Carbon Vol. 34, No. 10, pp. 1287-1291,1996
Copyright 0 1996 Elsevier Science Ltd
Printed in Great Britain. All rights reserved
0008-6223/96 $15.00 + 0.00
Pergamon
S0008-6223(96)00116-9
COMPARATIVE
NMR STUDY OF NEW CARBON FORMS
Y. MANIWA,‘**
M. SATO,~ K. KIJME,~ M.
‘Department
of Physics, Tokyo Metropolitan
University,
bElectrotechnical
(Received
Laboratory,
E.
KOZLOV~‘+
and
M.
TOKUMOTO~
Minami-osawa,
Hachi-oji, Tokyo
Umezono, Tsukuba, Ibaraki 305, Japan
l-l-4
13 March 1996; accepted in revised form
192-03, Japan
19 June 1996)
Abstract-A
comparative
13C-NMR study of several new phases of pressure/temperature-treated
C,,, a
bundle of carbon nanotubes,
graphite and diamond was carried out in the 4-300 K temperature
range.
It was found that face-centered
cubic and rhombohedral
high-pressure
phases of C, reveal very unusual
NMR lines at 110 and 70 ppm, respectively,
shifted from that of pristine C, (143 ppm). These lines
disappear in a hard carbon phase prepared by compression
of Cm at more severe conditions. Comparison
with NMR spectra of pristine C,,, diamond and graphite showed that the fullerenes acquire some partial
sp3 character when they are pressure-treated at 3-4.6 GPa and various temperatures (720°C). However
they are apparently not transformed into four-fold-coordinated diamond-like carbon clusters. NMR
spectra of carbon nanotubes exhibited an anomalous shift which may be attributed to their cylindrical
geometry. Copyright 0 1996 Elsevier Science Ltd
Key Words-NMR,
diamond, fullerenes, &,,, carbon nanotube, polymer, hard carbon.
1. INTRODUCTION
Since the first synthesis of macroscopic
amounts of
C,,, extensive studies on new forms of carbon have
been performed [l] and led to important discoveries
such as carbon nanotubes [2], Bucky onion [3], and
photoinduced
C6,, polymer [4]. Recently, it has been
shown that C, polymer phases can be also induced
from solid C,, by heating under high pressure [ 5,6].
A similar technique using non-hydrostatic
pressure
made synthesis of a new solid form of carbon, named
hard carbon (HC), possible [ 71. The hardness of HC
is more than 4000 kg/cm’, which is much larger than
that of glass and sapphire. The existence of these
various phases in carbon should come from the
flexibility of the carbon bond, which extends from
so-called sp to sp3 configuration.
In order to understand the physical properties
of these materials, it
would be of basic importance
to clarify their microscopic structure.
For this propose,
we performed
comparative
13C-NMR studies on new solid forms of carbon
including conventional
diamond
and graphite. We
found a NMR evidence that the Ceo solids heated
under high pressure
have noticeable
amounts
of
sp3-like (or four-fold-coordinated)
carbon atoms. This
confirms that the C,, polymer phase can be formed
by the high pressure treatment.
However, the shift
values suggest that the sp3 bond character is somewhat different from that of diamond. On the other
hand, there is no evidence for sp3 carbon in pressureinduced HC, suggesting its average microstructure
is
different from that of conventional
diamond. In the
carbon nanotube,
the NMR spectra exhibited
an
*Author to whom correspondence should be addressed.
+On leave from the Institute of Semiconductors, Kiev,
Ukraine.
1287
anomalous
cylindrical
shift which
geometry.
may be attributed
to their
2. EXPERIMENTAL
Two kinds of CeO “polymer” specimens, No. 1 and
No. 2, were prepared by a technique of heating under
high pressure, as previously reported by Iwasa et al.
[ 51. The synthesis conditions were 350°C at 4.6 GPa
for specimen No. 1 and 720°C at 4.6 GPa for specimen
No. 2. The pressure was generated by a cubic anvil
apparatus.
X-ray diffraction
(XRD) measurement
showed that specimen No. 1 is face-centered
cubic
(FCC) and specimen No. 2 has a rhombohedral
structure. As shown in Fig. 1, they are in good agreement
with those in Ref. 5. A pellet of hard carbon (HC) of
4 mm in diameter and 2 mm in length was obtained
from CeO solid using a belt-type apparatus under the
strictly nonhydrostatic
(uniaxial) pressure of 3 GPa
as described in Ref. 7. The treatment temperature was
experimentally
adjusted to be close to the high temperature limit of CGOstability (about 700°C according
to the calibration
of the apparatus).
In order to
increase the amount of the hard phase in the transformed material, admixture of aluminum with nominal composition
Al,&,, was introduced into starting
Cm. Due to large hardness and disordered structure,
the microscopic characterization
of the prepared specimen was difficult. Recorded X-ray, Raman and conductivity data were in good agreement with those in
Ref. 7. For comparison,
NMR spectra from graphite,
from microcrystals
of artificial diamond
of about
1 pm size, and from nanocrystalline
diamond powder,
were also recorded. The last material was obtained
by an explosive method and had a mean grain size
of about 4 nm. The graphite powder of spectroscopic
quality was obtained from Nippon Carbon Co. Ltd.
Y. M.~N~WAet al.
1288
useful to distinguish between the three and four-foldcoordinated
carbon atoms.
The graphite, consisting of sp2 carbon, shows a
very broad anisotropic
powder lineshape beyond a
typical sp’ region. This is understood
by the large
temperature-dependent
diamagnetic
susceptibility.
This comes from the interband transition, when the
magnetic field H, is applied perpendicular
to the
graphite plane [14]. That is, the broad anisotropic
lineshape
is a characteristic
of this semimetal.
It
should be noted, however, that the temperatureindependent
shift of - 180 ppm for the parallel magnetic field to the plane is very close to those of the
largest and intermediate
principal
values for the
typical chemical shift tensor for sp2 carbon. For
example, the principal values for chemical shift tensor
have been reported
to be (213, 182, 28) ppm in
pristine C,, [13] and (216.7, 140.5, 0.8) ppm in
benzene [15]. On this basis, we discuss the spectra
of HC, raw soot, pressure-induced
FCC and rhombohedral
phases
(No. 1 and No. 2) and carbon
nanotube.
lIiu!LJ
5
IO
20&
5
10
20
2QCdeg)
30
3s
30
35
Fig. 1. Cu Kcc X-ray diffraction patterns for two pressureinduced samples: (a) for No. 1; and (b) for No. 2.
We also measured
NMR in the carbon raw soot
obtained from MER Co. Ltd and in a non-refined
bundle of carbon nanotube
[ l,S] from Vacuum
Metallurgical
Co. Ltd. The bundle of nanotube with
needle like shape was aligned in the NMR sample
tube.
These specimens were evacuated up to 10m3 Torr
in an NMR glass tube and then sealed with He at
- 200 Torr, except the nanotube in the applied magnetic field perpendicular
to the needle. A conventional
pulse NMR apparatus was operated at 40.6 MHz for
r3C-NMR observation.
The frequencies are given in
parts per million (ppm) shift from the resonance
frequency in tetramethylsilane
(TMS) as the reference
substance.
3. RESULTS AND DISCUSSION
3.1 Diamond, graphite andpristine C,,
i3C-NMR
spectra
at low temperature,
where
molecular
rotation
cannot affect the spectra, are
shown in Figs 2(a) and (b). The resonance of diamond
powder (1 urn in diameter) was observed at - 35 ppm
from TMS in a typical region for sp3-coordinated
carbon [ 91. The rather narrow linewidth of - 30 ppm
is characteristic
of the sp3 carbon having the small
chemical shift anisotropy
due to the high symmetry
of its electronic environment
[9]. We also examined
nanocrystalline
diamond powder. There was no significant difference between these two diamond specimens in the i3C-NMR spectra. On the other hand,
the spectrum of pristine C,, solid gives one example
for the typical anisotropic
powder spectra for sp2
carbon (those in aromatic carbon atoms), as already
reported [10P13]. The clear difference between the
diamond and the pristine C,, solid demonstrates
that
the 13C-NMR spectra in the powder specimen are
3.2 Pressure-inducedphases,
and hard carbon
carbon raw soot
The spectra at room temperature
are shown in
Fig. 3. Those of HC, raw soot, and pressure-induced
specimens (Nos 1 and 2) are broad and essentially
temperature
independent.
These are contrast to the
sharp line of pristine CGO which indicates motional
narrowing
of the chemical shift anisotropy
due to
CeO molecular rotation [ 10-131. A slight broadening
in Nos. 1 and 2 in Fig. 2(a) is probably caused by
Curie-like spin which also contributes
to i3C spinlattice relaxation.
Therefore,
the large amplitude
molecular rotation may freeze in HC, raw soot, Nos. 1
and 2 even at room temperature
[ 161.
We observed sharp signals around 110 and 70 ppm
for Nos 1 and 2, respectively,
piled on the broad
sp2-like signal. This situation is well demonstrated
in
Fig. 4. There, the spectrum at 25 K is shown because
it has rather good signal-to-noise
ratio and rather
clear structure
without significant
line-broadening
due to Curie-like spins. It can be seen from the figure
that the spectrum of pristine C,, roughly produces
the broad signal of No. 2, if the former spectrum is
shifted by 15 ppm to the low-field (left) side and
expanded by 10% around the center of gravity (dotted
line). This means that the principal values for the
chemical shift tensor for the broad line are about
(235, 201, 32) ppm. The broad signal, therefore, is
due to the carbon atoms with almost the same
microstructure
as that of pristine C,,.
The intensities of the sharp lines, indicated by the
obliquely shaded regions in Fig. 2(a), are 14+ 5% for
No. 1 and 19k 5% for No. 2 to the total intensity.
These sharp lines would be attributed
to sp3-like
carbon in C,, cage, because they show small anisotropy (narrow linewidth), a characteristic
feature for
sp3 carbon. The observed shifts (70 and 110 ppm) are
Comparative
NMR study of new carbon
I
400
200
0
-200
-400
-600
2000
I,
1000
Shift (ppm from TMS)
,,I
”
I
600
1289
forms
I,
o
”
I
I”‘,”
I, I / I
-1000
I
-2000
Shift (ppm from TMS)
(a) Hard carbon at 4.2 K; (b) rhombohedral
phase (No. 2) at 4.2 K; (c) FCC
Fig. 2. r3C-NMR spectra at low temperature.
phase (No. 1) at 4.2 K; (d) pristine CsO at 77 K, (e) carbon raw soot obtained from MER Co. Ltd at 4.2 K; (f) diamond powder
of 1 urn diameter at 130 K; (g) graphite powder at 4.2 K; and (h) randomly oriented bundle of carbon tube at 4.2 K.
HC
500
400
300
200
100
shift (ppm)
0
-100
-200
Fig. 4. r3C-NMR spectrum for a rhombohedral
specimen,
No. 2, at 25 K, along with that of pristine C,, which is
expanded by 10% around the center of gravity and shifted
by 15 ppm to the low-field side.
800
400
0
-400
-800
Shift (ppm from TMS)
Fig. 3. r3C-NMR spectra at room temperature
in HC, rhombohedral C,, (specimen No. 2), raw soot, diamond powder
of 1 urn diameter, and pristine C,,.
also in rough agreement with those of the sp3 carbon
in C,, derivatives being located around 60-80 ppm
[1,17]. However, it should be noted that the value
of 110 ppm in the FCC phase is rather close to those
of aromatic carbon, e.g. 128.5 ppm for benzene and
143 ppm for C,,. This implies that these FCC phase
carbons may have a somewhat strong aromatic character, suggesting a close relation to the recent observation that the FCC and rhombohedral
phases show
different behaviors in ESR measurement
[IS].
In a previous experiment
of solid-state high-resolution NMR using magic angle sample spinning
UR34:10-F
(MAS), the pressure-induced
phases showed a broad
peak around 125 ppm together with several subpeaks
[5]. However, the assignment
of the peaks has not
been reported [ 191. In the present experiment without MAS, the sp3 carbon signal can be clearly
distinguished from those of the broad sp2 carbon due
to the different anisotropy in “C-NMR
spectra.
The sp3-like carbons in Nos 1 and 2 specimens are
probably attributed to those in C,,-C,,
intermolecular bonds in the “polymerized”
C,, network. The
signal intensity of the sharp line (19 f 5% for No. 2)
is in good agreement with the value of 20% expected
from the structural
model for the rhombohedral
phase [6].
To confirm the above assignment, the rhombohedral phase (specimen No. 2) was annealed at 225°C
for N 80 min, and then 250°C for N 110 min in the
sealed NMR tube. After each step, r3C-NMR spectra
Y. MANIWA et al
1290
were recorded
at room temperature,
as shown in
Fig. 5(a-c) . On this heating, a sharp line appeared
at 142 ppm, and the intensity of the broad structural
signal substantially decreased. This is quite consistent
with the previous reports [5,6], which described that
the polymerized
CGO is converted to pristine CeO by
reheating above 200-300°C at ambient pressure.
The other
possibility
for assignment
of the
sp3-like carbon is in the chemical bond with oxygen
[20] such as (&O
and C60-O-C60. To check this
r3C-NMR
was observed
in CeO solid
possibility,
specimen, which was heated in a glass tube sealed
with air under similar conditions to those for specimens Nos 1 and 2. The spectrum obtained was similar
to that of pristine C,, and could not produce line
shapes like (b) or (c) in Fig. 2(a). The signal intensity
of the sharp line cannot be explained by this model.
In the raw soot and HC, on the other hand, the
corresponding
sharp line was not observed and those
spectra seem to be understood
primarily with regard
to sp2 carbon. The results from the raw soot may be
consistent with a recent observation of soot structure
by high-resolution
transmission
electron microscopy
(TEM), where the soot is composed
of primary
graphitic layers [21]. On the contrary,
the result
from HC is somewhat different. In spite of the similarity of the preparation
processes between HC and
specimens Nos 1 and 2, the sp3-like signal was not
separated
in HC. The absence of large amplitude
molecular rotation, as well as preparation
conditions
close to the high temperature
limit of CeO stability,
suggests that the spherical structure of the C,, molecule is significantly distorted or completely destroyed.
As an extreme case, the C,, molecules may be fused
to each other and be converted
to rather large
I”““”
“““I
“’
‘I
graphitic domains, which may partially have microstructures of three-dimensional
sponge graphite consisting of mainly three-fold-coordinated
(sp”) carbon
atoms, as discussed by Fujita et al. [22]. The graphitic
nature in the raw soot and the HC may be seen in
the spectra (Figs 2 and 3) as the long tail at low
frequency side (to the right-hand side).
Another possible interpretation
in HC is related to
assumption of distribution of a different kind of bond
extending from typical sp2 to diamond-like
strong
sp3 bonds. The i3C-NMR spectrum may then not
show any distinct sharp line unlike the cases of C,,
polymers, and specimens Nos 1 and 2, because of a
distribution
of the shift.
3.3 Carbon nanotube
Finally, we discuss the 13C-NMR spectra of the
carbon nanotube (Figs 2(b) and 6). It is found that
the linewidth of the tube is much larger than graphite
powder. The spectrum for H, parallel to the needle
(graphitic plane) is slightly shifted to the high-field
(to the right) side from that for H, perpendicular
to
the needle. This is opposite to the case of graphite,
in which the shift values for H, perpendicular
and
parallel to the plane have been reported to be approx.
-450 and 180 ppm at low temperature
(- 100 K),
respectively,
in highly oriented
pyrolytic graphite
(HOPG)
[ 141. The present NMR results may be
closely related to recent static susceptibility (x) measurements.
Diamagnetic
susceptibility
of the tube,
xdia, is much larger than that of graphite [23], and
the xdia parallel to the tube is larger than that
perpendicular
to the tube. This is opposite to the
case of graphite [ 241. The differences between nanotube and graphite both in the xdia and the NMR
linewidth may have the same origin, probably due to
cylindrical
structure
and different interlayer
interaction. The cylindrical structure of nanotube may be
also responsible for the anomalous
large i3C-NMR
linewidth.
,““#““I”’
““I
4.2K
600
/
I,
I,
I,
400
200
0
-200
I,,
-400
-600
Shift (ppm from TMS)
Fig. 5. r3C-NMR
spectra in rhombohedral
C,, (specimen
No. 2): (a) before annealing; (b) after annealing at 225°C for
80 min; (c) after additional
annealing at 250°C for 110 min.
1000
0
-1000
-2000
-3000
Shift (ppm from TMS)
Fig. 6. 13C-NMR spectra at 4.2 K in the bundle of carbon
tube for the fields parallel and perpendicular
to the needle
shape specimen.
Comparative
NMR study of new carbon
4. CONCLUSIONS
In pressure-induced
FCC
and rhombohedral
sharp
isotropic
13C-NMR
signals
were
phases,
observed. These were assigned to sp3-like carbon in
“C,,-fullerene
network”. However the bond character
is somewhat different among the FCC phase, rhombohedral phase and diamond. That of the FCC is rather
close to those of aromatic carbon. On the other hand,
the hard carbon (HC) did not show clear evidence
Within
the present
pressure/
for sp3 carbon.
temperature
treatment
conditions
(3-4.6 GPa and
lower than 720”(Z), C,, solid cannot be apparently
transformed
into four-fold-coordinated
diamond-like
carbon clusters. While the origin of the hardness still
remains to be solved, it was shown that 13C-NMR
technique is one of the most powerful tools to study
conversion of carbon-carbon
bond character in fullerene-based
new solid forms of carbon.
13C-NMR spectra would also be useful to investigate graphitic
nature from the viewpoint
of the
detection of diamagnetic
susceptibility.
Acknowledgements-We
thank M. Hirabayashi,
H. Ihara
and A. Iyo for the use of high-pressure
apparatus
and X-ray
diffractometer.
Y. M. thanks T. Ishida for useful discussion
on 13C-NMR chemical shift in C,, derivatives, I. Shiozaki
for critical reading of the manuscript
and J. Fischer for
useful comments.
This work was supported
in part by a
Grant-in-Aid
from Ministry
of Education,
Science and
Culture. This work was also supported
in part by the fund
for special
Research
Project
at Tokyo
Metropolitan
University.
REFERENCES
1. For a review, see Physics and Chemistry of the Fullerenes,
(Edited by K. Prassides). Kluwer Academic Publishers,
The Netherlands
(1994).
2. S. Iijima, Nature 354, 56 (1991).
3. D. Ugarte, Nature 359, 707 (1992).
4. A. M. Rao, P. Zhou, K.-A. Wang, G. T. Hager, J. M.
Holden, Y. Wang, W.-T. Lee, X.-X. Bi, P. C. Eklund,
D. S. Cornett, M. A. Duncan and I. J. Amster, Science
259, 955 (1993).
5. Y. Iwasa, T. Arima, R. M. Fleming, T. Siegrist, 0. Zhou,
R. C. Haddon,
L. J. Rothberg,
K. B. Lyons, H. L.
Carter, Jr., A. F. Hebard, R. Tycko, G. Dabbagh,
J. J.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
forms
1291
Krajewski,
G. A. Thomas
and T. Yagi, Science 264,
1570 (1994).
M. N.-Regueiro,
L. Marques, J.-L. Hodeau, 0. Bethoux
and M. P&roux, Phys, Rev. Lett. 74, 278 (1995).
M. E. Kozlov. M. Hirabavashi.
K. Nozaki. M. Tokumoto and I. Ihara, Appl. Pkys. kt. 66, 1199 (1995).
T. W. Ebbsen, H. Hiura, J. Fujita, Y. Ochiai, S. Matui
and K. Tanigaki, Chem. Phys. Lett. 209,83 (1993).
M. Mehring,
High Resolution NMR Spectroscopy
in
Solid. Springer-Verlag,
Heidelberg, New York (1983).
Y. Maniwa,
K. Mizoguchi,
K. Kume, K. Kikuchi,
S. Suzuki and Y. Achiba, Solid State Commun. 80, 609
(1991).
R. D. Johnson, C. S. Yannoni, H. C. Dorn, J. R. Salem
and D. S. Bethune, Science 255, 1235 (1992).
R. Tvcko. G. Dabbaph.
R. M. Fleming. R. C. Haddon.
A. V: Makhija and ST h. Zahurak, P&s. Rev. Lett. 67;
1886 (1991).
Y. Maniwa, M. Nagasaka, A. Ohi, K. Kume, K. Kikuchi,
K. Saito, I. Ikemoto,
S. Suzuki and Y. Achiba, Jpn.
J. Appf. Phys. (Part 2) 33, L173 (1994).
Y. Hiroyama
and K. Kume, Solid State Commun. 65,
617 (1988).
M. Lender, A. Hohener and R. R. Ernst, J. Mag. Resonance 35, 379 (1979).
The apparent
stronger temperature
dependence
of the
sharp lines as compared
with the broad line can be
explained by a fact that the spectra are convolution
of
two contributions.
One is the anisotropy
and the other
is contribution
from the Curie-like spins. The temperature dependence is due to Curie-like spins, which gives
the same linewidth for the broad and sharp lines. However, because of the smaller anisotropy of the sharp line,
it has an apparent
stronger temperature
dependence
compared with the broad line.
T. fshida,
M. Tsuda,
T. Nogami,
S. Kurono
and
M. Ohashi. Fullerene Sci. Technol. 2. 155 (1994).
M. E. Kozlov,
A. A. Zakhidov,’
K. ?akushi
and
M. Tokumoto,
J. Phys. Chem., in press.
Very recently, F. Rachdi et al. reported detailed studies
on Cc,, polymers using MAS NMR (IWEPNM
‘96).
A. B. Smith III, H. Tokuyama,
R. M. Strongin, G. T.
Furst, W. J. Romanow,
B. T. Chait, U. A. Mirza and
I. Hailer, J. Am. Chem. Sot. 117, 9359 (1995).
S. Iijima, T. Wakabayashi
and Y. Achiba, J. Chem. Phys.
in press.
M. Fuiita, T. Umeda and M. Yoshida, Phys. Reo. B 51,
13778< 1995).
A. P. Ramirez. R. C. Haddon. 0. Zhou. R. M. Fleming.
J. Zhang, S. &I. McClure and R. E. Smalley, Science
265, 84 (1994).
0. Chauvet, L. Forro, W. Bacsa, D. Ugarte, B. Doudin
and W. A. de Heer, Phys. Rev. B 52, R6963 (1995).