A soluble hybrid material combining carbon nanohorns and C60

Article abstract of DOI:10.1039/c1cc15446j

A soluble hybrid nanomaterial that combines fullerenes and carbon nanohorns (CNHs) has been prepared and fully characterized. Electrochemical investigations revealed that the CNHs modify the electron accepting ability of C₆₀ in the hybrid mater

Full text of DOI:10.1039/c1cc15446j

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COMMUNICATION
A soluble hybrid material combining carbon nanohorns and C60w
a
b
c
a Vizuete,a Marı
´
a Jose Gomez-Escalonilla, J. L. G. Fierro, Masako Yudasaka,
¸
´
´
Marı
´
Sumio Iijima,d Maida Vartanian,e Julien Iehl,e Jean-Franc
ois Nierengarten*e and
Fernando Langa*a
Received 1st September 2011, Accepted 20th October 2011
DOI: 10.1039/c1cc15446j
A soluble hybrid nanomaterial that combines fullerenes and
carbon nanohorns (CNHs) has been prepared and fully characterized.
Electrochemical investigations revealed that the CNHs modify
the electron accepting ability of C60 in the hybrid material.
the existence of electronic communications between both kinds of
carbon nanostructures in the hybrid material.
The synthesis of the C60-CNH hybrid 8 is depicted in Scheme 1.
Treatment of diol 2 with carboxylic acid 1 and N,N0-dicyclohex-
ylcarbodiimide (DCC) in the presence of 4-dimethylaminopyridine
(DMAP) and 1-hydroxybenzotriazole (HOBt) gave bis-malonate 3
in 81% yield. A double Bingel cyclopropanation was used to
prepare fullerene bis-adduct derivative 4.6 Reaction of 3 with
C60, I2, and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in toluene
afforded 4 in 42% yield. Reaction of pristine CNHs with 57 in the
presence of isoamyl nitrite in N-methyl-2-pyrrolidone (NMP) gave
functionalized CNHs 6. Subsequent treatment with tetra-n-butyl-
ammonium fluoride (TBAF) gave the CNHs derivative 7 bearing
terminal alkyne subunits. Reaction of 7 with azide 4 in NMP under
the conditions optimized for fullerene-containing azides8 (CuSO4ꢀ
5H2O and sodium ascorbate) afforded the targeted CNH-fullerene
hybrid material 8. Owing to the four long alkyl chains per fullerene
subunit, compound 8 is soluble in chlorinated solvents (up to
0.5 mg mLꢁ1 in CH2Cl2) (see ESIw, Fig. S1).
Owing to their remarkable electronic, thermal and mechanical
properties, carbon nanostructures such as fullerenes, carbon
nanotubes (CNTs) and graphene are extremely promising
candidates for the preparation of new advanced materials
for a large number of applications.1 In recent years, hybrid
structures combining two different forms of carbon nanostructures
have been prepared with the aim to generate multifunctional
nanomaterials with enhanced properties.2 Examples include CNTs
filled with fullerenes, i.e. nanopeapods3 and supramolecular
ensembles in which fullerene derivatives are assembled around
the external surface of CNTs.4 A few derivatives in which the
fullerenes are covalently bonded to the outer surface of the CNTs
have been also reported.5 However, in most of the cases, the
resulting materials are completely insoluble thus making their
characterization difficult and limiting in-depth investigations of
their electronic properties. As part of this research, we now report
the synthesis and the characterization of a soluble hybrid material
that combines fullerenes and carbon nanohorns (CNHs) into a
single structure in which fullerene derivatives have been covalently
grafted to the outer surface of modified CNHs under the copper-
catalyzed alkyne–azide cycloaddition (CuAAC) conditions.
Owing to its good solubility, the electrochemical properties of
the C60-CNH conjugate could be investigated thus revealing
As shown in Fig. 1, the morphology of the newly prepared
CNH-fullerene 8 was observed with high resolution transmission
electron microscopy (HR-TEM, TOPCON 002B) at 120 kV.
The spherical form of the CNH aggregates was not destroyed
by the functionalization (Fig. 1), and C60-like spherical molecules
attached to the CNH surfaces were often found (Fig. 1, insets).
Raman spectroscopy provided important information on the
covalent modification.9 It is known that the Raman vibrational
spectrum of the CNTs family members, such as CNHs, has two
prominent bands, assigned as the disorder-induced mode (D-band)
and the tangential mode (G-band), centered at 1350 and 1590 cmꢁ1
,
a
´
Instituto de Nanociencia, Nanotecnologıa y Materiales Moleculares
respectively. The G band is associated with the vibrations of
sp2-hybridized carbon atoms, whereas the D band is attributed
to the sp3 carbon atoms existing within CNHs aggregates and
corresponds to the CNH core as well as to the defect sites of
the nanostructures. The Raman spectra (lexc = 532 nm) of
pristine CNHs and modified CNHs 6 and 8 are shown in Fig. 2
(see also ESIw, Fig. S2).
In the case of the modified CNHs 6 and 8, the intensity of
the D-band is enhanced with respect to pristine CNHs as a
result of the transformation of sp2 hybridized C atoms of the
CNH framework into sp3 ones.10 Specifically, after the first
covalent functionalization of CNHs, the ID/IG ratio increased
from 1.07 for pristine CNHs to 1.18 for 6, indicating a clear
(INAMOL), Universidad de Castilla-La Mancha, 45071, Toledo,
Spain. E-mail: Fernando.Langa@uclm.es
b
´
´
Instituto de Catalisis y Petroleoquımica, CSIC, 28049, Madrid,
Spain. E-mail: jlgfierro@icp.csic.es
c Nanotube Research Center, National Institute of Advanced Industrial
and Technology, Higashi, Tsukuba, Ibaraki 305-8565, Japan.
E-mail: m-yudasaka@aist.go.jp
d Department of Physics, Meijo University, Shiogamaguchi,
Tenpaku-ku, Nagoya 468-8502, Japan
e Laboratoire de Chimie des Mate´riaux Mole´culaires, Universite´ de
´
Strasbourg et CNRS, Ecole Europeenne de Chimie, Polymeres et
Materiaux, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France.
`
´
E-mail: nierengarten@unistra.fr
w Electronic supplementary information (ESI) available: Details of the
synthesis; Raman, XPS, TGA spectroscopies. See DOI: 10.1039/
c1cc15446j
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 12771–12773 12771

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Scheme 1 Synthesis of CNH-fullerene 8. Reagents and conditions: (i) DCC, DMAP, HOBt, CH2Cl2, 0 1C to rt, 60 h, 81%; (ii) C60, DBU, I2, PhMe,
rt, 12 h, 42%; (iii) isoamyl nitrite, NMP, 70 1C, 24 h; (iv) TBAF, THF, NMP, rt, 1 h; (v) CuSO4ꢀ5H2O, sodium ascorbate, NMP, 70 1C, 48 h.
conditions used for the grafting of the fullerene subunits onto
the modified CNHs 6. The reaction of azide 4 must therefore
effectively occur on the alkyne subunits of 6 thus leading to the
formation of triazole rings.11
X-Ray photoelectron spectroscopy was used to determine
the nature and the relative amount of functional groups
present on the CNHs surface.12 The survey spectra of CNH-
fullerene 8 and fullerene derivative 4 (see ESIw, Fig. S3) show
the photoelectrons collected from C1s, N1s and O1s core-levels
(together with the OKLL Auger emission). High resolution
spectra were then recorded and the corresponding binding
energy values are collected in Table S1 (see ESIw). All peaks
were decomposed into several symmetrical components
(i.e., five or six for C1s; two for O1s and one or two for N1s)
(see ESIw, Fig. S3). The C1s peak of 8 was satisfactorily fitted to
six components according to the peak assignment used by
Stankovich et al.13 The most intense peak at 284.8 eV is due to
sp2 C–C bonds of graphitic carbon whereas the component at
285.4 eV is originated from sp3 C–C bonds.14 The next three
components at 286.3, 287.6 and 289.3 eV have been often
assigned to C–O, CQO and COO groups12,15 present on the
surface of CNHs. In addition, CNH-fullerene 8 displays a weak
component at around 291.4 eV corresponding to p–p* transition
of carbon atoms in graphene structures,10,16 but is absent in
fullerene derivative 4. Finally, a weak C–N* component should
be expected somewhere around 286.3 eV, however it is over-
shadowed by the strongest C–O line and therefore it could not be
resolved.
Fig. 1 HR-TEM images of modified CNHs 8. Insets show tips of
SWNHs to which C60-like molecules (black arrows) are attached.
enhancement of the disruption of the skeleton of CNHs, thus
providing a first evidence for the covalent modification of the
surface of CNHs. At this point, it is also worth noticing that
D-band intensity for 8 is similar to that of 6 (Fig. 2), proving
that the CNH framework is unaffected by the reaction
For simplicity and sake of easy understanding the O1s
spectrum of 8 has been curve-resolved with only two components
(see ESIw, Table S1). A minor component at 531.7 eV corres-
ponds to OQC surface groups and major one at 533.1 eV is
likely associated with O–C bonds,10,11 demonstrating the
existence of oxygen functional groups on the CNHs surface.
The same components are observed for fullerene derivative 4
although both appear somewhat shifted toward higher binding
energies. The high-resolution N1s spectrum (Fig. S3, ESIw) of
8 displays two components, a major one at a binding energy of
Fig. 2 Comparative Raman spectra (lexc = 532 nm) of pristine
CNHs (black) and modified CNHs 6 (red) and 8 (blue), illustrating
D and G bands. The spectra are normalized to the peak intensity of
the G band.
c
12772 Chem. Commun., 2011, 47, 12771–12773
This journal is The Royal Society of Chemistry 2011

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preparation of related hybrid systems combining C60 with other
carbon nanostructures such as CNTs and graphene and thus
opens the door to explore the possible applications of such
multifunctional nanomaterials.
This research was supported by the Ministerio de Educacion y
´
Ciencia (CTQ2007-63363/PPQ), the EC (contract PITN-GA-
2008-215399–FINELUMEN), Consolider project HOPE
(CSD2007-00007) and JOINT MICINN-JST action: ‘‘Chemically
Functionalized Nano-Carbon for Photovoltaic Devices’’
(PLE2009-0038). We further thank M. Irie for TEM observations.
Notes and references
1 (a) Fullerenes principles and applications, ed. F. Langa and
J.-F. Nierengarten, Royal Society of Chemistry, 2008; (b) Carbon
Fig. 3 DPV (reduction run) of pristine C60 (black), fullerene
derivative 4 (magenta) and CNH-fullerene 8 (blue).
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Wiley-VCH, Weinheim, 2010.
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400.4 eV, belonging to two N-atoms bound to C-atoms as
nearest neighbours,17 and a minor one 399.2 eV originating
from the other remaining N-atom.11 In contrast, fullerene
derivative 4 bearing an azide group displays a single N1s
component at 401.0 eV. These observations unambiguously
confirm that the azide residue of fullerene building block 4 is
not anymore present upon covalent incorporation into the
CNHs and clearly supports the formation of triazole rings.
This is also in perfect agreement with the IR spectrum of 8
showing that no azide (2092 cmꢁ1) residues remain in the final
product (see ESIw, Fig. S4). The incorporation of the fullerene
moiety in the CNH framework through the formation of
triazole rings was further confirmed by thermogravimetric
analysis (TGA, see ESIw).
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chemical properties could be investigated by differential pulsed
voltammetry (DPV) (Fig. 3). The electrochemical studies were
performed at room temperature in o-dichlorobenzene/acetonitrile
4 : 1 containing tetra-n-butylammonium hexafluorophosphate
(TBAPF6) (0.1 M). The two characteristic reductions of C60
bis-adducts are observed for 8 (at ꢁ0.98 mV and ꢁ1.36 mV).
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reduction of 8 with that of its fullerene 4 revealed a cathodic
shift of 80 mV. Indeed, the redox potential of the fullerene
moiety in 8 is similar to that of pristine C60 (E1red = ꢁ0.99 mV)
measured under the same conditions. This shift may be
attributed to the existence of electronic interactions between
the CNHs and the fullerene cages. It is however worth noting
that molecular mechanic calculations show a distance of ca. 12 A
between the CNH surface and the C60 subunit. A similar shift has
been recently observed in endohedral C60@SWCNT,18 although
in this case, the distance between is much shorter, around 3.4 A.
In our case, it is also likely that the observed potential shift may
result from the specific nanoenvironment of the fullerene moieties
grafted on the CNHs.
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 12771–12773 12773