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Purification, Cutting, and Sidewall Functionalization of Multiwalled Carbon Nanotubes Using Potassium Permanganate Solutions Tony J. Aitchison,† Milena Ginic-Markovic,*,† Janis G. Matisons,† George P. Simon,‡ and Peter M. Fredericks§ Nanomaterials Group, School of Chemistry, Physics and Earth Sciences, The Flinders UniVersity of South Australia, G.P.O. Box 2100, Adelaide, SA 5001, Department of Materials Engineering, Monash UniVersity, Clayton, VIC 3800, and School of Physical & Chemical Sciences, Queensland UniVersity of Technology, G.P.O. Box 2434 Brisbane, QLD 4001, Australia ReceiVed: October 5, 2006; In Final Form: NoVember 28, 2006
In this work multiwalled carbon nanotubes were examined using a combination of thermal annealing followed by chemical treatments with potassium permanganate solution. Functionalization with carboxylic acid moieties through this newly devised process simultaneously purified and cut the nanotubes. A cutting procedure using potassium permanganate that could keep or remove the MnO2 has not been previously reported, making this study a stepping stone to further functionalization/application opportunities. One such further functionalization was explored here with alkyl groups, through the generation of an amide linkage. A very detailed standard suspension test was performed on this material. What was found was an increased suspension stability toward polar solvents for the nanotubes with high concentrations of MnO2 deposits, and in nonpolar solvents the nanotubes with large alkyl chains attached were observed, thus proving it to be a versatile treatment.
Introduction First discovered in 1991 by Iijima,1 carbon nanotubes generally occur in one of two forms; single-walled nanotubes (SWNTs) and multiwalled nanotubes (MWNTs). They are highly regarded as nanostructured materials due to their high aspect ratio (length to diameter ratio: diameter of tens of nanometers and lengths that can be as long as several micrometers2), high strength, and excellent mechanical and electrical properties.2 These nanotubes have also been described as onedimensional molecules due to their high aspect ratio or as the ultimate carbon fiber due to their excellent properties. The production of MWNTs leads to amorphous carbon and onion impurities, where the onions are essentially multishelled fullerenes. Amorphous carbon can be removed by methods similar to those used to purify SWNTs, but the continuing issue of a remnant presence of such onions remains. In research reported to date, all MWNT purification processes consume the amorphous carbon, onions, and nanotubes simultaneously, but each to a different extent, due to different degrees of reactivity (amorphous carbon and onions are consumed more readily than the nanotubes).4 If the process is particularly harsh (i.e., treatment with HNO3/H2SO4 (3:1)5), the nanotubes will also be “cut” (i.e., shortened in length) as the nanotube ends are more reactive than the sidewalls.6 The onions however are very difficult to remove without destroying the nanotube integrity, as the chemical treatments required alter the nanotube surface chemistry.7 As a result, future treatments are possible such as alkyl chain functionalization,8 but the integrity of the nanotube cylindrical structure is often damaged by such treatments. While * To whom correspondence should be addressed. Phone: +618 8201 5541. Fax: +618 8201 5571. E-mail: Milena.Ginic-Markovic@ flinders.edu.au. † The Flinders University of South Australia. ‡ Monash University. § Queensland University of Technology.
the reported literature shows much work on the functionalization of SWNTs, less has been accomplished with MWNTs, as functionalization here often requires harsher conditions. However, functionalization of MWNTs, importantly, maintains the mechanical and electrical integrity of all the inner nanotubes within the multiwalled structure, and this is a considerable advantage when using carbon nanotubes in device applications, as such properties remain relatively unaffected by the functionalization. A decrease in the strength and conductive properties for SWNTs can be caused by chemical oxidation occurring on the side walls, as it has no inner tube to preserve its excellent properties.9,10 To cause functionalization by a chemical oxidation method while maintaining the nanotube integrity, the relative reactivity of certain sites on the nanotubes is used. These sites are the end caps and defect sites, which are of particular interest because the end caps have high strain on their hexagon-heptagon pairs and can thus be chemically oxidized more easily than the defect sites along the nanotube wall.6 In a chemical oxidation process, these sites are converted into carboxylic acid (R-COOH) groups, which leave further functionalization opportunities.11 The impurities (i.e., amorphous carbon and onions) are chemically oxidized to form a gas (CO2) or removed by either filtration or centrifugation by virtue of their high degree of polarity resulting from the abundant surface hydroxyl, carbonyl, and carboxylic acid groups formed.12 Current treatments for the purification and cutting of carbon nanotubes are ozonolysis,1 nitric acid treatment,14 and combined nitric and sulfuric acid treatment,5 among many others. Treatment with potassium permanganate, which is a milder oxidant that can be used in either acidic or alkaline conditions, is one other such treatment. It allows a controllable degree of oxidation, as it is dependent on the reaction time and is capable of generating new defect sites, where the quinone groups formed are an intermediate toward oxidation to carboxylic acids in a
10.1021/jp066541d CCC: $37.00 © 2007 American Chemical Society Published on Web 01/23/2007
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further step.4 The nanotubes, however, become coated in MnO2(s) and require HCl washing to remove the deposit.15 This MnO2 deposit, however, allows a higher degree of suspension stability and thus could be beneficial for future applications. Previous research on potassium permanganate purification and functionalization of nanotubes has been reported, but has only occurred successfully for carbon nanotubes produced by chemical vapor deposition (CVD).16-18 From CVD, the nanotubes produced have a higher degree of purity than the nanotubes formed by arc discharge. In comparison, the CVD technique results in production of minimal graphitic impurities that are easier to remove than those experienced in arc discharge,7 and this study examines the use of such potassium permanganate purification on much more impure, arc-discharge nanotubes. On top of this, all the papers we have come across that describe the use of KMnO4 do not show clearly that purification has occurred or that they could successfully remove the MnO2. In addition, the ability of potassium permanganate to cut nanotubes and thus manipulate the length has not been previously reported. Experimental Methods The MWNTs were purchased from the Materials and Electrochemical Research (MER) Corp. with characteristics of 10-40% purity, 8-20 nm diameter, and 2-20 µm length. These nanotubes were used as received (AR), followed by an initial pretreatment of thermal annealing. Thermal annealing has the capacity to remove amorphous carbon by essentially oxidizing it, the impurities reacting to form carbon dioxide, carbon monoxide, methane, and hydrogen gas if sufficient heat is applied (i.e., temperatures of between 600 and 1000 °C).19,20 In our work AR nanotubes (1 g) were heated at 590 °C in air for 3 h in a platinum dish.21 Potassium Permanganate Treatments. Following the initial treatment, the now thermally treated (TT) nanotubes are subjected to potassium permanganate oxidation. Potassium permanganate treated nanotubes with a MnO2 deposit (PPWMnO2) were prepared by sonication of 25 mg of TT nanotubes for 20 min in a 30 mL solution of 0.2 M KMnO4 at pH 10 (0.2 M NaOH), and the solution subsequently was refluxed for 40 min, following which 0.7 g of Na2SO3 and 7 mL of a 1 M H2SO4 solution were added. This solution was then stirred for 20 h under ambient laboratory conditions, followed by filtration using a 200 nm pore size hydrolyzed PTFE membrane and rinsing with 0.4 g/L NaOH solution. The nanotubes were further rinsed with distilled water, followed by a dilute hydrochloric acid rinse to protonate any carboxylate ions. This was adapted from the unsuccessful method for purification used by Zhang et al.,4 which was performed on SWNTs with no prior thermal annealing and did not include a 20 h stirring step. To create the potassium permanganate treated nanotubes without a MnO2 deposit (PPWoutMnO2), a number of variations to the above procedure were undertaken. One such difference was that the potassium permanganate used was recrystallized from a saturated aqueous solution under nitrogen,22 resulting in a more stable solution than the as-received KMnO4 (Figure 1a). It is also important to mention that, during filtration, rinsing is consequently done with 0.4 g/L NaOH solution, Na2SO3, and 1 M H2SO4 to remove most manganese oxides. This rinsing stage was repeated until no brown color was visible in the resulting residue and was then followed by a final rinse with concentrated hydrochloric acid to remove any remaining MnO2. Functionalization. The resulting carboxylic acid groups were subsequently covalently reacted with alkylamines to form amide linkages. This was achieved using 5 mg of PPWoutMnO2
Figure 1. (a) The as-received KMnO4 solution (left vial) exhibits a larger degree of degradation in comparison to the recrystallized solution (right vial) after 1 month in laboratory conditions (i.e., room temperature, 1 atm, and in the dark). Dispersed samples of nanotubes, demonstrating the comparison made between (b) stable and (c) unstable dispersions.
dispersed in 25 mL of freshly distilled dichloromethane, to which 2 mol equiv of N-hydroxysuccinimide (NHS) and 1 mol equiv of hexylamine, decylamine, or hexadecylamine were added. Stirring rapidly, 30 mg of N,N′-dicyclohexylcarbodiimide (DCC) was added, and stirring was continued for 20 h in a sealed container to prevent solvent evaporation.23 The nanotubes were then filtered using a 200 nm pore size PTFE membrane, rinsed with freshly distilled dichloromethane, and henceforth referred to as “f-nanotubes” (i.e., functionalized MWNTs). The newly formed f-nanotubes were then dispersed into various solvents to determine their suspension stability. The solvents ranged from polar to nonpolar having both aromatic and alkyl groups. The following solvents tested are listed in order from their highest dielectric constant to their lowest: methanol (MeOH), ethanol (EtOH), dichloromethane, tetrahydrofuran (THF), chloroform, diethyl ether, triethylamine, toluene, cyclohexane, hexane. For each solvent tested, 0.1 mg of f-nanotubes was dispersed in 3 mL of the solvent and the sample sonicated for 40 min, after which it was left to settle. During this time, observations were made at regular intervals to determine whether the particles had aggregated visibly to the naked eye (i.e., the point where the dispersion was no longer held). The dispersion was classified as stable if the nanotubes had not visibly aggregated after 24 h (see Figure 1b) and unstable if they had (see Figure 1c). This was repeated for AR, TT, PPWMnO2, and PPWoutMnO2 for comparison. A scheme of the overall work presented in this paper has been supplied in Scheme 1. Characterization Techniques. The degree of purification and cutting extent of the MWNTs were characterized using TEM on a Phillips CM100, with a Soft Imaging System Mega View camera. Samples were held on a copper mesh disk with a carbon film. The chemical characterization was obtained using FT-IR
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SCHEME 1: Representation of the Work Presented in This Paper
spectroscopy, XPS, and Raman spectroscopy. FT-IR spectra were recorded in transmission mode in the spectral range 4000400 cm-1 by means of the KBr disk method. A Thermo-Nicolet Nexus 870 FT-IR spectrometer equipped with a DTGS detector was used. 128 scans were co-added at a resolution of 4 cm-1. XPS was performed on a Kratos Axis Ultra, with an Almonochromated X-ray source (1486.6 eV) and Raman spectroscopy on a Renishaw model 1000 equipped with a single grating and an electrically cooled CCD detector (excitation by a He-Ne laser ∼632.8 nm). Results and Discussion TEM was used to evaluate the purity and integrity of the nanotubes after the different treatments performed. In the TEM image of AR MWNTs (Figure 2a), impurities of amorphous carbon and onions are evident. Upon thermal annealing to produce the TT sample, the amorphous carbon was removed, but the carbonaceous onions remained, even after 3 h of treatment (Figure 2b). The potassium permanganate treatments PPWMnO2 and PPWoutMnO2 were used for both cleaning and cutting the nanotubes as shown by the TEM images, Figure 2e,f, respectively. This two-part process was also performed in separate stages for further analysis. The first or oxidation stage, performed by reflux, showed that the nanotubes were neither cleaned nor cut, as shown in Figure 2c. This oxidation step performed under alkaline conditions still helped to disperse the nanotubes and destroy any aggregated particles, enabling homogeneously cut nanotube samples to be produced by the second step. The reaction mechanisms of the oxidation step (reflux stage) are shown in Scheme 2, where importantly the permanganate ion attaches itself to the nanotubes and then requires sodium hydroxide to subsequently be removed while simultaneously being oxidized. Following this, the hydroxyl groups formed can be further oxidized to carbonyl groups, and hydrochloric acid is used to remove the MnO2.24,25 The second or cutting stage, carried out independently, resulted in nanotubes with an extremely high concentration of MnO2 that could not be removed by the simple filtration process. However, the nanotubes are cut, and most of the carbonaceous
Figure 2. TEM image of (a) AR MWNTs showing amorphous carbon, (b) TT MWNTs showing onions, (c) the first stage of KMnO4 treatment showing increased dispersion of the original material, (d) the second stage (no first stage) of KMnO4 treatment showing MnO2 aggregated particles with bundled nanotubes, (e) PPWMnO2-treated MWNTs, and (f) PPWoutMnO2-treated MWNTs.
SCHEME 2: KMnO4 Oxidation Treatment for Carbon Nanotubes
onions are removed, though some carbon impurities and uncut nanotubes in and around the aggregated bundles remain, as shown in Figure 2d. These aggregated bundles were analyzed using EDX, where they were found to contain high concentrations of manganese/MnO2. Therefore, without the vital first step, the nanotubes are still cut, but aggregated bundles of nanotubes and MnO2 remain, and there are much longer, uncut nanotubes inside these bundles (i.e., no homogeneous lengths are obtained). This conclusion is clearly seen in TEM, and it is important to note that pure, homogeneously cut nanotubes could only be achieved as a two-step process. After 20 h of cutting, PPWMnO2 had an average length of 64.77 nm (std dev 1.67 nm), and the yield by weight of the reaction was often greater than 100%, attributed to the associated MnO2 deposit. PPWoutMnO2 showed an average length of 118.8 nm (std dev 18.4 nm), and the difference in nanotube length results from using a solution that had a reduced oxidizing strength of potassium permanganate. The chemical structure of potassium permanganate treated nanotubes was examined using FT-IR spectroscopy. The FTIR spectra of AR, TT, PPWMnO2, and PPWoutMnO2 samples
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Figure 3. FT-IR spectra of AR (green), TT (pink), PPWMnO2 (red), and PPWoutMnO2 (blue) samples on a common normalized scale.
are shown in Figure 3. Three distinct peaks at 1380, 1460, and 1630 cm-1 are observed in all spectra, corresponding to the C-H bend, -CH2 vibrations, and the stretching mode of quinone groups, respectively.26 The peak at 1380 cm-1 is intense for the AR sample, due to the presence of the amorphous carbon and the defects in both the fullerenes and nanotubes. The intensity decreases upon thermal treatment as the amorphous carbon is removed; however, some defects still exist. With the potassium permanganate treatment, an increase in the peak intensity results from the sidewall functionalization and the opening of the remaining closed tip nanotubes. The absorbance band at 1460 cm-1 follows similar intensity changes, as it is similar in nature to the 1380 cm-1 peak.26 For the 1630 cm-1 peak, an intensity change between the AR and TT samples is not significant due to the thermal decomposition of the oxygenated species and a lack of carbonyl group generation on TT nanotubes.20 The potassium permanganate treatment causes the 1630 cm-1 peak to increase sharply in the PPWoutMnO2 sample, signifying an increase in surface oxygenated species. In contrast, the PPWMnO2 spectrum reveals a decrease in the absorbance at 1631 cm-1 due to the presence of the manganese surface functionality. The small shoulder at 1580 cm-1 arises from the stretching mode of carbon-carbon double bonds near oxygenated carbon atoms.27 This shoulder is not obvious in the AR spectra, but present in the TT, PPWMnO2 and PPWoutMnO2 spectra due to the removal of the amorphous carbon. However, a shift in the peak for PPWMnO2 is also evident, indicating the presence and interaction of the MnO2 with the carbon nanotubes. The 1730 cm-1 peak is assigned to ν(CdO) of carboxylic acid groups.26 The TT spectrum shows no change in this peak relative to that of the AR spectrum, as no new carbonyl groups are generated by this treatment.20 Treatment with potassium permanganate leads to an increase in the 1730 cm-1 peak (Figure 3), which results from an increase in carboxylic acid functionality. There is, however, the evident shift in the peak for the PPWMnO2 spectra, due to the polar-polar interaction of the MnO2 with the functionalized carbon nanotubes (Scheme 2), also observed in the 1580 cm-1 peak. The differences in the 1730 cm-1 peak demonstrate a significant increase in oxygenated species upon potassium permanganate treatment. XPS was used to determine the extent of oxygenated functionalization on the carbon nanotubes, and a survey spectrum of sample PPWMnO2 is shown in Figure 4a. The
Figure 4. (a) XPS survey spectra of PPWoutMnO2 and (b) scaled up scan of the C 1s peak.
spectrum exhibits three major peaks at 642, 531, and 285 eV, corresponding to Mn 2p, O 1s, and C 1s photoemission, respectively.28,29 The respective positions of each element did not change in the AR, TT, PPWMnO2, and PPWoutMnO2 samples, although the relative concentrations did. Atomic compositions obtained from multiplexed spectra of the carbon and oxygen photoemission were calculated and are presented in Table 1. Figure 4b shows the representative plot of the C 1s peak of the same sample in the range of 282-292 eV. It is clear from the C 1s peak that it exhibits a broad tail which corresponds to (after charge correction) 284.6, 285.3, 286, and 288 eV related to CdC, C-C, C-O, and CdO, respectively.28 The manganese (Mn 2p) envelope for the same sample also shows a broadening, and the following bonds can be assigned: MnO (640.7 eV) and MnO2 (642.1 eV).29 From Table 1 it is clear that the oxygen content of the TT nanotubes increases compared to that of the AR sample. The atomic concentration of oxygen increased from 0.59% to only 0.98% for the TT nanotubes, but increased further with the KMnO4 treatment, as supported by the FT-IR results. The oxygen content increased to 35.17% for the PPWMnO2 sample and 7.91% for the PPWoutMnO2 sample. The oxygen concentrations in the KMnO4-treated nanotubes, however, are also associated with manganese oxide. For the PPWMnO2 sample, 13.86% is due to the C-O and CdO groups, while 24.46% is due to MnO2. For the PPWoutMnO2 sample, 5.91% is attributed to the C-O and CdO groups, while 1.78% is attributed to residual MnO2. This is a drop from 24.46% to 1.78%, or a 13.7× drop of MnO2 with no other concernable impurities. This implies a purity of 98.22%, which is well within the acceptable industry standard of what is called “pure”. What can also be noticed from Table 1 is the C-O group concentration is always greater than the CdO group concentration. This is due to the detection
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Figure 5. (a) Raman spectra of PPWoutMnO2, showing the G and D modes, and (b) D to G area ratios, showing an increase with increasing oxygen functionalization.
TABLE 1: XPS Results of the Nanotubes, Showing Atomic Concentrations of Its Constituents atomic concn (%), survey scan
a
atomic concn (%), detailed scan
sample
carbon (BEa ) 285 eV)
oxygen (BE ) 530 eV)
C-O (BE ) 533 eV)
CdO (BE ) 531 eV)
MnO2 (BE ) 529 eV)
AR TT PPWMnO2 PPWoutMnO2
99.41 99.02 39.21 87.98
0.59 0.98 35.17 7.91
0.38 0.71 6.98 3.32
0.56 6.88 2.59
24.46 1.78
BE ) binding energy.
TABLE 2: Raman Peak Positions (cm-1) sample
G′ band
G band
D band
AR TT PPWMnO2 PPWoutMnO2
2655 2659 2664 2663
1576 1580 1581 1581
1329 1332 1334 1334
of both the C-O and CdO bonds within the carboxylic acid functionality and not the quinone group. The additional percentage in the C-O peak comes from other carbon-oxygen bonds (e.g., alcohols, ether, epoxy, etc.).30 Such results are in good agreement with the FT-IR spectra. The degree of functionalization was also examined by Raman spectroscopy (Figure 5a), by comparing the peak area ratio of the D to G modes. The G mode or tangential mode at 1580 cm-1 corresponds to the movement in opposite directions of two neighboring carbon atoms in a graphite sheet.31 The D mode or disorder mode at 1330 cm-1 is caused by sp3-hybridized carbon atoms (e.g., COOH) in the nanotube sidewalls, but is also activated by any defect that breaks the translational symmetry.32,33 A direct correlation in the degree of functionality to the intensity of the D to the G mode is observed in Figure 5b. It is clear that sidewall functionalization increases with the various treatments following the trend PPWoutMnO2 > PPWMnO2 > TT. The ratio signal for PPWMnO2 has been affected by the presence of the MnO2,34 so the relative difference between PPWoutMnO2 and PPWMnO2 remains uncertain, but in general, the ratio has increased and sidewall functionalization has occurred. Further information can also be drawn from the Raman spectra as Barros et al.35 have found that in SWNTs the radial breathing mode frequencies upshift, due to charge transfer from the nanotube to the functional groups (i.e., -COOH). A similar affect was also present in our data and can be seen in Table 2 with an overall upshift of approximately 5-9 cm-1. These results are well supported by both the FT-IR and XPS results, demonstrating that the nanotubes are functionalized at the end caps and along the sidewalls with carbonyl groups.
During the oxidative process with potassium permanganate, the MWNTs are functionalized with polar carbonyl groups. From this, an increase in their dispersability in polar solvents was expected,36,37 and the MWNTs were further studied by a suspension stability test. Li et al. 38 reported that by varying the linear alkyl chain length attached the degree of dispersion in CHCl3 changes, increasing as this alkyl chain becomes longer. The carbonyl groups found on the PPWoutMnO2 were subsequently covalently bound to different length alkylamines, forming amide bonds, so an improved dispersion was anticipated. Alkyl chain attachment was first proven with the use of TEM. In the TEM images (i.e., Figure 6 compared to Figure 2e) what can be noticed is that the nanotubes appear to be more clustered. This would be due to the loss of ionic repulsion/carboxylate ion, replaced with hydrogen bonding of amide groups and van der Waals forces. Further verification was performed with FT-IR and Raman spectroscopy.
Figure 6. TEM image of PPWMnO2 decylamine-coupled nanotubes.
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Figure 7. (a) Decylamine coupled to PPWoutMnO2 and (b) the curvefitted peak of the overlapping frequencies occurring approximately at 1630 cm-1. Similar results were obtained with hexyl- and hexadecylamines.
TABLE 3: Raman Peak Positions (cm-1) sample
G′ band
G band
D band
hexylamine decylamine hexadecylamine
2658 2656 2662
1577 1576 1580
1331 1331 1332
After functionalization with decylamine, only a slight change occurs in the FT-IR spectra (original, Figure 3 blue; functionalized, Figure 7a), due to the presence of the new secondary amide bonds, following the coupling treatment. One new peak observed is the interaction peak of N-H bending and C-N stretching of the C-N-H group (amide II band) at around 1630 cm-1, overlapping the quinone peak. These overlapping peaks were resolved using OriginPro 7.5 software and curve fitting the peaks (R2 ) 0.99927), shown in Figure 7b. The curve-fitted peak at 1628 cm-1 is assigned to the quinone group from the TABLE 4: Suspension Stability Results
nanotubes, and the 1652 cm-1 peak is assigned to the C-N-H group.26 The peak at 1258 cm-1 is related to the C-N-H group but is weaker than the peak at 1652 cm-1. Finally, a peak at around 800 cm-1 (not shown) results from an out-of-plane N-H wagging.26 All these peaks are an indication of successful amide linkage formation. The Raman spectroscopy ratios were determined for the coupled material, and a decrease in the D to G ratio was seen due to alkyl chain interference.34 This further supports the FTIR results demonstrating the successful attachment of the alkylamines. In addition to this a peak downshift was detected (i.e., comparing Tables 2 and 3), as the carboxylic acid charge transfer almost does not occur for amide bonds. This further supports the ab initio calculation study of Veloso and coworkers33 and the work of Barros et al.35 The suspension stability results are summarized in Table 4. From the table it is clear that the AR samples are stable in THF, chloroform, diethyl ether, and triethylamine, while the TT samples, in addition, are stable in EtOH. Such results reflect the results obtained in the literature, where triethylamine forms zwitterions39 and the aromatic solvents form more stable dispersions than the nonaromatic ones (i.e., stability was higher in toluene than hexane).37 It was also determined that our treated nanotubes were able to be dispersed well in the middle range of the solvents tested (i.e., between the polar and nonpolar solvents). The PPWMnO2 samples were stable in MeOH and EtOH, and the PPWoutMnO2 samples were stable in EtOH, dichloromethane, chloroform, and diethyl ether. These results suggest that the higher the concentration of MnO2, the more stable the suspensions were in polar solvents, and the lower the MnO2 concentration the closer the sample’s properties were to those of the TT samples. The hexylamine- and decylamine-coupled nanotubes showed high stability in THF, chloroform, and diethyl ether, but with hexadecylamine this stability extends to triethylamine and toluene. It is important to note though that the coupled material was synthesized from PPWoutMnO2, so the MnO2 concentration had minimal influence on the results. Overall, the results thus demonstrated improved dispersive capabilities over the entire dielectric range, with the longer alkyl chains being more stable, in terms of dispersion, at the lower dielectric range of the solvents tested (i.e., nonpolar). To some extent these results demonstrate a degree of control as to whether a suspension will prove to be stable or not. It can be controlled by the amount of MnO2 deposit and the coupled
2446 J. Phys. Chem. C, Vol. 111, No. 6, 2007 species attached; the higher MnO2 concentrations encourage stable nanotube dispersions in polar solvents, and the longer alkyl chains encourage stability in nonpolar solvents, with a general improvement throughout the dielectric range. For the lower concentrations of MnO2 and shorter alkyl chains, the nanotubes exhibit properties of the original material. Conclusion In conclusion, we have shown that thermal annealing can reduce the concentration of carbon contaminants that were formed during synthesis of the carbon nanotubes. When used in combination with the newly devised potassium permanganate treatment, however, it successfully cuts and further purifies the nanotubes, forming more carboxylic acid groups. This simultaneously deposits manganese oxide onto the nanotubes, which can be subsequently removed to an insignificant concentration. The carboxylic acid groups formed are then able to be successfully functionalized by alkylamines through a process called diimide-activated amidation, using DCC as the coupling agent. Their dispersive properties (i.e., capability to stay suspended in a solvent) were determined in a comprehensive study, and it was found that the stability could be controlled via the manganese oxide concentration and the alkyl chain length attached. Acknowledgment. This work was supported by the Australian Research Council (Discovery Grant DP0449692) and the International Science Linkages program established under the Australian Government’s innovation statement Backing Australia’s Ability (Grant CG0600). References and Notes (1) Iijima, S. Nature 1991, 354, 56-58. (2) Ebbesen, T. W. Annu. ReV. Mater. Sci. 1994, 24, 235-264. (3) Andrews, R.; Jacques, D.; Rao, A. M.; Rantell, T.; Derbyshire, R. Appl. Phys. Lett. 1999, 75, 1329-1331. (4) Zhang, J.; Zuo, H.; Qing, Q.; Tang, Y.; Li, Q.; Liu, Z.; Guo, X.; Du, Z. J. Phys. Chem. B 2003, 107, 3712-3718. (5) Kim, B.; Sigmund, W. M. Langmuir 2004, 20, 8239-8242. (6) Haddon. R. C. Acc. Chem. Res. 2002, 35, 997. (7) Ajayan, P. M.; Ebbesen, T. W. Rep. Prog. Phys. 1997, 60, 10251062. (8) Sun, Y.-P.; Huang, W.; Lin, Y.; Fu, K.; Kitaygorodskiy, A.; Riddle, L. A.; Yu, Y. J.; Carroll, D. L. Chem. Mater. 2001, 13, 2864-2869. (9) Sheeney-Haj-Ichia, L.; Basnar, B.; Willner, I. Angew. Chem. 2005, 117, 80-85. (10) Luong, J. H. T.; Hrapovic, S.; Liu, Y.; Yang, D. Q.; Sacher, E.; Wang, D.; Kingston, C. T.; Enright, G. D. J. Phys. Chem. B 2005, 109, 1400-1407. (11) Ravindran, S.; Bozhilov, K. N.; Ozkan, C. S. Carbon 2004, 42, 1537-1542.
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