High-yield carbon nanorods obtained by a catalytic copyrolysis process

Article abstract of DOI:10.1021/ic049812c

Carbon nanorods were produced with a yield of about 90% by the copyrolysis of C₆H₆ and C₅H₆ at 600 °C under the cocatalysis of Fe and Mg. Many novel Y-junction carbon nanorods were found in the products. The obtained carbon nanorods have a diameter in the range of 200-350 nm and are several micrometers in length. The effects of reactants, catalysts, and the temperature were investigated, and the experimental results indicate that C₅H₆ and cocatalysts Fe and Mg play crucial roles in the formation of carbon nanorods. The possible formation mechanism of the carbon nanorods is discussed.

Full text of DOI:10.1021/ic049812c

Inorg. Chem. 2004, 43, 5432−5435
High-Yield Carbon Nanorods Obtained by a Catalytic Copyrolysis
Process
Guifu Zou, Jun Lu, Debao Wang, Liqiang Xu, and Yitai Qian*
The Structure Research Laboratory and Department of Chemistry, UniVersity of Science and
Technology of China, Hefei 230026, PR China
Received February 13, 2004
Carbon nanorods were produced with a yield of about 90% by the copyrolysis of C H and C H at 600 °C under
6
6
5 6
the cocatalysis of Fe and Mg. Many novel Y-junction carbon nanorods were found in the products. The obtained
carbon nanorods have a diameter in the range of 200−350 nm and are several micrometers in length. The effects
of reactants, catalysts, and the temperature were investigated, and the experimental results indicate that C H and
5
6
cocatalysts Fe and Mg play crucial roles in the formation of carbon nanorods. The possible formation mechanism
of the carbon nanorods is discussed.
Introduction
have been largely synthesized with various methods. In
addition, carbon rods have largely been applied in anodic
materials of batteries because of their performance (large
discharge capacity, small irreversible capacity or high
Coulombic efficiency, and low discharge potential for
obtaining high voltage) and the low cost for mass produc-
tion.23 The great success of the microelectronics industry has
been based on the miniaturization of a few basic device
elements in which different types of junctions are used;24
therefore, Y-junction carbon nanorods with a complex three-
point junction structure have been proposed as the building
blocks of nanoelectronics. Various methods have been used
for the preparation of carbon nanorods. Liu et al. have
prepared carbon nanorods by the arc discharge method.25
Thien-Nga’s group have fabricated the carbon nanorods on
a high-Tc substrate via chemical vapor deposition (CVD).26
Template methods have been used to synthesize carbon
In recent years, the development of 1D materials has
become a focus in nanoscale research because their special
characteristics differ from those of respective bulk crystals.1-8
Among these 1D nanomaterials, carbon materials have been
especially emphasized because of their various structures and
correspondingly unique performance.9-17 So far, carbon
nanotubes,18,19 nanowires,20 nanoribbons,21 and nanofibers22
* Author to whom correspondence should be addressed. Tel: +86-551-
3601589. Fax: +86-551-3607402. E-mail: ytqian@ustc.edu.cn.
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5432 Inorganic Chemistry, Vol. 43, No. 17, 2004
10.1021/ic049812c CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/15/2004

High-Yield Carbon Nanorods
Figure 1. Typical X-ray powder diffraction pattern of the products.
Figure 2. Raman spectrum of the products.
nanorods.27 Chen et al. have prepared carbon nanorods using
the electron beam-induced route.28 However, to our knowl-
edge, a high-yield preparation of carbon nanorods by a
catalytic copyrolysis method has not been reported. Here,
we report that carbon nanorods with a yield of about 90%
have been prepared using the copyrolysis of C6H6 and C5H6
under the cocatalysis of Fe and Mg at 600 °C in the
autoclave; ca. 40% Y-junction carbon nanorods were found
for the first time in the products.
vibrations of carbon atoms with dangling bonds in plane
terminations of disordered graphite. The peak is relatively
high, indicating that in the basal planes 2D disorder exists,
which is quite common in pyrolytic production.30
Figure 3a and b shows SEM images of a typical sample
of carbon nanorods, indicating the large quantity of carbon
nanorods obtained via this approach. The yield of carbon
nanorods estimated through SEM and TEM observation of
the products is about 90%. These carbon nanorods have
diameters ranging from 200 to 350 nm and lengths ranging
from hundreds of nanometers to several micrometers. It is
worth noting that many Y-junction carbon nanorods (marked
with arrows in Figure 3b) can be found in the products. The
TEM images of the products and a typical Y-junction carbon
nanorod are shown in Figure 3c and d, respectively. The
ratio of the Y-junction carbon nanorods in the products is
estimated to be about 40% by the large number of TEM and
SEM observations. Because Y-junction carbon nanotubes
have displayed interesting nonlinear electric properties,31,32
we proposed Y-branch switches with the aim of realizing
low switching voltages in a single-mode, coherent regime
of operation.33-35 Therefore, we consider the Y-junction
carbon nanorods to be very significant to the building of the
future nanoelectronic industry. Figure 3e shows a TEM
image of an individual carbon nanorod with a diameter of
ca. 300 nm. The selected-area electron diffraction (SAED)
pattern (inset of Figure 3e) exhibits a pair of small but strong
arcs for 002 and a weak ring for 101 plane diffractions. The
appearance of 002 diffractions as a pair of arcs indicates
some orientation of the 002 planes in the carbon nano-
rods.25,36 The ED analysis is consistent with the XRD results.
Besides the carbon nanorods, a small quantity of carbon
nanotubes (Figure 3f) can be observed in the product.
Experimental Section
All of the reagents used were of analytical purity (Shanghai
Chem. Co.). In a typical experimental procedure, 10 mL of C6H6
and 5 mL of C5H6 together with a mixture of 0.25 g of Mg and
0.25 g of Fe powder were placed into a 20-mL stainless steel
autoclave. The autoclave was tightly sealed and heated on an electric
stove at a rate of 20 °C/min and maintained at 600 °C for 12 h,
and then it was naturally cooled to room temperature. The product
in the autoclave was collected and washed with absolute ethanol,
dilute hydrochloric acid, and distilled water several times. Finally,
the product was dried in a vacuum at 50 °C for 4 h. The final
products were characterized by powder X-ray diffraction (XRD,
Rigaku D with Cu KR1 radiation wavelength of λ ) 1.54178 Å),
scanning electron microscopy (SEM, HITACHIX-650 and JEOL
JSM-6700F), transmission electron microscopy (TEM, HITACHI
800), high-resolution transmission electron microscopy (HRTEM,
JEOL 2010 using an accelerating voltage of 200 kV), and Raman
spectroscopy (RS, Spex 1403 Raman spectrometer with an argon
ion laser at an excitation wavelength of 514.5 nm).
Result and Discussion
A typical XRD pattern of the products is shown in Figure
1. The intense peaks at ca. 26.4 and 43.4° can be indexed to
(002) and (101) diffraction planes of hexagonal graphite
(JCPDS card files, no. 41-1487), respectively. No impurity
is observed in the XRD pattern.
(29) Dresselhaus, M. S.; Dresselhaus, G.; Pimenta, M. A.; Eklund, P. C.
In Analytical Applications of Raman Spectroscopy; Pelletier, M.J., Ed.;
Blackwell Science: Oxford, U.K., 1999; Chapter 9.
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2002, 40, 2961.
(31) Gao, T.; Meng, G.; Zhang, J.; Sun, S.; Zhang, L. Appl. Phys. A. 2002,
74, 403.
Figure 2 shows the Raman spectrum of the carbon
nanorods. Two strong peaks exist at 1350 and 1570 cm-1,
corresponding to the typical Raman peaks of graphitized
carbon nanorods. The peak at 1570 cm-1 corresponds to an
E2g mode of graphite and is related to the vibration of sp2-
bonded carbon atoms in a 2D hexagonal lattice, such as in
a graphene layer.29 The peak at 1350 cm-1 is associated with
(32) Papadopoulos, C.; Rakitin, A.; Li, J.; Vedeneev, A. S.; Xu, J. M. Phys.
ReV. Lett. 2000, 85, 3476.
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C. T.; Ma, K. J. J. Phys. Chem. Solids 2001, 62, 1561.
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1998, 10, 260.
Inorganic Chemistry, Vol. 43, No. 17, 2004 5433

Zou et al.
over, the irregular graphite layers are almost perpendicular
to the respective axis of the Y-like carbon nanorod, and their
interlayer spacing is ca. 0.34 nm, which is typical for the
(002) lattice distance in hexagonal carbon. A carbon nanorod
with a similar microstructure was reported in the literature.26
The SAED (Figure 4b), taken at the junction of the Y-like
carbon nanrod, exhibited strong arcs for 002 diffractions and
weak arcs for 101 diffractions. At the same time, it also
reflected the disorder of the stacked carbon layers. The
further observation of the SAED found that it consisted of
two pairs of strong arcs for 002 diffractions, implying two
different orientations of the 002 planes in the Y-junction
carbon nanorod. It is worth noting that the angle of the strong
arcs was almost consistent with the Y-junction angle. On
the basis of the HRTEM and SAED analyses, the results
suggest that the junction was formed by curving carbon
layers. These observations were similar to those in the
Y-junction carbon nanotubes report.18
The reactions involved in our experiment are fairly
complex. To study the influences of the reactants and
catalysts on the formation and yield of carbon nanorods, we
carried out a series of experiments (as shown in Table 1)
with a process similar to that mentioned in the Experimental
Section. From experiment no. 1, it was found that the
products did not contain carbon nanorods; mainly, carbon
nanotubes were observed when only C6H6 was used as the
starting material. This result is similar to that in our previous
report.30 When C5H6 was added to the reaction system, some
carbon nanorods could be found in the products, and when
the C5H6 quantity increases, the yield of carbon nanorods
correspondingly increases (experiment no. 2). The yield of
ca. 90% carbon nanorods can be obtained when 10 mL of
C6H6 is added to 5 mL of C5H6 in experiment no. 3. Although
C5H6 was very important for the formation and yield of
carbon nanorods according to the above experiments, when
the amount of C5H6 increased, the yield of carbon nanorods
decreased in experiment no. 4; it is worth noting that the
yield of carbon nanorods obviously decreased when the
reactants did not contain C6H6 in experiment no. 5. This
shows that C6H6 and C5H6 have a synergetic effect on the
production of carbon nanorods. That is, the proper ratio of
C5H6 and C6H6 is a very important factor in carbon nanorod
formation. However, the catalysts also have important
influence on the formation of carbon nanorods. When the
catalysts are absent from the reaction system (no. 8), the
carbon nanorods cannot form. Moreover, the results of
experiments 3, 6, and 7 show that a high yield of carbon
nanorods was obtained only when Fe and Mg powder were
added simultaneously to the reaction system. We think that
this is a synergetic effect of cocatalysts Fe and Mg. The
phenomenon of the synergetic effect of catalysts can be also
found in some reports.37,38 Except for the above relative
factors, the reaction temperature is also an important factor
in carbon nanorod formation. When the reaction temperature
Figure 3. (a) Low-magnification and (b) field-emission SEM images of
the as-prepared sample. (c and d) TEM images of the sample and typical
Y-junction carbon nanorods, respectively. (e) TEM image and (inset) SAED
pattern of an individual carbon nanorod. (f) Typical TEM image of the
carbon nanotube.
Figure 4. (a) HRTEM image and (b) SAED pattern of the junction of a
Y-like carbon nanorod.
The structural characterization of the Y-junction carbon
nanorod was investigated in detail by HRTEM and SAED.
Typical images are shown in Figure 4. It can be seen that
the carbon nanorods are formed from graphitic layers, al-
though the carbon nanorods are not well crystallized. More-
(37) Lee, C. J.; Park, J.; Kim, J. M.; Huh, Y.; Lee, J. Y.; No, K. S. Chem
Phys. Lett. 2000, 327, 277.
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468.
5434 Inorganic Chemistry, Vol. 43, No. 17, 2004

High-Yield Carbon Nanorods
Table 1. Products Obtained by the Following Experiments at 600 °C for 12 h; and Ratios of the Products Estimated through Large Numbers of SEM
and TEM Observations of the Productsa
expt no.
reactant
C6H6 (15 mL)
C6H6(12 mL) + C5H6(3 mL)
C6H6 (10 mL) + C5H6 (5 mL)
C6H6 (5 mL) + C5H6 (10 mL)
C5H6 (15 mL)
C6H6 (10 mL) + C5H6 (5 mL)
C6H6 (10 mL) + C5H6 (5 mL)
C6H6 (10 mL) + C5H6 (5 mL)
catalyst
approximate contents of various products
1
2
3
4
5
6
7
8
Fe and Mg
Fe and Mg
Fe and Mg
Fe and Mg
Fe and Mg
Fe
75% NTs + 25% carbon particles
40% NTs + 50% NRs + 10% carbon particles
90% NRs + 10% NTs
30% NRs + 70% carbon particles
5% NRs + 95% carbon particles
85% carbon spheres + 15% NTs
90% carbon spheres + 10% NTs
carbon particles
Mg
a NRs and NTs are abbreviations for carbon nanorods and nanotubes, respectively.
is below 400 °C and other conditions are kept constant, we
cannot obtain any carbon products. Some short carbon
nanorods are produced when the temperature is increased to
500 °C. When the reaction temperature is further increased,
longer carbon nanorods can be obtained in larger quantity.
In the end, the ca. 90% yield of carbon nanorods is produced
at 600-700 °C. However, carbon particles are formed when
the reaction temperature is increased to 800 °C. In addition,
the entire yield of the carbon nanorods is changed by altering
the conditions of the reaction, but it is interesting that the
proportion of Y-junction carbon nanorods is almost identical
to that of whole carbon nanorods.
Although the exact formation mechanism of carbon
nanorods is not very clear, we can tentatively use a
mechanism that is similar to Gamaly and Ebbesen’s model39
to discuss the formation of carbon nanorods. In this model,
the velocity of carbon atom distribution on the surface of
catalysts mainly influences the formation of carbon nanorods.
In the reaction process, the starting materials are first
pyrolyzed into carbon atoms with the reaction temperature
increasing,and then the carbon atoms form a homogeneous
system under the assistance of catalysts. Next, some carbon
atoms that are attached to the surface of the catalysts form
graphite layers as a cap, which may be considered to be seed
structures for the growth of carbon nanorods. It is well known
that the carbon atom’s moving velocity is correlative with
many factors, including the concentration of carbon atoms,
the catalyst category, and the temperature. Thus, with the
reaction temperature increasing, part of C5H6 will be pyro-
lyzed into carbon atoms first because the pyrolysis temper-
ature of C5H6 is much lower than that of C6H6. Because of
the different pyrolysis speeds of C5H6 and C6H6, the carbon
atoms’ concentration is influenced by the ratio of C5H6 to
C6H6. That is, the ratio of C5H6 to C6H6 has an indirect
relationship with the moving speed of the carbon atoms. In
addition, the catalyst category and temperature also are
influences on the distribution speed of the carbon atoms in
our experiments. When carbon atoms with moderate veloci-
ties are distributed on the surface of the catalysts, the graphite
layers form carbon nanorods. When their velocity is slow,
the graphite layers form carbon nanotubes. However, when
the velocity is high, the graphite layers form carbon particles.
The above discussion well explains the experimental results;
of course, because of the complexity of the experimental
process, the exact formation mechanism of carbon nanorods
still needs further research.
Conclusions
In summary, C6H6 and C5H6 together with cocatalysts Fe
and Mg powder were successfully used to synthesize a yield
of about 90% carbon nanorods at 600 °C for 12 h. The yield
of carbon nanorods with diameters in the range of 200-350
nm and lengths in the range of 0.8-6 µm was ca. 90%. Many
Y-junction carbon nanorods that we found in the products
are significant to the basic building units for nanoelectronic
devices. The experimental results show that the synergic
effect of C5H6 and C6H6 and cocatalysts Fe and Mg affected
the yield of the carbon nanorods. In addition, the possible
formation mechanism of the carbon nanorods was discussed.
Acknowledgment. We acknowledge financial support
from the 973 Climbing Project Foundation of China.
(39) Gamaly, E. G.; Ebbesen, T. W. Phys. ReV. B 1995, 52, 2083.
IC049812C
Inorganic Chemistry, Vol. 43, No. 17, 2004 5435