J. Phys. Chem. C 2009, 113, 19237–19243
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Surface Structural Study on Ionic Liquids Using Metastable Atom Electron Spectroscopy Takashi Iwahashi,† Toshio Nishi,† Hiroyuki Yamane,† Takayuki Miyamae,‡ Kaname Kanai,§ Kazuhiko Seki,† Doseok Kim,§ and Yukio Ouchi*,† Department of Chemistry, Graduate School of Science, Nagoya UniVersity, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan, Nanotechnology Research Institute, National Institute of AdVanced Industrial Science and Technology (AIST), 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan, Research Center for Materials Science, Nagoya UniVersity, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan, Department of Physics and Interdisciplinary Program of Integrated Biotechnology, Sogang UniVersity, Seoul 121-742, Republic of Korea ReceiVed: June 17, 2009; ReVised Manuscript ReceiVed: September 11, 2009
The surface structures of ionic liquids 1-CnH2n+1-3-methylimidazolium tetrafluoroborate (BF4-), trifluoromethanesulfonate (OTf) and bis(trifluoromethylsulfonyl)amide (TFSA) with n ) 4, 8, and 10 were investigated by ultraviolet photoemission spectroscopy (UPS) and metastable atom electron spectroscopy (MAES). The MAES spectra reveal that the anions and cations share the surface of the room-temperature ionic liquids (RTILs) with a shorter alkyl chain at n ) 4 although the alkyl chain layer tends to cover the outermost surface in the case of RTILs with longer alkyl chains of n ) 8 and 10. Comparison of UPS and MAES spectra demonstrates that the nonpolar groups such as the alkyl chain and the CF3 group point toward the vacuum side, while the polar groups such as the SO3 and SO2 groups face toward the bulk side. A doublelayered structure at the surface, which consists of an alkyl chain layer and a polar layer containing anions and imidazolium rings, is proposed from these observations. 1. Introduction Room-temperature ionic liquids (RTILs) are salts that are in a liquid phase at room temperature and ambient pressure1 and attract much interest in various research fields because of unique properties such as nonvolatility, nonflammability, high ion transportation rates, and wide electrochemical windows.1-4 In recent years, both physicochemical and applied studies of RTILs have progressed energetically due to their importance not only for applications of RTILs5-12 but also for further understanding of the origin of their unique properties.13-15 Studies using Raman vibrational spectroscopy,16-19 neutron diffraction,20 X-ray scattering,17 and molecular dynamics (MD) simulation21-23 have suggested that the bulk liquid phase of RTILs spontaneously forms microscopic polar and nonpolar domains.22 It is thought that this domain formation may be the cause of many of the anomalous properties of RTILs.15 The interfacial structures of RTILs are widely recognized to be of fundamental importance in applications such as electrochemical cells, phase-separable catalysis, and chemical extraction processes.24-27 Thus, surface structures of 1-alkyl-3methylimidazolium ([Cnmim]+) RTILs have been intensively studied in recent years using techniques such as surface tension measurement,28,29 direct recoil spectrometry (DRS),28,30 neutron reflectometry,31 X-ray reflectivity measurement,32,33 nonlinear surface spectroscopy,33-42 electron spectroscopy,43-45 and MD simulations.46-49 However, the assertions of these studies include some contrarieties so that a complete understanding is still lacking. For example, G. Law et al. have reported in their early studies that both the anion and cation share the RTIL surface and the alkyl chains do not protrude from the surface.28,30 Studies * To whom correspondence should be addressed. E-mail: ohuchi@mat. chem.nagoya-u.ac.jp. Phone: +81-52-789-2485. Fax: +81-52-789-2944. † Graduate School of Science, Nagoya University. ‡ AIST. § Research Center for Materials Science, Nagoya University.
using IR-visible sum-frequency generation spectroscopy (IVSFG), however, have indicated that the alkyl chains of the [Cnmim]+ cations point away from the bulk into the air, which is in good agreement with the MD simulation results.33-41,46-49 The first X-ray reflectivity study proposed the existence of a higher electron density layer at the RTIL surface,32 while a more recent report suggests the existence of a nonpolar alkyl chain layer positioned directly above a higher-electron density layer composed of cation cores and anions.33 Recently, S. Krischok et al. have employed electron spectroscopies45 to investigate the surface structure of RTILs and concluded that at room temperature the surface structure of a RTIL is similar to that of the bulk, although some other studies have strongly suggested the presence of a structural difference between the surface and the bulk of RTILs.31-33,39,42 These problems likely stem from not only the limitation of the information which can be obtained from each technique, but also the limited number of molecular species of RTILs studied above. Thus, a comprehensive study of the interfacial structure for a series of RTILs having different molecular structures would certainly be valuable to understand the microscopic interfacial structures of RTILs, and since interfacial and bulk properties are never completely independent, such new data would solely contribute to an understanding of bulk properties as well. In this paper, we report UPS and metastable atom electron spectroscopy (MAES) studies on the surface structures of the [Cnmim]X RTILs having different alkyl chains from n ) 4, 8, 10 and the anion species X ) tetrafluoroborate (BF4), trifluoromethanesulfonate (OTf), and bis(trifluoromethylsulfonyl)amide (TFSA), shown in Figure 1. Because MAES uses a metastable atom with low kinetic energy as a probe beam, it is oftentimes referred to as an ultimate surface sensitive tool, while UPS may observe a little bit deeper surface layers defined by the effective escape depth of emitted electrons.50-52 By comparing the results of UPS and MAES, structural differences within
10.1021/jp9056797 CCC: $40.75 2009 American Chemical Society Published on Web 10/13/2009
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the outermost surfaces of RTILs with differing alkyl chain lengths and anion species are successfully demonstrated. The chain length and anion size dependence of the surface number density of molecules and their molecular orientation is also discussed on the basis of the MAES spectra and our results obtained by other techniques.33,39,42 We propose a double-layered structure for RTIL surfaces, in which an alkyl chain layer comes on top of a polar layer composed of anions and imidazolium rings. Protruding longer alkyl chains incorporate gauche defects in order to cover and hide the polar layer, thus reducing the surface energy. 2. Experimental Details 2.1. Synthesis. The RTILs examined in this study were prepared according to literature with a slight modification.1,39 [Cnmim]Br RTILs were prepared by the alkylation of 1-methylimidazole (Aldrich, purity 99+%, used as received) with a slight molar excess of CnH2n+1Br (TCI, purity >98%, n ) 4, 8, 10), which was distilled under low pressure. The reaction mixture was stirred at room temperature for 2-3 days. [Cnmim]Br RTILs obtained as viscous liquid were washed several times with ethyl acetate for n ) 4, 8, and with toluene for n ) 10. The anion exchange reaction of [Cnmim]Br was carried out by adding a slight molar excess of NaBF4 (Wako chemical, purity 98%, used as received), LiOTf (Aldrich, purity 99.995%, used as received) and LiTFSA (Aldrich, >99.95%, used as received) for the preparation of [Cnmim]BF4, [Cnmim]OTf and [Cnmim]TFSA, respectively. [Cnmim]BF4 and [Cnmim]OTf RTILs were extracted from water using dichloromethane. After the extraction, dichloromethane was evaporated under low pressure. [Cnmim]TFSA was washed repeatedly with water, followed by the evaporation under low pressure to remove residual water. The [Cnmim]X RTILs were further degassed by repeatedly heating and cooling in vacuum for several hours in order to remove traces of water and volatile solvent. We checked the purity using 1H NMR spectroscopy, and no trace of the starting materials, namely, 1-methylimidazole, CnH2n+1Br and the [Cnmim]Br RTILs, as well as the solvents, were found in the 1H NMR spectra. Water content of the [Cnmim]X RTILs was determined by Karl Fischer titration and confirmed to be less than 60 ppm. 2.2. Methods. MAES is one of the electron-emission spectroscopic techniques originating from the de-excitation of electronically excited metastable atoms (A*) with a target material (T) at surfaces.50-52 There are two types of the electron emission processes via an impact of A* with T. One is a combination of resonant ionization (RI) and corresponding Auger neutralization (AN) processes mostly observed on metal and semiconductor surfaces and is expressed by the following equations
Τ + Α* f Τ-+Α+(RI)
(1a)
Τ-+Α+ f Τ+ + Α + e-(ΑΝ)
(1b)
The other process, referred to as Penning ionization (PI), is described by the following equation
Τ + Α* f Τ+ + Α + e-(ΡΙ)
(2)
Figure 1. Structures of the ionic liquid components used in this paper: (a) 1-alkyl-3-methylimidazolium ([Cnmim]+) cation, (b) tetrafluoroborate (BF4-) anion, (c) trifluoromethanesulfonate (OTf) anion, and (d) bis(trifluoromethylsulfonyl)amide (TFSA) anion.
This process occurs dominantly for materials having no vacant levels corresponding to the excited level of A*, such as organic materials, and RTILs. When a metastable He (He*) atom is used, an electron in the valence band of the target material is transferred to the 1s hole of the He* atom accompanied by the electron ejection from the 2s orbital of the He*. The kinetic energy of the emitted electron Ek in the PI process is approximately described by
Ek ≈ E(Ηe*) - Eb
(3)
where E(He*) is the excitation energy of the He metastable state (23S, 19.82 eV; 21S, 20.62 eV), and Eb is the binding energy (BE) of the target material.53 Therefore, the BE can be obtained by measuring the kinetic energy of the emitted electrons, which is similar to the case of UPS and XPS. It should be noted that the metastable atom hardly penetrates into the bulk; thus, the electron emission spectrum obtained from MAES provides information about molecular orbitals (MO) at the outermost surface of the material. In this sense, MAES is often referred to as an ultimate surface sensitive tool. There are several studies in which MAES and UPS are employed together to investigate surface structures of complex molecular systems such as self-assembled monolayers (SAM),54,55 thin solid films of phthalocyanine derivatives,52,56,57 and RTILs.44,45 Though only a qualitative analysis of MAES and UPS spectra is performed in most of these reports, informative findings on surface structures of such media have been successfully obtained. The difficulties of quantitative analyses are mostly due to the fact that the ionization cross section of each MO is rarely obtainable for a target molecule. Removal of the contribution of secondary electrons from the spectra also poses practical and experimental difficulties for reliable quantitative analysis. Because the evaluation of these parameters is beyond the scope of this paper, we will confine ourselves to analyzing our spectra qualitatively. The MAES and UPS measurements were performed with a homemade photoelectron spectroscopy system.58,59 The RTIL samples for measurements in ultrahigh vacuum (UHV) environment were prepared by applying a thin film of RTIL on a gold sheet. The sample was installed in a vacuum chamber that was evacuated with an ion pump and a turbo molecular pump. As the excitation sources, we used helium resonance line (He I: 21.22 eV) for UPS and excited He atoms (He* (23S): 19.82 eV) for MAES. The emitted electron was detected with a hemispherical electron energy analyzer (AR65, Omicron NanoTechnology GmbH) with a total-energy
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Figure 2. (i) UPS and (ii) MAES spectra, and the corresponding simulated spectra derived from the DOS calculation for the (iii) anion and (iV) cation of each RTIL for (a) [C4mim]BF4, (b) [C8mim]BF4, (c) [C10mim]BF4, (d) [C4mim]OTf, (e) [C8mim]OTf, (f) [C10mim]OTf, (g) [C4mim]TFSA, (h) [C8mim]TFSA, and (i) [C10mim]TFSA. The base lines of the spectra are offset by appropriate intervals. The triangles on the UPS and MAES spectra represent the peaks originating from the anions, filled for the higher BE region, and open for the lower BE region.
resolution of about 0.1 eV at 300 K. The measurements were carried out under UHV conditions better than 10-9 Torr. Molecular orbital (MO) calculations of the isolated [Cnmim]+ (n ) 4, 8, 10) cations and BF4-, OTf, and TFSA anions with a 6-31G* basis set using density functional theory (DFT) were performed with Gaussian 98 package.60 The electron correlation was taken in Becke’s three-parameter exchange with a Lee, Young, and Parr correlation function (B3LYP).61 Geometry optimization was also performed. 3. Results Figure 2a-i summarizes the (i) UPS and (ii) MAES spectra and corresponding simulated density of states (DOS) of isolated
(iii) anions and (iV) cations for [Cnmim]X (n ) 4, 8, 10 and X ) BF4, OTf, TFSA). The abscissa is the BE relative to the Fermi level of the gold substrate. The simulated DOS is obtained by broadening the calculated MOs with 0.6 eV fwhm to reproduce the observed UPS and MAES spectra. Each (V) orbital energy is marked by a solid vertical bar. The energy axes of simulated spectra are shifted and adjusted for best fit as reported in our previous UPS and soft X-ray emission spectroscopy (SXES) studies.43,62-64 3.1. Results of DOS Calculations. As shown in Figure 2, there are some characteristic peaks in the simulated DOS for BF4-, OTf, and TFSA anions (blue lines), while those of [Cnmim]+ cations (red lines) show rather broad features
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Figure 3. Schematic illustrations of typical molecular orbitals and the simulated spectra for (a) BF4-, (b) OTf, and (c) TFSA anions.
originating from the C2p and N2p orbitals of the alkyl chain and the imidazolium ring. For later discussions, schematic illustrations of typical MOs along with the DOS of each anion are presented in Figure 3. All the peaks in the simulated spectrum for the BF4- anion (Figure 3a) are assigned to the contribution from F2p orbitals. The simulated DOS for the OTf anion (Figure 3b) is characterized by O2p and S3p as well as F2p orbitals; the three peaks in the region from 5 to 8 eV (lower BE region) are mostly due to the MOs localized at the SO3 group, while the other three peaks in the region ranging from 10 to 13 eV (higher BE region) are mainly attributable to the MOs at the CF3 group. The simulated DOS of the TFSA anion (Figure 3c) is also characterized by the contribution of F2p, O2p, and S3p orbitals. The peaks in the lower BE region (5-10 eV) originate from MOs localized mostly around the SO2 groups. The others in the higher BE region (10-13 eV) are dominated by the contribution from MOs more or less spreading over the CF3 groups of the TFSA anion. 3.2. MAES and UPS Spectra of [Cnmim]BF4 RTILs. In the observed MAES spectrum of [C4mim]BF4 shown in Figure 2a(ii; black line), the broad emission feature around 7 eV is assigned to the contribution from N2p and C2p orbitals of the [C4mim]+ cation and the peaks marked by filled triangles are attributed to F2p orbital of the BF4- anion. Similar spectral features are observed in the corresponding UPS spectrum (Figure 2a(i; green line)) in which the peaks assigned to the BF4- anion are also marked by filled triangles. These assignments are based on our previous studies of these compounds using UPS, XPS, and SXES.62-64 The fact that both the [C4mim]+ cation and BF4anion can be observed in the MAES spectra clearly demonstrates that the [C4mim]+ cation and the BF4- anion are exposed to the open side (vacuum side) at the outermost surface of [C4mim]BF4. Recent studies of surface structure of RTILs using MD simulations have suggested that both anions and cations share the outermost surface of [C4mim]PF6,48 which is in good agreement with our results. On the contrary, when the alkyl chain of the [Cnmim]+ cation is elongated to n ) 8 and 10 (Figure 2b,c), the features of the MAES spectrum marked by filled triangles in Figure 2a(ii), originating from the MOs of the BF4- anion diminish and disappear, while the corresponding features (marked also by triangles) observed in the UPS spectra are still noticeable as shown in Figure 2b(i) and c(i). Because there is no significant
Iwahashi et al. difference in the MAES spectra of [C8mim]BF4 and [C10mim]BF4 and also due to the ultimately high surface sensitivity of the MAES technique, this result indicates that the BF4- anions no longer show up at the outermost surfaces for the cases of chain length n ) 8 and 10. It should be noted here that the MAES spectral features of [Cnmim]BF4 RTILs with longer alkyl chains are very similar to those of an incomplete alkanethiol self-assembled monolayer (SAM) on a Au(111) surface,55 in which the emission feature shows a characteristic shape due to the existence of gauche defects in the alkyl chain. Our previous IVSFG study reveals that the alkyl chains of [Cnmim]BF4 with chain lengths n ) 8 and 10 contain gauche defects in their conformational structures.39 Spectral similarity of the incomplete SAM and [C8mim]BF4 and [C10mim]BF4 RTILs allows us to conclude that the octyl and decyl chains can form a loosely packed alkyl chain layer with gauche defects to cover the BF4- anion and the imidazolium ring located underneath. 3.3. MAES and UPS Spectra of [Cnmim]OTf RTILs. As in the case of [Cnmim]BF4, we obtained a series of MAES spectra (Figure 2d-f) for [Cnmim]OTf RTILs. Figure 2d shows a set of results of [C4mim]OTf for (i) UPS and (ii) MAES spectra and DOS of (iii) OTf anion and (iV) [C4mim]+ cation. The relative positions of DOS to the spectra are again calibrated by using our previous study.64 Contrary to the results of [C4mim]BF4 (Figure 2a), the MAES and UPS spectra in Figure 2d show different spectral shapes: the broad feature around 7 eV in MAES spectrum (Figure 2d(ii)) is assigned to the contribution from the [C4mim]+ cation while two peaks at 11 and 12 eV marked by filled triangles are attributed to the MOs mainly on the CF3 group of the OTf anion. As shown in Figure 3b, three peaks located within the lower BE region (from 5 to 8 eV) in the simulated DOS of the OTf anion are the contributions from the MOs localized at the SO3 group. It is interesting to see that, while these features are not observed in the MAES spectrum, they are observed in the UPS spectrum (marked by open triangles in Figure 2d), suggesting that the He* atoms do not reach the SO3 group to annihilate for electron emission. Furthermore, two prominent peaks at 11 and 12 eV in Figure 2d(ii) are assigned as anion contributions mostly from MOs on the CF3 group. Considering that MAES is a highly surface-sensitive technique, this strongly suggests that the CF3 group of the OTf anion is exposed to the vacuum side. Thus, we can conclude that OTf anions are polar oriented at the surface with the CF3 group pointing away from the bulk into the vacuum side, while the SO3 group of the OTf anion points to the bulk direction, and thus the impact of excited helium atoms with the SO3 group is hindered and no corresponding peaks are observed. As the number of carbon atoms in the alkyl chain increases, the two characteristic peaks (marked by filled triangles) around 11 eV originating from the MOs on the CF3 group gradually diminish and disappear at n ) 10 in the MAES spectra (Figure 2e(ii) and f(ii)). This result suggests that the outermost surfaces of the [Cnmim]OTf RTILs with longer alkyl chains also tend to be covered by the alkyl chains. The fact that the spectral features of the MAES spectra are very similar to those of an incomplete alkanethiol SAM on a Au(111) surface,55 clearly indicates that the protruding alkyl chains of [Cnmim]OTf RTILs have gauche defects as in the case of [Cnmim]BF4 RTILs. It is noticeable that the spectral features at 11 eV are still visible in the MAES spectra of n ) 8 (Figure 2e(ii)), which is unlike the case of [Cnmim]BF4 RTILs (Figure 2b(ii)), suggesting that coverage by the alkyl chain layer is incomplete for [C8mim]OTf. Our interpretation is that the difference in shielding by the octyl
Surface Structural Study on Ionic Liquids chains of the [C8mim]+ cations is a result of the different sizes of the anions. The larger OTf anion is expected to be less effectively shielded than the BF4 anion. 3.4. MAES and UPS Spectra of [Cnmim]TFSA RTILs. Representative data taken for [Cnmim]TFSA (n ) 4, 8, 10) are shown in Figure 2g-i. Again, each set of the result includes (i) UPS and (ii) MAES spectra and DOS of (iii) TFSA anion and (iV) [Cnmim]+ cation. Similar to the other RTILs with alkyl chain length n ) 4, both the contributions from the [C4mim]+ cation (the broad emission feature around 7 eV) and from the TFSA anion (the characteristic peaks in the higher BE region, marked by filled triangles) are observed in the MAES spectrum shown in Figure 2g(ii).64 Hence, we can reasonably conclude that the outermost surface of [C4mim]TFSA is also shared by the [C4mim]+ cation and the TFSA anion. It should be noted that the contributions from the MOs localized on the SO2 groups of the TFSA anion at the lower BE region, which appear in the UPS spectrum (marked by open triangles) are not observed in the MAES spectrum as shown in Figure 2g(ii). This result strongly supports the interpretation that the SO2 groups of the TFSA anion also are not present at the outermost surface of [C4mim]TFSA, as in the case of the SO3 group of [C4mim]OTf. For TFSA RTILs with alkyl chain lengths of n ) 8 and 10, Figure 2h,i shows relative intensity changes of the two peaks (filled triangles) originating from the MOs on the CF3 groups around 11 eV and their spectral features gradually become similar to those of the incomplete alkanethiol SAM.55 Thus, irrespective of the size of anions, the outermost surfaces of [Cnmim]X (X ) BF4, OTf, and TFSA) RTILs tend to be covered by the alkyl chains containing gauche defects. Because MAES peaks from the TFSA anion around 11 eV are still detectable even in n ) 10, it is easy to understand that a larger size TFSA anion is hardly covered even by the decyl chain. 4. Discussion In the MAES spectra of all the RTILs with the chain length n ) 4, the emission features of both the anions and cations are observed, indicating that the anions and cations share the outermost surfaces of the RTILs with alkyl chain length n ) 4. This interpretation is consistent with other surface structural studies.28,30 The comparison of MAES and UPS spectra with those of simulated DOS for [Cnmim]OTf and [Cnmim]TFSA RTILs, however, further indicates that OTf and TFSA anions polar orient at the surface: the CF3 groups point away from the bulk into the vacuum, while the SO3 and SO2 groups point toward the bulk due to the polar nature of these functional groups. The preferential orientations of these anions are also in good agreement with our previous results using IVSFG.42 As for the TFSA anion, it should be noted that two types of conformers (C1 and C2) may exist in the bulk.65 Because we do not observe the anion (SO2)-origin peaks within the low BE region around 7 eV in the MAES spectrum (Figure 2g(ii)), we can safely conclude that the C1 conformer dominates at the surface. In summary, the outermost surfaces of RTILs with alkyl chain length n ) 4 are shared by both the anions and the cations in which the nonpolar groups (butyl chain and CF3 groups) point away from the bulk to the vacuum side and the polar groups (SO3 and SO2 groups) toward the bulk liquid. Figure 4a shows a schematic surface structural model for RTILs with shorter alkyl chains. This model is also in good agreement with the MD simulation result of the surface structure of [C4mim]PF6, which predicts that the butyl chains prefer to be in the vacuum side and both the anions and cations are present at the outermost surface.48
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Figure 4. Schematic models for the surface structures of RTILs with (a) a shorter alkyl chain and (b) a longer alkyl chain. Here we chose the BF4- anion as a representative anion. When the alkyl chain is short, He* may hit either cations or anions and cause electron emission. When the alkyl chain is rather long, He* rarely hits the anion due to the presence of alkyl chain and no electron emission from the anion is observed.
For the RTILs with longer alkyl chains (n ) 8 and 10), a comparison of UPS and MAES results reveals that the longer alkyl chain preferentially orients at the outermost surface to cover the polar moieties of RTILs. This result is consistent with our previous IVSFG study that indicates a local clustering of alkyl chains at the air/RTIL interface of [Cnmim]BF4 RTILs with longer alkyl chains.39 X-ray diffraction as well as computational studies on the bulk structure of RTILs have shown that alkyl chains of n g 4 tend to aggregate in the liquid phase,22,66 and hence, such an aggregation of the alkyl chain may induce the formation of an alkyl chain layer or domain at the outermost surface of RTILs.39 Thus, it should be emphasized that RTIL outermost surface is formed such that the alkyl chain layer resides on top of the polar cation cores and the anions to reduce the surface energy. According to the previous study of RTIL surfaces using DRS, on the other hand, it is reported that both the cations and the anions are exposed to the vacuum side at the surface of [Cnmim]BF4 RTILs even for n ) 12.28,30 Though their interpretation seems to conflict with our MAES results, this discrepancy most likely arises from the difference in the probing depth of DRS and MAES. MAES uses metastable excited helium atoms with a large size (2.9 Å in radius) and low kinetic energy (
