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Covalent sidewall functionalization of single-walled carbon nanotubes: a photoreduction
approach
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2007 Nanotechnology 18 495703
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 18 (2007) 495703 (5pp)
doi:10.1088/0957-4484/18/49/495703
Covalent sidewall functionalization of
single-walled carbon nanotubes:
a photoreduction approach
Liangming Wei and Yafei Zhang
National Key Laboratory of Nano/Micro Fabrication Technology, Key Laboratory for Thin
Film and Microfabrication of Ministry of Education, Institute of Micro and Nano Science and
Technology, Shanghai Jiao Tong University, Shanghai 200030, People’s Republic of China
E-mail: lmwei@sjtu.edu.cn
Received 25 July 2007, in final form 4 October 2007
Published 2 November 2007
Online at stacks.iop.org/Nano/18/495703
Abstract
Covalent sidewall functionalization of single-walled carbon nanotubes
(SWNTs) via photoreduction of aromatic ketones by alcohols is reported for
the first time. Irradiation of benzophenone, benzhydrol and SWNTs in
benzene resulted in covalent attachment of benzhydrol to the sidewalls of the
SWNTs. A variety of tools were used to characterize the functionalized
SWNTs. Raman scattering, UV–visible and near-IR spectroscopy confirm the
covalent nature of the sidewall functionalization. Attenuated total reflection
(ATR) FTIR and NMR provided evidence for attachment of benzhydrol onto
the sidewalls of nanotubes. Thermogravimetric analysis (TGA) showed that
the degree of functionalization was about one benzhydrol in 52 sidewall
carbons. A long-chain hydrocarbon marker (n -C18 H35 ) was also grafted onto
the functional groups by esterification reaction for high-resolution TEM
(HRTEM) visualization.
of wet-chemical functionalization of SWNTs [1]. In this paper, we report a photoreduction method for covalent sidewall
functionalization of SWNTs via photoreduction of aromatic
ketones by alcohols (scheme 1). The photoreduction mechanism could be explained by a benzophenone (BP) in the excited
triplet state abstracting a hydrogen atom from the carbinol carbon within a benzhydrol (BZ) [14], leading to the formation of
BZ radicals which add readily to nanotubes. The BP/BZ system was chosen in this study because the bimolecular reaction
gave only one type of radical (BZ radical) [14], which makes
it easy to characterize the resulting production.
Most reports on addition of radicals onto SWNTs thus
far are based on decomposition of unique molecules, such
as organic peroxides [6], 4-methoxyphenylhydrazine [15],
heptadecafluoro-octyl iodides [10] or diazonium salts [16]. In
our case, the radicals were formed via hydrogen abstraction
from C–H bonds by excited BP triplets. Since the excited
BP triplet is reactive to many types of C–H bonds such as
alcohols [17], amines [18], ethers [19], hydrocarbons [20]
or aliphatic sulfides [21], a wide variety of radicals can be
generated using this photochemical method. In principle,
1. Introduction
The chemical sidewall functionalization of single-walled carbon nanotubes (SWNTs) has received much attention [1], as it
can improve solubility and processability and allows the unique
properties of SWNTs to be coupled with those of other types
of materials [2]. Several methods for sidewall functionalization
have been developed, using fluorine [3], carbenes [4], azomethine ylides [5] or organic radicals [6]. The photochemical
approach for functionalization of solid substrates is particularly interesting due to its significant advantages [7, 8]: mild
reaction conditions, low cost of operation, selectivity to absorb UV light without destroying the bulk structure and convenience to asymmetrical modification [9]. Recently, photochemical sidewall functionalization of SWNTs via photolysis
of some materials such as heptadecafluoro-octyl iodide [10],
azide reagents [11] or organosilanes [12] has been reported.
Argon plasma-assisted UV grafting of vinylimidazole has also
been developed for functionalization of carbon nanotubes [13].
However, the samples of photochemical sidewall functionalization of SWNTs are still limited, in contrast to a large variety
0957-4484/07/495703+05$30.00
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© 2007 IOP Publishing Ltd Printed in the UK
Nanotechnology 18 (2007) 495703
L Wei and Y Zhang
Scheme 1. Photochemical route for preparation of BZ-SWNTs and
their derivatives C18 –BZ-SWNTs.
Figure 1. Raman spectra of SWNT materials: (a) purified SWNTs;
(b) BZ-SWNTs.
these radicals are also feasible for functionalization of SWNTs,
which could lead to a large variety of functional systems.
Particularly, a molecule with chemical inertness, such as
polypropylene, is easy to change to a highly reactive radical
species via hydrogen abstraction by the excited BP triplet [7],
which makes it possible to covalently graft the molecule onto
the sidewalls of carbon nanotubes. Additional advantages of
BP systems are thermal stability, chemical inertness in the
absence of light [22], and the reactants are easily accessible.
A big challenge in the field of nanotube chemistry is the
characterization of the functionalized SWNTs [23]. Many
traditional tools of organic chemistry are of limited use
or cannot be applied [24]. Here, we provided a detailed
characterization of functionalized SWNTs using a variety of
tools. A long-chain hydrocarbon marker (n -C18 H35 ) was also
grafted onto the functional groups by esterification reaction for
high-resolution TEM (HRTEM) visualization.
2.3. Grafting of long-chain hydrocarbon onto the sidewalls of
BZ-SWNTs
In a typical experiment, BZ-SWNTs (5 mg), stearoyl chloride
(5 ml) and 10 ml of pyridine were added to a dried flask
and stirred for five days at 100 ◦ C. After the reaction was
completed, the mixture was centrifuged, washed with plenty
of tetrahydrofuran and dried in vacuum at 60 ◦ C.
2.4. Characterization and sample preparation
The Raman spectra of pristine and functionalized SWNTs were
collected with a Dilor Labram-1B Raman spectrometer with
He–Ne laser at 632.8 nm (1.96 eV). UV–visible spectra were
recorded on a Unicam UV300 spectrometer. The powder
samples were sonicated in DMF for 1 h and then centrifuged.
The supernatant was taken for characterization. Solution-phase
near-IR spectra were obtained on a Bruker MPA spectrometer
equipped with a CaF2 beam splitter and a InGaAs detector.
The samples were dispersed in DMF, and then sonicated for
1 h. FTIR spectra were recorded on a Nicolet Nexus 479
FTIR system with a ZnSe attenuated total reflectance (ATR)
accessory. The samples were dispersed in chloroform and
then dripped on a ZnSe crystal. Hydrogen nuclear magnetic
resonance (H1 NMR) spectra were measured with a Varian
Mercury Plus 400 MHz spectrometer. The samples were
dissolved in CDCl3 by sonication. The high-resolution TEM
images were obtained from a JEOL 2100 operating at 200 kV.
The samples were sonicated in chloroform for 10 min and
then centrifuged. The supernatant was dripped onto a Cu grid
coated with a lacy carbon film. TGA was conducted on a PE
TGA-7 instrument at a heating rate of 10 ◦ C min−1 from 50 to
800 ◦ C under nitrogen flow.
2. Experimental section
2.1. Materials
The purified SWNTs (purity 90%), stearoyl chloride,
benzhydrol and benzophenone were purchased from Aldrich.
Benzene and pyridine were purchased from Shanghai Reagents
Co. The 200 W mercury lamp was purchased from Advanced
Radiation Corporation.
2.2. Preparation of benzhydrol-functionalized carbon
nanotubes (BZ-SWNTs) via a photoreduction approach
The photochemical functionalization was carried out as
follows: 2.25 g of BP and 2.25 g of BZ were dissolved in 20 ml
of benzene. The solution was transferred into a quartz vessel,
and then 5 mg of purified SWNTs was added. The solution
was degassed by introducing nitrogen, and then sonicated for
10 min. The mixture was irradiated under a 200 W mercury
lamp for 4 h. During the irradiation, if the solvent was
evaporated, an additional 20 ml of benzene was added to
the quartz vessel. Following the irradiation, the mixture was
further stirred for 12 h and then filtered. The solid was flushed
with tetrahydrofuran, and dried in vacuum at 60 ◦ C.
3. Results and discussion
The covalent nature of the sidewall functionalization was
confirmed by Raman, UV–visible and near-IR spectroscopy.
The Raman spectrum shows that the purified SWNTs exhibit
a tangential mode (G band) at 1560 cm−1 , a weak disorder
mode (D band) at 1313 cm−1 and diameter-dependent radial
breathing modes (RBMs) at 140–253 cm−1 (figure 1). The
2
Nanotechnology 18 (2007) 495703
L Wei and Y Zhang
Figure 2. UV–visible spectra in dimethylformamide of (a) purified
SWNTs and (b) BZ-SWNTs.
Figure 3. Solution-phase NIR spectra of (a) purified SWNTs and
(b) BZ-SWNTs.
weak D band is attributed to the presence of disorder or
sp3 hybridized carbon atoms in the benzenoid framework of
the carbon nanotube walls [4]. Compared with the purified
SWNTs, the Raman spectrum of BZ-SWNTs shows a greatly
enhanced D band (figure 1(b)). This increase is due to the
covalent binding of the addends leading to sp3 defects in the
sidewalls [23].
The diameter (d) of the nanotube is empirically related to
the RBM position (ωR ) using the equation [12]
ωR (cm−1 ) = 238/d (nm)0.93 .
According to the Kataura plot [25, 26], the RBM features
at 140 and 253 cm−1 , corresponding to diameters of 1.77 and
0.93 nm, respectively, arise from semiconducting nanotubes
which are in resonance with the laser energy (1.96 eV),
and the features located between 170 and 230 cm−1 are
assigned to metallic nanotubes. For the purified SWNTs, the
semiconducting nanotube RBM features are pronounced, and
the metallic nanotubes show indistinct features (figure 1(a)).
The semiconducting nanotubes features decreased significantly
in intensity upon functionalization, whereas the features
located at 175, 195 and 221 cm−1 , ascribed to metallic
nanotubes with diameters of 1.39, 1.24 and 1.08 nm,
respectively, show the opposite trend (figure 1(b)). It is
well known that the addition of the functional groups to the
sidewall of nanotube disrupts the radial breathing oscillation
of the nanotubes, which causes the mode to decay accordingly
as the particular nanotube reacts [27]. Our results suggest
that BZ radicals are more reactive toward semiconducting
nanotubes than metallic nanotubes. The same behavior was
observed in chemical functionalization using pyrene-linked
azomethine ylides [28]. The reason for the preferential
reactivity for semiconducting nanotubes remains unclear.
However, we speculate that the phenyl groups within BZ could
π -stack preferentially with more aromatic semiconducting
nanotubes [28, 29].
The UV–visible spectrum of BZ-SWNTs shows that the
van Hove singularities typical for purified SWNTs [30] are
absent (figure 2). The loss of the van Hove singularities is
indicative of covalent sidewall functionalization [31], which
destroys the electronic structure of the nanotubes. It should
Figure 4. ATR-FTIR spectra of (a) BZ-SWNTs and
(b) C18 –BZ-SWNTs.
be mentioned that, for purified SWNTs, the UV–visible
spectra of dispersion of SWNTs are known to be sensitive to
aggregation state [32]. As the number of suspended individual
nanotubes increases, the absorption band becomes shaper
and more intense [33]. In our case, the UV–visible spectra
were measured in dimethylformamide (DMF). As compared
to the surfactants such as sodium dodecylbenzenesulfonate
(SDDBS), DMF is less effective to exfoliate the SWNTs
bundles [34]. As a result, even for the purified SWNTs, the
absorption bands are very weak. Indeed, the weak absorbance
bands were also observed by other research groups when DMF
was used as solvent [28].
The solution-phase NIR analysis (figure 3) shows that
the intensity of the S11 transition of the semiconducting
SWNTs [4] decreased significantly after functionalization,
which further confirmed the changes in the electronic structure
of the nanotubes. In brief, Raman, UV–visible and nearIR spectroscopy provided unambiguous proof of the covalent
sidewall functionalization of the SWNTs.
Figure 4 shows the ATR-FTIR spectrum of BZ-SWNTs.
The weak C–H stretching vibrations in the 3000–3100 cm−1
range are characteristic of the phenyl groups within BZ. The
band at 3328 cm−1 was ascribed to the O–H stretches of the
grafted BZ, and several absorptions in the 1400–1600 cm−1
3
Nanotechnology 18 (2007) 495703
L Wei and Y Zhang
Figure 5. High-resolution TEM image of (a) C18 –BZ-SWNTs and (b) C18 -SWNTs.
Figure 6. 1 H NMR spectra of (a) C18 –BZ-SWNTs in CDCl3 and
(b) CDCl3 solvent.
Figure 7. TGA results for (a) purified SWNTs and (b) BZ-SWNTs.
range were assigned to phenyl ring stretches [35]. The band at
1155 cm−1 was ascribed to the C–O stretching vibration, and
the band at 1212 cm−1 was assigned to C–O stretches or the
O–H deformations of the C–OH groups [36, 37]. It should
be mentioned that the peaks at 2923 cm−1 and 2856 cm−1 ,
which were assigned to aliphatic C–H stretching vibrations,
were observed in BZ-SWNTs. We also observed these peaks
in the purified SWNTs. These peaks could not be removed
even if we performed a high-temperature vacuum anneal on
the purified SWNTs (1200 ◦ C, 10−6 Torr, 2 h). These peaks
could be associated with hydrocarbon contamination in the
spectrometer and/or with C–H groups introduced by atomic
hydrogen generated in cold plasma [36, 38].
To further confirm the successful graft of BZ onto the
sidewalls of nanotubes, a long-chain hydrocarbon marker (n C18 H35 ) was grafted onto the BZ by esterification reaction
for HRTEM visualization, as outlined in scheme 1. FTIR
(figure 4(b)) shows that after BZ-SWNTs reacted with stearoyl
chloride the peak at 3328 cm−1 , assigned to O–H stretches in
BZ-SWNTs, disappeared, aliphatic C–H stretching vibrations
(3000–2800 cm−1 ) were greatly enhanced, and a new peak
at 1465 cm−1 , attributed to a long-chain hydrocarbon C–H
deformation mode [35], was observed. The resulting C18 –BZSWNTs exhibit high solubility (0.8 mg ml−1 ) in chloroform,
whereas the BZ-SWNTs show poor solubility (<0.2 mg ml−1 ).
Figure 5(a) shows the morphology of nanotubes with n -C18 H35
grafted onto the sidewalls of BZ-SWNTs. No n -C18 H35 groups
were grafted onto the sidewalls (figure 5(b)) when purified
SWNTs reacted with stearoyl chloride (the reaction production
denoted as C18 -SWNTs), suggesting the grafted BZ played a
key role in attachment of n -C18 H35 groups onto the sidewalls.
The solubility of BZ-SWNTs was not high enough to
allow solution state NMR spectra to be obtained. Fortunately,
C18 –BZ-SWNTs show high solubility in CDCl3 , and the
corresponding 1 H NMR spectrum is shown in figure 6. A
pronounced resonance signal at 7.26 ppm was observed for the
C18 –BZ-SWNT sample. A resonance signal ascribed to a few
protons of CHCl3 in CDCl3 was observed at this position, but
its intensity is low. The considerable increase in the intensity of
the resonance signal at 7.26 ppm after C18 –BZ-SWNTs were
dissolved in CDCl3 was due to the presence of the protons of
phenyl groups. Several new signals in the 1.0–1.4 ppm range
are ascribed to the protons of the long-chain hydrocarbon in
C18 –BZ-SWNTs.
The degree of functionalization was measured by TGA
(figure 7). For purified SWNTs, the weight loss of 6% is
attributed to the removal of the oxidized carbon groups in the
purified SWNTs [15]. The overall weight loss is 28.7% for
BZ-SWNTs. The increased weight loss (22.7%) is attributed
to the presence of BZ on the sidewalls. Based on the weight
loss (W %), the molecular weight of the addends (Madd ) and
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Nanotechnology 18 (2007) 495703
L Wei and Y Zhang
the atomic weight of carbon (Mc ), the degree of coverage ( X )
can be calculated from
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X = [Madd (1 − W %)]/(MC W %).
Here, the degree of functionalization is about one BZ in 52
carbon atoms.
In summary, we successfully developed a photoreduction
route based on the BP/BZ system for covalent sidewall
functionalization of SWNTs, as confirmed by Raman
scattering, UV–visible and near-IR spectroscopy, ATR-FTIR,
HRTEM, NMR and TGA. Application of other BP/hydrogen
donor systems for photochemical functionalization of SWNTs
is currently under investigation.
Acknowledgments
We thank the National Natural Science Foundation of China
No 20504021, 60576064, Shanghai Science and Technology
grant 0452 nm056 and the National Basic Research Program
of China No 2006CB300406 for the financial support. We
also thank the Instrumental Analysis Center of SJTU for
characterization of the samples.
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