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Ionic Conductivity in Ionic Liquid Nano Thin Films Shingo Maruyama, Ida Bagus Hendra Prastiawan, Kaho Toyabe, Yuji Higuchi, Tomoyuki Koganezawa, Momoji Kubo, and Yuji Matsumoto ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b06386 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018
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Ionic Conductivity in Ionic Liquid Nano Thin Films Shingo Maruyama†,§, Ida Bagus Hendra Prastiawan‡,§ , Kaho Toyabe†, Yuji Higuchi‡, Tomoyuki Koganezawa#, Momoji Kubo‡, and Yuji Matsumoto†,* †
Department of Applied Chemistry, School of Engineering, Tohoku University, Sendai 980-8579,
Japan ‡
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
#
Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Kouto, Sayo, Hyogo
679-5198, Japan KEYWORDS: ionic liquid, thin film, ionic conductivity, molecular dynamics simulation, interface, laser deposition
ABSTRACT: Thin film approaches are powerful methods for gaining a nanoscale understanding of interfacial ionic liquids (ILs) in the vicinity of solids. These approaches are used to directly elucidate the interfacial contributions to the physical properties of ILs since nanoscale thin films have significant proportions of the surface or interface region with respect to its total volume. Here we report the growth of a uniform [emim][TFSA] thin film ionic liquid (TF-IL) on chemically modified, well-wettable sapphire, thereby allowing the in situ measurement of its ionic conductivity on the nanoscale. We observed a thickness dependent behavior of the ionic
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conductivity, which was gradually decreasing especially when the thickness was less than 10 nm, and found it to be quantitatively analyzed well by using an empirical two-layer model. The molecular dynamics (MD) simulations show that the thickness dependent ionic conductivity originates from the solid-like structuring of the IL near the substrate, reproducing well such thickness dependent ionic conductivity. The MD simulation results suggest that the thickness of the low conductivity region determined in the two-layer model should roughly correspond to the thickness of the solid-like structuring of the IL near the substrate.
Ionic liquids (ILs) have various interesting properties, such as low melting points, low vapor pressures, and consequently lower volatilities. In addition, ILs are designable fluids due to the large selection of cation and anion pairs. The interfacial structures and dynamics of the ILs on solid surfaces1,2 must be understood to engineer IL-solid interfacial behaviors especially in the fields of electrochemistry,3 tribology,4 etc. In general, interfacial liquid molecules have a different solvation environment as compared to those in the bulk since they are in contact with either solids or a vacuum. There are two distinct experimental approaches for investigating IL-solid interfaces: (1) indirect measurements of the buried interface of a bulk IL and (2) direct investigations of the structure and physical properties of thin film ILs (TF-ILs) grown on the nanoscale. In the former approach, various sophisticated and sensitive interface analysis techniques, including atomic force microscopy (AFM),5–11 surface force apparatus (SFA),12–18 X-ray reflectometry (XRR),8,19,20 neutron reflectometry,21 sum-frequency generation spectroscopy (SFG),22–28 and surface-enhanced infrared absorption spectroscopy (SEIRAS),29,30 have been employed so far. Most of the results from these
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techniques indicate pronounced structuring and molecular layering in the region close to the solid surface within several nanometers. In particular, some SFA studies have revealed that the ILs confined between two charged solid surfaces with a gap less than 10 nm show a drastic enhancement of their effective viscosity.15,16 In the latter approach, there have been many attempts to determine the adsorption and the subsequent multilayered structures of IL molecules in the initial deposition of the IL on a solid surface by means of various sensitive surface analysis techniques, e.g., photoemission spectroscopy (PES),31–38 infrared reflection absorption spectroscopy (IRAS),39–43 time-of-flight secondary ion mass spectrometry (TOF-SIMS),44–46 scanning tunneling microscopy (STM),38,47–49 and AFM.37 However, further deposition of the IL to fabricate its thin film, owing to the poor wettability of ILs, especially on nonmetallic solid substrates, often results in a non-uniform dispersion of three-dimensional (3D) liquid droplets over the substrate. Such a non-uniformity of the IL thin film hampers the characterization of the IL/substrate interface structures as well as the macroscopic measurements of their physical properties. If a uniform TF-IL can be prepared with a well-defined nanoscale thickness, it will be a good model for studying the effects of the film thickness and the surface chemical properties of the substrate on the physical properties, such as ionic conductivity, of nanoscale TF-ILs on solid materials. Computational simulation is a powerful tool for investigating molecular-level phenomena that are often difficult to study only experimentally, though it should use simplified models in some cases. Classical molecular dynamics (MD) simulations have been used to study the structures50,51 and dynamics52–54 of ILs at the molecular level. MD simulations have predicted that the different lengths of the alkyl chains in the cation affect the detailed structure of ILs,50,51 which was later confirmed experimentally.55 The capability of MD simulations to estimate the macroscopic
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physical properties of ILs, such as density, ionic conductivity, viscosity, etc,53,54 allows for direct comparison with experimental results. Although many MD studies on ILs in the vicinity of solids have focused mainly on the interfacial structure of the ILs near the solids,56–58 information regarding the physical properties of interfacial ILs is still scarce, and the effects of the liquid thickness on the physical properties is unclear. In addition, to the best of our knowledge, there are no reports of a direct comparison of MD simulations and experimental results in terms of macroscopic physical properties of the nanoscale interfacial ILs. To obtain basic insight into the ionic conduction behavior of interfacial ILs, we employed a direct approach to investigate the structure and physical properties of a TF-IL, taking advantage of our original nanoscale deposition technique for ILs in a vacuum.59 In this study, we grew uniform IL thin films of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][TFSA]) with well-defined thicknesses on the nanoscale on a surface-modified, wellwettable sapphire (α-Al2O3(0001)) substrate. From a combined approach of in situ in-plane ionic conductivity measurements of the nanoscale TF-IL and MD simulations, the thickness dependent in-plane ionic conductivity was confirmed. The origin of this behavior is discussed in terms of the liquid structure and the dynamics of the constituent ions.
RESULTS Improvement of IL wettability on sapphire. As already mentioned above, ILs typically form 3D droplets on most solid surfaces, e.g., a single crystal sapphire substrate used in this study. To improve the wettability of the ILs on those substrates, we carried out a wetting layer treatment60 (Figure 1a): a bulk drop of [emim][TFSA] is evaporated in a tube furnace at 300 °C for 20 min in
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vacuum. After this treatment, the complete wetting of [emim][TFSA] (the contact angle became
