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Study on amino-functionalized multiwalled carbon nanotubes
Materials Science and Engineering A 464 (2007) 151–156
Study on amino-functionalized multiwalled carbon nanotubes Jianfeng Shen a , Weishi Huang a , Liping Wu a , Yizhe Hu a , Mingxin Ye a,b,∗ a
b
Department of Materials Science, Fudan University, Shanghai 200433, China The Key Laboratory of Molecular Engineering of Polymer, Ministry of Education, China
Received 30 June 2006; received in revised form 25 January 2007; accepted 23 February 2007
Abstract Functionalization with amine groups of MWNTs was achieved after such steps as carboxylation, acylation and amidation. XRD, Raman, FTIR, XPS, scanning electron microscopy (SEM) were used to investigate and determine the chemical structure and texture of the amino-functionalized MWNTs. By comparing with each other, it was found that the amino-functionalized MWNTs can improve their dispersion in H2 O. However, concerning the results of XPS, SEM and dispersity analyses, other reactions may also have occurred, which influenced their dispersity in organic solvents. © 2007 Elsevier B.V. All rights reserved. Keywords: Amino-functionalized MWNTs; Dispersity; Amidation
1. Introduction Carbon nanotubes have attracted great attention because of their unique structural, electronic, mechanical and thermal properties [1–3]. The incorporation of CNTs into polymer bulk materials will surely enhance their thermal and mechanical properties. However, the realization of nanotube-reinforced resin can only be achieved by solving following main problems: one is the lack of interfacial adhesion, which is critical for load transfer in composites. Due to the atomically smooth nonreactive surface of nanotubes built of rolled graphene sheets, lack of interfacial bonding inhibits load transfer from the matrix to nanotubes across the nanotube/polymer interface. Another problem is the poor dispersion of nanotubes in the matrix, which is also significant for the fabrication of reinforced composites. Because of the fine size, high surface energy of CNTs and with intrinsic van der Waals forces, the as-received CNTs were apt to aggregate and entangle together spontaneously. These problems can be overcome by using the functionalized nanotubes which can provide multiple bonding sites to the organic/inorganic polymer matrix so that the load can be transferred to nanotubes and thus inhibit separation between the surfaces of polymer and nanotubes [4–6].
∗
Corresponding author at: Department of Materials Science, Fudan University, Shanghai 200433, China. Tel.: +86 21 55664095; fax: +86 21 65642622. E-mail address: [email protected] (M. Ye). 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.02.091
Numerous methods for chemical functionalization of carbon nanotubes, either at the tips or sidewall of CNTs, have already been reported. The chemical covalent bond mechanism is proved to be very useful to maintain the stable bonds, and significant efforts have been directed towards the establishment of chemical functionalities on the surface of CNTs. Among them, aminofunctionalized CNTs have been researched because amino group has a high reactivity, a wealth of chemistry and can react with many chemicals. In this paper, we are mainly concerned with the functionalization of MWNTs containing amine groups by modifying the MWNTs with four different amines. Amine group has a high reactivity, a wealth of chemistry and can react with many chemicals, thus can be directly incorporated into such polymers as epoxy and polyimide. The aim of the present paper is to develop a relatively simple and effective process of modifying MWNTs and study the mechanism of it. 2. Experimental The MWNTs produced by chemical vapor deposition (CVD) were obtained from Shenzhen Nanotech Port Co. Purification of them was first performed with a selective oxidation step at 530 ◦ C in air for 30 min to remove amorphous. After being oxidized with a mixture of H2 SO4 (98%)/HNO3 (68%) (3:1, v/v) at 60 ◦ C for 4 h, the carboxylated MWNTs were filtered and washed with deionized water until pH was 7 and dried in vac-
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Scheme 1. The procedures used in this study for the functionalization of MWNTs. Table 1 Exact compounds for different functionalizations Compound
R
Ethylenediamine (e-MWNT) 1,6-Hexanediamine (h-MWNT)
H2 NH2 CCH2 NH2 H2 NH2 CCH2 CH2 CH2 CH2 CH2 NH2
4,4 -Diaminodiphenylmethane (p-MWNT) 4,4 -Diamino-dicyclohexylmethane (c-MWNT)
uum oven. One hundred and fifty milligrams of carboxylated MWNTs were stirred in 30 ml of a 20:1 mixture of thionyl chloride (SOCl2 ) and dimethylformamide (DMF) at 70 ◦ C for 24 h. After the acyl chlorination, the MWNTs were centrifuged and washed with anhydrous tetrahydrofuran (THF) for five times. The remaining solid was dried under vacuum. One hundred milligrams acyl-chlorinated MWNTs were reacted with 50 ml diamine solution at 100 ◦ C for 2 days. After cooling to room temperature, the MWNTs were washed with ethanol for five times to remove excess diamine. Finally, the black solid was dried at room temperature overnight under vacuum [7]. Corresponding chemical reactions are illustrated by Scheme 1. The exact compounds containing different amine groups are listed in Table 1. Functionalization of MWNTs were confirmed by FTIR spectra recorded on NEXUS 670 spectrometer, XRD spectra recorded with D/max-␥B, XPS spectra recorded on XR5 VG (U.K.), Raman spectra recorded with a Dilor LABRAM-1B multi-channel confocal microspectrometer and thermal properties were collected with Netzsch TG 209. The investigation of the structure had been performed by scanning electron microscopy (SEM) using a Philips XL30 FEG FE-SEM at 10 kV. 3. Results and discussion
MWNTs, indicating that amino-functionalized MWNTs still had the same cylinder wall structure as raw MWNTs and interplanner spacing of all samples remained the same. It can be concluded that the modification process would not change the general structure of MWNTs [8]. Raman spectra offer useful information concerning the slightly structural changes of MWNTs, especially the changes owing to significant sidewall modification. As can be seen in Fig. 2, the characteristic peaks of MWNT tangential modes, namely the D band at 1330 cm−1 and the G band at 1580 cm−1 slightly changed. The D band is activated in the first order scattering process of sp2 carbons by the presence of inplane substitutional hetero-atoms, vacancies, grain boundary or other defects and by finite size effects, all of which lower the crystalline symmetry of the quasi-infinite lattice. Comparing with each other, e-MWNTs have the lowest frequency (1324.14 cm−1 ) because of the strongest conjugated effect of ethylenediamine. The frequency of c-MWNTs (1329.75 cm−1 ) is lower than that of h-MWNTs (1331.58 cm−1 ) because of the tension effect of cyclohexyl groups. The frequency of p-MWNTs (1327.91 cm−1 ) is lower than that of h-MWNTs, owing to the conjugated effect of phenyl groups. Comparing the frequencies of p-MWNTs and c-MWNTs, the frequency of p-MWNTs is lower because its cyclic olefinic bond weakened the stretching
Fig. 1 shows XRD patterns of raw MWNTs and aminofunctionalized MWNTs. It could be seen that XRD patterns of amino-functionalized MWNTs were similar with that of raw
Fig. 1. XRD patterns of raw MWNTs and amino-functionalized MWNTs.
Fig. 2. Raman spectra of raw MWNTs and amino-functionalized MWNTs.
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Fig. 3. FTIR spectra of: (a) MWNT-COOH; (b) e-MWNT; (c) h-MWNT; (d) p-MWNT; (e) c-MWNT.
Fig. 4. TGA curves of the raw MWNTs and amino-functionalized MWNTs in N2 , 10 ◦ C/min.
mode of D band. Since Raman scattering is strongly sensitive to the electronic structure, this result is taken as obvious evidence for the chemical functionalization [9,15]. Fig. 3 shows the FTIR spectra of carboxylated carbon nanotube (MWNT-COOH) and amino-functionalized MWNTs. The C O and C–O stretching frequencies shifted from 1705 and 1200 cm−1 (MWNT-COOH) to 1650 and 1150 cm−1 (aminofunctionalized MWNTs), respectively. Peaks at 1600 cm−1 of amino-functionalized MWNTs are due to the N–H stretching of amine groups. Peaks at 2930 and 2860 cm−1 in the spectrum (c) are greatly enhanced because of the attachment of additional methyl groups. Peaks between 950 and 700 cm−1 in the spectrum (d) are due to the stretching mode of aromatic amine groups, and peaks at 2370 cm−1 may be because of the existence of ammonium ions [10,13,14]. Thermal behavior of the nanotubes is measured by TGA (Fig. 4). TGA trace of raw MWNTs shows little weight
Fig. 5. XPS spectra of p-MWNT (a) and c-MWNT (b).
loss, which is 4% below 600 ◦ C. Whereas that of aminofunctionalized MWNTs are all more than 15%, and the onset temperatures of amino-functionalized MWNTs become dramatically lower, both of which are due to the fact that the additional organic functional groups are decomposed before the onset of MWNTs’ weight loss [11]. XPS analysis was also applied to determine the chemical species introduced by modification. Fig. 5 shows XPS spectra of p-MWNTs (a) and c-MWNTs (b). The peaks at 294.8, 397.8 and 532.2 eV are attributed to C, N and O, respectively. The content of elements on surface of MWNTs was calculated by area of each element. The atomic ratios of oxygen and nitrogen in p-MWNTs are 9.724 and 4.741%, while those in c-MWNTs are 9.312 and 8.630%, respectively. It can be seen that the atomic
Fig. 6. Optical micrographs of the samples in: (a) H2 O, (b) methanol, (c) acetone, taken 6 h after the solution had been sonicated for 1 h. The concentration is 5 mg/10 ml and the samples are raw-MWNT, MWNT-COOH, e-MWNT, h-MWNT, p-MWNT and c-MWNT from right to left.
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ratios of N/O in these MWNTs were much lower than 2, which may be because of two reasons: (1) a steric effect of the ring from the attached amine could partially shield the COCl group nearby from chemical attack. Once some of the COCl groups are attached to the non-flexible ring structure of the amines, the steric effect will be created, and will prevent further reaction. (2) Other reactions more than the reaction in Scheme 1 may have been occurred, which is in good agreement with dispersibility and SEM analyses (see below). Dispersibility of MWNTs in different solvents (shown in Table 2 and Fig. 6) was semi-qualitatively determined by mixing 5 mg of MWNTs in 10 ml of solvent, followed by sonicating with SCS1200 sonicator for 1 h. As optical inspection of dispersions only indicates the presence or the absence of particles larger than 10 m, nanotube dispersions were further characterized by a UV–vis spectrophotometer (NEXUS 670 spectrometer) to obtain a quantitative result for the samples (see Fig. 7 and Table 3). The dispersibility of MWNTs is remarkably changed after the modification. It was observed that amino-functionalized MWNTs were relatively easily dispersed in H2 O, while poorly dispersed in acetone. However, comparing with MWNTs terminated with carboxylic acid groups (MWNTCOOH), the dispersibility of amino-functionalized MWNTs were much lower in all of the three solvents. This phenomenon is very interesting and the study of the reactions of aminofuctionalization is very important since these functionalized MWNTs are believed to be very promising in the fields such as preparation of composite materials and biological technologies. The much lower dispersibility of the amino-functionalized nanotubes also indicates that the chlorine has been substituted not only by amide groups, but also that other reactions may have occurred which link the nanotubes together to make big indispersible aggregates [3]. In order to observe the morphology of the aminofunctionalized MWNTs, SEM imaging of the p-MWNTs were carried out. Image (a) was obtained from powders directly dispersed on conducting tape, whereas image (b–d) were obtained after evaporating a drop of an aqueous suspension of the sample on a gold substrate. The homogeneous suspension of p-MWNTs in water (2 mg/10 ml) without any surfactant can be formed with light ultrasonification and remains stable over days, only few flakes precipitate. From SEM microphotograph (Fig. 8a), p-MWNTs show random, curled structure after modification. Cotton-like features on the p-MWNTs are seen in image (b and d) (white dots), mainly congregating on the tip of MWNTs. MWNTs after modification also shows such defects as “ragged” on the tube tips (Fig. 8c), with buckles and irreversible bends (Fig. 8a–d), which are consequence of the chemical processing. The breakdown of the nanotube structure which occurs during the reaction (Fig. 8c) may be caused by the great number of reaction sites on the tube surface. The close proximity of the sites does not leave enough space for many of the phenyl groups to dock to the tube face and so the steric hindrance may lead to the structural breakdown of the nanotubes [12]. It can also be seen in Fig. 8d that the p-MWNTs congregate with each other to form bundles or ropes, some with size larger than 50 nm. This could be due to the intermolecular
Fig. 7. UV–vis spectra of the samples in: (a) H2 O, (b) methanol, (c) acetone at a concentration of 1 mg/10 ml taken 6 h after the solution had been sonicated for 1 h.
hydrogen bonding of possible cross linking by amino functional groups or that other reactions more than the reaction in Scheme 1 may have occurred, which link the nanotubes together. So far, based on data from the above experiments, we proposed the possible mechanism of the dispersion of func-
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Table 2 The dispersity of MWNTs in different solvents after different time Solvent
Time
Raw-MWNT
MWNT-COOH
e-MWNT
h-MWNT
p-MWNT
c-MWNT
H2 O Methanol Acetone
6h 6h 6h
− − ++
+++ +++ +++
++ + +
+ + +
+++ + +++
++ + ++
H2 O Methanol Acetone
24 h 24 h 24 h
− − ++
+++ +++ +++
++ − +
− − +
++ − ++
+ − ++
H2 O Methanol Acetone
1 week 1 week 1 week
− − ++
+++ +++ +++
+ − +
− − −
+ − +
− − +
H2 O Methanol Acetone
4 weeks 4 weeks 4 weeks
− − ++
+++ ++ +++
− − −
− − −
+ − +
− − −
(+++) Completely dispersed; (++) partially dispersed; (+) mostly precipitated; (−) completely precipitated
Table 3 The absorbance at 600 nm (A600) of the samples with a concentration of 1 mg/10 ml Solvent
Raw-MWNT
MWNT-COOH
e-MWNT
h-MWNT
p-MWNT
c-MWNT
H2 O Methanol Acetone
0.0845 0.0884 0.7261
0.9446 0.9701 1.0822
0.4489 0.2627 0.5957
0.2050 0.2407 0.5910
0.6079 0.4917 0.9915
0.4165 0.2652 0.6798
tionalized MWNTs as follows: the dispersibility of the functionalized MWNTs strongly depends on the structure and amounts of the grafted organic moieties. The soluble chains pull the relatively huge MWNT backbones into the solvent phase and help enhance the dispersibility of carbon nanotubes. The much lower dispersibility of the amino-functionalized MWNTs
comparing to MWNTs terminated with carboxylic acid groups (MWNT-COOH) indicates that other reactions more than the reaction in Scheme 1 may have occurred. For example: M–CO–NH–R–NH2 + HOOC–M → M–CO–NH–R–NH3 + + − OOC–M
Fig. 8. (a–d) SEM images of p-MWNTs.
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M–CO–NH–R–NH2 + M–COCl → M–CO–NH–R–NH–CO–M where M = MWNT. Since functionalized MWNTs have multiple organic groups terminating each sidewall functionality as well as possibly more than one moiety bonded to an open end of the nanotube, M–CO–NH–R–NH–CO–M can go on reacting with other MWNTs, thus forming larger aggregates. Since the soft segments on the amino-functionalized MWNTs such as methylene radicals and hexylene groups can enhance these reactions, while the steric hindrance of the rigid segments such as benzene rings and cyclohexyl groups may prevent them, the dispersibilities of p-MWNT and c-MWNT are much better than that of e-MWNT and h-MWNT. 4. Summary and conclusion In conclusion, four different amino-functionalized carbon nanotubes were prepared after such steps as carboxylation, acylation and amidation. The amino-functionalized carbon nanotubes offer a pathway to a wide spectrum of nanotube derivatives suitable for numerous applications, and may be promising in applications such as polymer/carbon nanotube composites and coating. However, the dispersibility of aminofunctionalized MWNTs in common solvents is not ideal, which may be because of the reason that other reactions may have occurred which link the nanotubes to form indispersible aggre-
gates. Different amino groups on surface of the MWNTs have a great effect on their dispersibility in different solvents and it opens up new perspectives of functionalization by selecting different amines to functionalize MWNTs. Controlling the organic attachment content and the dispersion of carbon nanotubes still needs to be improved, which is our future work. References [1] J. Hone, M. Whitney, A. Zettl, Synth. Met. 103 (1999) 2498. [2] X. Sun, W. Zhao, Mater. Sci. Eng. A 390 (2005) 366. [3] E. Unger, A. Graham, F. Kreupl, M. Liebau, W. Hoenlein, Curr. Appl. Phys. 2 (2002) 107. [4] L. Valentini, D. Puglia, I. Armentano, J.M. Kenny, Chem. Phys. Lett. 403 (2005) 385. [5] I. Yamaguchi, T. Yamamoto, Mater. Lett. 58 (2004) 598. [6] W. Chen, X. Tao, P. Xue, X. Cheng, Appl. Surf. Sci. 252 (2005) 1404. [7] C. Jian, M.A. Hamon, Solution Sci. 282 (1998) 5386. [8] D. Zhang, L. Shi, J. Fang, X. Li, K. Dai, Mater. Lett. 59 (2005) 4044. [9] U. Dettlaff-Weglikowska, J.-M. Benoit, P.-W. Chiu, R. Graupner, S. Lebedkin, S. Roth, Curr. Appl. Phys. 2 (2002) 497. [10] H.-C. Kuan, C.-C.M. Ma, W.-P. Chang, S.-M. Yuen, H.-H. Wu, T.-M. Lee, Compos. Sci. Technol. 65 (2005) 1703. [11] M. Liu, Y. Yang, T. Zhu, Z. Liu, Carbon 43 (2005) 1470. [12] Y. Wang, Z. Iqbal, S.V. Malhotra, Chem. Phys. Lett. 402 (2005) 96. [13] J.L. Stevens, A.Y. Huang, H. Peng, I.W. Chiang, V.N. Khabashesku, J.L. Margrave, Nano Lett. 3 (2003) 331. [14] H. Peng, L.B. Alemany, J.L. Margrave, V.N. Khabashesku, J. Am. Chem. Soc. 125 (2003) 15174. [15] W. Huang, S. Fernando, L.F. Allard, Y.-P. Sun, Nano Lett. 3 (2003) 565.
Study on amino-functionalized multiwalled carbon nanotubes Jianfeng Shen a , Weishi Huang a , Liping Wu a , Yizhe Hu a , Mingxin Ye a,b,∗ a
b
Department of Materials Science, Fudan University, Shanghai 200433, China The Key Laboratory of Molecular Engineering of Polymer, Ministry of Education, China
Received 30 June 2006; received in revised form 25 January 2007; accepted 23 February 2007
Abstract Functionalization with amine groups of MWNTs was achieved after such steps as carboxylation, acylation and amidation. XRD, Raman, FTIR, XPS, scanning electron microscopy (SEM) were used to investigate and determine the chemical structure and texture of the amino-functionalized MWNTs. By comparing with each other, it was found that the amino-functionalized MWNTs can improve their dispersion in H2 O. However, concerning the results of XPS, SEM and dispersity analyses, other reactions may also have occurred, which influenced their dispersity in organic solvents. © 2007 Elsevier B.V. All rights reserved. Keywords: Amino-functionalized MWNTs; Dispersity; Amidation
1. Introduction Carbon nanotubes have attracted great attention because of their unique structural, electronic, mechanical and thermal properties [1–3]. The incorporation of CNTs into polymer bulk materials will surely enhance their thermal and mechanical properties. However, the realization of nanotube-reinforced resin can only be achieved by solving following main problems: one is the lack of interfacial adhesion, which is critical for load transfer in composites. Due to the atomically smooth nonreactive surface of nanotubes built of rolled graphene sheets, lack of interfacial bonding inhibits load transfer from the matrix to nanotubes across the nanotube/polymer interface. Another problem is the poor dispersion of nanotubes in the matrix, which is also significant for the fabrication of reinforced composites. Because of the fine size, high surface energy of CNTs and with intrinsic van der Waals forces, the as-received CNTs were apt to aggregate and entangle together spontaneously. These problems can be overcome by using the functionalized nanotubes which can provide multiple bonding sites to the organic/inorganic polymer matrix so that the load can be transferred to nanotubes and thus inhibit separation between the surfaces of polymer and nanotubes [4–6].
∗
Corresponding author at: Department of Materials Science, Fudan University, Shanghai 200433, China. Tel.: +86 21 55664095; fax: +86 21 65642622. E-mail address: [email protected] (M. Ye). 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.02.091
Numerous methods for chemical functionalization of carbon nanotubes, either at the tips or sidewall of CNTs, have already been reported. The chemical covalent bond mechanism is proved to be very useful to maintain the stable bonds, and significant efforts have been directed towards the establishment of chemical functionalities on the surface of CNTs. Among them, aminofunctionalized CNTs have been researched because amino group has a high reactivity, a wealth of chemistry and can react with many chemicals. In this paper, we are mainly concerned with the functionalization of MWNTs containing amine groups by modifying the MWNTs with four different amines. Amine group has a high reactivity, a wealth of chemistry and can react with many chemicals, thus can be directly incorporated into such polymers as epoxy and polyimide. The aim of the present paper is to develop a relatively simple and effective process of modifying MWNTs and study the mechanism of it. 2. Experimental The MWNTs produced by chemical vapor deposition (CVD) were obtained from Shenzhen Nanotech Port Co. Purification of them was first performed with a selective oxidation step at 530 ◦ C in air for 30 min to remove amorphous. After being oxidized with a mixture of H2 SO4 (98%)/HNO3 (68%) (3:1, v/v) at 60 ◦ C for 4 h, the carboxylated MWNTs were filtered and washed with deionized water until pH was 7 and dried in vac-
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Scheme 1. The procedures used in this study for the functionalization of MWNTs. Table 1 Exact compounds for different functionalizations Compound
R
Ethylenediamine (e-MWNT) 1,6-Hexanediamine (h-MWNT)
H2 NH2 CCH2 NH2 H2 NH2 CCH2 CH2 CH2 CH2 CH2 NH2
4,4 -Diaminodiphenylmethane (p-MWNT) 4,4 -Diamino-dicyclohexylmethane (c-MWNT)
uum oven. One hundred and fifty milligrams of carboxylated MWNTs were stirred in 30 ml of a 20:1 mixture of thionyl chloride (SOCl2 ) and dimethylformamide (DMF) at 70 ◦ C for 24 h. After the acyl chlorination, the MWNTs were centrifuged and washed with anhydrous tetrahydrofuran (THF) for five times. The remaining solid was dried under vacuum. One hundred milligrams acyl-chlorinated MWNTs were reacted with 50 ml diamine solution at 100 ◦ C for 2 days. After cooling to room temperature, the MWNTs were washed with ethanol for five times to remove excess diamine. Finally, the black solid was dried at room temperature overnight under vacuum [7]. Corresponding chemical reactions are illustrated by Scheme 1. The exact compounds containing different amine groups are listed in Table 1. Functionalization of MWNTs were confirmed by FTIR spectra recorded on NEXUS 670 spectrometer, XRD spectra recorded with D/max-␥B, XPS spectra recorded on XR5 VG (U.K.), Raman spectra recorded with a Dilor LABRAM-1B multi-channel confocal microspectrometer and thermal properties were collected with Netzsch TG 209. The investigation of the structure had been performed by scanning electron microscopy (SEM) using a Philips XL30 FEG FE-SEM at 10 kV. 3. Results and discussion
MWNTs, indicating that amino-functionalized MWNTs still had the same cylinder wall structure as raw MWNTs and interplanner spacing of all samples remained the same. It can be concluded that the modification process would not change the general structure of MWNTs [8]. Raman spectra offer useful information concerning the slightly structural changes of MWNTs, especially the changes owing to significant sidewall modification. As can be seen in Fig. 2, the characteristic peaks of MWNT tangential modes, namely the D band at 1330 cm−1 and the G band at 1580 cm−1 slightly changed. The D band is activated in the first order scattering process of sp2 carbons by the presence of inplane substitutional hetero-atoms, vacancies, grain boundary or other defects and by finite size effects, all of which lower the crystalline symmetry of the quasi-infinite lattice. Comparing with each other, e-MWNTs have the lowest frequency (1324.14 cm−1 ) because of the strongest conjugated effect of ethylenediamine. The frequency of c-MWNTs (1329.75 cm−1 ) is lower than that of h-MWNTs (1331.58 cm−1 ) because of the tension effect of cyclohexyl groups. The frequency of p-MWNTs (1327.91 cm−1 ) is lower than that of h-MWNTs, owing to the conjugated effect of phenyl groups. Comparing the frequencies of p-MWNTs and c-MWNTs, the frequency of p-MWNTs is lower because its cyclic olefinic bond weakened the stretching
Fig. 1 shows XRD patterns of raw MWNTs and aminofunctionalized MWNTs. It could be seen that XRD patterns of amino-functionalized MWNTs were similar with that of raw
Fig. 1. XRD patterns of raw MWNTs and amino-functionalized MWNTs.
Fig. 2. Raman spectra of raw MWNTs and amino-functionalized MWNTs.
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Fig. 3. FTIR spectra of: (a) MWNT-COOH; (b) e-MWNT; (c) h-MWNT; (d) p-MWNT; (e) c-MWNT.
Fig. 4. TGA curves of the raw MWNTs and amino-functionalized MWNTs in N2 , 10 ◦ C/min.
mode of D band. Since Raman scattering is strongly sensitive to the electronic structure, this result is taken as obvious evidence for the chemical functionalization [9,15]. Fig. 3 shows the FTIR spectra of carboxylated carbon nanotube (MWNT-COOH) and amino-functionalized MWNTs. The C O and C–O stretching frequencies shifted from 1705 and 1200 cm−1 (MWNT-COOH) to 1650 and 1150 cm−1 (aminofunctionalized MWNTs), respectively. Peaks at 1600 cm−1 of amino-functionalized MWNTs are due to the N–H stretching of amine groups. Peaks at 2930 and 2860 cm−1 in the spectrum (c) are greatly enhanced because of the attachment of additional methyl groups. Peaks between 950 and 700 cm−1 in the spectrum (d) are due to the stretching mode of aromatic amine groups, and peaks at 2370 cm−1 may be because of the existence of ammonium ions [10,13,14]. Thermal behavior of the nanotubes is measured by TGA (Fig. 4). TGA trace of raw MWNTs shows little weight
Fig. 5. XPS spectra of p-MWNT (a) and c-MWNT (b).
loss, which is 4% below 600 ◦ C. Whereas that of aminofunctionalized MWNTs are all more than 15%, and the onset temperatures of amino-functionalized MWNTs become dramatically lower, both of which are due to the fact that the additional organic functional groups are decomposed before the onset of MWNTs’ weight loss [11]. XPS analysis was also applied to determine the chemical species introduced by modification. Fig. 5 shows XPS spectra of p-MWNTs (a) and c-MWNTs (b). The peaks at 294.8, 397.8 and 532.2 eV are attributed to C, N and O, respectively. The content of elements on surface of MWNTs was calculated by area of each element. The atomic ratios of oxygen and nitrogen in p-MWNTs are 9.724 and 4.741%, while those in c-MWNTs are 9.312 and 8.630%, respectively. It can be seen that the atomic
Fig. 6. Optical micrographs of the samples in: (a) H2 O, (b) methanol, (c) acetone, taken 6 h after the solution had been sonicated for 1 h. The concentration is 5 mg/10 ml and the samples are raw-MWNT, MWNT-COOH, e-MWNT, h-MWNT, p-MWNT and c-MWNT from right to left.
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ratios of N/O in these MWNTs were much lower than 2, which may be because of two reasons: (1) a steric effect of the ring from the attached amine could partially shield the COCl group nearby from chemical attack. Once some of the COCl groups are attached to the non-flexible ring structure of the amines, the steric effect will be created, and will prevent further reaction. (2) Other reactions more than the reaction in Scheme 1 may have been occurred, which is in good agreement with dispersibility and SEM analyses (see below). Dispersibility of MWNTs in different solvents (shown in Table 2 and Fig. 6) was semi-qualitatively determined by mixing 5 mg of MWNTs in 10 ml of solvent, followed by sonicating with SCS1200 sonicator for 1 h. As optical inspection of dispersions only indicates the presence or the absence of particles larger than 10 m, nanotube dispersions were further characterized by a UV–vis spectrophotometer (NEXUS 670 spectrometer) to obtain a quantitative result for the samples (see Fig. 7 and Table 3). The dispersibility of MWNTs is remarkably changed after the modification. It was observed that amino-functionalized MWNTs were relatively easily dispersed in H2 O, while poorly dispersed in acetone. However, comparing with MWNTs terminated with carboxylic acid groups (MWNTCOOH), the dispersibility of amino-functionalized MWNTs were much lower in all of the three solvents. This phenomenon is very interesting and the study of the reactions of aminofuctionalization is very important since these functionalized MWNTs are believed to be very promising in the fields such as preparation of composite materials and biological technologies. The much lower dispersibility of the amino-functionalized nanotubes also indicates that the chlorine has been substituted not only by amide groups, but also that other reactions may have occurred which link the nanotubes together to make big indispersible aggregates [3]. In order to observe the morphology of the aminofunctionalized MWNTs, SEM imaging of the p-MWNTs were carried out. Image (a) was obtained from powders directly dispersed on conducting tape, whereas image (b–d) were obtained after evaporating a drop of an aqueous suspension of the sample on a gold substrate. The homogeneous suspension of p-MWNTs in water (2 mg/10 ml) without any surfactant can be formed with light ultrasonification and remains stable over days, only few flakes precipitate. From SEM microphotograph (Fig. 8a), p-MWNTs show random, curled structure after modification. Cotton-like features on the p-MWNTs are seen in image (b and d) (white dots), mainly congregating on the tip of MWNTs. MWNTs after modification also shows such defects as “ragged” on the tube tips (Fig. 8c), with buckles and irreversible bends (Fig. 8a–d), which are consequence of the chemical processing. The breakdown of the nanotube structure which occurs during the reaction (Fig. 8c) may be caused by the great number of reaction sites on the tube surface. The close proximity of the sites does not leave enough space for many of the phenyl groups to dock to the tube face and so the steric hindrance may lead to the structural breakdown of the nanotubes [12]. It can also be seen in Fig. 8d that the p-MWNTs congregate with each other to form bundles or ropes, some with size larger than 50 nm. This could be due to the intermolecular
Fig. 7. UV–vis spectra of the samples in: (a) H2 O, (b) methanol, (c) acetone at a concentration of 1 mg/10 ml taken 6 h after the solution had been sonicated for 1 h.
hydrogen bonding of possible cross linking by amino functional groups or that other reactions more than the reaction in Scheme 1 may have occurred, which link the nanotubes together. So far, based on data from the above experiments, we proposed the possible mechanism of the dispersion of func-
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Table 2 The dispersity of MWNTs in different solvents after different time Solvent
Time
Raw-MWNT
MWNT-COOH
e-MWNT
h-MWNT
p-MWNT
c-MWNT
H2 O Methanol Acetone
6h 6h 6h
− − ++
+++ +++ +++
++ + +
+ + +
+++ + +++
++ + ++
H2 O Methanol Acetone
24 h 24 h 24 h
− − ++
+++ +++ +++
++ − +
− − +
++ − ++
+ − ++
H2 O Methanol Acetone
1 week 1 week 1 week
− − ++
+++ +++ +++
+ − +
− − −
+ − +
− − +
H2 O Methanol Acetone
4 weeks 4 weeks 4 weeks
− − ++
+++ ++ +++
− − −
− − −
+ − +
− − −
(+++) Completely dispersed; (++) partially dispersed; (+) mostly precipitated; (−) completely precipitated
Table 3 The absorbance at 600 nm (A600) of the samples with a concentration of 1 mg/10 ml Solvent
Raw-MWNT
MWNT-COOH
e-MWNT
h-MWNT
p-MWNT
c-MWNT
H2 O Methanol Acetone
0.0845 0.0884 0.7261
0.9446 0.9701 1.0822
0.4489 0.2627 0.5957
0.2050 0.2407 0.5910
0.6079 0.4917 0.9915
0.4165 0.2652 0.6798
tionalized MWNTs as follows: the dispersibility of the functionalized MWNTs strongly depends on the structure and amounts of the grafted organic moieties. The soluble chains pull the relatively huge MWNT backbones into the solvent phase and help enhance the dispersibility of carbon nanotubes. The much lower dispersibility of the amino-functionalized MWNTs
comparing to MWNTs terminated with carboxylic acid groups (MWNT-COOH) indicates that other reactions more than the reaction in Scheme 1 may have occurred. For example: M–CO–NH–R–NH2 + HOOC–M → M–CO–NH–R–NH3 + + − OOC–M
Fig. 8. (a–d) SEM images of p-MWNTs.
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M–CO–NH–R–NH2 + M–COCl → M–CO–NH–R–NH–CO–M where M = MWNT. Since functionalized MWNTs have multiple organic groups terminating each sidewall functionality as well as possibly more than one moiety bonded to an open end of the nanotube, M–CO–NH–R–NH–CO–M can go on reacting with other MWNTs, thus forming larger aggregates. Since the soft segments on the amino-functionalized MWNTs such as methylene radicals and hexylene groups can enhance these reactions, while the steric hindrance of the rigid segments such as benzene rings and cyclohexyl groups may prevent them, the dispersibilities of p-MWNT and c-MWNT are much better than that of e-MWNT and h-MWNT. 4. Summary and conclusion In conclusion, four different amino-functionalized carbon nanotubes were prepared after such steps as carboxylation, acylation and amidation. The amino-functionalized carbon nanotubes offer a pathway to a wide spectrum of nanotube derivatives suitable for numerous applications, and may be promising in applications such as polymer/carbon nanotube composites and coating. However, the dispersibility of aminofunctionalized MWNTs in common solvents is not ideal, which may be because of the reason that other reactions may have occurred which link the nanotubes to form indispersible aggre-
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