Influences of 1-Propanol and Methanol Additives on Crystallization of Thin Amorphous Solid Water Films.

In general, randomly oriented ice crystallites are formed by heating amorphous solid water (ASW) films at ∼160 K via homogeneous nucleation. Here, we demonstrate that monolayers of methanol and 1-propanol additives incorporated in the multilayer ASW film lead to heterogeneous nucleation at the substrate interface of Pt(111), as evidenced by the occurrence of epitaxial ice growth. The mobility of water in direct contact with the Pt(111) substrate is decreased relative to that in the bulk, but it can be increased via interactions with hydrophobic moieties of alcohols that are segregated to the interfacial region. As a result, heterogeneous nucleation occurs at ca. 160 K along with homogeneous nucleation in the film interior. However, the template effect is quenched when the alcohols are in direct contact with the substrate. The methanol adspecies deposited onto the ASW film surface induces heterogeneous nucleation at a temperature as low as 145 K, but the 1-propanol adspecies has no such an effect. Their different ability of heterogeneous nucleation at the free ASW film surface, as well as their uptake behaviors in the near surface region, is associated with the hydrophobic hydration of the alcohols resulting from different lengths of the aliphatic moiety.

A temperature programmed desorption study of interactions between water and hydrophobes at cryogenic temperatures.

It is considered that hydrophobic solutes dissolve in water via the formation of icelike cages in the first hydration shell. However, this conventional picture is currently under debate. We have investigated how hydrophobic species, such as D2, Ne, Ar, Xe, CH4, and C3H8, interact with water in composite films of amorphous solid water (ASW) based on temperature programmed desorption (TPD). The D2 and Ne species tend to be incorporated in ASW without being caged, whereas two distinct peaks assignable to the caged species are identifiable for the other solutes examined here. The low-temperature peak is observed preferentially for Ar and CH4 prior to crystallization. The hydrophobes are thought to be encapsulated in porous ASW films via reorganization of the hydrogen bond network up to 100 K; most of them are released in a liquidlike phase that occurs immediately before crystallization at ca. 160 K. The nature of hydrophobic hydration at cryogenic temperature appears to differ from that in normal water at room temperature because the former resembles crystalline ices in the local hydrogen-bond structure rather than the latter. No ordered structures assignable to clathrate hydrates were identified before and after crystallization.

Nucleation and growth of water ice on oxide surfaces: the influence of a precursor to water dissociation.

We have investigated how nucleation and growth processes of ice are influenced by interfacial molecular interactions on some oxide surfaces, such as rutile TiO2(110), TiO2(100), MgO(100), and Al2O3(0001), based on the diffraction patterns of electrons transmitted through ice crystallites under the experimental configuration of reflection high energy electron diffraction (RHEED). The cubic ice Ic grows on the TiO2(110) surface with the epitaxial relationship of (110)Ic//(110)TiO2 and [001]Ic//[11[combining macron]0]TiO2. The epitaxial ice growth tends to be disturbed on the TiO2(110) surface under the presence of oxygen vacancies and adatoms. The result is not simply ascribable to small misfit values between TiO2 and ice Ic lattices (∼2%) because ice grains are formed randomly on TiO2(100). No template effects are identified during ice nucleation on the pristine MgO(100) and Al2O3(0001) surfaces either. The water molecules are chemisorbed weakly on these surfaces as a precursor to dissociation via the acid-base interaction. Such anchored water species act as an inhibitor of epitaxial ice growth because the orientation flexibility of physisorbed water during nucleation is hampered at the interface by the preferential formation of hydrogen bonds.

Reflection high energy electron diffraction (RHEED) study of ice nucleation and growth on Ni(111): influences of adspecies and electron irradiation.

How interfacial molecular interactions influence nucleation and growth processes of water ice is explored using pristine, oxygenated, and CO-adsorbed Ni(111) substrates based on RHEED, together with the effects of high-energy electron irradiation on the crystallization kinetics. A monolayer of amorphous solid water deposited onto the pristine Ni(111) substrate crystallizes into ice Ic at ca. 150 K, whereas ice Ih (Ic) is formed preferentially during water vapor deposition at 135 K (125 K). The ice nucleation tends to be hampered on the oxygenated Ni(111) surface because of the hydrogen bond formation with chemisorbed oxygen, leading to the growth of randomly-oriented ice Ic crystallites via spontaneous nucleation. The amorphization and recrystallization of initially crystalline ices are observed during prolonged RHEED measurements at 20 and 70 K, respectively, signifying that high-energy electron irradiation has both thermal and non-thermal effects on the water phase transition. The epitaxial growth (non-epitaxial growth) of ice occurs during electron irradiation of amorphous solid water formed on the pristine and oxygenated Ni(111) substrates (CO-adsorbed Ni(111) substrate) even at 100 K (120 K) because nucleation and growth are initiated at the substrate interface (in the ASW film interior).

Crystallization kinetics of thin water films on Pt(111): effects of oxygen and carbon-monoxide adspecies.

This paper describes nucleation, epitaxial growth, and wettability of water on Pt(111) and how they are influenced by oxygen and carbon-monoxide adspecies, based on reflection high energy electron diffraction (RHEED), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and temperature-programmed desorption (TPD). Amorphous solid water deposited onto the pristine Pt(111) substrate crystallizes into ice Ih together with a 2D layer at 150 K, whereas ice Ic (stacking disordered ice or a mixture of ice Ic and Ih) is formed preferentially onto oxygenated Pt(111) (CO-adsorbed Pt(111)) at 155-160 K (150 K). The ice nucleation and epitaxial growth tend to be hampered on the oxygenated Pt(111) surface via hydrogen bond formation with chemisorbed oxygen. The CO-adsorbed Pt(111) surface is hydrophobic, as evidenced by the fact that water forms a complex with CO during evaporation of crystallites at 160-165 K. A disordered 2D layer remains on pristine Pt(111) up to 175 K, whereas an ordered 2D layer exhibiting the (√3 ×√3)R30° structure formed on oxygenated Pt(111) up to 200 K.

Crystallization kinetics of water on graphite.

Graphite is hydrophobic in nature, but the crystallization kinetics and dewetting transition of thin water films deposited onto graphite are distinct from those on typical hydrophobic substrates. To clarify the origin of these behaviors, we investigated the crystallization kinetics of thin water films on graphite in terms of the initial film thickness, deposition temperature, and template effects of adspecies based on reflection high-energy electron diffraction (RHEED) images; the film morphology change was analyzed using time-of-flight secondary ion mass spectrometry (TOF-SIMS). The water monolayer nucleates after surface diffusivity occurs at ca. 120 K; the nucleation temperature and time increase with increasing initial film thickness. Crystallites of cubic and hexagonal ices are formed, having preferred orientation [cubic (111) or hexagonal (001)] along the surface normal direction; their relative quantity depends on the initial film thickness and the way of crystallization. Randomly oriented crystallites finally grow via spontaneous nucleation when the film thickness exceeds 7-10 monolayers. The template ordering effects of graphite are quenched when a monolayer of ordered n-octane preexists at the substrate interface. The crystalline ice tends to wet the graphite substrate immediately after nucleation, and the film morphology changes gradually at 130 K because of premelting. The crystallites are ripened via molecular transport through the quasiliquid layer formed at the free surface, grain boundaries, and substrate.

Interactions of methanol, ethanol, and 1-propanol with polar and nonpolar species in water at cryogenic temperatures.

Methanol is known as a strong inhibitor of hydrate formation, but clathrate hydrates of ethanol and 1-propanol can be formed in the presence of help gases. To elucidate the hydrophilic and hydrophobic effects of alcohols, their interactions with simple solute species are investigated in glassy, liquid, and crystalline water using temperature-programmed desorption and time-of-flight secondary ion mass spectrometry. Nonpolar solute species embedded underneath amorphous solid water films are released during crystallization, but they tend to withstand water crystallization under the coexistence of methanol additives. The CO2 additives are released after crystallization along with methanol desorption. These results suggest strongly that nonpolar species that are hydrated (i.e., caged) associatively with methanol can withstand water crystallization. In contrast, ethanol and 1-propanol additives weakly affect the dehydration of nonpolar species during water crystallization, suggesting that the former tend to be caged separately from the latter. The hydrophilic vs. hydrophobic behavior of alcohols, which differs according to the aliphatic group length, also manifests itself in the different abilities of surface segregation of alcohols and their effects on the water crystallization kinetics.

Decomposition of multilayer benzene and n-hexane films on vanadium.

Reactions of multilayer hydrocarbon films with a polycrystalline V substrate have been investigated using temperature-programmed desorption and time-of-flight secondary ion mass spectrometry. Most of the benzene molecules were dissociated on V, as evidenced by the strong depression in the thermal desorption yields of physisorbed species at 150 K. The reaction products dehydrogenated gradually after the multilayer film disappeared from the surface. Large amount of oxygen was needed to passivate the benzene decomposition on V. These behaviors indicate that the subsurface sites of V play a role in multilayer benzene decomposition. Decomposition of the n-hexane multilayer films is manifested by the desorption of methane at 105 K and gradual hydrogen desorption starting at this temperature, indicating that C-C bond scission precedes C-H bond cleavage. The n-hexane dissociation temperature is considerably lower than the thermal desorption temperature of the physisorbed species (140 K). The n-hexane multilayer morphology changes at the decomposition temperature, suggesting that a liquid-like phase formed after crystallization plays a role in the low-temperature decomposition of n-hexane.

Interfacial reaction of water ice on polycrystalline vanadium and its effects on thermal desorption of water.

Thermal desorption and decomposition of water ice deposited onto a polycrystalline V surface were investigated using temperature-programmed desorption and secondary ion mass spectrometry. The water molecules in multilayer films dissociate preferentially at the interface, whereas water desorption from the surface is depressed considerably. The oxygen atoms (hydrogen molecules) formed at the interface are incorporated into the substrate (released into the gas phase) sequentially at temperatures higher than 140 K. The crystallization kinetics of water multilayers is not influenced by the interfacial reaction, but the water desorption rate is depressed by the interfacial reaction even after crystallization. Consequently, thermal desorption of water from the surface and its reaction at the interface are found to be correlated across thin films. This behavior is explainable as dynamic heterogeneity of water in the deeply supercooled region and premelting of metastable ice Ic, where mobile water molecules play a dominant role in both thermal desorption and the interfacial reaction.

On sub-T(g) dewetting of nanoconfined liquids and autophobic dewetting of crystallites.

The glass transition temperature (T(g)) of thin films is reduced by nanoconfinement, but it is also influenced by the free surface and substrate interface. To gain more insights into their contributions, dewetting behaviors of n-pentane, 3-methylpentane, and toluene films are investigated on various substrates as functions of temperature and film thickness. It is found that monolayers of these molecules exhibit sub-T(g) dewetting on a perfluoro-alkyl modified Ni substrate, which is attributable to the evolution of a 2D liquid. The onset temperature of dewetting increases with film thickness because fluidity evolves via cooperative motion of many molecules; sub-T(g) dewetting is observed for films thinner than 5 monolayers. In contrast, monolayers wet substrates of graphite, silicon, and amorphous solid water until crystallization occurs. The crystallites exhibit autophobic dewetting on the substrate covered with a wetting monolayer. The presence of premelting layers is inferred from the fact that n-pentane crystallites disappear on amorphous solid water via intermixing. Thus, the properties of quasiliquid formed on the crystallite surface differ significantly from those of the 2D liquid formed before crystallization.

Adsorption, diffusion, dewetting, and entrapment of acetone on Ni(111), surface-modified silicon, and amorphous solid water studied by time-of-flight secondary ion mass spectrometry and temperature programmed desorption.

Interactions of acetone with the silicon surfaces terminated with hydrogen, hydroxyl, and perfluorocarbon are investigated; results are compared to those on amorphous solid water (ASW) to gain insights into the roles of hydrogen bonds in surface diffusion and hydration of acetone adspecies. The surface mobility of acetone occurs at ∼60 K irrespective of the surface functional groups. Cooperative diffusion of adspecies results in a 2D liquid phase on the H- and perfluorocarbon-terminated surfaces, whereas cooperativity tends to be quenched via hydrogen bonding on the OH-terminated surface, thereby forming residues that diffuse slowly on the surface after evaporation of the physisorbed species (i.e., 2D liquid). The interaction of acetone adspecies on the non-porous ASW surface resembles that on the OH-terminated Si surface, but the acetone molecules tend to be hydrated on the porous ASW film, as evidenced by their desorption during the glass-liquid transition and crystallization of water. The roles of micropores in hydration of acetone molecules are discussed from comparison with the results using mesoporous Si substrates.

Translational diffusion of cumene and 3-methylpentane on free surfaces and pore walls studied by time-of-flight secondary ion mass spectrometry.

Mobility of molecules in confined geometry has been studied extensively, but the origins of finite size effects on reduction of the glass transition temperature, T(g), are controversial especially for supported thin films. We investigate uptake of probe molecules in vapor-deposited thin films of cumene, 3-methylpentane, and heavy water using secondary ion mass spectrometry and discuss roles of individual molecular motion during structural relaxation and glass-liquid transition. The surface mobility is found to be enhanced for low-density glasses in the sub-T(g) region because of the diffusion of molecules on pore walls, resulting in densification of a film via pore collapse. Even for high-density glasses without pores, self-diffusion commences prior to the film morphology change at T(g), which is thought to be related to decoupling between translational diffusivity and viscosity. The diffusivity of deeply supercooled liquid tends to be enhanced when it is confined in pores of amorphous solid water. The diffusivity of molecules is further enhanced at temperatures higher than 1.2-1.3 T(g) irrespective of the confinement.

Structural relaxation of vapor-deposited water, methanol, ethanol, and 1-propanol films studied using low-energy ion scattering.

Glassy thin films of water, methanol, ethanol, and 1-propanol were prepared by deposition from the gas phase at 20 K. Relaxation of their surface and bulk structures was investigated by measuring temperature evolutions of H(+), H(-), and total yields in low-energy H(2)(+) scattering. The surface structure of a methanol film changes at temperatures of about 20 K below the glass transition temperature (T(g) = 103 K) because of enhanced diffusivity of molecules at the surface. Both surface and bulk structures change at around T(g) for ethanol, but the structure of the 1-propanol film is unchanged at temperatures higher than T(g), indicating that the vapor-deposited 1-propanol glass is more stable than the liquid-quenched one. These behaviors strongly suggest that the configurations of molecules in the vapor-deposited glass differ significantly from those in the liquid-quenched glass and that the aliphatic-chain length plays an important role in structural relaxation. The surface (bulk) structure of the water film changes gradually (is invariant) across T(g), suggesting that the relaxation and glass transition processes differ significantly between those of water and alcohols.

Roles of individual and cooperative motions of molecules in glass-liquid transition and crystallization of toluene.

Deeply supercooled fragile liquid is known to be dynamically heterogeneous, where super Arrhenius behavior of shear viscosity and alpha and beta relaxation processes have been observed. To clarify origins of these behaviors, we have investigated correlations between microscopic molecular diffusion and macroscopic hydrodynamics of vapor-deposited toluene films by using time-of-flight secondary ion mass spectrometry. The molecules are intermixed gradually at around 70 K on the film deposited at 15 K, which is followed by an abrupt film morphology change at around the calorimetric glass-transition temperature (T(g)) of 117 K. For the film deposited at 100 K, intermixing of the molecules occurs at temperatures over 100-130 K, but no abrupt increase in diffusivity is recognizable at T(g). This result can be explained as dynamical heterogeneity or decoupling between translational diffusion and viscosity. The self-diffusion is thought to occur in a sub-T(g) region as a precursor state of the glass-liquid transition; that is, individual motion in the solid-like state evolves into cooperative motion of liquid-like state at T(g). The supercooled liquid nucleates at 147 K, although the crystal can grow even at 117 K provided that nuclei preexist. The origins of unusual glass transition behaviors of amorphous solid water are also discussed in comparison with this standard.

Liquidlike nature of crystalline n-butane and n-pentane films studied by time-of-flight secondary ion mass spectrometry.

Crystallization of vapor-deposited thin films of n-butane and n-pentane has been investigated using temperature-programmed time-of-flight secondary ion mass spectrometry. The morphology of thin n-butane (n-pentane) films changes at around the calorimetric crystallization temperature of 65 K (85 K) as a result of crystallization of the supercooled liquid. The morphology of the crystal grains of n-butane changes at 85 K; the butane molecules permeate through porous amorphous-solid-water films above this temperature. The crystal grains of n-pentane are smaller in size than those of n-butane, forming a smoother crystalline film. However, the crystalline n-pentane film dewets abruptly at higher temperatures, depending on the film thickness. The liquidlike nature of crystalline n-pentane (n-butane) is attributable to premelting (coexisting second liquid).

Glass transition and crystallization dynamics of thin CCl(2)F(2) films deposited on Ni(111), graphite, and water-ice films.

The glass-liquid transition and crystallization of thin CCl(2)F(2) films, as well as the influence of substrates on the phase transition of a monolayer, have been investigated using temperature-programmed time-of-flight secondary ion mass spectrometry. The multilayer films of CCl(2)F(2) dewet a Ni(111) substrate abruptly at 57 K, which is explainable as immediate crystallization of supercooled liquid. The morphology of the crystalline CCl(2)F(2) film changes at 85 K; the molecules permeate through porous D(2)O films at temperatures higher than 70 K. These behaviors can be explained as the evolution of a second liquid or premelting of crystallites. The monolayer of CCl(2)F(2) formed on graphite undergoes a phase transition similar to that of the multilayer films, whereas the phase transition is quenched for the monolayer formed on the Ni(111) substrate. The phase transition of the CCl(2)F(2) monolayer formed on the D(2)O film is influenced by crystallinity, thickness, and morphology of the latter.

Phase transition of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide thin films on highly oriented pyrolytic graphite.

Thin glassy films of a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium bis[trifluoromethylsulfonyl]imide ([emim][Tf(2)N]), were deposited by thermal evaporation onto a substrate of highly oriented pyrolytic graphite. Their crystallization and fusion kinetics are discussed on the basis of results of time-of-flight secondary-ion mass spectrometry (TOF-SIMS) by measuring sputtered secondary-ion intensities as a function of temperature. Multilayer films crystallize at 205 K and then fuse at 255 K, as determined from temperature-programmed TOF-SIMS measurements, whereas crystallization occurs at around the glass-transition temperature (175-180 K) within 10 min, as shown by isothermal TOF-SIMS measurements. The ionic pairs in the [emim][Tf(2)N] monolayer tend to align over a wide temperature range of 180-220 K and retain crystal-like alignment up to 285 K. The weak van der Waals interaction at the interface is thought to be prerequisite for the aligned monolayer formation, because the ionic pairs on a Ni(111) substrate tend to be disordered. Thus, it is demonstrated that the alignment and wettability of the first monolayer, as well as the crystallization and fusion kinetics of thin films, are influenced by the substrate.

Comparative study of electron stimulated positive-ion desorption from LiCl and 1-ethyl-3-methylimidazolium bis[trifluoromethylsulfonyl]imide.

The mechanism of electron stimulated desorption (ESD) from LiCl has been investigated in comparison with that from a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium bis[trifluoromethylsulfonyl]imide, [emim][Tf(2)N]. The bonding natures of these materials are discussed based on the matrix effect in positive-ion yields. The [emim](+) and fragment ions are emitted from the [emim][Tf(2)N] molecule unless it is in direct contact with a metal surface, suggesting that the ions are emitted provided that the electronic excitation can be localized in each molecule. In contrast, the electronic excitation tends to be delocalized over the LiCl film, as evidenced by a monotonic increase of a Li(+) yield in the multilayer regime. The Li(+) ion is created via gas-phase ionization of desorbed neutrals or emitted directly from the surface, in which self-trapped excitons or hot carriers created in the bulk play a role. The Li(+) and Li(+)(LiCl) ions are emitted efficiently from LiCl nanoclusters formed on a rare-gas solid film via Coulombic fission. The delocalized nature of hot holes is also manifested by steep decay of the ion yields with increasing LiCl coverage. The structural transformation of [emim][Tf(2)N] during the phase transition is also revealed based on ESD positive-ion yields as a function of temperature.

Matrix effects on secondary ion emission from a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium bis[trifluoromethanesulfonyl]imide.

The ionization mechanism of room-temperature ionic liquids has been investigated using time-of-flight secondary ion mass spectrometry in the temperature range of 15-300 K. Analyses of 1-ethyl-3-methylimidazolium bis[trifluoromethanesulfonyl]imide ([emim][Tf(2)N]) deposited on a Ni(111) substrate revealed that the [emim](+) and [Tf(2)N](-) yields increase together with the Ni(+) yield at monolayer coverage; no such increase was observed for the films deposited on a D(2)O spacer layer. Results indicated that the [emim][Tf(2)N] molecule is not perfectly ionized; the Ni(111) surface accepts (for [emim](+)) or donates (for [Tf(2)N](-)) an electron with higher efficiency than the counterion because of the metal band effect. This phenomenon might be induced by electrostatic interactions between the separated cation and anion during sputtering. It is also suggested that the sputtered Ni atom can be ionized nonadiabatically by the formation of a quasimolecule with adspecies. The multilayer of [emim][Tf(2)N] deposited at 15 K has a porous structure, resembling that of polar molecules, because of nonionic intermolecular interactions. The phase transition is identifiable, together with the morphological change in the crystalline film, from temperature evolutions of the secondary ion yields.

Temperature-programed time-of-flight secondary ion mass spectrometry study of 1-butyl-3-methylimidazolium trifluoromethanesulfonate during glass-liquid transition, crystallization, melting, and solvation.

For this study, time-of-flight secondary ion mass spectrometry was used to analyze the molecular orientation of 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim][OTf]) and its interaction with the adsorbed Na and LiI species at temperatures of 150-300 K. A glassy [bmim][OTf] film crystallizes at around 230 K, as observed from the increase in the [bmim](+) yield. LiI and Na adsorbed on the glassy film are solvated, whereas they tend to form islands on a crystalline film. The crystalline surface inertness is ascribable to the termination with the CF(3) and C(4)H(9) groups, whereas the exposure of polar SO(3) and imidazole groups at the glassy film results in the solvation. Surface layering occurs during solvation of LiI on the glassy film in such a way that the [bmim](+) ([OTf](-)) moiety is exposed to the vacuum (oriented to the bulk). The LiI adsorbed on the glassy film is incorporated into the bulk at temperatures higher than 200 K because of the glass-liquid transition. No further uptake of LiI is observed during crystallization, providing a contrast to the results of normal molecular solids such as water and ethanol. The surface layers of the crystal melt at temperatures below the bulk melting point, as confirmed from the dissolution of adsorbed LiI, but the melting layer retains a short-range order similar to the crystal. The [bmim][OTf] can be regarded as a strongly correlated liquid with the combined liquid property and crystal-type local structure. The origin of this behavior is discussed.