aminesMWNTTaras

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Copyright 2005 American Scientic PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol. X, 17, 2005

Solvent-Free Derivatization of Pristine Multi-WalledCarbon Nanotubes with Amines

Elena V. Basiuk,1 Taras Y. Gromovoy,2 Andriy Datsyuk,2 Boris B. Palyanytsya,2

Valeriy A. Pokrovskiy,2 and Vladimir A. Basiuk31Centro de Ciencias Aplicadas y Desarrollo Tecnolgico, Universidad Nacional Autnoma de Mxico,

Circuito Exterior C.U., 04510 Mxico D.F., Mexico2Institute of Surface Chemistry, National Academy of Sciences of the Ukraine, Gen. Naumova 17, 03164 Kiev, Ukraine

3Instituto de Ciencias Nucleares, Universidad Nacional Autnoma de Mxico, Circuito Exterior C.U., 04510 Mxico D.F., Mexico

We performed direct solvent-free amination of multi-walled carbon nanotubes (MWCNTs) with nony-lamine, dodecylamine, octadecylamine, 4-phenylbutylamine and 1,8-ocanediamine at a temperatureof 150170 C and reduced pressure. Thermogravimetric analysis and temperature-programmeddesorptionmass spectrometry revealed that a major amine fraction decomposes in a tempera-ture interval of 250500 C, thus existing on multi-walled carbon nanotubes as chemically bondedspecies; a minor amine fraction was found in physisorbed form. The new derivatization techniquecombines simplicity in implementation and attractive features of green chemistry. It requires noadditional chemical activation, but thermal activation instead; it is relatively fast since it can becompleted in about 2 h; the high temperature allows one to spontaneously remove excess aminefrom the nanotube and minimize the possibility of physical adsorption; there is no need to use an(organic) solvent medium. In the case of diamines (represented in this study by 1,8-ocanediamine),the functional groups introduced can be potentially used as chemical linkers for anchoring metalcomplexes and nanoparticles to multi-walled carbon nanotubes, for adsorption and concentrationof trace metal ions.

Keywords: Carbon Nanotubes, Single-Walled, Multi-Walled, Temperature-Programmed Desorp-tionMass Spectrometry.

1. INTRODUCTION

Many potential applications of carbon nanotubes (CNTs),and especially of multi-walled carbon nanotubes (MWC-NTs), are hampered by their low solubility. Covalent func-tionalization (also called derivatization) is most frequentlyemployed to increase nanotube solubility and dispersibilityin organic (and sometimes aqueous) solvents. Accord-ing to the general classication of chemical reactionsused for this purpose (see recent reviews14 and refer-ences therein), they can be attributed to one of two largegroups. The derivatization (mainly amidation) of oxidizeddefect groups17 has to do with neither graphene sheetnor fullerene-cap chemistry, but instead with the chemistry

Author to whom correspondence should be addressed.

of oxygen-containing functionalities such as carboxylicgroups; the latter are introduced into CNT tips and side-walls by means of oxidative treatment with strong min-eral acids. However, the reactions of covalent sidewallderivatization employ the chemistry of folded graphenesheet.12811

At the same time, no general method for CNT deriva-tization is known, which would rely upon the chemistryof closed fullerene caps or pentagonal or hexagonal side-wall defects, and not of the ideal graphene sheet and car-boxylic acid chemistry. This is surprising in view of thepicture of CNTs as tubular fullerenes. There is a verysuitable chemical reaction, discovered more than a decadeago by Hirsch, Wudl and collaborators,1213 which employssuch common reagents as organic amines. Primary andsecondary amines, which are all neutral nucleophiles, are

J. Nanosci. Nanotech. 2005, Vol. X, No. xx 1533-4880/2005/X/001/007/$17.00+.25 doi:10.1166/jnn.2005.131 1

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Solvent-Free Derivatization of Pristine Multi-Walled Carbon Nanotubes with Amines Basiuk et al.

added onto fullerenic C60 at room temperature. The reac-tion is performed by dissolving C60 directly in liq-uid amines or in their solutions in organic solvents(dimethylformamide, dimethylsulfoxide, chlorobenzene,etc.).1216 Its stoichiometry depends on the sizes of theamine molecules, with the highest average amine:C60 ratio,10:1, found for 2-methylaziridine.16 A solvent-free reac-tion of silica-supported C60 with vaporous nonylamine at150 C produces a mixture of addition products as well.17

We attempted to apply the latter chemical principle fora direct amination of MWCNT closed caps by octade-cylamine (ODA).18 Thermogravimetric analysis (TGA)revealed a relatively high content of organics in the deriva-tization product, suggesting that a large octadecylaminefraction was distributed over the MWCNT sidewallsthrough chemical attachment. This was conrmed by high-resolution transmission electron microscopy (HRTEM)observations. Quantum chemical calculations at the AM1semi-empirical level showed that the presence of pyracy-lene units in the closed caps is not crucial for the amineaddition, although site-specicity of the reaction doesdepend on the mutual positions of ve-membered rings.If the caps contain pyracylene units, the addition prefer-entially takes place on their 6 6 bonds; if they do not,the preferential reaction sites are C C (5 5) bonds ofthe pentagons. While ideal nanotube sidewalls composedof solely benzene rings were found to be inert with respectto amines, the real nanotube sidewalls contain numerousreactive ve-membered rings as defects. OctadecylamineMWCNTs synthesized exhibited an enhanced dispersibil-ity/solubility in propanol.The direct amination reaction proposed by us18 is the

most direct link between carbon nanotube and fullerenechemistry, contrary to all derivatization methods designedup to now. It represents a new, third group of methodsfor CNTs chemical derivatization. Since such defects aspentagonal rings must be rather common in pristine car-bon nanotubes, our approach might become of wide use aswell. In addition, there is another aspect of primary impor-tance. The global trend of looking for green techniquesin chemistry has been manifested in CNT chemistry aswell. During the recent few years, a number of reportsappeared on the design of simple and efcient solvent-free approaches to the covalent derivatization of CNTs.Besides uorination of single-walled carbon nanotubes(SWCNTs)1921 (which is solvent-free since the nanotubetreatment is performed using elemental uorine gas), otherexamples are the derivatization of SWCNT sidewalls withdiazonium salts922 and aniline,23 mechanochemical link-ing of C60 fullerenes to SWCNTs,

24 direct amidationof oxidized SWCNTs2526 and, nally, amination of theclosed caps as well as any sidewall defects of MWCNTs18

with ODA vapor, mentioned above.We believe that our new approach might become of

wide use as well, combining simplicity in implementation

and attractive features of green chemistry. In order toexplore it further, in the present paper we varied aminereagents (nonylamine, dodecylamine, octadecylamine, 4-phenylbutylamine, and 1,8-ocanediamine) and analyzedMWCNTs derivatized by different techniques.

2. EXPERIMENTAL DETAILS

2.1. Chemicals

MWCNTs from chemical vapor deposition process (ILJINNanotech Co., Inc., Korea; 97%+purity, diameter of 1020 nm and length of 1050 m), nonylamine, dode-cylamine, octadecylamine, 4-phenylbutylamine and 1,8-ocanediamine (all of the amines coming from Aldrich)were used as received.

2.2. MWCNT Derivatization

To perform the derivatization with amines, we employedthe gas-phase solvent-free procedure designed anddescribed previously by us.1825 MWCNTs (100 mg) andamine (20 mg) were placed together into the reactor,and the reaction was performed at 150170 C for 2 h.During this procedure, amine vapors reacted with MWC-NTs. Amine excess condensed a few centimeters abovethe heated zone. Therefore, before extracting derivatizedMWCNTs, to avoid contaminating them with unreactedamines, the upper reactor was wiped with cotton wool wet-ted with ethanol. The reaction can be similarly performedby baking the MWCNT/amine mixture in a sealed vial;this is essentially a solvent-free process as well;26 how-ever, amine excess should afterwards be removed in someway, e.g., by washing or evacuating/heating.

2.3. Instruments

Fourier-transform infrared (FTIR) spectra were recorded inKBr pellets on a Nicolet 5SX FTIR spectrometer. Thermo-gravimetric analysis (TGA) was performed on a DuPontThermal Analyzer 951 (USA), with a heating ramp of10 C min1 until 1000 C, under air ow of 100 mLmin1.Temperature-programmed desorptionmass spectrom-

etry (TPDMS) measurements were performed using acustom-made system, the design and detailed explanationof which was presented in one of our earlier papers.25 Thesystem was based on an MX-7304A mass spectrometer(Sumy, Ukraine), with a mass range of 2360 Da, electronimpact energy of 70 eV, and sensitivity of 108 g. Temper-ature ramp was variable, in a range of 0.0530 C min1.The interface between the reactor and the mass spectrom-eter included a high-vacuum valve with a 5-mm-diameterorice and 20-cm-long tube kept at 150 C. The sam-ple tube was open in the ion-source direction, and underthe heating rate used the observed intensity of ion current

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Basiuk et al. Solvent-Free Derivatization of Pristine Multi-Walled Carbon Nanotubes with Amines

was expected to be proportional to the desorption rate,so that diffusion inhibition may be ignored. We assumedquasi-stationary conditions, in which the shape and posi-tion of desorption peaks do not depend on temperature ofthe spectrometer interface, sample mass, and the samplesgeometric characteristics.

2.4. Quantum Chemical Calculations

Gaussian 03W27 and GAMESS28 (version 6.4) suites ofprograms were used. All computations were performed atthe HF/STO-3G level of theory. Geometry optimizationsmet the default convergence criteria.

3. RESULTS AND DISCUSSION

Most methods of chemical derivatization of CNTs employchemical activation. In the present procedure, thermal acti-vation is used instead, similarly to the direct amidation ofoxidized SWCNT tips.25 In both cases, the procedures arerelatively fast, usually taking less than 2 h. We employeda variety of amines as derivatizing reagents; the necessarycondition is their sufcient thermal stability and volatilityunder reduced pressure. One can expect that most unre-acted amine is spontaneously removed from the productunder heating/pumping out. If this is true, there would beno major need of using (organic) solvents, which wouldbe especially attractive from an ecological point of view.We rst analyzed MWCNTs derivatized with amines by

means of infrared (IR) spectroscopy. One should note atypically poor quality of infrared spectra of MWCNTs,which is explained by inhomogeneity of the samples forIR measurements on the one hand, and by a low concen-tration and consequently negligible spectral contributionof the organic moieties on the other hand. Organic moi-eties generally manifest themselves in two regions, about1600 cm1 and 3000 cm1 (Fig. 1). The latter domain cor-responds to C H stretching vibrations. The bands herebecome more pronounced with increasing hydrocarbonradical size, so that the most intense C H absorption canbe observed in the case of octadecylamine (Fig. 1(c)). Inaddition to that, an increase in the absorption at 1640cm1 for 4-phenylbutylamine (Fig. 1(d)) is apparently dueto aromatic ring stretching vibrations, whereas for 1,8-ocanediamine (Fig. 1(e)), it can be explained by the pres-ence of the second amino group (NH2 deformation).The results of thermogravimetric analysis of the sam-

ples are shown in Figure 2. They are most illustrativefor dodecylamine-derivatized MWCNTs (Fig. 2(c)), due tobigger hydrocarbon radicals and the larger resulting contri-bution into the entire sample mass (TGA data for octade-cylamine were reported earlier18). A noticeable weight lossdue to organics decomposition begins after 200 C. It isslight and uniform until 300 C. Chattopadhyay et al.29

reported on TGA decomposition of octadecylamine phys-ically adsorbed on SWCNTs in a temperature interval of

Wavenumber (cm1)500 1000 1500 2000 2500 3000

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Fig. 1. FTIR spectra of MWCNTs: (a) pristine MWCNTs; andMWCNTs derivatized with (b) nonylamine, (c) dodecylamine, (d) 4-phenylbutylamine, and (e) 1,8-ocanediamine.

150300 C. Apparently, we observe the same pheno-menon due to a small (of a few per cent) physisorbedamine fraction. Then, after 300 C, the weight lossbecomes more pronounced, then very sharp after 500 Cuntil total decomposition of MWCNTs at almost 700 C.This interval corresponds to chemically-bonded dodecy-lamine; its content estimated from the second temperatureinterval of from 300 to 500550 C is about 10%. Otheramines do not exhibit such bright TGA features (Figs. 2(a),(b), (d), and (e)) because of smaller hydrocarbon radicals

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Temperature (C)Fig. 2. Thermogravimetric curves for MWCNTs: (a) pristine MWC-NTs; and MWCNTs derivatized with (b) nonylamine, (c) dodecylamine,(d) 4-phenylbutylamine, and (e) 1,8-ocanediamine.

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and thus smaller weight contribution into the derivatizedMWCNTs.The TGA curves correlate very well with the corre-

sponding TPDMS thermograms, summarized in Figure 3.The rst general feature found in the mass spectra ofamine-derivatized MWCNTs (Fig. 3, right column), is atypical pattern of hydrocarbon peaks. Especially commonones (found in the cases of nonyl-, dodecyl-, and octa-decyl-amines; Figs. 3(ac)) are those at m/z= 27 (C2H3),29 (C2H5), 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). Frag-mentation patterns are explained for MWCNTs deriva-tized with nonylamine and 1,8-ocanediamine. The seriesof peaks around m/z = 435771, and 85, result fromdifferent H-atom transfer processes during temperature-

I (log U) I (log U)

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Temperature (C) m/z

Fig. 3. TPDMS thermograms for selected peaks (left) and represen-tative mass spectra at 400C (where maximum evolution is observed;right) for MWCNTs derivatized with (a) nonylamine, (b) dodecylamine,(c) octadecylamine, (d) 4-phenylbutylamine, and (e) 1,8-ocanediamine.Fragmentation patterns are explained for the case of MWCNTs deriva-tized with nonylamine and 1,8-ocanediamine.

programmed desorption (TPD) cleavage and/or electronimpact fragmentation.The thermograms (i.e., plots of peak intensities vs. tem-

perature) for all of these peaks have a similar shape. Inall cases one can clearly distinguish two general evolu-tion steps. The rst one, corresponding to the removal ofphysisorbed amines from MWCNTs, begins at 50 C andceases after 200 C (Fig. 3). If we compare this intervalwith the rst weight-loss interval of TGA curves (Fig. 2),the former is obviously shifted to lower temperatures withrespect to the latter. This can be explained by high-vacuumconditions employed in the TPDMS method, when all thecompounds tend to volatilize at relatively low temperatures(the property crucial for our gas-phase solvent-free deriva-tization procedure). The second thermodesorption maxi-mum is observed between 250 and 500 C (Fig. 3). Itmatches much better the TGA interval of 300500 C(Fig. 2) due to thermal decomposition of the amine specieschemically bonded to MWCNTs, since it has nothing todo with the dependence of amine volatility on the pres-sure. Within both evolution intervals, in most cases onecan distinguish splitting of the maxima. This phenomenonapparently reects the existence of two general types ofsites on MWCNTs, where amines exist as physisorbed (theinterval of 50200 C) or chemically bonded species (theinterval of 250500 C).It is interesting to compare the TPDMS data for

amine-derivatized MWCNTs with those reported earlierfor amine-treated oxidized SWCNTs.25 In Figure 4, wereproduce TPDMS of volatile products evolved at 325 C

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Fig. 4. (a) TPD-mass spectrum of volatile products evolved at 325 Cfrom SWCNTs gas-phasetreated with nonylamine; (b) experimentalthermograms for selected hydrocarbon peaks at m/z= 42435557, and69. Reprinted with permission from,25 E. V. Basiuk et al., J. Phys. Chem.B 106, 1588 (2002). 2002, American Chemical Society.


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from SWCNTs gas-phase treated with nonylamine, alongwith experimental thermograms for selected hydrocarbonpeaks at m/z 42, 43, 55, 57, and 69. In the mass spectraof derivatized SWCNTs we detected a series of hydro-carbon peaks (similar to the one observed in the presentcase of MWCNTs) appearing at temperatures of 200 C(Fig. 4(a)), 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). TPD curves for all of thesepeaks had a similar shape with a maximum at 325 C,and total disappearance of the hydrocarbon fragments wasobserved at 400 C. For a detailed analysis of the thermalevolution of the hydrocarbon fragments we selected thepeak at m/z = 43 as one of the most abundant and illus-trative peaks, and plotted the logarithm of the desorptionrate k as a function of inverse temperature = KT 1,supposing rst, second, and third reaction orders.25 Thisdependence turned to be linear for the rst-order reac-tion. The calculated values of activation energy and pre-exponential factor (E = 796 kJ mol1 and k0 = 105 s1,respectively) were very low, suggesting the existence of anactivation-energy distribution for the destruction of nony-lamine species. An attempt to t the observed dependencewith a rectangular distribution of activation energies gaveunsatisfactory results. The reason became clear when weconsidered the shape of the thermodesorption curve (form/z= 43 as well as for other hydrocarbon peaks): it wasnoticeably asymmetric. A good agreement with the experi-mental curve was obtained when we admitted the existenceof two types of nonylamine species in SWCNTs (with acti-vation energies of E1 = 81 and E2 = 935 kJ mol1). Sincedetailed FTIR measurements proved that an overwhelm-ing amine fraction is actually physisorbed on SWCNTs,with a negligible spectral contribution of the chemicallybonded form, we concluded that the rst type is the ter-minal nonylamide derivative (corresponding to the rstthermodesorption maximum at 250 C and lower abun-dance of hydrocarbon decomposition products), and theother more abundant species is nonylamine physisorbed onSWCNTs due to strong hydrophobic interactions (from thesecond thermodesorption maximum at 325 C and higherabundance of hydrocarbon decomposition products). Theirabundance ratio estimated from the TPD maxima is 1:5.Comparison between the data for amine-derivatized

MWCNTs and SWCNTs raises a natural question: why arephysisorbed amine species released from SWCNTs afterthe chemically bonded ones, at temperatures approach-ing 300 C, whereas their desorption from MWCNTs isobserved at much lower temperatures, below 200 C?In part, this can be explained by the absence of closedcaps in oxidized SWCNTs, facilitating amine penetra-tion and accumulation inside the nanotubes. On the con-trary, MWCNTs used in the present study are pristine and

have closed caps, and amines are adsorbed on the outerwalls only. Other distinguishing characteristics of oxidizedSWCNTs are their typical narrow diameters of 11.5 nm,and the presence of voluminous carboxylic groups at theoxidized defects.To explain implications of these factors, we employed

molecular modeling. We constructed short armchair(10,10) and zigzag (16,0) SWCNT models having realis-tic diameters. Dangling bonds at their edges were lledwith the groups O, COOH, and OH, as shownin Figure 5. After optimizing the SWCNT geometries atthe HF/STO-3G theoretical level (their diameters were13.68 for (10,10) and 12.49 for (16,0) SWCNT), weplotted electrostatic potential maps30 by slicing the nano-tubes along and perpendicular to their axes through theoxygenated functionalities (Fig. 6). From Figure 5, onecan see that the effective entrance into the SWCNT cavitynarrows as a result of COOH group rotation almost per-pendicularly to the nanotube wall. Besides the steric hin-drance, an additional electronic obstacle is created due tothe highly negative electrostatic potential extending fromall of the oxygen containing functionalities, especiallyfrom COOH groups. It must restrict the penetrability ofany negatively charged molecule inside the nanotube cav-ity; this fully relates to organic amines, since NH2 groupbears partial negative charge due to its unshared electronpair. Apparently, a large number of amine molecules arestill able to enter SWCNT cavities during the initial stage

(a)

(b)

Fig. 5. Oxidized models of (a) armchair (10,10) and (b) zigzag (16,0)SWCNTs. Geometries were optimized at the HF/STO-3G level of theory.Atom colors: carbon, grey; hydrogen, white; oxygen, red.


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Fig. 6. Electrostatic potential maps for the oxidized models of (a) arm-chair (10,10) and (b) zigzag (16,0) SWCNTs (see models in Fig. 5),calculated at the HF/STO-3G level of theory, and obtained by slicing thenanotubes (upper maps) along, and (lower maps) perpendicular, to theiraxes through the oxygenated functionalities.

of the gas-phase derivatization procedure, at temperaturesbelow 100 C, temperatures high enough for amines tovolatilize, but insufcient for the formation of amides withCOOH groups. Then, after further temperature increase

to 150 C, C( O)NH R groups form and lockthe physisorbed molecules inside SWCNTs. Their releasebecomes impossible until C( O)NH R derivativesdecompose. That is why the second and not the rst TPDevolution step corresponds to physisorbed amine speciesin the case of SWCNTs.25

In conclusion, the solvent-free addition of amines is ageneral, simple, and efcient one-step method of derivati-zation of pristine MWCNTs. It has the following attractivefeatures: (a) it requires no additional chemical activation,(b) it is relatively fast since it can be completed in about2 h, (c) high temperature allows the spontaneous removalof excess amine from the nanotube and to minimize thepossibility of physical adsorption (although a minor frac-tion of physisorbed amine still can be detected by TGAand TPDMS), and (d) there is no need to use an (organic)solvent medium. In the case of diamines (represented inthis study by 1,8-ocanediamine), the functional groupsintroduced in this way can be potentially used as chemicallinkers for anchoring metal complexes and nanoparticles toMWCNTs, for adsorption and concentration of trace metalions.

Acknowledgments: Financial support from the Nat-ional Council of Science and Technology of Mexico(grants CONACYT-36317-E and -40399-Y) and from theNational Autonomous University of Mexico (grants DGA-PA-IN100402-3 and -IN100303) is greatly appreciated.

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Received: 17 January 2005. Accepted: 2 February 2005.


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