Coiling-chirality changes in carbon microcoils obtained by catalyzed pyrolysis of acetylene and its mechanism

Article abstract of DOI:10.1063/1.1516612

As can be seen in the double helix of DNA, the single helix of proteins, etc., the three-dimensional (3D) helical/spiral structure is a fundamental structure of living things, and affords them critical functionalities. Helically coiled carbon fibers, which usually take the peculiar form of either a microcoil or a helix or twisted form, referred to as carbon microcoils hereafter, are of great interest due to their novel functionality and various potential applications. They can potentially be used in electromagnetic absorbers and/or filters, 3D composites, smart tunable electrical devices, microsensors, chiral catalysts, etc.

Full text of DOI:10.1063/1.1516612

Coiling-chirality changes in carbon microcoils obtained by catalyzed pyrolysis of
acetylene and its mechanism
Shaoming Yang, Xiuqin Chen, and Seiji Motojima
Citation: Applied Physics Letters 81, 3567 (2002); doi: 10.1063/1.1516612
View online: http://dx.doi.org/10.1063/1.1516612
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APPLIED PHYSICS LETTERS
VOLUME 81, NUMBER 19
4 NOVEMBER 2002
Coiling-chirality changes in carbon microcoils obtained by catalyzed
pyrolysis of acetylene and its mechanism
Shaoming Yang
Department of Applied Chemistry, Faculty of Engineering, Gifu University, Gifu 501-1193, Japan
Xiuqin Chen
Department of Chemical Engineering, Huaqiao University, Quanzhou, Fujian 362011, China
Seiji Motojimaa)
Department of Applied Chemistry, Faculty of Engineering, Gifu University, Gifu 501-1193, Japan
͑Received 1 July 2002; accepted 3 September 2002͒
As can be seen in the double helix of DNA, the single helix of proteins, etc., the three-dimensional
͑3D͒ helical/spiral structure is a fundamental structure of living things, and affords them critical
functionalities. Helically coiled carbon fibers, which usually take the peculiar form of either a
microcoil or a helix or twisted form, referred to as ‘‘carbon microcoils hereafter,’’ are of great
interest due to their novel functionality and various potential applications. They can potentially be
used in electromagnetic absorbers and/or filters, 3D composites, smart tunable electrical devices,
microsensors, chiral catalysts, etc. © 2002 American Institute of Physics.
͓DOI: 10.1063/1.1516612͔
In 1956, Davis et al. reported the vapor growth of two
thin carbon fibers twisted together in the form of a rope.1
Since then, some researchers have reported observations of
spiral-form vapor grown carbon fibers. For example, Baker
et al. reported the morphology of a spring-like branched car-
bon filament obtained in a Sn/Fe catalysis system.2 Lee et al.
observed twisted carbon nanofibers among carbon nanofibers
grown on soda lime glass at 500 °C using thermal chemical
vapor deposition.3 Motojima et al. reported the preparation
of carbon microcoils ͑CMCs͒ with micron- to nanometer-
ordered coil diameters by metal-catalytic pyrolysis of acety-
lene containing a small amount of sulfur.4–9
The growth mechanism of the carbon microcoils is very
interesting. In 1970, Boehm proposed a bottom growth
mechanism for braided carbon filaments.10 Amelinckx et al.
proposed a formation mechanism of helix-shaped graphite
nanotubes based on a spatial velocity hodograph.11 Na-
kayama and co-workers proposed the difference effect of an
In and Fe bicatalyst.12 We proposed that the coiling driving
force of the carbon fiber grown by catalyzed chemical vapor
deposition ͑CVD͒ is the catalytic anisotropy of a catalyst
crystal face related to carbon deposition.13,14 However, the
reasons why two coiling patterns, right clockwise ͑R͒ and
left clockwise ͑L͒, formed have not been discussed, and the
coiling-chirality mechanism is still not known. The coiling
direction within a section of carbon coil is generally either
right clockwise or left clockwise. Up to now, no helical/
coiled carbon fibers with coiling chirality alternating be-
tween right clockwise and left clockwise within a coil sec-
tion have been reported. We have found that, by using a
catalyst, coiling chirality changing within a piece of coil
grown by catalytic pyrolysis of acetylene could take place.
In this study, twisting carbon microcoils with fiber diam-
eters of 0.3–1 ␮m and with changing coil chirality were first
prepared by metal-catalyzed pyrolysis of acetylene. The
preparation conditions, morphology, and microstructure of
the carbon microcoils were examined, and then the mecha-
nism of the changing coiling chirality was discussed.
A graphite or stainless steel ͑SUS 304͒ plate substrate
was placed in a horizontal quartz reaction tube ͑600 mm in
length and 40 mm in diameter͒. Fine powders of WS2 and
the SUS 304 stainless steel plate were used as the catalyst.
Commercial acetone-dissolved acetylene was used as the
carbon source. The reaction tube was heated to 700–800 °C
using external electrical heaters, then acetylene, hydrogen,
nitrogen, and thiophene gases were introduced into the reac-
tion tube at flow rates of 30 mL/min ͑sccm͒ and 20–120,
40, and 10–0.05 sccm, respectively. A high-resolution
ferroelectric-scanning electron microscope ͑FE-SEM͒ was
used to examine the morphologies of the as-grown carbon
coils.
Using a stainless steel plate as the catalyst as well as the
substrate, carbon microcoils with a coil diameter of several
microns and coil length of several millimeters were generally
obtained. The carbon microcoils generally have a double he-
lix form like DNA. The optimum reaction temperature, at
which the maximum coil yield was obtained, was 775–
785 °C. The optimum gas flow rates of acetylene and
thiophene were 60 and 0.15 sccm, respectively. Among the
common double helix carbon microcoils, a significant num-
ber of carbon coils of twisted single form were observed.
Representative twisted carbon coils are shown in Fig. 1. The
coil diameter of the twisted coils and the diameter of the
carbon fibers from which the carbon coils formed are 1 and
0.6 ␮m, respectively. It is very interesting to observe that the
twisting ͑coiling͒ direction of the coil changes from L to R
͑or R to L͒ at the point shown by the arrow. That is, the
coiling chirality changes during the growth. Figure 2 shows
the carbon coils with successively changing coiling direc-
tions from L to R ͑or R to L͒. Twisted carbon coils with
a͒
Electronic mail: motojima@apchem.gifu-u.ac.jp
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3568
Appl. Phys. Lett., Vol. 81, No. 19, 4 November 2002
Yang, Chen, and Motojima
FIG. 3. Twisted carbon coils.
FIG. 1. Twisted carbon coils with changed coiling chirality. Catalyst: Stain-
less steel ͑SUS 304͒. The arrow indicates the location of the change in
coiling chirality. ͑R͒ right-clockwise coiling; ͑L͒ left-clockwise coiling.
twisted single carbon coils with changing coiling chirality as
well as of general carbon microcoils of spring-like double
helical form was observed. Figure 5 shows the branched car-
bon coils obtained using the WS2 catalyst. The branched
carbon coils have different coiling directions from the origi-
nal coil as can be seen in Fig. 5͑c͒. Furthermore, both coils
of the original and branched coils also changed coiling
chirality during the growth ͓arrow in Fig. 5͑b͔͒. The growth
of twisted carbon coils with the changed coiling chirality was
more frequently observed than for that with the stainless
steel catalyst. The presence of the catalyst or impurity ele-
ments was not observed in the branched part or coiling-
chirality-changed part.
A well-formed catalyst grain was sometimes observed
on the tip of carbon coils.5,6 The catalyst grain is the exclu-
sive growing point of the carbon coils. Using a Ni catalyst,
the catalyst grain is a Ni3C single crystal with different crys-
tal faces, while a thin layer of Ni–C–S–O with an amor-
phous or solid solution phase is present on the surface of the
Ni3C crystal.5 Different crystal faces with different chemical
compositions have different catalytic activity for the growth
of carbon coils. It is considered that the chirality of the cata-
lytic activity of the catalyst grain strongly affects the chiral-
ity of the carbon coils grown and determines the coiling
direction, right-clockwise coiling or left-clockwise coiling.
That is, changing the coiling chirality may be caused by the
changing catalytic activity of the catalyst grain. Similar situ-
ations may reasonably be considered for the case using stain-
less steel or WS2 as the catalyst. Figure 6 shows a possible
changing coiling chirality were also obtained using a Fe–
Cr–Ni alloy with a small amount of other elements. Figure 3
shows that the carbon coil coiling chirality changed in the
brighter part, i.e., the bent part shown by the arrow. Small
amounts of Fe, O, and S as well as C were detected in this
brighter part by electron probe microanalysis ͑EPMA͒,
whereas Ni and Cr were not detected at all. That is, this
brighter part is a Fe catalyst grain, and the coiling chirality
changed at the location of this catalyst grain. This catalyst
grain may be the growing point as in the case of the spring-
like carbon microcoils obtained using the Ni catalyst. A cata-
lyst grain with Fe3C phase was generally observed on the tip
of straight vapor grown carbon fibers ͑VGCFs͒ which were
obtained using Fe as the catalyst and benzene as the carbon
source at 1000–1100 °C. However, in this study, the crystal-
line phase of the brighter part has not yet been determined.
On the other hand, the presence of Fe or other catalysts or
impurity elements was not observed in the coiling-chirality-
changed part of Figs. 1 and 2. This suggests that there are
two coiling-chirality-changing mechanisms.
It was sometimes observed that a carbon fiber grew lin-
early and then twisted in the same direction to form twisted
carbon coils ͑or twisted carbon coils grew and then stopped
twisting or coiling to form a straight fiber͒ as shown in Fig.
4. The reasons why the straight fiber suddenly entwines and
forms coils or why the coils stops entwining and form
straight fibers are not known.
Using the WS2 powder as the catalyst, the growth of
FIG. 2. Twisted carbon coils with changed coiling chirality. Catalyst: Fe–
FIG. 4. Stopping the twisting ͑coiling͒ to form a straight fiber ͑or initiating
the coiling to form a twisted coil͒.
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Cr–Ni alloy.
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Appl. Phys. Lett., Vol. 81, No. 19, 4 November 2002
Yang, Chen, and Motojima
3569
FIG. 6. 2D model for changing the coiling chirality.
smaller than that of crystal face B, and thus the catalytic
activity of crystal face A decreases more than that of crystal
face B with an increase of reaction time. This is caused by
the fact that crystal face A has higher catalytic activity and
thus a higher carbon deposition rate than crystal face B, re-
sulting in a higher accompanying rate of sulfur and oxygen
in the carbon deposited, and thus a higher decreasing rate
compared with the supply speed. On the other hand, the sul-
fur and oxygen contents in crystal face B increase with an
increase of reaction time, thus increasing the catalytic activ-
ity. At this point, the coiling chirality may change. However,
other changing mechanisms of coiling chirality cannot be
ruled out, and much research is needed to clarify the mecha-
nism.
In conclusion, we prepared carbon microcoils with
changing coiling chirality using stainless steel or WS2 as the
catalyst, and a changing mechanism based on changing the
catalytic activity of the catalyst grain was proposed.
FIG. 5. ͑a͒ SEM images of branched twisted carbon microcoils, ͑b͒ enlarged
view of arrow A in ͑a͒, ͑c͒ branched part and changing coiling chirality.
Catalyst: WS2 . The arrow in ͑c͒ indicates the part changed by coiling chiral-
ity.
1 W. R. Davis, R. J. Slawson, and G. R. Rigby, Nature ͑London͒ 171, 756
͑1953͒.
2 R. T. K. Baker, P. S. Harris, and S. Terry, Nature ͑London͒ 253, 37 ͑1975͒.
3 C. J. Lee, T. J. Lee and J. Park, Chem. Phys. Lett. 340, 413 ͑2001͒.
4 S. Motojima, M. Kawakuchi, K. Nozaki, and H. Iwanaga, Carbon 29, 379
͑1991͒.
two-dimensional ͑2D͒ model for changing the coiling chiral-
ity of the twisted carbon coils obtained using the stainless
steel catalyst. In this 2D model, the following four hypoth-
eses are postulated: ͑1͒ the catalyst grain present on the
coiling-chirality-changed point is the Fe3C crystal phase, ͑2͒
on this surface the Fe–C–S–O amorphous or liquid crystal-
line phase is present, ͑3͒ the higher S and O contents in the
Fe–S–C–O phase, the higher the catalytic activity, and ͑4͒
crystal face B has higher catalytic activity than that of face
A, and thus the carbon fibers grown curl toward the left. It is
considered that a small amount of sulfur and oxygen is in-
cluded in the carbon coils grown as in the case of the Ni
catalyst.14 The S and O are supplied from source gases via
the Fe3C catalyst grain and thin Fe–C–S–O layer. It is con-
sidered that the S and O content in crystal face A become
5 S. Motojima, S. Asakura, M. Hirata, and H. Iwanaga, Mater. Sci. Eng., B
35, L9 ͑1995͒.
6 X. Chen, T. Saito, M. Kusunoki, and S. Motojima, J. Mater. Res. 14, 4329
͑1999͒.
7 X. Chen and S. Motojima, J. Mater. Sci. 34, 3581 ͑1999͒.
8 X. Chen and S. Motojima, J. Mater. Sci. 34, 5519 ͑1999͒.
9 S. Motojima, M. Hirata, and H. Iwanaga, J. Chem. Vap. Deposition 3, 87
͑1994͒.
10 H. P. Boehm, Carbon 11, 583 ͑1973͒.
11 S. Amelinckx, X. B. Zhang, D. Bernaerts, X. F. Zhang, V. Ivanov, and J.
B. Nagy, Nature ͑London͒ 265, 635 ͑1994͒.
12 M. Zhang, Y. Nakayama, and L. Pan, Jpn. J. Appl. Phys., Part 2 39, L1242
͑2001͒.
13 S. Motojima and X. Chen, J. Appl. Phys. 85, 3919 ͑1999͒.
14 C. Kuzuya, W.-I. Hwang, S. Hirako, Y. Hishikawa, and S. Motojima, Adv.
Mater., Chem. Vap. Deposition 8, 57 ͑2002͒.
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