Rapid and controllable covalent functionalization of single-walled carbon nanotubes at room temperature

Article abstract of DOI:10.1039/b712299c

We report a rapid and efficient procedure to functionalize SWNT where free radicals generated at room temperature by a redox reaction between reduced SWNT and diacyl peroxide derivatives were covalently attached to the SWNT wall. The Royal Society of Chemistry.

Full text of DOI:10.1039/b712299c

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Rapid and controllable covalent functionalization of single-walled
carbon nanotubes at room temperature{
a
Yadienka Mart ´ı nez-Rub ´ı , Jingwen Guan, Shuqiong Lin, Christine Scriver, Ralph E. Sturgeon and
a
a
b
b
a
Benoit Simard*
Received (in Cambridge, UK) 10th August 2007, Accepted 2nd October 2007
First published as an Advance Article on the web 12th October 2007
DOI: 10.1039/b712299c
We report a rapid and efficient procedure to functionalize
SWNT where free radicals generated at room temperature by
a redox reaction between reduced SWNT and diacyl peroxide
derivatives were covalently attached to the SWNT wall.
SWNT in advanced materials. Chemical processing costs must
be reduced to make industrial utilization of SWNT a reality.
7
Recently, Penicaud et al. showed that SWNT can be readily
ex-foliated in standard solvents by reduction with an alkali metal
though electron transfer mediated by alkali-metal–naphthalene–
THF complexes. As the SWNT get charged negatively, they
exfoliate as a result of electrostatic repulsion. This is an important
practical advance because it readily allows chemistry at the single
tube, thus saving a considerable amount of time. In addition,
reduced tubes acquire higher nucleophilic character, thus allowing
chemistry under less stringent conditions. Here, we show that
reduced SWNT react readily within minutes at room temperature
with diacyl peroxide derivatives to yield side-walled covalently
functionalized SWNT. Furthermore, we show that the function-
alization level can be controlled through repetitive action of the
redox reaction.
Single-walled carbon nanotubes (SWNT) exhibit the best mechani-
cal, thermal and electrical properties of any known material.
Combined with their very high aspect ratios that can reach well
over 1000, SWNT is the dreamed material for the fabrication of
composites with ultimate performance. Unfortunately, despite the
wide availability of SWNT and despite many attempts, all SWNT-
based composites reported to date showed poorer than expected
1
performance, often worse than that of the pure matrices. The
main reasons for this are (1) highly variable purity and quality of
the SWNT samples used, (2) poor dispersion/exfoliation and, (3)
poor interface compatibility with the matrix. The second and
third reasons are related to the intrinsic strong van der Waals
interactions among the SWNT which lead to the formation of
large bundles and to the intrinsic chemical stability of SWNT
which makes binding to matrices rather difficult.
Our approach is depicted in Scheme 1 and consists of directly
adding an acyl peroxide derivative to a previously prepared
suspension of reduced SWNT. Reduced SWNT can be prepared
in one-step in either THF by electron transfer from alkali-
naphthalene salts (ESI{) or in toluene by electron transfer from
alkali-metal benzophenone salts. The acyl peroxide derivatives
used here are terminated with alkyl, phenyl, carboxylic acid and
Fmoc-protected amine functionalities, but the method should be
applicable to any acyl peroxide derivatives so that in principle any
functionality can be anchored on the SWNT at room temperature.
Similar alkyl, phenyl and carboxylic acid functionalization can
be achieved with neutral SWNT but it requires the thermal
activation (decomposition) of the peroxide derivatives and several
It is now widely accepted that chemistry is central to the
development of high performance materials based on SWNT.
Chemistry can solve the problems associated with bundles and the
lack of binding with the matrices. In the past five years, side-walled
covalent functionalization on neutral SWNT has received
2
considerable attention, notably by the groups led by Prato,
3
4
5
6
Hirsch, Tour, Margrave, Haddon, and many others. Strategies
to anchor all the practical functionalities have been developed.
Two major drawbacks are that completion of the reaction at the
single tube level requires substantial amounts of time and energy
because the SWNT are inherently unreactive and bundled. The
reaction proceeds on the outer tubes of the bundles first, which
eventually exfoliate to expose the second layer to the reagents and
so on. Reaction completion takes several hours, or even days,
under refluxing conditions. As a result, chemical processing is the
major and limiting cost associated with the integration of neutral
8–10
hours, or even days, of reaction time.
Fmoc-protected amine
a
Molecular and Nanomaterial Architectures Group, Steacie Institute for
Molecular Sciences, National Research Council of Canada, Ottawa,
Ontario, K1A 0R6. E-mail: Benoit.Simard@nrc-cnrc.gc.ca;
Fax: 613 991 2648; Tel: 613 990 0977
b
Chemical Metrology Group, Institute for National Measurement
Standards, National Research Council of Canada, Ottawa, Ontario,
K1A 0R6
{
Electronic supplementary information (ESI) available: Experimental
details, Na content, XPS and Raman spectra of various samples, LC-ESI-
MS of reaction byproducts. TGA results, MS-MALDI analysis, TEM
images and photograph of SWNT-GAP suspension. See DOI: 10.1039/
b712299c
Scheme 1
5
146 | Chem. Commun., 2007, 5146–5148
This journal is ß The Royal Society of Chemistry 2007

View Article Online
functionalization of neutral SWNT through the thermal decom-
position of the acyl peroxide derivative does not occur due to
rapid radical–radical reactions (ESI{). Thus, it appears that our
approach allows for new functionalizations, especially with amine
terminated ligands. Reduced SWNT are obviously good reducing
agents and hence are particularly reactive towards strong oxidizing
agents such as peroxides, thus allowing the reaction to occur
readily at room temperature. The limiting step is glassware
handling rather than reaction kinetics.
functionalized SWNT (SWNT-GAP; Fig. 1(B)) and re-subjecting
them to another reaction cycle with GAP. Similar data are
obtained with the other acyl peroxide derivatives (ESI{). However,
with GAP and SAP a higher degree of functionalization is
achieved, as indicated by higher intensity in the D-band of the
Raman spectra. This repetitive process also affords the possibility
of anchoring functional groups of different types; one type per
cycle, to produce multifunctional SWNT.
The degree of functionalization of carboxyl functionalized
The studies have been carried out with laser-grown materials
prepared according to the two-laser method developed in our
SWNT can be determined by first substituting the acidic proton
+
with Na using NaHCO
3
and then using microwave assisted
11,12
laboratory and reported previously.
+
The raw material has a
acidic leaching of the sample in 3 M HNO3 to free Na for
subsequent quantitation by ICP-AES (ESI{). The total concentra-
tion of Na in pristine SWNT (Fig. 1(A)) and SWNT-GAP after
the first reaction cycle (Fig. 1(B)) was 3350, and 16500 ppm
respectively, yielding a functionalization level of approximately
purity estimated to 70–80% using a combination of characteriza-
tion methods such as transmission electron (TEM) and scanning
electron microscopy (SEM), Raman and optical spectroscopy,
thermogravimetric analysis (TGA) and porosity measurements.
Raman spectroscopy is a sensitive and useful tool to assess side-
wall quality (graphitization) and side-wall functionalization. Our
Raman characterization is done using the 514 nm argon-ion laser
line with power density low enough for the Raman beam to be in
the non-invasive mode, thus providing an accurate assessment
of the wall structures. The 514 nm laser line excites mostly
semiconducting tubes which form two-thirds of the total popula-
tion of SWNT. The raw SWNT are purified to a purity of at least
1
C% after the first reaction cycle. A very similar value was
obtained with TGA by measuring the mass loss upon heating. The
CH CH CH COOH moiety desorbs at about 300 uC.
The identities of the functional groups anchored on the side-wall
–
2
2
2
of SWNT have been ascertained by IR spectroscopy and XPS.
Fig. 2 shows IR spectra of pristine SWNT, SWNT-GAP and
21
SWNT-NFP samples. The feature near 1715 cm in the spectrum
of SWNT-GAP can be readily associated with the CLO stretching
mode of the carboxyl acid moiety. In the case of SWNT-NFP,
the feature associated with the carbonyl group of Fmoc appears
90% through an in-house process that will be reported in detail
elsewhere. The purification does not employ acid treatments but
only flotation, sedimentation and extraction cycles that leave the
side-wall intact, as determined through Raman spectroscopy
2
1
at a lower frequency (1700 cm ), as expected. The bands in the
2
1
3
000–2800 cm region are attributed to the C–H stretching,
(ESI{). All chemicals (reagent grade, 97–99%) were purchased
whereas the features in the 1560–1550 region are due to CLC
stretching mode activated by sidewall attachment.
from Aldrich except for SOCl (Acros) and Fmoc-6-aminohexa-
2
noic acid (Genscript)
Liquid chromatography electrospray ionization mass spectro-
metry (LC-ESI-MS) analysis of the filtrate from the reaction
between reduced SWNT and NFP showed that the major
byproducts of the reaction are Fmoc-6-aminohexanoic acid
Fig. 1 shows the Raman spectra of pristine SWNT and SWNT-
GAP samples. The reaction of reduced SWNT with glutaric acid
acyl peroxide (GAP) leads to a substantial increase in the D-band
intensity, indicating side-wall functionalization. This D-band
increase is also correlated with an increase in the D + G
(C H NO ), Fmoc-6-aminohexanoic acid methyl ester
21 23 4
21
4
(C22H25NO )
and two compounds (C20
H
21NO
2
and
combination mode at ca. 2925 cm . The diameter distribution of
the SWNT probed by the 514 laser line is very narrow as
determined by a single radial breathing mode (RBMs) band at ca.
C H NO ) formed by the disproportion of the Fmoc-5-
23
20
2
aminopentyl radical (ESI{). The same byproducts were observed
after the second functionalization cycle and when the SWNT and
2
1
1
75 cm . The relative intensity of the RBM remains unchanged
upon functionalization. The figure also shows that the degree
of functionalization can be increased (Fig. 1(C)), hence controlled,
by repeating the redox process by reducing the previously
Fig. 1 Raman spectra of (A) pristine SWNT, (B) SWNT-GAP after the
Fig. 2 Infrared spectra of pristine SWNT (bottom), SWNT-NFP
first reaction cycle, (C) SWNT-GAP after the second reaction cycle.
(middle), and SWNT-GAP (top).
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 5146–5148 | 5147

View Article Online
NFP were refluxed for a few hours in THF. These results are
indicative of the radical pathway and also of the stability of the
ligand under the reaction conditions. MS-MALDI analysis on
SWNT-NFP confirmed that the anchored fragment is indeed
Notes and references
1
J. N. Coleman, U. Khan, W. J. Blau and Y. K. Gun’ko, Carbon, 2006,
4, 1624.
2 D. Tasis, N. Tagmatarchis, A. Bianco and M. Prato, Chem. Rev., 2006,
06, 1105.
4
1
–
(CH ) –NH–Fmoc (ESI{). The SWNT-NFP can be easily de-
2
5
3
4
5
A. Hirsch and O. Vostrowsky, Top. Curr. Chem., 2005, 245, 193.
J. L. Bahr and J. M. Tour, J. Mater. Chem., 2002, 12, 1952.
V. H. Khabashesku, W. E. Billups and J. L. Margrave, Acc. Chem. Res.,
2002, 35, 1087.
protected to reconstitute the primary amine for use in various
applications.
In summary, a simple, efficient, and cost-effective method has
been developed to functionalize SWNT at room temperature with
useful functional groups. The process is amenable to practically
any scales provided the reactions are carried out in oxygen-free
environment, a condition regularly encountered in industrial
settings.
6
7
8
9
S. Niyogi, M. A. Hamon, H. Hu, B. Zhao, P. Bhowmik, R. Sen,
M. E. Itkis and R. C. Haddon, Acc. Chem. Res., 2002, 35, 1105.
A. Penicaud, P. Poulin, A. Derre, E. Anglaret and P. Petit, J. Am.
Chem. Soc., 2005, 127, 8.
H. Peng, L. B. Alemany, J. L. Margrave and V. L. Khabashesku, J. Am.
Chem. Soc., 2003, 125, 15174.
Y. Ying, R. K. Saini, F. Liang, A. K. Sadana and W. E. Billups, Org.
Lett., 2003, 5, 1471.
10 P. Umek, J. W. Seo, K. Hernadi, A. Mrzel, P. Pechy, D. D. Mihailovic
Acknowledgment is made to J. Stupak for the MS-MALDI
analysis, D. Kingston for the XPS analysis and A. Schindler from
NETZSCH Applications Laboratory for running the TG analysis
and L. Forro, Chem. Mater., 2003, 15, 4751.
1
1 C. T. Kingston, Z. J. Jakubek, S. Denommee and B. Simard, Carbon,
004, 42, 1657.
12 C. T. Kingston and B. Simard, J. Nanosci. Nanotechnol., 2006, 6, 3677.
1
using a NETZSCH model STA 449 Jupiter simultaneous
2
thermal analyzer.
5
148 | Chem. Commun., 2007, 5146–5148
This journal is ß The Royal Society of Chemistry 2007