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Microwave-induced rapid chemical functionalization of single-walled carbon nanotubes
Carbon 43 (2005) 1015–1020 www.elsevier.com/locate/carbon
Microwave-induced rapid chemical functionalization of single-walled carbon nanotubes Yubing Wang, Zafar Iqbal, Somenath Mitra
*
Department of Chemistry and Environmental Science, New Jersey Institute of Technology, 138 Warren Street, Newark, NJ 07102, USA Received 9 September 2004; accepted 21 November 2004 Available online 21 January 2005
Abstract The microwave-induced chemical functionalization of single-walled carbon nanotubes (SWNTs) is reported. The major advantage of this high-energy procedure is that it reduced the reaction time to the order of minutes and the number of steps in the reaction procedure compared to that of conventional functionalization processes. Two successful model reactions, namely amidation and 1,3-dipolar cycloaddition of SWNTs were carried out. The amidation was completed in two steps as compared to three in the conventional approach. The step involving acid chloride formation was eliminated here, and the yield remained the same. The 1,3-dipolar cycloaddition of SWNTs was carried out in 15 min under microwave conditions, and the results were similar to what was achieved in 5 days using conventional methods. This finding opens the door to fast and inexpensive processing to produce functional SWNTs, which is extremely important for their use in real-world applications. Ó 2004 Elsevier Ltd. All rights reserved. Keyword: Carbon nanotubes
1. Introduction There has been intense interest in carbon nanotubes since their discovery [1] in 1991, in large part because of their unique structural and electronic properties. SWNTs are the fundamental form of these nanoscale tubes, with unique characteristics arising from their one-dimensional structure. The SWNTs usually aggregate into bundles (or ropes of tubes) that are held together by weak van der Waals forces. These bundles can contain up to several hundred tubes arranged in a hexagonal lattice [2]. SWNT sidewalls are largely defect-free and therefore rather inert to chemical attack. Limited reactivity occurs at defects on the sidewalls generated by curvature-induced stresses due to non-planar
*
Corresponding author. Tel.: +1 973 596 5611; fax: +1 973 596 3586. E-mail address: [email protected] (S. Mitra). 0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.11.036
sp2 carbons and the misaligned orbitals, and at dangling bonds located at the tube ends. Chemical functionalization of SWNTs is critical for applications of nanotubes. Firstly, the insoluble SWNTs are rendered soluble, resulting in easy and efficient chemical processibility. Functionalization can also lead to separation of tubes of differing chirality, and metallic SWNTs from the semiconducting ones. More importantly, functionalization leads to new classes of SWNT-based materials with specific physical and chemical properties. Extensive studies on the derivatization of the SWNTs [3–19] have therefore been carried out. Much of the effort so far has involved the use of conventional chemical techniques, such as refluxing and sonication. SWNTs are usually treated in different solvents by refluxing, or heating and stirring. Many of these reactions need to be carried out over long periods of time. For example, for carboxylation, it is often refluxed in conc. HNO3 for 10–50 h. Further functionalization, such as acyl chlorination and amidation
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Y. Wang et al. / Carbon 43 (2005) 1015–1020
Fig. 1. Model microwave induced reaction as: (a) amidation of SWNTs, (b) 1,3-dipolar cycloaddition of SWNTs.
which was lined with Teflon PFAÒ and fitted with a 0–200 psi pressure controller. The fourier transform infrared (FTIR) measurements were made either in highly purified KBr pellets (solid sample) or on NaCl crystal window (liquid sample) by using a Perkin–Elmer instrument. The Raman measurements were carried out using Horiba/Jobin Yvon LabRaman system with 632.8 nm excitation. The UV–vis-NIR spectra were obtained from Hewlett Packard, Model 8453 UV–Visible spectrophotometer. Proton nuclear magnetic resonance (NMR) data were acquired with a Varian INOVA 500 MHz NMR Spectrometer. And the scanning electron microscope (SEM) images were taken using a LEO 1530 instrument. All other chemicals were purchased from Sigmaaldrich. 2.1. Amidation reaction
[10–12], diimide-activated amidation [13] and 1,3-dipolar cycloaddition [3,4] require additional days of reaction time. Functionalization of carbon nanotubes by conventional methods is therefore a tedious and time consuming process. Consequently, there is an urgent need to develop techniques for rapid chemical functionalization of SWNTs. Chemistry under microwave radiation is known to be somewhat different, faster and more efficient [20–22] than under conventional chemical processing conditions. Microwave processing also reduces the need for solvents, thus it is eco-friendly. It has been exploited in a variety of organic syntheses involving heterocyclic [23] and organometallic [24] compounds, and in combinatorial chemistry [25,26]. Some of the reported advantages are high yields, and rapid reaction under controlled temperature and pressure (especially in a closed system), and high purity products due to the short residence times involved. Additionally, the chemical activation parameters are modified due to further polarization of the dipoles under microwave radiation. For example, it has been reported by Lewis [27] that during imidization of polyamic acid, the activation energy is reduced from 105 to 57 kJ/mol. Here we demonstrate the microwave-induced functionalization of SWNTs by two model reactions. The first reaction involves carboxylation (generation of –COOH groups) of SWNTs followed by amidation, as shown in Fig. 1(a) below. The second reaction involves 1,3-dipolar cycloaddition reaction of SWNTs together with an a-amino acid and an aldehyde, as shown in Fig. 1(b) above.
2. Experimental The experiments were carried out in a CEM Model 205 microwave oven with a 100 ml reaction chamber,
For the amidation reaction (Fig. 1(a)), the first step was the generation of carboxylic acid groups on the SWNT sidewalls and tube ends. In a typical reaction, 6–10 mg of pristine SWNTs (prepared by the HiPCO process and obtained from Carbon Nanotechnologies Inc [28]) were loaded into an extraction vessel, along with 20 ml of 70% HNO3. The microwave power was set to 75% of a total of 900 W, the pressure was set at 125 psi, and the reaction was carried out for 10– 15 min. After cooling to room temperature, the reacted mixture was filtered, washed and dried. About 5 mg of this carboxylic acid grafted SWNT sample was used to react with 2,6-dinitroaniline. For the amidation reaction, 20 ml of N,N-dimethylformamide (DMF) was used as the solvent, 15–20 mg of 2,6-dinitroaniline was added, and all other conditions remained the same as before. The reaction was carried out for 15–20 min. Once cooled, the mixture was filtered, washed with DMF and finally with anhydrous tetrahydrofuran (THF). After vacuum drying at room temperature for a few hours, the sample in powder form was analyzed by FTIR and Raman spectroscopy. 2.2. 1,3-Dipolar cycloaddition For the 1,3-dipolar cycloaddition reaction, about 10 mg of pristine SWNTs and 70 mg of salicylaldehyde were suspended in 20 ml DMF. Then, the mixture was reacted in the microwave reactor for 5 min at 90% microwave power, and at a pressure of 160 psi. After cooling, 2 ml of methionine suspension (70 mg in 4 ml DMF) was added to the reaction vessel. The reaction was then carried out for 5 min at the same power and pressure. Again the vessel was cooled, the other half of the methionine suspension was added, and the reaction was carried out for another 5 min under the same conditions. The reacted mixture was filtered, and the organic phase was vacuum-evaporated. The resulting dark
Y. Wang et al. / Carbon 43 (2005) 1015–1020
3. Results and discussion
(a)
Intensity (a.u.)
brown oil was extracted with a two-phase mixture of CHCl3/H2O. The organic phase was washed with water and dried over Na2SO4. It was evaporated, washed with ethyl ether, and yielded about 7 mg of dark brown solid. The solid was evaluated using FTIR, proton NMR, UV–vis-NIR spectroscopy and SEM.
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(b)
(c)
3.1. Microwave-induced amidation of SWNTs The conventional approach to amidation for SWNTs involves carboxylation, acyl chlorination, and amidation [10–12]. It involves three steps and typical reaction time is between 3 and 5 days. The amidation of SWNTs in the microwave was a two-step process (as shown in Fig. 1), and the total reaction time was between 20 and 30 min. SWNTs functionalized through amidation were characterized by FTIR and Raman spectroscopy. Fig. 2(a)–(c) shows the FTIR spectra of pristine SWNT, HNO3 treated SWNT and 2,6-dinitroaniline functionalized SWNT, respectively. In Fig. 2(a), the peak at 1580 cm 1 is assigned to the C@C stretching mode associated with SWNT sidewall defects. The line at 1626 cm 1 seen in all the spectra shown in Fig. 2 is assigned to traces of water in the KBr used for making the pellet. The line at 1730 cm 1 is clearly assigned to the C@O stretching mode in the HNO3 treated SWNT and indicate successful generation of –COOH groups on the nanotubes (Fig. 2(b)). The sharp peak at 1384 cm 1 is likely to be associated with the nitro group formed during the high pressure HNO3 treatment under microwave conditions. After the reaction with 2,6-dinitroaniline, the amide linkage formed at the C@O bond site, is associated with the line observed at 1650 cm 1
Transmittance (a.u.)
(a)
1626 1580
(b)
1730 1626
1384
(c) 1730 1650
3000
2500
2000
1500
1000
500
Wavenumbers (cm-1) Fig. 2. FTIR spectra from the amidation reaction of SWNTs: (a) pristine SWNTs, (b) microwave induced HNO3 treated SWNTs; (c) 2,6-dinitroaniline funtionalized SWNTs.
200
400
600
800 1000 1200 1400 1600 1800
Raman shifts (cm-1)
Fig. 3. Raman spectra from the amidation of SWNTs: (a) pristine SWNTs, (b) nitric acid treated SWNTs, (c) 2,6-dinitroaniline functionalized SWNTs. The Raman spectra were measured using identical parameters except for a factor of 2 lower spectral averaging for spectrum (c).
(Fig. 2(c)). The observation of the line at 1730 cm 1 in Fig. 2(c) indicates incomplete reaction with –COOH due to the steric effect of the rigid ring of the attached amine [11]. The Raman spectrum of the functionalized SWNTs (Fig. 3(c)) showed significant fluorescence background relative to that observed in the spectrum of the pristine and nitric acid treated SWNTs (Fig. 3(a) and (b)) due to the tethering of the photoluminescent amine on the SWNT structure. Similar observations have been reported by several other groups [13,29,30]. The increase in intensity of the defect mode at 1330 cm 1 was attributed to increased sp3-hybridization induced disorder on the nanotube framework [9,31,32]. 3.2. Microwave-induced dipolar cycloaddition of SWNTs 1,3-dipolar cycloaddition of SWNTs is a tedious and time-consuming process when carried out by conventional methods. Literature data showed that it took as long as five days to complete this reaction [3]. As mentioned in Section 2, the microwave-induced reaction time was only between 20 and 30 min. The final product comprising of 1,3-dipolar cycloaddition functionalized SWNTs was highly soluble in CHCl3, CH2Cl2 and dimethylformamide (DMF). The FTIR spectrum of the functionalized product along with that of L -methionine and salicylaldehyde, is shown in Fig. 4. The functionalized SWNTs (Fig. 4(c)) clearly showed the absence of the aldehyde C–H stretching peaks at 2749 cm 1 and 2845 cm 1, which were present in the original aldehyde (Fig. 4(b)). As shown in Fig. 1(b), the aldehyde group is expected to be removed by the functionalization process. Therefore, the absence of the aliphatic C–H stretching peaks in Fig. 4(c) demonstrated that the reaction had taken place successfully. The
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Transmittance (a.u.)
(a) 2911
(b) 2749 3176
3060
2845
(c) 3052
2849
2917
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm-1) Fig. 4. FTIR spectra from the 1,3-dipolar cycloaddition of SWNTs: (a) L -methionine, (b) salicylaldehyde, (c) final product from the microwave induced 1,3-dipolar cycloaddition of SWNTs.
aromatic C–H stretching line in Fig. 4(c) at 3052 cm 1 was slightly downshifted relative to its position at 3060 cm 1 in the spectrum for the aldehyde shown in Fig. 4(b). The peaks at 2917 cm 1 and 2849 cm 1 in Fig. 4(c) are from the attached amino acid. Note that these peaks are absent in the FTIR spectrum of pristine SWNTs shown in Fig. 2(a). This also demonstrated the functionalization of SWNTs. The proton-NMR measurements were made on the 1,3-dipolar cycloaddition product, and the spectrum was consistent with the proposed structure shown in Fig. 1(b). The measurements were carried out in CDCl3. The aromatic-H from salicylaldehyde still existed, but the aldehyde-H (CHO) disappeared from the chemical shift at about 9.9 ppm and a new peak appeared at 1.9 ppm. Likewise, the peak for proton alpha to the COOH group in methionine slightly shifted upward from 3.84 ppm to 3.80 ppm in the product. These observations are consistent with the FTIR measurements and indicate that the reaction did occur. SEM images of pristine SWNTs and their 1,3-dipolar cycloaddition functionalized products are presented in Fig. 5. As shown in Fig. 5(a), the pristine SWNTs exist as bundles. Fig. 5(b) shows a typical bundle of 20–30 nm diameter. Fig. 5(c) and (d) show the SWNTs after functionalization, they were obtained by evaporating a drop of the chloroform solution of the functionalized product. Although the cylindrical shape of the SWNTs is still intact, the attachment of the functional groups roughened the tube surface, and the diameter increased 10fold due to the molecules tethered on the sidewalls. The presence of only two SWNT bundles in the low magnification image in Fig. 5(d) demonstrates that the functionalized SWNTs became untangled and dispersed when dissolved in the solvent.
Fig. 5. SEM images of (a) pristine SWNT bundles, (b) single bundle of pristine SWNTs, (c) SWNT bundles dispersed in chloroform after functionalization, (d) single SWNT bundle after functionalization. Note increase in diameter. Images (a) and (b) were taken with the sample in the powder form, whereas (c) and (d) were obtained after evaporating a drop of a solution of the nanotubes in chloroform on a glass substrate.
UV–vis-NIR absorption spectra provided further evidence of the functionalization of the SWNTs. The spectra in the UV region is not presented here for brevity. These measurements were made in CHCl3. Spectra of the starting material taken in the same ratio as the reaction showed two broad absorption bands at about 250 and 330 nm. After the reaction, the bands remained, but shifted by 5–10 nm to lower wavelengths. This was in agreement with the observation reported in a previous study [3], the shift indicated a change in the electronic states of the starting material resulting from the interaction with SWNTs [33]. Measurements in the Visible-NIR region are shown in Fig. 6, which shows the solution phase absorption spectrum of both pristine SWNTs, and the ones after 1,3-dipolar cycloaddition. In case of the pristine SWNTs (Fig. 6(a)), the van Hove transitions features are clearly seen around 740, 820, and 890 nm. These have been
Fig. 6. UV–vis-NIR spectra of (a) pristine HiPco SWNTs suspended in DMF, (b) final product from the microwave induced 1,3-dipolar cycloaddtion of SWNTs in DMF.
Y. Wang et al. / Carbon 43 (2005) 1015–1020
attributed to the second bang gap transitions in semiconducting nanotubes [34,35]. In case the functionalized SWNTs, the van Hove absorption bands were not seen (Fig. 6(b)), indicating a dramatic change in the electronic structure. This observation is consistent with previous reports [34,35].
4. Conclusion In this paper we present microwave-induced rapid functionalization of SWNTs, where the reaction time is reduced to the order of minutes. Two model reactions, namely amidation and 1,3-dipolar cycloaddition of SWNTs were carried out successfully in relatively short times using a microwave reactor. The amidation was completed in two steps as compared to three in the conventional approach [10–12]. The 1,3-dipolar cycloaddition of SWNTs was carried out in 15 min under microwave conditions. Functionalization was demonstrated by their SEM images and analyses by UV, FTIR, UV–vis-NIR, Raman and NMR spectroscopy. Moreover, the observed solubility of the functionalized SWNTs in different organic solvents also demonstrated the successful functionalization of the pristine SWNTs in a microwave reactor.
Acknowledgments This research was partially funded by a STAR grant (Contract No: RD 830901) from the US Environmental Protection Agency (EPA). Dr. Ambarish Singh is acknowledged for his valuable suggestions.
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[8] Cai L, Bahr JL, Yao Y, Tour JM. Ozonation of single-walled carbon nanotubes and their assemblies on rigid self-assembled monolayers. Chem Mater 2002;14(10):4235–41. [9] Pan H, Liu L, Guo ZX, Dai L, Zhang F, Zhu D, et al. Carbon nanotubols from mechanochemical reaction. Nano Lett 2003;3(1):29–32. [10] Pompeo F, Resasco DE. Water solubilization of single-walled carbon nanotubes by functionalization with glucosamine. Nano Lett 2002;2(4):369–73. [11] Feng L, Li H, Li F, Shi Z, Gu Z. Functionalization of carbon nanotubes with amphiphilic molecules and their Langmuir– Blodgett films. Carbon 2003;41(12):2385–91. [12] Peng H, Alemany LB, Margrave JL, Khabashesku VN. Sidewall carboxylic acid functionalization of single-walled carbon nanotubes. J Am Chem Soc 2003;125:15174–82. [13] Huang W, Taylor S, Fu K, Lin Y, Zhang D, Hanks TW, et al. Attaching proteins to carbon nanotubes via diimide-activated amidation. Nano Lett 2002;2(4):311–4. [14] Matsuura K, Hayashi K, Kimizuka N. Lectin-mediated supramolecular junctions of galactose-derivatized single-walled carbon nanotubes. Chem Lett 2003;32(3):212–3. [15] Georgakilas V, Gournis D, Karakassides MA, Bakandritsos A, Petridis D. Organic derivatization of single-walled carbon nanotubes by clays and intercalated derivatives. Carbon 2004;42(4): 865–70. [16] Lin Y, Allard LF, Sun YP. Protein-affinity of single-walled carbon nanotubes in water. J Phys Chem B 2004;108(17):3760–4. [17] Sen R, Zhao B, Perea D, Itkis ME, Hu H, Love J, et al. Preparation of single-walled carbon nanotube reinforced polystyrene and polyurethane nanofibers and membranes by electrospinning. Nano Lett 2004;4(3):459–64. [18] Qin S, Qin D, Ford WT, Resasco DE, Herrera JE. Functionalization of single-walled carbon nanotubes with polystyrene via grafting to and grafting from methods. Macromolecules 2004; 37(3):752–7. [19] Lim JK, Yun WS, Yoon M, Lee SK, Kim CH, Kim K, et al. Selective thiolation of single-walled carbon nanotubes. Synthetic Metals 2003;139:521–7. [20] Bensebaa F, Zavaliche F, LÕEcuyer P, Cochrane RW, Veres T. Microwave synthesis and characterization of Co-ferrite nanoparticles. J Colloid Interface Sci 2004;277(1):104–10. [21] Zhou G, Pol VG, Palchik O, Kerner R, Sominski E, Koltypin Y, et al. A fast synthesis for zintl phase compounds of Na3SbTe3, NaSbTe2 and K3SbTe3 by microwave irradiation. J Solid State Chem 2004;177(1):361–5. [22] Loupy A. Solvent-free microwave organic synthesis as an efficient procedure for green chemistry. C R Chimie 2004;7(2):103– 12. [23] Shaabani A, Bazgir A. Microwave-assisted efficient synthesis of spiro-fused heterocycles under solvent-free conditions. Tetrahedron Lett 2004;45(12):2575–7. [24] Ardon M, Hogarth G, Oscroft DTW. Organometallic chemistry in a conventional microwave oven: the facile synthesis of group 6 carbonyl complexes. J Organomet Chem 2004;689:2429–35. [25] Lew A, Krutzik PE, Hart ME, Chamberlin AR. Increasing rates of reaction: microwave-assisted organic synthesis for combinatorial chemistry. J Comb Chem 2002;4(2):95–105. [26] Strohmeier GA, Kappe CO. Rapid parallel synthesis of polymerbound enones utilizing microwave-assisted solid-phase chemistry. J Comb Chem 2002;4(2):154–61. [27] Lewis DA, Summers JD, Ward TC, McGrath JE. Accelerated imidization reactions using microwave radiation. J Polym Sci A 1992;30:1647–53. [28] Nikolaev P, Bronikowski MJ, Bradley RK, Rohmund F, Colbert DT, Smith KA, et al. Gas-phase catalytic growth of single-walled carbon nanotubes from carbon monoxide. Chem Phys Lett 1999;313:91–7.
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[29] Lin Y, Rao AM, Sadanadan B, Kenik EA, Sun YP. Functionalizing multiple-walled carbon nanotubes with aminopolymers. J Phys Chem B 2002;106(6):1294–8. [30] Sun YP, Fu K, Lin Y, Huang W. Functionalized carbon nanotubes: properties and applications. Acc Chem Res 2002;35(12):1096–104. [31] Bahr JL, Yang J, Kosynkin DV, Bronikowski MJ, Smalley RE, Tour JM. Functionalization of carbon nanotubes by electrochemical reduction of aryl diazonium salts: A bucky paper electrode. J Am Chem Soc 2001;123:6536–42. [32] Mitchell CA, Bahr JL, Arepalli S, Tour JM, Krishnamoorti R. Dispersion of functionalized carbon nanotubes in polystyrene. Macromolecules 2002;35(23):8825–30.
[33] Zhu W, Minami N, Kazaoui S, Kim Y. p-Chromophorefunctionalized SWNTs by covalent bonding: substantial change in the optical spectra proving strong electronic interaction. J Mater Chem 2004;14(13):1924–6. [34] Zhang L, Kiny VU, Peng H, Zhu J, Lobo RFM, Margrave JL, Khabashesku VN. Sidewall functionalization of single-walled carbon nanotubes with hydroxyl group-terminated moieties. Chem Mater 2004;16(11):2055–61. [35] Qin S, Qin D, Ford WT, Resasco DE, Herrera JE. Functionalization of single-walled carbon nanotubes with polystyrene via grafting to and grafting from methods. Macromolecules 2004;37(3):752–7.
Microwave-induced rapid chemical functionalization of single-walled carbon nanotubes Yubing Wang, Zafar Iqbal, Somenath Mitra
*
Department of Chemistry and Environmental Science, New Jersey Institute of Technology, 138 Warren Street, Newark, NJ 07102, USA Received 9 September 2004; accepted 21 November 2004 Available online 21 January 2005
Abstract The microwave-induced chemical functionalization of single-walled carbon nanotubes (SWNTs) is reported. The major advantage of this high-energy procedure is that it reduced the reaction time to the order of minutes and the number of steps in the reaction procedure compared to that of conventional functionalization processes. Two successful model reactions, namely amidation and 1,3-dipolar cycloaddition of SWNTs were carried out. The amidation was completed in two steps as compared to three in the conventional approach. The step involving acid chloride formation was eliminated here, and the yield remained the same. The 1,3-dipolar cycloaddition of SWNTs was carried out in 15 min under microwave conditions, and the results were similar to what was achieved in 5 days using conventional methods. This finding opens the door to fast and inexpensive processing to produce functional SWNTs, which is extremely important for their use in real-world applications. Ó 2004 Elsevier Ltd. All rights reserved. Keyword: Carbon nanotubes
1. Introduction There has been intense interest in carbon nanotubes since their discovery [1] in 1991, in large part because of their unique structural and electronic properties. SWNTs are the fundamental form of these nanoscale tubes, with unique characteristics arising from their one-dimensional structure. The SWNTs usually aggregate into bundles (or ropes of tubes) that are held together by weak van der Waals forces. These bundles can contain up to several hundred tubes arranged in a hexagonal lattice [2]. SWNT sidewalls are largely defect-free and therefore rather inert to chemical attack. Limited reactivity occurs at defects on the sidewalls generated by curvature-induced stresses due to non-planar
*
Corresponding author. Tel.: +1 973 596 5611; fax: +1 973 596 3586. E-mail address: [email protected] (S. Mitra). 0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.11.036
sp2 carbons and the misaligned orbitals, and at dangling bonds located at the tube ends. Chemical functionalization of SWNTs is critical for applications of nanotubes. Firstly, the insoluble SWNTs are rendered soluble, resulting in easy and efficient chemical processibility. Functionalization can also lead to separation of tubes of differing chirality, and metallic SWNTs from the semiconducting ones. More importantly, functionalization leads to new classes of SWNT-based materials with specific physical and chemical properties. Extensive studies on the derivatization of the SWNTs [3–19] have therefore been carried out. Much of the effort so far has involved the use of conventional chemical techniques, such as refluxing and sonication. SWNTs are usually treated in different solvents by refluxing, or heating and stirring. Many of these reactions need to be carried out over long periods of time. For example, for carboxylation, it is often refluxed in conc. HNO3 for 10–50 h. Further functionalization, such as acyl chlorination and amidation
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Y. Wang et al. / Carbon 43 (2005) 1015–1020
Fig. 1. Model microwave induced reaction as: (a) amidation of SWNTs, (b) 1,3-dipolar cycloaddition of SWNTs.
which was lined with Teflon PFAÒ and fitted with a 0–200 psi pressure controller. The fourier transform infrared (FTIR) measurements were made either in highly purified KBr pellets (solid sample) or on NaCl crystal window (liquid sample) by using a Perkin–Elmer instrument. The Raman measurements were carried out using Horiba/Jobin Yvon LabRaman system with 632.8 nm excitation. The UV–vis-NIR spectra were obtained from Hewlett Packard, Model 8453 UV–Visible spectrophotometer. Proton nuclear magnetic resonance (NMR) data were acquired with a Varian INOVA 500 MHz NMR Spectrometer. And the scanning electron microscope (SEM) images were taken using a LEO 1530 instrument. All other chemicals were purchased from Sigmaaldrich. 2.1. Amidation reaction
[10–12], diimide-activated amidation [13] and 1,3-dipolar cycloaddition [3,4] require additional days of reaction time. Functionalization of carbon nanotubes by conventional methods is therefore a tedious and time consuming process. Consequently, there is an urgent need to develop techniques for rapid chemical functionalization of SWNTs. Chemistry under microwave radiation is known to be somewhat different, faster and more efficient [20–22] than under conventional chemical processing conditions. Microwave processing also reduces the need for solvents, thus it is eco-friendly. It has been exploited in a variety of organic syntheses involving heterocyclic [23] and organometallic [24] compounds, and in combinatorial chemistry [25,26]. Some of the reported advantages are high yields, and rapid reaction under controlled temperature and pressure (especially in a closed system), and high purity products due to the short residence times involved. Additionally, the chemical activation parameters are modified due to further polarization of the dipoles under microwave radiation. For example, it has been reported by Lewis [27] that during imidization of polyamic acid, the activation energy is reduced from 105 to 57 kJ/mol. Here we demonstrate the microwave-induced functionalization of SWNTs by two model reactions. The first reaction involves carboxylation (generation of –COOH groups) of SWNTs followed by amidation, as shown in Fig. 1(a) below. The second reaction involves 1,3-dipolar cycloaddition reaction of SWNTs together with an a-amino acid and an aldehyde, as shown in Fig. 1(b) above.
2. Experimental The experiments were carried out in a CEM Model 205 microwave oven with a 100 ml reaction chamber,
For the amidation reaction (Fig. 1(a)), the first step was the generation of carboxylic acid groups on the SWNT sidewalls and tube ends. In a typical reaction, 6–10 mg of pristine SWNTs (prepared by the HiPCO process and obtained from Carbon Nanotechnologies Inc [28]) were loaded into an extraction vessel, along with 20 ml of 70% HNO3. The microwave power was set to 75% of a total of 900 W, the pressure was set at 125 psi, and the reaction was carried out for 10– 15 min. After cooling to room temperature, the reacted mixture was filtered, washed and dried. About 5 mg of this carboxylic acid grafted SWNT sample was used to react with 2,6-dinitroaniline. For the amidation reaction, 20 ml of N,N-dimethylformamide (DMF) was used as the solvent, 15–20 mg of 2,6-dinitroaniline was added, and all other conditions remained the same as before. The reaction was carried out for 15–20 min. Once cooled, the mixture was filtered, washed with DMF and finally with anhydrous tetrahydrofuran (THF). After vacuum drying at room temperature for a few hours, the sample in powder form was analyzed by FTIR and Raman spectroscopy. 2.2. 1,3-Dipolar cycloaddition For the 1,3-dipolar cycloaddition reaction, about 10 mg of pristine SWNTs and 70 mg of salicylaldehyde were suspended in 20 ml DMF. Then, the mixture was reacted in the microwave reactor for 5 min at 90% microwave power, and at a pressure of 160 psi. After cooling, 2 ml of methionine suspension (70 mg in 4 ml DMF) was added to the reaction vessel. The reaction was then carried out for 5 min at the same power and pressure. Again the vessel was cooled, the other half of the methionine suspension was added, and the reaction was carried out for another 5 min under the same conditions. The reacted mixture was filtered, and the organic phase was vacuum-evaporated. The resulting dark
Y. Wang et al. / Carbon 43 (2005) 1015–1020
3. Results and discussion
(a)
Intensity (a.u.)
brown oil was extracted with a two-phase mixture of CHCl3/H2O. The organic phase was washed with water and dried over Na2SO4. It was evaporated, washed with ethyl ether, and yielded about 7 mg of dark brown solid. The solid was evaluated using FTIR, proton NMR, UV–vis-NIR spectroscopy and SEM.
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(b)
(c)
3.1. Microwave-induced amidation of SWNTs The conventional approach to amidation for SWNTs involves carboxylation, acyl chlorination, and amidation [10–12]. It involves three steps and typical reaction time is between 3 and 5 days. The amidation of SWNTs in the microwave was a two-step process (as shown in Fig. 1), and the total reaction time was between 20 and 30 min. SWNTs functionalized through amidation were characterized by FTIR and Raman spectroscopy. Fig. 2(a)–(c) shows the FTIR spectra of pristine SWNT, HNO3 treated SWNT and 2,6-dinitroaniline functionalized SWNT, respectively. In Fig. 2(a), the peak at 1580 cm 1 is assigned to the C@C stretching mode associated with SWNT sidewall defects. The line at 1626 cm 1 seen in all the spectra shown in Fig. 2 is assigned to traces of water in the KBr used for making the pellet. The line at 1730 cm 1 is clearly assigned to the C@O stretching mode in the HNO3 treated SWNT and indicate successful generation of –COOH groups on the nanotubes (Fig. 2(b)). The sharp peak at 1384 cm 1 is likely to be associated with the nitro group formed during the high pressure HNO3 treatment under microwave conditions. After the reaction with 2,6-dinitroaniline, the amide linkage formed at the C@O bond site, is associated with the line observed at 1650 cm 1
Transmittance (a.u.)
(a)
1626 1580
(b)
1730 1626
1384
(c) 1730 1650
3000
2500
2000
1500
1000
500
Wavenumbers (cm-1) Fig. 2. FTIR spectra from the amidation reaction of SWNTs: (a) pristine SWNTs, (b) microwave induced HNO3 treated SWNTs; (c) 2,6-dinitroaniline funtionalized SWNTs.
200
400
600
800 1000 1200 1400 1600 1800
Raman shifts (cm-1)
Fig. 3. Raman spectra from the amidation of SWNTs: (a) pristine SWNTs, (b) nitric acid treated SWNTs, (c) 2,6-dinitroaniline functionalized SWNTs. The Raman spectra were measured using identical parameters except for a factor of 2 lower spectral averaging for spectrum (c).
(Fig. 2(c)). The observation of the line at 1730 cm 1 in Fig. 2(c) indicates incomplete reaction with –COOH due to the steric effect of the rigid ring of the attached amine [11]. The Raman spectrum of the functionalized SWNTs (Fig. 3(c)) showed significant fluorescence background relative to that observed in the spectrum of the pristine and nitric acid treated SWNTs (Fig. 3(a) and (b)) due to the tethering of the photoluminescent amine on the SWNT structure. Similar observations have been reported by several other groups [13,29,30]. The increase in intensity of the defect mode at 1330 cm 1 was attributed to increased sp3-hybridization induced disorder on the nanotube framework [9,31,32]. 3.2. Microwave-induced dipolar cycloaddition of SWNTs 1,3-dipolar cycloaddition of SWNTs is a tedious and time-consuming process when carried out by conventional methods. Literature data showed that it took as long as five days to complete this reaction [3]. As mentioned in Section 2, the microwave-induced reaction time was only between 20 and 30 min. The final product comprising of 1,3-dipolar cycloaddition functionalized SWNTs was highly soluble in CHCl3, CH2Cl2 and dimethylformamide (DMF). The FTIR spectrum of the functionalized product along with that of L -methionine and salicylaldehyde, is shown in Fig. 4. The functionalized SWNTs (Fig. 4(c)) clearly showed the absence of the aldehyde C–H stretching peaks at 2749 cm 1 and 2845 cm 1, which were present in the original aldehyde (Fig. 4(b)). As shown in Fig. 1(b), the aldehyde group is expected to be removed by the functionalization process. Therefore, the absence of the aliphatic C–H stretching peaks in Fig. 4(c) demonstrated that the reaction had taken place successfully. The
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Transmittance (a.u.)
(a) 2911
(b) 2749 3176
3060
2845
(c) 3052
2849
2917
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm-1) Fig. 4. FTIR spectra from the 1,3-dipolar cycloaddition of SWNTs: (a) L -methionine, (b) salicylaldehyde, (c) final product from the microwave induced 1,3-dipolar cycloaddition of SWNTs.
aromatic C–H stretching line in Fig. 4(c) at 3052 cm 1 was slightly downshifted relative to its position at 3060 cm 1 in the spectrum for the aldehyde shown in Fig. 4(b). The peaks at 2917 cm 1 and 2849 cm 1 in Fig. 4(c) are from the attached amino acid. Note that these peaks are absent in the FTIR spectrum of pristine SWNTs shown in Fig. 2(a). This also demonstrated the functionalization of SWNTs. The proton-NMR measurements were made on the 1,3-dipolar cycloaddition product, and the spectrum was consistent with the proposed structure shown in Fig. 1(b). The measurements were carried out in CDCl3. The aromatic-H from salicylaldehyde still existed, but the aldehyde-H (CHO) disappeared from the chemical shift at about 9.9 ppm and a new peak appeared at 1.9 ppm. Likewise, the peak for proton alpha to the COOH group in methionine slightly shifted upward from 3.84 ppm to 3.80 ppm in the product. These observations are consistent with the FTIR measurements and indicate that the reaction did occur. SEM images of pristine SWNTs and their 1,3-dipolar cycloaddition functionalized products are presented in Fig. 5. As shown in Fig. 5(a), the pristine SWNTs exist as bundles. Fig. 5(b) shows a typical bundle of 20–30 nm diameter. Fig. 5(c) and (d) show the SWNTs after functionalization, they were obtained by evaporating a drop of the chloroform solution of the functionalized product. Although the cylindrical shape of the SWNTs is still intact, the attachment of the functional groups roughened the tube surface, and the diameter increased 10fold due to the molecules tethered on the sidewalls. The presence of only two SWNT bundles in the low magnification image in Fig. 5(d) demonstrates that the functionalized SWNTs became untangled and dispersed when dissolved in the solvent.
Fig. 5. SEM images of (a) pristine SWNT bundles, (b) single bundle of pristine SWNTs, (c) SWNT bundles dispersed in chloroform after functionalization, (d) single SWNT bundle after functionalization. Note increase in diameter. Images (a) and (b) were taken with the sample in the powder form, whereas (c) and (d) were obtained after evaporating a drop of a solution of the nanotubes in chloroform on a glass substrate.
UV–vis-NIR absorption spectra provided further evidence of the functionalization of the SWNTs. The spectra in the UV region is not presented here for brevity. These measurements were made in CHCl3. Spectra of the starting material taken in the same ratio as the reaction showed two broad absorption bands at about 250 and 330 nm. After the reaction, the bands remained, but shifted by 5–10 nm to lower wavelengths. This was in agreement with the observation reported in a previous study [3], the shift indicated a change in the electronic states of the starting material resulting from the interaction with SWNTs [33]. Measurements in the Visible-NIR region are shown in Fig. 6, which shows the solution phase absorption spectrum of both pristine SWNTs, and the ones after 1,3-dipolar cycloaddition. In case of the pristine SWNTs (Fig. 6(a)), the van Hove transitions features are clearly seen around 740, 820, and 890 nm. These have been
Fig. 6. UV–vis-NIR spectra of (a) pristine HiPco SWNTs suspended in DMF, (b) final product from the microwave induced 1,3-dipolar cycloaddtion of SWNTs in DMF.
Y. Wang et al. / Carbon 43 (2005) 1015–1020
attributed to the second bang gap transitions in semiconducting nanotubes [34,35]. In case the functionalized SWNTs, the van Hove absorption bands were not seen (Fig. 6(b)), indicating a dramatic change in the electronic structure. This observation is consistent with previous reports [34,35].
4. Conclusion In this paper we present microwave-induced rapid functionalization of SWNTs, where the reaction time is reduced to the order of minutes. Two model reactions, namely amidation and 1,3-dipolar cycloaddition of SWNTs were carried out successfully in relatively short times using a microwave reactor. The amidation was completed in two steps as compared to three in the conventional approach [10–12]. The 1,3-dipolar cycloaddition of SWNTs was carried out in 15 min under microwave conditions. Functionalization was demonstrated by their SEM images and analyses by UV, FTIR, UV–vis-NIR, Raman and NMR spectroscopy. Moreover, the observed solubility of the functionalized SWNTs in different organic solvents also demonstrated the successful functionalization of the pristine SWNTs in a microwave reactor.
Acknowledgments This research was partially funded by a STAR grant (Contract No: RD 830901) from the US Environmental Protection Agency (EPA). Dr. Ambarish Singh is acknowledged for his valuable suggestions.
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