Fabrication of Carbon Nanotube− Polyaniline Composites via

J. Phys. Chem. C 2007, 111, 4125-4131

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Fabrication of Carbon Nanotube-Polyaniline Composites via Electrostatic Adsorption in Aqueous Colloids Xing-bin Yan,* Zhao-jun Han, Yi Yang, and Beng-kang Tay School of Electrical and Electronic Engineering, Nanyang Technological UniVersity, Singapore 639798 ReceiVed: August 10, 2006; In Final Form: December 8, 2006

We report that, for the first time, multiwalled carbon nanotube (MWNT)-polyaniline (PANI) nanocomposites were readily prepared via electrostatic adsorption in their mixed aqueous media. The pristine MWNTs were first oxidized to add some negative charges, and the PANI nanofibers, to add some positive charges, were fabricated by aqueous/organic interfacial polymerization, and then the MWNT-PANI composites were prepared by rapidly mixing the negatively charged oxidized MWNT aqueous colloid and the positively charged PANI aqueous colloid. Scanning and transmission electron microscopies (SEM and TEM, respectively) and Raman and Fourier-transform infrared (FTIR) spectroscopies were used to characterize the resulting composites. The results show that the electrostatic adsorptive forces between the positively charged PANI nanofibers and the negatively charged MWNTs enable precipitation of MWNT-PANI composites in the mixed aqueous colloids. This work provides new strategy for fabricating other nanocomposites of carbon nanotubes with conducting polymers.

1. Introduction Iijima,1

Carbon nanotubes (CNTs), discovered by because of their extraordinary properties such as excellent Young’s modulus, good flexibility, and high electrical and thermal conductivity, are recognized as reinforcements for highperformance, multifunctional composites.2 Following the first report of the preparation of a CNT-polymer composite by Ajayan et al.,3 considerable achievements have been made in CNT-polymer composites, which show a remarkable enhancement in electrical and mechanical properties compared to those of monolithic polymers.3-8 It is well-known that polyaniline is a typical conducting polymer. Because of its mechanical flexibility, environmental stability, and controllable conductivity with acid/base modification (doping/undoping), PANI has potential applications in many fields such as lightweight battery electrodes, electromagnetic shielding devices, anticorrosion coatings, sensors, etc.9 Recently, extensive efforts have been made to prepare functional CNTPANI composites, which are expected to exhibit useful electrical and optical properties and superior mechanical strength compared to unprocessed PANI.10-14 For application in actual devices, two important factors must be addressed first: functionality and processability.15 These properties are related to (i) the effective aspect ratio, largely determined by the state of dispersion, and (ii) the interfacial adhesion between CNT additives and the PANI matrix. Good interfacial bonding is essential to ensure efficient transfer (charge or stress) from the PANI matrix to the nanotube lattice and is one of the critical issues related to the functionality of CNTPANI composites. With a view to enhance this interaction, two efficient methods have recently been studied more intensively, namely, preparing CNT-PANI composites by direct mixing and dispersal of PANI emeraldine base (EB) solutions of NMP (N-methylpyrrolidone) with CNTs11 and preparing CNT-PANI * Corresponding author. E-mail: [email protected]. Tel.: + (65) 6790 4533. Fax: + (65) 6793 3318.

composites based on the polymerization of the corresponding aniline monomer in the presence of CNTs in a solution (in situ process).9-14,16 The two routes, especially the in situ polymerization, have revealed the existence of effective site-selective interactions between the quinoid ring of the PANI and the multiwalled carbon nanotubes (MWNTs) facilitating transfer processes between the two components.16 As demonstrated by Huang and co-workers, high-quality PANI nanofibers with controlled distributions of diameters can be prepared using aqueous/organic interfacial polymerization, and stable, positively charged PANI nanofiber aqueous colloids can be made from these PANI nanofibers,17-21 which enables new possibilities for tackling the nanostructuring processability of PANI. Thus, in this work, we first demonstrate, by simply mixing a positively charged PANI nanofiber aqueous colloid and a negatively charged MWNT aqueous dispersion, the successful fabrication of uniform MWNT-PANI nanocomposites. The morphology and structure of the composites were analyzed using scanning and transmission electron microscopies (SEM and TEM, respectively) and Raman and Fourier-transform infrared (FTIR) spectroscopies. Interestingly, the experimental and analytic results revealed a strong electrostatic interaction between the C-N+ species of the PANI nanofibers and the COO- species of the MWNTs, which is another different interfacial adhesion than is found in CNT-PANI composites prepared by in situ polymerization. We expect that this simple method could open new opportunities for processing composites of CNTs and nanostructured conducting polymers. 2. Experimental Section 2.1. Preparation of MWNT-PANI Composites. (1) Comercially available MWNTs (600 mg; >95% purity, average diameter < 20 nm, CVD method; Chengdu Organic Chemicals Co., Ltd, Chinese Academy of Sciences, China) were oxidized via sonication (with a 15-W low-power-base sonicator) in 80 mL of 1:3 (v:v) concentrated nitric acid/sulfuric acid at

10.1021/jp0651844 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/23/2007

4126 J. Phys. Chem. C, Vol. 111, No. 11, 2007 60 °C for 10 h. The resultant solid was washed thoroughly with deionized water until the pH value was about 6, and then the black solid was collected and dried in a vacuum at 40 °C. (2) A typical interfacial polymerization reaction was performed in a 1-L glass beaker. Aniline (64 mmol) was dissolved in 200 mL of toluene, and ammonium peroxydisulfate (16 mmol) was dissolved in 400 mL of 1 M HCl solution. The two solutions were cooled to 0-5 °C, and then the organic phase was carefully spread onto the aqueous phase, forming an organic/aqueous interface. Reactions were carried out at 0-5 °C for 12 h, and then the aqueous phase was collected and purified by centrifugation against deionized water. When the deionized water bath reached a pH value of 6, the dark green products were collected and dried in a vacuum at 40 °C. (3) A fresh sample (40.0 mg) of the oxidized MWNTs was dispersed in 100 mL of aqueous HCl solution (0.0025 M, pH 2.6) and sonicated with a highpower (500-W) ultrasonic pole for 5 min, forming a stable black aqueous colloid (0.4 mg/mL). Separately, a fresh sample (120 mg) of PANI nanofibers was dispersed in 120 mL of aqueous HCl solution (pH 2.6) and sonicated for 5 min as well, forming a stable deep green colloid (1.0 mg/mL). (4) Aliquots of 0.5, 2.5, 5.0, 10, and 20 mL of MWNT colloids were poured into five PANI aqueous colloids with the same volume of 20 mL and immediately shaken to ensure sufficient mixing. By centrifugation (15 min, 4000 rpm) and drying in a vacuum at 40 °C, MWNT-PANI emeraldine salt (ES) composites with different CNT contents (0.1%, 0.5%, 1.0%, 2%, and 4%) were obtained. 2.2. Characterization of MWNT-PANI Composites. The morphology of the resulting MWNT-PANI composites was characterized by field-emission scanning electron microscopy (FE-SEM) and TEM. FE-SEM measurements were conducted on a JSM-6340F instrument (JEOL). Before FE-SEM imaging, the samples were sputtered with a thin layer of platinum. TEM images were obtained on a JEM-2010 instrument (JEOL), using an accelerating voltage of 200 kV. The samples for TEM imaging were prepared by casting a drop of the sample suspended in ethanol on a copper grid coated with a carbon support film. The molecular structures of all samples were measured by Raman and FTIR spectroscopies. The FTIR spectrum was recorded on an IFS3000 v/s Fourier-transform infrared spectrometer, and the Raman spectrum was recorded by a Renishaw Raman spectrometer (50 mW, 514.5-nm Ar+ laser) at room temperature. Samples used in electrical conductivity measurements had a uniform circular shape (diameter ) 10 mm) that was obtained by applying a hydraulic pressure about 10 MPa, and the electrical conductivity was measured by a standard four-probe method at the room temperature. 3. Results and Discussion It is known that the chemical oxidation of carbon materials is frequently used as a method to obtain a more hydrophilic surface structure with a relatively large number of oxygencontaining surface groups. These oxygen-containing groups behave as acids or bases that exhibit ion-exchange properties and improve the dispersibility.22 Previous reports23-28 that referred to the oxidation of CNTs with nitric acid and sulfuric acid have suggested the introduction of many functional groups, such as hydroxyl (-OH), carboxyl (-COOH), and carbonyl (>CdO), onto the surface of CNTs. In our experiments, the nature of the pristine and oxidized MWNT surface groups was investigated using FTIR spectroscopy (Figure 1). It is shown that there are some similar peaks in the two spectra: A band at 1585 cm-1 is assigned to O-H deformation vibrations in water,

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Figure 1. FTIR spectra of (a) pristine MWNTs and (b) oxidized MWNTs.

a broad band centered at 1153 cm-1 is associated with C-O stretching vibrations,26 and the peak at 2350 cm-1 is attributed to CO2. Also, compared to the spectrum of the pristine MWNTs, the spectrum of the oxidized MWNTs clearly shows the presence of oxygen-containing groups resulting from the oxidation: A broad band centered at around 3346 cm-1 with a shoulder at 3213 cm-1 is ascribed to O-H stretching vibrations in C-OH groups and water, respectively,26,27 and a band at 1721 cm-1 is attributed to CdO stretching vibrations in carboxyl and carbonyl groups.24-28 We found that the presence of carboxylic acid groups allowed the preparation of an aqueous MWNT colloid (0.4 mg/mL) after sonication in dilute acidic solution without any additional surfactant. The resulting colloid remained dispersed for at least 1 month; neither sedimentation nor aggregation of nanotube bundles was observed. It should be mentioned that, when the pristine MWNTs (8 mg) were dispersed in HCl solution (20 mL) with the same pH value using the high-power (500 W) ultrasonic pole for 5 min, it was impossible to form a uniform aqueous colloid. The gravity-driven sedimentation of large MWNT bundles was very fast, and almost all of the MWNTs settled at the bottom of the vial after 1 h (Figure S1). During the preparation of the PANI “colloid”, we found that, when asprepared nanofibers were dispersed in an HCl solution (pH 2.6) and sonicated for several minutes, a stable dark green PANI colloid (1.0 mg/mL) was created. This uniform supernatant could be maintained, even with centrifugation at 4000 rpm, for 30 min or longer. It is expected that PANI nanofibers can be colloidally stable for more than 2 days. The above results clearly indicate that stable MWNT and PANI nanofiber aqueous colloids can be prepared. The stability of the MWNT and PANI colloids was strongly dependent on concentration. In principle, with increasing concentration, the stability of a colloid decreases gradually. When the concentration of MWNT and PANI was above 10 mg/mL, we could not prepare stable colloids. Moreover, we found that commercially available PANI (EB) powder (purchased from Aldrich) cannot be dispersed in acidic aqueous solution at all, even after ultrasonication for 1 h. This is because the commercially available PANI powder is not a nanoscale material. Therefore, for preparing stable PANI aqueous colloids, there are two important conditions: one is the doped form of PANI, and the other is the nanoscale structure of PANI. The ζ potentials of the oxidized MWNT and PANI aqueous colloids are about -12.54 and 31.35, respectively, as measured

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Figure 2. Photographs showing the electrostatic adsorption of MWNTs and PANI nanofibers in acidic aqueous colloids. From a to f, the times are 0, 5, 10, 30, 60, and 300 min, respectively. The liquid phase from the left beaker to the right beaker is PANI nanofiber colloid (0.20 mg/mL, 40 mL), MWNT colloid (0.05 mg/mL, 40 mL), and the mixing of PANI and MWNT acidic colloids (10 mL of MWNT and 30 mL of PANI).

with a Zetaplus analyzer. This indicates that the oxidized MWNTs take some negative charges whereas the PANI nanofibers take some positive charges, and their aqueous colloids are good sources for fabricating MWNT-PANI composites using an electrostatic self-assembly process. When oxidized MWNT colloids with different volumes were poured into PANI colloids, after a short time, from several seconds to several minutes depending on the volume concentration of the MWNT colloid, some flocculation started to appear and the amount of the flocculation increased with time. Upon centrifugation to rapidly collect the flocculation, the solid powders were deposited at the bottom of the centrifuge tubes, and the color of suspension changed from dark green to colorless, which indicated that no residual MWNTs or PANI nanofibers remained in the suspension. To demonstrate this process more vividly, we used photographs to observe this process (Figure 2). In this experiment, the concentrations of the oxidized MWNT and PANI colloids were diluted to 0.05 and 0.20 mg/mL, respectively, to increase the transparency of the colloids and decrease the speed. A 10-mL sample of MWNT colloid was quickly added to another 30-mL sample of PANI colloid, and the resulting solution was mixed immediately, forming a uniform suspension (Figure 2a). It is interesting to note that, after 5 min, eye-visible flocculation started to appear in the mixture (Figure 2b), then aggregated with increasing time (Figure 2c,d), and slowly precipitated at the bottom of the vessel (Figure 2e,f). As the process proceeded, the mixture became lighter in color and finally became colorless as a transparent solution. The morphology of the products was detected using electron microscopy. Figure 3a shows a typical SEM image of the oxidized MWNTs, which were highly tangled with each other and had rarely visible ends. For the PANI sample (Figure 3b), more than 95% of the sample consisted of nanofibers, and the diameters were less than 100 nm. Most of the fibers were bent and tangled in length, indicating their high flexibility. For the composite sample (Figure 3c), many MWNTs were clearly present on the surface of the PANI nanofibers. Almost of them were individual, with no obvious bundles or agglomerations. Moreover, some PANI nanofibers were linked to one another by CNTs (marked with arrows), which can potentially be attributed to the interaction between PANI nanofibers and MWNTs. Shown in Figure 4a is a typical TEM image of oxidized MWNTs, which exhibited good dispersion, with different lengths (up to 1 µm) and diameters (up to 20 nm). High-resolution TEM images revealed the existence of defects on the CNTs after acid treatment (Figure S2). Also, shown in Figure 4b is a TEM image of pure PANI nanofibers. Typically, the fiber-like PANI nanostructures, with diameters ranging from 30 to 80 nm and lengths ranging from 150 to 400 nm, tended

to link into interconnected nanofiber networks, rather than bundles. In contrast, for the CNT-PANI composite sample shown in Figure 4c, the individual CNTs were dispersed in the PANI nanofiber network to form a porous CNT-PANI composite network. We suggest that the formation of the crosslinked network could be related to the interaction between PANI nanofibers and oxidized MWNTs. To confirm the nanostructure of MWNT-PANI (ES) composites, Figure 5 shows Raman spectra of pristine MWNTs, oxidized MWNTs, PANI (ES) nanofibers, and 4 atom % MWNT-PANI (ES) composite. It is clear that the peaks in Figure 5b are similar to those observed in pristine MWNTs (Figure 5a), except that the ratio of the peak intensity changed, as a result of the oxidization of the MWNTs. The strong peak at 1585 cm-1 (G lines) is the Raman-allowed phonon highfrequency E2g first-order mode, and the disorder-induced peak at 1350 cm-1 (D lines) might originate from defects in the curved graphene sheets, tube ends, and turbostratic structure.29,30 The strength of this peak is related to the amount of disordered graphite and the degree of conjugation disruption in the graphene sheet.31 For both PANI (ES) and the MWNT-PANI (ES) composite, C-H bending of the quinoid ring at 1176 cm-1, C-H bending of the benzenoid ring at 1228 cm-1, C-C stretching of the quinoid ring at 1489 cm-1, and C-C stretching of the benzene ring at 1617 cm-1 were observed, revealing the presence of doped PANI (ES) structures.16,32,33 Moreover, in the spectrum of the composite, peaks attributed to MWNTs (D and G peaks) cannot be identified because they are too weak and overlap. In comparing the spectrum derived from the MWNT-PANI composite to that of PANI, three points should be noted: (1) A clear trend in the spectral position of the peak assigned to the C-N stretching of the cation radical species (C-N•+)33 can be noticed. In the spectrum of PANI, this peak is at 1332 cm-1, whereas in the spectrum of composite, this peak is seen to shift to the higher wavenumber of 1347 cm-1. (2) The relative intensity of C-N•+ stretching increases markedly with respect to that of C-H bending of the quinoid ring. (3) The intensity and position of C-C stretching of the quinoid ring at 1489 cm-1 undergoes no obvious change. The observed upshift of C-N•+ stretching in the Raman spectra can be explained by the interaction between the C-N+ species of PANI and the COO- species of the MWNTs. The strong cation-anion attraction increases the energy necessary for C-N+ stretching vibrations in the PANI molecule to occur, which is reflected in the higher frequency of the Raman peaks. The increase of the IC-N•+/IC-H ratio indicates that the degree of protonic acid doping increases in the presence of MWNTs. Zhang et al.34 observed a pronounced decrease of the IC-N•+/IC-H ratio and an upshift of the CdN stretching band of the quinoid ring in

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Figure 3. SEM images of (a) oxidized MWNTs, (b) PANI nanofibers and (c) 4 wt % MWNT-PANI composite. All of the scale bars are 100 nm.

the Raman spectrum of the CNT-PANI composites. Cochet et al.16 also observed a decrease in intensity of the C-C stretching of the quinoid ring at 1485 cm-1. They explained that a site-selective interaction between the quinoid ring of the doped PANI and the CNTs occurred as a consequence of in situ polymerization, resulting in the above changes observed in the Raman spectra. Thus, we conclude that the interfacial bonding between the oxidized MWNTs and PANI nanofibers is another different interaction, in this case, the electrostatic adsorption of opposite charges of oxidized MWNTs and PANI nanofibers.

Figure 4. TEM images of (a) oxidized MWNTs, (b) PANI nanofibers and (c) 4 wt % MWNT-PANI composite. The scale bars are 50, 200, and 200 nm, respectively.

To further confirm that electrostatic adsorption had taken place between the MWNTs and PANI nanofibers, we investigated the infrared spectra of the PANI (ES) nanofibers and the MWNT-PANI composite. Both spectra exhibited the clear presence of quinoid and benzoid ring vibrations (CdC stretching) at 1580 and 1489 cm-1, respectively, thereby indicating

Fabrication of Carbon Nanotube-Polyaniline Composites

Figure 5. Raman spectra of (a) pristine MWNTs, (b) oxidized MWNTs, (c) PANI (ES) nanofibers, and (d) 4 wt % MWNT-PANI (ES) composite.

Figure 6. FTIR spectra of (a) PANI (ES) nanofibers and (b) 4 wt % MWNT-PANI (ES) composite.

the oxidation state of PANI (ES).34,35 As is commonly observed for PANI (ES) form (Figure 6a), the quinoid band at 1580 cm-1 was less intense than the benzenoid band at 1489 cm-1.9 The peaks at 1302 and 1140 cm-1 correspond to C-N stretching

J. Phys. Chem. C, Vol. 111, No. 11, 2007 4129 (-N-benzenoid-N-) and CdN stretching (-NdquinoiddN), respectively.34,35 The peak at 1240 cm-1 was assigned to the CsN•+ stretching vibration in protonic acid doped PANI.36 The band at 817 cm-1 was attributed to aromatic C-H bending out of the plane of the 1,4-disubstituted aromatic ring37,38 and the peaks between 800-500 cm-1 were assigned to the vibration of C-H bands in the benzene rings. These peaks are sharper in the spectra of composites than in the spectra of PANI. More importantly, a marked increase in the relative intensity of -Nd quinoiddN- stretching was revealed. This peak was described by MacDiarmid et al. as an “electronic-like band” and was considered to be a measure of the degree of delocalization of the electrons.35 This increase was expected to be relative to the interaction between the MWNTs and the PANI nanofibers, which could increase the effective degree of electron delocalization. The strong interaction might result in MWNTs functioning as a chemical dopant for PANI conductivity, which agrees well with the results derived from Raman analysis. Moreover, as demonstrated by Zengin9 and Baibarac et al.,11 a marked increase in the intensity ratio between the quinoid and benzoid ring vibrations is commonly observed for CNT-PANI composites prepared by the in situ process compared to that of the PANI (ES) without CNTs. In our MWNT-PANI composite, however, this change is not obvious. This indicates that the interaction might be not due to the π stacking, which takes place between the π-bonded surface of the MWNTs and the quinoid ring of PANI. According to the established theory for the stabilization of colloids,39 by introducing charge onto the nanoparticle surfaces, electrostatic repulsion between the charged nanoparticles can be utilized to stabilize a colloid. The backbone of the emeraldine form of polyaniline can be induced to accept positive charges by doping with a protonic acid such as HCl, changing to the emeraldine salt (ES) form. Because the size of PANI prepared by interfacial polymerization is on the nanoscale, a stable colloid can be formed through electrostatic repulsion of the backbone of PANI (ES) nanofiber dispersion in an acidic aqueous solution. Kaner et al.20 reported that the stability of the PANI (ES) colloid is strongly dependent on the pH value and that 2.6 was the most optimal value. In our experiments, to eliminate the influence coming from differing pH values, the pH values of the oxidized MWNTs and PANI nanofiber aqueous colloids were both 2.6. Although acid addition would destabilize the aqueous colloid of oxidized CNTs and accelerate the precipitation of CNTs, we still obtained a stable MWNT aqueous colloid (pH 2.6). Kim

SCHEME 1: Schematic of Simplified Electrostatic Adsorption between a Negatively Charged MWNT and Positively Charged PANI Molecules, Showing an Idealization of the First Adsorption Step, Depicting the Starting Generation of MWNT-PANI Composite

4130 J. Phys. Chem. C, Vol. 111, No. 11, 2007 et al.40 studied the ζ potentials of sulfuric and nitric acid oxidized MWNTs as a function of pH value and found that the isoelectric point was around pH 2. Therefore, oxidized MWNTs should be charged negatively when dispersed in an aqueous solution with a pH of greater than 2. For our oxidized MWNT aqueous colloid, the ζ potential is about -12.54. These negative charges explain the colloidal stability of oxidized MWNTs in aqueous HCl media (pH 2.6). We believe that the spontaneous interaction of the MWNT and PANI aqueous colloids is likely to involve the following mechanism: When the negatively charged MWNT colloid is added to the positively charged PANI colloid, MWNTs and PANI nanofibers become interconnected through electrostatic adsorption of the COO- species of the oxidized MWNT and the C-N+ species of the PANI (ES) nanofibers, which is also supported by our Raman and FTIR results. Scheme 1 shows a simplified electrostatic physical adsorption process taking place between an MWNT and backbone molecules of PANI (ES). In our experiments, we found that the speed of generating flocculation of the composite was not high. In other words, when the MWNT colloid was added to the PANI colloid, the flocculation did not appear until after a short time ranging from several seconds to several minutes. Therefore, a kind of homogeneous “solution” can be obtained simply by pouring the MWNT colloid into the PANI colloid all at once and rapidly mixing, such that all of the MWNTs and PANI nanofibers are evenly distributed. As the electrostatic adsorption begins, the positively charged PANI nanofibers are linked with negatively charged MWNTs in their vicinity. Finally, in this way, uniform MWNT-PANI composites can be successfully fabricated. Electrical conductivity was measured by a standard four-probe method. The electrical conductivities of pressed pieces of oxidized MWNTs and HCl-doped PANI nanofibers (ES) at room temperature were approximately 37.8 and 0.6 S/cm, respectively. In contrast, with a continual increase of the CNT content from 0 to 0.1, 0.5, 1.0, 2.0, and 4.0 wt %, the conductivities at room temperature gradually increased from 0.6 S/cm to 2.4, 14.3, 21.8, 25.7, and 27.9 S/cm, respectively, for the corresponding MWNT-PANI composites. The results show that a small number of MWNTs can efficiently connect PANI nanofibers to each other to form a conducting network in the composites, resulting in the enhancement of the electrical properties of the resulting MWNT-PANI composites. 4. Conclusion In summary, we have demonstrated that MWNT-PANI composites can be readily prepared by rapidly mixing of oxidized MWNTs and PANI nanofibers aqueous colloids. Electrostatic adsorptive forces between the positively charged PANI nanofibers and the negatively charged MWNTs enable precipitation of MWNT-PANI composites in their mixed aqueous media, as revealed by Raman and FTIR spectroscopies. The addition of a small number of MWNTs can enhance the electrical properties of the composites, as revealed by the measured electrical conductivity results. The present study indicates that charged nanomaterial colloids are a good source for fabricating nanocomposites consisting of a positively charged material and a negatively charged material via an electrostatic adsorption interaction. We expect that other nanocomposites of CNTs and conducting polymers (such as polypyrrole and polythiophene) could be made through nanostructuring, tailoring the positive charge on the polymer chains, mixing the aqueous

Yan et al. colloids with negatively charged CNT colloids. In addition, the newly fabricated MWNT-PANI composites might be of significant interest in the applications of lightweight electrodes, biosensors, and thermal-conductivity devices. Acknowledgment. The authors thank the Singapore Millennium Foundation for financial support. We also thank Dr. Peter C. T. Ha for fruitful discussions. Supporting Information Available: Optical micrographs (Figure S1) showing the dispersion of the pristine and the oxidized MWNTs in HCl solution (pH 2.6), respectively; highresolution TEM image (Figure S2) showing clearly the sites of the destruction of graphite multilayer by concentrated nitricsulfuric acids treatment. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Iijima, S. Nature 1991, 354, 56. (2) Peigney, A. Nat. Mater. 2003, 2, 15. (3) Ajayan, P. M.; Stephan, O.; Colliex, C.; Trauth, D. Science 1994, 265, 1212. (4) Jonghwan, S.; Nikhil, K.; Pawel, K. K.; Pulickel, A. Nat. Mater. 2005, 4, 134. (5) Kovtyukhova, N. I.; Mallouk, T. E. J. Phys. Chem. B 2005, 109, 2540. (6) Pradhan, B.; Batabyal, S. K.; Pal, A. J. J. Phys. Chem. B 2006, 110, 8274. (7) Datsyuk, V.; Guerret-Piecourt, C.; Dagreou, S.; Billon, L.; Dupin, J. C.; Flahaut, E.; Peigney, A.; Laurent, C. Carbon 2005, 43, 873. (8) Barrau, S.; Demont, P.; Peigney, A.; Laurent, C.; Lacabanne, C. Macromolecules 2003, 36, 5187. (9) MacDiarmid, A. G. Angew. Chem., Int. Ed. 2001, 40, 2581. (10) Wei, Z. X.; Wan, M. X.; Lin, T.; Dai, L. M. AdV. Mater. 2003, 15, 136. (11) Baibarac, M.; Baltog, I.; Lefrant, S.; Mevellec, J. Y.; Chauvet, O. Chem. Mater. 2003, 15, 4149. (12) Wu, T. M.; Lin, Y. W.; Liao, C. S. Carbon 2005, 43, 734. (13) Sainz, R.; Benito, A. M.; Martinez, M. T.; Galindo, J. F.; Sotres, J.; BarI, A. M.; Coraze, B.; Chauvet, O.; Maser, W. K. AdV. Mater. 2005, 17, 278. (14) Sainz, R.; Benito, A. M.; Martinez, M. T.; Galindo, J. F.; Sotres, J.; BarI, A. M.; Coraze, B.; Chauvet, O.; Dalton, A. B.; Baughman, R. H.; Maser, W. K. Nanotechnology 2005, 16, S150. (15) Zhi, C. Y.; Bando, Y.; Tang, C. C.; Honda, S.; Sato, K.; Kuwahara, H.; Golberg, D. Angew. Chem., Int. Ed. 2005, 44, 7929. (16) Cochet, M.; Maser, W. K.; Benito, A. M.; Callejas, M. A.; Martı´nez, M. T.; Benoit, J. M.; Schreiber, J.; Chauvet, O. Chem. Commun. 2001, 1450. (17) Huang, J.; Kaner, R. B. J. Am. Chem. Soc. 2004, 126, 851. (18) Huang, J.; Kaner, R. B. Angew. Chem., Int. Ed. 2004, 43, 5817. (19) Huang, J.; Virji, S.; Weiller, B. H.; Kaner, R. B. J. Am. Chem. Soc. 2003, 125, 314. (20) Li, D.; Kaner, R. B. Chem. Commun. 2005, 3286. (21) Huang, J.; Kaner, R. B. Chem. Commun. 2006, 367. (22) Shim, J.-W.; Park, S.-J.; Ryu, S.-K. Carbon 2001, 39, 1635. (23) Jia, Z.; Wang, Z.; Liang, J.; Wei, B.; Wu, D. Carbon 1999, 37, 903. (24) Shaffer, M. S. P.; Fan, X.; Windle, A. H. Carbon 1998, 36, 1603. (25) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95. (26) Kovtyukhova, N. I.; Mallouk, T. E.; Pan, L.; Dickey, E. C. J. Am. Chem. Soc. 2003, 125, 9761. (27) Li, Y. H.; Xu, C.; Wei, B.; Zhang. X.; Zheng, M.; Wu, D.; Ajayan, P. M. Chem. Mater. 2002, 14, 483. (28) Yu, R.; Chen, L.; Liu, Q.; Lin, J.; Tan, K.; Ng, S. C.; Chan, H.; Xu, G.; Andy, Hor, S. T. Chem. Mater. 1998, 10, 718. (29) Tan, P. H.; Zhang, S. L.; Yue, K. T.; Huang, F.; Shi, Z.; Zhou, X.; Gu, Z. J. Raman Spectrosc. 1997, 28, 369. (30) Hiura, H.; Ebbesen, T. W.; Tanigaki, K.; Takahashi, H. Chem. Phys. Lett. 1993, 202, 509. (31) Dresselhaus, M. S.; Dresselhaus, G.; Jorio, A.; Souza Filho, A. G.; Saito, R. Carbon 2002, 40, 2043.

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