Solvent Free Cnt

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CHAPTER 17

Solvent-Free Techniques for CovalentChemical Modification of CarbonNanotubes

Elena V. BasiukCentro de Ciencias Aplicadas y Desarrollo Tecnologico, Universidad Nacional Autonoma de Mexico,Circuito Exterior C.U., 04510 Mexico, D.F., Mexico.

CONTENTS

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 01

2. Fluorination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 03

3. Reactions with Aryl Diazonium species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 05

4. Amidation and Esterification of Oxidized Defects . . . . . . . . . . . . . . . . . . . . . . . . . 06

5. Functionalization of Closed Caps and Sidewall Defects . . . . . . . . . . . . . . . . . . . . . . 11

6. Miscellaneous Mechanochemical Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

7. Plasma Functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

8. Sonochemical Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

1. INTRODUCTIONAll the known types of carbon nanotubes (CNTs), namelysingle-walled, double-walled, and multi-walled CNTs(SWNTs, DWNTs, and MWNTs, respectively), are poorlysoluble in organic and especially aqueous solvents. Thisproperty limits processability of CNTs. It cannot affect theapplications where the bulk nanotube material is employed(e.g., the design and production of CNT-based flat-paneldisplays, thermal materials and fibers). However, someareas require either handling individual CNTs (fabricationof cantilevers for atomic force microscopy, nanoelectronic,and nanomechanical devices) or simply breaking the nano-tube bundles to provide access for versatile chemicalreagents and particles to their surface (e.g., grafting themoieties that allow CNT assembly onto different substratesfor electronic applications, grafting the functional groups

that allow chemical linking to host matrices in composites,as well as to catalyst molecules and nanoparticles).

The nanotube solubility and dispersibility can be dramat-ically improved by means of their chemical modification[18]. This term implies the modification of (chemical,physical, mechanical, etc.) properties of CNTs by changingthe chemical nature of the nanotube sidewalls and/or ends.This goal can be achieved in different ways, and corre-spondingly, there are several covalent as well as noncova-lent approaches to CNT chemical modification, which areschematically depicted in Figure 1. Note that along withchemical modification, other commonly used terms in thepresent context are functionalization and derivatization.The difference between these three terms is sometimesrather subtle, and there are no strict guidelines how to usethem. On one hand, chemical modification can be per-formed in some cases by invoking noncovalent interactions

ISBN: 1-58883-079-9Copyright 2008 by American Scientific PublishersAll rights of reproduction in any form reserved.

Chemistry of Carbon NanotubesEdited by V. A. Basiuk and E. V. Basiuk

Pages: 125

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with relatively unreactive molecules. On the other hand,functionalization usually means covalent introduction ofreactive (functional) groups into CNT structure. The termderivatization (formation of chemical derivatives of CNTs)is often used as a synonym of functionalization; althoughits narrower meaning can be a further chemical conversionof the functional groups already introduced into the idealsidewalls and/or defects, into other groups having differentchemical properties.

Although noncovalent modification methods can be veryuseful for some applications, a significant research effort isdone to design new and efficient covalent linking techniques.Within the latter group, one should emphasize covalent deriv-atization methods based on the chemistry of oxygenatedfunctionalities (mainly carboxylic groups COOH) intro-duced at the nanotube tips and sidewall defect sites bymeans of CNT oxidation with strong acids, which gained anespecially wide distribution. The COOH groups are rela-tively inactive under ambient conditions, and therefore needto be chemically activated. Their treatment with thionyl chlo-ride (SOCl2), carbodiimides, etc., produces chloroanhydrideand other reactive groups, which can be converted intoamides by reacting with aliphatic and aromatic amines[113]. In the case of the most popular SOCl2, the latter iscorrosive by itself, the whole reaction sequence requires theuse of an organic solvent medium and is quite tedious due tosuch auxiliary steps as washing, centrifugation, and drying.Most of this is also true for an overwhelming majority ofCNTcovalent derivatization techniques designed up to now.

The global trend of looking for more ecologicallyfriendly, green techniques in chemistry manifested itself inCNT chemistry as well. Here it is appropriate to recall thetwelve basic principles of green chemistry formulated byAnastas and Warner [14]:

1. Prevention: It is better to prevent waste than to treator clean up waste after it has been created.

2. Atom economy: Synthetic methods should bedesigned to maximize the incorporation of all mate-rials used in the process into the final product.

3. Less hazardous chemical syntheses: Wherever practi-cable, synthetic methods should be designed to useand generate substances that possess little or no tox-icity to human health and the environment.

4. Designing safer chemicals: Chemical products shouldbe designed to affect their desired function whileminimizing their toxicity.

5. Safer solvents and auxiliaries: The use of auxiliarysubstances (e.g., solvents, separation agents, etc.)should be made unnecessary wherever possible andinnocuous when used.

6. Design for energy efficiency: Energy requirements ofchemical processes should be recognized for theirenvironmental and economic impacts and should beminimized. If possible, synthetic methods should beconducted at ambient temperature and pressure.

7. Use of renewable feedstocks: A raw material or feed-stock should be renewable rather than depletingwhenever technically and economically practicable.

8. Reduce derivatives: Unnecessary derivatization (useof blocking groups, protection/ deprotection, tempo-rary modification of physical/chemical processes)should be minimized or avoided if possible, becausesuch steps require additional reagents and can gen-erate waste.

9. Catalysis: Catalytic reagents (as selective as possible)are superior to stoichiometric reagents.

10. Design for degradation: Chemical products should bedesigned so that at the end of their function theybreak down into innocuous degradation productsand do not persist in the environment.

11. Real-time analysis for pollution prevention: Analyticalmethodologies need to be further developed to allow

Figure 1. General classification for the methods of CNT chemical modification, based on different types of interactions and attachment sitesemployed.

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2 Solvent-Free Techniques for Covalent Chemical Modification of Carbon Nanotubes


for real-time, in-process monitoring and controlprior to the formation of hazardous substances.

12. Inherently safer chemistry for accident prevention: Sub-stances and the form of a substance used in a chemi-cal process should be chosen to minimize thepotential for chemical accidents, including releases,explosions, and fires.

From the principles 1, 3 and 5, it is clearly seen howimportant would be avoiding the use, or at least reducingthe consumption of organic solvents for a chemical process.And it is precisely this aspect where significant improve-ments have been achieved in CNT chemistry during last fewyears, namely, a number of reports appeared on the designof simple and efficient solvent-free approaches to the cova-lent derivatization of CNTs. These approaches can beexemplified by the functionalization of CNT sidewalls withfluorine atoms, diazonium salts and aniline, mechanochemi-cal linking of C60 fullerene to SWNTs, direct amidation ofoxidized SWNTs, and direct amination and thiolation of theclosed caps along with sidewall defects of MWNTs withvapors of aliphatic amines and thiols, respectively. Thepresent chapter presents a detailed discussion of the devel-opments in this area of carbon nanotube chemistry.

2. FLUORINATIONFluorination of CNTs (usually SWNTs) [1540] is a solvent-free process performed by means of the nanotube treat-ment with elemental fluorine gas. As a matter of fact, itdoes not have a liquid-phase analogue for the obvious rea-son of extreme reactivity of F2 toward organic solvents. Thisis perhaps one of the most explored techniques of CNTchemical modification at all, and not only of their solvent-free functionalization. There are many reasons for suchinterest towards the nanotube fluorination. While unfunc-tionalized SWNTs are poorly soluble in organic solvents,fluorinated SWNTs (F-SWNTs) produce metastable solu-tions in dimethylformamide (DMF), tetrahydrofuran(THF) and alcohols after ultrasonication. Conductivityproperties of the nanotubes become strongly modified,leading to potential new applications in electronics. Also,fluorination is considered to be a chemical gateway to fur-ther chemical conversions on the nanotube sidewalls, facili-tated by the enhanced solubility of F-SWNTs.

The synthesis of F-SWNTs is usually performed by reac-tion of solid SWNTs with fluorine gas [1526, 32, 33, 37].The fluorine content increases with increasing the tempera-ture, which is maintained between 150 and 250C in mostcases. Extensive studies were carried out to establish optimalconditions (reaction temperatures and times, addition of HFgas as a catalyst) to provide a saturation stoichiometry with-out destruction of the nanotube integrity. It was found thatSWNTs are essentially all destroyed when they are fluori-nated at 400C and above [15, 17, 19]. At the same time, thetubular structure at 325C was retained and it appeared thatthe F/C ratio of 0.5 (C2F stoichiometry) is an upper limit fornondestructive thermal fluorination of SWNTs.

Plank et al. [29] first suggested a principally newapproach to SWNT fluorination, by exposing them to CF4

plasma. The highest F/C ratio found in their experimentswas 0.22. A little after, Valentini et al. [36, 41] performedsimilar studies. Felten et al. [42] used MWNTs instead ofSWNTs and analyzed in detail the variation of diverseplasma parameters, such as power, treatment time, pres-sure, as well as position of the nanotube sample inside thereaction chamber. Depending on the plasma conditions it ispossible either to have a functionalization of the MWNTsurface or a polymerization of the monomer on this surface.In particular, if the nanotubes are treated at a power of 30W, pressure of 0.1 Torr, with a treatment time of 30 s, themeasured amount of fluorine is very high (57 at. %) com-pared to the carbon (40 at. %); this readily suggests that afluoropolymer was polymerized around the surface ofMWNTs. When the nanotubes are treated under 0.1 Torrand 50-W power during 5 min, the atomic concentration offluorine is lower, namely, 20 at. %, suggesting a softerfunctionalization.

Infrared (IR) spectroscopy was used to confirm the pres-ence of covalently bound fluorine due to the peaks in12001250 cm1 region, which can be compared to theCAF band at 1215 cm1 found for fluorinated graphite[1517, 20, 21, 2527, 36, 41]. Raman spectroscopy is evena more powerful tool to characterize F-SWNTs [1521, 25,26, 29, 33]. In particular, an intensity increase of disordermode (1292 cm1) due to large amounts of sp3-type carbonatoms was observed, as well as changes in the radial breath-ing modes (RBM) in the region of 170270 cm1 and a tan-gential mode at 1592 cm1 [20], which indicate thatbonding of fluorine to the carbon on the nanotube sidewallsmodifies the overall symmetry and bonding structure ofSWNTs. At an early stage of fluorination, intensity of thehigher-wave number side of RBM peaks was noticed todecrease, and the mean peak position to show downshift[18]. This means that smaller nanotubes, which are morehighly strained and thus have less stable CAC bonds, areselectively fluorinated at an early stage of fluorination.Marcoux et al. [23] observed a 7 cm1 downshift and broad-ening of the E2g tangential modes, and noted that the bandat 1546 cm1, typical for metallic SWNTs, vanished uponfluorination. Other methods that proved to be useful forthe characterization of F-SWNTs are X-ray photoelectronspectroscopy (XPS) [18, 23, 24, 26, 28, 29, 39, 42], ultravioletphotoelectron spectroscopy (UPS) [39], UVvisiblenear-IRspectroscopy [16, 27], thermogravimetric analysis (TGA)[16, 20, 36, 41], X-ray diffraction (XRD) [18], and high-resolutionelectron energy loss spectroscopy (HREELS) [32]. In partic-ular, according to the XPS data by Mickelson et al. [24], an F1s binding energy of 687 eV found for F-SWNTs is lower thanthe corresponding value of 691.5 eV for poly(tetrafluoroeth-ylene). Therefore, fluorine atoms in the fluorinated nano-tubes are more ionic than F atoms in alkyl fluorides. Marcouxet al. [23] came to the same conclusion. On the other hand,from the HREELS measurements Hayashi et al. [32] con-cluded that F establishes two different types of bonds withcarbon atoms: covalent and ionic.

Of all microscopic methods employed to study F-SWNTs,scanning tunneling microscopy (STM) provided the mostvaluable data on the possible fluoronanotube structure[15, 22, 41], especially taken together with the results of molec-ular modeling. On the basis of the experimentally established

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Solvent-Free Techniques for Covalent Chemical Modification of Carbon Nanotubes 3


limiting C2F stoichiometry, at which fluorinated tubes canstill maintain their tubular structures [19], two isomericstructures, resulting from fluorine addition to either 1,2 or1,4 positions within a hexagonal ring on the graphene-likesidewall of SWNT, have been proposed [Figure 2]. Accord-ing to molecular mechanical (MM force field) and quan-tum mechanical calculations (semiempirical AM1 andCNDO, as well as density functional theory PBE/3-21G andLSDA/3-21G [43]), the energy difference between thesetwo isomers is very small, so that one can assume that bothisomers coexist in F-SWNTs [15]. The STM images showedthat the fluorinated regions typically form bands aroundthe nanotube circumference [Figure 3(a)], which mayimply that this way of fluorine addition is more favorable.Nevertheless, semiempirical calculations showed that theaddition along SWNT axis for the 1,2-isomer is a moreexothermic process than circumferential 1,2-addition,whereas in the case of the 1,4-isomer the addition aroundthe circumference is more energetically favorable thanpropagation along the nanotube axis [15, 22]. On that basis,the abrupt band boundaries observed in STM images ofF-SWNTs, could be explained by a circumferential additionmechanism proceeding via initiation of the 1,4-isomer atmultiple sites along the nanotube and propagating on alter-nate pairs of rows [Figure 3(b)]. However, taking intoaccount the small calculated energy difference betweenthe 1,2- and 1,4-isomers, one can admit that both types offluorine addition occur simultaneously to form discrete iso-meric domains on the nanotube [Figure 3(c)]. The authorssuggested an important role of defects in the sidewalls foreither initiating or terminating such domains. Other exam-ples of atomistic simulation of F-SWNT formation andproperties at different theoretical levels (density functionaltheory-based methods, with and without applying periodicboundary conditions) can be found in [33, 40, 4449].

Despite that historically CNT fluorination was first per-formed for MWNTs [30], up to now MWNT fluorinationdid not gain a wide distribution. Nevertheless, the fewknown examples are interesting from a phenomenological

point of view. In particular, the fluorination of arc-produced carbon material containing MWNTs was per-formed at room temperature by using a gas mixture of BrF3and Br2 [31]; elemental composition of the samples accord-ing to XPS data was CF0.3Br0.02. A combined MWNToxyfluorination with F2, O2 and N2 gases at temperatures of25C300C [34] allows introduction of oxygen functional-ities along with fluorine atoms, which is believed to beimportant for improving the mechanical and interfacialproperties of MWNT-reinforced composites. A very inter-esting example is the reaction of MWNTs with xenon diflu-oride, achieved at ambient temperature under the influenceof sunlight or a halogen lamp [35]. Due to its extreme sim-plicity in performance and safety (contrary to fluorinationwith F2 and other aggressive gaseous reagents), it can beconsidered as the most ecologically friendly technique forCNT fluorination. As was mentioned above, when MWNTsare subjected to CF4 plasma treatment at a power of 30 Wand a pressure of 0.1 Torr for 30 s, a fluoropolymer formsaround the nanotube surface; increasing the power up to50 W and the treatment time up to 5 min results in a moreuniform fluorination of the nanotube surface (the atomicconcentration of fluorine of 20 at. %) [42]. In a recent com-munication by Muramatsu et al. [37], fluorine atoms wereselectively attached to the sidewalls of the outer shell ofDWNTs. In this way, the double-layered morphology wasnot disrupted; the stoichiometry of the fluorinated DWNTswas CF0.3.

The finding that F atoms turn to be localized in certainSWNT regions and leave their other regions intact, leadGu et al. [20] to the idea of chemical scissors for cutting thenanotubes. The fluorination of purified SWNTs to a stoichi-ometry CFx (x < 0.2) followed by pyrolysis of the partiallyfluorinated nanotubes up to 1000C was found to have cutthem to a range of short lengths (average


who optimized the cutting conditions. They employedliquid-phase etching of SWNTs, prepared by the reductionof F-SWNTs with hydrazine (see below), with room-temperature piranha (H2SO4-H2O2 mixture) and ammo-nium persulfate solutions. The final average length of theshortened SWNTs was approximately 100 nm with carbonyields as high as 70%80%.

The fluorination gives rise to a dramatic change inSWNT solubility. While pristine SWNTs do not dissolve inalcohols, Mickelson et al. [24] found that F-SWNTs dissolvewell in alcohol solvents by short time ultrasonication. Thesolutions produced are stable for a couple of days toover one week. A solubility limit of at least 1 mgmL1 in2-propanol was measured; at the same time, it was notedthat particulate matter was precipitating out after a few days.As no indication of alkoxy substitution or HF evolution wasobserved, formation of CAF. . .HAO hydrogen bonds wasinvoked to explain F-SWNT dissolution in alcohols.

What is especially important for the CNT chemistry is theaddition of fluorine drastically enhances the chemical reac-tivity of the nanotube side walls [1517, 19, 21, 36, 38, 41].F-SWNTs can be defluorinated using anhydrous hydrazineat room temperature [15, 19, 38], according to Scheme 1:

4CnF N2H4 ! 4Cn 4HF N2 Scheme 1The starting nanotube sidewall structure is almost com-

pletely recovered in this way. On the contrary, the use oforganic nucleophiles allows further derivatization steps[1517, 21, 41]. In particular, Stevens et al. [16] and Zhuet al. [21] developed the method of SWNT sidewall CANfunctionalization, which is based on the enhanced reactivityof F-SWNTs towards terminal diamines, according to thefollowing scheme:

SWNT-FH2N-Y-NH2 !Py;70170C

-HFSWNT-HN-Y-NH2

Scheme 2

where Py is pyridine, and H2N-Y-NH2 can be aliphatic,cycloaliphatic, aromatic diamine, etc. The SWNT materialsderivatized with amines can by subjected to further cou-pling reactions, for example with amino acids and DNA

bases [16], or used as reactive monomers in a broad rangeof polycondensation reactions leading to the production ofnew covalently integrated nanotube-reinforced polymers[36]. Monofunctional amines can be used for F-SWNT deri-vatization as well, as was recently shown by Valentini et al.[41]. The authors performed butylamine attachment toF-SWNT sidewalls by ultrasonication for 1 h at room tem-perature. The peak at 1221 cm1 in the IR spectrum ofF-SWNTs disappeared after the above treatment, and abroad absorption band at 3400 cm1 was attributed to themNH vibrations in secondary amine.

3. REACTIONS WITH ARYL DIAZONIUMSPECIES

Very reactive aryl diazonium species can be generated from4-substituted anilines (R-C6H4-NH2, where R Cl, Br, tert-butyl, CO2CH3, NO2, etc.) by adding isoamyl nitrite orsodium nitrite in sulfuric or acetic acid. Their in situ reac-tion with SWNTs was reported by Dyke and Tour [5052].Isoamyl nitrite, H2SO4 and acetic acid are liquids, whichcan serve as the reaction medium, without the use ofcommon organic solvents. The general scheme is shown inFigure 4. The reaction was performed in a flask equippedwith a reflux condenser and a magnetic stir bar.

The resulting compounds were characterized by Ramanspectroscopy. For SWNT-(4-Cl-C4H4)n, the disorder modeat 1290 cm1 was greatly enhanced, which is indicative ofcovalent functionalization. IR spectroscopy in attenuatedtotal reflection mode showed the presence of functionaladdends for the above and other products. The levels ofsolubility achieved were similar to those obtained previouslywith the solvent-based techniques [1, 53].

Dyke and Tour [50] tried another variant of the samesolvent-free functionalization reaction for SWNTs, employingball-milling (for a detailed consideration of mechanochemi-cal reactions, see Section 6). The Raman spectrum suggesteda high degree of functionalization, although much of theSWNT structure could be compromised by the high shear,pulverizing action of the process. The intensity of theRaman-disorder mode might suggest either the nanotube

Figure 4. Solvent-free functionalization of SWNTs with various 4-substituted anilines and isoamyl nitrite or sodium nitrite/acid. Reprinted with per-mission from Ref. [50], C. A. Dyke and J. M. Tour, J. Am. Chem. Soc. 125, 1156 (2003). 2003, American Chemical Society.

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destruction or a very high degree of functionalization. How-ever, since atomic force microscopic (AFM) analysis showedno rope-like structure, the nanotube scaffold was probablycompromised.

Using the solvent-free stir bar conditions, the functional-ization of MWNTs can be achieved as well. TGA analysis(10C/min to 750C in argon) of MWNT-(4-Cl-C4H4)n gave8% weight loss, while the pristine MWNTs gave no weightloss under the same TGA conditions. The authors believethat the lower functionalization percentage in the MWNTsis likely due to their larger outer diameter causing them tobe less reactive than the smaller-diameter SWNTs. Second,the sheathed nature of MWNTs makes many of their side-walls inaccessible. Summarizing their observations, theauthors suggested that for the CNT functionalization tooccur, the bundles must be partially de-roped prior to reac-tion, and in the solvent-free techniques such initial de-ropingare probably affected by mechanical means.

Recently, the same research group reported on the reac-tion of SWNTs with diazonium salts in imidazolium andpyridinium-based ionic liquids [54]. It results in the produc-tion of functionalized nanotube individuals, which are com-parable to those derived from other processes using harshreaction conditions. The method utilizes the unique solventproperties of ionic liquids. It can be completed in minutesat room temperature, and does not require adverse solventconditions. Thus, despite the fact that this method doesemploy solvents, it has a certain value from the point ofview of the green chemistry of CNTs.

4. AMIDATION AND ESTERIFICATIONOF OXIDIZED DEFECTS

All the reactions considered above are based on the chem-istry of pristine CNT sidewalls. On the contrary, in the nextgroup of methods nanotube chemical modification employsthe chemistry of oxidized defects. The standard strong-acidpurification process terminates the CNT open ends withcarboxylic groups, COOH (among other oxygen-containingfunctionalities) [18], which can be subjected to furtherderivatization reactions. The most common reaction is theformation of amide derivatives. Nevertheless, COOHgroups by themselves are unreactive under ambient temper-ature and thus require chemical activation. It is convention-ally performed by the treatment with thionyl chlorideSOCl2 in aprotic solvent media, which converts carboxylicgroups into their chloroanhydrides C(@O)-Cl. At the sec-ond step, the latter condense with aliphatic and aromaticamines producing amide derivatives according to the fol-lowing scheme:

CNT-------COOH SOCl2! CNT-------COCl !H2NR

-HClCNT-------CONHR Scheme 3

where R is alkyl or aryl substituent. Thionyl chloride is acorrosive compound, and is sometimes substituted withmore ecologically-friendly carbodiimides, which are softamide-coupling agents frequently used in peptide chemis-try. On the other hand, SOCl2 is volatile, and its excess can

be easily removed, whereas substituted ureas (final prod-ucts of carbodiimide conversion) are not volatile com-pounds, and need to be washed off.

An analogous esterification reaction can be carried outby substituting amine reagent with an alcohol HOR, underthe same reaction conditions as for amidation:

CNT-------COOH SOCl2! CNT-------COCl !HOR

-HClCNT-------COOR Scheme 4

In addition to the common liquid-phase derivatizationprocedure, the acid chloride-functionalized SWNTs werefound to be susceptible to solvent-free amidation with long-chain aliphatic amines octadecylamine (ODA)CH3(CH2)17NH2, and alkyl-aryl amine 4-dodecyl-aniline(4-CH3(CH2)13C6H4NH2) [55, 56]. It was performed at ele-vated temperature of 100C for 4 days. The solubility prop-erties of derivatized SWNTs were similar, regardless ofwhether the amide function derives from an alkylamine oran alkyl-arylamine. The authors also pointed that, while theSWNT-derivative is represented as an amide for conven-ience, it appears that the products exist primarily as imides.

Aimed to the design of a nondestructive and scalableprocedure for dissolution and purification of SWNTs, thevery same reaction via condensation between amine groupsof ODA and COOH groups of oxidized raw SWNTs wasalternatively carried out with the assistance of dicyclohexyl-carbodiimide (DCC) as a carboxyl activation reagent[5759]. Because of their difference in surface area andreactivity, the carbon forms contained in the starting mate-rial should be modified to a varying extent, bringing differ-ence in their solubility. This makes it possible to separatevarying carbon impurities and metal catalysts from solubleSWNTs by a dispersioncentrifugation recycle (up to 10cycles). From the results of Raman and optical absorptionspectra, AFM, scanning electron microscopy (SEM), trans-mission electron microscopy (TEM), and TGA analyses,60% of SWNTs in the starting material was recovered,and the purity of the final product was quantitatively esti-mated to be >90 wt%. As a whole, the reaction of carboxyl-activated oxidized SWNTs with ODA and other long-chainamines during a prolonged period of heating without sol-vents is one of the most efficient ways of preparation offunctionalized SWNTs, which have substantial solubility inchloroform, dichloromethane, aromatic solvents, and CS2.On the contrary, Hamon et al. [56] attempted to producesoluble SWNTs through the amide formation with aniline,having a short hydrocarbon radical, but the resultingSWNTs were almost completely insoluble in CH2Cl2, and avery small fraction was soluble in THF.

The solvent-free reaction conditions were used for ami-dation and esterification (according to Schemes 3 and 4,respectively) of acid chloride-functionalized SWNTs andMWNTs with lipophilic and hydrophilic dendron species,shown in Figure 5 [60, 61]. The general procedure includedheating of the component mixture at 75C, and vigorousstirring for 48 h under nitrogen protection. All the dendronspecies can be used as solvents in the functionalizationreaction because of their nearly ambient melting points.The reactions were found to be sensitive to residual water

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in the reactants, and therefore careful drying of them wasrequired for best results. Since the dendra were terminatedwith long alkyl chains and oligomeric poly(ethylene glycol)

moieties, the dendron-functionalizied nanotubes were solu-ble in hexane, chloroform, and water to form coloredhomogeneous solutions. These amide and ester derivatizedSWNTs and MWNTs can be defunctionalized in homoge-neous solutions under base- and acid-catalyzed hydrolysisreaction conditions [61]. The authors also explored the pos-sibility of encapsulating silver nanoparticles inside thederivatized MWNTs [60]. While no Ag-filled nanotubeswere found, the experiments did produce some interestingresults concerning the trapping of silver nanoparticles bythe nanotube-bound dendron moieties in THF solutions ofsilver nitrate AgNO3. In a typical experiment, a solution ofthe derivatized MWNTs in anhydrous THF was prepared,and to the solution was added AgNO3 until saturation.After a brief ultrasonication, THF was removed and theremaining solid sample was repeatedly washed with waterto remove AgNO3, and then was redissolved in anhydrousTHF to form a clear solution. Hydrazine was added to thesolution to reduce Ag cations, and the clear solutionturned into more of a suspension. A small aliquot of thissuspension was diluted and deposited on a TEM grid, andthe remaining suspension was evaporated to produce asolid sample for X-ray powder diffraction analysis. Theresulting X-ray diffraction pattern was characteristic ofnanocrystalline Ag particles. The TEM images showed thatthe nanocrystalline silver particles are aligned next to cer-tain sections of a nanotube [Figure 6], whereas the resultsfrom the control experiments with purified raw nanotubeswere rather different, showing instead separate aggregatesof MWNTs and Ag particles. According to the depositionprocedure, the solid sample containing dendron-derivatizedMWNTs and AgNO3 was repeatedly washed with waterbefore the reduction with hydrazine. Thus, there is likely

Figure 5. Lipophilic and hydrophilic dendron species used for the ami-dation and esterification of acid chloride-functionalized SWNTs andMWNTs. Reprinted with permission from Ref. [60], Y. P. Sun et al.,Chem. Mater. 13, 2864 (2001). 2001, American Chemical Society.

Figure 6. TEM images showing the trapping of nanocrystalline silver particles by dendron-derivatized MWNTs. The two images are for two differ-ent sections of the same nanotube (see the four particles in the middle of both images). Reprinted with permission from Ref. [60], Y. P. Sun et al.,Chem. Mater. 13, 2864 (2001). 2001, American Chemical Society.

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something special with those sections of the nanotube,which allows the trapping of either Ag cations beforechemical reduction or Ag nanoparticles. The authorsbelieve that these special sections of the nanotube containthe sites linking the dendron functionalities, or in otherwords, it is the MWNT-bound dendron moieties that facili-tate the trapping of nanoparticles. In this regard, thealigned Ag nanoparticles may actually be considered ashigh electron density markers for indicating the branchingpoints or regions on the dendron-functionalized MWNTs.

Chen et al. [62] attempted another possible route to thedissolution of oxidized SWNTs by directly heating ODAwith both shortened and full-length SWNT-COOH, by pro-ducing zwitterions through a simple acid-base interaction:

SWNT-COOHH2N CH2 17CH3! SWNT-COO NH3CH217CH3 Scheme 5

Full-length SWNTs are very important for nanotube-based composites and copolymers, because of their highaspect ratio. The procedure included mixing oxidizedSWNTs with ODA, heating the mixture for 48 days at120C130C (the melting point of ODA is 55C57C),washing with ethanol to remove ODA excess, membrane fil-tering, and drying. The resulting derivatized nanotubes(typically longer than 1lm) were soluble in dichlorome-thane, chlorobenzene, and other similar organic solvents.The authors idea was that the ionic functionalization cangive a much higher yield of soluble SWNTs than the cova-lent amide derivatization. Unlike the covalent amide bond,the cation NH3(CH2)17CH3 in the ionic bond of SWNT-COO NH3(CH2)17CH3 can be readily exchanged forother organic and inorganic cations. Therefore it would bepossible to use soluble SWNTs as versatile building blocksfor advanced nanotube-based materials via supramolecularchemistry. Furthermore, such an ionic feature may allowelectrostatic interactions between SWNTs and biologicalmolecules, and thus could serve as the basis for developingbiocompatible SWNTs. While all the above considerationsare understandable and valid in principle, one should bearin mind one of the fundamental properties of carboxylicacids and amines; they can form amides under higher tem-peratures (>100C) directly, i.e. via thermal activation andwithout chemical activation of COOH groups. The reactionof oxidized CNTs with amines in this case can be describedby the following general scheme:

CNT-------COOHH2NR !-H2O CNT-------CONHR Scheme 6

It is quite obvious from the IR spectrum shown in Figure 7,along with the disappearance of COOH band at 1709 cm1,an increased absorption around 1670 cm1 due to amidebonds (amide I band mC@O). At the same time, the band at1614 cm1 (attributed by the authors to COO groups) ismore likely due to the dNH vibrations in adsorbed octadecyl-amine molecules.

Contrary to Chen et al. [62], Lin et al. [63] intentionallyemployed the solvent-free melting method to derivatizeoxidized MWNTs with poly-(propionylethylenimine-co-ethylenimine, PPEI-EI) through amidation reaction. Theytested two options: (a) amidation of SOCl2-activated

MWNTs (Scheme 3), and (b) direct amidation of COOH-containing MWNTs (Scheme 6). In both cases, theMWNTpolymer melt was heated at 160C180C for12 h under nitrogen protection, followed by chloroformextraction and precipitation into hexane, solvent removal,and drying. After a comparative study of the derivatizedMWNTs by SEM, TEM, nuclear magnetic resonance(NMR), TGA, UV-visible, and Raman spectroscopy, theauthors came to the conclusion that both methods arealmost equally effective. They also believe that COOHgroups are likely associated with the surface defects on thetips and throughout the sidewall, which were oxidized dur-ing the oxidative purification under strongly acidic condi-tions. In fact, the MWNT sample produced by chemicalvapor deposition (CVD) seemed more readily derivatizedand solubilized than the sample obtained from the arc dis-charge method, based on a comparison of the resultingnanotube solutions. This is consistent with the understand-ing that the CVD-produced MWNTs contain more defects.Figure 8 shows a TEM image of a PPEI-EI-derivatizedMWNT. According to this image, there are definitelydefects on the nanotube tip that are dislocated from thegraphite fringes. The polymer-attached MWNTs were solu-ble in many common organic solvents as well as in water.The aqueous solubility of these derivatized MWNTs as aresult of the hydrophilicity of the aminopolymers, may findapplications in introducing carbon nanotubes into biologi-cally significant systems.

As it could be expected, the amidation of SOCl2-activated oxidized CNTs with molten PPEI-EI is a fasterprocess as compared with the direct amidation according toScheme 6, and does not need such long exposure times.There are two reported examples where similar PPEI-EI-derivatized SWNTs and MWNTs, forming highly coloredhomogeneous solutions in both organic solvents and inwater, were prepared by heating the melts at 165C for only20 min [64, 65].

All the techniques on CNT amidation and esterificationdiscussed up to this point employ solvent-free reaction

Figure 7. Fourier-transform IR (FTIR) spectra of oxidized SWNTs (p-SWNTs(C)) after their treatment with molten ODA for 48 days at120C130C (s-l-SWNTs(C)). Adapted with permission from Ref.[62], J. Chen et al., J. Phys. Chem. B 105, 2525 (2001). 2001, Ameri-can Chemical Society.

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conditions, but nevertheless, the derivatized nanotubematerial does need further purification from excess organicreagent. The auxiliary purification operations are extrac-tion, centrifugation, precipitation, filtration, solvent evapo-ration, drying (often in vacuum), and so forth, which arequite tedious and usually take an incomparably longer timethan the derivatization reaction by itself. As a result, theygive no gain from the point of view of green chemistry.This circumstance motivated our work on the developmentof more sophisticated solvent-free techniques for CNT deriv-atization, which would also minimize the auxiliary purifica-tion operations.

A suitable approach was already proposed more than adecade ago for the chemical modification of silica stationaryphases for liquid chromatography. Systematic studies werecarried out on the design of the gas-phase chemical deriv-atization of silica with polyazamacrocyclic ligands, pyrimi-dine bases, and crystalline carboxylic acids [6669]. All ofthese reagents are not volatile compounds under ambienttemperature and pressure. However, decreasing the pres-sure to a moderate vacuum (101102 Torr) and, simulta-neously, increasing the temperature to >150C providesefficient formation of the chemically bonded surface deriva-tives. The most relevant reaction in the present context isthe reaction between silica-bonded aminoalkyl groups andvaporous carboxylic acids to form surface amides (similarlyto Scheme 6). It proceeds smoothly under 150C180Cwithout chemical activation of the COOH groups, is rela-tively fast (0.51 h) and produces high yields of the amidederivatives (>50% based on the starting surface concentra-tion of aminoalkyl groups). Since excess derivatizing re-agent is spontaneously removed from the reaction zone due

to a simultaneous heating and pumping out, there is noneed to use (organic) solvents to purify the derivatizationproduct.

With the above advantages of the gas-phase chemicalmodifications in mind, we attempted to apply it first for thedirect amidation of oxidized SWNT defects [Scheme 6][70]. Experimental setup can be variable, but should neces-sary contain the following basic elements: (a) a pump capa-ble of producing vacuum of 101102 Torr; (b) a vesselserving as reactor; (c) a heating element such as heatingmantle. In most of our experiments (on this and other reac-tions of gas-phase modification of SWNTs and MWNTs) weemployed the custom-made Pyrex glassware shown inFigure 9. In a typical experiment, 100 mg of CNTs areplaced into the bigger reactor 11. To remove volatilecontaminants (which are always adsorbed from the environ-ment) from the nanotube material, the reactor is pumpedout to a vacuum of 102 Torr (valves 2 and 5 open; 1, 3and 4 closed), and its bottom is heated for 0.5 h at100C120C by means of heating mantle 13. Then thereactor is cooled, open and 50 mg of amine is dropped tothe bottom containing CNTs. After pumping the reactor outto 1 Torr at room temperature, valve 2 is closed, and thebottom is heated at 150C170C for 12 h. During this pro-cedure, amine evaporates, reacts with CNTs, and its excesscondenses a few centimeters above the heating mantle. Asin the case of silica derivatization, the high temperature notonly facilitates the reaction itself, but also helps to minimizethe amount of amine physically adsorbed on the nanotubematerial. After finishing the procedure, valve 2 shouldbe open again for 1560 min to remove volatiles. Then theheating mantle is removed, the reactor is cooled and discon-nected from the manifold. Before extracting derivatizedCNTs, the upper reactor part with condensed excess amineon the wall should be wiped with cotton wool wet withethanol to avoid contact with CNTs and the possibility ofcontaminating them. For milligram-scale derivatization pro-cedures, smaller reactor 12 can be used in a similar way. Ifneeded, the reaction setup can be extremely simplified; forexample, direct amidation of oxidized SWNTs with ODAwas performed by baking the two components at 170C for2 h in a vial sealed under vacuum [71]. However, since thevolume is closed, excess amine should be removed afteropening the vial; it can be done by evacuating/heating usingthe same vacuum manifold [Figure 9]. The reaction temper-ature can be even lower, e.g. 110C, but then the reactiontime should be substantially increased, to 40 h [72]. Amide-derivatized SWNTs acquired an enhanced solubility in manyorganic solvents, for example in THF. The strongest effectwas found for ODA [71], although the derivatization withnonylamine [70] gave good results as well.

We analyzed applicability of IR spectroscopy for charac-terization of the amide-derivatized SWNTs. This technique ismost routinely used in related studies, but at the same timeoften produces controversial data. We compared IR spectraof the gas phase-synthesized nonylamine derivative to thoseof oxidized SWNT samples impregnated with nonylamineunder room temperature and different nonylamine:SWNTsratios [70]. Based on spectral band intensities, we concludedthat IR spectra of oxidized SWNTs treated with aminesunder both liquid-phase and solvent-free conditions cannot

Figure 8. High-resolution TEM image of a PPEI-EI polymer-derivat-ized MWNT. The scale bar represents 15 nm. Reprinted with permis-sion from Ref. [63], Y. Lin et al., J. Phys. Chem. B 106, 1294 (2002). 2002, American Chemical Society.

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correspond to amide derivatives on SWNT tips, due to a verylow concentration of the terminal groups relative to thewhole sample mass, which implies a negligible contributionto the IR spectra. The bands detectable in the case of long-chain amines must correspond to amine molecules, physi-sorbed due to strong hydrophobic interactions of theirhydrocarbon chains with SWNT walls. According to MMmolecular mechanics modeling, the energetically preferableadsorption sites are the nanotube cavities [70, 73].

An additional information on the chemical state ofamines in the derivatized SWNTs was obtained fromtemperature-programmed desorption mass spectrometry(TPD-MS) measurements [70]. As an example, Figure 10shows TPD-mass spectrum of volatile products evolved at325C from oxidized SWNTs treated with nonylaminevapors, along with experimental thermograms for selectedhydrocarbon peaks at m/z 42, 43, 55, 57, and 69. The massspectra within a wide temperature range contained a seriesof hydrocarbon peaks, in particular those at m/z 41 (C3H5),42 (C3H6), 43 (C3H7), 55 (C4H7), 56 (C4H8), 57 (C4H9), 67(C5H7), 69 (C5H9), 70 (C5H10), 71 (C5H11), 79 (C6H7), 81(C6H9), 83 (C6H11), 84 (C6H12), 85 (C6H13), 97 (C7H13), 98(C7H14), and 99 (C7H15) [Figure 10(a)]. Both the massnumbers and peak intensity distribution corresponded tononene, formed as a result of thermal decomposition ofnonylamide terminal groups and physisorbed nonylamine inSWNTs, according to Schemes 7 and 8, respectively:

SWNT-CONH CH2 8CH3! SWNT-CONH2 H2CCH CH2 6CH3 Scheme 7

H2N CH2 8CH3 ! NH3 H2CCH CH2 6CH3Scheme 8

TPD curves for all hydrocarbon peaks had a similar,notably asymmetric shape with a principal maximum at

325C. Total disappearance of these peaks was observedat 400C. Since the IR measurements proved that an over-whelming amine fraction is actually physisorbed on SWNTs,with a negligible contribution of the chemically bondedform, the results were interpreted in the following way. The

Figure 9. Vacuum manifold designed for the gas-phase covalent modification of CNTs with amines and thiols. (15) Teflon valves; (68) 10-mmI.D. O-ring joints; (9,10) 41.4-mm I.D. O-ring joints; (11) gram-scale reactor; (12) milligram-scale reactor; (13) heating mantle; (14) vacuum gauge;and (15) vacuum pump.

Figure 10. (a) TPD-mass spectrum of volatile products evolved at325C from SWNTs gas-phase treated with nonylamine; (b) experimen-tal thermograms for selected hydrocarbon peaks at m/z 42, 43, 55, 57and 69. Reprinted with permission from Ref. [70], E. V. Basiuk et al.,J. Phys. Chem. B 106, 1588 (2002). 2002, American ChemicalSociety.

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first thermodesorption maximum at 250C and lowerabundance of hydrocarbon decomposition products corre-sponds to the nonylamine species present as the terminalnonylamide derivative. The second thermodesorption maxi-mum at 325C and higher abundance of hydrocarbondecomposition products corresponds to the nonylaminephysisorbed on SWNTs due to strong hydrophobic interac-tions, which is a more abundant species. Their abundanceratio estimated from the TPD maxima is 1:5.

Both IR and TPD-MS results practically imply that evenunder high post-reaction temperatures of the gas-phasederivatization procedure along with continuous evacuation,it is impossible to remove all unreacted amine. While thepresence of physically adsorbed amine cannot negativelyaffect solubility properties of SWNTs, it must inevitablyalter physical properties (e.g., conductivity) of the nano-tubes and should always be taken into account for elec-tronic applications. These considerations remain valid forthe liquid-phase-derivatized CNTs as well.

5. FUNCTIONALIZATION OF CLOSEDCAPS AND SIDEWALL DEFECTS

In view of the above observations, pristine MWNTs grownby most methods (in particular by CVD) have certainadvantages as compared to open-end acid-oxidized SWNTs.The reason is that the tips of pristine MWNTs are closedwith fullerene hemispheres, and as a rule, organic reactantscannot penetrate inside the nanotube. Pristine MWNTs donot have reactive functional groups like COOH, whichcould be easily derivatized, but instead they have pentago-nal (and apparently heptagonal) defects on the tips andsidewalls (causing nanotube kinking). It is these sites thatare oxidized first under the strong acid treatment.

More than a decade ago, Hirsch, Wudl, and collaborators[74, 75] reported on a chemical reaction of spheric fullereneC60 with primary and secondary amines. Being all neutralnucleophiles, they add onto C60 cage producing a mixture ofproducts. The reaction is carried out at room temperatureby dissolving C60 directly in liquid amines or in their solu-tions in organic solvents (DMF, dimethylsulfoxide, chloro-benzene, etc.) [7478]. A solvent-free treatment of silica-supported C60 with nonylamine vapor at 150C was reportedas well [79]. The addition stoichiometry depends on the sizeof amine molecule, with the highest average amine:C60 ratioof 10:1 found for 2-methylaziridine [78]. According to theresults of quantum chemical DFT calculations [79, 80], thereaction preferably takes place across the 6,6 bonds of C60pyracylene units, and not across the 5,6 bonds.

We attempted to apply this chemical reaction for a directamination of MWNT closed caps with a variety of amines,namely with nonylamine, dodecylamine, octadecylamine,4-phenyl-butylamine, and 1,8-ocanediamine [81, 82], usingthe same reaction conditions and experimental setup as forthe direct amidation of oxidized SWNTs [Figure 9]. Thefirst indication of successful attachment of hydrophobiclong-chain amines to MWNTs was an increase in the nano-tube solubility/dispersibility in some organic solvents. Forexample, for ODA-MWNTs it was tested by ultrasonicatingin isopropanol for 20 min, and comparing to the behavior

of pristine MWNTs [81]. While the latter began to precipi-tate almost immediately after ultrasonication, ODA-MWNTsolution/dispersion did not exhibit visible changes for morethan one month.

The presence of organic moieties in the functionalizedMWNT samples can be detected by IR spectroscopy, due tothe absorption bands about 1600 and 3000 cm1 (stretchingvibrations mCAH). They become more pronounced withincreasing the size of hydrocarbon radical, so that the mostintense mCAH absorption can be observed in the case ofoctadecylamine [82]. In addition to that, an increase in theabsorption at 1640 cm1 for 4-phenyl-butylamine wasapparently due to aromatic ring stretching vibrations,whereas for 1,8-octanediamine the same band wasexplained by the presence of second amino group (NH2deformation vibrations, or dNH2). Unfortunately, IR spec-troscopy is unable to provide any information on the sitesof attachment or the chemical state of amine molecules(chemical bonding vs. physical adsorption).

TGA analysis of the amine-treated MWNTs was per-formed as well [81, 82]. For ODA-MWNTs, the steepestweight loss of 5% due to organics decomposition wasobserved in a temperature interval of 250C400C[Figure 11] [81]. Taking into account the high aspect ratiosof 103 typical for MWNTs, this weight loss cannot corre-spond to ODA molecules attached to the fullerene capsonly. Therefore, one can conclude that a major ODAfraction is distributed over MWNT sidewalls. These ODAmolecules might be either simply physisorbed on the side-walls, or chemically attached there, in the way similar to

0 200 400 600 800Temperature (C)

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Figure 11. TGA curves for (a) pristine MWNTs and (b) ODA-MWNTs. The temperature interval (250C400C) of the steepestweight loss due to organics decomposition for ODA-MWNTs is shownwith a rectangle. Reprinted with permission from Ref. [81], E. V.Basiuk et al., Nano Lett. 4, 863 (2004). 2004, American ChemicalSociety.

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their attachment to the closed fullerene caps. The firstexplanation seems less likely, since physisorbed ODA mole-cules must decompose at lower temperatures than250C400C, as supported by the observation that TGAdecomposition of ODA physically adsorbed on SWNTsoccurs in a temperature interval of 150C300C [83]. Thesecond explanation is hardly compatible with the chemistryof ideal, defectless graphene sheet, which is known to bechemically unreactive in these kind of reactions. However,this might be possible if to admit the presence of suchdefects as five-membered rings (pentagons) typical forspherical fullerenes.

High-resolution TEM (HRTEM) observations of ODA-MWNTs provided useful information on the addition sites[81]. As it is seen from Figure 12(a), pristine MWNTs arecomposed of about 10 coaxial tubes. Their closed caps havean irregular shape, however the outer graphene sheet frag-ments are relatively ordered. While similar closed caps wereclearly distinguished in ODA-MWNTs, they were found tobe covered with 2-nm layer of an amorphous material[Figure 12(b)]. It apparently originates from electron beamburning of ODA molecules chemically bound to the pentag-onal defect sites. In addition to that, HRTEM observationof the sidewalls of ODA-MWNTs revealed similar amor-phous formations [Figure 12(c)], which were concentratedat the sites with well-pronounced curvature (or kinks). Onthe contrary, almost ideal MWNT sidewalls did not containany additional material.

To explain these results, we employed theoretical calcula-tions by the AM1 semiempirical method. We tested ener-getic feasibility of amine addition to different types ofcarbon atoms [81], using methylamine as model amine andtwo simple SWNT structures. One of them [armchair (5,5)]was capped with C60 hemisphere and thus contained pyra-cylene units, in which pentagons are directly connected.The second nanotube model [zigzag (10,0)] was cappedwith C80 hemisphere, where all pentagons are separated bytwo CAC bonds. We compared the formation energies cal-culated for different isomeric methylamine monoadducts,and came to the following conclusions:

(a) The presence of pyracylene units in the closed nano-tube caps is not crucial for the amine addition to be possi-ble. (b) Site-specificity of the reaction does depend on themutual position of five-membered rings. If the caps containpyracylene units, the addition preferentially takes place ontheir 6,6 bonds; if they do not, the preferential reactionsites are CAC bonds of the pentagons. (c) In all the cases,ideal CNT sidewalls composed of benzene rings only(derived from ideal graphene sheet) are relatively unreac-tive toward amines. (d) Since introducing pentagonaldefects into the ideal sidewalls causes a distortion of thecylindrical curvature (kinking), one can expect that thesedefects would be reactive towards amines as well. The highcontent of organics in the amine-functionalized MWNTsfound by TGA apparently results from the enhanced reac-tivity of the real sidewalls containing numerous pentagonal

Figure 12. HRTEM microphotographs showing closed caps of pristine MWNTs (a), of ODA-MWNTs (b), and sidewalls of ODA-MWNTs (c).Black arrows point to an amorphous material originating from ODA molecules bound to the defect sites; white arrow points to almost ideal side-walls where no similar material can be observed. Reprinted with permission from Ref.[81], E. V. Basiuk et al., Nano Lett. 4, 863 (2004). 2004,American Chemical Society.

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defects. Our conclusions were supported by later DFT cal-culations by Lin et al. [80].

Physical adsorption of amines on closed-cap MWNTscannot be discarded completely. Even TGA [81, 82] wasable to detect some weight loss below 200C, presumablydue to a small (of a few percent) physically adsorbed aminefraction. Further information on the chemical state ofamine molecules in the functionalized MWNTs wasobtained by TPD-MS [82]. The first general feature foundin all the mass spectra [nonylamine-functionalized MWNTsas an example; Figure 13(a)] was a typical pattern of hydro-carbon peaks, very similar to the one observed for thenonylamide-derivatized SWNTs [Figure 10]. TPD curves forall of these peaks have a similar shape [Figure 13(b)],where one can clearly distinguish two general evolutionsteps. The first step, which corresponds to the removal ofphysisorbed amines from MWNTs, begins at about 50Cand ceases after 200C. It matches well the first weight-lossinterval of TGA curves, being somewhat shifted to lowertemperatures with respect to the latter due to the high-vacuum TPD-MS conditions (where all the compounds arevolatilized at lower temperatures). The second thermode-sorption maximum is observed between 250C500C. Itmatches much better the second TGA interval of300C500C due to thermal decomposition of the aminespecies chemically bonded to MWNTs, since the latter pro-cess does not depend on the amine volatility of the pressureAQ1 .

The abundance ratio of physisorbed to chemically-bondedamine species can be roughly estimated from areas of theTPD maxima as 1:2, whereas for nonylamide-derivatizedSWNTs it was 5:1 (see previous Section).

Two questions arise: why the two ratios are so different,and why physically adsorbed amine species are evolvedfrom SWNTs after the chemically bonded ones, at tempera-tures approaching 300C, whereas their desorption fromMWNTs is observed at much lower temperatures, below200C? Both observations can be explained by the absenceof closed caps in oxidized SWNTs, facilitating amine pene-tration and accumulation inside the nanotubes. On the con-trary, MWNTs used were pristine and had closed caps, andamines were adsorbed on the outer walls only. Other distin-guishing characteristics of oxidized SWNTs are their typicalnarrow diameters of 11.5 nm, along with the presence ofvoluminous carboxylic groups at the oxidized defects. Asquantum chemical modeling showed [82], the effectiveentrance into the SWNT cavity becomes very narrow as aresult of COOH group rotation almost perpendicular to thenanotube wall. Besides the steric hindrance, an additionalelectronic obstacle is created due to a high negative electro-static potential extending from all the oxygenated function-alities, especially from COOH groups. It limits thepenetrability of any negatively charged molecule insidethe nanotube cavity; this fully relates to organic amines,since the NH2 group bears partial negative charge due toits lone electron pair. Apparently, a large number of aminemolecules is still able to enter SWNT cavities during thevery first moments of the gas-phase derivatization, at a tem-perature


reagents, namely bifunctional aliphatic thiols [86, 87],employing the same gas-phase procedure, under very similarreaction conditions. We expected dithiols (1,4-butanedithiol,1,6-hexanedithiol and 1,8-octanedithiol), as well as 2-aminoethanethiol, to react with defect sites of the pristinenanotubes according to essentially the same addition mecha-nism suggested for amines [81, 82]. The first indication ofsuccessful reaction was an increased solubility/dispersibilityof the functionalized MWNTs, tested by ultrasonicatingthem in isopropanol. Despite that spectral (in particular, IRand Raman) confirmation for the covalent attachmentturned to be problematic as usual, a way was found tovisualize it directly by HRTEM [86]. The thiols used (1,4-butanedithiol, 1,6-hexanedithiol and 1,8-octanedithiol) areall bifunctional molecules, so that one SH group is expectedto form a covalent bond with the nanotube defect, but toremain capable of coordinating to transition metal cations.The second SH group can exchange H for the same cations.The simple test we made relied upon this affinity of SH andS groups to many transition metals, and to zinc ions inparticular. Ultrasonicating the samples of pristine and 1,4-butanedithiol-derivatized MWNTs in ethanol for HRTEMobservations, we added a very small amount of ZnCl2solution (the final concentration was ca. 1 lgmL1). Thedithiol-MWNTs after the reaction with ZnCl2 were coveredwith a dense amorphous layer, making the coaxial nanotubestructure almost completely indistinguishable [Figure 15].The elemental composition of this amorphous layer wasdetermined by means of energy dispersive X-ray spectros-copy (EDS). It revealed the presence of a sharp peak at2.2 keV due to sulfur in the derivatized nanotube samplesbefore and after treating them with ZnCl2 [Figure 16(a andb)]. In addition to that, the latter exhibited a peak corre-sponding to zinc at about 8.7 keV along with a chlorine peakat 2.6 keV [Figure 16(b)], both due to chemisorbed ZnCl2.At the same time, none of them can be detected in pristineMWNTs, even after ZnCl2 treatment [Figure 16(c)]. Therelative uniformity of ZnCl2 layer seen in Figure 15(b and c)suggests that the kinks and closed caps are not the only sitesfor dithiol addition. Apparently, reactive pentagonal ringsare much more widely distributed than commonly thought;the kinks represent only a small fraction of them, whereasmost pentagons are incorporated in the defect sites withoutwell-manifested spherical curvature (e.g., Stone-Walesdefects).

The nanotubes obtained were used as supports for thedeposition of gold nanoparticles [87]. Small Au particles,with a narrow particle size distribution 1.7 nm, wereobtained on 1,6-hexanedithiol-functionalized MWNTs[Figure 17(a and c)]. For MWNTs functionalized with theaminothiol, the average Au particle size was larger, 5.5nm [Figure 17(b and d)]. In Figure 17(b), an overlapping of

the nanoparticles is evident (white solid arrow), which insome cases gives rise to their coalescence (dashed arrow),explaining the larger average particle size. Looking fordirect information on the location of the sulfur-containingfunctional groups, a Gatan Image Filter attached to themicroscope was employed to obtain the filtered imagesaround sulfur L-edge (165 eV) and carbon K-edge (284 eV).Figure 18(a) shows a bent nanotube with a gold nanoparticleof about 6-nm size. The corresponding filtered image at car-bon K-edge peak is shown in Figure 14(b), and the filteredimage around the sulfur L-edge is presented in Figure 18(c).From the latter image, it is evident that sulfur is present atthe nanotube surface, however its distribution is not homoge-neous, and the contrast differences on the sulfur mappingare evident from the bright contrast differences over theMWNT surface; a notably higher sulfur concentration isobserved around the gold nanoparticle location. Besides thedeposition of metal nanoparticles, dithiol groups introducedin the way proposed can be used as chemical linkers foranchoring metal complexes, attaching the nanotubes to goldtips for AFM and STM, and potentially for adsorption andconcentration of trace metal ions.

6. MISCELLANEOUS MECHANOCHEMICALREACTIONS

Mechanochemical reactions refer to the processes in whichmechanical motions/energy initiate chemical reactions. Thisreaction technique can generate local high-pressure spotsand has been presumed to bring the reacting species intothe closest contact to cause novel chemical reactions tooccur. Konya et al. [88] were apparently first who used themechanochemical approach for the chemical modificationof CNTs. They reported on a large-scale ball-milling pro-duction of short MWNTs with simultaneous attachment ofthiol, amine, amide, carbonyl, and other groups. Two differ-ent mills were used. The first mill was an agate mortar witha large agate ball. The second was a special metal mortarwith several small metal balls and the possibility of heating.In the agate mortar, the maximum weight of the samplewas 500 mg, whereas in the metal mill, 100 g of the nano-tubes were treated. The MWNT functionalization was per-formed in the following way. First, the nanotubes wereplaced in a ball-mill, and the system was degassed by heat-ing either under nitrogen atmosphere or in vacuum. Then,the reactant gas (H2S, NH3, Cl2, CO, CH3SH, or COCl2)was flown over the sample for the entire duration of theball-milling process. After finishing the process, the excessreactant gas was removed from the system either by nitro-gen stream or by evacuating for 1 h, and the resultingMWNT samples were washed with ethanol.

Figure 14. Proposed reaction mechanism between SWNT sidewalls and aromatic amines. Reprinted with permission from Ref. [84], Y. Sun et al.,J. Am. Chem. Soc. 123, 5348 (2001). 2001, American Chemical Society.

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The changes in nanotube length were estimated fromTEM images of MWNTs before and after different ball-milling processes [Figure 19]. Pristine MWNTs were very

long (>10 lm). The metal mill produced broken MWNTswith an average length of 200300 nm, whereas the nano-tubes from the agate mortar were 34 times longer. TheMWNT samples before and after functionalization werealso characterized by XPS, IR, and volumetric adsorption.The spectral techniques provided information on the natureof functional groups chemically bonded to MWNTs. Forexample, in different samples IR absorption peaks wereobserved at 791, 1490, 1675, 615 and 1785 cm1, whichwere assigned to SH, NH2, C@O, SCH3 andC(@O)Cl groups, respectively.

Very interesting observations were made by scanning tun-neling microscopy. STM measurements on MWNT samples

Figure 15. HRTEM microphotographs of 1,4-butanedithiol-MWNTsamples prepared by ultrasonicating in ethanol without (a) and withZnCl2 added (b, sidewalls; c, an amplified view of the nanotube tip),along with pristine MWNT sample prepared with ZnCl2 (d). Repro-duced with permission from Ref. [86], E. V. Basiuk et al., Mater. Lett.in press (2006). 2006, Elsevier Science.

Figure 16. EDS spectra of 1,4-butanedithiol-MWNTs (a) before and(b) after treatment with ZnCl2 solution, along with (c) the spectrum ofpristine MWNTs after treatment with ZnCl2 for comparison. Repro-duced with permission from Ref. [86], E. V. Basiuk et al., Mater. Lett.In press (2006). 2006, Elsevier Science.

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ball-milled under ammonia atmosphere showed smallislands of functional groups which were associated withsidewall defects [Figure 20]. The functional groups attachedin a grouped-together way indicate that the anchoringpoints of the functionalities are the regions with a highdefect concentration; unfortunately, STM measurements

cannot determine whether these defects originated fromthe nanotube growth, or were created during the ball-milling process. The typical apparent height of the islandsof functional groups is on the order of 0.3 nm. The authorsbelieve that MWNT cleavage starts not only at the alreadyexisting defects, but also the mechanical stress induces the

Figure 17. HRTEM images of gold nanoparticles on (a) 1,6-hexanedithiol-functionalized and (b) 2-aminoethanethiol-functionalized MWNTs, alongwith the corresponding size histograms for Au nanoparticles (c and d, respectively). Reprinted with permission from Ref. [87], R. Zanella et al., J.Phys. Chem. B 109, 16290 (2005). 2005, American Chemical Society.

Figure 18. (a) TEM image of a bent MWNT with a gold nanoparticle of about 6-nm size; (b) the corresponding image filtered around carbon K-edge (284 eV), and (c) filtered image around the sulfur L-edge (165 eV). Reprinted with permission from Ref. [87], R. Zanella et al., J. Phys.Chem. B 109, 16290 (2005). 2005, American Chemical Society.

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formation of new defects and, finally, the nanotube cleav-age. If the cleavage of the CAC bonds takes place in thepresence of NH3, Cl2, H2S, etc., new bonds betweenMWNTs and the reactant can form easily, albeit the

reaction efficiency strongly depends on the reactant. Anexample of possible applications of the amino-derivatizedMWNTs is the interconnecting of MWNTs with carboxy-substituted bipiridyl complex of ruthenium for potentialsensor applications [89].

The ball-milling method was tested for the chemicalmodification of SWNTs with halocarbons such as

Figure 19. TEM images of MWNTs before treatment (a) and after ballmilling in Cl2 (b), NH3 (c) and CO (d) atmosphere. Reprinted withpermission from Ref. [88], Z. Konya et al., Chem. Phys. Lett. 360, 429(2002). 2002, Elsevier Science.

Figure 20. Two consecutive topographic STM images of a functional-ized nanotube shifted by the scanning tip on HOPG. The marker (hori-zontal line, arrow labeled 1) was cut in the HOPG surface with theSTM tip at a bias of 10 V. The horizontal arrows, 2 and 3, point toislands of functional groups on the nanotube. The region from thedashed square is shown with a higher magnification in the inset.Reprinted with permission from Ref. [88], Z. Konya et al., Chem. Phys.Lett. 360, 429 (2002). 2002, Elsevier Science.

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trifluoromethane, trichloromethane, tetrachloroethylene,and hexafluoropropene, which can easily form reactive radi-cal species [90]. The chemical modification was carried outunder mild ball-milling conditions in an atmosphere ofhalocarbon, at room temperature. Chlorination by chlorinegas was also performed for comparison. The cleavage ofnanotube CAC bonds with Cl2 produces active sites, whichcan activate molecules in gas phase or those adsorbed onSWNT surface. After characterizing the halogenatedSWNTs by means of TEM, TGA, XPS, and particleinduced c-ray emission, the authors concluded that thismethod produces functionalized SWNTs, containing 0.33.5wt.% fluorine and 5.517.5 wt.% chlorine.

The above two examples of ball-milling proceduresemployed reagents, which are gaseous andAQ2 easily volatileunder ambient temperature. A report by Li et al. [91] dem-onstrated that even non-volatile compounds such as fuller-enes could be used for the chemical modification of CNTs.The authors proposed a very simple solid-phase mechano-chemical treatment of SWNTs with fullerene C60 at roomtemperature under nitrogen atmosphere, which producesC60-decorated nanotubes. Explaining motivation for theirexperiments, the authors fairly noted that since CNTs arescarcely soluble in common solvents, the solvent-freeor/and solid-phase reactions should have privilege in the nano-tube modification. In a typical experiment, 5 mg of SWNTs,25 mg of C60, and 100 mg of potassium hydroxide (KOH)were placed into a stainless steel capsule containing a stain-less steel milling ball. After the capsule was vigorouslyshaken for 50 min under nitrogen atmosphere at room tem-perature, the reaction mixture was washed with a largeexcess of toluene followed by centrifugation until the exces-sive C60 was removed. The resulting black solid was furtherwashed with excess of o-dichlorobenzene to remove com-pletely any C60 oligomers, which probably formed as by-products. To ensure a complete removal of the excessiveKOH, the insoluble residue was washed repeatedly withdeionized water and centrifuged, until the aqueous solutionbecame neutral. Thus, as in many solvent-free modificationtechniques described in the previous sections, (organic) sol-vents were still necessary in auxiliary purification steps.

The authors performed a systematic spectroscopic studyof SWNTC60 [91]. The UV-visible absorption spectrumwas generally featureless, although an additional peak at326 nm suggested the existence of C60 fragments in thenanotubes. FTIR spectra of pristine SWNTs did not showany meaningful peaks (as usual), whereas the spectrum ofSWNTC60 exhibited two clear absorption bands at 526and 746 cm1. Although a broad band at 1050 cm1 wasobserved as well, as in the FTIR spectra of polymerizedfullerenes, the spectrum of SWNTC60 was significantly dif-ferent from those of C60 polymers. The absorption at 526cm1 corresponds to the strongest intramolecular F1u modeof C60. The peaks at 1124 and 1431 cm

1 were likelyderived from other F1u modes of C60 at 1182 and 1428cm1, respectively. The peak shifts were explained by chem-ical linking of the C60 cages, most likely to SWNTs. TheRaman spectrum [Figure 21] of pristine SWNTs showed anintense peak at 1591 cm1, attributable to the E2g tangen-tial mode, with a small disorder-induced peak at 1309 cm1

(D line) and a series of characteristic peaks (192, 217, 257,

and 282 cm1) in the RBM region. The spectrum forSWNTC60 revealed an E2g tangential mode at 1590 cm

1

and a relatively strong D line at 1308 cm1. The tangential/disordered band intensity ratio decreased from 20:1 forpristine SWNTs to 7:1 for SWNTC60. This is a quiteexpected result of the introduction of covalently bound C60moieties into the nanotube framework, where some sp2 car-bon atoms changed their hybridization to sp3. The Ramanspectrum of SWNTC60 also showed characteristic RBMpeaks at 196, 217, 255, and 282 cm1, strongly suggestingthat the entire tubular structure in the SWNTAC60 sampleremained unaltered. A small but clear peak at 1459 cm1

corresponds to the Ag(2) mode due to the C60 cage.TEM observations provided a more direct evidence for

the attachment of C60 to SWNTs. The pristine SWNTs bun-dles had an average diameter of 1030 nm and relativelyclean sidewalls [Figure 22]. The TEM image of SWNT-C60revealed that the average bundle size remained essentiallyunchanged. At the same time, some clusters with 5-nmdiameters were observed clearly upon the bundles. Sincethe size of a single C60 molecule is below 1 nm, it was sug-gested that these clusters are C60 polymers attached toSWNT surface. Based on the TEM images and the spectro-scopic results, the authors suggested the following

Figure 21. Raman spectra of pristine SWNTs (dash line) andSWNTC60 (solid line). The insets show the enlarged radial breathingmode regions. Reprinted with permission from Ref. [91], X. Li et al.,Chem. Phys. Lett. 377, 32 (2003). 2003, Elsevier Science.

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mechanism for the mechanochemical reaction [Figure 23].First, single C60 molecules attach to the nanotube sidewalls.Since fullerene molecules are more reactive than SWNTs,next C60 molecules react with the already bonded C60 toform C60 oligomers, thus resulting in the C60 cluster-modified SWNTs. No possible explanation was given onwhat sites on the nanotube surface react with C60 (only atthose sites on the surface of SWNTs where the reaction iseasier to take place [91]).

The high-speed vibration mill technique was used in thereaction of SWNTs with potassium hydroxide, by a simplemilling of SWNTs and KOH together in air at room tem-perature for 2 h [92]. After the milling, the reaction mixturewas treated with 10 mL of water and precipitated into 100mL of methanol. Precipitation was repeated until the meth-anol solution became neutral to ensure a complete removalof excess KOH. After centrifuging, the upper layer of liquidwas removed and the resulting black solid was readily solu-ble in water (up to 3 mgml1). The authors named thematerial obtained single-walled carbon nanotubols(SWNTols), that is, direct-modified SWNTs with multiplehydroxyl groups. Different from other solubilized CNTs,SWNTols are insoluble in common organic solvents such asdichlorobenzene, chloroform, etc., which implies a highlyhydrophilic nature. FTIR spectra of SWNTols are remark-ably different from those of pristine SWNTs. While no well-manifested bands are usually observed for pristine SWNTsin the spectral range of 5004000 cm1, the spectrum forSWNTols showed a very broad band centered at 3405 cm1,characteristic of hydrogen bonded OH [Figure 24]. Theband at 1177 cm1 was due to CAO stretching vibrations,while the band at 1372 cm1 was attributed to the bendingstretching band of OH groups. The strong band around1580 cm1 was attributed to the stretching mode ofAC@CA in enols AC@CAOH. The FTIR results were con-sistent with the introduction of OH groups into SWNTstructure, as was also supported by XPS and Ramanspectra.

Based on the high content of OH groups in SWNTolsand the FTIR hydrogen bonding feature, which suggested astrong intermolecular interaction between single nanotubes,the authors expected the self-assembling of SWNTols totake place. The latter was indeed demonstrated by SEM[Figure 25]. While the starting nanotubes showed a usualrandomly entangled morphology [Figure 25(a)], the SEMimages of a mashed solid sample of SWNTols revealed awell-aligned structure [Figure 25(b and c)]. As schemati-cally depicted in Figure 25(d), the strong hydrogen bondinginteraction between SWNTols is believed to be the drivingforce for the formation of highly oriented, self-assemblednanotubol arrays. The authors consider this method as asimple and versatile approach toward large-scale produc-tion of functionalized SWNTs and self-assembled SWNT

Figure 23. Proposed reaction route to SWNTC60. HSV, high-speedvibration. Reprinted with permission from Ref. [91], X. Li et al., Chem.Phys. Lett. 377, 32 (2003). 2003, Elsevier Science.

Figure 22. Typical TEM images of pristine SWNTs (upper image; scalebar, 200 nm) and SWNTC60 (lower image; scale bar, 300 nm).Reprinted with permission from Ref. [91], X. Li et al., Chem. Phys.Lett. 377, 32 (2003). 2003, Elsevier Science.

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films with ordered structures. Such nanomaterials might beattractive for many potential device applications rangingfrom nanotube electronics to sensor chips.

7. PLASMA FUNCTIONALIZATIONThis technique was already briefly mentioned in Section 2.As an alternative method to direct SWNT fluorination,Plank et al. [29] first suggested a principally new approach,by means of exposing the nanotubes to CF4 plasma. Valentiniet al. [36, 41] reported similar results on SWNT plasma fluo-rination. Felten et al. [42] used MWNTs instead of SWNTsand studied in detail the variation of diverse plasmaparameters, such as power, treatment time, pressure, andposition of the nanotube sample inside the reaction cham-ber. Depending on the plasma conditions it was possibleeither to have a functionalization of the MWNT surface ora polymerization of the monomer on this surface (see Sec-tion 2).

A wider applicability of the plasma technique was dem-onstrated in the same work by Felten et al. [42]. The use ofO2 and NH3 atmosphere instead of CF4 allowed for graft-ing oxygen- and nitrogen-containing functionalities toMWNTs. XPS analyses showed that for too high oxygenplasma power, chemical etching occurs at the surface ofMWNTs, thus destroying its structure. On the other hand,for optimal values of the plasma parameters, functionalgroups (OH, C@O, COOH, NH2, etc.) were found to bondto the nanotube surface, suggesting that both the concen-tration and type of the functional groups are in close con-nection with the plasma conditions. These results werecompared to interaction energies predicted by quantumchemical calculations for different functional groups underconsideration, showing that functionalization by O2 plasma

Figure 24. FTIR spectra of the pristine SWNTs (solid curve) and theSWNTols (dashed curve). Reprinted with permission from Ref. [92], H.Pan et al., Nano Lett. 3, 29 (2003). 2003, American Chemical Society.

Figure 25. SEM images of (a) the starting SWNT sample (scale bar, 100 nm); (b) cross-section view of the self-assembled SWNTol sample shownin (c) (scale bar, 100 nm); (c) top view of the self-assembled SWNTol sample (scale bar, 1000 nm); and (d) schematic representation of the self-assembling process. Reprinted with permission from Ref. [92], H. Pan et al., Nano Lett. 3, 29 (2003). 2003, American Chemical Society.

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produces mainly functional groups with lower interactionenergy. Although only a few milligrams of MWNT materialwas functionalized at a time during this study, scalability ofthis method to larger quantities of 2 g is anticipated.

A cold plasma approach under NH3 atmosphere was alsoused by Khare et al. [93] to functionalize SWNTs. IR evi-dence of successful functionalization was found just after 20min of discharge by the formation of peaks in expectedregions corresponding to NAH stretching and bending ofprimary and secondary amines, C@N type bonds, and C@Ctype bonds[Figure 26]. Further evidence was supplied byfluctuations of the peaks around 2900 cm1 correspondingto CAH bonding; hydrogen atoms initially bonded to thewalls of the SWNTs were being displaced in favor of thespecies produced in the plasma. After 20 min of dischargethere was a decrease in the CAH band intensities of CAH

bands, while an increase was observed after 40 min of dis-charge. The authors assumed that, at longer times, themore reactive H atoms are preferentially reacting with theSWNTs surface. NAH type stretching was observed around3343 and 3198 cm1. These bands were consistent with thecomputed values for the NAH symmetric and asymmetricstretches in nanotube-NH2, with some contribution fromthe nanotube-NH systems. IR spectroscopy revealed addi-tional features at 1670 and 1571 cm1 after the functionali-zation. The data from XPS and Raman analysis supportedthe IR band assignment. In particular, SWNTs exposed toNH3 and ND3 discharges displayed RBM frequency shiftsfrom that of the pristine nanotubes [Figure 27]. In bothcases, the percentage of smaller diameter nanotubes (0.9 nmfor example) decreased since the nanotubes were func-tionalized. The tangential modes (TMs) between 1100 and

Figure 26. FTIR spectra of SWNTs after exposure to NH3 microwave plasma: (a) SWNT on CaF2 substrate after heating; (b) after 20 min ofplasma exposure; (c) after 40 min of exposure. Reprinted with permission from Ref. [93], B. N. Khare et al., J. Phys. Chem. B 108, 8166 (2004). 2004, American Chemical Society.

Figure 27. Raman spectra showing that the high-frequency RBM (267, 241, 232, 208, and 172 cm1 in pristine SWNTs) decreases, indicating thepercentage of smaller diameters decreases as likely the functionalized nanotubes. The frequency of the G-band (1594 cm1 in pristine SWNT) inthe tangential mode region shifts toward the higher energy (1583 cm1 in the NH3-functionalized SWNTs), and line shape is broadened, correlatingwith the SWNT sidewall functionalization. Reprinted with permission from Ref. [93], B. N. Khare et al., J. Phys. Chem. B 108, 8166 (2004). 2004,American Chemical Society.

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1800 cm1 displayed the following behavior for both NH3and ND3 precursors. The frequency of the G-band(1594 cm1 in pristine SWNTs) shifted toward the lowerenergy (1583 cm1 in the NH3-functionalized SWNTs), andthe line shape was broadened. In the case of ND3 discharge,additional TM features were seen, possibly due to the multi-ple interactions from different NDx species, or differentreaction sites. Another interesting Raman observation in theTM region was that the intensity of sp3 carbon increased inthe ND3 functionalized SWNTs, while this intensity in thecase of NH3-functionalized SWNTs decreased. As sidewalldefect sites, sp3 carbon often acts as an active site for func-tionalization. The observed sp3 intensity charge is consistentwith the decrease in D-band intensity for SWNTs exposed toNH3 discharge from that of pristine SWNTs, whereas theD-band intensity increased for ND3-exposed SWNTs. Theobserved shift of the band at 1553 to 1563 cm1 for NH3-exposed nanotubes also suggested functionalization; how-ever, a similar change did not occur with ND3. The increasein the relative intensity ratio of sp3 to sp2 carbon from ND3-functionalized SWNTs compared to pristine SWNTs sug-gested ND and ND2 coexist on the SWNT sidewalls.

Later, the same research group exposed SWNTs tomicrowave-generated N2 plasma [94]. The results stronglydepended on the distance between the discharge sourceand the nanotube sample, since nitrogen atoms generatedcan be lost due to recombination. No functionalization wasobserved when this distance was 7.0 cm. At intermediatedistances (2.5 cm), the incorporation of nitrogen and oxy-gen (from impurities) onto the SWNT was observed, whileat short distances (1 cm), products containing CBN groupswere also observed. As a whole, the nature of chemicalgroups generated was very much similar to those identified(e.g., by IR spectroscopy) in the previous study with NH3glow discharges [93].

The method of MWNT chemical modification proposedby Chen et al. [95] involved radio frequency glow-dischargeplasma activation with acetaldehyde or ethylenediaminevapor. Acetaldehyde plasma polymerization was carried outat 200 kHz, 20 W, and a monomer pressure of 0.3 Torr for5 min (the optimized conditions maximizing bonded alde-hyde functionalities), whereas ethylenediamine vapor wasplasma-polymerized at 0.3 Torr, 200 kHz, and 20 W for 1min. Figure 28(a) shows a typical SEM micrograph for thealigned MWNTs as-synthesized on a Si wafer or thosetransferred onto Scotch tape from the quartz surface. Byplasma polymerization (e.g., acetaldehyde), a concentriclayer of (acetaldehyde) plasma polymer film was homoge-neously deposited onto each of the constituent alignednanotubes. The SEM image for the plasma-polymer-coatednanotubes [Figure 28(b)] shows the similar features as thealigned nanotube array of Figure 28(a), but with a largertubular diameter and smaller inter-tube distance due to thepresence of the plasma coating. TEM images of the constit-uent nanotubes before and after the plasma treatment aregiven in the insets of Figure 28, parts a and b, respectively.Comparing the insets clearly shows the presence of a homo-geneous coating along the MWNT sidewall. The coatingthickness was determined from the TEM image shown inthe inset of Figure 28(b) to be 2030 nm for this particularsample, but it can be varied in a controllable fashion by

changing the plasma polymerization conditions (e.g., treat-ment time). The plasma activation was followed by chemicalreactions characteristic of the plasma-generated functionalgroups. For instance, amino-dextran chains were immobi-lized onto acetaldehyde-plasma-treated aligned MWNTsthrough the formation of Schiff-base linkages, which werefurther stabilized by reduction with sodium cyanoborohy-dride. Using the same reaction, the authors also chemicallygrafted periodate-oxidized dextran chains pre-labeled withfluorescein onto ethylenediamine-plasma-treated MWNTs.The fluorescein labeling allows the surface immobilizationreaction to be followed simply by photoluminescence meas-urements. The resulting polysaccharide-grafted carbonnanotubes were very hydrophilic, as demonstrated by XPSand air/water contact angle measurements. Owing to thehighly generic nature characteristic of the plasma treatment/polymerizatio

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