Controlled Attachment of Ultrafine Platinum Nanoparticles on

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Controlled Attachment of Ultrafine Platinum Nanoparticles on Functionalized Carbon Nanotubes with High Electrocatalytic Activity for Methanol Oxidation Aditi Halder,† Sudhanshu Sharma,‡ M. S. Hegde,‡ and N. Ravishankar*,† Materials Research Center, and Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India ReceiVed: August 14, 2008; ReVised Manuscript ReceiVed: NoVember 25, 2008

Platinum nanoparticles with average diameters of 2-3 nm have been coated on multiwalled carbon nanotubes by controlled attachment of an intermediate on the walls of the nanotube. Reduction of the intermediate leads to the formation platinum nanoparticles which are uniformly coated on the nanotubes. This composite shows excellent electrocatalytic activity toward methanol oxidation and hence is a candidate material for fuel cell applications. Cyclic voltammetry and chronoamperometry studies indicate that these composites have a better electrocatalytic activity for methanol oxidation compared with a commercial Pt-carbon mixture with similar Pt loading. The intermediate approach for the formation of hybrids is general and can be extended to other systems. Introduction The environmental issues related to the increased use of fossil fuel and the pressure on the available fossil fuel reserves are important factors that have influenced the development of fuel cells.1,2 Fuel cells are galvanic cells in which the free energy of a chemical reaction is converted into electrical energy. In direct methanol fuel cells (DMFC), methanol is electrochemically oxidized directly without the intermediate step of converting alcohol into hydrogen. One of the principal advantages of using fuel cells for energy over conventional fossil fuels is the reduced release of harmful emission. The cheap resource and easy transportability make methanol a potential material in the application of portable power sources. However, issues like methanol crossover, poor reaction kinetics, and CO poisoning at the cathode are still problems that need to be overcome. Several studies over the past decade have been carried out for the development of better catalysts for methanol oxidation.3-6 Metallic platinum has been undisputedly considered as the best catalyst for methanol oxidation. However, problems related to CO poisoning associated with the platinum electrode implies that pure Pt may not be the best option. To improve the catalytic activity of platinum, Pt-Ru alloys have been used which show an increased electrocatalytic activity compared with pure platinum electrode.7-9 The catalytic activity of Pt-Ru proceeds mainly by the bifunctional mechanism10,11 whereby Ru adsorbs oxygen-containing species like CO at a lower potential compared with pure platinum and enables the removal of CO poisoning by converting CO into CO2. Carbon nanotubes (CNTs) are one of the most promising materials for a variety of applications.12 In particular, they are considered as good catalyst supports for direct methanol fuel cells.13-20 The substitution of carbon black particles with multiwalled CNTs as the catalyst support material improves the performance in DMFCs.4,21 While there are several methods available for decorating CNTs with Pt nanoparticles, controlling * Corresponding author. Phone: 91-80-2293 3255. Fax: 91-80-2360 7316. E-mail: [email protected]. † Materials Research Center. ‡ Solid State and Structural Chemistry Unit.

Figure 1. FTIR spectra of the functionalized carbon nanotubes.

the dispersion and the stability has been an important issue. The importance of surface modification of CNTs with the functional groups by various methods1,22-29 has been pointed out in several studies. Generating defects on the wall of the CNTs by treatment with strong HNO3, KMnO4, or other strong oxidizing agents is important as those defects can serve as anchor groups for functionalization and/or can provide sites for the coordination chemistry. The attachment of metal catalyst on the wall of CNTs can improve the electrocatalytic activity of the catalyst drastically. Recently, Yang et al. have modified the surfaces of CNTs by thiolation prior to the deposition of Pt nanoparticles.30 Molecular linker in the form of benzyl mercaptan was used to bind the Pt nanoparticles to the CNT walls. Another approach is to derivatize CNTs with nanoparticles synthesized in situ by metal-ion reduction during functionalization by microwave radiation.27 Electrodeposition of Pt particles has the advantage of being a simple method and yielding high purity deposits.31-34 However, the main problem is the concurrent reduction of H+ during the electrodeposition process. The loading mass of the metallic catalyst is not easy to estimate according to the deposition charge. The problems

10.1021/jp8072574 CCC: $40.75  2009 American Chemical Society Published on Web 01/06/2009

Attachment of Ultrafine Platinum Nanoparticles

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Figure 2. (a) C1s core-level spectrum and (b) O1s core-level spectrum observed in the XPS spectra of the functionalized CNTs. The inset spectra show the same for nonfunctionalized CNT.

related to most of the methods are either the sparse coating of metal particles or the formation of aggregates on the nanotube walls. Deposition using supercritical carbon dioxide medium has been shown to be a versatile technique for coating a variety of metals on multiwalled nanotubes.5,35-37 Hydrogen reduction of metal precursors in supercritical carbon dioxide results in a uniform deposition of metal catalyst nanoparticles. The better adhesion and stabilization of metal nanoparticles on functionalized CNTs is attributed to the strong interaction between the functionalized groups on CNTs and the metal particles and to the zero surface tension of supercritical carbon dioxide which gives good wetting. Here, we demonstrate a novel method of attaching ultrafine platinum nanoparticles of 2-3 nm in diameter on the walls of functionalized, multiwalled CNTs. The method is based on controlled attachment of a precursor phase on the walls of the nanotube. The intermediate contains Pt in a nonzero oxidation state and can be effectively distributed uniformly on the tube walls. Thus, subsequent reduction of the intermediate ensures uniform distribution of the metal on the walls of the nanotube. Except for the functionalization of CNTs, the complete experiment is carried out in an organic medium.

Figure 3. TEM micrograph of (a) platinum intermediate, (b) platinum nanoparticles synthesized by reducing the intermediate, and (c) the platinum intermediate attached on the CNTs.

Experimental Section Carbon nanotubes (Sigma-Aldrich) having diameters between 110 to 170 nm and lengths of 5-9 µm are functionalized by sonicating them in the mixture of concd HNO3 and concd H2SO4 (1:3 ratio) for 1 h. These functionalized CNTs are washed several times with distilled water, centrifuged, and dried under vacuum. FTIR studies are performed to confirm the generation of functionalized group on the wall of CNTs. In our earlier work, we have reported38 a crystalline cubic intermediate of gold(I) generated by the reduction of HAuCl4 · 3H2O by oleyl amine in toluene medium that was subsequently used to produce ultrafine Au nanowires.39 A similar protocol has been used to prepare the platinum intermediate. In a typical experiment, 30 mg of hexachloroplatinic acid is mixed with 400 µL of oleyl amine and 200 µL of oleic acid in 50 mL of dry toluene. The mixture is refluxed at 120 °C for

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Figure 5. Cl2pcore level XPS spectra from the CNT-Pt intermediate. Figure 4. Pt4f core level XPS spectra of (a) the CNT-Pt intermediate and (b) the CNT-Pt nanocomposites after reduction.

different times (typically 1 to 6 h) under vigorous stirring. The solution turns pale yellow and is then cooled to room temperature. To synthesize platinum nanoparticles from the intermediate platinum solution, 15 mg of sodium borohydride in 50 mL of methanol is dissolved, and this borohydride containing methanol solution is directly added into the intermediate solution under vigorous stirring. The product is centrifuged, cleaned in methanol, and dried for characterization. In order to obtain platinum nanoparticles attached with CNTs, the functionalized CNTs are added to the platinum intermediate solution and sonicated for 3 min. After sonication, the solution with CNTs is stirred for 18 h. In the next step, 20 mg of NaBH4 is dissolved in 50 mL of methanol, and the solution is slowly added to this mixture. The resultant mixture is stirred for 4 h. The platinum-coated CNTs settle at the bottom of the reaction medium. The product is centrifuged, washed several times with methanol, and dried under vacuum, and the powder is used for all subsequent characterization. The platinum-coated CNTs are dispersed in hexane, drop-cast on a carbon-coated Cu grid, dried for several hours, and used for transmission electron microscopy investigation. The platinum-CNTs nanocomposites are characterized by powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). XPS data are collected in a ThermoScientific Multilab 2000 instrument, and the binding energies are with respect to graphitic C1s at 284.5 eV. The binding energy is accurate to within (0.1 eV. Transmission electron microscopy (TEM) is done on Tecnai F30 TEM operated at 300 kV. The electrochemical characterization of the Pt-coated CNTs is carried out using cyclic voltammetry (CV) in a conventional three electrode system (PG26250, Techno Science Instrument Bangalore) where SCE (saturated calomel electrode) is used as the reference electrode, and a platinum foil is used as the counter electrode. Preparation of Electrode for Electrochemical Studies. Seven milligrams of Pt-CNT nanocomposites is mixed with 14 mg of Nafion, and isopropyl alcohol is added to the mixture followed by the sonication for half an hour to make a catalyst ink. A known amount of the resulting catalyst ink is coated on a 1 cm × 1 cm area on a graphite sheet. The electrodes are dried and weighed after drying. Each electrode has 2 mg of Pt-CNT nanocomposites. From the gravimetric analysis, we

have calculated the platinum content in the nanocomposites, and it is estimated that the Pt content of individual electrodes is about 0.56 mg/cm2. The geometric surface area of the electrode is taken for calculating the current density. Results and Discussion For the preparation of Pt-CNT nanocomposites, the functionalization of CNTs is an important step. The attachment of Pt nanoparticles on nonfunctionalized CNTs is found to be insignificant. Not only is the surface functionalization important for the attachment of nanoparticles, but also the catalytic activity is related to the active sites on the carbon surfaces.25 By increasing the number of oxygen containing functional groups (defects) on the carbon surface by several different ways of oxidative treatment, the properties of carbon-supported metal catalysts can be tuned.15,23,27,28 An acid mixture of concd H2SO4 and concd HNO3 (3:1) ratio gives very good surface functionalization on the wall of CNTs40 and has been used in the present work. The FTIR spectra of the functionalized CNTs are given in Figure 1. The FTIR data clearly indicates the characteristic features of the oxygen containing groups present in the functionalized CNTs. The presence of peaks at 3547-3417 cm-1 and at 1188-1100 cm-1 show the generation of hydroxyl groups while the peaks around 1705 cm-1 and at 1570-1470 cm-1 indicate carboxylate groups. The strong peaks around 2917-2633 cm-1 are indicative of the presence of aliphatic -CH2 groups. From the FTIR data, the presence of functional groups on the CNTs is confirmed. X-ray photoelectron spectroscopic investigation reveals the exact surface composition/nature of the functionalized CNTs. We have carried out XPS measurements on the untreated CNTs and functionalized CNTs. The presence of hydroxyl, carbonyl, and carboxylic groups has been confirmed by the C1s and O1s peaks observed in XPS spectra which are shown in Figure 2. Figure 2a shows the curve fitted C1s peak. The main peak at 284.5 eV is due to the C1s of the graphitic carbon. The shoulder of the main peak is composed of three peaks which can be attributed to the C1s of hydroxyl carbon (286.1 eV), C1s of carbonyl carbon (287.7 eV), and C1s of carboxyl carbon (289.1 eV). Figure 2b shows the presence of oxygen in hydroxyls at 532.1 eV which is slightly higher at value and the oxygen atoms in carboxyl groups at 534.6 eV in O1S spectra. The presence of



Figure 6. (a) Dark-field TEM image of platinum nanoparticles on the wall of CNTs; bright-field TEM images of the platinum nanoparticles on (b) the wall and (c) end-cap of the CNTs (d) lattice image of the Pt-CNT nanocomposites.

a very small amount of oxygen of carboxylate group is detected in as-received CNTs, but in functionalized CNTs, the O1s peak is much broader and higher in intensity. This clearly confirms the effect of acid treatment on the CNTs to generate functional groups. In the nonfunctionalized CNTs, the presence of small amount of oxygen has been detected because of adsorbed oxygen species. This is consistent with the earlier reported works.14,41 The second step of the nanocomposite preparation is the preparation of the platinum intermediate/precursor solution. The starting metal salt chloroplatinic acid (H2PtCl6 · 6H2O) is insoluble in toluene. On refluxing with the oleylamine and oleic acid in toluene medium at 120 °C for 6 h, a pale yellow colored transparent solution is obtained. This transparent yellow color solution is the precursor solution for the synthesis of platinum nanoparticles as well as for the preparation of CNT-Pt nanocomposites. Transmission electron microscopy reveals that precursor solution contains a crystalline cubic intermediate (Figure 3a; similar to the gold(I) intermediate already reported in our previous work38). By adding sodium borohydride containing methanol solution to the platinum intermediate solution, highly monodisperse platinum nanoparticles of 2-3 nm diameter can be synthesized (Figure 3b). The functionalized CNTs and the platinum intermediate are mixed together in toluene medium with constant stirring for several hours. The transmission electron microscopy shows that this resulted in the attachment of platinum intermediates on the wall of the functionalized CNTs (Figure 3c). Experimental evidence indicates that better attachment of these cubic Pt precursors on the wall of CNTs is the key step for the good coating of platinum nanoparticles on them. Hence, understanding the cubic Pt intermediate deposited on the functionalized on CNTs is vital.

Figure 7. XRD pattern of (a) the functionalized CNT and (b) CNT-Pt nanocomposites.

XPS analysis indicates the presence of both +4 and +2 oxidation state of platinum on the intermediate-coated CNTs. Figure 4a shows the Pt4f core level XPS spectrum of the intermediate-coated CNTs. The spectrum shows a doublet with an energy band (4f7/2) at 73.25 eV and one at 76.45 eV (4f5/2) corresponding to the +2 oxidation state. The energy bands at 75.5 and 78.85 eV correspond to the +4 oxidation state of Pt


Halder et al.

Figure 8. (a) TGA-DTA data of the Pt-CNT nanocomposites and (b) XRD pattern of the residue obtained after the TGA-DTA analysis.

consistent with the NIST online database. We believe that the +2 oxidation state corresponds to the cube-shaped structures seen in the images while the +4 state could be due to the presence of unreduced H2PtCl6 or the formation of some other intermediate with the same oxidation state (PtCl4, for instance). The cubic intermediates, after attachment on the CNT walls, are reduced by the methanol solution of sodium borohydride. After reduction, the 4f band corresponding to metallic Pt is seen, as shown in Figure 4b. The presence of Cl- group in the XPS spectra of the precursor coated CNTs suggests that the intermediate may be a chloride salt of Pt (Figure 5). The peaks obtained at 198.32 eV and 199.9 eV are due to Cl2p1/2 and Cl2p3/2 respectively which is attributed to Cl-.42,43 The reduced CNT-Pt nanocomposites show no trace of chlorine. Surprisingly, no nitrogen peak has been detected in the intermediate-CNTs sample. This clearly rules out the formation of platinum-amine complex as the precursor/ intermediate. We are currently undertaking investigation to understand the nature and composition of this intermediate precursor of platinum.

The formation of uniform coating of nanoparticles on the nanotubes is primarily due to the attachment of the intermediate cubes on the tube walls. The nonfunctionalized CNTs do not show appreciable attachment of platinum intermediate, and hence, the attachement of platinum nanoparticles on the nonfunctionlized CNTs is found to be less. Another indirect evidence of the mechanism of platinum nanoparticles coating on the CNTs comes from the fact that mixing nanoparticles with the CNTs does not lead to attachment of the particles on the walls. Hence, we can conclude that the attachment of platinum intermediate on the CNTs is the critical step for the formation of uniformly coated nanotubes. Figure 6a shows the dark field image of the platinum nanoparticles on the wall of CNTs and clearly shows the crystalline nature of these 2-3 nm size platinum nanoparticles. Figure 6b shows the bright field image of nanoparticles on the end caps of CNTs. The functionalized carbon nanotubes shown in the image have a diameter of 110 nm and length of 5-9 µm. Figure 6c shows the platinum nanoparticles on the side walls of CNTs. The TEM micrographs clearly indicate that the



Figure 9. (a) Cyclic voltammograms of of Pt-CNT nanocomposites in 1.0 M H2SO4, (b) cyclic voltammograms of of Pt-CNT nanocomposites in the mixture of 1.0 M H2SO4 and 1.0 M methanol, (c) cyclic voltammograms of the commercial Pt/C catalysts in the mixture 1.0 M H2SO4 and 1.0 M methanol, and (d) chronoamperometry measurement of (i) the Pt-CNT nanocomposites and (ii) commercial Pt/C catalysts (charge consumed in 2000 s).

platinum nanoparticles are well-distributed on the carbon nanotubes. The high resolution electron microscopy confirms the crystalline nature of the carbon nanotubes (Figure 6d). The lattice fringes of the carbon nanotubes are clearly noticeable and the d spacing is 0.341 which corresponds to the (002) planes of graphitic carbon. The crystalline nature of the Pt-coated CNT sample is confirmed by X-ray diffraction studies. Figure 7 is powder XRD patterns of functionalized multiwalled CNTs and platinumcoated carbon nanotubes, respectively. The diffraction peaks at 26.16°, 42.7°, 54.12°, and 77.9° observed in the diffraction of MWNTs can be attributed to the hexagonal graphitic planes (002), (100), (004), and (110) (Figure 7a) indicating the crystallinity of the functionalized nanotubes. The pattern from the Pt-CNT nanocomposites (Figure 7b) show all of the major peaks of platinum at 39.863 (111), 45.065 (200), 67.76 (220), and 81.649 (311) along with the peaks due to the CNTs. The XRD patterns are in excellent agreement with that of the reference pattern ((JCPDS 75-1621) for graphitic carbon and (JCPDS 87-0646) for platinum. From the thermogravimetric analysis, the weight of the platinum content in the Pt-CNT nanocomposite is measured. The TGA experiment was carried out in oxygen atmosphere under the flow rate of 100 mL/min and heating rate of 10 °C/ min from room temperature to 800 °C. The observed charac-

teristic peaks around 150 and 550 °C are due to the removal of organic molecules/capping agents present in the material and the decomposition of CNTs, respectively. The complete decomposition of CNTs into CO2 at 650 °C is utilized to calculate the accurate loading of Pt on CNTs. From the TGA curve (Figure 8a), it has been calculated that the amount of residue, platinum in this nanocomposite, was 28% by weight. We have carried out X-ray diffraction on the residue obtained after TGADTA to confirm the presence of platinum. The pattern (Figure 8b) corresponds to FCC Pt with no evidence for any oxide/ compound formation. The absence of other peaks in the X-ray diffraction data helps in the direct quantification of platinum content in the platinum-CNTs nanocomposites from the TGA data. The broadening in X-ray peaks is reduced which clearly indicates the coarsening of the platinum nanoparticles due to the thermal treatment up to 800 °C during thermogravimetric analysis. The absence of peak for carbon nanotubes clearly indicates the process of combustion is complete at these temperatures. Electrochemical Activity of Pt-CNT Nanocomposites. The electrocatalytic activity of Pt-CNT nanocomposites is investigated out by cyclic voltammetry (CV) and chronoamperometry measurements for methanol oxidation.The cyclic voltammetry measurement is a convenient and efficient tool for the characterization of electrocatalytic activity of different materials. The


Figure 10. Pt4f core level XPS measurement has been done on the CNT-Pt nanocomposites after cyclic voltammetry measurements.

redox electrocatalytic activity of the Pt-CNT nanocomposites is measured in the 1.0 M H2SO4 in the range of voltage -0.3 to 1.2 V with scan rate of 40 mV s-1. The cyclic voltammograms (Figure 9a) shows usual Pt nanoparticle behavior in acid medium.44 The peaks clearly indicate the weak and strong adsorption/desorption of hydrogen on the platinum nanoparticles attached on CNTs. We did not carry out any heat treatment for CNT-Pt nanocomposites to avoid the decomposition of functionalized groups on the CNTs surfaces. For the same reason, CNT-Pt nanocomposites have been directly used in the methanol oxidation. The cyclic voltammetry experiments for methanol oxidation over the metal-CNT nanocomposites have been done in 0.5 M H2SO4 and 0.5 M methanol at a scan rate of 80, 40, 20, 10 mV/s, respectively. Figure 9b shows the volatammogram taken at a scan rate of 40 mV/s. The activity of metal-CNT nanocomposites has been compared under similar condition with the conventional Pt/C catalyst where platinum is dispersed over high surface area carbon (Figure 9c). In the cyclic voltammetry of methanol oxidation, the forward anodic peak is associated with the oxidation of methanol, and the reverse anodic peak is related with the oxidation of carbonaceous species (like CO). The ratio of forward anodic peak current (If) to reverse anodic peak current (Ib) can be employed to evaluate catalyst tolerance to carbonaceous species. Higher the value of If/Ib, tolerance to carbonaceous species will be more. Although, from the cyclic voltammograms of the Pt-CNT nanocomposites shows less current than that of the commercial Pt/C catalyst and Pt-coated CNT obtained by supercritical CO2 method,5 the If/Ib ratio for the nanocomposites is found to be 1.5 times more than that of the commercial Pt/C. This higher If/Ib ratio of Pt-CNT nanocomposites suggests that it has less carbonaceous accumulation and hence is much more tolerant toward CO poisoning. The steady state experiments (chronoamperometry, Figure 9d) also suggest that the activity (charge consumed in 2000 s) of metal-CNT nanocomposites is almost three times that of commercial Pt/C. The presence of a functional group like -COOH and -OH groups is actually responsible for better electrocatalytic activity of Pt-CNT nanocomposites. Recently, it has been reported that those functional groups of CNTs can improve the electrocatalytic activity of Pt and they can play the role of Ru as secondary catalyst.24 The oxygen containing surface defects may be believed to be responsible for the better catalytic activity.45 We

Halder et al. also have come to the same inference that the electrochemical activity of these Pt-CNT nanocomposites shows the improved catalytic activity than the commercial catalyst due to the functional groups on CNTs. The increasing electrocatalytic activity due to the surface defects of the CNTs are already discussed in literature.15,24,25 Our experimental results of cyclic voltammetry study also support the role of surface defects (functionalized groups) on the CNTs. These defects on the wall of CNT, to some extent, play the role of Ru in the case of Pt-Ru catalyst. The decrease in the reverse anodic peak current (Ib) in the case of Pt-CNT nanocomposites is attributed to lesser accumulation of carbonaceous species like CO that could lead to poisoning of the catalyst. The hydroxyl and carboxyl groups on the functionalized CNTs play a major role to remove those carbonaceous species and contribute to the low catalyst poisoning in the Pt-CNT nanocomposites. The XPS measurement, which was carried out for the Pt-CNT nanocomposite after 200 cycles of the cyclic voltammetric measurement, reveals that there is no difference in the metallic state of Pt confirming their long-term stability and electrocatalytic activity (Figure 10). Conclusion In this article, we have reported a novel method of synthesis of platinum nanoparticles and attachment of platinum nanoparticles of 2-3 nm size on the functionalized CNT. This is an efficient method to attach platinum nanoparticles on the CNT surfaces. The Pt-CNT nanocomposites show good electrocatalytic activity toward methanol oxidation and hence may be useful for the low temperature DMFCs. It was found that the functionalization of CNTs significantly enhances the electrocatalytic activity of Pt nanoparticles and plays a major role in decreasing catalyst poisoning. Acknowledgment. A.H. thanks the Council of Scientific and Industrial Research (CSIR) for senior research fellowship. N.R. acknowledges financial support through the NSTI programme of DST. The Tecnai F30 microscope is a part of the National Electron Microscopy Facility, Institute Nanoscience Initiative, IISc. The XPS is a part of the Institute Surface Science Facility at the Indian Institute of Science. References and Notes (1) Linda Carrette, K. A. F. U. S. ChemPhysChem 2000, 1, 162. (2) Wasmus, S.; Kuver, A. J. Electroanal. Chem. 1999, 461, 14. (3) Rajesh, B.; Ravindranathan Thampi, K.; Bonard, J. M.; Xanthopoulos, N.; Mathieu, H. J.; Viswanathan, B. J. Phys. Chem. B 2003, 107, 2701. (4) Wang, C.; Waje, M.; Wang, X.; Tang, J. M.; Haddon, R. C.; Yan, Y. S. Nano Lett. 2004, 4, 345. (5) Lin, Y.; Cui, X.; Yen, C.; Wai, C. M. J. Phys. Chem. B 2005, 109, 14410. (6) Liu, Y. G.; Shi, S. L.; Xue, X. Y.; Zhang, J. Y.; Wang, Y. G.; Wang, T. H. Appl. Phys. Lett. 2008, 92, 203105. (7) Chrzanowski, W.; Wieckowski, A. Langmuir 1998, 14, 1967. (8) Park, K. W.; Sung, Y. E. J. Phys. Chem. B 2005, 109, 13585. (9) Roth, C.; Benker, N.; Theissmann, R.; Nichols, R. J.; Schiffrin, D. J. Langmuir 2008, 24, 2191. (10) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 275. (11) Kua, J.; Goddard, W. A. J. Am. Chem. Soc. 1999, 121, 10928. (12) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (13) Li, W.; Liang, C.; Qiu, J.; Zhou, W.; Han, H.; Wei, Z.; Sun, G.; Xin, Q. Carbon 2002, 40, 791. (14) Liu, Z.; Lin, X.; Lee, J. Y.; Zhang, W.; Han, M.; Gan, L. M. Langmuir 2002, 18, 4054. (15) Guha, A.; Lu, W.; Zawodzinski Jr, T. A.; Schiraldi, D. A. Carbon 2007, 45, 1506. (16) Li, L.; Xing, Y. J. Phys. Chem. C 2007, 111, 2803. (17) Maiyalagan, T. Appl. Catal., B 2008, 80, 286.

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