Synthesis of SiC nanorods using floating catalyst

Article abstract of DOI:10.1016/S0038-1098(01)00181-8

The beta-silicon carbide (β-SiC) nanorods have been synthesized by a floating catalyst method. Iron particles, decomposed from ferrocene vapor while being carried into the reaction chamber by the flowing gases, are very tiny. These small Fe particles act as catalyst to promote the growth of SiC nanorods in the SiCl₄-C₆H₆-H₂-Ar system at 1100-1200°C. The diameters of the β-SiC in the products are less than 100 nm, and the SiC nanorods with uniform diameters are single crystals with the stacking faults on the {111} crystal planes.

Full text of DOI:10.1016/S0038-1098(01)00181-8

PERGAMON
Solid State Communications 118 .2001) 595±598
www.elsevier.com/locate/ssc
Synthesis of SiC nanorods using ¯oating catalyst
*
Yingjiu Zhang, NanLin Wang, Rongrui He, Xihua Chen, Jing Zhu
Electron Microscopy Laboratory, School of Materials Science and Engineering, Tsinghua University,
Beijing 100084, People's Republic of China
Received 25 October 2000; accepted 20 March 2001 by Z.Z. Gan
Abstract
The beta-silicon carbide .b-SiC) nanorods have been synthesized by a ¯oating catalyst method. Iron particles, decomposed
from ferrocene vapor while being carried into the reaction chamber by the ¯owing gases, are very tiny. These small Fe particles
act as catalyst to promote the growth of SiC nanorods in the SiCl4±C6H6±H2±Ar system at 1100±12008C. The diameters of the
b-SiC in the products are less than 100 nm, and the SiC nanorods with uniform diameters are single crystals with the stacking
faults on the {111} crystal planes. q 2001 Published by Elsevier Science Ltd.
PACS: 61.46.1w; 81.15.Gh; 81.16.Be; 81.16.Hc
Keywords: A. Nanostructures; B. Chemical synthesis; B. Crystal growth
1. Introduction
The size of the metal catalysts usually affects the diameter
of the nanotubes/nanorods. The larger the metal catalysts,
the thicker the nanotubes/nanorods. Generally, the metal
catalysts are fabricated .for example, deposited by CVD)
before the nanotubes or nanaorods are grown. Nanoscale
metal catalysts are dif®cult to synthesize and are easy to
coarsen when they are heated to the desired reaction
temperature. Thus, SiC whisker diameters less than 50 nm
were not produced directly [10,16] until recently. In order to
obtain nanoscale metal catalysts, H.M. Cheng et al.
exploited a new method to synthesize single-wall carbon
nanotubes by Fe powders pyrolyzed from ferrocene [17].
In this new method, the Fe metal particles were very small
because Fe atoms were introduced from decomposed ferro-
cene vapor and collected as Fe catalysts through a very short
time at high temperature. In the present work, we report an
approach to the synthesis of the SiC nanorods by using the Fe
catalysts pyrolyzed from ferrocene in the SiCl4±C6H6±H2±
Ar system.
Since the discovery of carbon nanotubes by Iijima [1],
one-dimensional nanoscale materials have received wide-
spread interests due to their potential applications in both
mesoscopic research and the development of nanodevices.
Although carbon nanotubes are studied with most detail,
other nanowires of Si [2±6], Ge [7,8], SiC [9,10] and
Si3N4 [11,12] are also noticed for their special properties.
For example, besides high strength and good ¯exibility [9],
SiC nanorods are also found to have great potential in the
area of electron ®eld-emitting devices [10]. Thus, several
methods to synthesize SiC nanorods have also been
developed by now. Dai et al. [13] and Han et al. [14] ®rstly
produced SiC nanorods by chemical vapor reaction of
Si/SiO2 with carbon nanotube templates. Recently, b-SiC
nanorods of 5±20 nm diameter were grown on a porous
silicon substrate by chemical vapor deposition .CVD) with
iron catalysts [10], straight b-SiC nanowires of 20±70 nm
diameter were produced by a hot ®lament assistant chemical
vapor deposition [15] and b-SiC nanowires of 10 nm
diameter were synthesized directly in an arc-discharge
[16]. In these processes of synthesis, at least one of the
reactants is solid.
2. Experimental process
The schematic representation of the SiC nanorods growth
apparatus is shown in Fig. 1. An Al2O3 boat about 10 cm in
length, full of ferrocene in a quartz tube, was put about 5 cm
away from the left end of the furnace. Then the horizontal
quartz tube .reaction chamber) of 35 mm diameter was
* Corresponding author. Tel.: 186-10-62773998; fax: 186-10-
62772507.
E-mail address: jzhu@mail.tsinghua.edu.cn .J. Zhu).
0038-1098/01/$ - see front matter q 2001 Published by Elsevier Science Ltd.
PII: S0038-1098.01)00181-8

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Y. Zhang et al. / Solid State Communications 118 /2001) 595±598
Fig. 1. Schematic representation of the SiC nanorods growth apparatus, .1)±.3) gas entrance; .4) furnace; .5) quartz tube; .6) Al2O3 boat full of
ferrocene; .7) waste gases.
heated to 1100±12008C in a ¯owing H2 gas. A H2 gas with
300 standard cm3/min .sccm) ¯owed through C6H6 liquid
.2±3 vol% thiophene) and an argon gas with 200 sccm passed
through SiCl4 liquid. Both these gases entered into the quartz
tube together and in the meantime, the quartz tube was pulled
from the right end until the left end of the Al2O3 boat reached
the left end of the furnace at the rate of 0.5 mm/min so that the
ferrocene could be gradually volatilized. The vapor of ferro-
cene was carried by the Ar and H2 ¯ow and turned into Fe
clusters at the center of the furnace. Catalyzed by the Fe
clusters, SiCl4 and C6H6 reacted with each other. The reaction
time was 30 min and a gray zone was deposited on the inner
surface of the quartz tube after the reaction. The product was
observed on a JEOL-2010F ®eld emission gun transmission
microscope .TEM) equipped with electron energy loss spec-
trum .EELS) system and energy dispersive spectrum .EDS).
Fig. 2. The different morphologies and SAED of the products .a) SiC nanorods bundle; .b) SAED of the SiC nanorods bundle; .c) a long and
thin SiC nanorod with uniform diameter; .d) SiC nanorods with slightly uneven diameters.

Y. Zhang et al. / Solid State Communications 118 /2001) 595±598
597
Fig. 3. The HREM photo of an individual SiC nanorod with stacking faults. Its growth direction is perpendicular to one of the {111} crystal
planes.
3. Results and discussion
by several techniques. It implies that some of the decom-
posed C6H6 does not react with Si. The second product is the
bundle of the SiC nanorods .Fig. 2a). The SiC nanorods
seem to grow from a big catalyst particle, and thus form a
hairlike bundle. However, these nanorods with less than
70 nm in diameter are not very well shaped, their diameters
are not uniform along their axes. The corresponding selected
area electron diffraction .SAED) shown in Fig. 2b reveals
that the nanorods are cubic b-SiC. The rings in the SAED
correspond to the {111}, {220}, {311} crystal planes of
cubic b-SiC.
The morphologies of the products are shown in Fig. 2.
The products can be classi®ed into three types. The ®rst is
the particles and they appear in Fig. 2a and c. Their sizes are
about several tens to several hundreds of nanometers, and
their shapes are anomalous. Some of the particles are
crystalline SiC while others are amorphous carbon identi®ed
The third product is the nanorods with high aspect ratio
.length/diameter). Their lengths are usually several mm and
their diameters are less than 100 nm, or even less than
10 nm. In Fig. 2c, a typical morphology of such a kind of
SiC nanorod, which is more than 1200 nm in length and less
than 15 nm in diameter, is shown. Most of these kind of
nanorods have uniform diameters along their axes, although
some nanorods have slightly uneven diameters .Fig. 2d).
The nanorods with uniform diameters are usually single
crystalline. A typical high-resolution electron microscopy
.HREM) photo of the nanorod with single crystal is shown
in Fig. 3. Its crystalline core is about 30 nm in diameter and
is covered by an incoherent amorphous or a small crystalline
Fig. 4. EELS of an individual SiC nanorod.

598
Y. Zhang et al. / Solid State Communications 118 /2001) 595±598
layer. Its growth direction is normal to one of the {111}
lattice planes. There are some stacking faults in this SiC
nanorod. The stacking fault planes are parallel to the {111}
crystal planes and the angle between the two groups of stack-
ing fault planes is 70.58. The number of the stacking faults is
less than that in the SiC nanorods synthesized by carbon
nanotube-con®ned reaction [18] but more than that in the
SiC nanorods fabricated by arc-discharged [16]. The number
of the stacking faults may be affected by the synthesis process.
EELS in Fig. 4 is taken from an individual nanorod. This
spectrum is very similar to that of SiC nanorods produced by
T. Seeger et al. [16], not only in the positions of the peaks,
but also in the shape of the EELS. Two groups of the peaks
in this spectrum can be identi®ed as Si±L2,3 and C±K edges.
Thus, the EELS clearly shows that the nanorods in the
present work mainly consists of SiC phase.
C6H6±H2±Ar system by using the ¯oating catalyst. The Fe
catalyst particles used are derived from the decomposition
of ferrocene. Ferrocene is decomposed in the short time that
it ¯ows into the reaction chamber together with the carrier
gases .H2 and Argon), the Fe catalyst particles thus formed
are small enough to grow thin nanorods. The SiC nanorods
fabricated in the present work are smaller than 100 nm in
diameter and can be identi®ed as cubic b-SiC. The b-SiC
nanorods with uniform diameter are single b-SiC crystal
with stacking faults on the {111} crystal planes. The method
may provide a new way to produce one-dimensional nano-
scale materials.
Acknowledgements
Similar to the growth of the single-wall carbon nanotubes
by the ¯oating catalyst ferrocene, the Fe catalyst clusters
nucleate and grow during the ferrocene vapor being carried
by the H2 and Ar ¯ow. Therefore, the sizes of most of the Fe
catalyst particles are small enough to grow thin SiC nano-
rods .Fig. 2c). As the catalyst particles may have different
size, the SiC nanorods with different diameters are fabri-
cated in the present work. In some case, the catalyst particles
may be as large as a few micrometers. However, these cata-
lyst particles are anomalously shaped and may be full of
sharp-angled surfaces. Thus, SiC nanorods have thin
diameters and form a hairlike bundle .Fig. 2a) when they
grow on this kind of surfaces.
This work was supported by Nature Science Foundation
of China, National 973 Project, 985 Project of Tsinghua
University.
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4. Conclusion
The SiC nanorods have been synthesized in the SiCl4±