Oxidation, Deformation, and Destruction of Carbon Nanotubes in

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J. Phys. Chem. B 2005, 109, 1400-1407

Oxidation, Deformation, and Destruction of Carbon Nanotubes in Aqueous Ceric Sulfate John H. T. Luong,*,† Sabahudin Hrapovic,† Yali Liu,† De-Quan Yang,‡ Edward Sacher,‡ Dashan Wang,§ Christopher T. Kingston,| and Gary D. Enright| Biotechnology Research Institute, National Research Council Canada (NRCC), Montreal, Quebec, Canada H4P 2R2, Regroupement Que´ be´ cois de Mate´ riaux de Pointe and Department of Engineering Physics, Ecole Polytechnique, Montreal, Quebec, H3C 3A7, Institute of Chemical Process and EnVironmental Technology, NRCC, Ottawa, Ontario, K1A 0R6, and Steacie Institute of Molecular Sciences, NRCC, Ottawa, Ontario, K1A 0R6 ReceiVed: October 6, 2004; In Final Form: NoVember 17, 2004

A simple wet chemical method involving only ultrasonic processing in dilute ceric sulfate (CS) was used to functionalize carbon nanotubes (CNTs). Unexpectedly, single-walled and multiwalled carbon nanotubes (SWCNTs and MWCNTs) were cut, oxidized, and disintegrated by sonication in 0.1 N CS for 2-5 h. Transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction spectroscopy (XRD), Raman scattering, and photoacoustic Fourier transform infrared spectroscopy (FTIR) were used to probe wall damage during the chemical processing. Cyclic voltammetry and impedance spectroscopy were used to evaluate the conductivity of the CS-treated CNTs. This one-step process resulted in the destruction of SWCNTs to produce nonconducting amorphous carbon. MWCNTs were oxidized and converted to graphitic materials and amorphous carbon with retained conductivity.

Introduction Since their discovery in 1991,1,2 carbon nanotubes (CNTs) have attracted remarkable attention because of their extraordinary properties and potential applications in various fields.3 With extremely high mechanical strength and chemical stability, CNTs represent attractive possibilities for developing ultrastrong composite materials. Indeed, it is generally believed that CNTs are extraordinarily stable and almost impervious to radiation and chemical destruction. Single-walled CNTs, particularly, are entangled by van der Waals forces to form a dense, robust, network structure. Considerable attention has been devoted to improving the solubility and processability of CNTs. Fluorinating CNTs, followed by elevated heating (1000 °C), can cut the tubes into shorter segments (20-300 nm).4 Sonicating CNTs in a mixture of concentrated sulfuric and nitric acids (3:1, 98% and 70%, respectively) results in short, open CNT pipes (100300 nm) with acidic functional groups on their ends and walls.5 SWCNTs become more soluble via functionalization with longchain amines6a or diazonium salts.6b SWCNTs can be oxidized by sonicating in concentrated H2SO4 containing (NH4)2S2O8 and P2O5, followed by H2SO4 and KMnO4, yielding individual oxidized tubes, 40-500 nm long.7 Despite all of these harsh and destructive treatments, CNTs generally come out relatively unscathed with their tubular structure intact and only partial exfoliation of the ropes can be obtained. Nanotubes can be converted to a diamond-like super-hard material only under high temperature and high-pressure treatment.8 We also note that vigorous sonication in CH2Cl2 can damage the sides of MWCNTs and the extent of structural deformation is solvent* Corresponding author. E-mail: [email protected]. † Biotechnology Research Institute, NRCC. ‡ Ecole Polytechnique. § Institute of Chemical Process and Environmental Technology, NRCC. | Steacie Institute of Molecular Sciences, NRCC.

dependent.9 In the presence of strong electric fields, carbon nanotubes decay by unraveling monatomic carbon “wires” from the exposed end.10 Molecular dynamics simulation further predicts that, under high temperature and, in the presence of defects, the tubes exhibit mainly plastic deformation, with the appearance of medium sized chains as the final stage before fracture.11 Carbon exhibits a very rich dynamics of bondbreaking and bond-reconstruction, which permits transformations from fullerenes to tubes to chains. A carbon nanotube can collapse radially into a flattened tube with bulbs on either edge,1 and radiation damage destroys the circumferential periodicity and transforms a collapsed edge-on tube into a multilayered graphitic ribbon.12 In the course of nanotube functionalization, we report here the unexpected transformation of CNTs to nanocrystalline materials and amorphous carbon and, most surprisingly, the complete destruction of SWCNTs. This process involves only treatment in dilute aqueous ceric sulfate (CS), 0.1 N Ce(SO4)2, with sonication at ambient temperature for 2-5 h. As a strong oxidizer and the only nontrivalent lanthanide ion stable in solution, CS has been used to oxidize inorganic and organic compounds.13a,b To our knowledge, the Ce (IV) oxidation of CNTs has not been attempted, and we note that the first example of the Ce(IV) oxidation of malonic acid was first investigated as early as 1930.13c Experimental Section SWCNTs (0.79-1.2 nm outer diameter) and MWCNTs (hollow structure carbon nanotubes with purity >95%, outer diameter 15 ( 5 nm, length 1-5 µm) were purchased from Carbon Nanotechnologies (Houston, TX) and NanoLab (Newton, MA), respectively. Commercial SWCNTs are produced by a modified gas phase process and purified to remove large catalyst particles with the purity of about 90% (http://

10.1021/jp0454422 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/08/2005

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Figure 1. Typical TEM micrographs, showing the morphology of SWCNTs: (a) CS-treated for 15 min, (b) an enlarged view of Figure 1a, (c) CS-treated for 2 h, (d) an enlarged view of Figure 1c, (e) HRTEM for SWCNTs after 5 h of sonication in CS, and (f) a typical AFM image of SWCNTs after 5 h of sonication in CS. The inset is a SWCNT image obtained after 5 h of sonication in DMF.

cnanotech.com). Ceric sulfate (CS, 0.1 N) was obtained from Fisher (Fair Lawn, NJ). Poly(diallyldimethylammonium chloride) (PDDA, average MW ∼ 200-350 kDa, 20 wt. %) was obtained from Aldrich (Milwaukee, WI). Transmission electron microscopy (TEM) data were recorded using a Philips CM20 200 kV electron microscope (Hillsboro, OR) equipped with an Oxford Instruments energy-dispersive X-ray spectrometer (Link exI and II) with an Ultrascan 1000 CCD camera. Samples were dropped onto porous carbon films on 300 mesh copper grids, and allowed to dry in air. A Hitachi 9000 R high-resolution TEM (HRTEM), with energy-dispersive X-ray spectroscopy, operating at 300 kV, was used. Atomic force microscopy (AFM) micrographs were obtained using a Nanoscope IV system (Digital Instruments-Veeco, Santa

Barbara, CA), using tapping mode (TM) and operating in air. A silicon AFM probe, with a cantilever length of 125 mm and a drive frequency ranging from 235 to 255 kHz, was employed for imaging. The scan speed was 0.50 Hz at 512 lines per scan. All height measurements and size distribution were obtained from the instrument analysis software package. DMF-treated CNTs were spin-coated on a piece of silicon wafer (Virginia Semiconductor, Fredericksberg, VA), whereas CS-treated CNTs were immobilized on PDDA-activated glass microscope slides, using the procedure described by Liu et al.14 Micro-Raman spectra were collected using a custom-made spectrometer, consisting of a Leitz Metalux 3 microscope and a SPEX 1877c Triplemate monochromator fitted with a Princeton Instruments CCD detector (model LN/ CCD-1340/

1402 J. Phys. Chem. B, Vol. 109, No. 4, 2005 100-EB/1). The laser was a Coherent I-308C argon ion laser operating at 514.5 nm and attenuated to give 2-5 mW at the sample. X-ray photoelectron spectroscopy (XPS) was carried out using a VG ESCALAB 3 Mark II (Thermo VG Scientific), using nonmonochromated Mg KR X-rays (1253.6 eV). The base pressure in the analysis chamber was less than 10-10 Torr. Highresolution spectra were obtained at a perpendicular takeoff angle, using a pass energy of 20 and 0.05 eV steps. The instrument resolution was ∼0.7 eV. After Shirley background removal, the component peaks were separated by the VG Avantage software. Drops of solution containing the SW- and MWCNT samples for XPS analysis were placed onto substrates and dried in air. The substrates were Si wafers, onto which 10 nm Ti, followed by 100 nm Au, were evaporated by electron beam heating at a deposition rate of 0.5 A/s, under high vacuum. The Au4f7/2 peak (84.0 eV) was used for calibration. The X-ray diffraction patterns of pristine, DMF- and CStreated CNTs were obtained on a Scintag ×2 powder diffractometer (Cupertino, CA) using graphite-monochromatized CuKR radiation (1.5405 Å) with a θ-θ scan mode. In this geometry, the sample was fixed and the scattering vector was normal to the surface of the powder layer. Photoacoustic Fourier transform infrared spectroscopy (FTIR) was carried out using a He-purged MTEC 300 photoacoustic cell in a Bio-Rad FTS 6000 spectrometer (Cambridge, MA) at a resolution of 4 cm-1. The 5 kHz modulation frequency used probed the entire sample thickness. The samples were prepared for analysis by dropping and drying, as was done for XPS, onto freshly cleaned Si wafers. Cyclic voltammetry measurements were performed using an electrochemical analyzer coupled with a picoamp booster and Faraday cage (CHI 601A, CH Instruments, Austin, TX). AC impedance analysis were performed with an EG&G potentiostat (Perkin-Elmer, formally EG/G, Princeton Applied Research, Model 6310, Oak Ridge, TN). All electrolyte solutions were purged for 20 min in argon prior to the measurement, and a blanket of argon was maintained over the solutions during the measurement. In both cases, a Pt wire (Aldrich, 99.9% purity, 1 mm diameter) and an Ag/AgCl, 3 M NaCl (BAS, West Lafayette, IN) electrode were used as counter and reference electrodes, respectively. The data were measured and collected for 31 harmonic frequencies from 0.1 Hz to 100 kHz at 5 steps/ decade, then was analyzed using ZSimpWin software (Princeton Applied Research, Oak Ridge, TN). Samples containing 1 mg/mL of MWCNTs or SWCNTs were sonicated in DMF or 0.1 N aqueous CS, at room temperature. Resulting samples were centrifuged (4000 rpm, 20 min), and the DMF- or CS-containing supernatants were removed. The CNTs were redispersed in water and centrifuged again. To completely remove DMF or CS from the samples, this procedure was repeated 10 times. Both the UV spectra (peak at 264 nm) and cyclic voltammetry in phosphate buffer did not reveal any ceric sulfate peaks (CS is electroactive). Such purified carbon nanomaterials were immobilized on the glassy carbon electrode by drying 8 µL drops at room temperature for 1 h, using thin Nafion film. The active surface area of the DMF and ceric sulfate-treated MWCNT and SWCNT-modified GC electrodes was determined by steady-state cyclic voltammetry in a solution of 20 mM K4Fe(CN)6 (99% purity, A&C Ltd, Montreal, QC, Canada), with 0.2 M KCl as the supporting electrolyte. Results and Discussion Deformation and Disintegration of CNTs. During the tube formation, there are some defects of the six-membered-ring

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Figure 2. CNT samples were prepared by sonication in DMF or ceric sulfate, centrifuged and resuspended in water. From left to right: 1st SWCNT (in DMF), 2nd MWCNT (in DMF), 3rd SWCNT (in ceric sulfate), 4th MWCNT (in ceric sulfate). Samples treated in DMF precipitated rapidly upon redispersion in water (2-3 min), whereas those treated in ceric sulfate remained in solution for several weeks (the pictures were taken after 5 days upon redispersion in water).

carbon structure of the nanotubes. In addition to an inclusion of five-or seven-membered rings in the carbon network, sp3hybridized defects in the sidewall introduce -CH and C-OH groups.6c The open ends of the tubes are often closed by catalyst particles in the crude material which can be removed by oxidative workup with HNO3 or HNO3/H2SO4. Therefore, the tube ends are largely decorated with -COOH groups.5 Such defects, however, are a promising staring point for the derivatization chemistry of CNTs. Sonication produced microscopic domains of high temperature, leading to localized sonochemistry that attacked the surfaces of the SWCNTs at defect sites, to oxidize the tube wall and/or remove C-atoms from the graphene cylinder. The mechanism for the oxidation of CNTs by CS is very complex, and it is a subject of future endeavor. It should be noted that the formation of radicals plays an important role in the oxidation of glyoxylic acid by cerium (IV).13a Highly tangled tubes were wet by CS, seen as several dark spots in Figure 1a. Only 15 min into the experiment, considerable structural damage was already observed. The tubes were cut into shorter segments, and their close examination indicated the formation of amorphous carbon (Figure 1b). Further sonication for 2 h was able to cut CNT bundles in segments (Figure 1c) and significantly reduce the population of highly tangled tubes. The inset of Figure 1c also suggests (to us) that CS penetrates the CNT bundles and reduces the van der Waals interaction between the tubes in the bundle. In general, tubes were severely damaged, as indicated by significant amounts of structural defects. These might be areas most vulnerable toward subsequent attack by CS-aided ultrasonic cavitation, converting CNTs to amorphous carbon layers (Figure 1d). After 5 h of treatment, no tube structures were observed (Figure 1e), indicating the efficiency of energy transfer during cavitation in CS, compared to other solvents or acids. The solution pH decreased from 2.26 to 1.55, indicating the liberation of H+. It should be noted that the oxidation of water by Ce4+ could also lead to the decrease of pH value (2Ce4+ + H2O f 2Ce3+ + O2 + 2H+). Carbon nanoparticles, recovered by repeated centrifugation in water and dispersed in DMF, had an average diameter of 11.2 nm (Figure 1f). By comparison, sonication in N,N-dimethylformamide (DMF) for 5 h did not inflict any noticeable damage on the tubes (Figure 1f, inset). This observation is in agreement with the results obtained by Furtado et al., who observed no evidence of damage to the tube wall structure when SWCNTs are dispersed by ultrasonication in amide

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Figure 3. AFM and TEM micrographs of MWCNTs: (a) CS-treated for 2 h, (b) an enlarged view of Figure 1a, (c) HRTEM for MWCNTs after 2 h of sonication in CS, (d) TEM imaging of DMF-treated MWCNTs with 2 h of sonication, and (inseet) a typical AFM image of MWCNTs after 2 h of sonication in DMF.

Figure 4. XPS spectra of SWCNTs: (a) pristine, (b) sonication in DMF, and (c) sonication in 0.1 N CS. XPS spectra of MWCNTs: (d) pristine, (e) sonication in DMF, and (f) sonication in 0.1 N CS.

solvents such as DMF and N-methyl-2-pyrrolidone (NMP).15 The amorphous carbon nanomaterials were very stable and well dispersed in DMF and even water for months (Figure 2). The stable colloidal suspensions were very resistant to centrifugation. In comparison with K2S2O8, CS shows much more severe

damage toward the structure of carbon nanotubes. Lian et al.16 describe a nondestructive and scaleable procedure for dissolution and purification of SWCNTs using aqueous K2S2O8. This chemical plays an important roles in modifying and unbundling SWCNTs without obvious damage to the tube structure.

1404 J. Phys. Chem. B, Vol. 109, No. 4, 2005 In comparison to SWCNTs, the much larger diameter of MWCNTs is expected to be an important factor for the varying destruction caused by CS. Nevertheless, sonication in the presence of CS was also able to cut and distort the sidewalls of multiwalled CNTs, progressing from the outer to inner layers (Figure 3a). The TEM micrographs revealed a very high level of defects, such as bending and buckling. A further effect of this treatment was noted as the outer layers were stripped off, resulting in the thinning of the nanotubes. Such thin walled areas became more vulnerable to combined ultrasound and CS attacks, giving opened areas along the sidewall and, thereby, exposing these thin tubes to new sonication-induced damage, and subsequent further cutting and de-roping. Several tube structures were severely damaged, turning them to graphite and, to a lesser extent, amorphous carbon (Figure 3b). Although some segments of MWCNTs survived the treatment, none of them remained intact and they became asymmetrical. A closer inspection of some isolated tubes indicated their apparent thickness was not uniform along the tube length, illustrating the effective etching power of CS to disrupt the C-C covalent bond. This is somewhat surprising, as many of the tubes in the interior of the bundle are considered to be protected from the acids or oxidative cleanup by the outer tubes. HRTEM imaging further revealed the presence of several dark spots which were not observed with DMF sonicated CNTs (Figure 3c). Energydispersive X-ray microanalysis (EDX) measurements confirmed that there were no metal particles such as nickel, iron, cobalt, etc. inside MWCNTs. However, as discussed later, XRD data confirmed a broad, weak peak at 44° due to trace amounts of Co-Ni particles from the pristine CNTs. Therefore, further study is need to explain the rationale behind the emergence of such dark spots. Sonication in DMF did not result in any significant tube damage, as expected (Figure 3d). However, prolonged sonication (24 h) in ethanol, at high intensity, was able to convert MWCNTs to carbon fibers.17 In this very preliminary report, the extent of conversion is not discussed. As mentioned earlier, a carbon nanotube can collapse radially into a flattened tube1 and tangential irradiation of edge-on flattened nanotubes can produce nanometer-wide, micrometerlong carbon ribbons.13 Under high temperatures and in the presence of defects, the tubes exhibit mainly plastic deformation before fracture.12 Characterization of CNTs. Chemical oxidation leads to the formation of oxygen-containing groups, such as hydroxyl, ether, quinone and carboxylate groups, on the tube walls.5,18 For HNO3 treatment, carboxylic groups have been reported as the most abundant group.18a Therefore, we first conducted XPS to characterize CS-treated CNTs. High resolution C1s X-ray photoelectron spectra (XPS) of SWCNTs are shown in Figure 4a-c for different treatments. Peak separations were carried out, on the basis of the analysis of the C1s XPS spectrum of the freshly cleaved HOPG surface.19 Component peaks are found at 284.6 eV (C1, extensively delocalized sp2-hybridized carbon), 285.6 eV (C2, defect-containing sp2-hybridized carbon) and 286.5 eV (C3, sp3 defects). The C1 π* r π shake-up (C4) appears at 291.4 eV, and that for C2 (C5) at 287.8 eV. Small amounts of oxygen contaminant (∼2-3%) are not normally visible in the C1s spectrum (Figure 4a, pristine), because they fall in the same range of energies as the C1-C5 peaks, and are difficult to distinguish; they are detected from the O1s spectrum. Larger amounts (Figure 4c, CS-treated) may be detected in the C1s spectrum, at ∼286.5 eV for alcohols (C-OH), ∼288 eV for carbonyls (aromatic, ∼287 eV; aliphatic, ∼289 eV) and ∼289.5 eV for carboxylic acids (O-CdO).20,21 The fwhm of

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Figure 5. Photoacoustic IR spectra of (A) SWCNTs and (B) MWCNTs.

the C1 component, which is associated with the electron density of the sp2-hybridized carbon skeleton, increases with treatment. Its slight increase after DMF sonication, from 1.25 to 1.35 eV for SWCNTs, suggests the creation of some mild defect, especially when compared to the severe defects indicated by the 1.75 eV fwhm on CS treatment. A similar trend was observed for MWCNTS. However, MWCNTs evince commensurately smaller changes since only the outer layer is attacked (Figure 4 d-f). The relative oxygen concentrations estimated by XPS sensitivity factors and core level intensities are summarized in Table 1. Photoacoustic FTIR spectra are shown in Figure 5. Carbon skeletal vibrations are found in untreated samples at ∼1530 and ∼1650 cm-1 (assigned to the CdO stretching mode),22 and C-Hn stretching vibrations at ∼2800-3000 cm-1. Both DMF and CS sonications cause diminutions in the skeletal vibration peaks, indicating the loss of the ability to participate in such motions. XPS indicates sonication by CS to be due to chemical oxidation; this involves the C-Hn defect structures, as they are lost on CS sonication. In addition, a carbonyl stretching vibration appears at ∼1740 cm-1, indicating extensive oxidation or the production of carboxylic acid groups22 (it is not clear whether an -OH stretching vibration appears at ∼3400 cm-1). However,

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TABLE 1: The Relative Oxygen Concentrations Were Estimated by XPS Sensitivity Factors and Core Level Intensities samples

oxygen concentration (atomic %)a

fwhm of C1 componentb

untreated SWCNTs DMF treatment CS treatment Untreated MWCNTs DMF treatment CS treatment fresh HOPG surfacec

5.9 31.6 25.9 2.4 24.2 53.4 2.5

1.25 1.35 1.75 1.20 1.25 1.30 1.1

a From XPS sensitivity factors and core level intensities. b Main component of the C1s peak; see, also, ref 18. The values are directly associated with the surface defect density, as we found there. c Highly ordered pyrolytic graphite after surface peel by tape and immediate insertion into the UHV chamber of the XPS instrument.

the C-Hn stretching vibrations are not lost in the case of DMF sonication, indicating that they do not participate in the loss of the skeletal vibrations. The mild effect of DMF sonication on the C1s XPS spectrum indicates little change in the C1-C5 components, compared to the untreated samples, although the oxygen concentration has increased markedly (Table 1). This could be due to π orbital overlap interactions, such as, e.g., between the 2A′′ π2 orbital of the DMF molecule and the 1E2utype πCC/ orbital of the sp2-hybridized carbon structure. Such interaction would make the electrons less able to participate in the skeletal vibrations, decreasing their intensities. It would also account for the high oxygen content found for the DMF-treated samples in Table 1, as well as for the N1S spectrum (not shown) that persists even in the high vacuum of the XPS instrument. A study of this behavior is in progress and will be reported on later.

The Raman spectra of the pristine and DMF-sonicated SWCNTs were very similar (Figure 6A), both showing the characteristic features of SWCNTs.23 Therefore, sonication in DMF does not introduce any significant physical or chemical modification to the tubular structure, which is consistent with the TEM and XPS/photoacoustic FTIR data. The CS-treated sample, however, manifested a substantial decrease in the intensity of the tangential G-band (stretching vibrations of sp2hybridized carbon near 1580 cm-1) relative to the disorderinduced D-band (∼1325 cm-1, attributed to defect carbon atoms) and a complete disappearance of the radial breathing modes at 150-300 cm-1. The D-band has also become significantly broader, while the G-band has taken a symmetrical line shape, indicating the reduction of long-range order in the material; these are consistent with the formation of graphitic and amorphous carbon.24 We note that similar changes in the Raman spectrum can be achieved through covalent functionalization of the nanotube sidewall while maintaining the tubular structure.7,25 However, the supporting data, particularly the TEM micrographs, clearly point to the interpretation that the changes in the Raman spectrum upon CS treatment result from a loss of tubular structure and a conversion of sp2-hybridized carbon into amorphous carbon. The interpretation of the Raman spectra for the MWNT samples (Figure 6B) is less clear since the pristine tubes (>8 nm) are too large to show any significant quantum confinement effects, and as a result, their Raman signature closely resembles those of graphite7,26 or carbon fiber.27 Nevertheless, the DMF-treated MWCNTs show an increase in D-band intensity relative to the G-band, as compared to the unprocessed material, indicating that the sonication process could introduce additional defect sites into the outer MWNT walls. A similar profile is obtained for the CS-treated MWCNTs,

Figure 6. Raman comparison of (A) the single-walled carbon nanotube (SWCNT) samples and (B) the multiwalled carbon nanotube (MWCNT) samples (the spectra have been normalized to the G-band and slightly offset for clarity). The blue traces represent the unprocessed pristine material; red traces are for nanotube samples processed by ultrasonication in DMF; green traces are from samples processed in an identical fashion but using the CS solution. The radial modes (150-300 cm-1) for the CS-treated SWCNT have completely disappeared indicating a loss of tubular structure. Also, the D-band (1350 cm-1) has become broader, and the G-band (1590 cm-1) has become weaker and symmetrical, which are both consistent with the formation of graphitic and amorphous carbons. The Raman spectrum for the CS-treated SWCNT shows no significant differences from the pristine sample.

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Figure 7. PXRD pattern of (a) SWCNT CS-treated (b) SWCNT DMF (c) MWCNT CS-treated and d) MWCNT DMF-treated. Pattern a is weak due to the limited mass available (less than 7 mg compared to 20 mg for the others). Pattern c intensity scale is enhanced 10× compared with that of d.

except that the intensity ratio of the D- to G-band is closer to unity, which is more consistent with the spectrum of nanocrystalline graphite or strongly disordered sp2 carbon materials, such as glassy carbon.28 The peak at ∼9°, the peak from the PXRD pattern, due to the 2D rope lattice7 is clearly present on the DMF-treated SWCNTs, indicating that the basic structure of SWCNTs remains basically intact. This peak, however, is absent from the CS-treated SWCNTs (Figure 7). In addition, the PXRD

Luong et al. pattern of the CS-treated SWCNTs only shows a broad peak from 15° to 25° attributed to amorphous carbon and a very faint peak at 26° from graphitic carbon (Figure 7). Such features indicates the destruction of SWCNTs to amorphous carbon, in corroboration with the TEM, AFM, XPS, and Raman results. The graphitic carbon peak (26°), the (002) reflection of graphite, dominates the DMF-treated MWCNT pattern. Although a similar pattern is observed for the CS-treated MWCNTs, the signal intensity is only about one-tenth of the DMF-treated MWCNTs (Figure 7). We note that all patterns exhibit a broad, weak peak at 44° due to trace amounts of Co-Ni particles from the pristine CNTs. Conductivity of CNTs. A series of cyclic voltammetry experiments was conducted to evaluate the electrical conductivity of CNTs since graphite is conducting whereas amorphous carbon is not. With CS-treated MWCNTs, the modified electrode displayed only a small decrease in the signal response. As illustrated by TEM, XPS, and Raman measurements, the decomposed products of such MWCNTs consist mainly of graphite-like materials and some amorphous carbon. Therefore, the conduction of CS-treated MWCNTs was still largely preserved (Figure 8A). When modified with DMF-treated SWCNTs, a glassy carbon electrode displayed a significant increase in the redox waves (anodic and cathodic peak currents), an inherent property of pristine metallic SWCNTs. In contrast, an electrode modified with CS-treated SWCNTs displayed a complete blockage of the signal, indicating the presence of amorphous carbon, the main product of highly damaged SWCNTs (Figure 8B). Since CS is electroactive, such a result confirmed that CS-treated SWCNTs, after extensive washing was practically free from any significant CS contamination. Any disruption of the conjugated system of these nanotubes (introducing sp3 carbon and/or shortening the tubes) and exfoliation of the tube bundles should increase the impedance of the

Figure 8. Cyclic voltammetry of the glassy electrode modified with CS-treated MWCNTs (A) and CS-treated SWCNTs (B). Curve a, bare electrode; curve b, modified with CS-treated and curve (c) modified with DMF-treated. The inset of Fig. 8b illustrates the difference between the CS-treated SWCNTs and the CS-treated MWCNTs electrodes. AC impedance analysis of the CNT modified electrodes. (C) CS-treated MWCNTs (C: inset) DMF-treated MWCNTs. (D) CS-treated SWCNTs and (D: inset) DMF-treated SWCNTs.

Carbon Nanotubes in Aqueous Ceric Sulfate material. The measured impedance of CS-treated SWCNTs was significantly higher than that of DMF-treated SWCNTs, again confirming the insulating behavior of CS-treated SWCNTs. Such behavior, however, was less pronounced for CS- or DMF-treated MWCNTs, in agreement with cyclic voltammetry data (Figure 8C,D). Conclusions To our knowledge, this is the first report of the progressive conversion of SWCNTs into individual amorphous nonconducting carbon nanoparticles, and MWCNTs to mainly graphite and some amorphous carbon with retained conductivity. The presence of oxidizing groups aids the attachment of organic, inorganic, and biomolecular materials to the surface that is important to nanotube solubilization, self-assembly on surfaces, drug delivery, and biochemical sensing. Acknowledgment. The authors thank the Natural Sciences and Engineering Research Council of Canada for funding. References and Notes (1) Iijima, S. Nature 1991, 354, 56. (2) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (3) Thess, A. Science 1996, 273, 483. (4) Gu, Z.; Hauge, R. H.; Smalley, R. E.; Margrave, J. L. Nano Lett. 2002, 2, 1009. (b) Mickelson, E. T.; Chiang, I. W.; Zimmerman, B. P. J.; Lozano, J.; Liu, J.; Smalley, R. E.; Hauge, R. H.; Margrave, J. L. J. Phys. Chem. B 1999, 103, 4318-4322. (c) Boul, P. J.; Liu, J.; Mickelson, E. T.; Huffman, C. B.; Ericson, L. M.; Chiang, I. W.; Smith, K. A.; Colbert, D. T.; Hauge, R. H.; Margrave J. L.; Smalley, R. E. Chem. Phys. Lett. 1999, 310, 367-372. (5) Liu, J.; Rinzler, A. G.; Dai, H. J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; Rodriguez-Macias, F.; Shon, Y.-S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253. (6) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95. (b) Bahr, J. L.; Tour, J. M. Chem. Mater. 2001, 13, 3823. (c) Hirsch, A. Agnew Chem., Int. Ed. Engl. 2002, 41, 1853. (d) Basiuk, V. A.; Basiuk (Golovataya-Dzhymbeeva), E. V. In Encyclopedia of Nanoscience and Nanotechnology; Nalwa, H. S., Ed.; American Scientific Publishers: Stevenson Ranch, CA, 2004; Vol. 1, p 761. (7) Kovtyukhova, N. I.; Mallouk, T. E.; Pan, L.; Dickey, E. C. J. Am. Chem. Soc. 2003, 125, 9761. (8) Chow, L.; Wang, H.; Kleckley, S.; Daly, T. K.; Buseck, P. R. Appl. Phys Lett 1995, 66, 430.

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