Functionalization of multi-walled carbon nanotubes

A. Gergely, J. Telegdi and E. Kálmán

Chemical Research Center of the Hungarian Academy of Sciences, P.O. Box 17, 1525 Budapest, Hungary

Email: doohan11@chemres.hu

Keywords: modification of carbon nanotubes, Diels-Alder and Sand-Meyer reaction, catalytic oxidation Abstract In order to alter the physical/chemical characteristics of multi-walled carbon nanotubes (MWNTs) we modified them by different organic reactions (Diels-Alder and Sand-Meyer reaction, oxidation) and their d properties were characterized by thermogravimetry/mass spectrometry, photoelectron spealterectroscopy, and nuclear magnetic resonance spectroscopy, as well as by dispersion. The results proved that, depending on the groups built in the MWNTs, the modified carbon nanotubes are more dispersible either in polar or apolar solvents and the suspensions are stable for long time. The presence of the substituents in the MWNTs was proved by methods listed above, e.g. high concentration of sulfur was detected when SO3H groups were inserted onto the MWNTs. The enhanced thermal stability of the modified carbon nanotubes allows their further application. Introduction In 1991 Iijima discovered a complex, elongated type of fullerene: the carbon nanotube (CNT) [1]. It can be imagined as a C70 fullerene with many thousands of carbon rings inserted across its equator, giving a tiny tube with about 1.5 nm of diameter and a length of several microns. In other words, CNTs are rolled-up versions of the original graphene sheets. Basically there are three types of carbon nanotubes: armchair, zig-zag, and chiral CNTs. The tubes may be single-walled (SWNT), double-walled (DWNT) and multiple walled (MWNT) objects. The unique structure of CNTs explains their unusual electrical and mechanical properties: CNTs are conductors or semiconductors depending on the tube structure, they have the highest known Young-modulus and yield strength, and they have other remarkable properties, too [2]. Like graphite, CNTs are also composed of sp2 carbons, meaning that each carbon is engaged in a single double (C=C) bond and two single (C-C) bonds. Therefore, like graphite and many other systems, CNTs have “benzenoid” structure. The stability of benzenoids may be approximated by determining the Kekule´ structure count (K) [3] that is the number of ways the double bonds can be placed in the network consisting of hexagons. Since all preparative methods yield in mixtures of metallic and semi-conducting nanotubes, of different diameters and chiralities, extensive research has been devoted to the modification of electronic structure and the separation of different electronic types by both physical and chemical processes [4, 5]. The latter requires suitable chemical process that is selective for metallic versus semi conducting carbon nanotubes. Understanding the interplay between electronic structure and chemical reactivity is thus critical. A few recent reports describe some degree of metallic/semi conducting selectivity toward the non-covalent adsorption of surfactants [5].

Materials Science Forum Vols. 537-538 (2007) pp 623-630Online available since 2007/Feb/15 at www.scientific.net© (2007) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.537-538.623

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The carbon nanotubes have been in the focus of research in the last decades because of their outstanding properties (significant aspect ration, chemical resistivity, very high specific surface etc.). Single, double and multi-walled nanotubes prepared by different methods are applied in a lot of cases to alter/improve the original chemical/physical/mechanical properties of the bulk materials and extend their use. The problem is that the CNTs are in bundles and that’s why they do not form proper homogeneous blends with the bulk material. In order to improve their dispersibility surface modification is necessary. The functionalization depends on the hydrophobicity/hydrophilicity of the basic components. Many efforts have been made to work out modification processes that result in a properly modified CNT [6]. Mechano-chemical modification was applied when -SH, -NH2, -CO, -Cl functional groups were introduces into the CNTs by ball-milling [7]. When the reactant NaOH was the functionalization resulted in a ‘water-soluble’ CNTs [8]. The modification usually starts at the defects and kinks of the CNTs. The oxidation of CNTs is one of the easiest and most common ways for insert ionic (carboxyl) group in the CNTs. When strong oxidizers (e.g. ozone) or other substituents alter the structure of the CNTs at high degree, - and, parallel, some original characteristics (like electronic conductivity) - is destroyed [9]. Radical modification was performed by fluoridization of CNTs [10] and in the second step all of the functionalizing groups were removed by hydrazination. Cyclopentadiene was the reactant in Diels-Alder reaction when five-membered rings modified the CNTs [11]. Radicals of electrochemically reduced diazonium salts were the reactants in other cases [12]. The CNTs are able to react with diazonium salts synthesized from aryl amines and isoamyl nitrite without solvent [13]. All these modifications alter the dispersibility of the CNTs, in most cases increase it. The aim of our work was to modify MWNTs moderately in order to be able to blend it easily with hydrophobic and hydrophilic polymers, to elaborate a simple, ‘one-pot’ functionalization of MWNTs by environmentally friend reaction partners. Experimental Materials The aligned-MWNTs-10 (A-MWNT, MWNTs quantity greater than 95% vol. pure, average diameter 10nm and length 5-15μm, amorphous carbon less than 2% (in weight), measured surface by BET method: 182 m2/g) and L.MWNTs-1030 (L-MWNTs, diameter 10-30nm and length 5-15μm, amorphous carbon quantity less than 3%, surface: 86 m2/g) were purchased from Shenzhen Nanotech Port Co., Ltd., Shenzhen Guangdong, China. Nickel, lanthane and silicate catalysts and other sort of impurities were less than 0.2% in the material. MWNTs were used without any further purification. Naphthylamine (98%), hydroquinone (98%), sodium nitrite (99%), sodium hydroxide (99%), oleum (20%), hexane, chloroform, toluene, tetrahydrofurane, dioxane, acetone, ethanol, acetonitrile, dimetyl formamide, carbon tetrachloride and ethyl acetate were obtained from Reanal (Budapest, Hungary). 1-amino-2-hydroxy-4-naphthalene sulfonic acid (98%) is the product of Chinoin (Budapest, Hungary) and acetic acid (96%) was purchased from Acidum GMK (Debrecen, Hungary). Ammonium iron(II) sulfate (98%) and iron(II) sulfate (98%), copper(I) chloride, ruthenium(III)•3H2O (99%) and sodium (meta)periodate (99%) were Fluka products. The naphthylamine was purified by sublimation. Preparation of modified MWNTs Functionalization by Diels-Alder reaction on two ways Carbon nanotube was added to the solid blend of maleic anhydride (used in high excess), and some quantity of hydroquinone. The molten reaction mixture was well stirred, heated up and kept at 130oC for 40 hours. In the other Diels-Alder modification, MWNTs were dispersed in tetrahydrofurane.

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The suspension was added to the mixture of maleic anhydride (dissolved in dioxane) and aluminium chloride (dissolved in tetrahydrofurane). The reaction mixture was stirred and refluxed on 110oC for 48 hours. For filtration of modified nanotubes, 0.2 µm hydrophilic (polyamide) and 0.22 µm hydrophobic (polytetrafluoroethylene) membranes were applied. After separation the modified nanotubes were washed thoroughly with acetone and ethyl acetate and dried in vacuum at 60oC. Functionalization of carbon nanotubes by Sand-Meyer (radical) reaction was also carried out in acetic acid. Functionalization of MWNTs with naphthylamine (SM) was also carried out by Sand-Meyer reaction with naphthylamine in acetic acid aqueous solutions. After adding nitrite the mixture was stirred. Modified nanotubes were filtrated and washed with acetic acid, acetone and ethanol. After drying, nanotubes were mixed with excess of oleum and mixture was refluxed. The suspension of sulfonated (two or threefold) naphthalene modified nanotubes was poured onto ice and neutralized by solution of sodium hydroxide. Finally, nanotubes were filtrated and dried [14]. Catalytic oxidation of nanotubes was carried out in a mixture of carbon tetrachloride, acetonitril, water and acetic acid. Sodium (meta)periodate was dissolved in the suspension of nanotubes and Ru(III) chloride was added to the suspension at room temperature. The reaction mixture was stirred for three hours. Different kinds of modifications are given in Table 1. Table 1 Summary of abbreviations of differently modified MWNTs and the used reagents for synthesis Characterization and instrumentation The dispersibility of modified CNTs was checked in nine different solvents: water (a), dimethyl formamide (b), ethanol (c), acetone (d), dioxane (e), tetrahydrofurane (f), toluene (g), chloroform (h), hexane (i). These experiments showed the change in polarity of modified CNTs.

Codes of modified carbon nanotubes Reagents Reaction conditions

Diels-Alder reaction

DA1

Maleic acid, hydroquinone, L-MWNTs

In molten state

DA2 Maleic acid, aluminium chloride, L-MWNTs

In solution of dioxane and tetrahydro-furane

Sand-Meyer reaction

SM naphthylamine, A-MWNTs, Fe(II)

In solution of acetic acid and water

Catalytic oxidation

OX Ruthenium(III) chloride, sodium (meta)periodate,

A-MWNTs

In mixture of solvents

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The TG/DTG measurements were performed by a Perkin-Elmer TGS-2 thermobalance connected to a Hiden HAL quadrupole mass spectrometer. Most of the measurements were carried out under argon gas. The mass spectrometer operates in electron impact ionization mode with 70 eV electron energy. X-ray photoelectron spectroscopy measurements were carried out using an electron spectrometer (OMICRON Nanotechnology GmbH (Germany). The photoelectrons were excited by AlKα (1486.6 eV) radiation. Spectra were recorded at normal emission in the Constant Analyser Energy mode: for survey spectra 50 eV pass energy was used, while detailed scans of the nanotube related peaks were measured at 30 eV pass energy, with resolution better than 1 eV. Spectra were evaluated by fitting with Gaussian-Lorentzian sum peaks after removing a Shirley or linear background. Binding energies were referenced to the main component of the C 1s spectrum (284.6 eV binding energy for graphite [15] or multi-walled nanotubes [16]). Results Dispersibility investigations Figure 1. Suspensions of Diels-Alder modified MWNTs: in molten state (a) and in solution (b). [water (a), dimethyl formamide (b), ethanol (c), acetone (d), dioxane (e), tetrahydrofurane (f), toluene (g), chloroform (h), hexane (i)] The modification of multiwalled carbon nanotubes by Diels-Alder reaction significantly alters the dispersability of the functionalized MWNTs. The unmodified MWNTs are dispersible in acetone and tetrahydrofurane but not in the other solvents. The functionalization in molten state gave well dispersible MWNTs (Fig. 1.a) mostly in polaric solvents such as dimethyl formamide, ethanol, acetone, dioxane and tetrahydrofurane. The MWNT modified ‘in solution’ (Fig. 1.b) was dispersible almost all cases, nearly in all solvents. The different dispersibility could be the consequence of the percentage in the modification, i.e. in solvent the contact between the reactant and the MWNTs was much better. Figure 2. Suspensions of MWNTs modified by Sand-Meyer reaction (three-sulfonated naphthalene group (a) catalytically oxidized MWNTs (b). The MWNTs modified by three-sulfonated naphthalene are dispersable almost in all solvents except chloroform and hexane (Fig. 2.a). The catalytical oxidation resulted also in well-dispersable nanotubes in water, acetone, dioxane, tetrahydrofurane and toluene (Fig. 2.b). The suspensions were stable for weeks. The change in polarity of functionalized MWNTs is reflected in the differences in dispersibility.

a) a g b c d e f h i b) a b c d e f g h i

b) a b c d e f g h i a) a b c d e f g h i

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Results of TG/MS measurements This technique is very useful to monitor the modification and to analyze the functional groups on the MWNTs. The TG curves show the extent of the modification. Figure 3. TG/DTG curves of unmodified MWNTs and the modified CNTs (a), DTG and MS curves, evolution profiles of some relevant volatile products (b) [DA1: Diels-Alder reaction in molten state; DA2: Diels-Alder reaction in solvent; SM: three-sulfonated naphthalene modification by Sand-Meyer reaction] The TG and DTG curves of modified nanotubes are shown in Figs. 3.a, b. Approximately two percent of mass decrease was measured during heating up till 900oC. Very pronounced mass change was detected measuring the DA1 sample comparison with unmodified CNTs. Significant mass change was detected when the modification was carried out in solvent. The more functional groups are in the modified MWNTs the better is the dispersibility as it was seen in the dispersibility test (Fig. 1.a, b). The cycloaddition modified nanotubes (DA1, DA2) emitted a lot of carbon dioxide with some carbon monoxide and water from carboxyl group contents (Fig. 3.a). At higher than 200oC, fragments of chemically bonded groups of CNTs could be observed. The first fragments of alkyl carboxyl modified substrate were appeared at around 300oC and the second one nearly 400oC. In the case of solution modified (DA2), there was a third peak in the spectra at about 500oC. The naphthalene three-sulfonic acid functionalized nanotubes (SM) showed very wide decomposition temperature range from 250-650oC (Fig. 3.b). The reason of this behavior is given by analysis of the mass spectra. Two main decomposition peaks were observed when heating up the sample. This temperature range is so wide probably because the modifying sulfonil groups were emitted step by step from the less stable state to the thermodynamically more stable one containing fewer electronegative functional groups. Morphological information about the modified carbon nanotubes is given in Fig. 4.

T (°C)

MWNT

• • • DA1

DA2

OX

100 200 300 400 500 600 700 8000

20

40

60

80

m(%)

0.000

0.005

0.010

0.015

0.020

0.025

-dm/dt(%/s)

T (°C)

-dm

/dt

(%/s

), in

tens

ity (

arbi

trar

y un

its)

DTG• • • H2O

CO (×0.2)

CO2

SO2

100 200 300 400 500 600 700 800100 200 300 400 500 600 700 800

(a) (b)

Materials Science Forum Vols. 537-538 627

Figure 4. Schematic presentation of the functionalization of a nanotube with two-sulfonated naphthalenes XPS investigations Although the dispersability and TG/MS measurements gave clear evidence for the successful surface modification of carbon nanotubes, samples were also studied by XPS. XPS data give important information not only on the elemental composition, but about the chemical state of the atoms participating in the photoemission process. Some results of the XPS analysis are displayed here. Figure 5. O 1s and C 1s spectra of nanotubes modified by catalytic oxidation (OX). Chemically bonded oxygen was found in the functionalized nanotubes. Since the oxygen signal of the untreated nanotubes is small (Fig. 5.a), the oxygen was clearly introduced during the functionalization process.

305 300 295 290 285 280 275

π-π* satellite

Ru 3d5/2

untreatednanotube

OX

C 1s

Inte

nsity

(arb

. uni

ts)

Binding Energy (eV)

560 555 550 545 540 535 530 525 520

from the glasssubstrate

untreated nanotube

OX

O 1s

Inte

nsity

(arb

. uni

ts)

Binding Energy (eV)

(a) (b)

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XPS results confirm the successful catalytic oxidation, as the O 1s signal associated with oxygen bound to the nanotubes significantly increased (Fig. 5.b). The binding energy of this component was found to be around 532 eV, which is characteristic of C=O, C-O-C, C-OH bonds [15, 16]. The O 1s signal at 537 eV (peak in the spectrum of the untreated nanotubes and shoulder in case of the OX sample) can be assigned to the glass substrate. The surprisingly high binding energy of this peak is due to charging effects. The high binding energy of the C 1s spectrum of the OX sample also indicates the presence of oxidated carbon species (Fig. 5.b). The ruthenium signal is due to the catalyst. Summary Some special modification experiments of MWNTs were presented. Reactions used for functionalization were Diels-Alder cycloaddition (under two different conditions) Sand-Meyer reaction (with different reactants) as well as catalytic oxidation. The functionalizations were monitored by change in the dispersibility in solvents of different polarity, by TG and XPS techniques. Results got by complementary techniques proved that the MWNTs were modified and the substituents in the carbon chain altered the chemical property of the MWNTs. Acknowledgements The authors are gratefully acknowledged to E. Jakab, E. Mészáros, Z. Pászti, I. Lukovits and L. Trif for the TG/MS, XPS investigations, their technical help and useful discussions. References [1] S. Iijima: Nature Vol. 354 (1991) pp. 56-58. [2] M.S. Dresselhaus, G. Dresselhaus: Adv. Phys. Vol. 51 (2002) pp. 1-186; R. Saito, G. Dresselhaus, M. Dresselhaus, Physical Properties of Nanotubes; Imperial College Press: London, (1998) [3] H. Sachs, P. Hansen, M. Zheng: Commun. Math. Chem. (MATCH) Vol. 33 (1996) pp. 169 241;. I. Lukovits, A. Graovac, E. Kálmán, Gy. Kaptay, P. Nagy, S. Nikolić, J. Sytchev, N. Trinajstić: J. Chem. Inf. Comput. Sci. Vol. 43 (2003) pp. 609-614 [4] R. Krupke, F. Hennrich, H. von Lohneysen, M. M. Kappes: Science Vol. 301 (2003) pp. 344 347. [5] M. Zheng, A. Jagota, E. D. Semke, B. A. Diner, R. S. Mclean, S. R. Lustig, R. E. Richardson, N. G. Tassi, Nature Mater. Vol. 2 (2003) pp. 338-342 [6] S. Iijima, T. Ichihashi, Nature Vol. 363 (1993) pp. 603-605 [7] Z. Kónya, I. Vesselényi, K. Niesz, A. Kukovecz, A. Demortier, A. Fonseca, J. Delhalle, Z. Mekhalif, J.B. Nagy, A.A. Koós, Z. Osváth, A. Kocsonya, L.P. Biró, I. Kiricsi, Chem. Phys. Lett. Vol. 360 (2002) pp. 429-435 [8] H.L. Pan, L.Q. Lin, Z.X. Geo, L.M. Dai, F.S. Zhang, D.B. Zhu, R. Czern, D.L. Conell, Nanoletters Vol. 3 (1) (2003) pp. 29-32 [9] L. Cai, J.L. Bahr, Y.X. Yao, J.M. Tour, Chemistry of Materials Vol. 14 (10) (2002) pp. 4235-4241

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[10] E.T. Mickelson, I.W. Chiang, J.L. Zimmerman, P.J. Boul, J. Lozano, J. Liu, R.E. Smalley, R.H. Hauge, J.L. Margrave, J. Phys. Chem. B Vol. 103 (21) (1999) pp. 4318-4322 [11] M.T. Beck, J. Szépvölgyi, P. Szabó, E. Jakab, Carbon Vol. 39 (1) (2001) pp. 147-149 [12] J.L. Bahr, J.P. Yang, D.V. Kosynkin, M.J. Bronikowski, R.E. Smalley, J.M. Tour, J. Am. Chem. Soc. Vol. 123 (27) (2001) pp. 6536-6542 [13] C.A. Dyke, M.P. Stewart, F. Maya, J.M. Tour, Synlett Vol. 1 (2004) pp. 0155-0160 [14] Know-how CRC-HAS, KH 2005/02/01, Patent application is in progress [15] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C. J. Powell, J.R. Rumble Jr.,

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Materials Science, Testing and Informatics III 10.4028/www.scientific.net/MSF.537-538

Functionalization of Multi-Walled Carbon Nanotubes

10.4028/www.scientific.net/MSF.537-538.623 DOI References[1] S. Iijima: Nature Vol. 354 (1991) pp. 56-58.doi:10.1038/354056a0

[2] M.S. Dresselhaus, G. Dresselhaus: Adv. Phys. Vol. 51 (2002) pp. 1-186; R. Saito, G.

Dresselhaus, M. Dresselhaus, Physical Properties of Nanotubes; Imperial College Press:

London, (1998)

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[4] R. Krupke, F. Hennrich, H. von Lohneysen, M. M. Kappes: Science Vol. 301 (2003) pp.

344 347.

doi:10.1126/science.1086534

[5] M. Zheng, A. Jagota, E. D. Semke, B. A. Diner, R. S. Mclean, S. R. Lustig, R. E.

Richardson, N. G. Tassi, Nature Mater. Vol. 2 (2003) pp. 338-342

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[6] S. Iijima, T. Ichihashi, Nature Vol. 363 (1993) pp. 603-605

doi:10.1038/363603a0

[9] L. Cai, J.L. Bahr, Y.X. Yao, J.M. Tour, Chemistry of Materials Vol. 14 (10) (2002) pp.

4235-4241

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[10] E.T. Mickelson, I.W. Chiang, J.L. Zimmerman, P.J. Boul, J. Lozano, J. Liu, R.E.

Smalley, R.H. Hauge, J.L. Margrave, J. Phys. Chem. B Vol. 103 (21) (1999) pp. 4318-4322

doi:10.1021/jp9845524

[12] J.L. Bahr, J.P. Yang, D.V. Kosynkin, M.J. Bronikowski, R.E. Smalley, J.M. Tour, J. Am.

Chem. Soc. Vol. 123 (27) (2001) pp. 6536-6542

doi:10.1021/ja010462s

[13] C.A. Dyke, M.P. Stewart, F. Maya, J.M. Tour, Synlett Vol. 1 (2004) pp. 0155-0160

doi:10.1055/s-2003-44983

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Carbon Vol. 43 (2005) pp. 153-161

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