Preparation and optical properties of fullerene/ferrocene hybrid hexagonal nanosheets and large-scale production of fullerene hexagonal nanosheets

Article abstract of DOI:10.1021/ja901032b

The supramolecular nanoarchitectures, C₆₀/ferrocene nanosheets, were prepared by a simple liquid-liquid interfacial precipitation method and fully characterized by means of SEM, STEM, HRTEM, XRD, Raman and UV-vis-NIR spectra. The highly crystal

Full text of DOI:10.1021/ja901032b

Published on Web 07/01/2009
Preparation and Optical Properties of Fullerene/Ferrocene
Hybrid Hexagonal Nanosheets and Large-Scale Production of
Fullerene Hexagonal Nanosheets
Takatsugu Wakahara,*,† Marappan Sathish,† Kun’ichi Miyazawa,† Chunping Hu,‡
Yoshitaka Tateyama,*,‡,⊥ Yoshihiro Nemoto,‡ Toshio Sasaki,# and Osamu Ito§
Fullerene Engineering Group, Exploratory Nanotechnology Research Laboratories, National
Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, International
Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science,
1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, PRESTO, Japan Science and Technology
Agency, 4-1-8 Honcho, Kawaguchi, Saitama 333-0012, Japan, ICYS, National Institute for
Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, and CarbonPhotoScience
Institute, Kita-Nakayama2-1-6, Izumi-ku, Sendai, 981-3215, Japan
Received February 18, 2009; E-mail: wakahara.takatsugu@nims.go.jp
Abstract: The supramolecular nanoarchitectures, C60/ferrocene nanosheets, were prepared by a simple
liquid-liquid interfacial precipitation method and fully characterized by means of SEM, STEM, HRTEM,
XRD, Raman and UV-vis-NIR spectra. The highly crystallized C60/ferrocene hexagonal nanosheets had
a size of ca. 9 µm and the formulation C60(ferrocene)2. A strong charge-transfer (CT) band between ferrocene
and C60 was observed at 782 nm, indicating the presence of donor-acceptor interaction in the nanosheets.
Upon heating the nanosheets to 150 °C, the CT band disappeared due to the sublimation of ferrocene
from the C60/ferrocene hybrid, and C60 nanosheets with an fcc crystal structure and the same shape and
size as the C60/ferrocene nanosheets were obtained.
Introduction
because of their unique chemical and physical properties and
their possible application in the fields of materials and medical
Because of their unique physical and chemical properties,
fullerenes have tremendous potential as building blocks for new
materials.1 Recently, fullerene-based supramolecular nanoar-
chitectures such as nanowhiskers,2 nanotubes,3 nanorods,4
nanowires,5 and nanosheets6 have attracted special interest
sciences. The unique properties are a result of the high symmetry
of the nanoarchitectures and the presence of novel π-conjugated
systems in them. Very recently, flower-like supramolecular
assemblies were also prepared by using C60 derivatives with
long aliphatic chains.7
Fullerenes form a wide variety of donor-acceptor complexes
with different classes of organic and organometallic donors.8
These complexes show a wide range of physical properties,
including metallic,9 photoconducting,10 and unusual magnetic
properties.11 Ferrocene (Fc) is one such very important donor
because of its strong electron-donating ability and high stability
under redox conditions. Therefore, supramolecular hybrids
formed by the combination of fullerene and Fc are expected to
have fascinating properties. Indeed, C60/Fc hybrid molecules in
solution12-14 and liquid crystals with supramolecular organized
structures15-17 have been reported to show interesting properties.
† Fullerene Engineering Group, Exploratory Nanotechnology Research
Laboratories, National Institute for Materials Science.
‡ International Center for Materials Nanoarchitectonics (MANA), Na-
tional Institute for Materials Science.
⊥ PRESTO, Japan Science and Technology Agency.
# ICYS, National Institute for Materials Science.
§ CarbonPhotoScience Institute.
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KGaA: Weinheim, Germany, 2005.
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Mater. Res. 2002, 17, 83–88. (b) Miyazawa, K.; Hamamoto, K.;
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10.1021/ja901032b CCC: $40.75  2009 American Chemical Society

Optical Properties of Fullerene/Ferrocene
A R T I C L E S
linear response (MLR) scheme for the time-dependent (TD) DFT.20
All the calculations were performed using the ABINIT code.21 The
calculation details are described in Supporting Information.
As a crystalline state, C60/Fc hybrid was first reported by Crane
et al. in 1992;18 they determined the crystal structure by
assuming motionless C60 molecules. These authors inferred that
Fc does not donate fully its electron to C60 and that contribution
of the weak intermolecular charge-transfer (CT) interaction to
the stability of the hybrid is very small.18 However, the
electronic properties of the hybrid are not still clear.
We herein report a simple method for the preparation of C60/
Fc hybrid hexagonal nanosheets with a size of ca. 9 µm by a
liquid-liquid interfacial precipitation (LLIP) method. This
method was introduced by our group for the preparation of C60
nanowhiskers at a toluene/isopropyl alcohol (IPA) interface.2a
In this paper, we also discuss the CT interaction between C60
and Fc in nanosheets on the basis of CT-band observations in
the solid state. Interestingly, the CT band disappears after heat
treatment, which is accompanied by the sublimation of Fc. After
the heat treatment, face-centered cubic (fcc) C60 nanosheets are
observed to be left behind.
Results and Discussion
Preparation and Structure of C60/Fc Nanosheets. In the
preparation of C60/Fc nanosheets by the LLIP method through
the formation of an interface between IPA and toluene solution
containing C60 and Fc, it is very interesting to note that the
concentration of Fc in toluene must be more than 30 mg/mL.
When the concentration of Fc is less than 20 mg/mL, C60
nanowhiskers are formed and Fc molecules just cover the
external surface of the C60 nanowhiskers, as explained in our
previous report.22 This indicates that, at low Fc concentrations,
Fc molecules cannot enter the C60 crystals due to the strong
interaction between the C60 molecules. Consequently, C60
nanowhiskers are surrounded by Fc molecules. However, at high
Fc concentrations, Fc and C60 form precipitates with quite
different morphologies. These precipitates are found to be C60/
Fc hybrid hexagonal nanosheets in the present study.
Experimental Section
SEM images of C60/Fc precipitates prepared at high Fc
concentrations are shown in Figure 1. These representative
images show the hexagonal morphology of the precipitates,
which are highly transparent to electron beams. The average
size of the precipitates is 9.1 ( 6.2 µm and the thickness is
about 250-550 nm, confirming the formation of hexagonal
nanosheets. It is very interesting to note that many self-standing
nanosheets are observed in the SEM images in Figure 1,
indicating rigid crystalline materials. The morphology of these
nanosheets is very similar to that of hexagonal C60 nanosheets
prepared solely from C60 molecules using the LLIP method,
which was reported by our group recently.6
To locate the doped Fc in the nanosheets, a STEM mapping
analysis was carried out (Figure 2). Carbon and iron atoms were
detected in the nanosheets, as shown in Figure 2, panels b and
c, respectively. The distribution and concentration of carbon
and iron were uniform in the nanosheets. This result confirms
the fine dispersion of iron atoms in the nanosheets, indicating
that the nanosheets are C60/Fc nanosheets.
The C60/Fc nanosheets were prepared by the LLIP method, in
which an interface containing C60 and excess Fc was formed
between IPA and toluene. An amount of 3 mL of C60-saturated
toluene was taken in a 10 mL glass bottle and 120 mg of Fc was
added to this solution. After 10-min ultrasonication at room
temperature, the C60/Fc solution was cooled to 10 °C in an
ice-water bath. To this toluene solution, 3 mL of IPA was added
slowly, and the mixture was maintained at 10 °C for 5 min without
disturbing the interface. The resulting mixture was stored at 10 °C
for 24 h for the growth of C60/Fc nanosheets. C60/Fc nanosheets
were obtained by filtering the solution and were heated at 80 °C
for 15 min to remove the excess Fc.18
The structure and morphology of the obtained nanosheets were
characterized using scanning transmission electron microscopy
(STEM; JEOL JEM-2100F, 200 kV), field emission scanning
electron microscopy (FE-SEM; Hitachi S-4800, 15 kV, FE-SEM;
JEOL JSM-6700F, 5 kV), and a micro-Raman system (NRS-3100,
JASCO, Japan) equipped with a semiconducting laser with a
wavelength of 532 nm. Diffuse reflectance spectra of the nanosheets
were measured by a UV-vis-NIR spectroscope (V-570, JASCO,
Japan) equipped with an integrating sphere. The prepared C60/Fc
nanosheets were taken on a quartz plate and dried at room
temperature prior to the measurement of the spectra. UV-vis-NIR
spectra of C60 and Fc powder samples were also measured in a
similar manner.
Figure 3 shows an HR-TEM image of C60/Fc nanosheets. A
clear lattice structure is observed. The distance between lattices,
d, is calculated to be 0.90 nm.
X-ray diffraction (XRD) patterns of the C60 powder and C60/
Fc nanosheets are shown in Figure 4. The XRD pattern of the
C60/Fc nanosheets is quite different from that of the C60 powder,
but identical to that of triclinic C60(Fc)2 prepared directly from
a solution of C60 in molten Fc.23 This observation indicates that
the C60/Fc nanosheets contain C60(Fc)2 units, which have two
Fc molecules near every C60 molecule.18 The inset in Figure 4
shows the selected-area electron diffraction pattern of a C60/Fc
nanosheet. The observed pattern is indexed for the (010) and
Calculations were carried out for ascertaining the structural
stability and determining the electronic states within the local
density approximation (LDA)19 to the density functional theory
(DFT). A plane wave basis set and a pseudopotential scheme were
adopted for the crystal system. For determining the CT excitation
energy, we used the Slater transition (ST) method and the modified
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C. P.; Guldi, D. M. ChemPhysChem 2002, 3, 195–205.
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(210) planes. The d value calculated from the HR-TEM image
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in Figure 3 is indicated as the (010) plane.
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A R T I C L E S
Wakahara et al.
normalized C60 spectrum from the spectrum of the C60/Fc
nanosheets, which can be attributed to the CT band of C60/Fc
nanosheets with absorption maximum at 782 nm, since CT
interaction between C60 and Fc has been frequently reported in
solution and liquid-crystalline.12-16 Interestingly, the disap-
pearance of the 782 nm band when the nanosheets are heated
to 150 °C confirms that the band is a CT band. The CT transition
energy corresponding to the maximum at 782 nm is estimated
as 1.59 eV. It has been reported that the position of the CT
band of C60 complexes depends on the ionization potentials of
the donors;25 from reported linear relation, the observed CT band
of 1.59 eV for C60/Fc nanosheets can be obtained using the
ionization potential of 7.5 eV for Fc.
Theoretical Calculations. By considering the unit cell struc-
ture determined by XRD in this study, we perform the structural
optimization of the C60(Fc)2 crystal. The relaxed geometry is
shown in Figure 7 and is consistent with the structure reported
on the basis of XRD analysis of single crystal.18 The nearest
distance between a C5H5 ring of the Fc and a pentagonal face
of the C60 is calculated to be 3.23 Å, and the eclipsed Fc is
displaced with respect to the pentagonal center by 1.23 Å. The
highest occupied molecular orbital (HOMO) mainly consists
of Fe 3d orbitals of the two Fc molecules, while the triply
degenerated lowest unoccupied molecular orbital (LUMO)
mainly comprises t1u orbitals of C60 and a small part of Fe 3d
orbitals of the two Fc molecules (see Supporting Information).
Thus, the electronic transition from the HOMO to LUMO is
the cause of the CT excitation. At the DFT Kohn-Sham level,
intramolecular valence excitations in Fc and C60 are calculated
to be 2.4 and 1.6 eV, respectively, which is consistent with
experimental absorption edges around 500 and 700 nm. The
ST method and (TD)-DFT-MLR scheme give the CT excitation
energy values as 1.005 and 1.016 eV, respectively, which can
be reasonably assigned to the broad absorption with peak around
800 nm and edge at 1000 nm in the diffuse reflectance spectra
of the nanosheets.
Figure 1. SEM images (left, on the grids; right, on the silicon wafer) and
size distribution histograms of C60/Fc nanosheets. (The size of the sheets is
defined as the length of the longest diagonal line of the sheet.)
Conversion of C60/Fc Nanosheets to C60 Nanosheets. Subse-
quent to the preparation of C60 nanosheets, they have attracted
great attention for their potential application to nanodevices and
superhard materials.6 However, synthetic methods continue to
have limitations and the sizes and shapes of the nanosheets are
not fully controllable. Recently, it has been reported that C60(Fc)2
solids thermally decompose into fcc C60 crystals and Fc vapor
at elevated temperature.23 Therefore, the preparation of C60
nanosheets from hexagonal C60/Fc nanosheets is an interesting
challenge. Figure 8 shows SEM images of the thermal decom-
position products of C60/Fc nanosheets. These representative
images show that the hexagonal morphology is retained even
after the heat treatment; in addition, the size is also almost
unchanged. The Raman spectrum of the heat-treated nanosheets
shows the disappearance of the Fc lines, indicating the conver-
sion to hexagonal C60 nanosheets.
Recently, the electronic interaction between C60 and Fc
molecules has been clarified; for example, it has been proposed
that supramolecular organization in fullerene-Fc liquid crystals
gives rise to a smectic A phase.15,16 C60 imposes the arrangement
of other molecular moieties to form a partial bilayer and the Fc
moieties are localized between the C60 units and the dendric
core.15,16 In the case of C60/Fc nanosheets, each C60 is
surrounded by two Fc molecules, showing a triclinic C60(Fc)2
structure.18
Raman Spectra of C60/Fc Nanosheets. Figure 5 shows the
Raman spectra of the prepared C60/Fc nanosheets, pristine C60
powder, and Fc powder. The Raman spectrum of the prepared
C60/Fc nanosheets is almost identical to the summed spectra of
the pristine C60 powder and Fc powder. The Raman lines
observed in the case of the prepared C60/Fc nanosheets are a
To investigate the crystal structure of the produced nanosheets,
we carried out XRD measurements. The XRD patterns of the
nanosheets, shown in Figure 4c, are quite different from those
of C60/Fc nanosheets, but almost identical to the patterns of fcc
C60 powder, indicating that the nanosheets obtained are hex-
agonal C60 nanosheets. In Figure 4c, we observe an additional
peak (indicated by *); since this peak is not related to fcc C60,
combination of those of the C60 molecule (269, 430, 491, 709,
24
1422, 1465, and 1568 cm-1
)
and the Raman lines of the Fc
molecule (317and 1105 cm-1). These observations indicate that
the nanosheets contain C60 and Fc.
Optical Properties of C60/Fc Nanosheets. Figure 6 shows the
diffuse reflectance spectra of the C60/Fc nanosheets and C60
powder. The extra spectrum is obtained by the subtraction of a
(24) Kuzmany, H.; Pfeiffer, R.; Hulman, M.; Kramber, C. Philos. Trans.
R. Soc. London, Ser. A 2004, 362, 2375–2406.
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Optical Properties of Fullerene/Ferrocene
A R T I C L E S
Figure 2. (a) STEM image of a C60/Fc nanosheet and (b and c) STEM mapping images (b, carbon; c, iron) of C60/Fc nanosheet.
Figure 3. HRTEM images of C60/Fc nanosheets. Inset: magnified images.
Figure 5. Raman spectra of (a) ferrocene, (b) pristine C60 powder, and (c)
the C60/Fc nanosheets.
Figure 4. XRD patterns of (a) C60/Fc nanosheets, (b) C60 powder, and (c)
fcc C60 nanosheets. The inset is the selected-area electron diffraction pattern
of C60/Fc nanosheets. The peaks indicated by open circles correspond to
the fcc structure of C60.
it may be associated with defects and stacking faults caused by
the escape route of Fc molecules.26
Figure 6. Diffuse reflectance UV-vis-NIR spectra (K/M: Kubelka-Munk
function) of (a) the C60/Fc nanosheets, (b) pristine C60 powder, and (c)
subtracted spectrum (a - b). Spectra a and b are normalized at 610 nm.
These results indicate that triclinic C60/Fc nanosheets are
converted to fcc C60 nanosheets by the heat treatment, with the
hexagonal morphology being retained through the conversion.
This is in contrast to the formation of fibrous superstructures
of C60 from the C60/Fc system that has been reported by Shinkai
and co-workers.5b There are two methods that have been
presented for the formation of C60 nanosheets, but it is very
difficult to prepare C60 nanosheets on a large scale using these
methods. In one method, C60 nanosheets are prepared by the
precipitation method using CCl4 as a solvent; the solubility of
C60 is low in this solvent.6a In the other method, the preparation
is by the slow evaporation of m-xylene solution of C60 on
(26) Vaughan, G. B. M.; Chabre, Y.; Dubois, D. Europhys. Lett. 1995, 31,
525–530.
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A R T I C L E S
Wakahara et al.
Figure 8. SEM images of the C60 nanosheets.
is possibly the driving force for the formation of C60/Fc
nanosheets. These results are also supported by the theoretical
calculations. The C60/Fc hexagonal nanosheets can be converted
to fcc C60 hexagonal nanosheets by heat treatment at 150 °C, at
which both components C60 and Fc are thermally stable. The
successful preparation of nanoarchitectures based on the CT
interaction is expected to be an important stepping stone to the
fabrication of novel fullerene nanodevices and to lead to a new
field pertaining to fullerene engineering in nanoscience.
Figure 7. Relaxed geometry of C60(ferrocene)2 by ab initio DFT-LDA
calculation.
substrates.6b The method for the preparation of hexagonal C60
nanosheets presented here is a very simple LLIP method using
toluene/IPA and involving heat treatment at 150 °C. This method
shows potential for the large-scale production of fcc C60
nanosheets with a range of sizes up to ca. 9 µm.
Acknowledgment. Part of this research was financially sup-
ported by a Grant-in-Aid for Scientific Research from the Ministry
of Education and Culture, Sports, Science and Technology, Japan.
Conclusions
We have succeeded in the construction of supramolecular
nanoarchitectures, C60/Fc hexagonal nanosheets, with an average
size ranging up to 9 µm by a simple LLIP method. Diffuse
reflectance spectra show a strong CT band at 782 nm, from
which the CT transition energy is estimated as 1.59 eV. The
presence of the CT band allows us to consider that the nearest
C60-Fc pair in the C60/Fc hybrid interacts, and this interaction
Supporting Information Available: Complete refs4c and6b MO
calculation details; optical microscope images; IR spectrum of
nanosheets; HOMO-LUMO of C60(Fc)2. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA901032B
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