced by such floaters and counter holes created in the surface layer. For supported crystallites, quasi-liquid is expected to spill over the substrate as well, thereby forming a wetting monolayer of liquid-like species. On the contrary, grain boundaries are known to comprise an array of dislocations with equilibrium thickness and a structure that is less ordered and less dense than the crystal phase.48 They were originally presumed as some form of amorphous or liquid layer between the grains; however, it is now considered that properties of grain boundaries are determined by the misorientation of the two grains and the plane of the interface.47 In any case, it is apparent that mobile species are created at grain boundaries in addition to the free surface. In fact, the crystal grains can grow with increasing temperature via coalescence of crystallites or the movement of grain boundaries. This growth occurs because smaller grains are expected to have smaller Tm. After scattered grains are formed via coalescence, they continue to grow via viscous flow of molecules on the substrate surface (Ostwald ripening). In the IR absorption spectra, a broad, liquid-like band is not identifiable after crystallization (see Figure 6), perhaps because quasi-liquid formed at grain boundaries is spectroscopically indistinguishable from crystals. Alternatively, the IR sensitivity to the liquidlike species with a monolayer thickness is poor relative to the crystal grains. Using TOF-SIMS, the presence of mobile species in the grain boundaries (Figure 2a) and on the crystallite surface (Figure 2b) is identifiable through the increased uptake rate of LiI at T > 140 K. Liquid-like layers are known to evolve near bulk Tm (T > ∼0.9 Tm) on the surface of macroscopic systems.21−25 When temperature rises toward Tm, the surface layer structure approaches that of bulk liquid phase. Its thickness diverges upon reaching Tm. However, the melting behaviors of crystallites observed here are apparently distinct from this

respectively, in Figures 2a and 4. It is therefore confirmed that a liquid-like methanol is formed after crystallization. Two narrow peaks evolve from the broad band at ca. 3300 and 3400 cm−1 at temperatures higher than 145 K, indicating that phase separation occurs during recrystallization of the solution. The former corresponds to the pure methanol crystal (see Figure 5), so that it evaporates up to 160 K. The latter is assignable to crystalline methanolate of LiI. This species remains at temperatures higher than 200 K under the UHV condition. The RAIR spectra are also presented in Figure 8, obtained using a thinner methanol film (10 ML) deposited on solid LiI.

Figure 8. Temperature evolution of the OH stretching band for the CH3OH film (10 ML) deposited onto the polycrystalline LiI film at 90 K.

In this case, no peak of crystalline methanol occurs at 120 K from the broad band of amorphous methanol. The blue-shifted band is observed at 120−130 K because of the formation of the methanolic solution of LiI. Then, the crystalline methanolate of LiI starts to form at temperatures higher than ca. 140 K without the occurrence of the pure crystalline methanol. The crystalline methanolate of LiI dissociates at 200 K because methanol is liberated, in agreement with the TPD result in Figure 5.

4. DISCUSSION The occurrence of liquid-like methanol from the initially glassy film is manifested not only by self-diffusivity (Figure 1) but also by uptake of LiI adspecies in the film interior (Figure 2a). However, their onset temperatures are apparently lower than Tg = 103 K because surface mobility of methanol commences at ca. 80 K,47 thereby incorporating adspecies in the subsurface site in the sub-Tg region. In contrast, the dissolution of LiI from the substrate interface occurs fundamentally at Tg, as revealed from the experimental results in Figures 3 and 4. This result occurs because the uptake is induced by supercooled methanol 17425

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The Journal of Physical Chemistry C behavior in terms of the onset temperature (140 K or 0.8 Tm) and melting layer thickness (∼1 ML) because they are characteristic of the premelting of nanoparticles and their grain boundaries.14−20 The submonolayer LiI additives incorporated in supercooled methanol are thought to be accumulated in grain boundaries upon crystallization. They are delivered to deeper positions at T > 140 K (Figure 2) because quasi-liquid is formed at boundaries of crystal grains. Premelting also occurs at the interface of crystallites with solid LiI. The quasi-liquid layer dissolves in LiI at ca. 140 K, forming a concentrated LiI solution, as revealed in Figure 7. The uptake of methanol continues until crystal grains disappear, indicating that the quasi-liquid layer is equilibrated with crystal cores of methanol. The formation of crystalline methanol is quenched by the thinner methanol film (10 ML; see Figure 8), suggesting that quasi-liquid is formed immediately after crystallization at Tc = 120 K. These results imply that the premelting temperature can be reduced to Tc for smaller grains descended from thinner films. In this respect, the melting temperatures of clusters are known to exhibit large size-dependent fluctuations, so that their melting transitions are broad.31−34 The morphology of a pure methanol film (50 ML) deposited onto Ni(111) is also revealed to change gradually after crystallization under an isothermal condition at 120 K.49 This result implies that smaller grains melt at 120 K and that they tend to coalesce into larger ones via the molecular diffusion at grain boundaries. Consequently, premelting of crystalline methanol is not induced by interaction with LiI. The supercooled methanol formed at Tg dissolves LiI additives, but it does not develop into concentrated LiI solution even when a large amount of LiI is present, as revealed from the RAIR spectra in Figure 7. The hydrogen-bond structure of methanol after mixing with LiI is fundamentally identical to that of the pure methanol in the amorphous phase, as revealed from invariance of the OH stretching band. Consequently, only a dilute LiI solution can be formed for supercooled methanol on the solid LiI film. The solvation ability of supercooled methanol is low relative to normal liquid methanol. In contrast, quasi-liquid methanol formed after crystallization resembles normal liquid methanol in terms of the formation of concentrated LiI solution. It is known that supercooled liquid is spatially and dynamically heterogeneous:1 Correlated motion of groups of neighboring particles produces domain structures. The overall fluidity is given by an average of the dominant caged domain and the minor domain exhibiting high fluidity. Because of the coexistence of liquid-like and solidlike domains in the deeply supercooled region (T < 1.2 Tg), decoupling occurs between self-diffusion coefficient and viscosity embodied in the Stokes−Einstein equation. The methanol molecules move forming clusters or strings in liquidlike domains. For that reason, the solvation ability of LiI by supercooled methanol is likely to be deficient. These behaviors contrast markedly against the interaction of water with LiI, where the LiI uptake by water is almost quenched at Tg < T < Tc.40 This occurs because a distinct liquid with local hydrogenbond structures resembling those of crystalline ice is created in this temperature range as a result of polyamorphism.44,50 The additives are incorporated steeply during water crystallization because supercooled liquid might be formed transiently.40,45 No such behavior is observed at all in the case of methanol. Consequently, the inefficient uptake of LiI in methanol is explainable as the spatially heterogeneous dynamics of

supercooled liquid rather than the occurrence of a distinct liquid. The ensembles of crystallites are also heterogeneous, having solid and liquid domains at temperatures higher than the premelting temperature, but their ability for LiI uptake is much higher than that of supercooled liquid and resembles that of normal liquid methanol. Probably, the molecules at grain boundaries can diffuse independently without forming clusters or tagged species, thereby enabling the reconstruction of molecules for the solvation of larger amount of LiI. It is also possible that the mobile region of crystal grains has much faster relaxation time and lower viscosity than even the fastest regions in the supercooled methanol because the overall dynamics in the deeply supercooled region can be orders of magnitude slower because of non-Arrhenius kinetics. The quasi-liquid layer is also formed at the interface with metal substrates, as manifested by interactions of thin methanol films with a deoxygenated V substrate.51 Almost complete decomposition of a 10 ML methanol film occurs on V, as evidenced by the fact that hydrogen molecules desorb predominantly relative to methanol. The hydrogen desorption commences at Tc = 120 K because quasi-liquid is formed immediately after crystallization, as demonstrated here. The decomposition continues until the multilayer methanol is consumed completely, not only because the quasi-liquid layer is supplied from equilibrated crystal cores but also because the fragments including carbon and oxygen atoms can be incorporated continually into the V substrate. Consequently, the underlying mechanism of multilayer methanol decomposition on V is fundamentally identical to that of the formation of the concentrated solution on LiI, in which the continual uptake of quasi-liquid methanol or its fragments in the substrates plays a decisive role.

5. CONCLUSIONS Interactions of thin methanol films with LiI were investigated in terms of the glass−liquid transition and premelting using TOFSIMS, TPD, and RAIRS. The supercooled methanol was formed in the film interior at Tg = 103 K, but the CD3OD and LiI adspecies on the surface tended to incorporate into the film at lower temperatures because surface mobility occurs in the sub-Tg region. The solid LiI tends to dissolve in supercooled methanol at the substrate interface at temperatures higher than Tg without the formation of a dead layer. The OH stretching band was found to be almost unchanged until crystallization occurred at Tc = 120 K, indicating that the amount of LiI incorporated in supercooled methanol was small. The film morphology changed during crystallization because of the grain growth. The LiI additives incorporated from the surface into the 250 ML methanol film were probably accommodated in grain boundaries after crystallization. Subsequently, they became mobile at 140 K because of grain boundary diffusion. The crystalline methanol also dissolved completely in solid LiI at this temperature, thereby resulting in a concentrated LiI solution that exhibited a blue-shifted OH stretching band. This result was obtained because the premelting layers and crystal cores were equilibrated. The premelting temperature decreased for thinner methanol films because smaller grains were formed via viscous flow of supercooled liquid on the substrate. In fact, the 10 ML methanol premelted at 120 K formed the concentrated LiI solution without apparent grain growth. Consequently, the properties of the liquid-like methanol formed during premelting were found to be distinct from 17426

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those of supercooled methanol in interactions with LiI. Correlated motion of molecules or the formation of domain structures in supercooled methanol might be responsible for the poor solubility of LiI.

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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.

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DOI: 10.1021/acs.jpcc.7b07389 J. Phys. Chem. C 2017, 121, 17421−17428