Detonation chemistry of a CHNO explosive: Catalytic assembling of carbon nanotubes at low pressure and temperature state

Article abstract of DOI:10.1039/b207166e

The detonation of a CHNO explosive was used for the first time to synthesize carbon nanotubes effectively at low pressure and temperature by introducing a cobalt catalyst and/or paraffin into the detonation system.

Full text of DOI:10.1039/b207166e

Detonation chemistry of a CHNO explosive: catalytic assembling of
carbon nanotubes at low pressure and temperature state
Yi Lu, Zhenping Zhu,* Weize Wu and Zhenyu Liu
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences,
Taiyuan 030001, P. R. China. E-mail: zpzhu@sxicc.ac.cn; Fax: +86 351 4041153; Tel: +86 351 4048310
Received (in Cambridge, UK) 24th July 2002, Accepted 3rd October 2002
First published as an Advance Article on the web 21st October 2002
The detonation of a CHNO explosive was used for the first
time to synthesize carbon nanotubes effectively at low
pressure and temperature by introducing a cobalt catalyst
and/or paraffin into the detonation system.
of all H atoms to H2O, thus a large amount of CO is produced.
In addition, the obtained solid product consists of sp2 graphite-
like amorphous nanoparticles, with sizes of 10–70 nm. No
tubular structure is formed likely due to too low temperature,
corresponding to the employed low loading density (0.2 g
cm23) of PA, to allow the development of sp2 networks.
When cobalt acetate, Co(Ac)2, is introduced with PA, with
Co(Ac)2+PA molar ratios of 1+20–1+30, the solid carbon yield
is significantly enhanced after detonation and carbon nanotubes
are well grown. In the obtained product, tubular structural
carbon is the dominant component, with a content of 80–90%.
The main impurities are cobalt-encapsulated onion-like nano-
particles and a minority of graphite-like amorphous carbon
particles. Additionally, the gaseous detonation products mainly
contain CO2, N2 and H2O, with little CO. These results indicate
that cobalt nanoparticles, formed by a quick decomposition of
Co(Ac)2 during the detonation, play an excellent catalytic role
in the disproportion reaction of CO and in the nucleation and
growth of CNTs. Both the direct explosive decomposition of PA
and the subsequent CO disproportion reaction on the surface of
cobalt particles provide the small carbon building blocks for
CNT assembling. The former process is similar to the catalysis-
assisted arc-discharge and laser-evaporation of graphite carbon
for CNT syntheses,4,5 and the latter process is similar to the
catalytic CO disproportionation approach.6
Considering part of carbon atoms involved in PA being
consumed by CO2 formation, liquid paraffin† is introduced into
the PA-Co(Ac)2 system, with a paraffin–PA weight ratio of 1+4.
The heat produced from the detonation quickly decomposes the
paraffin (this may be also catalyzed by the formed cobalt
particles7) into small carbon species and hydrogen radicals. The
former directly participates in CNT assembling and the latter
captures oxygen from PA and forms H2O, which effectively
enhances the solid carbon formation and depresses the carbon
consumption from CO2 formation as indicated by the analyses
of gaseous product.
Using detonation chemistry for peaceful purposes is an
interesting and challenging issue, especially for nanostructure
constructions due to simple processing and low production
cost.1 Typical detonation of nitro-hydrocarbons (CHNO) can
produce nano-sized pure carbon particles with both sp3 and sp2
bonded structures.1a Detonation synthesis of sp3 diamond
nanoparticles has been intensively studied and successfully
industrialized.1a,2 A diamond content as high as 80 wt% is
available following this procedure.2c The synthesis generally
employs high-energy explosives and operates at very high
loading densities (2–3 g cm23) to reach a detonation Chapman–
Jouguet (CJ) state with extremely high pressures and tem-
peratures, typically a few tens GPa and thousands degrees.1a,2
The reasons are that the diamond-to-graphite phase transforma-
tion often occurs in the normal range of pressure and that the
size reduction and the large difference between surface energies
of diamond and graphite forms lead to great changes in phase
diagrams of nano-sized carbon particles compared to that of
bulk carbon and significantly raise the pressure level of
graphite–diamond kinetic boundary line.3 Nano-sized diamond
nanoparticles are formed at a CJ point that is above the raised
graphite–diamond kinetic line, at which sp3 diamond structure
is thermodynamically stable.
On the other hand, the raised graphite–diamond kinetic
boundary line of the size-reduced carbon widens the graphite
phase zone, that is, sp2 graphite structures can be well formed
under mild detonation conditions. If proper conditions are
selected, it is possible for the detonation-resulting small carbon
species to assemble into carbon nanotubes (CNTs), which are
constructed by sp2 bonded pure carbon walls (multi-walled or
single-walled) and have drawn great attention in the scientific
community due to their unique properties and wide potential
applications.4 Here we report an efficient synthesis of sp2
carbon nanotubes by catalysis-assisted detonation of picric acid
(PA) at very low CJ pressure and temperature.
Elemental analyses using energy dispersive X-ray spectrom-
etry shows that the detonation product of PA–Co(Ac)2–paraffin
consists mainly of carbon and cobalt; very little oxygen and
nitrogen are also evident, possibly resulting from adsorbed N2
and H2O. X-Ray diffraction results reveal that carbon exhibits
an sp2 graphite structure, with a diffraction line at a d value of
3.342 Å (002), and that cobalt has a metallic cubic structure,
with diffraction lines at d values of 2.052 (111), 1.776 (200) and
1.258 Å (220). HNO3 and HClO4 treatment8 of the detonation
product was performed to remove cobalt and sp2 bonded
carbon, and showed no residue, suggesting there is no sp3
diamond carbon in the detonation product.
The detonation of PA (2 g) were performed in a sealed
stainless steal pressure vessel (10.8 ml), connected with a
pressure gauge, and induced by rapid heating (20 °C min21) to
310 °C. When the detonation occurs, about 40 MPa pressure
(shock wave) and 900 °C temperature are produced inside the
vessel. After the detonation, the vessel was cooled in air and
emptied of gaseous products, and then the solid products were
collected.
Initially, we attempted to synthesize CNTs by a detonation of
PA alone, but this failed. Very little solid carbon (about 5 wt%
of carbon in fed PA) was collected due to the high O+C atomic
ratio (7+6) and low hydrogen content (H+C = 1+2) in PA, most
of the produced small carbon species being oxidized to gaseous
CO. In the situation of diamond synthesis, in which detonation
occurs at high CJ pressure and temperature, and equilibrium
favours the reaction 2CO ? CO2 + C (solid).1a Under our
experimental conditions, however, CO is kinetically stable and
the O/C atomic ratio is still high (0.92) after complete oxidation
The introduction of paraffin enhances the synthesis effi-
ciency due to its high carbon content and generating additional
carbon building blocks for CNT assembling. A content of CNTs
as high as 80–90% in the detonation product of PA–Co(Ac)2–
paraffin can be obtained reproducibly, as observed by scanning
electron microscopy (SEM) and transmission electron micros-
copy (TEM) (Fig. 1).‡ The produced CNTs are multi-walled,
having outer diameters of 12–50 nm and lengths of 0.5–40 mm.
All tubes are separated from each other and show tangled
morphologies. Most of the tube walls are well crystallized and
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This journal is © The Royal Society of Chemistry 2002

microsecond time-scale), and a high gas pressure (tens of MPa)
is produced. Under such conditions, the instant density of
carbon species inside the reactor is very high, about 0.009 mol
cm23 carbon atoms, while the tube growth is very effective so
that 80–90% tube content is available. This means that the CNT
growth rate is extremely high, in accord with the theoretical
estimation: a multi-wall CNT with 5 nm diameter and 1000 mm
length grows in 1023–1024s.10 On the other hand, the
fluctuating system temperature and the high density of carbon
species are perhaps responsible for the formation of the
structural defects. The introduction of paraffin increases the
number of defects, especially for high paraffin–PA ratios.
Our synthesis of CNTs by a detonation process is certainly
not the first. Kroke et al.11 recently observed CNTs being
formed after a detonation of 2,4,6-triazido-s-triazine (C3N12),
although they employed a specially prepared explosive with
very low carbon content and operated at a high CJ pressure
(1.63 GPa), which resulted in a very low content of CNTs of
only 2%.
In our approach, a very common CHNO explosive, PA, is
employed, a catalyst and additional hydrocarbon are introduced
into the detonation system, and the detonation occurs at very
low CJ density and pressure. Such a detonation process is
chemically much different from that for pure C/N explosives
and undoubtedly facilitates practical operation. On the other
hand, compared to the other processes for CNT syntheses, our
process is characterized by high-density and high-pressure
conditions, which experimentally shows that CNTs can grow in
such an environment and provides an alternative process for
CNT synthesis and a new route for theoretical studies on tubule
growth, especially in high density environments and in the
presence of metal catalysts.
Fig. 1 SEM (A) and TEM (B) images of a typical product from the
detonation of a PA–Co(Ac)2–paraffin mixture, showing that tubular
structures are dominant. Inset of B shows one tubule capped by a cobalt
particle.
Notes and references
† The employed paraffin has a boiling point of 300 °C and carbon and
hydrogen content of 82 wt% and 18 wt%, respectively.
‡ SEM and TEM analyses were carried out at a scanning electron
microscope (Philips XL30-FEG) and transmission electron microscope
(Philips CM200-FEG), respectively.
constructed by several to tens graphite layers parallel to the tube
axis, with an interlayer spacing of 0.34 nm as revealed by high-
resolution TEM (Fig. 2), but some of them have obvious defects
in the graphite networks. Cobalt nanoparticles are frequently
located at the ends of the CNTs, with sizes close to the diameters
of the tubes (inset of Fig. 1B), suggesting that the CNT growth
mechanism in the present process is similar to that in the
catalytic decomposition of hydrocarbons,9 in which the tubules
grow by the extrusion of carbon, dissolved in a metallic catalyst
particle that is over-saturated in carbon at one part of the
surface.
However, there are large differences between the present
detonation process and the traditional catalytic decomposition
of hydrocarbons or CO. In our process, the system temperature
is generated by the exothermic decomposition of explosive
following a self-heating process and drops quickly after the
detonation.§ Additionally, all carbon species for CNT assem-
bling are supplied in an extremely short period (on a
§ During experiments, we stop heating while the detonation occurs and
place the reactor in air for cooling.
1 (a) N. R. Greiner, D. S. Phillips, J. D. Johnson and F. Volk, Nature,
1988, 333, 440; (b) S. Eidelmann and A. Altshuler, Nanostruct. Mater.,
1993, 3, 31; (c) A. C. Frank, F. Stowasser, H. Sussek, H. Pritzkow, C.
R. Miskys, O. Ambacher, M. Giersig and R. A. Fischer, J. Am. Chem.
Soc., 1998, 120, 3512.
2 (a) T. Xu, K. Xu and J. Z. Zhao, Mater. Sci. Eng. B, 1996, 38, L1; (b)
V. L. Kuznetsov, Carbon, 1994, 32, 873; (c) V. M. Titov, V. F.
Anisichkin and I. Y. Mal’kov, Combust. Explos. Shock Waves, 1989, 25,
373.
3 (a) F. H. Ree, N. W. Winter, J. N. Glosli and J. A. Viecelli, Physica B,
1999, 265, 223; (b) J. A. Viecelli and F. H. Ree, J. Appl. Phys. B, 2000,
88, 683.
4 (a) S. Iijima, Nature, 1991, 354, 56; (b) P. M. Ajayan, Chem. Rev., 1999,
99, 1787; (c) M. S. Dresselhaus, G. Dresselhaus and P. C. Eklund,
Science of Fullerenes and Carbon Nanotubes, Academic Press, New
York, 1996.
5 (a) T. W. Ebbesen and P. M. Ajayan, Nature, 1992, 358, 220; (b) A.
Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y. H. Lee,
S. G. Kim, A. G. Rinzler, D. T. Colbert, G. E. Scuseria, D. Tomanek, J.
E. Fisher and R. E. Smalley, Science, 1996, 273, 483.
6 P. Nikolaev, M. J. Bronikowski, R. K. Bradley, F. Rohmund, D. T.
Colbert, K. A. Smith and R. E. Smalley, Chem. Phys. Lett., 1999, 313,
91.
7 (a) J. Kong, H. Soh, A Cassell, C. F. Quate and H. Dai, Nature, 1998,
395, 878; (b) W. Z. Li, S. S. Xie, L. X. Qian, B. H. Chang, B. S. Zou,
W. Y. Zhou, R. A. Zhao and G. Wang, Science, 1996, 274, 1701.
8 R. S. Lewis, T. Ming, J. F. Wacker, E. Anders and E. Steel, Nature,
1987, 326, 160.
9 (a) S. Amelinckx, X. B. Zhang, D. Bernaerts, X. F. Zhang, V. Ivanov
and J. B. Nagy, Science, 1994, 265, 635; (b) J. C. Charlier and S. Iijima,
Top. Appl. Phys., 2001, 80, 55.
10 E. G. Gamaly and T. W. Ebbesen, Phys. Rev. B, 1995, 52, 2083.
11 E. Kroke, M. Schwarz, V. Buschmann, G. Miehe, H. Fuess and R.
Riedel, Adv. Mater., 1999, 11, 158.
Fig. 2 High-resolution TEM image showing that the tube walls are well
crystallized.
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