Published on Web Date: October 20, 2010

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Room-Temperature Ionic Liquid. A New Mediumfor Material Production and Analyses underVacuum ConditionsSusumu 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 Acharacteristic of negligible vapor pressure that a room temperatureionic liquid (RTIL) possesses enables us to introduce RTILs in the apparatusrequiring vacuum conditions for material production and analyses. This combina-tion creates a path toward development of new techniques under vacuum condi-tions. As formaterial production, especiallymetal nanoparticle synthesis, those aremagnetron sputtering onto RTILs, plasma reduction in RTILs, physical vapordeposition onto RTILs, and electron beam and γ-ray irradiation to RTILs. Interest-ingly, the nanoparticles prepared in RTILs without any stabilizing agent do notaggregate in the RTILs. Also, we can introduce RTILs in analytical instrumentsrequiring vacuum conditions such as X-ray photospectroscopy (XPS), matrix-assisted laser desorption/ionization mass spectroscopy (MALDI-MS), scanningelectron microscopy (SEM), and transmission electron microscopy (TEM). Thefact that the RTIL is not charged under irradiation by quantum beams enables us toestablish new analytical techniques. Furthermore, homogeneous conditions thatare obtainable by dissolving a substance in a RTIL are quite useful for conductinganalyses using the instruments described above, for example,MALDI-MS,with highreproducibility.

S ome readers may know that there are a great numberof factors behind the worldwide interest in room-temperature ionic liquids (RTILs), which are liquid salts

having a liquid phase at 298 K. Some of these factors includehigh ionic conductivity (e120 mS cm-1), a wide liquidustemperature range (173-450 K), wide electrochemical win-dows (e5.8 V), a negligible vapor pressure (e5 � 10-9 Torr),and easily tunable physicochemical properties.1-4 Only 10years ago, we could follow all of the RTILs, but now, we neverdo that because numerous RTILs have been produced in thisdecade. However, most RTILs can be classified into sevenfamilies on the basis of their cationic structures, as depicted inFigure 1 along with typical side chains and anions. (In thisPerspective, the abbreviation formsexhibited in this figure areexploited to represent RTILs.)

There are many technologies that can take advantage of thephysicochemical inertness, that is, highphysicochemical stability,of the RTILs. Examples are the use of theRTILs as electrolytes fora Li secondary battery and low-temperature PEM fuel cell, asreaction solvents for organic synthesis and nanoparticle pre-paration, as extract agents for rare metal ions and CO2, and aslubricants for space technology. There isno longeranydoubt thatRTILs contribute to create future science and technologies.1-4

Compared with “modern” RTILs having fluoroanions such

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 aro-matic halide, for example, AlCl3-1-ethyl-3-methylimidazo-lium chloride ([EtMeIm]Cl) and AlCl3-1-(1-butyl)pyridi-nium chloride ([BuPy]Cl), exhibits relatively low viscosityand high conductivity but is highly sensitive tomoisture, that is,not air-stable, especially at higher Lewis acidity.5 Thus, currentRTIL studies in most cases use modern RTILs.

The negligible vapor pressure of most RTILs at room tem-perature invented a new technological concept. Only recently,RTILswere beginning to be applied to vacuum technology. Thismust be a revolutionary incident in science history becausepeople never would have imagined a wet world in vacuum.There aremanymanufacturingmachines and analytical instru-ments that require vacuum conditions. Of course, they aredesigned under the premise that materials treated in themaredryandsolid. Conventional procedures cannotbeapplied toa wet sample, although we occasionally get carried away with

Received Date: June 29, 2010Accepted Date: October 8, 2010

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our desire to dealwith it in the vacuumequipment.On the otherhand, RTILs can be put in the vacuum equipment withoutparticular care, as will be introduced in this Perspective. As amatter of fact, there are some other liquids having very lowvapor pressure, like lubricants including silicone grease, whichcan be introduced into the vacuum chamber. These liquids arenot adequate for chemical and physical reaction media due totheir extremely highviscosity. Thus,webelieve that RTILs,whichhavenegligible vapor pressure and lower viscosity,must create amysterious wet world in vacuo. At this point, the numbers ofresearchers who exploit RTILs in vacuum equipment are quitelimited. We hope that this Perspective will trigger further ex-plosive progress of the vacuum technology using the RTIL.

Material Production under Vacuum Conditions. A greatnumber of articles onRTILs have been reported to date, but as

hard as it may be to believe, the research related to thematerial production in the RTIL under vacuum conditions isstill in its early stages. To our knowledge, the first materialproduction-related study in RTILs under vacuum conditionswas reported by Scherson et al. in 2005.6 They succeededin revealing the Al electrodeposition process on ultrapure Auand Welectrodes 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-baseor Brønsted acid-base type RTILs strongly depends on thecomposition of the RTILs because some kind of equilibriumreaction that alters with the composition exists. Such anequilibrium reaction often releases neutral molecules in theRTILs. Especially, the neutral molecules are readily sublimedin avacuumathigher temperature. Forexample, in theAlCl3-[EtMeIm]Cl RTIL, two main equilibrium reactions exist.

AlCl3 þ ½EtMeIm�Cl a ½AlCl4�- þ ½EtMeIm�þAlCl3 molar fraction e 50 mol % ð1Þ

½AlCl4�- þAlCl3 a ½Al2Cl7�-

50 mol % < AlCl3 molar fraction ð2Þwhere [Al2Cl7]

- is the dominant acidic species in the Lewisacidic RTIL containing below ∼65 mol % AlCl3.

7 The exis-tence of excess AlCl3makes the vacuum stability lower due toreaction 2, which can release AlCl3 in a stronger Lewis acidicRTIL with relative ease. Thus, now non-acid-base type liquid

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

The negligible vapor pressure ofmost RTILs at room temperatureinvented a new technological con-cept. Only recently, RTILs were

beginning to be applied to vacuumtechnology.

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salt, that is, a single liquid salt system, is mostly used as asolvent formaterial productionunder vacuumconditions.Onlyrecently, a Cu electrode reaction in [(N-methylacetate)-4-picolinium][Tf2N] RTIL under ultrahigh vacuum condition (5 �10-9 Torr) was reported for development of a new spectro-electrochemical method.8

We believe that metal nanoparticle preparation inRTILs will contribute to the development of future tech-nology because those metal nanoparticles are not cov-ered with any covalently adsorbed stabilizing agents thatoften adversely affect the physicochemical properties ofthe nanoparticles. The stabilization mechanism is notentirely clear, but it would not be an exaggeration to saythat the RTIL has a relatively strong interaction with thesurface of the metal nanoparticles. The metal nanoparti-cles have been prepared in RTILs using various reactionmodes. Those preparation methods and characteristics ofthe prepared nanoparticles were painstakingly reviewedby Dupont and Scholten.9 In the cases of metal nanopar-ticle preparations by chemical reduction of metal ions ormetal complexes, the stabilizing agent is not required inthe RTIL, but several kinds of byproducts must be dis-solved in the resulting nanoparticle-suspended liquids.Metal nanoparticle preparations in RTILs under vacuumconditions, which are introduced here, are groundbreaking

techniques that enable synthesis of target nanoparticles with-out a significant amount of byproduct.

Magnetron Sputtering onto RTILs. This procedure was estab-lished by our research group.10 In principle, all elements thatcan be ejected by Arþ and N2

þ plasma bombardment arenanoparticulated by this method. Figure 2 shows a schematicillustration of this method that uses a common magnetronsputteringapparatuss except for theuseofaRTILasa substrate.This method has achieved the preparation of various puremetal nanoparticles, such as Au,10-14 Ag,15,16 Pt17,18 and so

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

Metal nanoparticle preparations inRTILs under vacuum conditions,which are introduced here, aregroundbreaking techniques that

enable synthesis of target nanopar-ticles without a significant amount

of byproduct.

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forth, possessing particle sizes less than 10 nm in diameterwithout any specific stabilizing agent. A small-angle X-rayscatting study revealed the initial formation mechanism ofthe gold nanoparticles during the sputtering process ontoseveral [1,3-dialkylimidazolium][BF4].

14 The proposed forma-tion mechanism is divided into two phases, as shown inFigure 2, where it was concluded that both surface tensionand viscosity of the RTIL are important factors for the Aunanoparticle growth and its stabilization.

Interestingly, this technique also enables production ofalloy nanoparticles by placing different elements as a target.Figure 3 shows the first attempt to prepare Au-Ag alloynanoparticles by the sputtering method with a target havingAu and Ag plates of the same area.15 As recognized fromTEMimages, the mean particle size enlarges with an increase inthe Ag area in the target. In addition to this, the chemicalcomposition and the optical properties of the deposited alloynanoparticles vary with the surface area ratio of Au to Ag, too.It implies that this approach can directly control the alloycomposition by changing the ratio of the metal areas in thetarget.

Plasma Deposition Method. This technique was proposedby Endres and co-workers, who have succeeded in prepara-tion of Ag, Cu, and Al nanoparticles using this technique.19-21

This approach, once called glow discharge electrolysis, isbased on historical articles reported about 100 years ago.Schematic illustration of the system for the plasma depositionis illustrated in Figure 4A.More accurately, plasma generationdoes not need vacuum conditions, but appropriate gas of lowpressure is required to generate a stable plasma.However, theuse of RTILs is also essential in this case because the presenceof vapor of a volatile liquid in the gas phase would inhibit theplasma generation. Photographs of a typical plasma experi-ment conducted under Arþ plasma irradiation are shown inFigure 4B.21 The reaction media was [EtMeIm][Tf2N] with62mmolL-1Cu(I).Obviously, adarkbrown layer thatappearedat the interphase between the RTIL and Arþ plasma phasewas growing with plasma irradiation time, indicating thatCu(I) was reduced to Cu metal that formed nanoparticleswith an average size of ∼11 nm. However, somehow,the surface was covered with a copper oxide layer. If themetal nanoparticles are yielded under vacuum or inert gas

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

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condition, this is a common issue for which it is very difficultto collect metallic state nanoparticles, especially base metalnanoparticles that are oxidized readily under atmosphericcondition. One of solution methodologies about this will beintroduced in a later section.

Very recently, the plasma deposition method was adoptedfor preparationofAu22,23 andPt23 nanoparticles. The relation-ship between deposition conditions and the characteristics ofthe prepared nanoparticles was studied in detail. One ofinteresting findings is that nanoparticles were prepared evenif the cathode was placed in the RTIL phase and not the gasphase.

Physical Vapor Deposition Method. This method was de-veloped based on the solvated metal atom dispersion techni-que that is used for the preparation of low-valent main grouphalides and transition-metal arene complexes. This physicalvapor deposition method can synthesize pure metal nano-particles such as Au and Cu24 as well as EuF2 showing lumi-nescent behavior.25 The TEM images revealed that the mean

nanoparticles size was ∼3 nm, and the particles were about2 nm apart from each other. The UV spectra for the RTILcontaining the produced Cu nanoparticles showed a clearabsorption peak at 576nmcorresponding to surface plasmonresonance for Cu nanoparticles soon after the preparation.However,when the liquidwas left in air, the peak reducedwithtime, suggesting formation of a copper oxide layer on thenanoparticles due to oxygen oxidation.

Electron Beam and γ-ray Irradiation. These methods ex-ploit solvated electrons and/or radicals yielded during verystrong electron beam26,27 and γ-ray27 irradiation of the RTILcontainingmetal salts so as to synthesizemetal nanoparticles.Note that primary the electron beam and γ-rays themselvescannot directly contribute to nanoparticle preparation be-cause of their considerably strong energy. In other words,no solvated electrons and no radicals would result in nonanoparticles. This has been already verified through theexperiments using a variety of [Tf2N]

--based RTILs. As atypical example, variation in UV-vis spectra of the RTILswith0.5mmol L-1 NaAuCl4 3 2H2O after accelerator electron beamirradiation is shown in Figure 5.27 The accelerator electronbeam 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 nanoparti-cles, whereas irradiation of [Bu3MeN][Tf2N] caused almost nospectrum change. In the case of electron beam irradiation at6 kGy, only [BuMeIm][Tf2N] showed spectral change. Theseresults indicate that Au nanoparticle generation in the RTILsbecomes easier in the following order

½Bu3MeN�½Tf2N� < ½MePrPip�½Tf2N� < ½BuMeIm�½Tf2N�Considering the fact that [Bu3MeN][Tf2N] is more radioche-mically stable than [BuMeIm][Tf2N],

28-31 it is highly likely thatradiochemically unstable RTILs tend to generate Au nanopar-ticles. TEM observation revealed that the Au nanoparticlesprepared in the experiments using the [BuMeIm][Tf2N] solu-tion had amean particle size of 26.4 nm at 20 kGy irradiationand 7.6 nm at 6 kGy. Again, such solvated electrons and/orradicals will be yielded only under very strong electron beamand γ-ray irradiation, not moderate irradiation described inthe next section.

Figure 4. Schematic illustration of the experimental setupfor plasma electrochemical reduction of metal ions dissolvedin RTILs (A) and photographs of the plasma electrochemicalreduction experiment of Cu(I) dissolved in a RTIL at differentreduction times (B).21 Reproduced by permission of the PCCPOwner Societies.

Figure 5. UV-vis spectra of several RTILs with 0.5 mmol L-1

NaAuCl4 3 6H2O after accelerator electron beam irradiation experi-ments at 6 or 20 kGy.

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Future Challenges in This Field. As you may understand, intheory, due to the high physicochemical stability of the RTILs,above-mentioned RTIL vacuum techniques can produce evenbasemetal nanoparticles that cannot be produced in conven-tional aqueous or organic solvents. Future challenge in thisfieldwill behowwecollect themetal nanoparticles suspendedin RTILs to make effective utilization of their functions andhow we develop a metal nanoparticle mass productionmethod. Regarding the former matter, we found a facileway, which is adsorption of the suspended nanoparticles ona solid substance.12,17,18 Nanoparticles are stably suspendedin RTILs by the interaction with ionic species exciting aroundthe nanoparticles. Heating of the suspension on a carbonplate seems to weaken the interaction, resulting in theiradsorption on the plate.When a glassy carbon plate onwhichPt nanoparticles were adsorbed was used as an electrode, itexhibited high catalytic activity against O2 reduction due tothe adsorbed Pt nanoparticles.17,18On the other hand, a recentapproach carried out at an existing common γ-ray or accel-erator electron beam irradiation industrial plant for sterilizingmedical kits may be a key to overcome the mass productionissue.27 As illustrated in Figure 6, if the glass ampules, in whichRTIL solutionswithmetal salts are encapsulated under vacuumcondition or inert gas condition, placed on the container areautomatically transferred to the irradiation position,metal saltsshould be reduced to the metal state in the ampules withoutany contamination derived from air. This would be one way tochurn out metal nanoparticles because the industrial plant canirradiate 200 glass ampules�100mL at a time. The irradiationtimes of the accelerated electron beam and γ-ray are 7 s and 3h, respectively, if the irradiation dose is 20 kGy.

Analyses under Vacuum Condition. Vacuum condition isone of the essential requirements for ultraprecise analyses at themolecular and atomic levels because all atoms andmoleculesother than target species are excluded under the conditionand/or itmakespossible to irradiate the stable quantumbeamand to detect the generated signals with high S/N ratio.Therefore, to introduce a volatile substance is normally pro-hibited into the sample chamber of the vacuum analyticalinstrument; that is, the matrixes for the instruments are

limited to solid materials to keep the vacuum. If we need toanalyze a liquid or a wet sample by using one of thoseinstruments, it must be required to follow complicated pro-cedures including preservation of the frozen condition of thesample in a vacuum chamber during the analysis. As some ofyou may know, the procedure is very tough, and the extrainstrument for the frozen preservation is expensive. A limitednumber of persons are able to make the analysis.

Negligible vapor pressure has become common sense as oneof characteristics that RTILs possess. However, from the discov-ery of RTILs, it took a long time for us to introduce RTILs into thevacuum chamber of the ultraprecise analytical instruments,which dislike extreme contamination in the chamber. Recently,thermodynamicsandkinetics of thevaporizationand/ordecom-position of RTILs have been revealed by heating experiments ofthe RTILs under ultravacuum conditions.32-35 Now, on the basisof these data, we can safely put nonvolatile RTILs into thevacuuminstruments.Combinationof theRTILand theanalyticalinstruments,which had almost no relation to liquid chemistry sofar, opens up an innovative analytical method without compli-cated procedures and expensive additional equipment.

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

Negligible vapor pressure has be-come common sense as one of

characteristics that RTILs possess.However, from the discovery of

RTILs, it took a long time for us tointroduce RTILs into the vacuumchamber of the ultraprecise analy-tical instruments, which dislike ex-

treme contamination in thechamber.

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XPS Analysis. This instrument is remarkably useful foranalyzing the composition of solidmaterials and the chemicalstate of each element. Vacuum condition is inevitable foravoiding contamination and detecting generated photoelec-tronswith high sensitivity. Once, analyses of the liquid surfacewere attempted under vacuum conditions, but intricatelydesigned sample stages were necessary to reduce the influ-ence of vaporization during the analysis.36-38 In cases of aRTIL that is not vaporized under ultravacuum conditions atroom temperature, the liquid can be put in the chamber of theXPS without any specific technique or modification.

[EtMeIm][EtOSO3] was the first RTIL subjected to XPS ana-lyses,39 and XPS analyses of several kinds of RTILs were con-ducted.40-43AllRTILs gavehigh-resolutionXPSspectrawithoutany charge compensation, but the peak intensities decreasedby freezing the liquids,39,40 indicating that RTILs behave aselectrical conductors under appropriate conditions. The peakintensities obtained by usual XPS analyses of the RTIL itselfcorresponded to ratios of elements included in cationic andanionic species of the liquid. When XPS spectra were taken bygrazing electron emission, changes in the peak intensity withelectron emission angle provided significant information onthe surface structures of the RTIL. In imidazolium-based RTILshaving long alkyl chains, the obtained XPS spectra suggestedthat long alkyl chains jut out from the bulk liquid.40,43

XPS analysis is a useful approach for in situ monitoring ofchemical reactions in RTILs, too. Reduction of Pd(II) to Pd(0)was monitored as the first demonstration.39 The Pd(II) spe-cies used was a Heck catalyst (Pd(OAc)2(PPh3)2) dissolved in[EtMeIm][EtOSO3] (ECOENG 212). Figure 7 shows variationin the peak intensity for the Pd(II) and Pd(0) before and afterthe XPS experiment. After the experiment, the peak for thePd(II)decreased, and thePd(0) increased. The results indicatethat the Heck catalyst is unstable in this RTIL. In addition tothis, in situ monitoring of electrochemical reactions was alsocarried out recently by introducing an electrochemical cell inan XPS chamber.44

MALDI Mass Spectroscopy. A mass spectrometer is animmensely useful instrument for determining the elementalcomposition of a sample or amolecule in a vacuum chamber,and it is composed of a part for evaporating and ionizingchemical compounds to generate charge molecules or mole-cule fragments and a part formeasuring their mass-to-chargeratios. These days, both parts have been considerably im-proved to enable analysis of large molecular species likebiologically relevant molecules. The matrix-assisted laserdesorption/ionization mass spectrometry (MALDI-MS) is oneof the groundbreaking techniques that vaporize large com-pounds by laser irradiation with assistance of an appropriatematrix.

Matrix selection is very important, and an ideal matrix is amaterial possessing a sufficient absorption coefficient at thelaser wavelength region, low vapor pressure, the ability todissolve or cocrystallize with the sample, and the ability topromote ionization of the sample without its significantdecomposition. Although both solid and liquid have beenemployed as a matrix, the former is used more widelybecause 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 matrixand sample. Irradiation of a laser shot induces vaporization of apart of the analyte spot, as schematically shown in Figure 8a,and samples contained in the part are vaporized. If samplesare not homogeneously dispersed in the matrix, MS signalsshould be affected by the position of the laser shot, resultingin poor shot-to-shot reproducibility. This reason makes itvery difficult to use MALDI-MS for quantification analysis.From this viewpoint, a liquid matrix should bemore desirable.So far, some liquids possessing low vapor pressure, such asglycerol and 3-nitrobenzyl alcohol, have been examined as aliquidmatrix for MALDI-MS. Shot-to-shot reproducibility wasactually improved because of their homogeneous condition,but their inherent volatility still causes some problems, suchas a decrease in their amountswith time. Another problem isthat these liquids possess no UV absorbability.

Because RTILs possess both no volatility and UVabsorbabil-ity, it seemed to be an ideal solution matrix for MALDI-MS, butusual RTILs such as [BuMeIm][BF4] and [BuMeIm][PF6] wereunfortunately unable to ionize samples dissolved in them.45

Armstrong et al. designed anew ionic liquid family for the liquidmatrixes using solid acidic compounds of R-cyano-4-hydroxy-cinnamic acid (CHCA), sinapinic acid (SA), and 2,5-dihydroxy-benzoic acid (DHB), which are widely used as solid matrixesfor MALDI-MS.45,46 It was then found that some of them keptthe liquid state at room temperature and worked as liquidmatrixes for detection of the polymer and some biomoleculesby MALDI-MS.45-48 Figure 8b shows a change in signal inten-sities of [MþH]þ obtained at 90 different positions on a spot ofthe sample-matrix mixture. As expected, the RTIL matrix ofR-cyano-4-hydroxycinnamic acid butylamine (CHCAB) gavemuch narrower data dispersion than that obtained for the solidmatrix of CHCA, indicating, evidently, the usefulness of theRTILmatrix for improvement of reproducibility.46 Another feature ofthe RTIL matrix is the higher ability to suppress decompositionof the sample than the conventional solidmatrix. Use of CHCA-based 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 experi-ment, and the black line presents data recorded 6 h later.39

Reproduced by permission of The Royal Society of Chemistry.

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enabled detection of oligosaccharides, which exhibit poorionization efficiencies and tend to have thermal fragmen-tation through the loss of SO3 groups, with suppression ofthe loss of SO3.

47,48

SEM Observation and EDX Analysis. Scanning electronmicroscopy enables us to observe electrically conductingsamples with highmagnification in vacuum. If one would liketo observe an insulating sample, it is required to give electricconductivity to the sample by vacuumdeposition of ametal orcarbon. Otherwise, the insulating samplemust be charged byirradiation of the electron beam, giving a low-quality imageaccompanied with lots of noise and distortion.

Our research group first attempted to observe RTILs bySEM because RTILs can be put in the vacuum chamber of theSEM. A silicone oil droplet exhibited awhite imagewith somenoise caused by charging behavior (Figure 9a), but surpris-ingly, all RTILs gave dark contrast images (Figure 9b-d).

Figure 8. Schematic illustration of MALDI (a). [M þ H]þ ionintensities from 90 positions on a human angiotensin II prepara-tion with a RTIL matrix CHCAB (black triangles) and with atraditional CHCA matrix (gray diamonds) (b, left). The black barindicates relative standard deviation (RSD) values found usingRTIL matrixes, and the gray bar indicates RSD values of the dataseries yielded by the respective traditional MALDI matrixes(b, right). Reproduced with permission from ref 46.

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

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

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It suggests that the RTILs behave as electrically conductivematerials for SEM observations.49 It has been revealed bypulse radiolysis experiments that electrons injected in RTILswith high accelerated voltage are stabilized in condensed ionspecies, allowing electrons to move in the liquid.50 Similarbehavior may occur during the SEM observation, consideringavoidance of charging of the liquid. This property is useful as away to give electrical conductivity to insulating samples.Figure 10 shows a typical example, SEM images of the surfaceand cross-sectional surface of a wad of cotton. On the cottonsubjected to Au sputtering, the surface is observed by SEMwithout charging, but the cross-sectional view gives a chargedimage due to presence of Au-free cotton fibers. On the otherhand, cotton that was immersed in 0.1 mol L-1 [BuMeIm]-[Tf2N] ethanolic solution and dried under vacuum givesuncharged clear images in both cases.51

RTILs are well-known as a favorable electrolyte forseveral kinds of electrochemical reactions. If nonvolatilespecies are used in the reaction, the desired reaction shouldtake 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 hasbeen attempted by introducing an electrochemical cell in acommon SEM system. As the first demonstration, a cross-sectional view of a polypyrrole (PPy) film deposited on a Ptelectrode was observed while applying various potentials tothe electrode.52 [BuMeIm][Tf2N] was the electrolyte in thiscase. Figure 11a shows SEM images, indicating variations inthe PPy film thickness caused by the electrode potentialsteps. Increase in the film thickness at negative potentialssuggests incorporation of electrolyte cations in the film byreduction of PPy. This assumption was evidenced by in situenergy-dispersiveX-ray spectroscopy (in situ EDX). To revealthe cation behavior during application of the potential,KTf2N as a marker was dissolved in the RTIL, and the Kcontent in the PPy film at oxidized and reduced states wascompared by the line analyses, as shown in Figure 11b. TheK content in the PPy film obviously increased at morenegative potential. It supports the aforementioned assump-tion. In situ SEM observation of electrochemical deposition

Figure 11. In situ ECSEM observation of thickness variations in a PPy film in [BuMeIm][Tf2N] caused by changing the electrode potential(vsAg(I)/Ag) (a). EDX line analysis along awhite line drawn in the SEM image byX-ray intensity at 3.310 keV corresponding toK-KR. Resultsfor PPy polarized at-1.70 (solid line) andþ0.87 V versus Ag(I)/Ag (broken line) are shown (b). Redox reactionmechanism determined by insitu ECSEM and EDX measurements using binary [BuMeIm][Tf2N]-K[Tf2N] RTIL (c).

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of metals has been also examined using an electrochemicalcell, which was specially designed for the in situ electroche-mical SEM observation.53,54

TEM Observation. As already mentioned in the MaterialProduction under Vacuum Conditions paragraphs, the metalnanoparticles produced in RTILs can be directly observedby TEM if the RTILs do not vaporize even under ultra-vacuum. By this finding, we could know the interesting factthat metal nanoparticles were stably dispersed in RTILswithout significant aggregation and that the TEM observa-tion is a useful technique to reveal the aggregation preven-tion mechanism of the nanoparticles in the RTIL. It wasrecently found by combination of optical measurementsand TEM observation that RTILs containing a small amountof impurity such as 1-methylimidazole prevents aggrega-tion of Au nanoparticles much more efficiently than that ofvery pure RTILs.55

Watanabe et al. have contended with the aggregationprevention effects using silica nanoparticles as model parti-cles. As well as bare silica nanoparticles having hydrophilicSi-OH groups on their surface, they prepared hydrophobicnanoparticles by grafting poly(methyl methacrylate) chainson the silica nanoparticles. Dispersion of them in [EtMeIm]-[Tf2N] gave apparently different TEM images, as shown inFigure 12. The hydrophilic nanoparticles tended to aggre-gate (Figure 12a), whereas the hydrophobic ones kepttheir dispersive state even at higher silica concentration(Figure 12b).56,57 These different behaviors are discussedbased on the colloidal stabilization theory.56

The RTIL is the first liquid that works well as reactionmedia even under vacuum conditions. New RTILs andtheir relatives, for example, a urea-choline chloridemixture and zwitterionic compounds, are being synthe-sized every day because their properties can be readilydesigned by introducing functional groups on the ioniccomponent. On the other hand, modern vacuum techno-logy already supports our comfortable daily lives. How-ever, to apply RTIL science to vacuum technology likelycontributes to further development of science and tech-nology, and the wet condition in vacuum obtained byintroducing RTILd must fascinate researchers dealing

with the vacuum equipment. Unfortunately, however,the RTIL still is not widely recognized, except in thechemical field, and the researchers who employ vacuumequipment hesitate to put the RTIL in the equipment evenif the liquid is involatile. We hope some of the readersare motivated to begin wet science and technology invacuum conditions, of course using RTILs. We welcomethe newcomers of great promise in vacuum technologywith RTILs. It is time to voyage to the RTIL Sea in vacuumequipment!

AUTHOR INFORMATION

Corresponding Author:*Towhomcorrespondence should be addressed. E-mail: kuwabata@chem.eng.osaka-u.ac.jp.

BiographiesSusumu Kuwabata has been a Professor of the Graduate School

of Engineering at Osaka University since 2002. He received hisDr. Eng. from Osaka University in 1991. His current researchinterests are centered on electrochemistry and functional nano-materials, including design of the solid/liquid interface on thenanometer scale to enhance electron transfer and visualization ofthe electron-transfer reactions using ionic liquids. His researchcredentials include over 150 original papers, 6 book chapters, andover 15 patents.

Tetsuya Tsuda is an assistant professor in the Graduate School ofEngineering at Osaka University. He received his Ph.D. in EnergyScience from Kyoto University, Japan, in 2001. He started hisacademic career at The University of Mississippi under the directionof Professor Charles L. Hussey, who is one of the fathers of modernRTIL science. His research interests are energy science and materi-als science related to electrochemistry in RTILs.

Tsukasa Torimoto has been a Professor of the Graduate School ofEngineering at Nagoya University since 2005. He received his Ph.D.from Osaka University in 1994. He started an academic career atOsaka University in 1994 as a research associate. From 2000 to2005, he worked at Hokkaido University as an associate professor.His main research interests are the preparation of novel semicon-ductor and metal nanoparticles and their application to the energyconversion systems.

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

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

We hope some of the readersare motivated to begin wet scienceand technology in vacuum condi-tions, of course using RTILs. Wewelcome the newcomers of greatpromise in vacuum technologywithRTILs. It is time to voyage to theRTIL

Sea in vacuum equipment!

r 2010 American Chemical Society 3187 DOI: 10.1021/jz100876m |J. Phys. Chem. Lett. 2010, 1, 3177–3188

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