High magnetization helical carbon nanofibers produced by nanoparticle catalysis

Article abstract of DOI:10.1021/jp060004b

Helical carbon nanofibers were synthesized by means of acetylene pyrolysis at 450°C using Fe nanoparticles as catalyst. There is no need to modify the Fe nanoparticles by a chiral reagent. The carbon nanofibers generated are crystalline and symmetric, with the (110) plane of Fe particle being the mirror plane. There would be a change in the shape of the nanofibers if the temperature at the beginning or during the synthesis were altered. Compared to the helical carbon nanofibers reported elsewhere in the literature, our samples show higher magnetization.

Full text of DOI:10.1021/jp060004b

11772
J. Phys. Chem. B 2006, 110, 11772-11774
High Magnetization Helical Carbon Nanofibers Produced by Nanoparticle Catalysis
†
,†
‡
†
Nujiang Tang, Wei Zhong,* Aharon Gedanken, and Youwei Du
National Laboratory of Solid State Microstructures and Jiangsu ProVincial Laboratory for NanoTechnology,
Nanjing UniVersity, Nanjing 210093, P. R. China, and Department of Chemistry and Kanbar Laboratory for
Nanomaterials at the Bar-Ilan UniVersity Center for AdVanced Materials and Nanotechnology, Bar-Ilan
UniVersity, Ramat-Gan 52900, Israel
ReceiVed: January 1, 2006; In Final Form: April 28, 2006
Helical carbon nanofibers were synthesized by means of acetylene pyrolysis at 450 °C using Fe nanoparticles
as catalyst. There is no need to modify the Fe nanoparticles by a chiral reagent. The carbon nanofibers generated
are crystalline and symmetric, with the (110) plane of Fe particle being the mirror plane. There would be a
change in the shape of the nanofibers if the temperature at the beginning or during the synthesis were altered.
Compared to the helical carbon nanofibers reported elsewhere in the literature, our samples show higher
magnetization.
Introduction
Experimental Section
To prepare the catalyst precursor, 0.01 mol FeCl2‚4H2O and
The investigation of chirality and chiral molecules is primarily
an attempt to understand the origin of life. The studies are mostly
conducted at molecular levels and related to chemical as well
as physical aspects of chirality. With the detection of nanosized
chiral structures, questions are raised as related to their
formation. Scientists have paid much attention to coiled carbon
0
6
.015 mol citric acid monohydrate were well mixed (stirring at
0 °C for 6 h) with 100 mL of absolute ethanol. The xerogel
obtained after ethanol removal (at 80 °C) was heated in air at
50 °C for 3 h for the generation of ferric oxide. Then 50 mg
4
of the ferric oxide powder was dispersed on a ceramic plate,
which was transferred to a horizontal reaction tube located inside
a tubular furnace. The whole system was equipped with proper
temperature and gas-flow control facilities. After the ferric oxide
was reduced in H2 at 450 °C for 4 h, acetylene was introduced
into the reaction tube and the pyrolysis of acetylene at 450 °C
was retained for 6 h under atmospheric pressure. Usually, about
1.582 g of the coiled nanofibers sample was produced in each
run.
The morphologies of the samples were examined by a
transmission electron microscope (TEM) and high-resolution
TEM (HRTEM, model JEOL-2010, Japan) operated at an
accelerating voltage of 120 kV and 200 kV, respectively, and
by field-emission scanning electron microscopy (FE-SEM model
fibers and tubes because of their novel chiral morphologies and
distinct physical properties.1-11 By the addition of a dilute gas
such as hydrogen, argon, thiophene, or H2S, coiled carbon fibers
were prepared in the catalytic decomposition of organic vapors
such as acetylene and benzene at high temperatures (>600 °C)
over transition metals or alloys (e.g., Ni and its alloys).
Furthermore, Qin et al.1
2-15
reported the synthesis of helical
polyacetylene nanofibers at 250 °C over copper nanoparticles
that had been modified with chiral tartaric acid and showed that
helical carbon nanofibers could be obtained by heating the
polyacetylene nanofibers at or above 900 °C. It was said that
the decomposition process occurred at relatively low temper-
atures, and the resulting nanofibers were amorphous. Herein,
we report the first time the synthesis of helical carbon nanofibers
1
530VP, LEO, Germany) operated at an accelerating voltage
of 15 kV and was equipped with a JEOL JFC-1600 auto fine-
coater. The magnetic properties of the samples were measured
at 300 K by a Quantum Design MPMS SQUID magnetometer
(referred hereafter as coiled nanofibers) of symmetric structures
by the pyrolysis of acetylene over Fe nanoparticles derived from
a combined sol-gel/reduction method. The pyrolysis reaction
conducted at a relatively low temperature of 450 °C is highly
reproducible. No dilute gas, such as argon, nitrogen, or sulfur
compounds, was needed. Synthesized at this temperature, the
coiled nanofibers are crystalline.
(
Quantum Design MPMS-XL, U.S.A.) equipped with a super-
conducting magnet capable of producing fields of up to 50 kOe.
FTIR spectra of samples (in KBr pellets) were recorded using
a Nicolet 510P spectrometer.
Fe catalysts have been widely used in the preparation of one-
Results and Discussion
16
dimensional carbon nanostructures such as carbon nanofibers,
Figure 1 shows the TEM image of two coiled nanofibers
attached to an iron nanoparticle. It is common to have two coiled
nanofibers grown on one catalyst nanoparticle. With one coiled
nanofiber on each side, the iron nanoparticle appears to be at
the node of the nanofibers. The diameter of the coiled nanofibers
is roughly the size of the Fe nanoparticle. A closer examination
reveals that the iron nanoparticle inside the nanofiber is oval in
shape and is about 40 nm in size, and the two “arms” of coiled
nanofibers are mirror-image to each other. They are identical
in cycle numbers and are almost equal in coil diameters, coil
lengths, and coil pitch; the direction of coiling, however, is
opposite.
carbon nanohorns,17 and three-dimensional microcoils. The
fibers obtained are generally straight, tubular, and/or micro-
coiled. To the best of our knowledge, the use of Fe nanoparticles
for the synthesis of coiled nanofibers of high symmetry without
the employment of a chiral reagent has never been reported
before.
18
*
Corresponding author. E-mail: tangnujiang@yahoo.com.cn. Tele-
phone: +86-25 83594588. Fax: +86-25 83595535.
†
National Laboratory of Solid State Microstructures and Jiangsu
Provincial Laboratory for NanoTechnology.
‡
Department of Chemistry and Kanbar Laboratory for Nanomaterials.
1
0.1021/jp060004b CCC: $33.50 © 2006 American Chemical Society
Published on Web 05/27/2006

High Magnetization Helical Carbon Nanofibers
J. Phys. Chem. B, Vol. 110, No. 24, 2006 11773
Figure 1. Typical TEM image of a pair of coiled nanofibers grown
on an iron nanoparticle.
Figure 4. Microstructure of a coiled nanofiber; (a) HRTEM image;
(b) selected area electron diffraction pattern; (c and d) magnified images
of the areas marked c and d in figure (a), respectively.
Figure 2. FE-SEM image of representative coiled nanofibers. (The
bar denotes 200 nm.)
Figure 5. IR spectrum of coiled nanofibers.
Figure 3. Microstructure of a catalyst nanoparticle located at the node
of coiled nanofibers; (a) TEM image; (c) HRTEM image; (b, d, and e)
magnified images of areas marked in figure (c), respectively.
Figure 2 shows the FE-SEM image of the coiled nanofibers.
Most of the nanofibers are regularly and tightly coiled and hence
show very short coil pitches. The diameter of the coils (i.e., the
outer diameter of the helix) is about 80 nm, whereas that of the
fibers is ca. 40 nm.
Figure 6. TEM images of coiled nanofibers; (a), (b), and (c) coiled
nanofibers identical in cycle number (7 cycles) but with interangles θ
of about 70°, 35°, and 130°, respectively; (d) coiled nanofibers with θ
of about 35° and a change in growth direction at the third cycle.
Figure 3 shows the microstructure of an Fe nanoparticle. The
HRTEM images (Figure 3b, d, and e) reveal that the (110) plane
of the single-crystalline Fe nanoparticle appears to be the mirror
plane of the two coiled nanofibers. Parts b and e of Figure 3
show the graphitic layers wrapping the Fe nanoparticle; the
interplanar distance of the graphitic layers is ca. 0.34 nm.
From Figure 4, one can see that the coiled nanofiber is not
amorphous but rather polycrystalline (Figure 4b), although at a
relatively low reaction temperature. The coiled nanofiber
develops along the direction of growth and is composed of
graphitic layers (Figure 4c and d) that are peripheral and
circulate around the axes of the fiber.The magnified HRTEM
images of the end and coiled sections show again an interplanar
distance of graphitic layers of ca. 0.34 nm.
and -CH3- entities. In other words, the coiled nanofibers are
completely carbonized and the method adopted in this study is
capable of producing coiled carbon nanofibers effectively.
Most of the pairs of coiled nanofibers show an interangle θ
of about 70° (Figure 6a), and there are minorities with θ equal
to 35° or 130° (Figure 6b, c, and d). It is interesting to observe
that no coiled nanofibers with θ other than these three have
been found. It is also interesting to observe that most of the
coiled nanofibers stopped at the seventh cycle and the coiling
direction is opposite within each pair set. It seems as if there
are some unknown factors that are in control. As demonstrated
in Figure 6b, the two coiled nanofibers changed simultaneously
in coiling direction at the sixth cycle. On the other hand, if one
Figure 5 shows the IR spectrum of as-prepared coiled
nanofibers. There are no IR signals of -CHdCH-, -CH2-,

11774 J. Phys. Chem. B, Vol. 110, No. 24, 2006
Tang et al.
Figure 9. TEM images of carbon products in acetylene pyrolysis at
7
00 °C.
The effect of decomposition temperature on the morphology
Figure 7. Typical magnetization curves of coiled nanofibers measured
at 300 K.
of carbon products was also investigated. If the pyrolysis of
acetylene was conducted at 700 °C (Figure 9), there would be
no coiled nanofibers but fibers with straight and/or irregular
shapes. The diameters of these carbons are irregular and fall in
the 100-200 nm range. The yield of carbon products was found
to be around 15.753 g in each run.
Conclusion
This study reveals that the Fe nanoparticles prepared by the
combined sol-gel/reduction method are effective for the growth
of coiled nanofibers. Hence, compared to the Fe catalysts
reported elsewhere, the Fe nanoparticles prepared in such a way
are unique and coiled nanofibers can be synthesized from
acetylene at low pyrolysis temperature. Compared to the coiled
nanofibers obtained over nonmagnetic transition metals, the
samples containing magnetic iron nanoparticles show higher
magnetization.
Figure 8. TEM images of carbon products obtained when acetylene
was introduced initially at room temperature.
fiber changed the direction of growth, the other changed also
at the same cycle number (Figure 6d).
The FE-SEM and TEM investigations reveal that the Fe
nanoparticles prepared by the combined sol-gel/reduction
method are effective for the growth of these coiled nanofibers.
The experiment is a highly reproducible product and product
selectivity is high. The results reported so far review the unique
catalytic property of the Fe nanoparticles prepared by this
method. The products of acetylene decomposition over the Fe
Acknowledgment. We thank Prof. C. T. Au (Department
of Chemistry, Hong Kong Baptist University) for valuable
suggestions. Y.W.D. and A.G. thank the Chinese and Israeli
Ministries of Science for a Binational Grant through the Sino-
Israeli Program in Materials Science. This work was supported
by the National Key Project for Basic Research (grant no.
2005CB623605) and the National Natural Science Foundation
of China (grant no. 50471049), China.
1
6
catalysts reported elsewhere are carbon nanofibers, carbon
1
7
18
nanohorns, carbon microcoils, and straight and/or tubular
carbon nanofibers of irregular helical form.
The magnetization dependence of coiled nanofibers on field
was measured at room temperature. Figure 7 shows the M-H
curves at an applied maximum field of 50 KOe. The saturation
magnetization is 4.99 emu/g, and the coercivity is 91.61 Oe.
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