Effects of the size of nano-copper catalysts and reaction temperature on the morphology of carbon fibers

Article abstract of DOI:10.1016/j.materresbull.2007.03.022

In this study, carbon fibers with different morphologies, including coiled carbon nanofibers and straight carbon fibers, were obtained by the chemical vapor deposition using a Cu-catalytic pyrolysis of acetylene at 250 °C. The influences of nano-copper catalyst particle size and the reaction temperature on the morphology of carbon fibers were investigated. Under the same reaction condition, coiled carbon nanofibers generally were synthesized using nano-copper catalyst with smaller particles size, and bigger copper particles are apt to produce straight carbon fibers. With decreasing of reaction temperature to 200 °C, straight carbon fibers were obtained, instead of coiled carbon nanofibers at 250 °C. The product was characterized by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and X-ray powder diffraction (XRD).

Full text of DOI:10.1016/j.materresbull.2007.03.022

Materials Research Bulletin 43 (2008) 735–742
www.elsevier.com/locate/matresbu
Effects of the size of nano-copper catalysts and reaction
temperature on the morphology of carbon fibers
*
Qian Zhang, Liyan Yu, Zuolin Cui
Key Laboratory of Shandong Province for Nanostructured Materials, Qingdao University of Science & Technology,
Qingdao 266042, People’s Republic of China
Received 5 February 2007; received in revised form 15 March 2007; accepted 21 March 2007
Available online 24 March 2007
Abstract
In this study, carbon fibers with different morphologies, including coiled carbon nanofibers and straight carbon fibers,
were obtained by the chemical vapor deposition using a Cu-catalytic pyrolysis of acetylene at 250 8C. The influences of
nano-copper catalyst particle size and the reaction temperature on the morphology of carbon fibers were investigated. Under the
same reaction condition, coiled carbon nanofibers generally were synthesized using nano-copper catalyst with smaller particles
size, and bigger copper particles are apt to produce straight carbon fibers. With decreasing of reaction temperature to 200 8C,
straight carbon fibers were obtained, instead of coiled carbon nanofibers at 250 8C. The product was characterized by field
emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and X-ray powder diffraction
(XRD).
# 2007 Elsevier Ltd. All rights reserved.
Keywords: A. Amorphous materials; B. Vapor deposition; C. Electron microscopy
1. Introduction
The related material of nanotubes, which was discovered by Iijima [1], carbon fibers attracted the attention of many
scientists in the wide fields of science and nanotechnology as an important component in the realization of
nanotechnology due to their outstanding mechanical and electromagnetic properties, surface properties, etc. With the
appearance of various forms of carbon fibers such as whisker-like, branched, bi-directional, spiral, coiled, many kinds
of carbon fibers growth models and mechanisms were established and discussed [2–10].
In order to investigate further the growth mechanism of carbon fibers, many researchers studied the influence of
reaction conditions on the morphology of carbon fibers. Motojima and coworkers [11] reported that the yield of carbon
fibers decreased by 35–50% due to the substrate rotation and irregular carbon coils were obtained, while regular coils
were obtained without rotation. The influences of temperature and gas environment on the morphology of carbon
deposition were studied by McCaldin et al. [12], they suggested that the type of carbon deposited is affected by the
initial CVD deposition conditions, and changing the CVD environment during deposition only alter the rate of
* Corresponding author. Tel.: +86 532 84022869; fax: +86 532 84022869.
E-mail address: cuizl@public.qd.sd.cn (Z. Cui).
0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2007.03.022

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deposition but not the type of carbon deposited. The role of hydrogen in this system is so important that it can
encourage the growth of filamentous carbon.
Recently, nano-copper catalysts were used to catalytically synthesize carbon fiber and carbon nanotube
[13–16]. In this contribution, we have studied the effect of nano-copper catalyst particles size and reaction
temperature on the morphology of carbon fibers with the use of different catalysts and different reaction
temperatures.
2. Experimental
2.1. Catalysts preparation
In this paper, three catalysts were used for the preparation of carbon fibers, the preparation methods of catalysts as
follows.
2.1.1. Copper tartrate catalyst precursor (A)
Hundred milliliters of potassium sodium tartrate aqueous solution (0.0586 mol/l) was slowly added to 100 ml of
0.0586 mol/l copper dichloride aqueous solution under vigorous stirring, copper tartrate (light blue precipitate) was
formed quickly and then filtered and washed with anhydrous ethanol, dried for 1 h at 100 8C.
2.1.2. Nano-copper catalyst (B)
The preparation of nano-copper (B) was carried out with hydrogen-arc plasma method. This method was described
in detail in Ref. [17].
2.1.3. Nano-copper catalyst (C)
Firstly, 2 g sodium hypophosphite and 1.5 g gelatin were added to 60 ml distilled water, and prepared 100 ml of
0.625 mol/l cupric sulphate aqueous solution and 100 ml of 1.25 mol/l sodium hypophosphite aqueous solution,
respectively; Secondly, the three kinds of solutions were heated to 85 8C; Lastly, the cupric sulphate solution and the
sodium hypophosphite solution were slowly added to a container at the same rate under vigorous stirring, the
precipitate was filtered and washed with distilled water and anhydrous ethanol in turn.
2.2. Sample preparation
In the present work, a horizontal quartz tube (9 cm in diameter and 90 cm in length) was used as the reaction
tube. The catalysts, placed on a ceramic substrate, were located in the central part of the reaction tube, and the
carbon fibers were obtained by the catalytic decomposition of acetylene at 250 8C for 30 min, the pressure
within the reaction tube was atmospheric. At last, the reaction tube was cooled in a vacuum (5 Pa) to room
temperature.
In addition, we have successfully synthesized carbon fibers at 200 8C for 1 h using copper tartrate as a catalyst
precursor to investigate the effect of reaction temperature on the morphology of carbon fibers.
2.3. Sample characterization
FE-SEM micrographs were obtained with a cold field emission scanning electron microscope (FE-SEM, JEOL,
JSM-6700F), operating at 5 kVat a working distance of 8 mm; transmission electron microscopy (TEM) was carried
out on a JEOL JEM-2000EX at an accelerating voltage of 160 kV; X-ray powder diffraction (XRD) patterns were
˚
recorded on a Philips X’Pert MPD diffractometer with Cu Ka radiation (l = 1.5418 A).
3. Results and discussion
Fig. 1A shows a FE-SEM image of copper tartrate catalyst precursor (A), which has many parallel cracks in its
sides. The inset (scale bar: 100 nm) displays a TEM image of the thermal decomposed product of copper tartrate
catalyst precursor (A) obtained at the same temperature as acetylene polymerization reaction. From this TEM image,

Q. Zhang et al. / Materials Research Bulletin 43 (2008) 735–742
737
Fig. 1. FE-SEM photos of copper catalysts. (A) Copper tartrate precursor; (B) nano-copper; (C) nano-copper. The inset in (A) TEM photo of the
thermal decomposed product of copper tartrate precursor.

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we have found that the size of the decomposed product was ranged in 30–60 nm, smaller than that of nano-copper
catalyst (B) particles, which is or above 120 nm. Nano-copper catalyst (B) particles, as shown in Fig. 1B, have uniform
size, smooth surface, and exist as spheres or approximate spheres. Fig. 1C displays the morphology of nano-copper
catalyst (C), the size of particles ranging from 60 to 110 nm.
It is well known that metal catalysis is the key for the formation of carbon fibers, and the physical and chemical
characters of catalyst have an intimate relation with the morphologies of fibers.
Fig. 2. Coiled carbon nanofibers (A), straight carbon fibers (B), the mixture of coiled carbon nanofibers and straight carbon fibers (C).

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The synthesis reactions of carbon fibers were carried out using the above-mentioned three kinds of catalysts. It was
observed that coiled carbon nanofibers were obtained using copper tartrate catalyst precursor (A) as a catalyst
precursor, as shown in Fig. 2A. On the other hand, we have prepared straight carbon fibers over nano-copper catalyst
(B) particles. Interestingly, nano-copper catalyst (C) can catalytically synthesize coiled carbon nanofibers and straight
carbon fibers at the same time.
From a previous publication [18], we known that the shape of catalyst particles included in carbon fibers has an
influence on the morphologies of carbon fibers, but the extent of which is limited, that is, the shape of catalyst particles
is not determined factor to the morphology of carbon fibers [19].
In this study, we found that the morphology of carbon fibers is related to the size of catalyst particles, namely, the
smaller catalyst particles are apt to synthesize coiled carbon nanofibers and the bigger ones to straight carbon fibers.
Then, nano-copper catalyst (C), including smaller particles and bigger ones, can prepare coiled carbon nanofibers and
straight carbon fiber simultaneously.
In order to investigate the relation between the morphology of carbon fibers and the size of catalyst particles in more
detail, we have observed the morphologies of carbon fibers using nano-copper catalyst (B) as a catalyst after the
growth period 3 and 15 min, respectively. Fig. 3A shows a FE-SEM image of the carbon fibers prepared at 250 8C for
3 min, a mass of nano-copper catalyst (B) particles can be observed obviously, which was coated with a layer of
carbon. When increasing reaction time from 3 to 15 min, the typical carbon fibers, shown in Fig. 3B, were synthesized
Fig. 3. FE-SEM photos of carbon fiber synthesized for different reaction periods: (A) 3 min; (B) 15 min.

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at 250 8C, and the growth rate of carbon fibers is about 4.5 nm/s. An interesting phenomenon is that catalyst particles in
carbon nanofibers bundles are almost in the same plane (rectangular area in Fig. 3B), which indicates that the cluster of
catalyst particles were coated into a agglomeration during the initial growth period of carbon fibers; with the
continuing growth of carbon fibers, the inner stress among fibers, which generated from the different growth directions
of carbon fibers, became stronger and stronger until the catalytic particles were separated slowly, then typically
straight carbon fibers formed.
From the XRD patterns of carbon fibers, as shown in Fig. 4A and B, we know that both samples have the
same components, the sharp peaks and the disperse peak correspond to the nano-Cu catalyst and carbon fibers,
respectively. XRD analysis showed that there was a difference in the relative content of metal Cu and carbon in both
samples.
In addition, the straight carbon fibers were prepared successfully where the copper tartrate was used as a catalyst
precursor, and the reaction temperature and reaction time in this experiment are 200 8C and 1 h, respectively. The
morphologies of the prepared carbon fibers are shown in Fig. 5. The growth rate of carbon fibers is about 0.5 nm/s. One
possible explanation is that the growth rate of carbon fibers, as a result of the lower reaction temperature 200 8C, is so
slow that the agglomeration of catalyst particles were formed even before the start of the carbon fibers growth
effectively, and the agglomeration of catalyst particles behaves isotropy to the growth of carbon fibers, so straight
carbon fibers have been synthesized, not coiled carbon nanofibers.
Fig. 4. XRD patterns of carbon fibers for different reaction periods: (A) 3 min; (B) 15 min.

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Fig. 5. FE-SEM photos of straight carbon fibers prepared using copper tartrate as a catalyst precursor at 200 8C for 1 h.
4. Conclusion
In our work, we have successfully synthesized straight carbon fibers and coiled carbon nanofibers using nano-
copper as a catalyst. The influence of catalyst particles size on the morphology of carbon fibers has been investigated
and discussed, that is, the smaller catalyst particles are apt to synthesis coiled carbon nanofibers, the bigger ones to
straight carbon fibers. Straight carbon fibers, not coiled carbon nanofibers, will be obtained at 200 8C using copper
tartrate as a catalyst precursor, lower than typical reaction temperature 250 8C.
Acknowledgment
The Natural Science Foundation of Shandong Province supported this work financially.
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