Tsvaygboym, J Phys Chem C 2008 v112 pp 695-700

1. J. Phys. Chem. C 2008, 112, 695-700 695
Reaction of Single-Walled Carbon Nanotubes with Organic Peroxides
Paul S. Engel,* Wilbur E. Billups, David W. Abmayr Jr., Konstantin Tsvaygboym, and
Runtang Wang
Department 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 electron
transfer (SET). Phthaloyl peroxide functionalizes SWNTs to the greatest extent of the four. It was also found
that t-butoxy radicals add to SWNTs but that SWNTs fail to inhibit cumene autoxidation. Thus, SWNTs are
reactive to alkoxy radicals but not to alkylperoxy radicals.
The great potential value of functionalizing single-walled
nanotubes (SWNTs) has led a number of workers to explore
free radical attack on the sidewalls of these unique nanometer-
sized objects.1,2 The most common radical precursors are
diazonium salts3 and peroxides,4-8 but other approaches are
based on Fenton’s reagent,6 perfluoroalkyl iodides,9,10 perfluoro
azo compounds,11,12 microwave discharge of ammonia,13 and
attack of growing polymer chains.14 Presently, we report the
kinetics of diacyl peroxide thermolysis in the presence of
SWNTs, which reveal moderate to large rate accelerations
attributed to induced decomposition. We further report the attack
of t-butoxy radicals on SWNTs and the failure of SWNTs to
inhibit the autoxidation of cumene.
While studying the thermolysis of benzoyl peroxide (BP) with
SWNTs, we noticed that the rate of gas evolution, as monitored
by a pressure transducer, was accelerated by inclusion of
purified, pristine HiPco SWNTs.15 Thus, a solution of 75 mg
BP in 10 mL ortho-dichlorobenzene (o-DCB) exhibited a 67%
greater pressure rise over the course of 2 h at 80 °C when
SWNTs (5 mg) were included than a control experiment without
SWNTs. This rate enhancement was confirmed by iodometric
titration16 of the BP remaining after thermolysis. All titration
studies discussed below were performed in non-degassed o-DCB Figure 1. Effect of added SWNTs on the thermolysis of benzoyl
using purified HiPCo SWNTs batch no. 164-2 produced in the peroxide (BP) in o-DCB at 80 °C (solid circles) and 90 °C (open circles)
Rice University Carbon Nanotechnology Laboratory.17 Com- and on p-methoxybenzoyl peroxide (p-MeO-BP) at 70 °C (solid squares)
and 80 °C (open squares) for 1 h.
parison of non-degassed with degassed o-DCB gave essentially
the same percent peroxide decomposition whether SWNTs were
present or not. thermolysis was important. After this initial jump, BP consump-
Figure 1 shows the percent BP consumed in 1 h at 80 °C tion rises more or less linearly with increasing amounts of
and 90 °C in 50 mL o-DCB as the initial weight of SWNTs SWNTs.
was increased from 0 to 5 mg. Although the rate enhancement The same effect was observed in p-methoxybenzoyl peroxide
due to SWNTs is apparent, a number of control experiments (p-MeO-BP), whose inherent thermolysis rate is about twice as
were required. The thermolysis of 0.006 M BP in o-DCB at fast as that of BP (Figure 1).19 The greater rate necessitated
80 °C is slow enough that hardly any decomposition (∼1%/hr) using lower temperatures, but the curve shape is similar for the
was detectable. We ran the thermolysis in benzene as a control two peroxides. The rates, especially at 80 °C and somewhat at
and obtained about 6%/hr decomposition, corresponding to the 70 °C, are elevated by a contribution from ordinary thermolysis
rate reported in the literature.18,19 Therefore o-DCB does not of p-MeO-BP. Several data points were obtained below 1 mg
induce BP thermolysis as much as benzene does and no SWNT, giving a steeper slope than the one at larger amounts
correction was needed for residual thermolysis at 80 °C. (cf. of SWNTs.
Figure 1). However, at 90 °C BP in o-DCB thermolyzes at a Much greater rate enhancements were found with two other
moderate rate, as evidenced by the 17% decomposed in 1 h peroxides: phthaloyl (PhP)20 and trifluoroacetyl (TFAP).21 As
even without added SWNTs. The steep rise in % BP consumed seen in Figure 2, the plot of the percent peroxide consumption
at low [SWNT] is seen in most runs where normal peroxide versus weight of SWNT at 80 °C exhibits a slope about 2.3
times greater with PhP than with BP. Moreover, TFAP is nearly
* Corresponding author. E-mail: engel@rice.edu. as sensitive to SWNTs as PhP. Initially, we changed solvents
10.1021/jp0770054 CCC: $40.75 © 2008 American Chemical Society
Published on Web 01/03/2008

2. 696 J. Phys. Chem. C, Vol. 112, No. 3, 2008 Engel et al.
TABLE 1: Average Raman D/G Area Ratios (%) of
Degassed SWNTs Subjected to Peroxides
peroxide D/G (633 nm) D/G (780 nm) no. samples
nonea 8.3 13.8 2
BP 14.4 19.6 5
p-MeO-BP 8.3b 12.5c 4, 2d
PhP 26.7 30.9 5
TFAP 6.1 9.1 3
a
Blank consisted of purified SWNTs stirred in o-DCB at 80 °C for
1 h then filtered. b A fifth sample gave D/G )17.2. c A third sample
gave D/G )24.9. d Four measurements at 633 nm and two at 780 nm.
TABLE 2: Average Atomic Percent of Elements in SWNTs
Exposed to Peroxidesa
peroxide %C %O %Cl %F no. samples
none 93.3 3.2 3.6 0 2
BP 93.6 3.9 1.6 0.9 5
p-MeOBP 91.4 5.2 2.4 1.0 8
PhP 85.6 12.7 1.7 0 5
TFAP 92.4 4.4 1.2 2.0 5
a
The data were obtained on 1-5 mg degassed SWNT samples
exposed to amounts of peroxide ranging from 0.15 to 0.6 mmol.
Figure 2. Effect of added SWNTs on the thermolysis of phthaloyl
peroxide (PhP) at 70 °C (solid triangles) and 80 °C (open triangles)
and on the thermolysis of trifluoroacetyl peroxide (TFAP) at 40 °C To determine whether the reaction of SWNTs with BP could
(solid diamonds) and 50 °C (open diamonds) for 1 h. be induced photochemically, we irradiated an o-DCB suspension
with a 500-W quartz-halogen lamp through a water filter. No
from o-DCB to nitrobenzene (NB) to suppress the solvent- change in rate was noticed, hardly a surprising result in view
induced decomposition of TFAP.22,23 In NB solvent at 50 °C, of the very short lifetime of excited SWNTs.25 In contrast, it
7% of TFAP was consumed in 1 h; but with 5 mg SWNTs, was reported recently that visible light irradiation of SWNTs
this figure rose to 75%. Because further experiments revealed with hydrogen peroxide caused the disappearance of their
unexpected reactions with NB, it was necessary to conduct the characteristic near-infrared fluorescence.8
remaining studies in o-DCB and to simply tolerate the ther-
molysis of TFAP in the blank run. We found that rapid addition Product Characterization
of TFAP to o-DCB without SWNTs gave more peroxide Although the product of phenyl radical attack on SWNTs
decomposition (25-35%) than dropwise addition (15-20%) has been characterized already,4,5 we analyzed the products of
This effect may be due to autoinduction caused by high local our peroxides with SWNTs by Raman and XPS spectroscopy.
TFAP concentrations because the droplets of TFAP did not These methods were employed because SWNTs do not allow
immediately dissolve in o-DCB. The problem was minimized us the luxury of using some of the powerful analytical techniques
by dropwise addition of TFAP, in contrast to the other three applicable to the far more soluble C60.26,27
peroxides, which were added all at once as solids. A few of the Raman D/G area ratios are often determined to ascertain the
peroxide + SWNT rate experiments were repeated with a degree of SWNT functionalization.3,12,28,29 Such measurements
different batch of SWNTs (no. 162-1). These runs showed less were carried out here using laser wavelengths of 633 and 780
peroxide consumption for a given weight of SWNTs but nm on SWNT sample no. 162-1 recovered from degassed
otherwise the same trends as in Figures 1 and 2. titration runs, as summarized in Table 1 and shown in more
The rate acceleration seen on addition of SWNTs to peroxides detail in the Supporting Information. While 780 nm probes
might be attributed to traces of iron left in the purified SWNTs. mainly semiconducting SWNTs, 633 nm begins to see metallic
However, such an explanation is unlikely at the outset because tubes as well. In accord with the literature on BP, we observe
these SWNTs had been subjected to multiple oxidations during an increase in the Raman D band. The largest increase appears
purification17 and they contained only ∼1.5% Fe by TGA with PhP, whose high D/G ratio is comparable to other SWNT
analysis. Moreover, any residual Fe is encapsulated in a layer studies in the literature12,28,30 and implies that PhP strongly
of carbon so that we could never see an iron signal by XPS. functionalizes SWNTs. The D/G ratio at 633 nm versus that at
Control experiments with deliberate addition of 2 mg of iron 780 nm is close to the value for the blank except for PhP.
powder or 50 µg of Fe(II) or Fe(III) (as chlorides) to BP in Another approach to look for SWNT functionalization is
o-DCB showed the same decomposition rate as the blank. X-ray photoelectron spectroscopy (XPS). The same degassed
Although 50 µg is the approximate concentration of iron in the SWNT samples as in Table 1 were analyzed by XPS, yielding
SWNT samples, we also tried 2 mg of Fe(II) and Fe(III). These the results summarized in Table 2. Although different weights
large doses of oxidized iron caused greatly enhanced the of SWNTs were exposed to varying concentrations of peroxides,
peroxide decomposition rates but are unrealistic control experi- there was no correlation of elemental composition with these
ments. We reason that if residual Fe is inaccessible to oxygen parameters. The most obvious effect is the high oxygen
during purification and to the X-ray beam of XPS it is surely incorporation with PhP, which will be discussed below.
inaccessible to solution-phase peroxides. Although it is still Although the fluorine content was greatest for TFAP, the
possible that the rate acceleration is caused by an impurity in averages in Table 2 do not tell the whole story (cf. Supporting
the HiPco SWNTs, such an impurity would have to be a much Information for all data). Every TFAP-SWNT sample contained
more effective catalyst than Fe(II).24 ∼2% F, but several of the p-MeO-BP and BP samples showed

3. Reaction of Single-Walled Carbon Nanotubes J. Phys. Chem. C, Vol. 112, No. 3, 2008 697
TABLE 3: Percent BP Consumed on Thermolysis of BP for
2 h at ∼86 °C in o-DCB
SWNT type SWNT weight, mg % BP consumed
none 0 25
SWNT 5 79
Ph-SWNT 5 52
C12H25-SWNT 5 53
SCHEME 1: Autoxidation of Cumene
Figure 3. XPS C1s spectrum of SWNTs subjected to peroxides. The
typical position of the bold, underlined carbons is shown on the plot. Reaction of SWNTs with t-Butoxy Radicals
little or no F. This variability almost certainly arises because Highly reactive t-butoxy radicals33 are conveniently generated
the smaller samples did not completely coat the Teflon filter by mild thermolysis of di-t-butyl hyponitrite (DTBHN).34 To
membrane used to isolate reacted SWNTs.3 Therefore, part of check the reactivity of SWNTs toward t-BuO•, a suspension of
the membrane was sometimes accidentally included in the 5 mg of SWNTs and 50 mg of DTBHN in 4-5 mL of benzene
analysis. We are confident that only the TFAP-exposed SWNTs was heated under nitrogen at 55 °C for 2 days. The Raman D
contained an elevated level of fluorine. The chlorine in all band was found to increase approximately 10-fold, indicating
samples comes from the purification step, which involves HCl.17 considerable attack of radicals on the SWNTs. Repeating this
experiment with 150 mg of DTBHN and 2 mg of SWNTs in
The data in Table 1 indicate that PhP and BP functionalize ∼5 mL of benzene again increased the D band 10-fold to 44%
SWNTs while Table 2 shows that same result with PhP, p-MeO- of the G band. In the breathing mode region, the pristine SWNTs
BP, and TFAP. Apparently, Raman spectroscopy is not sensitive exhibited a group of three bands at 213, 224 (sh), and 232 cm-1,
enough to detect the small amount (<1 CF3 per 100 carbons) but after treatment with DTBHN the lower frequency bands
of added groups from TFAP. A small shoulder at 291.2 eV on had greatly decreased relative to the one at 266 cm-1. Because
the carbon 1s band supports the presence of a CF3 group.9 the frequency of these modes is inversely proportional to SWNT
Although BP does not add enough phenyl groups to change diameter,13 it appears that t-BuO• is selective for larger diameter
the carbon content seen by XPS, we observe an increase in the SWNTs. The ratio of benzene to SWNTs is over 300; hence,
Raman D/G ratio, in accord with previous reports.4-6 Peroxides t-BuO• must preferentially attack SWNTs35 or we would see
RCOOOCOR might introduce either R or RCOO groups, and no increase in the D/G ratio.
the latter would lead to more oxygen in the recovered SWNTs
than in the blank (Table 2). Such an increase is obvious with The Effect of SWNTs on Cumene Autoxidation
PhP but still occurs with the other peroxides, especially p-MeO-
BP. The 5.2% oxygen found with p-MeO-BP can be explained Literature reports36,37 on the reaction of “reactive oxygen
in part by p-methoxyphenyl radicals adding to SWNTs.12 In species” with fullerenes prompted us to determine whether
fact, a shoulder due to the methoxy carbon is clearly visible at SWNTs would behave similarly. We chose to study the
286.5 eV. (cf. Figure 3). Because the XPS measurements were autoxidation of cumene, which proceeds by the chain mechanism
done on samples that were degassed and never exposed to air shown in Scheme 1.38 The experimental approach was to
determine manometrically whether SWNTs would inhibit the
until workup, there is little chance that atmospheric oxygen is
AIBN-initiated uptake of gaseous oxygen by cumene in o-DCB
captured by reacting SWNTs. In contrast, non-degassed samples
at 70 °C. As shown in the lowest curve of Figure 4, AIBN with
did exhibit an elevated oxygen content and samples deliberately
SWNTs in o-DCB exhibited an apparent rapid volume increase
exposed to O2 gave 8-9% oxygen by XPS. XPS of SWNTs +
over 5 min, which we attribute to the rise in solvent vapor
BP and p-MeO-BP showed a small peak at 289 eV attributed
pressure after the reaction vessel was placed into the hot oil
to carbonyl carbon but this peak was very clear with PhP (cf.
bath. This rapid drop in the curve (volume increase) was
Figure 3). Additional support for the carbonyl group is found followed by a much slower decline due to nitrogen evolution
in the ATR IR spectrum of SWNTs that had been thermolyzed from AIBN. When AIBN was omitted (“SWNTs only”), the
with PhP, where we observed a distinct band centered at 1704 curve was flat after the equilibration period. The curve labeled
cm-1 (half width 84 cm-1). “BHT” shows the typical behavior of an autoxidation inhibitor,
SWNTs have been functionalized by various methods, among where BHT is 2,6-d-t-butylcresol. In this case, hardly any
which are thermolysis with excess benzoyl peroxide5,31 and oxygen was taken up for the first 40 min. Once the inhibitor
reductive alkylation with alkyl halides.32 To ascertain whether was exhausted, oxygen was absorbed at a rate of ∼9 mL/hr.
previously functionalized SWNTs31,32 were capable of inducing SWNTs do not behave like BHT, for the curve shows no
BP decomposition, we ran a set of four experiments in o-DCB inhibition period but instead shows the steady uptake of oxygen
at ∼86 °C for 2 h, as summarized in Table 3. Although after equilibration. The same behavior was found for dodecyl
unfunctionalized SWNTs roughly tripled the percent of BP functionalized SWNTs.
consumed, those bearing phenyl or dodecyl groups only doubled The SWNTs and C12-SWNTs that were re-isolated after the
the BP consumption. Clearly, functionalized SWNTs still induce attempted autoxidation inhibition showed no change in their
BP decomposition but not as effectively as SWNTs themselves. Raman spectra. It is therefore likely that they were not destroyed

4. 698 J. Phys. Chem. C, Vol. 112, No. 3, 2008 Engel et al.
Figure 4. Oxygen volume change in the autoxidation of cumene in o-DCB at 70 °C.
and that SWNTs are not autoxidation inhibitors. Attributing this SCHEME 2: Electron-Transfer-Induced Decomposition
negative result to poor suspendability of SWNTs in o-DCB is of PhP
not a viable argument because such suspensions are stable for
months and they react nicely with peroxides.
Discussion
BP at 90 °C, p-MeO-BP at 80 °C , TFAP at both 40 and
50 °C, but not PhP, exhibit a greater initial slope than the one
attained at higher amounts of added SWNT (cf. Figures 1 and
2). This effect seems to be confined to peroxides that are already
decomposing at the experimental temperature. When the rate
is high, autoinduction may contribute to the overall decomposi-
tion rate and SWNTs may serve as radical scavengers, analogous C60.26 We do not know whether the ester moiety attached to
to the decomposition of BP in aromatic solvents with added SWNT carbon suffers hydrolysis, remains intact, or whether
styrene.19 Thus, at low SWNT levels there are multiple the SWNT cation is captured by adventitious water. Either an
competing and interacting pathways that complicate and enhance ester group or an OH would account for the elevated oxygen
the overall rate. content in Table 2. The high efficiency of PhP in functionalizing
We attribute the rate acceleration of peroxide thermolysis SWNTs might be due to electrostatic attraction between radical
caused by SWNTs to electron-transfer-induced decomposition, anion 1 and SWNT+• because the corresponding reaction in
which has been seen previously with electron-rich aromatics39 acyclic peroxides involves neutral radicals attacking SWNT+•s.
and C60.26 For example, addition of 5 equiv of benzene to TFAP The SET mechanism proposed here is new for SWNTs plus
or other perfluoroacyl peroxides causes rate enhancements of peroxides and is a likely contributor to earlier such studies.4-7
2.7 to 4.2.23 These peroxides have low-lying antibonding M.O.’s However, SET need not be the exclusive mechanism because
that make them particularly good electron acceptors.40 Studies reactive radicals are known to attack SWNTs.6,9-12
of PhP with polynuclear aromatic compounds41 and of SWNTs Thermolysis of di-t-butylhyponitrite (DTBHN) leads to
with aryl diazonium salts3 also supported initial electron-transfer. radicals that add to SWNTs, as judged from the Raman spectra.
We propose that SWNTs reduce peroxides to radical anions, This result is plausible because t-BuO• also attacks C60,42 though
which immediately undergo O-O bond scission. Following SWNTs are in general less reactive. Because there is far more
decarboxylation, the radicals react rapidly with SWNT+• and benzene solvent than SWNTs, one might expect predominant
lead to functionalization, probably via the pathway depicted in attack on benzene. However, the reactivity of t-BuO• is
Scheme 2 below for PhP. For simplicity, only one benzene ring diminished in benzene, possibly affecting selectivity.33 The
of SWNTs is shown. The experimental support for Scheme 2 question remains whether the attacking species is t-BuO• or
consists of an enhanced Raman D band (Table 1), an elevated methyl radicals arising from β-scission. The rate of β-scission
oxygen content (Table 2), and especially the carbonyl carbon can be calculated as 7.6 × 103 s-1 at 55 °C,43,44 ignoring any
seen by IR and by XPS in Figure 3. solvent effect. The reaction rate of t-BuO• with SWNTs is
The mechanism for the acyclic peroxides, which is similar unknown, not to mention the problem that SWNT “solutions”
to Scheme 2, has already been set forth by Yoshida et al. for are actually suspensions, making it difficult to know the

5. Reaction of Single-Walled Carbon Nanotubes J. Phys. Chem. C, Vol. 112, No. 3, 2008 699
concentration. An XPS spectrum of the DTBHN functionalized Cumene Autoxidation. Commercially available o-DCB
SWNTs suggests that at least part of the radical attack is by (Fisher Scientific) and butylated hydroxytoluene (BHT) (Acros)
t-BuO•. Thus, two measurements of the raw SWNTs used in were used without purification. Solutions of recrystallized AIBN
this experiment revealed 4.3% and 4.9% oxygen. SWNTs (0.1N) and BHT (0.1N) were prepared in o-DCB. Cumene was
thermolyzed with DTBHN showed 5.2% and 5.63% oxygen, a distilled from calcium hydride (50 mmHg at 70 °C). To ensure
small but discernible increase that supports t-BuO• attack. uniformity, all experiments used the same size flask (25 mL)
As seen in Figure 4, neither SWNTs nor C12H25-SWNTs and the same stir bar, stirring speed, and oxygen pressure (1
inhibit the autoxidation of cumene. This result means that atm). Efficient stirring and constant heating bath temperature
SWNTs do not react with the chain propagating radicals cumyl were required to avoid volume fluctuations. To decrease the
and cumylperoxy (cf. Scheme 1). Because cumyl radicals are partial pressure of solvent, a water chilled condenser was
both delocalized and hindered, their inertness in not surprising. attached directly to the reaction flask. A low-power bath
The failure of SWNTs to scavenge cumylperoxy radicals stands sonicator was used to make the SWNT dispersion in o-DCB.
in contrast to the behavior of C60, which traps both this radical37 In a typical procedure, a solution of AIBN in o-DCB (10
and t-butylperoxy radicals.27,36,37,45 A single report46 that C60 is mL, 0.1 N, 1 mmol), and a solution of BHT in o-DCB (1 mL,
inert toward alkylperoxy radicals, as determined by chemilu- 0.1 N, 0.1 mmol) was added to a 25 mL flask containing cumene
minescence quenching, stands in contrast to the aforementioned (1.322 g, 11 mmol). The flask was attached to the volumetric
work. The enhanced reactivity of C60 can be attributed to its apparatus, evacuated to 20 mmHg, and filled with oxygen three
greater curvature than SWNTs. times. The solution was immersed into an oil bath preheated to
In summary, we find that SWNTs induce the thermolysis of 70 °C and the volume of oxygen consumed was monitored with
diacyl peroxides and are subject to attack by t-butoxy radicals. time. For experiments where SWNTs and C12-SWNTs are used
On the other hand, they do not inhibit the autoxidation of instead of BHT, SWNTs were sonicated for 20 min in a solution
cumene, indicating that they are far less reactive than C6037 and of AIBN in o-DCB (10 mL, 0.1 N, 1 mmol) in a bath sonicator.
are unlikely to serve as oxidation inhibitors. As a control, SWNTs (1.2 mg, 0.1 mmol) in o-DCB (10 mL)
sonicated for 20 min were tested for oxygen consumption.
Experimental Section
SWNTs (1.2 mg, 0.1 mmol) in a solution of AIBN in o-DCB
Commercial benzoyl peroxide (Luperox A98) was found to (10 mL, 0.1 N, 1 mmol) were also sonicated for 20 min and
be >99% pure by iodometric titration. p-MeO-BP and PhP were were tested for oxygen uptake.
prepared according to the literature19,20,47 and were purified as
needed by multiple recrystallizations to >99%. o-Dichloroben- Acknowledgment. We gratefully acknowledge the Robert
zene (o-DCB) was washed with aq. Na2S2O3 to remove any A. Welch Foundation (C-0490 and C-0499) and the National
peroxides, then with water, 2 M NaOH, water, saturated Science Foundation (CHE-0450085) for support of this work.
NaHCO3, water, and brine. After drying over MgSO4, it was We also thank Steven Ho and Dr. Robert H. Hauge of the Rice
distilled over CaH2 at 1 atm. Carbon Nanotechnology Laboratory for the SWNT samples.
The air-free peroxide titration experiments were carried out
as follows. A stock solution of 100 mg of SWNTs in 1000 mL Supporting Information Available: Table of percent
of 3x freeze-thaw degassed o-DCB was sonicated in a bath peroxide consumed for all runs shown in Figures 1 and 2, and
sonicator for 18 h. Appropriate volumes were removed via a all Raman and XPS data shown in Tables 1 and 2. This material
volumetric pipet and diluted with pure, degassed o-DCB to is available free of charge via the Internet at http://pubs.acs.org.
produce SWNT solutions containing 0-5 mg of SWNTs. These
solutions were flushed with N2, sonicated for 15 min, then placed References and Notes
in an oil bath at the appropriate temperature. After a 15 min
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