194 Letters to the Editor / Carbon 41 (2003) 179  198
make the interlayer spacing difficult to decrease even if the
graphitization is proceeding. We showed that the extrapo-
lated interlayer spacing, d, increased a little with increasing
HTT in Figs. 3 5. If the volume change, DV is caused by
the broadening of interlayer spacing, the volume change is
shown by Eq. (2)
DV5 (d3 2 d3 )/d3 (2)
t 1000 1000
where d1000 and dt are the extrapolated interlayer spacing
at HTT 1000 8C and t 8C, respectively. In the case of
volume change at HTT 2800 8C, DV50.045 is obtained
from Eq. (2). This value almost agrees with that of volume
change from 1000 to 2800 8C in Fig. 1a. The result
supports that the volume expansion comes from the
broadening of mean interlayer spacing of the stacking
layers.
References
Fig. 5. Effect of heat treatment times, 1 h and 9 h on the
extrapolated interlayer spacing, d.
[1] Jenkins GM, Kawamura K, Ban LL. Formation and structure
of polymeric carbons. Proc R Soc Lond 1972;A327:501 17.
[2] Mochida I. Review on the structural concepts of carbons.
Molecular, nano, meso and micro-scopic views of mesophase
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pitch/graphitized fiber including nano phased fibers as
carbon layers in the three-dimensional entangling structure.
models of carbons. TANSO 2001;200:206 16.
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New York: John Wiley, 1973, pp. 111 490.
obtained from the lattice image directly using a high-
[4] Azároff LV, Buerger MJ. In: The powder method in X-ray
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like carbon heat-treated at low temperature shows that
[5] Mehrotra BN, Bragg RH, Rao AS. Effect of heat treatment
small carbon layers gather at random in dense clusters [1].
temperature (HTT) on density, weight and volume of glass-
This observation is consistent with the high density of the
like carbon (GC). J Mater Sci 1983;18:2671 8.
glass-like carbon heat-treated at 1000 8C as shown in Fig.
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carbon. Carbon 1978;16(5):373 6.
layers grow, and a structure is established, which is
[7] Kawamaura K, Bragg RH. Graphitization of pitch coke:
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Catalytic formation of carbon nanotubes during detonation of
m-dinitrobenzene
Yi Lu, Zhenping Zhu*, Weize Wu, Zhenyu Liu
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China
Received 13 May 2002; accepted 1 October 2002
Keywords: A. Carbon nanotubes; B. Catalyst; C. Transmission electron microscopy; Raman spectroscopy; X-ray diffraction
*
Corresponding author. Tel.: [phone]; fax: 186- Owing to unique electrical, mechanical, gas-storing, and
351-404-1153.
catalytic properties, as well as a variety of significant
E-mail address: [e-mail] (Z. Zhu).
potential applications [1,2], carbon nanotubes (CNTs) have
0008-6223/03/$  see front matter © 2003 Elsevier Science Ltd. All rights reserved.
PII: S0008-6223(02)00335-4
Letters to the Editor / Carbon 41 (2003) 179  198 195
Fig. 3. Cone-shaped metal particles existing at ends of MWCNTs.
Fig. 1. TEM image of a typical detonation product. The molar
ratio of DNB to Co is 6:1 and loading density of DNB is 0.4
g/ml.
these methods are high energy- and hardware-intensive,
which leads to high cost of CNTs and thus constrains their
been an area of intense research since their discovery in practical applications.
1991 [3]. At present, syntheses of CNTs are normally Recently, Boese et al. [10] and Kroke et al. [11] reported
conducted via arc-discharge [3 6], laser ablation [7] and the formation of tubular products after explosive decompo-
catalytic decomposition of hydrocarbons [8,9]. However, sition. This detonation approach may arouse an energy-
Fig. 2. Raman spectrum of the sample shown in Fig. 1.
196 Letters to the Editor / Carbon 41 (2003) 179  198
and hardware-saved method for CNT synthesis, since the diameter, 70 mm in length) equipped with a pressure
needed high temperature is spontaneously produced from gauge, under an inert gas (Ar) of 0.1 MPa. Before the
the detonation. However, the employed experiment, DNB and cobalt acetate (or nickel formate)
1,2:5,6:11,12:15,16-tetrabenzo-3,7,9,13,17,19-hexadehydro were mixed in an appropriate molar ratio (DNB:metal
annulene [10] and 2,4,6-triazido-s-triazine [11] were spe- salt58:1 1:2), ground physically and then loaded into the
cially synthesized, and the obtained products contained pressure vessel to a desired loading density (the amount of
only 2% of CNTs [11]. Here we present a more effective explosive employed for the detonation in a given volume,
synthesis of CNTs using m-dinitrobenzene (DNB) as a 0.02 0.4 g/ml) for DNB. The detonation was induced by
starting explosive and cobalt (or nickel) as a catalyst for heating (20 8C/min) to 420 8C and evidenced by a sudden
CNTs growth. Such a detonation system can produce pressure rise (shock wave, from 0 to 20 40 MPa, depend-
CNTs with purity of 30 40%. ing on the loading density of DNB). After the detonation,
The detonation experiments were performed in a her- the reactor was cooled to room temperature naturally,
metic stainless steel pressure vessel (14 mm in inner argon and the gaseous products were vented. The solid
Fig. 4. XRD patterns of detonation products. The molar ratio of DNB to metal is 6:1 and loading density of DNB is 0.4 g/ml, cobalt (a) and
nickel (b) are the catalysts.
Letters to the Editor / Carbon 41 (2003) 179  198 197
products, very voluminous black powders, were collected pounds and subsequent reduction by carbon species re-
for further characterization. Transmission electron micro- sulting from detonation. Such a procedure resembles those
scopy (TEM; Hitachi H-600), X-ray diffraction (XRD; of the catalysis-assisted arc-discharge [5,6] and laser
Rigaku X-ray diffractometer using Cu Ka radiation) and ablation [7] approaches and is superior to catalytic de-
Raman scattering (Renishaw Micro-Raman 2000 equipped composition of hydrocarbons unless the alignment of
with a 209 objective and 632.8 nm line of helium neon synthesized nanotubes is requested. Furthermore, most of
laser as excitation resource) were employed to investigate the metal particles located at the tube ends show a cone
the as-synthesized solid products. shape, suggesting that the metal particles are likely in a
Fig. 1 is a typical TEM image of the solid products from quasi-liquid phase or fluxion state during the tube growth,
detonation of a mixture containing DNB and cobalt acetate which may be important to understand the carbon metal
at a molar ratio of 6:1. The loading density of DNB is 0.4 interaction. Cobalt exhibits high catalytic activity in con-
g/ml. It reveals that the products mainly consist of multi- trast to nickel, which may be related to the difference in
wall carbon nanotubes (MWCNTs) and carbon-encapsu- structure. XRD patterns indicate that in the solid products,
lated metal nanoparticles. The MWCNTs are separated the cobalt particles show cubic structures (Fig. 4a) while
from each other with inner diameters of 12 30 nm and nickel particles show both cubic and hexagonal structures
outer diameters of 20 40 nm. The tube length is between (Fig. 4b).
0.5 and 8 mm. Most of the MWCNTs are coiled to In conclusion, the detonation of nitro-containing organic
different extents. Based on the TEM images, it is estimated explosives as common as m-dinitrobenzene can be used to
that the sample contains 30 40% MWCNTs. Higher synthesize CNTs in the presence of a catalyst. The
loading densities of DNB and appropriately higher molar approach is simple, low-cost and universal. From the
ratios of DNB to the metal salt are preferable to yield experimental observation, it is estimated that some other
CNTs-rich products, possibly due to larger heat release nitro-containing organic explosives possessing higher
[12] and slower local heat diffusion, which are very chemical energy than DNB, such as picric acid and
important for the nucleation and thermal annealing of trinitrotoluene, may be more effective for CNTs growth
CNTs [12,13]. In the experiments where DNB loading because their detonations can generate higher tempera-
density is lower than 0.05 g/ml and/or the molar ratio of tures. In addition, a further increase in CNTs selectivity
DNB/metal salt is lower than 1:1, no CNTs are found in and tailoring of structure defects may be available by
the products, carbon-encapsulated metal nanoparticles and elaborate design of detonation experiments.
amorphous carbon particles are the main products.
Fig. 2 is a Raman spectrum of the sample shown in Fig.
1. There are two main peaks in the region between 200 and
Acknowledgements
2000 cm21. The one located at 1595 cm21 (G-band [14])
can be designated as the stretching vibration mode of
The authors thank the National Scientific Foundation of
graphite crystals indicating the formation of graphitized
China for partial support (No. 59872047) and Profs. Bing
CNTs. The one located at 1329 cm21 (D-band [14]) can be
Zhong and Yongwang Li for some valuable discussions.
designated as disorder-induced mode. Kang et al. [15]
attributed the D-band to the carbonaceous particles existing
near CNTs or adhering to the walls of CNTs. The
relatively low intensity of the G-band with respect to that
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