Effect of synthesis method of nanocopper catalysts on the morphologies of carbon nanofibers prepared by catalytic decomposition of acetylene

Article abstract of DOI:10.1016/j.jcat.2004.02.004

Carbon fibers, prepared by metal-catalyzed decomposition of hydrocarbons, e.g., acetylene, benzene, and methane, present various growth models including whisker-like, branched, bi-directional, and multidirectional types of growth. The addition of additives or a second metal to the metal catalyst, the use of a support, the pretreatment of the catalyst, and the catalyst source may have considerable effects on carbon deposition. Carbon nanofibers were synthesized by the decomposition of acetylene with nanocopper catalysts to determine the effects of synthesis method on the morphologies of carbon nanofibers obtained. Copper nanoparticles catalyzed the carbon deposition of acetylene to yield regularly coiled fibers with a novel symmetric growth type, whereas ribbon-like fibers formed over nanocopper particles prepared by hydrogen-arc plasma method. The copper nanocrystals changed from irregular or spherical shape to faceted shapes after fiber growth. The regularity of the shapes of the faceted catalyst particles included in the fibers accounted for the different morphologies of the resulting products.

Full text of DOI:10.1016/j.jcat.2004.02.004

Journal of Catalysis 223 (2004) 389–394
www.elsevier.com/locate/jcat
Effect of synthesis method of nanocopper catalysts on the morphologies
of carbon nanofibers prepared by catalytic decomposition of acetylene
Yong Qin,a,b Qian Zhang,a and Zuolin Cui a,∗
a
Key Laboratory of Nanomaterials, Qingdao University of Science and Technology, Qingdao, 266042, People’s Republic of China
College of Environmental Science and Engineering, Ocean University of China, 266002, Qingdao, People’s Republic of China
b
Received 16 December 2003; revised 20 January 2004; accepted 7 February 2004
Abstract
Regularly coiled carbon nanofibers with a novel “V”-type symmetric growth mode were synthesized by the decomposition of acetylene
with nanocopper catalysts. These were obtained from an aqueous copper sulfate solution by borohydride reduction and from the decompo-
sition of copper tartrate, butyrate, oxalate, and lactate precursors, whereas copper nanoparticles prepared by the hydrogen-arc plasma only
revealed ribbon-like fibers under identical reaction conditions. The morphology of the catalyst particles before and after fiber growth has
been investigated and the effect of the synthesis method on the morphology and growth of the fibers is discussed.
2004 Elsevier Inc. All rights reserved.
Keywords: Carbon fibers; Nanocopper; Decomposition; Growth mode
1. Introduction
coiled forms in which two fibers intertwine with each other
as in a double helix of a DNA. In addition, variation of the
catalyst precursor and the presence of impurities have an ef-
fect in the synthesis of coiled carbon nanotubes [14,15]. It
was proposed that small metal particles could be prepared
in a particular orientation favorable toward carbon deposi-
tion by selecting the optimum catalyst precursor, and that
the pretreatment of metal in a particular environment would
result in the formation of oxide, nitride, or sulfide and so on,
being precursor for small metal particles [1].
In this paper, we describe the synthesis of carbon nanofi-
bers by the decomposition of acetylene with nanocopper
catalysts under mild reaction conditions. The effect of syn-
thesis method of the catalyst on the morphologies of carbon
nanofibers obtained was examined.
Carbon fibers, prepared by metal-catalyzed decompo-
sition of hydrocarbons such as acetylene, benzene, and
methane, present various growth modes, including whisker-
like, branched, bidirectional, and multidirectional types of
growth [1–3]. The addition of additives or a second metal
to the metal catalyst, the use of a support, the pretreat-
ment of the catalyst in a particular environment, and the
catalyst source may have considerable effects on the car-
bon deposition. Bernardo and co-workers observed whisker-
like growth from catalyst particles containing less than 80%
copper and multidirectional carbon deposits at higher cop-
per concentrations [4,5]. Bidirectional growth was reported
for several binary catalysts, for example, Fe–Sn and Fe–Ni
[6,7]. Multidirectional carbon deposits were also observed
in filaments formed on nickel catalysts of high sulfur con-
tent [8–10]. Motojima and co-workers prepared microcoiled
carbon fibers by metal powder-activated pyrolysis of acety-
lene containing a small amount of H2S or thiophene as an
impurity [11–13]. The effect of the metal catalyst source and
presulfidation of the metals on the growth of the coiled car-
bon fibers were examined in detail. These coils are double-
2. Methods
Nanocopper catalysts were prepared by these methods as
follows:
1. Copper tartrate catalyst precursor. To prepare the pre-
cursor, 100 ml of 0.02 M sodium hydroxide aqueous
solution was added to 100 ml of well-stirred 0.01 M tar-
taric acid aqueous solution. Then 100 ml copper dichlo-
*
Corresponding author.
E-mail address: cuizl@public.qd.sd.cn (Z. Cui).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2004.02.004
2004 Elsevier Inc. All rights reserved.

390
Y. Qin et al. / Journal of Catalysis 223 (2004) 389–394
ride aqueous solution (0.01 M) was slowly added to
the obtained sodium tartrate solution under vigorous
stirring. A light blue precipitate (copper tartrate) was
formed. The precipitate was filtered and washed with
anhydrous ethanol. Then it was dried for 1 h at 110 ◦C
and transferred to the reaction tube.
2. Copper butyrate, oxalate, and lactate catalyst precur-
sors. These were prepared in analogous ways.
3. Borohydride reduction of cupric ion. An amount of
100 ml of aqueous solution of copper sulfate (0.01 M)
was brought in a conical flask and purged with nitro-
gen for 10 min to remove the dissolved oxygen. Then
100 ml of 0.01 M ice-cold sodium borohydride aqueous
solution was added under stirring. Metallic copper parti-
cles formed immediately and were filtered, washed with
anhydrous ethanol, and then transferred to the reaction
tube.
4. Hydrogen-arc plasma method. This method was de-
scribed in detail in a previous publication [16].
The catalytic decomposition of acetylene was carried out
at atmospheric pressure in a horizontal quartz tube (9 cm
in diameter, 90 cm in length) which was heated from the
outside through an electric furnace. A ceramic plate con-
taining the catalysts was placed in the reactor. After the tube
was heated to 250 ◦C in a vacuum (for catalyst precursor,
holding at 250 ◦C for 30 min in order that all the precur-
sor particles decomposed to form metallic copper particles),
acetylene was introduced into the reactor for a selected reac-
tion period.
For transmission electron microscopy analysis (TEM),
the sample powders were dispersed in ethanol, agitated in
an ultrasonic bath, and finally deposited on a copper grid,
which was coated with a carbon film. TEM was carried
out on a JEOL JEM-2000EX operating at an accelerating
voltage of 160 kV. High-resolution transmission electron
micrographs (HRTEM) were obtained by imaging samples
on carbon mesh copper grids using a JEOL JEM 2010F
microscope operated at an accelerating voltage of 200 kV.
The carbon nanofiber specimens for field-emission scanning
electron microscopy (FE-SEM) were platinum-coated using
a JEOL JFC-1600 auto fine coater prior to imaging. Sam-
ples were examined on a JEOL JSM-6700F operating at
an accelerating voltage of 10 kV. Energy-dispersive X-ray
(EDX) analysis was performed on an Oxford Inca X-ray
energy detector. XRD patterns of the nanocopper catalysts
were obtained on a Philips X’Pert MPD diffractometer us-
ing Kα radiation.
Fig. 1. TEM image of the nanocopper particles, (A) obtained from the ther-
mal decomposition of copper tartrate at 250 C, and (B) prepared by the
hydrogen-arc plasma method. (C) XRD pattern of copper nanoparticles.
◦
mogravimetric (TG) analysis revealed that copper tartratre,
butyrate, oxalate, and lactate precursors all start to decom-
pose at a temperature of about 250 ◦C. According to TEM
results, copper nanoparticles prepared from aqueous copper
sulfate solution by borohydride reduction, and from decom-
position of copper butyrate, oxalate, and lactate precursors
were all identical to copper tartrate-derived particles. How-
ever, the nanocopper particles prepared by the hydrogen-arc
plasma method have a spherical or ellipsoidal shape and a
grain size ranging from 50 to 100 nm (Fig. 1B). These parti-
cles exhibit a chain-like shape due to the interaction between
the particles. Powder XRD analysis was used to examine the
crystal structure of the samples. The copper nanoparticles
obtained by all the above methods were crystalline. As an ex-
ample, the XRD pattern of the copper nanoparticles derived
from copper tartrate decomposition is shown in Fig. 1C. The
diffraction pattern shows the standard copper reflections, in-
dicating that the sample is highly crystalline. No peaks due
to impurities were found.
The experiments showed that the copper nanocrystals,
prepared by all the above methods, possess good catalytic
activity for the growth of carbon nanofibers under selected
reaction conditions, enabling high fiber yields to be ob-
tained. There was no obvious difference in fiber yields when
copper nanocrystals of different source were used as a cata-
lyst. In general, the fiber yield of a gram of copper nanocrys-
tals or an equimolar amount of precursor was about 75 g for
a 2-h reaction period at a reaction temperature of 250 ◦C.
3. Results and discussion
TEM images showed that the copper particles, obtained
from the thermal decomposition of copper tartrate precur-
sor, had a rather irregular shape and a size of 30–80 nm (see
Fig. 1A). Differential scanning calorimetry (DSC) and ther-

Y. Qin et al. / Journal of Catalysis 223 (2004) 389–394
391
contain hydrogen. This indicates that the growth of the fibers
occurs by both decomposition and polymerization of acety-
lene.
Fig. 3 shows the TEM and FE-SEM images of coiled
nanofibers obtained using copper tartrate precursor as a cat-
alyst for a reaction period of 3 min. Fig. 3A shows that two
regularly coiled fibers have grown over the rhomb-shaped
copper nanocrystal, which is about 50 nm in size. The fibers
have identical coil number, coil length, coil pitch, coil di-
ameter, and fiber diameter. Most important of all, it must
be noted that the helical senses of the two coiled fibers are
different, as one fiber coils left-handed and the other right-
handed. The fiber diameter is approximately equal to the
grain size of the copper nanocrystal, and the coil diame-
ter is about twice the size of the particle. Generally, two
coiled fibers grown from a single particle have opposite heli-
cal sense, resulting in the occurrence of helical reversals. As
shown in Fig. 3B, the two fibers both have 42 beautiful coils
and change their helical senses at the 21st coil (numbered
from the catalyst particle). This induces that the two fibers
coil always in opposite helical senses. Therefore, we con-
clude that the two fibers follow a mirror-symmetric growth
mode.
The catalyst particles included in the fibers (located at
the nodes of the two fibers) always exhibit regularly faceted
shapes. In addition to rhombic particles (Fig. 3C), copper
nanocrystals of a variety of faceted shapes, such as triangu-
lar (Fig. 3D) and quadrangular(Fig. 3E), were also observed.
Selected area electron diffraction patterns indicated that they
were single crystalline. Because these particles exhibited ir-
regular shapes (Fig. 1A) before fiber growth, we conclude
that they changed from an irregular to a regular faceted shape
during fiber growth. Detailed TEM investigations revealed
that only two coiled fibers, with identical fiber diameter, coil
diameter, coil number, coil pitch, and different helical sense,
grow from a single copper nanocrystal, irrespective of the
shape it had originally. Thus, the faceted shape of the cat-
alyst particle is not the determining factor for the helical
morphology and symmetric growth mode of the fiber. Mul-
tidirectional (including tridirectional) growth has not been
observed in our work. Of course, the symmetrical relation-
ship between the twin nanohelices resulted in an identical
number of left- and right-handed coils. EDX analysis of cat-
alyst particles included in the coiled fibers gave no evidence
for impurities (in particular Ni or Co). Therefore, the copper
nanocrystals had high purity and were indeed responsible for
the growth of the fibers.
Fig. 2. Field-emission secondary electron microscopy (FE-SEM) images of
representative coiled carbon nanofibers prepared using copper tartrate as
a catalyst precursor. (A) After a growth period of 3 min, and (B) after a
growth period of 2 h.
FE-SEM investigations revealed that the copper nano-
crystals, obtained from the decomposition of copper tartrate
precursor, were extraordinarily effective for the growth of
regularly coiled carbon nanofibers. The coiled-fiber con-
tent in the resulting fibers was very high and reproducible.
The FE-SEM images of Figs. 2A and B show representa-
tive coiled nanofibers after a growth period of 3 min and
2 h (the residual acetylene was pumped off immediately
after reaction), respectively. The catalyst particles were in-
cluded in the coiled fibers and are clearly seen in Fig. 2A.
Almost all these fibers are regular coiled in the form of sin-
gle coils. They are tightly coiled, and hence have very short
coil pitches. These fibers are about 100 nm in coil diam-
eter. Every fiber exhibited constant coil diameter, coil pitch
and fiber diameter, indicating that the fiber growth was stable
and not sensitive to external influence. XRD analysis showed
that the coiled fibers prepared at 250 ◦C were amorphous
(not graphite-like). The C/H molar ratio of the fibers was
1.11/1 (by Flash EA 1112), demonstrating that the products
The FE-SEM images, given in Figs. 3F and G, show the
nodes of two coiled fibers at which catalyst particles located.
The two nodes were imaged in different directions, that is,
Fig. 3F presented the concave of one node and Fig. 3G the
convex of the other. We infer that the helical morphology of
the fibers results from the dispersion of the growth rate of
fibers over the catalyst particles. The magnitude of the dis-
persion could be embodied by and large via the FE-SEM
image of the node, from the curvature of the convex or con-

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Y. Qin et al. / Journal of Catalysis 223 (2004) 389–394
Fig. 3. (A and B) TEM images of two coiled fibers grown over single copper nanocrystals. (C–E) TEM image of copper nanocrystals located at the node of
two coiled fibers. (F and G) FE-SEM images of the node at which catalyst particles are located.
cave nodes. The considerable dispersion of growth rate over
the surface of catalyst particle induced a curly node, small
coil diameter, and tight coil pitch of the fibers [17].
In order to examine the effect of carboxyl anion of pre-
cursor on the growth forms of carbon nanofibers, copper
nanoparticles derived from the decomposition of copper bu-
tyrate, oxalate, and lactate precursor were also used as cat-
alysts to catalyze the carbon deposition. In brief, the results
revealed that the fibers obtained were coiled and had an iden-
tical symmetric growth mode. Further examination by FE-
SEM and TEM indicated that coiled nanofibers of symmetric
growth mode could also be produced with nanocopper cat-
alysts obtained from the borohydride reduction of copper
sulfate. Thus, we conclude that the form of the coiled mor-
phology and the symmetric growth mode were not induced
by carboxyl anions of precursor.
Fibers from nanocopper particles prepared by hydrogen-
arc plasmas are shown in Fig. 4. Comparing the FE-SEM
images of Fig. 4A and Fig. 2 shows that the morphologies
of the resulting fibers are different, as under identical reac-
tion conditions, only ribbon-like fibers with a width of about
100 nm were obtained. TEM investigations showed that after
fiber growth the catalyst particles were also faceted and had
a variety of shapes with a grain size of about 100 nm. Se-
lected area electron diffraction patterns indicated that they
were also single crystals. The majority of these particles
exhibited irregular shapes, as shown in Figs. 4B–D. In addi-
tion, very few particles seemed to have a regular projected
faceted shape, such as rhombic (Fig. 4E) and triangular
(Figs. 4F and G). Besides bidirectional growth, the tridirec-
tional growth form was usually found (Figs. 4D and G). Ob-
viously, the copper nanoparticles prepared by the hydrogen-
arc plasma method also changed from initial spherical or
ellipsoid to faceted shapes after fiber growth. However, their
Fig. 4. (A) FE-SEM image of ribbon-like carbon nanofibers obtained using
nanocopper catalysts prepared by the hydrogen-arc plasma method. TEM
images of copper nanocrystals included in the fibers with (B–D) irregular
shapes and (E–G) regular projected shapes.

Y. Qin et al. / Journal of Catalysis 223 (2004) 389–394
393
Fig. 5. Microstructure of a rhombic-projected copper nanocrystal located at the node of two coiled fibers. (A) TEM image. (B) Selected area electron diffraction
pattern. (C) HRTEM image. (D–G) Magnified images of those corresponding positions marked in (C). (The carbon fibers are amorphous because of the low
reaction temperature.)
Fig. 6. Microstructure of a rhombic-projected copper nanocrystal included in a straight fiber. (A) TEM image. (B) Selected area electron diffraction pattern.
(C) HRTEM image. (D–F) Magnified images of those corresponding positions marked in (C).
faceted shapes were generally irregular and the resulting
fibers had a ribbon-like shape.
6 show the microstructure of a rhombic-projected copper
nanocrystal located at the node of two coiled fibers, and
one included in a straight fiber, respectively. Obviously, the
two particles have identical rhombic shapes according to
TEM investigation (Figs. 5A and 6A). Also, they exhibit
nearly identical electron diffraction patterns (Figs. 5B and
6B). This reveals that the two copper nanocrystals exposed
their crystal faces in the same form, as (111) index faces.
Regularly faceted catalyst particles were suited for the
growth of regularly coiled fibers [17]. In order to clarify
the reason why very few copper nanocrystals prepared by
the hydrogen-arc plasma method seemed to have a regu-
larly faceted shape after fiber growth, although ribbon-like
fibers formed, HRTEM analysis was applied. Figs. 5 and

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Y. Qin et al. / Journal of Catalysis 223 (2004) 389–394
The HRTEM images (Figs. 5C–G and 6C–F) further con-
firm this. The HRTEM images indicate that the carbon fibers
were mainly extruded from the (111) planes. At the same
time, the HRTEM investigations revealed that the rhombic
particle located at the node of two coiled fibers had a uniform
thickness (Fig. 5C) and a relatively clean and structurally
perfect surface (Figs. 5D–G). The lattice fringes were vis-
ible throughout the whole particle (Fig. 5E). However, the
rhombic-projected particle included in the straight fibers had
a variable thickness (Fig. 6C). Its lattice fringes were visi-
ble only at the thicker part of the particle. It did not have a
clean and structurally perfect surface (Figs. 6D–F). Based
on these analyses we conclude that even if this rhombic-
projected particle included in the straight fibers had a regular
projected shapes, it did not possess a regular shape as to a
three-dimensional structure.
Many researchers described the deposition of hydrocar-
bon over metal catalysts and the possible growth mechanism
for the fibers. The metal particles were usually faceted after
the growth of carbon filament. Yang and Chen reported that
small particles of Ni, Fe, and Co were faceted single crys-
tals upon carbon filament growth by reaction with methane
and the faceting was caused by the surface reconstruction
due to precipitation of graphite. All particles, however, had
an irregular shape [3]. Motojima et al. reported the synthesis
of carbon microcoils by metal-catalyzed pyrolysis of acety-
lene and the effect of the source of metal powder catalyst on
the coil yield [11]. After the growth of the coiled fibers, the
catalyst particles were regular rhombic single crystals (mi-
crometer sized) and the fibers grown over them had a DNA-
like form. A three-dimensional growth mechanism was pro-
posed for the fibers based on the three-dimensional structure
of the rhombic catalyst particles [12]. Baker and Waite re-
ported that platinum–iron alloy particles underwent a rapid
change from an irregular to a more spheroidal form while
catalyzing the carbon deposition of acetylene [2]. In our
study, nanocopper catalysts, obtained by borohydride reduc-
tion of copper sulfate and by thermal decomposition of pre-
cursors including copper tartrate, butyrate, oxalate, and lac-
tate, all catalyzed the formation of coiled carbon nanofibers
from acetylene. These fibers exhibited a symmetric growth
mode. TEM investigations showed that the catalyst parti-
cles changed from an irregular to a regular faceted shape
after fiber growth. However, nanocopper particles prepared
by the hydrogen-arc plasma method could only catalyze the
growth of ribbon-like fibers. These particles showed irreg-
ular faceted shapes after fiber growth. Therefore, it could
be concluded that the helical structure and the symmetric
growth mode of coiled fibers synthesized in our study were
not induced by carboxyl anions of the precursor and the
grain size of copper nanoparticles. However, the original
shape of the catalyst particles seems to have a considerable
effect on the morphology of the fibers obtained. We infer that
the decomposition of precursors and borohydride reduction
of copper sulfate could produce a special form of metastable
copper nanocrystals, which would change from irregular to
regular faceted shapes in a way that favors the growth of
coiled fibers upon contacting the acetylene gas. In this case a
large number of regularly coiled fibers was obtained. Copper
nanoparticles prepared by the hydrogen-arc plasma method
also change shape during growth of fibers. However, these
particles changed to irregular shapes and were not suited for
the growth of regularly coiled fibers; only ribbon-like fibers
formed.
4. Conclusion
Nanocopper catalysts were prepared by the decompo-
sition of several copper carboxylate precursors, by boro-
hydride reduction of copper sulfate, and by the hydrogen-
arc plasma method. These copper nanoparticles catalyze
the carbon deposition of acetylene to yield regularly coiled
fibers with a novel symmetric growth type, whereas ribbon-
like fibers formed over nanocopper particles prepared by
the hydrogen-arc plasma method. The copper nanocrystals
change from irregular or spherical shape to faceted shapes
after fiber growth. The regularity of the shapes of the faceted
catalyst particles included in the fibers accounts for the dif-
ferent morphologies of the resulting products.
Acknowledgment
The Natural Science Foundation of Shandong Province
supported this work financially.
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