Observation of Molecular Ordering at the Surface of

The surface structure of trimethylpropylammonium bis(trifluoromethanesulfonyl)imide ([TMPA] [TFSI]) is studied by high-resolution Rutherford backscatt...
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Langmuir 2008, 24, 4482-4484

Observation of Molecular Ordering at the Surface of Trimethylpropylammonium Bis(trifluoromethanesulfonyl)imide Using High-Resolution Rutherford Backscattering Spectroscopy Kaoru Nakajima, Atsushi Ohno, Motofumi Suzuki, and Kenji Kimura* Department of Micro Engineering, Kyoto UniVersity, Yoshida-honmachi, Sakyo, Kyoto 606-8501, Japan ReceiVed February 16, 2008. In Final Form: March 6, 2008 The surface structure of trimethylpropylammonium bis(trifluoromethanesulfonyl)imide ([TMPA] [TFSI]) is studied by high-resolution Rutherford backscattering spectroscopy at room temperature. The results provide direct evidence of the molecular ordering at the surface. The C1 conformer of the [TFSI] anion is dominant among two stable conformers, and the anions are oriented with their CF3 groups pointing toward the vacuum in the outermost molecular layer. The anions in the second molecular layer also show preferred orientation although it is rather weak.

1. Introduction Exploring the surface and interface structures is a key issue in order to understand many important properties of liquids, such as the solubility, the vapor pressure, the viscous behavior, the liquid supercooling, and so on. Nevertheless, little is known about the surface structures of liquids. This is because the conventional surface analysis techniques are usually incompatible with liquids due to their high vapor pressures. Consequently, even the most fundamental information on the surface structures, such as the composition depth profile, is still lacking. Room-temperature ionic liquids (ILs) are thermally stable, nonvolatile, nonflammable solvents. Because of these unique and excellent properties, ILs have attracted increased attention. A number of studies have focused on the potential applications of ILs as alternatives to the traditional volatile organic solvents in synthetic and catalytic processes.1 Other applications of ILs as new electrolytes for electrodeposition2-4 and electrochemical devices such as lithium ion batteries,5,6 capacitors7,8 and fuel cells9,10 are also extensively studied. From a fundamental point of view, ILs can provide a unique opportunity to study the liquid surfaces because of their extremely low vapor pressure. In spite of this advantage and the immense importance of the surface properties of ILs for a number of technological applications, there were only a limited number of studies on the surface structures of ILs to date.11-17 An X-ray reflectivity (XR) study has shown the existence of a surface * To whom correspondence should be addressed. E-mail: kimura@ kues.kyoto-u.ac.jp. (1) Chiappe, C. In Ionic Liquids in Synthesis; Wasserscheid, P., Welton, T., Eds.; Wiley-VCH: Weinheim, Germany, 2007; Chapter 5. (2) Lin, Yu-Feng; and Sun, I. Wen, Electron. Acta 2000, 45, 3163. (3) Endres, F. Chem. Phys. Chem. 2002, 3, 144. (4) Freyland, W.; Zell, C. A.; Abedin, S.; Zein, El.; Endres, F. Electron. Acta 2003, 48, 3053. (5) Garcia, B.; Lavalle’e, S.; Perron, G.; Michot, C.; Armand, M. Electrochim. Acta 2004, 49, 4583. (6) Matsumoto, H.; Sakaebe, H.; Tatsumi, K. J. Power Sources 2005, 146, 45. (7) Ue, M.; Takeda, M.; Toriumi, A.; Kominato, A.; Hagiwara, R.; Ito, Y. J. Electrochem. Soc. 2003, 150, A499. (8) Kim, Y.; Matsuzawa, Y.; Ozaki, S.; Park, K. C.; Kim, C.; Endo, M.; Yoshida, H.; Masuda, G.; Sato, T.; Dresselhaus, M. S. J. Electrochem. Soc. 2005, 152, A710. (9) Noda, A.; Susan, A.; Kudo, K.; Mitsushima, S.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2003, 107, 4024. (10) Hagiwara, R.; Nohira, T.; Matsumoto, K.; Tamba, Y. Electrochem. SolidState Lett. 2005, 8, A231. (11) Sloutskin, E.; Ocko, B. M.; Tamam, L.; Kuzmenko, I.; Gog, T.; Deutsch, M. J. Am. Chem. Soc. 2005, 127, 779. (12) Bowers, J.; Vergara-Gutierrez, M. C. Langmuir 2004, 20, 309. (13) Gannon, T. J.; Law, G.; Watson, P. R. Langmuir 1999, 15, 8429.

layer of 0.6 - 0.7 nm thickness which has an electron density 10-12% higher than the bulk density in the surface of 1-butyl3-methylimidazolium hexafluorophosphate ([bmim] [PF6]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim] [BF4]).11 It was suggested that the origin of the observed electron density enhancement could be the surface enrichment of the anion whose electron density is about three times larger than the cation. A neutron reflectivity (NR) study also revealed that there is an inhomogeneous surface structure extending ∼4 nm into 1-CnH2n+13-methylimidazolium hexafluorophosphate ([Cnmim] [PF6]) and 1-CnH2n+1-3-methylimidazolium tetrafluoroborate ([Cnmim] [BF4]).12 These techniques, however, can access only density distributions and the obtained distribution depends on the model used in the analysis. As a result, the origin of the observed density enhancement was not clarified yet. Compositional analyses were performed using several techniques, such as direct recoil spectroscopy (DRS),13,14 X-ray photoelectron spectroscopy (XPS),15,16 low-energy ion scattering (LEIS),15 and metastable impact electron spectroscopy (MIES).16 The results of the XPS measurements on 1-CnH2n+1-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([Cnmim] [TFSI]) were in good agreement with the stoichiometric composition, indicating that neither the surface segregation nor the preferred molecular orientation exists within the depth probed by XPS (a few nm).15,16 Other techniques used so far (DRS, LEIS, and MIES) can probe primarily only the outermost atomic layer and gave results different from XPS. The measurements of LEIS and MIES have shown that the outermost atomic layer of [Cnmim] [TFSI] is enriched with fluorine.15,16 On the other hand, DRS measurements on [Cnmim] [PF6] have shown that the outermost atomic layer is not occupied by fluorine but that hydrogen is dominant.13,14 Using the observed DRS results, the preferred orientation of [Cnmim] at the surface was discussed.13,14 These techniques, however, provide only qualitative information and cannot analyze the subsurface region precisely. Clearly, more quantitative techniques that are able to probe the surface region with a good depth resolution are required to explore the detailed surface structure. (14) Law, G.; Watson, P. R.; Carmichael, A. J.: Seddon, K. R. Phys. Chem. Chem. Phys. 2001, 3, 2879. (15) Caporali, S.; Bardi, U.; Lavacchi, A. J. Elec. Spectrosc. Relat. Phenomena 2006, 151, 4. (16) Ho¨fft, O.; Bahr, S.; Himmerlich, M,; Krischok, S.; Schaefer, J. A.; Kempter, V. Langumuir 2006, 22, 7120. (17) Iimori, T.; Iwahashi, T.; Kanai, K.; Seki, K.; Sung, J.; Kim, D.; Hamaguchi, H.; Ouchi, Y. J. Phys. Chem B 2007, 111, 4860.

10.1021/la800509f CCC: $40.75 © 2008 American Chemical Society Published on Web 03/25/2008

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Figure 1. Structures of [TMPA] cation and two stable conformers of [TFSI] anion.

We have used high-resolution Rutherford backscattering spectroscopy (HRBS)18,19 to study the surface structure of trimethylpropylammonium bis(trifluoromethanesulfonyl)imide ([TMPA] [TFSI]). The molecular structure of [TMPA] [TFSI] is schematically shown in Figure 1. The [TFSI] anion has two stable conformers of C1 and C2 symmetry and the C2 conformer is more stable than the C1 conformer by 3.5 kJ mol-1.20 Our HRBS measurements reveal an oscillatory structure in the surface composition profile, indicating that there is molecular ordering at the surface. Although ILs having alkyl chains longer than C12H25 may form liquid crystalline phases, ILs containing the [TFSI] anion do not exhibit any mesophase structure,21 indicating that the observed structure cannot be ascribed to the liquid crystalline phase. 2. Experimental Section [TMPA] [TFSI] was purchased from Kanto Reagent (Japan) and used without further purification. The nominal water content reported by the manufacturer was less than 50 ppm. A slowly rotating wheel (diameter 38 mm) was partially immersed in a reservoir of [TMPA] [TFSI] to form a thin fresh layer of [TMPA] [TFSI] on the wheel surface.22 This wheel system was mounted on a precision gomiometer in a UHV scattering chamber (base pressure 1 × 10-10 Torr), which was connected to a 400 kV Cockcroft Walton type accelerator via a differential pumping system. A beam of 400 keV He+ ions produced by the accelerator was collimated to 2 mm × 2 mm and impinged onto the IL thin layer formed on the surface of the rotating wheel at room temperature (RT). The He+ ions scattered from the IL at 51° were energy analyzed by a 90° sector type magnetic spectrometer and detected by a one-dimensional position sensitive detector (1DPSD) placed on the focal plane. Nonuniformity of the efficiency of the 1D-PSD was carefully corrected so that a precise composition analysis can be performed.

3. Results and Discussion Figure 2 shows examples of the HRBS spectra observed at two different incident angles, R ) 41.9° and 44.2°. Each spectrum shows five steps at ∼310, ∼323, ∼332, ∼342, and ∼364 keV, which correspond to the leading edges of carbon, nitrogen, oxygen, fluorine, and sulfur, respectively. The solid lines show the results of spectrum simulation for randomly distributed molecules. The agreement between the measurement and the simulation is rather good, indicating that the overall composition is close to the stoichiometric composition. Looking at the spectra closely, however, there are some discrepancies. There are fine structures near the fluorine and sulfur edges in the observed spectra. These (18) Kimura, K.; Ohshima, K.; Mannami, M. Appl. Phys. Lett. 1994, 64, 2232. (19) Kimura, K.; Joumori, S.; Oota, Y.; Nakajima, K.; Suzuki, M. Nucl. Instrum. Methods B 2004, 219-220, 351. (20) Fujii, K.; Fujimori, T.; Takamuku, T.; Kanzaki, R.; Umebayashi, Y.; Ishiguro, S. J. Phys. Chem. B 2006, 110, 8179. (21) Hardacre, C. In Ionic Liquids in Synthesis; Wasserscheid, P., Welton, T., Eds.; Wiley-VCH: Weinheim, Germany, 2007; Chapter 4. (22) Saecker, M. E.; Govoni, S. T.; Kowalski, D. V.; King, M. E.; Nathanson, G. M. Science 1991, 252, 1421.

Figure 2. HRBS spectra of [TMPA] [TFSI] observed at incident angles R ) 41.9° (circles) and 44.2° (triangles). The spectrum for R ) 44.2° is displaced vertically by 1000. The error is smaller than the size of the symbols. The solid lines show the results of spectrum simulation for randomly distributed molecules. The open circles show the fluorine spectrum for R ) 41.9° derived by subtracting the sulfur contribution (dashed line) from the observed spectrum.

structures show that the molecular distribution and/or orientation deviate from the random distribution at the surface. Assuming that the deeper region is stoichiometric, the depth profiles of elements at the surface region were derived by the following procedure. The observed spectrum at energies larger than the fluorine edge (∼342 keV) directly corresponds to the sulfur spectrum. The spectra of other elements can be derived by subtracting the calculated spectrum of heavier elements from the observed spectrum. For example, the dashed line in Figure 2 shows the sulfur spectrum calculated with the random molecular distribution. The fluorine spectrum can be derived by subtracting the sulfur spectrum from the observed one, and the result is shown by the open circles. The spectra of other elements can be derived through similar procedures. Using the Bragg’s rule23 and the tabulated stopping cross sections given by Ziegler,24 the stopping power of [TMPA] [TFSI] (density 1.44 g/cm325) was calculated to be 0.228 keV/nm for 400 keV He+. Using the stopping power and the scattering cross sections calculated with the universal potential,24 the energy spectrum of each element was converted into a depth profile. Figure 3 shows the obtained depth profiles. While the profiles of oxygen, nitrogen, and carbon are almost flat and structureless within the experimental error, the profiles of fluorine and sulfur show clear structures. The most pronounced structure is an oscillatory structure seen in the fluorine profile. There is a sharp fluorine peak at a depth of 0.2 nm and a broad second peak at 1.1 nm. The sulfur profile also shows a clear peak at 0.5 nm and a somewhat unclear peak at ∼1.5 nm. It should be noted that the depth shown here was calculated in the basis of the energy loss. Consequently, the origin of the depth scale corresponds to the electronic surface, which is located outside the atomic surface by 0.1∼0.2 nm. The observed fluorine and sulfur peaks suggest that the anions predominantly occupy the surface. In order to examine this possibility, the average composition in the surface layer of half a nanometer thickness was estimated by integrating the profiles. Considering the average volume of one pair of the molecular ions (0.44 nm3), this layer corresponds to one molecular layer (23) Bragg, W. H.; Kleeman, R. Phil. Mag. 1905, 10, 318. (24) Ziegler, J. F.; Biersack, J. P.; Littmark, U. The Stopping and Ranges of Ions in Solids; Pergamon Press: New York, 1985. (25) Sakaebe, H.; Matsumoto, H. Electrochem. Commun. 2003, 5, 594.

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Figure 3. Composition depth profiles in [TMPA] [TFSI] derived from the observed HRBS spectra. The horizontal dashed lines show the stoichiometric concentrations. Typical errors are shown by error bars.

or less. The obtained composition, S2.2F7.1O3.8N1.7C7.9, is close to the stoichiometric composition (S2F6O4N2C8), showing that neither ion is enriched in the surface layer. The sharp fluorine peak at 0.2 nm indicates that the CF3 groups of the anions are pointing toward vacuum in the outermost layer. The sulfur peak seen at 0.5 nm indicates that sulfur atoms are located at slightly deeper positions than fluorine by 0.3 nm. This separation is close to the theoretical separation (∼0.23 nm20) between the fluorine plane and the sulfur in the [TFSI] anion, indicating that the S-C bonds tend to align perpendicular to the surface. The small difference between 0.23 and 0.3 nm will be discussed later. Figure 1b schematically shows possible orientations of the [TFSI] anions at the surface for both C1 and C2 conformers. The observed second fluorine peak at 1.1 nm seems to originate from the lower CF3 groups in the C2 conformers. If this is the case, the sulfur peak should appear at the middle point between the two fluorine peaks. The observed sulfur peak, however, is closer to the first fluorine peak. In addition, the separation between the observed fluorine peaks (∼0.9 nm) is much larger than the separation of two CF3 groups in the C2 conformer (the theoretical separation between the two fluorine planes is smaller than 0.5 nm20). These results indicate that the C2 conformer is not dominant at the surface, although the abundance of the C2 conformer is estimated to be 80% at RT from the observed energy difference between two conformers (3.5 kJ mol-120). This discrepancy can be explained in terms of the extremely low surface energy of fluorine.26 The small energy difference between the C1 and C2 conformers can be easily compensated by the surface energy reduction. (26) Zangwill, A. Physics at Surfaces; Cambridge University Press: Cambridge, U.K., 1988; p 11.

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As was mentioned above, the observed separation ∼0.3 nm between the first fluorine peak and the first sulfur peak is slightly larger than the theoretical value (0.23 nm). This discrepancy can be resolved if the density of IL is enhanced by ∼30% at the surface. The observed separation was derived on the basis of the stopping power calculated with the bulk density. If the surface density is enhanced by 30%, the stopping power is also enhanced. Consequently, the above obtained separation should be corrected and the resulting separation is in good agreement with the theoretical one. It is likely that the observed molecular ordering at the surface allows the anions to pack more closely and results in the enhancement of density. The molecular ordering and the resulting density enhancement at the surface were actually predicted by atomistic simulations of IL surfaces,27,28 although the simulations were done for ILs other than [TMPA] [TFSI]. The present result together with the atomistic simulation suggests that the electron density enhancement observed by XR is caused by the molecular ordering. Finally, we will discuss the orientation of the anion in the second molecular layer. The second fluorine peak seen at 1.1 nm indicates that the anions in the second molecular layer also show preferred orientation. The large separation, ∼0.9 nm, between the first and second fluorine peaks, suggests that the CF3 groups of the anions in the second layer have a tendency to point toward the bulk, although the broad second peak indicates that the degree of ordering is rather weak. In summary, the surface of [TMPA] [TFSI] was observed by HRBS. The overall composition is in good agreement with the stoichiometric composition, even in the outermost surface layer, indicating that the surface is shared equally between cations and anions. The observed fluorine and sulfur depth profiles, however, reveal the molecular ordering. It is found that the C1 conformer of the [TFSI] anion is dominant at the surface of [TMPA] [TFSI]. The anions are oriented with their CF3 groups pointing toward the vacuum in the outermost molecular layer. The anions in the second molecular layer also show preferred orientation although it is rather weak. The observed molecular ordering cannot be ascribed to the liquid crystalline phase because the ordering is limited only in the surface region. The application of highresolution RBS is not limited to [TMPA] [TFSI]. Further studies on various ILs are now in progress. Acknowledgment. This work was supported in part by Center of Excellence for Research and Education on Complex Functional Mechanical Systems (COE program) of the Ministry of Education, Culture, Sports, Science and Technology, Japan. LA800509F (27) Lynden-Bell, R. M. Mol. Phys. 2003, 201, 2625. (28) Lynden-Bell, R. M.; Del Popolo, M. Phys. Chem. Chem. Phys. 2006, 8, 949.