Room-Temperature Ionic Liquid. A New Medium for Material

PERSPECTIVE pubs.acs.org/JPCL

Room-Temperature Ionic Liquid. A New Medium for Material Production and Analyses under Vacuum Conditions Susumu Kuwabata,*,†,‡ Tetsuya Tsuda,†,§ and Tsukasa Torimoto‡,^ †

Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan, ‡Japan Science and Technology Agency, CREST, Kawaguchi, Saitama 332-0012, Japan, § Frontier Research Base for Global Young Researchers, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan, and ^Department of Crystalline Material Sciences, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya, Aichi 464-8603, Japan

ABSTRACT A characteristic of negligible vapor pressure that a room temperature ionic liquid (RTIL) possesses enables us to introduce RTILs in the apparatus requiring vacuum conditions for material production and analyses. This combination creates a path toward development of new techniques under vacuum conditions. As for material production, especially metal nanoparticle synthesis, those are magnetron sputtering onto RTILs, plasma reduction in RTILs, physical vapor deposition onto RTILs, and electron beam and γ-ray irradiation to RTILs. Interestingly, the nanoparticles prepared in RTILs without any stabilizing agent do not aggregate in the RTILs. Also, we can introduce RTILs in analytical instruments requiring vacuum conditions such as X-ray photospectroscopy (XPS), matrixassisted laser desorption/ionization mass spectroscopy (MALDI-MS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The fact that the RTIL is not charged under irradiation by quantum beams enables us to establish new analytical techniques. Furthermore, homogeneous conditions that are obtainable by dissolving a substance in a RTIL are quite useful for conducting analyses using the instruments described above, for example, MALDI-MS, with high reproducibility.

as [BF4]-, [PF6]-, [(CF3SO2)2N]- (=[Tf2N]-), [CF3SO3](=[TfO]-), and other water-stable anions including [CH3CO2]-, [N(CN)2]- , and so forth, a “classic” RTIL, which is composed of an anhydrous metal halide combined with a heterocyclic aromatic halide, for example, AlCl3-1-ethyl-3-methylimidazolium chloride ([EtMeIm]Cl) and AlCl3-1-(1-butyl)pyridinium chloride ([BuPy]Cl), exhibits relatively low viscosity and high conductivity but is highly sensitive to moisture, that is, not air-stable, especially at higher Lewis acidity.5 Thus, current RTIL studies in most cases use modern RTILs. The negligible vapor pressure of most RTILs at room temperature invented a new technological concept. Only recently, RTILs were beginning to be applied to vacuum technology. This must be a revolutionary incident in science history because people never would have imagined a wet world in vacuum. There are many manufacturing machines and analytical instruments that require vacuum conditions. Of course, they are designed under the premise that materials treated in them are dry and solid. Conventional procedures cannot be applied to a wet sample, although we occasionally get carried away with

S

ome readers may know that there are a great number of factors behind the worldwide interest in roomtemperature ionic liquids (RTILs), which are liquid salts having a liquid phase at 298 K. Some of these factors include high ionic conductivity (e120 mS cm-1), a wide liquidus temperature range (173-450 K), wide electrochemical windows (e5.8 V), a negligible vapor pressure (e5 10-9 Torr), and easily tunable physicochemical properties.1-4 Only 10 years ago, we could follow all of the RTILs, but now, we never do that because numerous RTILs have been produced in this decade. However, most RTILs can be classified into seven families on the basis of their cationic structures, as depicted in Figure 1 along with typical side chains and anions. (In this Perspective, the abbreviation forms exhibited in this figure are exploited to represent RTILs.) There are many technologies that can take advantage of the physicochemical inertness, that is, high physicochemical stability, of the RTILs. Examples are the use of the RTILs as electrolytes for a Li secondary battery and low-temperature PEM fuel cell, as reaction solvents for organic synthesis and nanoparticle preparation, as extract agents for rare metal ions and CO2, and as lubricants for space technology. There is no longer any doubt that RTILs contribute to create future science and technologies.1-4 Compared with “modern” RTILs having fluoroanions such

r 2010 American Chemical Society

Received Date: June 29, 2010 Accepted Date: October 8, 2010 Published on Web Date: October 20, 2010

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DOI: 10.1021/jz100876m |J. Phys. Chem. Lett. 2010, 1, 3177–3188


Figure 1. Typical cations and anions used to prepare RTILs. Abbreviation forms are indicated by bold type.

our desire to deal with it in the vacuum equipment. On the other hand, RTILs can be put in the vacuum equipment without particular care, as will be introduced in this Perspective. As a matter of fact, there are some other liquids having very low vapor pressure, like lubricants including silicone grease, which can be introduced into the vacuum chamber. These liquids are not adequate for chemical and physical reaction media due to their extremely high viscosity. Thus, we believe that RTILs, which have negligible vapor pressure and lower viscosity, must create a mysterious wet world in vacuo. At this point, the numbers of researchers who exploit RTILs in vacuum equipment are quite limited. We hope that this Perspective will trigger further explosive progress of the vacuum technology using the RTIL.

hard as it may be to believe, the research related to the material production in the RTIL under vacuum conditions is still in its early stages. To our knowledge, the first material production-related study in RTILs under vacuum conditions was reported by Scherson et al. in 2005.6 They succeeded in revealing the Al electrodeposition process on ultrapure Au and W electrodes in a Lewis acidic 52.4-47.6 mol % AlCl3[EtMeIm]Cl RTIL under ultrahigh vacuum conditions (5 10-9 Torr). Note that the vacuum stability of Lewis acid-base or Brønsted acid-base type RTILs strongly depends on the composition of the RTILs because some kind of equilibrium reaction that alters with the composition exists. Such an equilibrium reaction often releases neutral molecules in the RTILs. Especially, the neutral molecules are readily sublimed in a vacuum at higher temperature. For example, in the AlCl3[EtMeIm]Cl RTIL, two main equilibrium reactions exist.

The negligible vapor pressure of most RTILs at room temperature invented a new technological concept. Only recently, RTILs were beginning to be applied to vacuum technology.

AlCl3 þ ½EtMeImCl a ½AlCl4 - þ ½EtMeImþ AlCl3 molar fraction e 50 mol % ½AlCl4 - þ AlCl3 a ½Al2 Cl7 -

50 mol % < AlCl3 molar fraction

ð2Þ

where [Al2Cl7]- is the dominant acidic species in the Lewis acidic RTIL containing below ∼65 mol % AlCl3.7 The existence of excess AlCl3 makes the vacuum stability lower due to reaction 2, which can release AlCl3 in a stronger Lewis acidic RTIL with relative ease. Thus, now non-acid-base type liquid

Material Production under Vacuum Conditions. A great number of articles on RTILs have been reported to date, but as


ð1Þ

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Figure 2. Schematic illustration of the Au nanoparticle formation mechanism during Au sputtering onto a RTIL.

salt, that is, a single liquid salt system, is mostly used as a solvent for material production under vacuum conditions. Only recently, a Cu electrode reaction in [(N-methylacetate)-4picolinium][Tf2N] RTIL under ultrahigh vacuum condition (5 10-9 Torr) was reported for development of a new spectroelectrochemical method.8 We believe that metal nanoparticle preparation in RTILs will contribute to the development of future technology because those metal nanoparticles are not covered with any covalently adsorbed stabilizing agents that often adversely affect the physicochemical properties of the nanoparticles. The stabilization mechanism is not entirely clear, but it would not be an exaggeration to say that the RTIL has a relatively strong interaction with the surface of the metal nanoparticles. The metal nanoparticles have been prepared in RTILs using various reaction modes. Those preparation methods and characteristics of the prepared nanoparticles were painstakingly reviewed by Dupont and Scholten.9 In the cases of metal nanoparticle preparations by chemical reduction of metal ions or metal complexes, the stabilizing agent is not required in the RTIL, but several kinds of byproducts must be dissolved in the resulting nanoparticle-suspended liquids. Metal nanoparticle preparations in RTILs under vacuum conditions, which are introduced here, are groundbreaking


techniques that enable synthesis of target nanoparticles without a significant amount of byproduct.

Metal nanoparticle preparations in RTILs under vacuum conditions, which are introduced here, are groundbreaking techniques that enable synthesis of target nanoparticles without a significant amount of byproduct. Magnetron Sputtering onto RTILs. This procedure was established by our research group.10 In principle, all elements that can be ejected by Arþ and N2þ plasma bombardment are nanoparticulated by this method. Figure 2 shows a schematic illustration of this method that uses a common magnetron sputtering apparatuss except for the use of a RTIL as a substrate. This method has achieved the preparation of various pure metal nanoparticles, such as Au,10-14 Ag,15,16 Pt17,18 and so

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Figure 3. Photographs of [BuMeIm][PF6] RTILs after sputtering experiments at Au-Ag targets having different surface area ratios and TEM images of the resulting nanoparticles obtained at each Au-Ag target.

forth, possessing particle sizes less than 10 nm in diameter without any specific stabilizing agent. A small-angle X-ray scatting study revealed the initial formation mechanism of the gold nanoparticles during the sputtering process onto several [1,3-dialkylimidazolium][BF4].14 The proposed formation mechanism is divided into two phases, as shown in Figure 2, where it was concluded that both surface tension and viscosity of the RTIL are important factors for the Au nanoparticle growth and its stabilization. Interestingly, this technique also enables production of alloy nanoparticles by placing different elements as a target. Figure 3 shows the first attempt to prepare Au-Ag alloy nanoparticles by the sputtering method with a target having Au and Ag plates of the same area.15 As recognized from TEM images, the mean particle size enlarges with an increase in the Ag area in the target. In addition to this, the chemical composition and the optical properties of the deposited alloy nanoparticles vary with the surface area ratio of Au to Ag, too. It implies that this approach can directly control the alloy composition by changing the ratio of the metal areas in the target.


Plasma Deposition Method. This technique was proposed by Endres and co-workers, who have succeeded in preparation of Ag, Cu, and Al nanoparticles using this technique.19-21 This approach, once called glow discharge electrolysis, is based on historical articles reported about 100 years ago. Schematic illustration of the system for the plasma deposition is illustrated in Figure 4A. More accurately, plasma generation does not need vacuum conditions, but appropriate gas of low pressure is required to generate a stable plasma. However, the use of RTILs is also essential in this case because the presence of vapor of a volatile liquid in the gas phase would inhibit the plasma generation. Photographs of a typical plasma experiment conducted under Arþ plasma irradiation are shown in Figure 4B.21 The reaction media was [EtMeIm][Tf2N] with 62 mmol L-1 Cu(I). Obviously, a dark brown layer that appeared at the interphase between the RTIL and Arþ plasma phase was growing with plasma irradiation time, indicating that Cu(I) was reduced to Cu metal that formed nanoparticles with an average size of ∼11 nm. However, somehow, the surface was covered with a copper oxide layer. If the metal nanoparticles are yielded under vacuum or inert gas

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Figure 5. UV-vis spectra of several RTILs with 0.5 mmol L-1 NaAuCl4 3 6H2O after accelerator electron beam irradiation experiments at 6 or 20 kGy.

nanoparticles size was ∼3 nm, and the particles were about 2 nm apart from each other. The UV spectra for the RTIL containing the produced Cu nanoparticles showed a clear absorption peak at 576 nm corresponding to surface plasmon resonance for Cu nanoparticles soon after the preparation. However, when the liquid was left in air, the peak reduced with time, suggesting formation of a copper oxide layer on the nanoparticles due to oxygen oxidation. Electron Beam and γ-ray Irradiation. These methods exploit solvated electrons and/or radicals yielded during very strong electron beam26,27 and γ-ray27 irradiation of the RTIL containing metal salts so as to synthesize metal nanoparticles. Note that primary the electron beam and γ-rays themselves cannot directly contribute to nanoparticle preparation because of their considerably strong energy. In other words, no solvated electrons and no radicals would result in no nanoparticles. This has been already verified through the experiments using a variety of [Tf2N]--based RTILs. As a typical example, variation in UV-vis spectra of the RTILs with 0.5 mmol L-1 NaAuCl4 3 2H2O after accelerator electron beam irradiation is shown in Figure 5.27 The accelerator electron beam irradiation at 20 kGy of [BuMeIm][Tf2N] and [MePrPip][Tf2N] resulted in appearance of a broad absorption peak, which is attributable to plasmon absorption of Au nanoparticles, whereas irradiation of [Bu3MeN][Tf2N] caused almost no spectrum change. In the case of electron beam irradiation at 6 kGy, only [BuMeIm][Tf2N] showed spectral change. These results indicate that Au nanoparticle generation in the RTILs becomes easier in the following order

Figure 4. Schematic illustration of the experimental setup for plasma electrochemical reduction of metal ions dissolved in RTILs (A) and photographs of the plasma electrochemical reduction experiment of Cu(I) dissolved in a RTIL at different reduction times (B).21 Reproduced by permission of the PCCP Owner Societies.

condition, this is a common issue for which it is very difficult to collect metallic state nanoparticles, especially base metal nanoparticles that are oxidized readily under atmospheric condition. One of solution methodologies about this will be introduced in a later section. Very recently, the plasma deposition method was adopted for preparation of Au22,23 and Pt23 nanoparticles. The relationship between deposition conditions and the characteristics of the prepared nanoparticles was studied in detail. One of interesting findings is that nanoparticles were prepared even if the cathode was placed in the RTIL phase and not the gas phase. Physical Vapor Deposition Method. This method was developed based on the solvated metal atom dispersion technique that is used for the preparation of low-valent main group halides and transition-metal arene complexes. This physical vapor deposition method can synthesize pure metal nanoparticles such as Au and Cu24 as well as EuF2 showing luminescent behavior.25 The TEM images revealed that the mean


½Bu3 MeN½Tf2 N < ½MePrPip½Tf2 N < ½BuMeIm½Tf2 N Considering the fact that [Bu3MeN][Tf2N] is more radiochemically stable than [BuMeIm][Tf2N],28-31 it is highly likely that radiochemically unstable RTILs tend to generate Au nanoparticles. TEM observation revealed that the Au nanoparticles prepared in the experiments using the [BuMeIm][Tf2N] solution had a mean particle size of 26.4 nm at 20 kGy irradiation and 7.6 nm at 6 kGy. Again, such solvated electrons and/or radicals will be yielded only under very strong electron beam and γ-ray irradiation, not moderate irradiation described in the next section.

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Figure 6. Schematic illustration of an existing common γ-ray or accelerator electron beam irradiation industrial plant. The RTIL solution with metal salts is encapsulated in the glass ampules under vacuum or Ar atmosphere conditions.

Future Challenges in This Field. As you may understand, in theory, due to the high physicochemical stability of the RTILs, above-mentioned RTIL vacuum techniques can produce even base metal nanoparticles that cannot be produced in conventional aqueous or organic solvents. Future challenge in this field will be how we collect the metal nanoparticles suspended in RTILs to make effective utilization of their functions and how we develop a metal nanoparticle mass production method. Regarding the former matter, we found a facile way, which is adsorption of the suspended nanoparticles on a solid substance.12,17,18 Nanoparticles are stably suspended in RTILs by the interaction with ionic species exciting around the nanoparticles. Heating of the suspension on a carbon plate seems to weaken the interaction, resulting in their adsorption on the plate. When a glassy carbon plate on which Pt nanoparticles were adsorbed was used as an electrode, it exhibited high catalytic activity against O2 reduction due to the adsorbed Pt nanoparticles.17,18 On the other hand, a recent approach carried out at an existing common γ-ray or accelerator electron beam irradiation industrial plant for sterilizing medical kits may be a key to overcome the mass production issue.27 As illustrated in Figure 6, if the glass ampules, in which RTIL solutions with metal salts are encapsulated under vacuum condition or inert gas condition, placed on the container are automatically transferred to the irradiation position, metal salts should be reduced to the metal state in the ampules without any contamination derived from air. This would be one way to churn out metal nanoparticles because the industrial plant can irradiate 200 glass ampules 100 mL at a time. The irradiation times of the accelerated electron beam and γ-ray are 7 s and 3 h, respectively, if the irradiation dose is 20 kGy.

limited to solid materials to keep the vacuum. If we need to analyze a liquid or a wet sample by using one of those instruments, it must be required to follow complicated procedures including preservation of the frozen condition of the sample in a vacuum chamber during the analysis. As some of you may know, the procedure is very tough, and the extra instrument for the frozen preservation is expensive. A limited number of persons are able to make the analysis. Negligible vapor pressure has become common sense as one of characteristics that RTILs possess. However, from the discovery of RTILs, it took a long time for us to introduce RTILs into the vacuum chamber of the ultraprecise analytical instruments, which dislike extreme contamination in the chamber. Recently, thermodynamics and kinetics of the vaporization and/or decomposition of RTILs have been revealed by heating experiments of the RTILs under ultravacuum conditions.32-35 Now, on the basis of these data, we can safely put nonvolatile RTILs into the vacuum instruments. Combination of the RTIL and the analytical instruments, which had almost no relation to liquid chemistry so far, opens up an innovative analytical method without complicated procedures and expensive additional equipment.

Negligible vapor pressure has become common sense as one of characteristics that RTILs possess. However, from the discovery of RTILs, it took a long time for us to introduce RTILs into the vacuum chamber of the ultraprecise analytical instruments, which dislike extreme contamination in the chamber.

Analyses under Vacuum Condition. Vacuum condition is one of the essential requirements for ultraprecise analyses at the molecular and atomic levels because all atoms and molecules other than target species are excluded under the condition and/or it makes possible to irradiate the stable quantum beam and to detect the generated signals with high S/N ratio. Therefore, to introduce a volatile substance is normally prohibited into the sample chamber of the vacuum analytical instrument; that is, the matrixes for the instruments are


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MALDI Mass Spectroscopy. A mass spectrometer is an immensely useful instrument for determining the elemental composition of a sample or a molecule in a vacuum chamber, and it is composed of a part for evaporating and ionizing chemical compounds to generate charge molecules or molecule fragments and a part for measuring their mass-to-charge ratios. These days, both parts have been considerably improved to enable analysis of large molecular species like biologically relevant molecules. The matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is one of the groundbreaking techniques that vaporize large compounds by laser irradiation with assistance of an appropriate matrix. Matrix selection is very important, and an ideal matrix is a material possessing a sufficient absorption coefficient at the laser wavelength region, low vapor pressure, the ability to dissolve or cocrystallize with the sample, and the ability to promote ionization of the sample without its significant decomposition. Although both solid and liquid have been employed as a matrix, the former is used more widely because of its low vapor pressure and UV adsorption ability. However, the solid matrix possesses an inevitable drawback, which is heterogeneity of the prepared mixture of the matrix and sample. Irradiation of a laser shot induces vaporization of a part of the analyte spot, as schematically shown in Figure 8a, and samples contained in the part are vaporized. If samples are not homogeneously dispersed in the matrix, MS signals should be affected by the position of the laser shot, resulting in poor shot-to-shot reproducibility. This reason makes it very difficult to use MALDI-MS for quantification analysis. From this viewpoint, a liquid matrix should be more desirable. So far, some liquids possessing low vapor pressure, such as glycerol and 3-nitrobenzyl alcohol, have been examined as a liquid matrix for MALDI-MS. Shot-to-shot reproducibility was actually improved because of their homogeneous condition, but their inherent volatility still causes some problems, such as a decrease in their amounts with time. Another problem is that these liquids possess no UV absorbability. Because RTILs possess both no volatility and UV absorbability, it seemed to be an ideal solution matrix for MALDI-MS, but usual RTILs such as [BuMeIm][BF4] and [BuMeIm][PF6] were unfortunately unable to ionize samples dissolved in them.45 Armstrong et al. designed a new ionic liquid family for the liquid matrixes using solid acidic compounds of R-cyano-4-hydroxycinnamic acid (CHCA), sinapinic acid (SA), and 2,5-dihydroxybenzoic acid (DHB), which are widely used as solid matrixes for MALDI-MS.45,46 It was then found that some of them kept the liquid state at room temperature and worked as liquid matrixes for detection of the polymer and some biomolecules by MALDI-MS.45-48 Figure 8b shows a change in signal intensities of [M þ H]þ obtained at 90 different positions on a spot of the sample-matrix mixture. As expected, the RTIL matrix of R-cyano-4-hydroxycinnamic acid butylamine (CHCAB) gave much narrower data dispersion than that obtained for the solid matrix of CHCA, indicating, evidently, the usefulness of the RTIL matrix for improvement of reproducibility.46 Another feature of the RTIL matrix is the higher ability to suppress decomposition of the sample than the conventional solid matrix. Use of CHCAbased guanidium salt and its analogous salts as RTIL matrixes

Figure 7. XPS spectra of a solution of Pd(Oac)2 in ECOENG 212. The red line shows data recorded at the start of the XPS experiment, and the black line presents data recorded 6 h later.39 Reproduced by permission of The Royal Society of Chemistry.

XPS Analysis. This instrument is remarkably useful for analyzing the composition of solid materials and the chemical state of each element. Vacuum condition is inevitable for avoiding contamination and detecting generated photoelectrons with high sensitivity. Once, analyses of the liquid surface were attempted under vacuum conditions, but intricately designed sample stages were necessary to reduce the influence of vaporization during the analysis.36-38 In cases of a RTIL that is not vaporized under ultravacuum conditions at room temperature, the liquid can be put in the chamber of the XPS without any specific technique or modification. [EtMeIm][EtOSO3] was the first RTIL subjected to XPS analyses,39 and XPS analyses of several kinds of RTILs were conducted.40-43 All RTILs gave high-resolution XPS spectra without any charge compensation, but the peak intensities decreased by freezing the liquids,39,40 indicating that RTILs behave as electrical conductors under appropriate conditions. The peak intensities obtained by usual XPS analyses of the RTIL itself corresponded to ratios of elements included in cationic and anionic species of the liquid. When XPS spectra were taken by grazing electron emission, changes in the peak intensity with electron emission angle provided significant information on the surface structures of the RTIL. In imidazolium-based RTILs having long alkyl chains, the obtained XPS spectra suggested that long alkyl chains jut out from the bulk liquid.40,43 XPS analysis is a useful approach for in situ monitoring of chemical reactions in RTILs, too. Reduction of Pd(II) to Pd(0) was monitored as the first demonstration.39 The Pd(II) species used was a Heck catalyst (Pd(OAc)2(PPh3)2) dissolved in [EtMeIm][EtOSO3] (ECOENG 212). Figure 7 shows variation in the peak intensity for the Pd(II) and Pd(0) before and after the XPS experiment. After the experiment, the peak for the Pd(II) decreased, and the Pd(0) increased. The results indicate that the Heck catalyst is unstable in this RTIL. In addition to this, in situ monitoring of electrochemical reactions was also carried out recently by introducing an electrochemical cell in an XPS chamber.44


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Figure 9. SEM images of droplet of silicone oil (a), [BuMeIm][BF4] (b), [EtMeIm][BF4] (c), and [EtMeIm][Tf2N] (d).

Figure 8. Schematic illustration of MALDI (a). [M þ H]þ ion intensities from 90 positions on a human angiotensin II preparation with a RTIL matrix CHCAB (black triangles) and with a traditional CHCA matrix (gray diamonds) (b, left). The black bar indicates relative standard deviation (RSD) values found using RTIL matrixes, and the gray bar indicates RSD values of the data series yielded by the respective traditional MALDI matrixes (b, right). Reproduced with permission from ref 46.

enabled detection of oligosaccharides, which exhibit poor ionization efficiencies and tend to have thermal fragmentation through the loss of SO3 groups, with suppression of the loss of SO3.47,48 SEM Observation and EDX Analysis. Scanning electron microscopy enables us to observe electrically conducting samples with high magnification in vacuum. If one would like to observe an insulating sample, it is required to give electric conductivity to the sample by vacuum deposition of a metal or carbon. Otherwise, the insulating sample must be charged by irradiation of the electron beam, giving a low-quality image accompanied with lots of noise and distortion. Our research group first attempted to observe RTILs by SEM because RTILs can be put in the vacuum chamber of the SEM. A silicone oil droplet exhibited a white image with some noise caused by charging behavior (Figure 9a), but surprisingly, all RTILs gave dark contrast images (Figure 9b-d).


Figure 10. SEM images of the surface and the cross-sectional surface of a wad of cotton that was subjected to Au sputtering (a) and was immersed in a 0.1 mol L-1 [BuMeIm][Tf2N] ethanolic solution, followed by drying (b).

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Figure 11. In situ ECSEM observation of thickness variations in a PPy film in [BuMeIm][Tf2N] caused by changing the electrode potential (vs Ag(I)/Ag) (a). EDX line analysis along a white line drawn in the SEM image by X-ray intensity at 3.310 keV corresponding to K-KR. Results for PPy polarized at -1.70 (solid line) and þ0.87 V versus Ag(I)/Ag (broken line) are shown (b). Redox reaction mechanism determined by in situ ECSEM and EDX measurements using binary [BuMeIm][Tf2N]-K[Tf2N] RTIL (c).

It suggests that the RTILs behave as electrically conductive materials for SEM observations.49 It has been revealed by pulse radiolysis experiments that electrons injected in RTILs with high accelerated voltage are stabilized in condensed ion species, allowing electrons to move in the liquid.50 Similar behavior may occur during the SEM observation, considering avoidance of charging of the liquid. This property is useful as a way to give electrical conductivity to insulating samples. Figure 10 shows a typical example, SEM images of the surface and cross-sectional surface of a wad of cotton. On the cotton subjected to Au sputtering, the surface is observed by SEM without charging, but the cross-sectional view gives a charged image due to presence of Au-free cotton fibers. On the other hand, cotton that was immersed in 0.1 mol L-1 [BuMeIm][Tf2N] ethanolic solution and dried under vacuum gives uncharged clear images in both cases.51 RTILs are well-known as a favorable electrolyte for several kinds of electrochemical reactions. If nonvolatile species are used in the reaction, the desired reaction should take place even in the SEM chamber. It is then possible to


observe progress of these electrochemical reactions by SEM. Such in situ electrochemical SEM (ECSEM) observation has been attempted by introducing an electrochemical cell in a common SEM system. As the first demonstration, a crosssectional view of a polypyrrole (PPy) film deposited on a Pt electrode was observed while applying various potentials to the electrode.52 [BuMeIm][Tf2N] was the electrolyte in this case. Figure 11a shows SEM images, indicating variations in the PPy film thickness caused by the electrode potential steps. Increase in the film thickness at negative potentials suggests incorporation of electrolyte cations in the film by reduction of PPy. This assumption was evidenced by in situ energy-dispersive X-ray spectroscopy (in situ EDX). To reveal the cation behavior during application of the potential, KTf2N as a marker was dissolved in the RTIL, and the K content in the PPy film at oxidized and reduced states was compared by the line analyses, as shown in Figure 11b. The K content in the PPy film obviously increased at more negative potential. It supports the aforementioned assumption. In situ SEM observation of electrochemical deposition

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with the vacuum equipment. Unfortunately, however, the RTIL still is not widely recognized, except in the chemical field, and the researchers who employ vacuum equipment hesitate to put the RTIL in the equipment even if the liquid is involatile. We hope some of the readers are motivated to begin wet science and technology in vacuum conditions, of course using RTILs. We welcome the newcomers of great promise in vacuum technology with RTILs. It is time to voyage to the RTIL Sea in vacuum equipment!

We hope some of the readers are motivated to begin wet science and technology in vacuum conditions, of course using RTILs. We welcome the newcomers of great promise in vacuum technology with RTILs. It is time to voyage to the RTIL Sea in vacuum equipment!

Figure 12. TEM images of bare silica particles (a) and (PMMA)grafted silica particles (b) dispersed in [EtMeIm][Tf2N]. (a) Reproduced with permission from refs 56 and (b) 57. Reproduced by permission of The Royal Society of Chemistry.

of metals has been also examined using an electrochemical cell, which was specially designed for the in situ electrochemical SEM observation.53,54 TEM Observation. As already mentioned in the Material Production under Vacuum Conditions paragraphs, the metal nanoparticles produced in RTILs can be directly observed by TEM if the RTILs do not vaporize even under ultravacuum. By this finding, we could know the interesting fact that metal nanoparticles were stably dispersed in RTILs without significant aggregation and that the TEM observation is a useful technique to reveal the aggregation prevention mechanism of the nanoparticles in the RTIL. It was recently found by combination of optical measurements and TEM observation that RTILs containing a small amount of impurity such as 1-methylimidazole prevents aggregation of Au nanoparticles much more efficiently than that of very pure RTILs.55 Watanabe et al. have contended with the aggregation prevention effects using silica nanoparticles as model particles. As well as bare silica nanoparticles having hydrophilic Si-OH groups on their surface, they prepared hydrophobic nanoparticles by grafting poly(methyl methacrylate) chains on the silica nanoparticles. Dispersion of them in [EtMeIm][Tf2N] gave apparently different TEM images, as shown in Figure 12. The hydrophilic nanoparticles tended to aggregate (Figure 12a), whereas the hydrophobic ones kept their dispersive state even at higher silica concentration (Figure 12b).56,57 These different behaviors are discussed based on the colloidal stabilization theory.56 The RTIL is the first liquid that works well as reaction media even under vacuum conditions. New RTILs and their relatives, for example, a urea-choline chloride mixture and zwitterionic compounds, are being synthesized every day because their properties can be readily designed by introducing functional groups on the ionic component. On the other hand, modern vacuum technology already supports our comfortable daily lives. However, to apply RTIL science to vacuum technology likely contributes to further development of science and technology, and the wet condition in vacuum obtained by introducing RTILd must fascinate researchers dealing


AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: kuwabata@ chem.eng.osaka-u.ac.jp.

Biographies Susumu Kuwabata has been a Professor of the Graduate School of Engineering at Osaka University since 2002. He received his Dr. Eng. from Osaka University in 1991. His current research interests are centered on electrochemistry and functional nanomaterials, including design of the solid/liquid interface on the nanometer scale to enhance electron transfer and visualization of the electron-transfer reactions using ionic liquids. His research credentials include over 150 original papers, 6 book chapters, and over 15 patents. Tetsuya Tsuda is an assistant professor in the Graduate School of Engineering at Osaka University. He received his Ph.D. in Energy Science from Kyoto University, Japan, in 2001. He started his academic career at The University of Mississippi under the direction of Professor Charles L. Hussey, who is one of the fathers of modern RTIL science. His research interests are energy science and materials science related to electrochemistry in RTILs. Tsukasa Torimoto has been a Professor of the Graduate School of Engineering at Nagoya University since 2005. He received his Ph.D. from Osaka University in 1994. He started an academic career at Osaka University in 1994 as a research associate. From 2000 to 2005, he worked at Hokkaido University as an associate professor. His main research interests are the preparation of novel semiconductor and metal nanoparticles and their application to the energy conversion systems.

ACKNOWLEDGMENT The research of the authors was supported by Core Research for Evolution Science and Technology (CREST) from the Japan Science and Technology Agency ( JST).

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Room-Temperature Ionic Liquid. A New Medium for Material

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