J. Phys. Chem. B 2008, 112, 15349–15354
15349
Glass-Liquid Transition, Crystallization, and Melting of a Room Temperature Ionic Liquid: Thin Films of 1-Ethyl-3-methylimidazolium Bis[trifluoromethanesulfonyl]imide Studied with TOF-SIMS Ryutaro Souda* Nanoscale Materials Center, National Institute for Materials Science 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ReceiVed: April 8, 2008; ReVised Manuscript ReceiVed: September 12, 2008
To discuss the relationship between liquid, crystalline, and glassy states of ionic liquids, TOF-SIMS was used to analyze the glass-liquid transition, crystallization, and melting of 1-ethyl-3-methylimidazolium bis[trifluoromethanesulfonyl]imide ([emim][Tf2N]) at the molecular level at temperatures of 150-280 K. The [emim][Tf2N] molecules can be deposited thermally on a Ni(111) surface without decomposition. LiI was adsorbed onto the thin film in order to investigate the glass-liquid transition; it was incorporated in deeper layers at temperatures higher than 180 K. Crystallization of the film at around 200-220 K was identifiable from the abrupt increase in the [emim]+ yield, which probably results from the steric effect of the structured cations and anions forming anisotropic bonds in a specific layered structure. The glass-liquid transition and crystallization of [emim][Tf2N] differ significantly from those of water and alcohol in terms of the morphological change of the film and the interaction with adsorbed LiI. This behavior might be explained by the absence of a liquid-liquid phase transition for [emim][Tf2N]. The vapor-deposited thin films (2.5 and 5.0 monolayers) crystallize at around 200 K, but they melt gradually at temperatures considerably lower than the bulk melting point (ca. 260 K) because of the evolution of a quasi-liquid layer and the disappearance of a crystal template. 1. Introduction Room temperature ionic liquids (ILs) have physicochemical properties that differ from those of common ambient-temperature liquids. This class of compounds might be useful for numerous applications such as clean solvents and catalysts for green chemistry,1 electrolytes for battery,2 and dye-sensitized solar cells.3 This enormous range of applications of ILs requires a deep understanding not only of their bulk properties but also of their interfacial behavior. Regarding the bulk structure of ILs, X-ray and neutron diffraction studies revealed that a considerable degree of order exists in the liquid phase;4,5 its local structure resembles that found in the crystalline state. On the other hand, thermodynamic studies have demonstrated that ILs are fragile glass formers, similar to normal molecular liquids.6,7 Indeed, the features resembling those of glass transition are observed for supercooled amorphous ILs using quasielastic neutron scattering.8 Therefore, further insight into the properties of ILs might be gained from detailed investigations of their glass-liquid transition and crystallization. This goal is the intention for the present study. A liquidlike phase is expected to be present in the deeply supercooled region above the glasstransition temperature (Tg). However, very little is known about its properties because of the occurrence of crystallization. To date, the glass-liquid transition and crystallization of vapordeposited amorphous solids have been investigated using temperature-programmed time-of-flight secondary ion mass spectrometry (TOF-SIMS).9-11 Therefore, the characteristics of the ILs are expected to be highlighted from comparison with previously reported results related to simple molecular solids. * E-mail:
[email protected].
The gas-liquid interface has also been studied extensively using ultrahigh-vacuum (UHV)-based techniques.12-18 A common notion expressed among these reports is that the cation and anion share the surface of pure ILs in their fluid states. However, the orientation of the ions at the gas-liquid interface remains controversial;19,20 little is known about the properties of the solid-liquid interface.21,22 Important problems in this respect are the molecular orientation in the first monolayer, surface diffusion, wettability of thin films, and substrate effects on the phase transition of thin films. Very recently, a conventional belief that ILs exert no measurable vapor pressure was challenged;23 the results of related studies demonstrated that many ILs can be distilled at low pressure without decomposition. This result strongly suggests that a thin IL film can be grown on a substrate from the vapor phase. The decomposition of ILs results from dealkylation or transalkylation of the cation by the attack of a nucleophilic anion.23 In this study, we specifically investigate 1-ethyl-3-methylimidazolium bis[trifluoromethanesulfonyl]imide ([emim][N(SO2CF3)2] or [emim][Tf2N]) because the bistriflimide anion is expected to have an extremely low nucleophilicity. We discuss the structural transformation of the thin films during crystallization and melting and the glass-liquid transition via the dissolution of adsorbed LiI species into the bulk at temperatures of 150-280 K using TOF-SIMS. 2. Experimental Section The experiments were performed in a UHV chamber with base pressure of less than 1 × 10-8 Pa. The TOF-SIMS spectra were taken using a combination of an electron-impact-type ion source and a linear TOF analyzer. A primary He+ ion beam was chopped with electrostatic deflection plates into a pulse of 40 ns in width and 15 kHz in repetition rate. The positive
10.1021/jp805120m CCC: $40.75 2008 American Chemical Society Published on Web 11/08/2008
15350 J. Phys. Chem. B, Vol. 112, No. 48, 2008 secondary ions were extracted into a field-free, linear TOF tube by applying a bias voltage (+500 V) to the sample and placing a grounded stainless steel mesh close to the sample surface. The secondary ions were detected using a channel electron multiplier and were pulse-counted using a multichannel scaler with a time resolution of 16 ns. The mass resolution of ca. 70 was attained using this simple TOF-SIMS setup. The temperature-programmed TOF-SIMS spectra were taken continually every 30 s at a ramping speed of ca. 4 K/min. All spectra were acquired using a total ion dose of less than 1012 ions/cm2 to avoid sample surface damage. The [emim][Tf2N] sample was purchased in high purity quality (99.9%, including 28.0 ppm of H2O). For further purification, the sample was dried under vacuum and stirring conditions for 12 h at a temperature of 80 °C to a water content of less than 3 ppm. Thin films of [emim][Tf2N] were prepared in two ways: a spin-coated film with submicron thickness deposited on a polycrystalline Ni plate and in situ deposited thin films on the clean Ni(111) substrate in UHV. The former was introduced into the UHV chamber via a load-lock system that was evacuated using oil-free pumps. A liquid film of [emim][Tf2N] is expected to be formed initially because its melting temperature is -16 °C. It was vitrified by inserting the Ni plate into the sample holder that was cooled to 150 K using a closed-cycle helium refrigerator. LiI was evaporated on this film from a resistively heated Ta boat placed in front of the surface. The spin-coated film charged up at temperatures of less than 170 K, so that the TOF-SIMS measurements were made at temperatures higher than this. The Ni(111) substrate was cleaned by heating to 1200 K in UHV using electron bombardment from behind. The [emim][Tf2N] molecules were evaporated from the Ta boat onto the clean Ni(111) surface that was maintained at 150 K. The coverage of the adsorbates was estimated from the evolution curves of secondary ion yields as a function of the deposition time. The sample temperature was determined as an average of the sample holder’s temperatures measured at two points using Au-Fe chromel thermocouples. 3. Experimental Results The TOF-SIMS spectra from the [emim][Tf2N] molecules adsorbed on the Ni(111) surface are typically presented in Figure 1; the spectra that are obtained at deposition times of (a) 0, (b) 3, and (c) 14 min are compared to the spectrum from the spincoated thin film (d). They were taken at temperatures of 150 K (a-c) and 170 K (d). The cleaned Ni(111) surface after flash heating exhibits no secondary ions. Small secondary ion peaks in Figure 1a come from contaminants adsorbed from the vacuum during the cooling process; no indications of other contaminants, such as C and S, are recognizable in the present study. Upon adsorption of the [emim][Tf2N] molecules, various fragment ions are sputtered in addition to the intact [emim]+ cation. Smaller ions, such as CH3+, C2H3+, and C2H5+, are typical fragment ions from alkanes; they are expected to originate from the methyl and ethyl groups of the [emim]+ moiety. The fragment ions including the N atom are also sputtered, together with larger fragment ions having an imidazolium ring (indicated with asterisks). Very few cations are emitted from the [N(SO2CF3)2]anion, except for the F+, CF+, and CF3+ ions. The [emim]+ ion peaks at 111 amu, but tails are recognizable toward both shorter and longer TOF sides; they are caused by the fragment ions or ion adducts of [emim]+ with the loss or gain of a few hydrogen atoms; they cannot be separated completely in the present experiment because of the limited ion mass resolution. We integrated the peak area for ion mass of 105-113 amu and
Souda
Figure 1. TOF-SIMS spectra of vapor-deposited thin films of [emim][Tf2N] formed on the clean Ni(111) surface (a-c) and the spincoated thin film (d). The spectral change during deposition at time of (a) 0, (b) 3, and (c) 14 min is typically shown. Measurement was made at 150 K (a-c) and 170 K (d). A schematic of the [emim][Tf2N] molecule is shown in the inset.
used it as the [emim]+ yield. It is noteworthy that the [emim]+ yield is enhanced considerably relative to the fragment ion yields at the low coverage regime (see Figure 1b). Moreover, the Ni+ ion, which is almost absent from the clean Ni(111) surface, becomes clearly recognizable upon adsorption of the molecules. With increasing coverage, the Ni+ ion disappears and the [emim]+ yield decreases. The fact that the TOF-SIMS spectrum from the vapor-deposited sample (c) is identical to that from the thicker, spin-coated film (d) clearly indicates that the [emim][Tf2N] molecules can be evaporated without decomposition. Figure 2 shows evolutions of typical secondary ion intensities as a function of the deposition time of the [emim][Tf2N] molecules on the Ni(111) substrate. The measurement was made at 150 K. The [emim]+ and Ni+ intensities exhibit a maximum at 3 min and then decrease with increasing coverage. No such behavior is observed for the fragment CH3+ and C2H5+ ions. The secondary ions are emitted from the surface or subsurface regions; their intensities are strongly dependent on the ionization and neutralization processes that are influenced by the chemical environment or local structure of the parent species. In fact, the Ni+ ions are not emitted from the clean surface because they undergo efficient resonance neutralization. The Ni+ ions are formed during collisions of the sputtered Ni atoms with the [emim][Tf2N] molecules, but their emission is disturbed when the film thickens, thereby forming a maximum in the Ni+ intensity as a function of coverage. The yields of the other ions tend to be saturated when the substrate effect disappears by the formation of a multilayer. We tentatively assume that one monolayer (1 ML) is attained at the deposition time of 3 min,
Thin Films of [emim][Tf2N]
Figure 2. Intensities of typical secondary ions sputtered from the [emim][Tf2N] adsorbed Ni(111) surface as a function of the deposition time. The molecule was deposited on the surface at temperature of 150 K.
where the Ni+ yield exhibits a maximum. Actually, the CF3+ ion appears only after the multilayer is formed. The same is true for the CF+ ion, but the F+ ion is sputtered from the submonolayer coverage regime as well (see Figure 1b). The F+ ion is sputtered via the potential energy of the primary He+ ion (potential sputtering), whereas the other secondary ions are formed during collisions (kinetic sputtering). The F+ yield is known to be enhanced from the chemisorbed species on the transition metal surface.24 The [emim]+ yield increases when the [emim][Tf2N] species is in direct contact with the Ni(111) surface because the metal surface plays a better role as an electron acceptor than the [Tf2N]- anion. In fact, no enhancement of [emim]+ occurs when [emim][Tf2N] is adsorbed on a spacer layer of water formed on the Ni(111) surface (not shown) because electron tunneling to the metal conduction band state is prohibited. The intensities of the typical secondary ions from the vapordeposited [emim][Tf2N] films are measured as a function of temperature (temperature-programmed TOF-SIMS). Figure 3 displays the experimental result obtained using the initially glassy film formed at 150 K with thickness of 2.5 ML. Upon heating, the [emim]+ intensity increases at temperatures higher than 200 K. The Ni+ yield increases gradually and peaks at around 250 K, indicating that the film morphology changes with increasing temperature. From the analogy of the result presented in Figure 2, it might be presumed that the increase in the [emim]+ yield is induced by the evolution of the monolayer patches as a result of the morphological change of the film. However, this possibility is denied because the evolutions of Ni+ and [emim]+ ions are not mutually correlated and the CF3+ yield is not changed at all. We ascribe the increase in the [emim]+ yield to crystallization because glassy [emim][Tf2N] is known to crystallize at around 210 K.25 The formation of a specific layered structure of crystalline [emim][Tf2N] is thought
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Figure 3. Temperature-programmed TOF-SIMS intensities from the vapor-deposited [emim][Tf2N] film. 2.5 ML of the molecules was deposited on the Ni(111) surface at 150 K. The temperature was increased at a rate of 4 K min-1.
Figure 4. Same as in Figure 3, but for the [emim][Tf2N] film with thickness of 5 ML.
to be responsible for the enhanced [emim]+ emission, as will be discussed later. The same measurements were made using a vapor-deposited film with 5 ML thickness; the results are portrayed in Figure 4. The [emim]+ yield increases at 200-205 K because of crystallization. The gradual decay of the [emim]+ yield at temperatures of 210-255 K might result from melting of the crystal.
15352 J. Phys. Chem. B, Vol. 112, No. 48, 2008
Souda 260 K due to melting. This phenomenon is most likely explainable in terms of surface melting. Surface melting is now known to occur in many systems including Pb(110)28 and molecular solids.29-31 4. Discussion
Figure 5. Temperature-programmed TOF-SIMS intensities from the [emim][Tf2N] film on which LiI (ca. 0.2 ML) was adsorbed at 170 K. The glassy film was prepared by vitrification of the spin-coated thin film at 150 K.
Compared to the thinner film (Figure 3), the rise and drop of the [emim]+ yield during the phase transition are steeper for the thicker film. The Ni+ ion is detected at temperatures higher than 240 K where the crystalline film melts. The film morphology is not altered during crystallization at around 200 K. This behavior contrasts sharply to the results of the simple molecular solids,9,10 in which the droplets are formed prior to or during crystallization. The morphological change of the film has been interpreted as the formation of a fluidized liquid phase prior to crystallization. However, this seems not the case for [emim][Tf2N]. The absence of dewetting during crystallization suggests that the [emim][Tf2N] film crystallizes directly from the glassy state without forming any liquid phases. In reality, however, the liquid phase is characterized not only by the cooperative motion of many molecules, leading to the fluidity and morphological change of the film, but also by the self-diffusion of individual molecules. To date, the latter has been explored extensively for amorphous molecular solids using TOF-SIMS based on the uptake of the adsorbed species into the bulk.9-11 Figure 5 shows the temperature evolutions of the typical secondary ions from the spin-coated [emim][Tf2N] film on which ca. 0.2 ML of LiI was deposited at 170 K. The amount of the adsorbed LiI species can be reduced, so that the solvation effect is minimized. In fact, the TOF-SIMS result in Figure 5 is fundamentally identical to that for the pure [emim][Tf2N] film except for Li+.26 The Li+ ion decreases in intensity at temperatures higher than 180 K, where the LiI species are incorporated in the deeper layers of the film. This phenomenon is explainable as self-diffusion of the molecules because Tg of [emim][Tf2N] is most likely to be 175 K.27 The Li+ yield decreases until the crystallization is completed at around 220 K. It is therefore confirmed that a liquidlike phase which is characterized by self-diffusion without apparent fluidity is formed prior to crystallization. The Li+ intensity decreases at 253 K before the [emim]+ yield drops at
In this study, we demonstrate that the analysis of the secondary ion yields as a function of the coverage and temperature reveals the glass-liquid transition, crystallization, and melting of [emim][Tf2N]. Regarding the vapor deposition experiment (Figure 2), the [emim]+ yield increases considerably up to the formation of the first monolayer. For the cation species with a small ionization potential, the metal surface acts as a better electron acceptor than the molecular anion because the electron can be transferred resonantly to the open conduction band state (the band effect): the diffusion of the electron into the bulk results in a higher ionization probability than the localization in the counteranion site because the transferred electron never returns to [emim]+. On the other hand, the enhanced [emim]+ yield during crystallization of multilayer films must be explained by the other mechanism. Crystallographic data show that a specific ABAB... type layered structure is formed in the bulk of [emim][Tf2N];25 each layer contains an equal number of cations and anions located as mutually adjacent, thereby ensuring charge neutrality. Probably, the crystalline [emim][Tf2N] surface is terminated with such a layer. In contrast to the simple ionic salts, both anion and cation of ILs are nonspherical. For that reason, their interaction is highly anisotropic, especially for the layered crystalline system discussed here. Therefore, the steric effects are thought to be responsible for the enhanced ion emission probability from the crystalline film relative to the glassy and liquid films.11,26 The imidazolium ring of the crystal is parallel to the plane and is hydrogen-bonded to oxygens of the adjoining anion in the same plane, whereas the interlayer interaction is thought to be ionic between the imidazolium ring and the SO2 moiety of the [N(SO2CF3)2]- anion having a cisoid conformation (see the inset of Figure 1; also refer to Figure 3 of ref 23 for the crystal packing). It is known that hydrogen bonding has some covalency by which the ionic hole can be delocalized,32,33 thereby reducing the ionization probability. From the crystalline [emim][Tf2N] surface, the [emim]+ cation can be emitted perpendicular to the surface via a highly ionic collision with the second-layer anion. On the other hand, the glassy and liquid surfaces contain nonparallel imidazolium rings, so that hydrogen bonding is reinforced during collisions. This reinforcement is thought to be responsible for the reduction of the [emim]+ yield from the disordered system. Next we must address the effects of the Ni(111) substrate on the phase transition of the thin films. The crystallization temperature, as estimated from the jump of the [emim]+ yield, is higher for the thicker spin-coated film than the vapordeposited film. This phenomenon can be ascribed to the kinetics of crystallization for the nonequilibrium amorphous materials; the thicker film requires a longer aging time for crystallization to occur. It is also possible that the structural transformation of the top surface layer is delayed because of subsurface freezing, as proposed for homogeneous nucleation of water.34 The substrate effect is recognizable more clearly in the melting behavior of the crystal. The melting point of the spin-coated film (Figure 5) is identifiable as a sharp drop of the [emim]+ yield, whereas that of the vapordeposited films (Figures 3 and 4) is difficult to assign because the [emim]+ yield decreases gradually. The melting behavior
Thin Films of [emim][Tf2N] estimated from the evolution of the Ni+ yield is also gradual, as is the typical situation depicted in Figure 3. Consequently, the crystallinity of the thinner film is thought to be poor even at temperatures considerably lower than the bulk melting point. The thin crystal layer is strained because of the misfit with the substrate lattice. It is also likely that the disordering of the thin film is related to surface melting: the surface layer of crystals is known to melt gradually prior to the bulk melting point because of the formation of quasi-liquid.28-31 The quasi-liquid layer is highly correlated with the underlying crystal; the molecules are thought to jump among lattice sites. Indeed, the fact that the [emin]+ yield is kept higher from the spin-coated film up to the bulk melting point (see Figure 5) indicates that the molecules in the quasi-liquid layer tend to retain the orientation of the underlying crystal. In contrast, the molecules in the vapor-deposited film tend to lose preferred orientation during surface melting (Figure 3) because the crystal template disappears by evolution of the melting layer. Consequently, the instability at the interface of the vapor-deposited thin film might be responsible for the formation of the quasi-liquid layer at lower temperatures. The liquidlike phase is also present in the deeply supercooled region before crystallization occurs, as evidenced by selfdiffusion of the molecules at T > 180 K. This liquid is characterized by ultrahigh viscosity, as inferred from the invariance of the film morphology. It is noteworthy that the crystallization behavior of the [emim][Tf2N] film contrasts sharply with that of simple molecular solids: The formation of droplets during crystallization is commonly observed for thin films (
