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J. Phys. Chem. C 2008, 112, 738-740
Functionalization of Single-Walled Carbon Nanotubes with 1,4-Benzenediamine Using a Diazonium Reaction Mark D. Ellison* and Patrick J. Gasda Department of Chemistry, Ursinus College, CollegeVille, PennsylVania 19426 ReceiVed: August 29, 2007; In Final Form: NoVember 2, 2007
Single-walled carbon nanotubes (SWCNTs) have been functionalized with a phenyl amine group in a singlestep diazonium reaction process. Raman and UV/vis spectroscopies verify that the SWCNTs have been functionalized, and Fourier transform infrared spectroscopy confirms the presence of an -NH2 group attached to the SWCNTs that is available for further chemistry. This -NH2 group reacts with a cross-linking molecule, confirming that it does undergo additional chemical reactions. These results illustrate a direct pathway to functionalize SWCNTs for building nanostructures.
Introduction Single-walled carbon nanotubes (SWCNTs) have interesting mechanical, electrical, and optical properties1 and show potential for use in integrated devices, such as solar cells.2 The use of SWCNTs in integrated devices relies on covalent functionalization of the nanotubes so that they can be connected to other parts of the device. Consequently, significant effort has been made to investigate methods of covalently functionalizing SWCNTs.3-19 Of particular interest are functionalization processes that provide amine groups for further reaction. This is because amine groups are versatile and undergo a variety of reactions,20 allowing for numerous further functionalization possibilities on the SWCNTs. Two common methods for functionalizing SWCNTs with -NH2 groups for further reactions are fluorination of the SWCNTs followed by reaction with diamines,18,21 and reaction of amines with carboxylic acid groups already present on the SWCNTs.6,7,17,22,23 The latter process has been used for several applications, such as the attachment of nanoparticles to single- and multiwalled carbon nanotubes21,22,24,25 and the attachment of DNA to SWCNTs7,26 and carbon fibers.27 The functionalization strategies listed above involve multistep processes to yield SWCNTs with amine groups ready for further chemistry. We report here a single-step process to achieve such a product. It is straightforward and produces nanotubes that can easily be further functionalized, as described herein. Experimental Section The functionalization procedure is based directly on the diazonium reactions developed by Tour et al., who were able to attach SWCNTs benzene rings with -F, -Cl, -CO2H, -NO2, and -t-butyl functional groups, among others, para to the bond to the SWCNT.3-5,8-10,16 We have found that it is possible to attach 1,4-benzenediamine using a similar diazonium reaction, which results in an attached benzene ring with an -NH2 functional group para to the bond to the SWCNT. Typically, 15 mg (1.2 mmol of C) of purified28,29 HiPco SWCNTs (Carbon Nanotechnologies, Inc.) were mixed with 20 * Corresponding author. E-mail:
[email protected].
Figure 1. FTIR spectrum of SWCNTs reacted with 1,4-benzenediamine.
Figure 2. Raman spectra of (a) purified SWCNTs and (b) functionalized SWCNTs.
mg (0.30 mmol) of NaNO2 (Baker Analyzed Reagent, 99.1%) and 30 mg (0.30 mmol) of 1,4-benzenediamine (Aldrich, 99%). A 0.013 mL (0.25 mmol) portion of concentrated H2SO4 (J. T. Baker) was added, and the mixture was stirred and heated at 60 °C for 1 h, as shown in Scheme 1. The mixture was then allowed to cool to room temperature, dissolved in dimethylformamide (DMF, Aldrich, >99.8%), and filtered through a 0.45-µm PTFE membrane (Millipore). The solid was sonicated in DMF and filtered again, and the process was repeated until the DMF was colorless after sonication. This process removed any unreacted 1,4-benzenediamine from the product. Also, the pH of the washings was tested to ensure that no acid remained in the sample.
10.1021/jp076935k CCC: $40.75 © 2008 American Chemical Society Published on Web 12/29/2007
Single-Step Functionalization of SWCNTs
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SCHEME 1. Functionalization of SWCNTs with 1,4-Benzenediamine
SCHEME 2. Attachment of SMCC to Phenyl-NH2-functionalized SWCNTs
Functionalization was verified using Fourier transform infrared (FTIR), Raman, and UV/vis spectroscopies. Infrared spectra were obtained by placing the sample on CaF2 plates and placing them in the path of the infrared beam (transmission mode) of a ThermoNicolet 6700 FTIR spectrometer with a liquid-nitrogen-cooled HgCdTe detector. Background scans were obtained using just the CaF2 plates, and the spectrometer computed the ratio of the sample to the background scans. Typically, 1000 scans were averaged, and the resolution was 4 cm-1. The CaF2 plates absorb IR light below about 1200 cm-1, so some peaks in the fingerprint region are not visible in the spectra. Raman spectra were collected using a ThermoFisher NXR FT-Raman spectrometer using 1064 nm excitation and a liquid-nitrogen-cooled Ge detector. The typical excitation power was 10 mW, and 128 scans were averaged. UV/vis spectra were collected using a Varian Cary 1 UV/vis spectrometer by dispersing the SWCNTs in DMF. Results and Discussion Figure 1 shows an FTIR spectrum of the functionalized SWCNTs. The peaks at 3737 and 3664 cm-1 arise from N-H stretches, and the splitting is indicative of a primary amine.30 These peaks are about 200 cm-1 higher in frequency than those for solid 1,4-benzenediamine, indicating that the -NH2 groups are not involved in hydrogen bonding. This suggests that the -NH2 groups are in an environment quite different from that of solid 1,4-benzenediamine, which would be expected if the molecules were attached to the nanotubes. The peak at 1630 cm-1 is attributed to the CdC stretch of the benzene ring.30 The -NH2 scissors motion for an aryl-NH2 group occurs between 1300-1500 cm-1 and is not likely to be the source of this peak.30 These data, then, indicate the presence of phenyl-
NH2 functional groups in the SWCNT sample. An oxidative coupling of phenylamine compounds has been reported.31 Such a process is unlikely under these conditions for the following reasons. First, that reaction was carried out under electrochemical conditions significantly different from the conditions used in this work. Second, the oxidative coupling resulted in the phenylamine being connected to the SWCNTs via a linking -NH group. Such a secondary amine group exhibits a single N-H stretch around 3400-3500 cm-1,30 which is not consistent with the observed IR spectrum. Thus, the data suggest that the reaction proceeds by conversion of one of the -NH2 groups to a diazonium group, followed by attachment to the SWCNT. Raman spectra of purified and functionalized SWCNTs are shown in Figure 2, traces a and b, respectively. Trace b shows significant intensity in the D band, which is indicative of sidewall functionalization.32 For the functionalized SWCNTs, the ratio of peak areas for the D and G bands is 0.58, which is consistent with other highly functionalized SWCNTs obtained using a similar diazonium reaction.16 The UV/vis spectrum, shown in Figure 3, also verifies that the SWCNTs are functionalized. Trace a is from purified SWCNTs and shows peaks resulting from transitions between van Hove singularities. Trace b is from SWCNTs that have undergone reaction with 1,4-benzenediamine. The absence of peaks in trace b is consistent with sidewall-functionalized SWCNTs.8,16 The step in trace b at 350 nm occurred because of an instrument glitch in the lamp switch during the scan. Thus, the Raman and UV/vis data confirm that the SWCNTs have been functionalized in the reaction with 1,4-benzenediamine. To demonstrate that the amine group on the functionalized SWCNTs can undergo further chemical reactions, the functionalized SWCNTs were reacted with a heterofunctional cross-
740 J. Phys. Chem. C, Vol. 112, No. 3, 2008
Ellison and Gasda attaches a phenyl -NH2 group, simplifying the attachment of additional functionalities to SWCNTs. Acknowledgment. The authors acknowledge Erica Ellison for a critical proofreading of the manuscript. Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research. This work was also supported by funds from Ursinus College. References and Notes
Figure 3. UV/vis spectra of (a) purified SWCNTs and (b) functionalized SWCNTs.
Figure 4. FTIR spectrum of phenyl-NH2-functionalized SWCNTs reacted with SMCC.
linker, succinimidyl-4-(N-maleimidomethyl)cyclohexane-1carboxylate (SMCC, Pierce Biotechnologies, >92%), which has been previously used to link DNA to amine-functionalized SWCNTs.26 The reaction we employed is shown in Scheme 2 and is similar to that used in reference 26. Briefly, 0.0116 g of functionalized SWCNTs was added to 25 mL of a 60 mM SMCC solution and stirred in the dark for 2 h. A drop of triethylamine was added. The product was filtered, washed with diethyl ether, and suspended in a triethanolamine buffer. Figure 4 shows the FTIR spectrum of the product of this reaction. The peak at 3300 cm-1 arises from the N-H stretch of the amide formed when SMCC reacts with the -NH2 group on the SWCNT. Peaks at 2800-3000 cm-1 are C-H stretches from the aliphatic portions of the SMCC. Also visible is a CdO carbonyl stretch of the SMCC at 1650 cm-1. The peaks at 3600-3700 cm-1 indicate unreacted amine from the functionalized SWCNTs. These data are consistent with the product of the reaction of SMCC with the -NH2 group of the functionalized SWCNTs, indicating that the amine group is indeed available to undergo further reaction. In summary, we have used a single-step process to functionalize SWCNTs with 1,4-benzenediamine, resulting in a phenyl -NH2 group available for further chemistry. The functionalization chemistry is straightforward and results in highly functionalized SWCNTs. The phenyl -NH2 group is able to undergo additional reactions, such as with the heterofunctional cross-linker SMCC. This functionalization process directly
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