Reaction of Single-Walled Carbon Nanotubes with Organic Peroxides

Paul S. Engel,* Wilbur E. Billups, David W. Abmayr Jr., Konstantin Tsvaygboym, andRuntang WangDepartment of Chemistry, Rice UniVersity, P.O. Box 1892, Houston, Texas 77251

ReceiVed: August 31, 2007; In Final Form: October 1, 2007

Single-walled carbon nanotubes (SWNTs) induce the decomposition of four diacyl peroxides by single electrontransfer (SET). Phthaloyl peroxide functionalizes SWNTs to the greatest extent of the four. It was also foundthat t-butoxy radicals add to SWNTs but that SWNTs fail to inhibit cumene autoxidation. Thus, SWNTs arereactive to alkoxy radicals but not to alkylperoxy radicals.

The great potential value of functionalizing single-wallednanotubes (SWNTs) has led a number of workers to explorefree radical attack on the sidewalls of these unique nanometer-sized objects.1,2 The most common radical precursors arediazonium salts3 and peroxides,4-8 but other approaches arebased on Fenton’s reagent,6 perfluoroalkyl iodides,9,10perfluoroazo compounds,11,12 microwave discharge of ammonia,13 andattack of growing polymer chains.14 Presently, we report thekinetics of diacyl peroxide thermolysis in the presence ofSWNTs, which reveal moderate to large rate accelerationsattributed to induced decomposition. We further report the attackof t-butoxy radicals on SWNTs and the failure of SWNTs toinhibit the autoxidation of cumene.

While studying the thermolysis of benzoyl peroxide (BP) withSWNTs, we noticed that the rate of gas evolution, as monitoredby a pressure transducer, was accelerated by inclusion ofpurified, pristine HiPco SWNTs.15 Thus, a solution of 75 mgBP in 10 mLortho-dichlorobenzene (o-DCB) exhibited a 67%greater pressure rise over the course of 2 h at 80 °C whenSWNTs (5 mg) were included than a control experiment withoutSWNTs. This rate enhancement was confirmed by iodometrictitration16 of the BP remaining after thermolysis. All titrationstudies discussed below were performed in non-degassedo-DCBusing purified HiPCo SWNTs batch no. 164-2 produced in theRice University Carbon Nanotechnology Laboratory.17 Com-parison of non-degassed with degassedo-DCB gave essentiallythe same percent peroxide decomposition whether SWNTs werepresent or not.

Figure 1 shows the percent BP consumed in 1 h at 80°Cand 90°C in 50 mL o-DCB as the initial weight of SWNTswas increased from 0 to 5 mg. Although the rate enhancementdue to SWNTs is apparent, a number of control experimentswere required. The thermolysis of 0.006 M BP ino-DCB at80 °C is slow enough that hardly any decomposition (∼1%/hr)was detectable. We ran the thermolysis in benzene as a controland obtained about 6%/hr decomposition, corresponding to therate reported in the literature.18,19 Thereforeo-DCB does notinduce BP thermolysis as much as benzene does and nocorrection was needed for residual thermolysis at 80°C. (cf.Figure 1). However, at 90°C BP in o-DCB thermolyzes at amoderate rate, as evidenced by the 17% decomposed in 1 heven without added SWNTs. The steep rise in % BP consumedat low [SWNT] is seen in most runs where normal peroxide

thermolysis was important. After this initial jump, BP consump-tion rises more or less linearly with increasing amounts ofSWNTs.

The same effect was observed inp-methoxybenzoyl peroxide(p-MeO-BP), whose inherent thermolysis rate is about twice asfast as that of BP (Figure 1).19 The greater rate necessitatedusing lower temperatures, but the curve shape is similar for thetwo peroxides. The rates, especially at 80°C and somewhat at70 °C, are elevated by a contribution from ordinary thermolysisof p-MeO-BP. Several data points were obtained below 1 mgSWNT, giving a steeper slope than the one at larger amountsof SWNTs.

Much greater rate enhancements were found with two otherperoxides: phthaloyl (PhP)20 and trifluoroacetyl (TFAP).21 Asseen in Figure 2, the plot of the percent peroxide consumptionversus weight of SWNT at 80°C exhibits a slope about 2.3times greater with PhP than with BP. Moreover, TFAP is nearlyas sensitive to SWNTs as PhP. Initially, we changed solvents* Corresponding author. E-mail: engel@rice.edu.

Figure 1. Effect of added SWNTs on the thermolysis of benzoylperoxide (BP) ino-DCB at 80°C (solid circles) and 90°C (open circles)and onp-methoxybenzoyl peroxide (p-MeO-BP) at 70°C (solid squares)and 80°C (open squares) for 1 h.

695J. Phys. Chem. C2008,112,695-700

10.1021/jp0770054 CCC: $40.75 © 2008 American Chemical SocietyPublished on Web 01/03/2008

from o-DCB to nitrobenzene (NB) to suppress the solvent-induced decomposition of TFAP.22,23 In NB solvent at 50°C,7% of TFAP was consumed in 1 h; but with 5 mg SWNTs,this figure rose to 75%. Because further experiments revealedunexpected reactions with NB, it was necessary to conduct theremaining studies ino-DCB and to simply tolerate the ther-molysis of TFAP in the blank run. We found that rapid additionof TFAP to o-DCB without SWNTs gave more peroxidedecomposition (25-35%) than dropwise addition (15-20%)This effect may be due to autoinduction caused by high localTFAP concentrations because the droplets of TFAP did notimmediately dissolve ino-DCB. The problem was minimizedby dropwise addition of TFAP, in contrast to the other threeperoxides, which were added all at once as solids. A few of theperoxide + SWNT rate experiments were repeated with adifferent batch of SWNTs (no. 162-1). These runs showed lessperoxide consumption for a given weight of SWNTs butotherwise the same trends as in Figures 1 and 2.

The rate acceleration seen on addition of SWNTs to peroxidesmight be attributed to traces of iron left in the purified SWNTs.However, such an explanation is unlikely at the outset becausethese SWNTs had been subjected to multiple oxidations duringpurification17 and they contained only∼1.5% Fe by TGAanalysis. Moreover, any residual Fe is encapsulated in a layerof carbon so that we could never see an iron signal by XPS.Control experiments with deliberate addition of 2 mg of ironpowder or 50µg of Fe(II) or Fe(III) (as chlorides) to BP ino-DCB showed the same decomposition rate as the blank.Although 50µg is the approximate concentration of iron in theSWNT samples, we also tried 2 mg of Fe(II) and Fe(III). Theselarge doses of oxidized iron caused greatly enhanced theperoxide decomposition rates but are unrealistic control experi-ments. We reason that if residual Fe is inaccessible to oxygenduring purification and to the X-ray beam of XPS it is surelyinaccessible to solution-phase peroxides. Although it is stillpossible that the rate acceleration is caused by an impurity inthe HiPco SWNTs, such an impurity would have to be a muchmore effective catalyst than Fe(II).24

To determine whether the reaction of SWNTs with BP couldbe induced photochemically, we irradiated ano-DCB suspensionwith a 500-W quartz-halogen lamp through a water filter. Nochange in rate was noticed, hardly a surprising result in viewof the very short lifetime of excited SWNTs.25 In contrast, itwas reported recently that visible light irradiation of SWNTswith hydrogen peroxide caused the disappearance of theircharacteristic near-infrared fluorescence.8

Product Characterization

Although the product of phenyl radical attack on SWNTshas been characterized already,4,5 we analyzed the products ofour peroxides with SWNTs by Raman and XPS spectroscopy.These methods were employed because SWNTs do not allowus the luxury of using some of the powerful analytical techniquesapplicable to the far more soluble C60.26,27

Raman D/G area ratios are often determined to ascertain thedegree of SWNT functionalization.3,12,28,29Such measurementswere carried out here using laser wavelengths of 633 and 780nm on SWNT sample no. 162-1 recovered from degassedtitration runs, as summarized in Table 1 and shown in moredetail in the Supporting Information. While 780 nm probesmainly semiconducting SWNTs, 633 nm begins to see metallictubes as well. In accord with the literature on BP, we observean increase in the Raman D band. The largest increase appearswith PhP, whose high D/G ratio is comparable to other SWNTstudies in the literature12,28,30 and implies that PhP stronglyfunctionalizes SWNTs. The D/G ratio at 633 nm versus that at780 nm is close to the value for the blank except for PhP.

Another approach to look for SWNT functionalization isX-ray photoelectron spectroscopy (XPS). The same degassedSWNT samples as in Table 1 were analyzed by XPS, yieldingthe results summarized in Table 2. Although different weightsof SWNTs were exposed to varying concentrations of peroxides,there was no correlation of elemental composition with theseparameters. The most obvious effect is the high oxygenincorporation with PhP, which will be discussed below.Although the fluorine content was greatest for TFAP, theaverages in Table 2 do not tell the whole story (cf. SupportingInformation for all data). Every TFAP-SWNT sample contained∼2% F, but several of thep-MeO-BP and BP samples showed

Figure 2. Effect of added SWNTs on the thermolysis of phthaloylperoxide (PhP) at 70°C (solid triangles) and 80°C (open triangles)and on the thermolysis of trifluoroacetyl peroxide (TFAP) at 40°C(solid diamonds) and 50°C (open diamonds) for 1 h.

TABLE 1: Average Raman D/G Area Ratios (%) ofDegassed SWNTs Subjected to Peroxides

peroxide D/G (633 nm) D/G (780 nm) no. samples

nonea 8.3 13.8 2BP 14.4 19.6 5p-MeO-BP 8.3b 12.5c 4, 2d

PhP 26.7 30.9 5TFAP 6.1 9.1 3

a Blank consisted of purified SWNTs stirred ino-DCB at 80°C for1 h then filtered.b A fifth sample gave D/G)17.2. c A third samplegave D/G)24.9. d Four measurements at 633 nm and two at 780 nm.

TABLE 2: Average Atomic Percent of Elements in SWNTsExposed to Peroxidesa

peroxide %C %O %Cl %F no. samples

none 93.3 3.2 3.6 0 2BP 93.6 3.9 1.6 0.9 5p-MeOBP 91.4 5.2 2.4 1.0 8PhP 85.6 12.7 1.7 0 5TFAP 92.4 4.4 1.2 2.0 5

a The data were obtained on 1-5 mg degassed SWNT samplesexposed to amounts of peroxide ranging from 0.15 to 0.6 mmol.

696 J. Phys. Chem. C, Vol. 112, No. 3, 2008 Engel et al.

little or no F. This variability almost certainly arises becausethe smaller samples did not completely coat the Teflon filtermembrane used to isolate reacted SWNTs.3 Therefore, part ofthe membrane was sometimes accidentally included in theanalysis. We are confident that only the TFAP-exposed SWNTscontained an elevated level of fluorine. The chlorine in allsamples comes from the purification step, which involves HCl.17

The data in Table 1 indicate that PhP and BP functionalizeSWNTs while Table 2 shows that same result with PhP,p-MeO-BP, and TFAP. Apparently, Raman spectroscopy is not sensitiveenough to detect the small amount (<1 CF3 per 100 carbons)of added groups from TFAP. A small shoulder at 291.2 eV onthe carbon 1s band supports the presence of a CF3 group.9

Although BP does not add enough phenyl groups to changethe carbon content seen by XPS, we observe an increase in theRaman D/G ratio, in accord with previous reports.4-6 PeroxidesRCOOOCOR might introduce either R or RCOO groups, andthe latter would lead to more oxygen in the recovered SWNTsthan in the blank (Table 2). Such an increase is obvious withPhP but still occurs with the other peroxides, especiallyp-MeO-BP. The 5.2% oxygen found withp-MeO-BP can be explainedin part by p-methoxyphenyl radicals adding to SWNTs.12 Infact, a shoulder due to the methoxy carbon is clearly visible at286.5 eV. (cf. Figure 3). Because the XPS measurements weredone on samples that were degassed and never exposed to airuntil workup, there is little chance that atmospheric oxygen iscaptured by reacting SWNTs. In contrast, non-degassed samplesdid exhibit an elevated oxygen content and samples deliberatelyexposed to O2 gave 8-9% oxygen by XPS. XPS of SWNTs+BP andp-MeO-BP showed a small peak at 289 eV attributedto carbonyl carbon but this peak was very clear with PhP (cf.Figure 3). Additional support for the carbonyl group is foundin the ATR IR spectrum of SWNTs that had been thermolyzedwith PhP, where we observed a distinct band centered at 1704cm-1 (half width 84 cm-1).

SWNTs have been functionalized by various methods, amongwhich are thermolysis with excess benzoyl peroxide5,31 andreductive alkylation with alkyl halides.32 To ascertain whetherpreviously functionalized SWNTs31,32were capable of inducingBP decomposition, we ran a set of four experiments ino-DCBat ∼86 °C for 2 h, as summarized in Table 3. Althoughunfunctionalized SWNTs roughly tripled the percent of BPconsumed, those bearing phenyl or dodecyl groups only doubledthe BP consumption. Clearly, functionalized SWNTs still induceBP decomposition but not as effectively as SWNTs themselves.

Reaction of SWNTs with t-Butoxy Radicals

Highly reactivet-butoxy radicals33 are conveniently generatedby mild thermolysis of di-t-butyl hyponitrite (DTBHN).34 Tocheck the reactivity of SWNTs towardt-BuO•, a suspension of5 mg of SWNTs and 50 mg of DTBHN in 4-5 mL of benzenewas heated under nitrogen at 55°C for 2 days. The Raman Dband was found to increase approximately 10-fold, indicatingconsiderable attack of radicals on the SWNTs. Repeating thisexperiment with 150 mg of DTBHN and 2 mg of SWNTs in∼5 mL of benzene again increased the D band 10-fold to 44%of the G band. In the breathing mode region, the pristine SWNTsexhibited a group of three bands at 213, 224 (sh), and 232 cm-1,but after treatment with DTBHN the lower frequency bandshad greatly decreased relative to the one at 266 cm-1. Becausethe frequency of these modes is inversely proportional to SWNTdiameter,13 it appears thatt-BuO• is selective for larger diameterSWNTs. The ratio of benzene to SWNTs is over 300; hence,t-BuO• must preferentially attack SWNTs35 or we would seeno increase in the D/G ratio.

The Effect of SWNTs on Cumene Autoxidation

Literature reports36,37 on the reaction of “reactive oxygenspecies” with fullerenes prompted us to determine whetherSWNTs would behave similarly. We chose to study theautoxidation of cumene, which proceeds by the chain mechanismshown in Scheme 1.38 The experimental approach was todetermine manometrically whether SWNTs would inhibit theAIBN-initiated uptake of gaseous oxygen by cumene ino-DCBat 70°C. As shown in the lowest curve of Figure 4, AIBN withSWNTs ino-DCB exhibited an apparent rapid volume increaseover 5 min, which we attribute to the rise in solvent vaporpressure after the reaction vessel was placed into the hot oilbath. This rapid drop in the curve (volume increase) wasfollowed by a much slower decline due to nitrogen evolutionfrom AIBN. When AIBN was omitted (“SWNTs only”), thecurve was flat after the equilibration period. The curve labeled“BHT” shows the typical behavior of an autoxidation inhibitor,where BHT is 2,6-d-t-butylcresol. In this case, hardly anyoxygen was taken up for the first 40 min. Once the inhibitorwas exhausted, oxygen was absorbed at a rate of∼9 mL/hr.SWNTs do not behave like BHT, for the curve shows noinhibition period but instead shows the steady uptake of oxygenafter equilibration. The same behavior was found for dodecylfunctionalized SWNTs.

The SWNTs and C12-SWNTs that were re-isolated after theattempted autoxidation inhibition showed no change in theirRaman spectra. It is therefore likely that they were not destroyed

Figure 3. XPS C1s spectrum of SWNTs subjected to peroxides. Thetypical position of the bold, underlined carbons is shown on the plot.

TABLE 3: Percent BP Consumed on Thermolysis of BP for2 h at ∼86 °C in o-DCB

SWNT type SWNT weight, mg % BP consumed

none 0 25SWNT 5 79Ph-SWNT 5 52C12H25-SWNT 5 53

SCHEME 1: Autoxidation of Cumene

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and that SWNTs are not autoxidation inhibitors. Attributing thisnegative result to poor suspendability of SWNTs ino-DCB isnot a viable argument because such suspensions are stable formonths and they react nicely with peroxides.

Discussion

BP at 90°C, p-MeO-BP at 80°C , TFAP at both 40 and50 °C, but not PhP, exhibit a greater initial slope than the oneattained at higher amounts of added SWNT (cf. Figures 1 and2). This effect seems to be confined to peroxides that are alreadydecomposing at the experimental temperature. When the rateis high, autoinduction may contribute to the overall decomposi-tion rate and SWNTs may serve as radical scavengers, analogousto the decomposition of BP in aromatic solvents with addedstyrene.19 Thus, at low SWNT levels there are multiplecompeting and interacting pathways that complicate and enhancethe overall rate.

We attribute the rate acceleration of peroxide thermolysiscaused by SWNTs to electron-transfer-induced decomposition,which has been seen previously with electron-rich aromatics39

and C60.26 For example, addition of 5 equiv of benzene to TFAPor other perfluoroacyl peroxides causes rate enhancements of2.7 to 4.2.23 These peroxides have low-lying antibonding M.O.’sthat make them particularly good electron acceptors.40 Studiesof PhP with polynuclear aromatic compounds41 and of SWNTswith aryl diazonium salts3 also supported initial electron-transfer.We propose that SWNTs reduce peroxides to radical anions,which immediately undergo O-O bond scission. Followingdecarboxylation, the radicals react rapidly with SWNT+• andlead to functionalization, probably via the pathway depicted inScheme 2 below for PhP. For simplicity, only one benzene ringof SWNTs is shown. The experimental support for Scheme 2consists of an enhanced Raman D band (Table 1), an elevatedoxygen content (Table 2), and especially the carbonyl carbonseen by IR and by XPS in Figure 3.

The mechanism for the acyclic peroxides, which is similarto Scheme 2, has already been set forth by Yoshida et al. for

C60.26 We do not know whether the ester moiety attached toSWNT carbon suffers hydrolysis, remains intact, or whetherthe SWNT cation is captured by adventitious water. Either anester group or an OH would account for the elevated oxygencontent in Table 2. The high efficiency of PhP in functionalizingSWNTs might be due to electrostatic attraction between radicalanion 1 and SWNT+• because the corresponding reaction inacyclic peroxides involves neutral radicals attacking SWNT+•s.The SET mechanism proposed here is new for SWNTs plusperoxides and is a likely contributor to earlier such studies.4-7

However, SET need not be the exclusive mechanism becausereactive radicals are known to attack SWNTs.6,9-12

Thermolysis of di-t-butylhyponitrite (DTBHN) leads toradicals that add to SWNTs, as judged from the Raman spectra.This result is plausible becauset-BuO• also attacks C60,42 thoughSWNTs are in general less reactive. Because there is far morebenzene solvent than SWNTs, one might expect predominantattack on benzene. However, the reactivity oft-BuO• isdiminished in benzene, possibly affecting selectivity.33 Thequestion remains whether the attacking species ist-BuO• ormethyl radicals arising fromâ-scission. The rate ofâ-scissioncan be calculated as 7.6× 103 s-1 at 55°C,43,44 ignoring anysolvent effect. The reaction rate oft-BuO• with SWNTs isunknown, not to mention the problem that SWNT “solutions”are actually suspensions, making it difficult to know the

Figure 4. Oxygen volume change in the autoxidation of cumene ino-DCB at 70°C.

SCHEME 2: Electron-Transfer-Induced Decompositionof PhP

698 J. Phys. Chem. C, Vol. 112, No. 3, 2008 Engel et al.

concentration. An XPS spectrum of the DTBHN functionalizedSWNTs suggests that at least part of the radical attack is byt-BuO•. Thus, two measurements of the raw SWNTs used inthis experiment revealed 4.3% and 4.9% oxygen. SWNTsthermolyzed with DTBHN showed 5.2% and 5.63% oxygen, asmall but discernible increase that supportst-BuO• attack.

As seen in Figure 4, neither SWNTs nor C12H25-SWNTsinhibit the autoxidation of cumene. This result means thatSWNTs do not react with the chain propagating radicals cumyland cumylperoxy (cf. Scheme 1). Because cumyl radicals areboth delocalized and hindered, their inertness in not surprising.The failure of SWNTs to scavenge cumylperoxy radicals standsin contrast to the behavior of C60, which traps both this radical37

andt-butylperoxy radicals.27,36,37,45A single report46 that C60 isinert toward alkylperoxy radicals, as determined by chemilu-minescence quenching, stands in contrast to the aforementionedwork. The enhanced reactivity of C60 can be attributed to itsgreater curvature than SWNTs.

In summary, we find that SWNTs induce the thermolysis ofdiacyl peroxides and are subject to attack byt-butoxy radicals.On the other hand, they do not inhibit the autoxidation ofcumene, indicating that they are far less reactive than C60

37 andare unlikely to serve as oxidation inhibitors.

Experimental Section

Commercial benzoyl peroxide (Luperox A98) was found tobe>99% pure by iodometric titration.p-MeO-BP and PhP wereprepared according to the literature19,20,47and were purified asneeded by multiple recrystallizations to>99%.o-Dichloroben-zene (o-DCB) was washed with aq. Na2S2O3 to remove anyperoxides, then with water, 2 M NaOH, water, saturatedNaHCO3, water, and brine. After drying over MgSO4, it wasdistilled over CaH2 at 1 atm.

The air-free peroxide titration experiments were carried outas follows. A stock solution of 100 mg of SWNTs in 1000 mLof 3x freeze-thaw degassedo-DCB was sonicated in a bathsonicator for 18 h. Appropriate volumes were removed via avolumetric pipet and diluted with pure, degassedo-DCB toproduce SWNT solutions containing 0-5 mg of SWNTs. Thesesolutions were flushed with N2, sonicated for 15 min, then placedin an oil bath at the appropriate temperature. After a 15 minequlibration period under N2, 0.3 mmol peroxide was addedand thermolysis was carried out in a closed system. Iodometrictitration16 was employed to determine the percent peroxideremaining after thermolysis. Specifically, the hot SWNT solutionwas vacuum filtered through a 0.2µm PTFE filter, at whichtime the SWNTs received their first exposure to air. The receiverwas a 250 mL sidearm flask containing 10 mL acetic anhydride(Ac2O) and∼0.5 g NaI. The reaction flask was rinsed withanother 5 mL ofAc2O, which was filtered as well. The filtercake was then washed with a final 5 mL of Ac2O and stored ina vial under air for analysis. The resultant dark yellow-brownsolution of 50 mL ofo-DCB and 20 mL of Ac2O was swirledperiodically for 30 min, at which time 40 mL of DI water wasadded. The two-phase mixture was titrated to the water-whiteendpoint with ∼0.07 M thiosulfate (exact concentration ofthiosulfate determined by standardization with KIO3).

The re-isolated SWNTs were examined spectroscopically,leading to the results in Tables 1 and 2. XPS characterizationwas carried out using a Physical Electronics Quanteras with amonochromated Al source (100× 100µm2 analysis area) whilethe Raman spectrometer was a Renishaw Raman Spectroscopymicroscope. According to XPS, the filtered SWNT samplescontained the same percent oxygen after months of storage asthey did when fresh.

Cumene Autoxidation. Commercially availableo-DCB(Fisher Scientific) and butylated hydroxytoluene (BHT) (Acros)were used without purification. Solutions of recrystallized AIBN(0.1N) and BHT (0.1N) were prepared ino-DCB. Cumene wasdistilled from calcium hydride (50 mmHg at 70°C). To ensureuniformity, all experiments used the same size flask (25 mL)and the same stir bar, stirring speed, and oxygen pressure (1atm). Efficient stirring and constant heating bath temperaturewere required to avoid volume fluctuations. To decrease thepartial pressure of solvent, a water chilled condenser wasattached directly to the reaction flask. A low-power bathsonicator was used to make the SWNT dispersion ino-DCB.

In a typical procedure, a solution of AIBN ino-DCB (10mL, 0.1 N, 1 mmol), and a solution of BHT ino-DCB (1 mL,0.1 N, 0.1 mmol) was added to a 25 mL flask containing cumene(1.322 g, 11 mmol). The flask was attached to the volumetricapparatus, evacuated to 20 mmHg, and filled with oxygen threetimes. The solution was immersed into an oil bath preheated to70 °C and the volume of oxygen consumed was monitored withtime. For experiments where SWNTs and C12-SWNTs are usedinstead of BHT, SWNTs were sonicated for 20 min in a solutionof AIBN in o-DCB (10 mL, 0.1 N, 1 mmol) in a bath sonicator.As a control, SWNTs (1.2 mg, 0.1 mmol) ino-DCB (10 mL)sonicated for 20 min were tested for oxygen consumption.SWNTs (1.2 mg, 0.1 mmol) in a solution of AIBN ino-DCB(10 mL, 0.1 N, 1 mmol) were also sonicated for 20 min andwere tested for oxygen uptake.

Acknowledgment. We gratefully acknowledge the RobertA. Welch Foundation (C-0490 and C-0499) and the NationalScience Foundation (CHE-0450085) for support of this work.We also thank Steven Ho and Dr. Robert H. Hauge of the RiceCarbon Nanotechnology Laboratory for the SWNT samples.

Supporting Information Available: Table of percentperoxide consumed for all runs shown in Figures 1 and 2, andall Raman and XPS data shown in Tables 1 and 2. This materialis available free of charge via the Internet at http://pubs.acs.org.

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