Functionalized Carbon Nanotubes: Synthesis of Meltable and Amphiphilic Derivatives

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Meltable nanotubesDOI: 10.1002/smll.200600221Functionalized Carbon Nanotubes: Synthesisof Meltable and Amphiphilic Derivatives**Athanasios B. Bourlinos, Vasilios Georgakilas,Vasilios Tzitzios, Nikolaos Boukos, Rafael Herrera,and Emmanuel P. Giannelis*Carbon nanotubes (CNs) undoubtedly occupy a unique po-sition among advanced materials due to their novel electri-cal, mechanical, and chemical properties.[1] Regarding thechemistry of CNs, significant work has focused on their sur-face functionalization with the aim of enhancing dispersibili-ty, providing colloidal dispersions, improving compatibilitywith polymers, and designing novel derivatives with evenmore complex behavior.[2] In general, the chemical function-alization of CNs takes place either via covalent grafting tothe graphitic surface, structural defects, and end cups of thetubes, through molecular stacking (e.g., p stacking) on thewalls, or by wrapping of the tubes with polymers.[3, 4]In each case, functionalized CNs exhibit solidlike behav-ior in the absence of solvents and do not undergo any mac-roscopic solid-to-liquid transition. On the other hand, thefunctionalization often gives organophilic or hydrophiliccharacter to the tubes, depending on the nature of the modi-fier, and thus favors dispersibility in organic or aqueousmedia.[3,4] Nevertheless, very few derivatives have beenshown to possess amphiphilic properties, that is, being effec-tively dispersible in both aqueous and organic media.[5]Recently, we have developed a series of functionalizednanoparticles that exhibit liquidlike behavior in the absenceof a diluent or solvent.[6] The solventless nanoparticle fluidsare synthesized by attaching a corona of flexible chains ontoan inorganic oxide core such as SiO2, g-Fe2O3, TiO2, or ZnO.In addition to the oxide nanoparticles, polyoxometalateclusters and layered organosilicate nanoparticles have beendemonstrated. The nanoparticle fluids possess flow proper-ties (viscosities and diffusivities) that are remarkably similarto those of simple molecular liquids. Unlike simple liquids,however, they do not possess a measurable vapor pressure,which dramatically increases their range of potential appli-cations. Also, since the nanofluids are hybrid systems, theycan be engineered to combine specific properties (e.g., re-fractive index, viscosity, conductivity, magnetism) that aredifficult or impossible to achieve with molecular-basedfluids.In an effort to further expand the gallery of materialsthat undergo a solid-to-liquid transition at low temperatures,we report here a new system based on functionalized CNs.The new CN hybrids possess amphiphilic properties and aredispersible in both aqueous and organic media. The newmethod presents an alternative synthetic pathway towardssolvent-free nanofluids and is different from those previous-ly reported in both the nature of the organic modifier andthe type of interaction.[6]The new molten CN derivatives are distinguished fromconventional colloidal suspensions in that the particles andsuspending medium have been combined into a single, ho-mogeneous phase. As such, the presence of a molecular sol-vent or an ionic liquid as the suspending medium is nolonger necessary.[35,7]The functionalization, shown in Scheme 1, is based on atwo-step process. The first step involves the acid oxidationof standard CNs, which leads to nanotubes with open endsand bearing polar hydrophilic groups (COOH, C=O,ACHTUNGTRENNUNGOH) on the surface. The oxidation step is necessary inorder to create surface functional groups required for fur-ther reaction. In the second step, a poly(ethylene glycol)-(PEG-) substituted tertiary amine reacts with the carboxylicgroups on the oxidized surface via an acidbase reaction[4c]or via hydrogen bonding between the surface OH or C=Ogroups and the amine groups. Direct interaction of theamine molecules with the carbon nanotubes cannot be ruledout based on the electron-donating ability of the aminegroups[8] and the defect sites of the nanotubes. Thin-layerchromatography (TLC) on silica in tetrahydrofuran shows asingle band with a retention factor Rf=0 for the CN deriva-tive and no evidence for the PEG-substituted tertiary amine(Rf=0.55), suggesting there is no excess unreacted amine inthe final product. If there were, the TLC of the CN deriva-tive would be expected to show two bands: one at Rf=0due to the bulky, functionalized CNs and another at Rf=0.55 due to the faster moving amine.The CN derivative was isolated as a black, waxy solidthat melted at 35 8C to give a highly viscous, tar-like liquid(Figure 1a). The low density and flexibility of CNs, as wellas the lubricating action provided by the molten organiccorona among adjacent nanotubes, impart mobility. As ex-pected, the molten organic modifier possesses a consider-ACHTUNGTRENNUNGably lower viscosity under the same conditions. The moltenderivative is a homogeneous material with no evidence ofphase separation. Solidification of the liquid upon coolingunder ambient conditions takes place within a minute. Themelting and solidification are reversible over many cycles.Due to its ionic nature as well as the presence of the PEG-substituted chains, the CN derivative is readily dispersible athigh concentration (20 mgmL1) in both aqueous and or-ganic solvents (e.g., acetone, ethanol, tetrahydrofuran) andprovides clear, black/brown sols (Figure 1b). The sols, inparticular the aqueous-based sol, are stable for a long[*] Dr. A. B. Bourlinos, Dr. V. Georgakilas, Dr. V. Tzitzios,Dr. N. BoukosInstitute of Materials Science, NCSR DemokritosAg. Paraskevi Attikis, Athens 15310 (Greece)R. Herrera, Prof. E. P. GiannelisDepartment of Materials Science and EngineeringCornell University, Ithaca, NY 14853 (USA)Fax: (+1)607-255-2365E-mail:[**] We gratefully acknowledge the support of the Air Force Office ofScientific Research (AFOSR), the Cornell Center for MaterialsResearch (CCMR), the Office of Naval Research (ONR), and theFuel Cell Institute at Cornell.1188 A 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim small 2006, 2, No. 10, 1188 1191communicationsperiod of time: approximately one month for organosolsand at least six months for hydrosols. In addition, the solsleave no residue upon filtration (100-mm-mesh filter paper),indicating that the nanotubes disperse in the medium andform small, isolated bundles void of any large aggregates.The enhanced dispersibility and colloidal stability, in con-junction with the TLC test, suggest the presence of stronginteractions between the organic corona and the oxidizednanotubes.Increasing the carbon nanotube content results in non-meltable powders (>35% w/w). On the other hand, de-creasing the CN content (nanotubes with the PEG-substituted amine, the IR spectrashow the characteristic absorption peaks of the amine dueto CH3, CH2, and CO (ether) vibrations. Moreover, thefunctionalized derivative shows an additional band at1580 cm1 that marks the formation of carboxylate groupsafter protonation of the tertiary amine.Although the X-ray diffraction (XRD) patterns of boththe tertiary amine and the functionalized CNs lack any re-flections in the low 2q region, they display two sharp peakscorresponding to d spacings of 3.8 and 4.6 E, respectively(Figure 2, right). These values are typical for the lateralACHTUNGTRENNUNGinterchain spacing observed for densely packed PEGchains.[10] Most importantly, transmission electron microsco-py (TEM) imaging of the sample (Figure 3) reveals the pres-ence of hollow multi-walled carbon nanotubes (MWCNTs),suggesting that the nanotubes remain intact after oxidationand functionalization.The Raman spectrum of the sample indicates two peaksthat are characteristic of MWCNTs[11] (Figure 4). The G(graphite) band at 1595 cm1 corresponds to the Raman-active E2g mode of graphite due to sp2-hybridized carbons.The strong D1 (defect) band at 1314 cm1 is attributed toeither sp3-hybridized carbons or to structural defect sites ofthe sp2-hydridized carbon network. The high intensity ratioID1/IG indicates that the nanotubes contain a high defectconcentration. The spectra before and after functionaliza-tion are slightly different, exhibiting a small increment inthe ID1/IG ratio for the final derivative (Figure 4). This indi-cates that the oxidation process induces slight structuralchanges in the nanotubes, where oxidation with HNO3 andthe subsequent formation of surface functional groups af-fects the outer graphene layers. Recently, Murphy et al.have demonstrated similar changes in the Raman spectra ofmulti-walled carbon nanotubes after oxidation with variousacids.[12]Thermogravimetric analysis (TGA) demonstrates thatthe CN derivative is virtually solvent-free (Figure 5). TheTGA trace under O2 shows a complete weight loss up to600 8C that takes place in two steps, one at 250 8C and an-other at 350 8C, attributable to the thermal decompositionof the PEG-containing organic modifier and to CN combus-tion, respectively.[13] The corresponding tracemeasured under a nitrogen atmosphere re-veals a content of 80% w/w organic with theremaining 20% w/w attributed to CNs. Sucha composition corresponds to a dense surfacecoverage of one modifying molecule per 50carbon atoms of the nanotubes. For compari-son, other functionalization processes typical-ly lead to a surface coverage of one modify-ing molecule per 100200 carbon atoms.[14]However, in another report, Dyke et al. havedemonstrated an even denser surface cover-age of one modifying molecule per 10 carbonatoms.[15] Differential scanning calorimetry(DSC) analysis shows a first-order transition at 35 8C and acrystallization peak at 20 8C (Figure 5). Note that the organ-ic corona exhibits similar melting and crystallization temper-atures.In summary, a meltable, amphiphilic CN derivative wasobtained in a two-step process comprising oxidation of thetubes with HNO3 followed by a surface reaction with aPEG-substituted tertiary amine. The material underwent areversible, macroscopic solid-to-liquid transition at 35 8Cand was highly dispersible in both aqueous and organicmedia, providing clear, stable sols. These unusual featuresmay prove useful in potential nanotube applications, for ex-ample, as lubricants, polymer nanocomposites, and nanopar-ticle fluids.Figure 3. TEM images of the amine-treated CNs.Figure 4. Raman spectra of the original (left) and functionalized CNs(right).Figure 5. TGA traces of the amine-treated carbon nanotubes underoxygen (left) and nitrogen atmosphere (center), and the correspond-ing differential scanning calorimetry (DSC) profile (right).1190 A 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim small 2006, 2, No. 10, 1188 1191communicationsExperimental SectionSynthesis: In a typical procedure, commercial MWCNTs(0.45 g, Aldrich, 90+%) were refluxed in concentrated HNO3(65%, 50 mL) for 24 h. The mixture was centrifuged and the re-maining solid washed repeatedly with water prior to dispersionin water (50 mL, pH 3). The pH value of the dispersion was thenadjusted to 7 by the dropwise addition of a 10% w/v aqueoussolution of the PEG-substituted tertiary amine (C18H37)N-ACHTUNGTRENNUNG(CH2CH2O)nH ACHTUNGTRENNUNG(CH2CH2O)mH (Ethomeen 18/60, Akzo Nobel, m+n=50). The resulting black/brown sol was centrifuged at3600 rpm and the supernatant liquid was collected, concentrat-ed, and dried at 70 8C. The residual material was carefully rinsedtwice with water, five times with toluene, and once with acetone(20 mL each time) to remove excess unreacted amine and driedat 70 8C before the rinsing procedure was repeated once more.Finally, the product was dried at 70 8C for two days; the yieldwas 0.5 g.Characterization: XRD patterns were recorded on a SiemensXD-500 diffractometer using CuKa radiation. The patterns werecollected using background-free holders. IR measurements weremade on an FTIR spectrometer (Bruker Equinox 55/S) using KBrpellets. TEM images were obtained with a CM20 Phillips electronmicroscope. For this study, a few drops of an aqueous disper-sion of the material (0.2% w/v) were placed on a copper gridand evaporated prior to observation. The Raman spectrum wasrecorded using a Raman microscope system (Renishaw System1000) consisting of an optical microscope (Leica) coupled to aRaman spectrometer (532 nm, 23 mW). 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