Article pubs.acs.org/Langmuir
Interface Controls Spontaneous Crystallization in Thin Films of the Ionic Liquid [C2C1Im][OTf] on Atomically Clean Pd(111) Stefan Schernich,† Valentin Wagner,‡ Nicola Taccardi,‡ Peter Wasserscheid,‡,§ Mathias Laurin,*,† and Jörg Libuda†,§ †
Lehrstuhl für Physikalische Chemie II, ‡Lehrstuhl für Chemische Reaktionstechnik, and §Erlangen Catalysis Resource Center, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Egerlandstraße 3, 91058 Erlangen, Germany S Supporting Information *
ABSTRACT: A total of 5−30 monolayer thick films of the ionic liquid (IL) [C2C1Im][OTf] were vaporized in vacuo onto an atomically clean Pd(111) single crystal surface at 220 K. Time- and temperature-resolved infrared reflection−absorption spectroscopy reveals growth, interactions with the metallic support, and the macroscopic phase behavior of the layer. At 220 K, the IL layer first grows in the form of a glassy phase. Crystallization of the IL was witnessed above a critical thickness of about 10 monolayers. On the basis of the known bulk crystal structure of the IL, we suggest the formation of well-oriented checkerboard-like crystalline film structures on the surface. The preferential orientation manifested by the crystal phase with regard to the macroscopic metallic surface is attributed to strong interactions between anionic headgroups and the metal.
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their crystal structure in the frozen state,24−33 only a few publications address the thermal behavior of thin IL films on solid substrates34−36 and even a lower number of studies deal with films prepared under UHV conditions.37,38 The properties of the thin IL films depend upon the structure and phase of the IL at the surface, and we concentrate on their identification in this study. [C2C1Im][OTf] is chosen as a prototype of the popular imidazolium-based ILs. Furthermore, the [OTf]− anion has strong absorption bands in infrared (IR). Similar ILs, such as [C8C1][OTf], have been shown to vaporize intact as ion pairs,39 and despite reports of a lower thermal stability for the present IL,40 no sign of decomposition was observed during our experiments. The structural formula of the IL is shown in Scheme 1. The anion having C3v symmetry, the dynamic dipole moments of the symmetric stretching of the end groups, is strictly aligned with the molecular axis. It is strictly perpendicular for the asymmetric stretching. Such a symmetry combined with the metal surface selection rule (MSSR),41,42
INTRODUCTION Ionic liquids (ILs) are salts with melting points below 100 °C. They combine interesting physicochemical properties, such as extremely low volatility, exceptional miscibility behavior, high thermal stability, nonflammability, and electrical conductivity.1 These properties have stimulated research toward their use in several fields, such as catalysis,2,3 lubrication4−6 electrochemistry,7,8 photovoltaics,9,10 sensor technology,11,12 and molecular electronics.13 In most of these applications, a thin IL film is spread onto a solid surface to modify its performance. The interactions at the IL−support interface therefore largely determine the macroscopic properties of the system. However, understanding such interfaces at the atomic scale is experimentally challenging and far from complete. To that aim, our group and others have started applying surface science methods to supported ILs.14−22 Indeed, the low vapor pressure and good thermal stability of many ILs allows for the preparation of supported IL films in situ under ultrahigh vacuum (UHV) conditions, allowing for spectroscopic investigations under well-defined conditions. As a consequence, the new field of IL surface science has emerged.23 In this work, we studied thin films of 1-ethyl-3methylimidazolium trifluoromethanesulfonate [C2C1Im][OTf] supported on a clean, single crystalline Pd(111) surface using a rigorous surface science approach. The systems were prepared in UHV, and the IL films were deposited by physical vapor deposition (PVD). Time-resolved infrared reflection−absorption spectroscopy (TR-IRAS) was used to investigate the film during deposition and temperature ramps. While a number of studies deal with the thermal behavior of ILs in the bulk and © 2014 American Chemical Society
Scheme 1. Structural Formula of [C2C1Im][OTf]
Received: March 4, 2014 Revised: April 30, 2014 Published: May 22, 2014 6846
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which states that only vibrations with a dynamic dipole moment perpendicular to the surface absorb, allows for the determination of the orientation of the anion at the surface.43 Weaker IR absorptions in the cation (Cs symmetry) allow for a similar but more qualitative analysis. IRAS is therefore sensitive to probe the specific molecular interactions and the orientation of the IL at the metallic surface. In our previous work, we have identified the interaction mechanism between [C2C1Im][OTf] and Pd(111).43 In this work, we use IRAS to explore how these interactions affect order−disorder phase transitions of supported thin films.
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EXPERIMENTAL SECTION
Figure 2. Assignment of the IR features of a [C2C1Im][OTf] multilayer adsorbed onto Pd(111).
IRAS Apparatus and Sample Preparation. The IRAS experiments were performed in an UHV system described elsewhere (base pressure around 2 × 10−10 mbar).44 Briefly, the system allows for up to four effusive beams and one supersonic beam to be superimposed on the sample surface. For the present experiments, a home-built IL evaporator replaced one effusive beam source.15 The system is equipped with a Fourier transform infrared (FTIR) spectrometer (Bruker IFS66/v) and all necessary preparation tools. The Pd(111) surface was cleaned by several cycles of Ar+ sputtering and annealing in vacuum. The quality of the film was checked by low-energy electron diffraction (LEED). Synthesis of [C2C1Im][OTf]. The IL was synthesized via an alkylation carried out under dry argon conditions using standard Schlenk techniques. A flask was charged with freshly distilled 1ethylimidazole (7.38 g, 7.69 mmol) and 50 mL of dry dichloromethane. Commercial methyl trifluoromethanesulfonate (SigmaAldrich) was slowly added (9.1 mL, 80.7 mmol, 1.05 equiv) and was heated until reflux for 5 days. The product was obtained after drying the reaction mixture under reduced pressure (0.01 mbar, 40 °C) as a clear to pale yellow, low-viscosity liquid in an 86% yield (17.2 g). Proton nuclear magnetic resonance (1H NMR) (DMSO-d6) δ: 1.37 (t, J = 7.4 Hz, 3, CH3), 3.80 (s, 3, CH3), 4.14 (q, J = 7.4 Hz, 2, CH2), 7.65 (t, J = 1.7 Hz, 1, CH), 7.74 (t, J = 1.7 Hz, 1, CH), 9.06 (s, 1, CH). 13C {1H} NMR δ: 15.7, 36.4, 44.5, 122.4, 124.1, 136.7. 19F NMR δ: −77.8. Experimental Protocol. The IL was deposited by PVD onto the sample cooled to 220 K. After growth of a film of the suitable thickness, the sample was heated to 340 K and cooled back to 220 K at a rate of 1.5 K/min. The cycles were repeated in some experiments. This procedure is schemed in Figure 1. IR spectra were acquired in time-resolved mode during the deposition, the temperature cycles at a spectral resolution of 2 cm−1, and an acquisition time of 1 spectrum/ min.
[OTf]− anion and are assigned as follows:45−47 The peaks at 1038 and 1285 cm−1 correspond to the symmetric and asymmetric stretching vibrations of the SO3 group [νs(SO3) and νas(SO3)], respectively. Analogously, the peaks at 1231 and 1176 cm−1 correspond to νs(CF3) and νas(CF3). The smaller peak at 1013 cm−1 is attributed to interactions of the SO3 group with Pd(111) [νs,surface(SO3)].43 Absorption at 1578 cm−1 is attributed to a stretching vibration within the aromatic imidazolium ring [ν(CC)].34 The first experiments show coverage- and temperaturedependent measurements of thick [C2C1Im][OTf] films on Pd(111). Figure 3a displays the spectra acquired during the deposition of approximately 30 ML at the fixed sample temperature of 220 K. In a previous publication, we have shown that the intensity ratios r = Is/Ias, where Is and Ias are the integrated intensities of corresponding symmetric and asymmetric stretching, were directly related to the molecular orientation.43 Figure 3b plots rs/as against deposition time
Figure 1. Experimental procedure of this work. The IL was deposited onto Pd(111) by PVD at 220 K, after which two cycles of heating and cooling (1.5 K/min) between 220 and 340 K were performed. TRIRAS spectra were recorded during deposition, heating, and cooling.
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RESULTS AND DISCUSSION Thick Films on Pd(111). Figure 2 displays the assignment of the IR absorption band for approximately 30 monolayers (ML) of [C2C1Im][OTf] adsorbed at 220 K on Pd(111). Overall, the spectra exhibit large absorption features between 1000 and 1300 cm−1 that can be attributed to vibrations of the
Figure 3. (a) IRAS spectra recorded during the deposition of [C2C1Im][OTf] onto Pd(111) at 220 K. (b) Development of rs/as as a function of the deposition time. (“surf” stands for “surface”). 6847
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(and, therefore, the thickness of the film). Four distinct regions are identified. The first 3 min of deposition only feature absorption bands attributed to interactions of the anion with the surface in the ML. At deposition times between 3 and 14 min, we observe the growth of νs(SO3), νas(SO3), νs(CF3), and νas(CF3) bands, at nearly constant relative intensities rs/as. The presence of every symmetric and asymmetric vibration in the spectra indicates the growth of a disordered layer.43 Because deposition was performed below the melting point, the IL film is likely frozen in an amorphous glassy structure without longrange order.34,38,48 A third regime is observed after 14 min. In the spectra, ν(CC) vanishes and new bands are observed at 1470, 1386, 866, 782, and 758 cm−1. Among these, the band at 782 cm−1 is the most intense and is identified as the C−H outof-plane bending mode [ω(CH)] in the imidazolium ring.34,49 At the same time, rs/as increases quickly until 19 min and keeps increasing slowly thereafter. The larger values for rs/as indicate dominant symmetric intensities and, therefore, reorientation of the IL layer with the molecular axis of the anion oriented normal to the surface. The appearance of ω(CH) and disappearance of ν(CC) also indicate a preferred orientation of the imidazolium ring parallel to the surface. The increased order of the layer therefore provides evidence of the crystallization of the layer. Choudhury et al. proposes a crystal structure of frozen [C2C1Im][OTf] in the bulk.50 Briefly summarized, [C2C1Im][OTf] crystallizes in the orthorhombic Pbca space group. Cations and anions interact with each other via C−H···O hydrogen bonds forming a layered structure, where the molecular plane of the imidazolium ring of the cation is perpendicular to the molecular axis of the anion. Adjacent layers are connected via F···F interactions involving anions from both layers. The authors also report that the imidazolium rings are not stacked but offset in an ABAB... arrangement. This bulk crystal structure is compatible with our own observations (see Scheme 2). Interestingly, the arrangement implies that the first IL layer contacts Pd(111) with a checkerboard structure, as proposed for [C1C1Im][Tf2N] on Au(111)17 but not on Ni(111).20
Figure 4. (a) TR-IRAS spectra recorded during the heating of a roughly 30 ML thick film of [C2C1Im][OTf] on Pd(111) from 220 to 340 K (1.5 K/min) and values of rs/as as a function of the temperature during (b) heating and (c) cooling.
largest changes are observed near the melting point (247 K reported in the bulk50). Indeed, a decrease of the intensity ratios r to values close to the glass state (Figure 4b, as compared to Figure 3b) is observed in the spectra between 245 and 260 K. Above this temperature, the spectra are comparable with bulk spectra acquired in transmission, such as the spectra presented in Figure S1 of the Supporting Information. The changes therefore indicate a transition from an ordered to a disordered phase and are unambiguously attributed to melting of the crystalline layer. Upon cooling the sample back from 340 to 220 K, crystallization is again observed between 245 and 240 K, as presented in Figure 4c. We therefore find no evidence for hysteresis. Thin Film on Pd(111). In the previous section, we identified crystallization of the IL layer on Pd(111). Annealing time, increased thickness, or both may be responsible for this phase transition. To identify the kinetic or thermodynamic driving force for the crystallization, we proceed to a comparable experiment at 220 K but stopped the deposition at 5 ML or about half the thickness where crystallization occurred during the previous experiment. The new deposition lasted 7 min, and the experiment is comparable to the beginning of the experiment presented Figure 3. The surface was then kept at 220 K for another 10−15 min for a total time of about 20 min. Figure 5a (top spectrum at 220 K) shows the spectrum recorded thereafter. Apart from an overall loss of intensity attributed to partial desorption of the IL at the highest temperatures, the spectrum is very similar to the spectrum
Scheme 2. Checkerboard Structure of the IL in the Crystalline State on Pd(111)a
a
The crystal structure was drawn after the study by Choudhury et al.50
Furthermore, our experiments indicate a preferential macroscopic orientation of the crystal phase on the metallic surface. The νs(SO3) shift to νs,surface(SO3) observed in the ML indicates strong interactions of the anion with Pd(111). We therefore postulate that the strong microscopic interactions of the SO3 groups of the anion with the surface determine the macroscopic orientation of the supported crystal phase. Figure 4a shows the spectra recorded upon heating the thick IL layer from 220 to 340 K at 1.5 K/min. As expected, the 6848
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this and the presumably layered structure of the liquid phase14,16,51 by increasing the temperature by a few degrees can be expected to dramatically modify the physicochemical properties of the IL layer.52 A different IL with a higher melting point would nevertheless be required here for most applications.
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CONCLUSION We have studied the growth and temperature-dependent behavior of 5−30 ML thick films of [C2C1Im][OTf] on Pd(111). The IL was vaporized in situ and in vacuo by PVD onto the single crystal at 220 K. The film starts growing in a glassy phase without long-range order. At a critical thickness of about 10 ML, we observed crystallization of the film and ordering of the crystal with regard to the metallic surface. IRAS allows us to identify the orientation of the anion and, to a lesser extent, the cation. Comparing our observation to the bulk crystal of the IL reported in the literature, we can conclude that the IL film arranges with a checkerboard structure at the metallic surface. Interestingly, the long-range order and preferential macroscopic orientation of the crystal probably originate from strong interactions between the anion and the surface at the microscopic scale. Increasing the sample temperature results in melting of the crystalline phase at a temperature close to the temperature reported for the bulk. No hysteresis was observed upon cooling the film back to 220 K.
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ASSOCIATED CONTENT
* Supporting Information S
Figure 5. (a, Top) TR-IRAS spectra of a 5 ML thick film of [C2C1Im][OTf] on Pd(111) after deposition at 220 K and during heating from 220 to 340 K (1.5 K/min). (a, Bottom) Comparison between the IRAS spectra recorded at 220 and 340 K, respectively. (b) Development of rs/as as a function of the temperature.
Transmission infrared (TIR) spectrum of [C2C1Im][OTf] diluted in KBr (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
recorded above the melting point at 340 K. Therefore, even after 20 min at 220 K, the thin film shows no evidence of crystallization. We can therefore conclude that there is a critical film thickness slightly above 10 ML that, when exceeded, energetically favors the crystalline state. Figure 5b also displays the spectra acquired during the ramp between 220 and 340 K. Contrary to the thick film case, only minor changes are observed in the IRAS spectra. The small, continuous shifts as well as the small decrease of the intensity ratios are attributed to effects of the temperature, such as spectral broadening and a slight attenuation of certain bands. Whereas the absence of the crystalline phase is unambiguous, the glass−liquid phase transition is a transition between two disordered phases and it is possible that our technique is not sensitive enough to detect such transitions.
*Telephone: +49-9131-8527310. E-mail: mathias.laurin@fau. de. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This project was financially supported by the “Deutsche Forschungsgemeinschaft” (DFG) within the Excellence Cluster “Engineering of Advanced Materials” in the framework of the excellence initiative and within the DFG Priority Program 1708 “Material Synthesis near Room Temperature”. We also acknowledge support by Clariant AG. Stefan Schernich gratefully acknowledges a grant from the “Fonds der Chemischen Industrie”.
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DISCUSSION The structure of the thin IL films at surfaces therefore depends upon not only the IL itself, the composition of the surface, or the temperature but also the thickness of the film. This demonstrates that the structure depends upon inter- and intramolecular forces and the interaction with the surface with forces that are close in magnitude. Prediction of the crystal structure of such films is therefore particularly difficult, and experimental investigations will have to be performed in any case. Nevertheless, the identification of the checkerboard structure in the crystalline phase and the possibility to switch between
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dx.doi.org/10.1021/la500842c | Langmuir 2014, 30, 6846−6851
