Yi Lu, Zhenping Zhu and Zhenyu Liu- Catalytic growth of carbon nanotubes through CHNO explosive detonation

8/3/2019 Yi Lu, Zhenping Zhu and Zhenyu Liu- Catalytic growth of carbon nanotubes through CHNO explosive detonation

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Catalytic growth of carbon nanotubes throughCHNO explosive detonation

Yi Lu, Zhenping Zhu *, Zhenyu Liu

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China

Received 7 April 2003; accepted 4 November 2003

Abstract

Multi-walled carbon nanotubes (CNTs) have been efficiently synthesized by a self-heating detonation process, operated at low

loading densities of picric acid (PA), which acts as the explosive to provide needed high temperatures and parts of carbon sources.

Paraffin or benzene provides additional carbon source for tube assembling and hydrogen source to capture oxygen in PA to form

H2O and thus to survive some carbon species from oxidation. Cobalt nanoparticles, in situ formed from a detonation-assisted

decomposition and reduction of cobalt acetate, show good catalytic activity for nanotube nucleation and growth and for dispro-

portionation reaction of CO generated from the PA detonation. The nanotubes and catalyst particles are characterized by SEM,

TEM, EDX, SAED, XRD, and Raman spectroscopy techniques. Some tubes are well crystallized but others have lots of structural

defects, especially for the tubes with thin walls and bamboo-like shapes. The catalyst particles show conical shapes and exhibit a fcc

crystalline structure of parent cobalt. These data also experimentally show that tube growth is at a very high rate and suggest that it

is possible for a large-scale synthesis of CNTs under high-density and high-pressure conditions.

2003 Elsevier Ltd. All rights reserved.

Keywords: A. Carbon nanotubes; B. Catalyst; C. Electron microscopy; Raman spectroscopy; X-ray diffraction

1. Introduction

Carbon nanotubes (CNTs) can be thought as

graphite sheets built from the hexagonal lattice of sp 2

bonded carbon wrapped into seamless cylinders. CNTs

are intriguing one-dimensional systems with nano-scale

diameters and micron-scale or greater lengths, and are

ideally suitable for fundamental studies of atomically

well-defined materials. Strong in-plane CC bond, un-

ique dimension, and hollow channel endow CNTs with

excellent electronic, mechanical, gas-storing and cata-

lytic properties. It has been suggested that CNTs can

find important applications in numerous fields, such as

enforced polymer composites, electron field emitters,

quantum nanowires, nano-scale gas store devices, cata-

lysts and catalyst supports [1,2].

Since the synthesis of CNTs by Iijima [3], many

methods have been developed and proven effective for

CNTs growth, such as arc-discharge of graphite elec-

trode [4,5], laser ablation of graphite [6], catalytic

decomposition of hydrocarbon [79], and catalytic dis-

proportionation of CO [10,11]. Although there are many

differences among these methods in technique as well as

in chemical and physical principles, one common

ground is that CNTs always grow under high-tempera-

ture conditions, from several hundreds to a few thou-

sands. At high temperatures, carbon precursors are

decomposed or evaporated and then condensed to build

the sp2 graphite networks of CNTs. The needed high

temperatures are normally obtained from external

heating, which is highly energy-consumed. Theoreti-

cally, such a problem can be solved by employing hugely

exothermic reaction systems. The detonation of explo-

sives represents such a reaction system. During the

detonation, an enormous amount of heat is released by

transformation of chemical energy involved in explosive

molecules to thermal energy, which results in very high

temperatures. In addition, most of explosives are car-

bon-rich organic compounds, which can fully or par-

tially provide carbon building blocks for the CNTs

construction. Therefore, the detonation of carbon-rich

explosives can be designed to provide a self-heating

method for CNTs synthesis.

* Corresponding author. Tel.: +86-351-404-8310; fax: +86-351-404-

1153.

E-mail address: [email protected] (Z. Zhu).

0008-6223/$ - see front matter 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.carbon.2003.11.001

Carbon 42 (2004) 361370

www.elsevier.com/locate/carbon
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It is well known that the detonation of nitro-hydro-

carbon (CHNO) explosives can produce nano-sized pure

carbon particles with both sp3 and sp2 bonded structures

[12]. Detonation synthesis of sp3 diamond nanoparticles

has been studied extensively and industrialized success-

fully [1216]. The synthesis generally employs high-en-

ergy explosives and operates at very high loadingdensities (23 g/cm3) to reach a ChapmanJouguet (CJ)

state 1 with extremely high pressures and temperatures,

typically a few tens GPa and thousands degree [1216].

In theory [17,18], there is a large difference between the

surface energies of bulk graphite (40 Kcal/mol) and

diamond (70 Kcal/mol) phases. A size reduction of

carbon particles to nano-scale results in a further

development of the difference in surface energy and

enlarges the differences of graphite and diamond phases

in surface tension and in Gibbs free energy and thus

significantly raise the pressure level of the graphite

diamond boundary line in phase diagram compared

with that of bulk carbon. Diamond nanoparticles are

formed at a CJ point that is above the raised graphite

diamond boundary line, at which sp3 diamond structure

is thermodynamically stable. On the other hand, the

raised graphitediamond boundary line of the size-re-

duced 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-resulted small carbon

species to assemble into CNTs.

Following this idea, we recently synthesized CNTs

successfully using the detonation of explosives under

much mild conditions [19,20]. Before our reports,tubular carbon products have also been observed in the

soot from the detonation of special explosives,

1,2:5,6:11,12:15,16-tetrabenzo-3,7,9,13,17,19-hexadehy-

dro annulene [21] and 2,4,6-triazido-s-triazine [22], with

2% CNTs in the products [22]. We employed some

common CHNO explosive such as m-dinitrobenzene

(DNB) [19] and picric acid (PA) [20], introduced cata-

lysts into the detonation system and operated at very

low pressure and temperature, typically a few tens MPa

and several hundreds degree. Under such conditions,

CNTs can well grow, with 3040% and 8090% contents

for the cases of DNB and PA, respectively. Transition

metal catalysts such as Fe, Co and Ni were found to be

indispensable for the CNTs growth under the mild

detonation conditions employed, and the activity of Co

is much higher than Ni and Fe. Moreover, the intro-

duction of hydrocarbons such as paraffin into PACo

system further increases CNTs yield.

The present paper focuses the attention on some re-

cent experimental results for PACo detonation system

and provides outlined descriptions on the chemistry of

the catalysis-assisted detonation process, the relation-

ship between the experimental conditions and tubesgrowth, structure and morphologies as well as on the

formation, structure and roles of the catalyst particles.

2. Experimental

All of the chemical reagents used, including PA,

benzene, and cobalt acetate (Co(AC)2 4H2O) were of

analytical grade without further pre-treatment. The li-

quid paraffin employed has a boiling point of 300 C,

and carbon and hydrogen contents of 82 wt% and 18

wt%, respectively. Before the detonation experiments,

the starting materials were mixed mechanically in de-

sired ratios.

The detonation was performed in a sealed stainless

steel pressure vessel (10.8 cm3) connected with a pressure

gauge, and induced by external heating (20 C/min) to

310 C (ignition temperature of PA). The experiments

conditions are summarized in Table 1. When the deto-

nation occurs, pressure and temperature peaks of 2040

MPa and several hundreds to one thousand degree were

generated inside the vessel, varied with the loading

density of PA (the amount of PA loaded in unit volume

of reactor, g/cm3). After the detonation, the vessel was

cooled in air to ambient temperature. The gaseousproducts were vented and the solid products were col-

lected for further characterizations. In some cases, the

gaseous products were also collected and analyzed by

gas chromatography.

The as-synthesized samples were examined using a

Hitachi H-600 transmission electron microscopy (TEM)

operated at 75 kV. High-resolution TEM imaging were

performed at a Philips CM 200FEG high resolution

TEM (HRTEM) operated at 200 kV, which is equipped

with energy dispersive X-ray spectrometer (EDX) for

elemental analysis. Scanning electron microscopy

(SEM) was carried out on a Philips XL 30-FEG oper-

ated at 30 kV. For TEM analyses, the samples were

prepared by sonicating in ethanol for 10 min, followed

by depositing one drop of the resulted suspension on

thin carbon film supported on holey copper grid. X-ray

diffraction (XRD) patterns of the products were re-

corded using a Rigaku D/max 2500X X-ray diffrac-

tometer operated at a step size of 0.02 with Cu Ka

radiation (k 154:178 pm) and a Ni filter at 40 kV and100 mA anode current. Diffraction profiles were ob-

tained by the usual h2h scan. Raman scattering was

conducted using Renishaw Micro-Raman 2000 equip-

ped with a 20 objective to give an illuminated spot.

1 CJ theory assumes that thermodynamic equilibrium of the

detonation products is reached instantaneously and that all products

are consumed completely. In the CJ state, the shock velocity of the

detonation wave equals to the sound speed behind the detonation

wave. CJ pressure is an intrinsic property of an explosive and is an

indicator of explosive performance.

362 Y. Lu et al. / Carbon 42 (2004) 361370


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The 632.8 nm line of heliumneon laser is used as

excitation source. The CNTs content is estimated as the

volume ratio of CNTs to the whole carbonaceous

products based on systematic TEM and SEM investi-

gations (random sampling and repeated observations).

3. Results and discussion

3.1. Detonation reaction systems and CNTs growth

The detonation of PA alone (sample 1) results in very

little solid carbon, about 5 wt% of carbon in fed PA. The

obtained solid product shows no tubular structure, and

contains only spherical carbon nanoparticles with sizes

of 1070 nm (Fig. 1a). Its XRD pattern (Fig. 1b)

exhibits single weak and broad diffraction peak, cen-

tered at 24.5 with interplanar distances of 0.360.37

nm, indicating that the sample is highly disordered

amorphous carbon. Analyses of the gaseous products

show that CO is dominant, followed by N2. Minority of

the gases are CO2, CH4 and H2O (noted that H2O

should be at high concentration but, during the cooling,

it heavily condenses on reactor wall). NH3 is also found

but with very low concentration. Therefore, the low

solid carbon yield is clearly associated with the high O/C

atomic ratio (7:6) and low hydrogen content (H/C 1:2)in PA molecule. These make most of the produced small

carbon species to be oxidized into gaseous CO because

the O/C atomic ratio is still high (0.92) after complete

oxidation of all H atoms into H2O. In the situation of

diamond synthesis, the detonation occurs at highFig. 1. TEM image (a) and XRD pattern (b) of the sample prepared

from the detonation of PA alone (sample 1).

Table 1

Experimental parameters and the related data about the detonation products

Sample Loading density

of PAa (g/cm3)

Co(AC)2/PA mass

ratio

Paraffin/PA mass

ratio

Benzene/PA mass

ratio

Solid carbon

yieldb (%)

CNTs contentc

(%)

1 0.2 0 0 0 5.12 0

2 0.2 1/5 0 0 11.18 40

3 0.2 1/10 0 0 9.17 50

4 0.2 1/20 0 0 8.58 905 0.2 1/30 0 0 5.44 80

6 0.2 1/20 1/2 0 50.25 40

7 0.2 1/20 1/4 0 33.31 80

8 0.2 1/20 1/6 0 21.38 80

9 0.2 1/20 1/8 0 16.67 90

10 0.2 1/20 0 1/2 40.21 30

11 0.2 1/20 0 1/4 26.60 50

12 0.2 1/20 0 1/6 18.78 80

13 0.2 1/20 0 1/8 14.78 90

14 0.1 1/20 1/4 0 26.0 50

15 0.05 1/20 1/4 0 20.6 20

16 0.02 1/20 1/4 0 14.4 5

a The amount of PA loaded in unit volume of the reactor. For the calculation, the total volume of the reactor was roughly taken as 10 cm3.b

Calculated as follows: Carbon yield M

P M

Co=M

C, whereM

P is the total amount of the detonation product;M

Co is the amount of metalcobalt involved in the product and is nearly equal to the amount of metal cobalt in starting material; MC is the amount of carbon involved in the

starting materials.c Estimated from the TEM and SEM observations.

Y. Lu et al. / Carbon 42 (2004) 361370 363


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pressure and temperature, and thermodynamics favors

the reaction of 2COfiCO2 + C (solid) [12]. Under the

experimental conditions used in this work, however, CO

is kinetically stable. The disordered spherical carbon

structures are likely associated with the relatively low

temperatures (several hundreds degree) generated from

the mild detonation, at which the carbon species aredifficult to spontaneously develop into organized sp2

networks and tubular structures without inducement of

a catalyst.

To enhance the solid carbon yield and development

of tubular graphite structure, Co(AC)2 is mixed with PA

at Co(AC)2PA mass ratios of 1/51/30 before the det-

onation experiment. The introduction of Co(AC)2 sig-

nificantly enhances the solid carbon yield and CNTs

growth (Table 1). In the products obtained (samples 4

and 5), tubular structural carbon with contents up to

8090% is dominant. Correspondingly, the amount of

CO is obviously decreased in the gaseous detonation

products whereas the concentration of CO2 is increased.

These results indicate that cobalt nanoparticles, in situ

formed from a detonation-assisted decomposition and

reduction of cobalt acetate (see below), play important

catalytic roles in the disproportionation reaction of CO

and in the nucleation and growth of CNTs. Both the

direct PA decomposition and the subsequent CO dis-

proportionation reaction on the surface of cobalt par-

ticles provide carbon building blocks for CNTs

assembling.

Considering the consumption of part of carbon

atoms from PA into carbon oxides, carbon- and

hydrogen-rich hydrocarbons such as benzene and liquidparaffin are introduced into the PACo(AC)2 system,

with hydrocarbonPA mass ratio from 1/8 to 1/2. The

heat produced from the detonation quickly decomposes

the hydrocarbon (may be also catalyzed by the cobalt

particles [7]) into small carbon species and hydrogen

radicals. The carbon species directly participate in CNTs

assembling, and the hydrogen radicals capture oxygen

to form H2O, which effectively depresses the oxidation

of carbon species and enhances the solid carbon for-

mation. The addition of hydrocarbons greatly increases

the product yield (Table 1).

Finally, an explosive mixture can be formulated,

including explosive PA, additive hydrocarbon (paraffin

or benzene), and catalyst precursor Co(AC)2, from

which CNTs can be effectively synthesized. In this sys-

tem, the introduction of paraffin or benzene results in

similar products (see below) despite significant differ-

ences in their molecular structures and in hydrogen

contents. Three parameters mainly influence the CNTs

growth. One is Co(AC)2PA mass ratio, with an optimal

value of about 1/20 by mass. Higher ratio would result

in the formation of large-sized cobalt particles, which

show low catalytic activity for CNTs growth (see below

for a detailed description). In this case, the obtained

products contain less CNTs and the produced CNTs are

obviously short, thick and poorly crystallized. Another

factor is the hydrocarbonPA mass ratio. Low ratio

(less than or equal to 1/4) leads to low carbon yield but

does not impact the content of CNTs, and about 80%

tube content can be obtained (Table 1). Higher hydro-

carbonPA mass ratios bring high carbon yields butheavily decrease tube contents. The other factor is the

loading density of PA, which determines the system

temperature and thus is crucial for CNTs growth and

the subsequent annealing process. A systematic TEM

examination on CNTs synthesized at different PA

loading densities (from 0.02 to 0.2 g/cm3) shows that the

CNTs content in the obtained products nearly increases

linearly from 5% to 80% (Table 1). At a low loading

density, the obtained CNTs obviously have more

structural defects, although the defects are still evident

in the cases of 0.2 g/cm3 (see below). Based on this

information, further tailoring of the structural defects is

possible by raising the PA loading density to generate

higher temperatures.

3.2. Morphologies and basic aspects of CNTs

The detonation products are very voluminous, uni-

form, and fine black powders. Elemental analyses by

EDX show that the products mainly consist of carbon

and cobalt. In addition, very little amounts of oxygen

and nitrogen are also detectable, possibly from adsorp-

tion of H2O, N2 and/or other oxygen- and nitrogen-

containing gases. HNO3 and HClO4 treatments [23] ofthe detonation products were performed to remove co-

balt and sp2 bonded carbon. The result shows no resi-

due, suggesting that no sp3 diamond carbon is

produced.

Figs. 2 and 3 are typical SEM and TEM images of the

samples obtained from the detonations of PA

Co(AC)2paraffin and PACo(AC)2 benzene mixtures,

respectively. These images indicate that these two dif-

ferent mixtures result in similar detonation products.

CNTs are dominant in the products with about 7090%

in content. Carbon-encapsulated cobalt nanoparticles,

some short and thick carbon fibers, and amorphous

carbon particles are also evident in the samples.

The inner and outer diameters of the CNTs are in the

range of 825 and 1250 nm, peaked at about 20 and 35

nm, respectively. The lengths of the CNTs are up to 40

lm, peaked at about 20 lm. No single-walled tubules

are observed under the used preparation conditions. The

tube ends are normally closed on one side and capped by

catalyst particles on the other side. Some open ends are

also observed, which may be as grown naturally or

generated during the subsequent sample preparation for

TEM analyses. All tubes are separated from each other

and frequently show tangled morphologies.

364 Y. Lu et al. / Carbon 42 (2004) 361370


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The CNTs normally show cylindrical morphologies,

with hollow channel along the full length (Figs. 2b and

3b), but some bamboo-like tubes, constructed by many

jointed hollow compartments are also present in small

amount for the samples obtained under optimal condi-

tions. The bamboo-like tubes (Fig. 4) are primarily

found at high hydrocarbon/PA ratios (e.g. higher than 1/

3 by mass) or at low PA loading densities (e.g. less than

0.1 g/cm3), which seem mean that the formation of

bamboo-like tubes is associated with relatively high

carbon density and low temperature environments since

the decomposition of the massive hydrocarbon mole-

cules engenders considerable carbon species and con-

sumes large amount of the heat generated from the PA

detonation. This suggestion is supported by other

observations that the amounts of amorphous carbon

particles and thick carbon fibers with poor graphite

structures are visibly high under these conditions.

3.3. Structures of CNTs

Fig. 5 shows typical XRD patterns of the as-synthe-

sized samples obtained from the detonations of PA

Co(AC)2paraffin and PACo(AC)2 benzene mixtures.

Unlike the XRD pattern of the sample without catalyst

(Fig. 1b), both of the two patterns exhibit one strong

and narrow diffraction peak at about 26.2, which is

corresponding to an interlayer spacing of 0.34 nm

associated with the (0 0 2) planes of hexagonal graphite

structure, suggesting the formation of sp2 bonded

graphite crystals. However, the peak is rather asymme-

try and broader than that of the well-crystallized

graphite, indicating a relatively low stacking order (Lc)

and crystalline dimension (La) [24] in the tube struc-

tures. The other three diffraction peaks are generated by

the cobalt catalyst particles and will be described below

in detail.

Raman spectra taken from the samples obtained

from the two different mixtures are depicted in Fig. 6.

These two spectra are rather similar and exhibit mainly

two bands at 1328 cm1 (D-band) and 1593 cm1 (G-

band). The origin of D-band has been explained as

disorder-induced features due to the finite particle size

effect, lattice distortion [25], or amorphous carbon

background signals [26]. The G-band has been explained

as the stretching vibration mode of graphite crystals,

Fig. 2. Typical SEM (a) and TEM (b) images of the product obtained

from the detonation of PACo(AC)2paraffin (sample 7).

Fig. 3. Typical SEM (a) and TEM (b) images of the product obtained

from the detonation of PACo(AC)2benzene (sample 12).

Y. Lu et al. / Carbon 42 (2004) 361370 365


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which is also a characteristic of nanotubes [27]. It is well

known that the ratio (ID=IG) of the D-band intensity tothe G-band intensity is inversely proportional to La

value of graphite crystalline [25]. For our samples, ID is

higher than IG, indicating the faulty long-range ordered

crystalline and the existence of disordered carbon

structures, in agreement with the XRD results. The

disordered carbon structures are not only associated

with some defect-rich nanotubes but also with the

impurities including amorphous carbon particles, short

carbon fibers, and amorphous carbon layers on the

outer tube surfaces (see below). However, the position

and the well-defined shape of the D and G bands point

to a well-ordered graphene-based texture of the ob-

tained CNTs.

HRTEM analyses provide more clear and detailed

information for the CNTs wall structures. Fig. 7a is a

representative image of the walls of most tubes, which

reveals the multi-walled structure of the obtained CNTs.

The tube walls are organized by several to tens carbon

layers, which are clearly resolved and are parallel to the

tube axis. The distance between the planes is about 0.34

nm, which is consistent with XRD results and associated

with the (0 0 2) planes of well-crystallized graphite

structures. Selected-area EDX analyses confirm that

carbon is the exclusive element of the tube walls. No

nitrogen is incorporated into the graphite structures,

although the used PA contains lots of nitro groups,which are reduced to N2 and even NH3 during the

detonation.

Fig. 7a also reveals that the inner surfaces of the tubes

are clean, but the outer surfaces are covered with thin

amorphous carbon layers. Similar phenomena were also

observed frequently for the CNTs prepared from cata-

lytic decomposition of hydrocarbon and explained as

the rate of hydrocarbon feedstock pyrolysis over-

whelming the rate of tubes growth [11], in agreement

with the situation in our detonation process, in which

the generation of all the carbon species for tube con-

struction are extremely fast, on a microsecond time scale

[28]. In addition, the formation of the amorphous car-

bon layers is also possibly caused by the great fluctua-

tion in temperature, which is mainly derived from the

energy emitted from the exothermic decomposition of

explosive and drops continuously after the detonation

due to the lack of external heating (during the experi-

ments, the reactor was heated externally to 310 C to

induce the detonation, after which the heating was

stopped). Under such conditions, it is easy to imagine

that some un-reacted carbon species would deposit on

the already-formed tube surfaces as disordered struc-

tures at decreased temperatures. In the sense of appli-

Fig. 4. Bamboo-like tubes produced from the detonation of PA

Co(AC)2 paraffin mixture with a mass ratio of 2:0.11:1 and at a

loading density of 0.2 g/cm3

for PA (sample 6).

Fig. 5. XRD patterns of the detonation products prepared from (a)

PACo(AC)2 paraffin (sample 9) and (b) PACo(AC)2benzene

(sample 12).

Fig. 6. Raman shift spectra of the samples obtained from the deto-

nation of (a) PACo(AC)2paraffin (sample 9) and (b) PACo(AC)2

benzene (sample 12).

366 Y. Lu et al. / Carbon 42 (2004) 361370


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cation, the covered amorphous carbon layers are trou-

blesome for electronic purposes but may be helpful for

chemical surface modification and for catalysis purposes

since they are more reactive than graphite crystals.

The structure of the tube walls seems to have a rela-

tionship with their thickness. Well-crystallized structures

normally appear on the tubes with relatively thick walls,

for example, the tubule shown in Fig. 7a has a wall with

40 carbon layers. Fig. 7b shows a tubule with 7 carbon

layers, which appear wavy and discontinued, suggesting

that lots of structural defects are involved and may be

associated with the jumps in spacing at points where the

helix angle changes [29]. In addition, defects are also

frequently observed on the walls of the bamboo-like

tubes (Fig. 7c), especially at the joints of the compart-

ments. The greatly fluctuated temperature as mentioned

above and the high rate of carbon deposition (derived

from both the decreased temperature and the high

density of carbon species as shown below) are likely

responsible for the formation of the structural defects.

Such defects and the covering amorphous carbon layers

are believed to be tailorable by controlling the reactor

temperature to a proper level or by further annealing the

as-synthesized samples in inert atmosphere at high

temperatures [30].

3.4. Formation and structures of the catalyst particles

The presence of catalysts is necessary for the CNTs

growth in this method, at least under the experimental

conditions used. Cobalt shows a high catalytic activity.

In the detonation products, the cobalt catalyst exists as

nano-sized particles with various morphologies and lo-

cated at various positions (Fig. 8). Most of the cobalt

nanoparticles are located at the tube ends and show

conical shapes (Fig. 8a), or filled inside of spherical

onion-like carbons (Fig. 8c). Some particles are filled

inside compartments of bamboo-like tubes and located

near the joints of different compartments, with conical

appearances (Figs. 4 and 8b). Some particles are con-

tinuously filled inside of some tubules, forming as metal

wires (Fig. 8c).

In the XRD patterns (Fig. 5), three peaks at 44.3,

51.5 and 75.9 are indexed to the diffraction lines from

the (11 1), (20 0) and (22 0) planes of face-centered

cubic (fcc) cobalt, respectively. Signals for cobalt

Fig. 7. Typical HRTEM images of the normal tubes with thick (a) and thin (b) walls and of bamboo-like tubes (c). These images were taken from the

sample 7.

Y. Lu et al. / Carbon 42 (2004) 361370 367


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carbides or oxides are not detected. Selected-area elec-tron diffraction (SAED) of individual catalyst particles

as shown in the inset of Fig. 9 further confirms that the

catalyst nanoparticles have a fcc single crystal structure

of parent cobalt, independent on their morphologies and

the locations. From the TEM analyses (Figs. 8 and 9), it

has been also observed that nearly all the catalyst par-

ticles are covered with carbon layers (including the tube

walls), which coincides with the fact that treating the

samples with 1 mol/l HNO3 solution at ambient tem-

perature shows no cobalt dissolution.

In the formation of the nano-sized cobalt particles,

the PA detonation plays an important role. Despite theadopted precursor (Co(AC)2 4H2O) can decompose at

298 C before the ignition of PA at 310 C, however, the

decomposition is rather uncompleted because the tem-

perature increase from 298 C to 310 C only needs very

short time at the used high heating rate of 20 C/min. In

addition, the decomposition of cobalt acetate (microm-

eter-sized powders) alone results in the formation of

micrometer-sized cobalt oxide particles, as shown by theTEM and XRD analyses of the product obtained from a

2-h decomposition reaction of cobalt acetate at 310 C.

This result suggests that the nano-sized cobalt particles

in the detonation products are really formed in situ. The

heat and the carbon species produced from the deto-

nation promote the decomposition and the reduction of

cobalt acetate, respectively. The detonation wave likely

helps the size control.

The presences of the coating carbon layers around the

catalyst particles and the conical catalyst particles at the

tube ends suggest that the mechanism of tube growth in

this process is similar to that in the catalytic decompo-

sition of hydrocarbons. For the latter process, it has

been widely accepted [3137] that the tube growth fol-

lows a way in which carbon atoms adsorb, dissolve, and

diffuse into the catalyst particle interior to form a metal

carbon solid state solution. Tube growth occurs when

supersaturation leads to carbon precipitation into a

crystalline tubular form. According to the mechanism,

the sizes of the catalyst particles regulate diameters of

CNTs, which is supported by the fact that the sizes of

the catalyst particles located at the tube ends are nor-

mally close to the tube diameters (left of Fig. 8a).

However, the catalyst particles with sizes larger than

tube diameters are also observed frequently at the endsof some tubules, for example, the size of the metal

particle shown at the right in Fig. 8a is about two times

of the tube diameter. This phenomenon especially ap-

pears in the cases with high Co(AC)2PA mass ratios, at

which both the contents and lengths of CNTs are re-

duced concomitantly. These observations suggest that

during the tubules growth, the catalyst particles grow as

well, possibly due to a continuous sorption of additional

metal species from vapor phase. The catalyst particle

growth is likely responsible, at least partly (combined

with the decreased local temperature) for the termina-

tion of tube growth by lengthening diffusion path and

blocking the diffusion of carbon species in the Co par-

ticles (because the diffusion of carbon through the cat-

alyst particles limits the tube growth [35]), which results

in a complete encapsulation of catalyst particles with

carbon layers and deactivates the catalyst.

It is of importance to emphasize that the physical and

chemical environments generated from the detonation

for the CNTs growth are much different from those of

other processes, the latter are normally at a relatively

clean atmosphere, ambient pressure, and low density

of feedstock. In the present process, the detonation-

generated atmosphere is rather complex. Particularly, all

Fig. 8. Some typical TEM images of Co catalyst particles presented in

various morphologies and at various positions, (a) and (c) were taken

from sample 7, (b) from sample 6.

Fig. 9. HRTEM images of the carbon-covered Co particles (sample 7)

at tube end, the inset shows a SAED pattern of the core.

368 Y. Lu et al. / Carbon 42 (2004) 361370


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the carbon species for tube assembling are supplied on a

microsecond time-scale at a high gas pressure (tens

MPa) [28]. Under such conditions, the density of carbon

species just behind the detonation wave (at this time,

carbon deposition starts) is extremely high, typically

larger than 0.008 mol/cm3 for carbon atom, while the

tube growth is much effective so that 80% tube content isobtainable. This fact implies that the rate of the cata-

lyzed CNTs growth is considerably high, supporting the

theoretical estimation from Gamaly and Ebbesen: a

multi-wall CNT with 5 nm diameter and 1000 lm

length grows in 103104 s [38]. Therefore, the results

obtained in this work also suggest that CNTs can be

effectively synthesized under rather harsh condi-

tions with very high pressure and feedstock density,

which is of importance for production of CNTs in large-

scale.

4. Conclusion

CNTs have been efficiently synthesized by a self-

heating detonation process, using PAcobalt acetate

paraffin/benzene as the explosive mixture. The formed

CNTs show multi-walled and tangled morphologies,

with inner diameters of 825 nm, outer diameters of 12

50 nm and lengths up to 40 lm. Some of the tubes are

well crystallized but others have lots of structural de-

fects, especially for the tubes with thin walls and bam-

boo-like shapes. The nanotubes are normally capped by

catalyst particles at one end. The catalyst particles showconical shapes and exhibit a fcc crystalline structure of

parent cobalt. It is also found that the formed cobalt

nanoparticles have a catalytic role in the dispropor-

tionation reaction of CO generated from the PA deto-

nation.

In this detonation approach, three parameters,

Co(AC)2 PA ratio, hydrocarbonPA ratio, and the

loading density of PA, are of special importance in the

tube growth. The former two parameters exhibit optimal

values of about 1/20 (by mass) and 1/4 (by mass),

respectively, while, for the latter parameter, the best

results appear at 0.2 g/cm3 that is the highest value we

have used. Co(AC)2 PA ratio determines the amount

and the size of catalyst nanoparticles. HydrocarbonPA

ratio greatly influences product yield and also the CNTs

content and morphology. The loading density of PA,

which determines the system temperature, is crucial for

CNTs growth.

Although the samples obtained currently do not

contain single-walled nanotubes and fullerenes, it is

believed that their syntheses are possible by similar

processes through designing the reactor and explo-

sive mixture and/or adjusting the detonation condi-

tions.

Acknowledgements

Authors thank Natural Scientific Foundation of

China for partial financial support (no. 59872047), Mr.

Hermann Sauer at Fritz Haber Institute of MPG, Ger-

many, for his helps in some TEM analyses, and the

referees for their nice suggestions.

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Yi Lu, Zhenping Zhu and Zhenyu Liu- Catalytic growth of carbon nanotubes through CHNO explosive detonation