Sidewall Amino-Functionalization of Single-Walled Carbon Nanotubes

Jan 28, 2003 - Single-walled carbon nanotubes (SWNTs) with the N-alkylidene amino groups covalently attached to their side walls have been prepared st...
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NANO LETTERS

Sidewall Amino-Functionalization of Single-Walled Carbon Nanotubes through Fluorination and Subsequent Reactions with Terminal Diamines

2003 Vol. 3, No. 3 331-336

Joel L. Stevens, Aaron Y. Huang, Haiqing Peng, Ivana W. Chiang, Valery N. Khabashesku,* and John L. Margrave Department of Chemistry, Rice Quantum Institute and Center for Nanoscale Science and Technology, Rice UniVersity, Houston, Texas 77005-1982 Received December 16, 2002; Revised Manuscript Received January 9, 2003

ABSTRACT Single-walled carbon nanotubes (SWNTs) with the N-alkylidene amino groups covalently attached to their side walls have been prepared starting from colloidal solutions of fluorinated SWNTs (fluoronanotubes) in terminal alkylidene diamines followed by heating at 70−170 °C. On the basis of data from thermal gravimetric and energy-dispersive X-ray analyses, the degree of SWNT functionalization achieved was estimated to be as high as 1 in 8 to 12 sidewall carbons. The demonstrated new C−N functionalization method provides a synthetic tool for binding amino acids, DNA, and polymer matrices to the side walls of the SWNTs as well as yields sidewall amino-functionalized nanotube precursors for the preparation of nylon−SWNT polymer materials.

Various applications of single-walled carbon nanotubes (SWNTs) are expected to emerge from their unique structural, electronic, and mechanical properties.1-5 The remarkable tensile strength of SWNTs has made them good candidates for particular use in the fabrication of SWNTreinforced fibers and polymer composites.6 For these applications, the chemically derivatized SWNTs will be especially useful because of their potential ability to bind to polymer organic and inorganic matrices either through van der Waals interactions or hydrogen or covalent bonding. The preparation, processing, and manipulation of such nanoengineered composites and copolymers require the dispersion and solubilization of SWNTs, which in their pristine form are not soluble in most common organic solvents and water. To address this problem, current work is in progress on noncovalent surfactant6c,7 or polymer-wrapping8 modifications of SWNTs and on covalent functionalization9 utilizing open-end10 and sidewall11 chemistry. Besides a general improvement in the solubility and processibility achieved by these approaches, the sidewall functionalizations, in particular, most significantly alter the structural and electronic properties of the SWNTs, yielding new nanotube derivatives with useful properties of their own. The direct addition of fluorine,11a-c hydrogen,12 aryl groups,11d,e nitrenes, carbenes, and radicals11f,g as well as the * To whom correspondence should be addressed. E-mail: khval@ rice.edu. 10.1021/nl025944w CCC: $25.00 Published on Web 01/28/2003

© 2003 American Chemical Society

1,3-dipolar11h,j and electrophilic11k additions to the side walls of pristine SWNTs have been reported. In the earliest reports on sidewall functionalization chemistry,11a-c it was shown that fluorine substituents on SWNTs can be substituted by alkyl groups from corresponding Grignard and alkyllithium reagents, resulting in the covalent attachment of alkyls to the SWNT sidewalls through the C-C bonds. These reactions are facilitated by a weakened C-F bonds relative to those in alkylfluorides and a stronger electron-accepting ability of fluoronanotubes in comparison with that of pristine SWNTs.13 Such enhanced reactivity of fluoronanotubes opens new opportunities for the sidewall attachment of nucleophilic substituents carrying terminal functional groups (e.g., NH2, OH, or COOH), which are capable of further chemical modification. In this paper, we demonstrate the first functionalization of SWNTs by N-alkylidene amino groups covalently attached to the SWNT side walls through the C-N bonds created in the course of reactions where the fluoronanotubes were used as precursors and terminal diamines, as nucleophilic reagents. The amino-functionalized SWNTs prepared by this fairly simple method can be further modified via reactions with acyl chloride derivatives to form peptide linkages for the preparation of nylon-type cross-linked SWNTs and covalent sidewall binding to polymer matrices or DNA. The fluoronanotubes were prepared by the direct fluorination of purified HiPco SWNTs14 at 150 °C to approximately

Scheme 1

C2F stoichiometry according to a procedure described elsewere.15 Fluoronanotubes 1 were heated with terminal alkylidene diamines 2a-d, such as ethylene 2a, propylene 2b, butylene 2c, and hexamethylenediamine 2d (Scheme 1), in the solution phase by stirring the reactants at elevated temperatures (70-170 °C) in the presence of pyridine (Py) as a catalyst. The intermolecular elimination of HF in the reaction of 1 with 2 resulted in the formation of corresponding amino-functionalized SWNTs 3a-d. Figure 2. UV-vis-NIR spectra of purified HiPco SWNTs (A) and N-alkylidene amino-functionalized SWNTs 3a (B) and 3d(C).

Figure 1. Raman spectra of purified HiPco SWNTs (A), fluoronanotubes 1 (B), N-alkylidene amino-functionalized SWNTs 3a (C), and nylon-nanotube polymer derivative 5 (D), a reaction product of 3a with adipyl chloride 4. 332

In these experiments, the following procedure was found to achieve the best functionalization results. A milligram quantity (normally 10-20 mg) of precursor 1 was placed into the reaction vessel, and 5 mL of the corresponding diamine 2 added. The subsequent ultrasonication for 3 min caused almost a complete dispersion of fluoronanotubes to form a black solution; thereafter, five drops of Py were added, and the reaction mixture was stirred under a nitrogen atmosphere for 3 h at ∼150-170 °C. After the completion of the reaction and cooling to room temperature, the insoluble black material that formed was separated from the soluble material by centrifugation. The decanted solution was filtered onto a 0.2-µm pore size Teflon membrane (Cole Palmer), and the amino-functionalized SWNTs 3, collected on a filter, were washed with large amounts of ethanol and dried overnight in a vacuum oven at 70 °C. FTIR, Raman, UV-vis-NIR, SEM/EDX, TEM, variable temperature mass spectrometry (VTP-MS), and TGA analyses were used to determine the sidewall attachment of the N-alkylidene amino groups. Evidence for the significant alteration of the electronic state of 3 due to sidewall functionalization was obtained by Raman and UV-vis-NIR spectroscopy. In the Raman spectra (Figure 1), the typical peaks for purified HiPco SWNTs breathing and tangential mode peaks at 200-263 and 1591 cm-1, respectively (Figure 1A), were observed to decrease in fluoronanotubes 1, and the peak at 1291 cm-1, to increase dramatically (Figure 1B) owing to the large presence (nearly 50%) of the sp3-hybridized carbons in the C2F composition structure of 1. The substantial relative intensity of the sp3 carbon peak at 1291 cm-1, observed in 3a (Figure 1C) and similarly in the other samples (3b-d), provides a diagnostic indication of the disruption of the graphene π-bonded electronic structure of the side walls, suggesting their covalent functionalization. This is further confirmed by the solution-phase UV-vis-NIR spectra taken for samples of 3a and 3d (Figure 2B and C) in ethanol. These spectra are Nano Lett., Vol. 3, No. 3, 2003

Figure 3. ATR-FTIR spectra of fluoronanotubes 1 (A) and N-alkylidene amino-functionalized SWNTs 3a (B), 3b (C), 3c (D), 3d (E).

typical for sidewall functionalized SWNTs,9,11d,15 showing a complete loss of the van Hove absorption band structures routinely observed in purified HiPco SWNTs (Figure 2A).14b The ATR-FTIR spectra (Figure 3) allow for the identification of N-alkylidene amino functionalities on the SWNTs. The two intense peaks at 1214 and 1102 cm-1, characteristic of the C-F bond stretches in fluoronanotubes 1 (Figure 3A), disappear after the reaction with diamines 2 as a result of fluorine displacement. The appearance of new peaks in the 3400-3100, 3000-2800, and 1250-1000 cm-1 regions (Figure 3B-E) attributed to the N-H, C-H, and C-N stretches, respectively,16 give a strong indication of the N-alkylidene amino group attachment to the side walls in the nanotube derivatives 3a-d. The intensities of the NH2 and C-H peaks were observed to be weaker in 3b-c relative to that in 3a, which can be related to the possibility of partial dual buckling of diamines 2b-c to the sidewall surface or wall-to-wall cross linking of the SWNTs by longer-chain alkylidenediamino groups. The elemental analyses by SEM/EDAX yielded for all samples 3a-d significant nitrogen content (within 12-17 at. %) as well as a very low (1-2 at. %) fluorine content, validating the efficient displacement of F by the N-alkylidene amino functionalities. The thermal degradation studies of 3a-d have provided further evidence for covalent functionalization. As an example, the TGA analysis of 3a (Figure 4) shows on a derivative plot a major peak at 300 °C and a minor peak at 450 °C, which correspond to the two-step loss of ethylenediamine 2a evolving at temperatures obviously too high to be due to physisorbed species. The same analysis for 3d also yielded a two-step degradation mechanism with

Figure 4. TGA/DTA data plots for 3a. The inset shows the TG-MS ion current vs time data plots for ions with m/z ) 43 (A) and 29 (B), characterizing volatile products of the thermal degradation of 3a. Nano Lett., Vol. 3, No. 3, 2003

333

Scheme 2

Figure 5. TEM images of N-alkylidene amino-functionalized SWNTs 3a (A) and 3d (B).

the loss of hexamethylenediamine 2d in the 400-550 °C temperature range. The VTP-MS analyses of volatile species evolving from 3a-d show a major loss of corresponding attached groups under vacuum conditions at ∼350-500 °C. The detaching N-alkylidene amino groups were detected in the mass spectra by a peak at m/z ) 59 (HNCH2CH2NH2) for 3a and by peak at m/z ) 73 (HNCH2CH2CH2NH2) for 3b. The peak at 85 m/z, detected for 3c, is probably indicative of the formation of NCH2CH2CH2CH2NH fragments from diamine 2c units, each attached by a single or by two C-N linkages to the side wall in 3c. The observed peaks in the VTP-MS spectrum of 3d at m/z ) 100 and 99 due to (CH2)6334

NH2 and (CH2)6NH fragments, respectively, evolving at ∼450 °C imply that the N-alkylidene bond cleavage is likely preferred over the sidewall C-N bond. The similar and even deeper fragmentation of the SWNT-attached groups probably occurs when the residence time of the sample in the heated zone is increased, as, for instance, in the course of the TGMS analysis of 3a, which leads to the predominant observation of peaks with m/z ) 43 and 29 (inset in Figure 4) due to the CH2CHNH2 and CH2NH species, respectively, arising at the same temperature (450 °C). On the basis of the data obtained from thermal gravimetric and energy-dispersive X-ray analyses, the degree of SWNT functionalization achieved in 3a-d was estimated to be as high as 1 in 8 to 12 sidewall carbons. The TEM studies have revealed, in addition to the individual functionalized SWNTs, which were successfully imaged for the 3a specimen (Figure 5A), a large number of the cross-linked nanotube bundles, which are abundant in the longer-chain N-alkylidene amino-functionalized SWNTs, such as 3d (Figure 5B). Nevertheless, all of the samples 3a-d tested positive by the Kaiser testing procedure17 for the free NH2 groups. Their availability helps to improve the solubility of nanotubes in amines, alcohols, water, and dilute acids and also provides access to various solution-phase reactions involving functional group interchange, for example, condensation reactions with the dicarboxylic acid chlorides to form new “nylon-nanotube” polymer materials. To test this idea, the amino-functionalized SWNTs 3a have been reacted with the adipyl chloride 4 (Scheme 2). The reaction was carried out with 11.8 mg of 3a, which after sonication in 18 mL of ethanol and the addition of 2 mL of 4 have been refluxed at 60 °C for 3 h and stirred at room temperature for 24 h. The resulting SWNT polymer derivative 5 was collected on a PTFE membrane after filtering the reaction mixture and washing the black precipitate on a filter with chloroform. The formation of the amide -C(dO)NHlinkages, as a result of the condensation reaction of 3a with 4, was established by observing in the IR spectrum of 5 (Figure 6) the presence of medium-intensity bands at 3290, 1645, and 1340 cm-1 due to N-H, CdO, and C-N stretches, which are typical for secondary amides.16 The Raman spectrum of 5 (Figure 1D) shows a substantial decrease of the peak intensities of the SWNT breathing and tangential modes with respect to the intensity of the disordered sp3 carbon mode at 1291 cm-1, likely caused by the cross linking of the nanotube side walls via N-alkylidene diamido chains in the ladder polymer structure of 5. It seems reasonable to correlate the formation of this type of microNano Lett., Vol. 3, No. 3, 2003

Figure 6. ATR-FTIR spectrum of nylon-nanotube polymer derivative 5.

These preliminary results suggest the possibility of using amino-functionalized SWNTs 3a-d as reactive monomers in a broad range of polycondensation reactions leading to new polymer materials. The developed method of SWNT sidewall C-N functionalization, which is based on the enhanced reactivity of fluoronanotubes, can also be extended to applications in the other coupling reactions, such as the binding of the amino acids and DNA base to the SWNTs. This work and other exciting modifications of the SWNTs are in progress. Acknowledgment. This work was supported by the Robert Welch Foundation, Texas Advanced Technology Program and by a NASA Johnson Space Center research grant. We thank B. Brinson for conducting TEM analysis, Z. Gu for generous assistance, and Professor R. E. Smalley and Dr. R. H. Hauge for supplying HipCO single-walled carbon nanotubes. References

Figure 7. SEM images of fluoronanotubes 1 (A) and nylonnanotube polymer derivative 5 (B).

structure with the considerable modification, observed by SEM, of the morphology of nanotube materials from bundles, which is typical of sidewall-functionalized SWNTs (e.g., fluoronanotubes 1) (Figure 7A), to the raftlike morphology of polymer 5 (Figure 7B), which somewhat resembles the morphology of the wall-to-wall polymerized SWNTs recently produced under high-pressure-high-temperature conditions.18 Nano Lett., Vol. 3, No. 3, 2003

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Nano Lett., Vol. 3, No. 3, 2003