Synthesis and characterization of SiC nanowires through a reduction-carburization route

Article abstract of DOI:10.1021/jp000124y

Cubic silicon carbide (3C-SiC) nanowires were synthesized through a reduction-carburization route by using silicon powders and tetrachloride (CCl₄) as Si and C sources, and metallic Na as the reductant at 700 °C. The as-prepared SiC nanowires were characterized and studied by X-ray powder diffraction, transmission electron microscopy, X-ray photoelectron spectra, Raman backscattering, and photoluminescence spectra at room temperature. The SiC nanowires produced from the present route typically have diameters of 15-20 nm and lengths of 5-10 μm. The influencing factors of the formation of the SiC nanowires were discussed and a possible growth mechanism for the SiC nanowires was proposed.

Full text of DOI:10.1021/jp000124y

J. Phys. Chem. B 2000, 104, 5251-5254
5251
Synthesis and Characterization of SiC Nanowires through a Reduction-Carburization
Route
J. Q. Hu, Q. Y. Lu, K. B. Tang,* B. Deng, R. R. Jiang, Y. T. Qian,* W. C. Yu, G. E. Zhou,
X. M. Liu, and J. X. Wu
Structure Research Laboratory and Department of Chemistry, UniVersity of Science and Technology of China,
Hefei, Anhui 230026, P. R. China
ReceiVed: January 10, 2000; In Final Form: March 6, 2000
Cubic silicon carbide (3C-SiC) nanowires were synthesized through a reduction-carburization route by
using silicon powders and tetrachloride (CCl4) as Si and C sources, and metallic Na as the reductant at 700
°C. The as-prepared SiC nanowires were characterized and studied by X-ray powder diffraction, transmission
electron microscopy, X-ray photoelectron spectra, Raman backscattering, and photoluminescence spectra at
room temperature. The SiC nanowires produced from the present route typically have diameters of 15-20
nm and lengths of 5-10 µm. The influencing factors of the formation of the SiC nanowires were discussed
and a possible growth mechanism for the SiC nanowires was proposed.
Introduction
One-dimensional (1D) structures with nanometer diameters,
such as nanowires and nanotubes are attracting increasing
interest due to their remarkable optical, electrical, and mechan-
ical properties and their potential applications ranging from
probe microscopy tips to interconnections in nanoelectrical
devices.1,2 Therefore, current attention has focused on the
development of new methods for the preparation of nanoscale
one-dimensional structures.
Silicon carbide belongs to IV-IV semiconductors and there
exist more than 130 polytypes due to the different orders of
stacking of the SiC layers.3 They have wide energy band gaps
from 2.3 eV for 3C-SiC to 3.3 eV for 2H-SiC at room
temperature, and have good physical stability, high thermal
conductivity, and high breakdown electric field.4 These excellent
physical properties make SiC a promising semiconductor
material for device applications at high temperature, high power,
and high frequency.5 There are many reports on the synthesis
of silicon carbide whiskers. The conventional methods fall into
the following three standard categories: (1) thermal decomposi-
tion of organic silicon compounds,6,7 (2) reaction between SiCl4
and CCl4 in the presence of hydrogen,8 and (3) carbonthermal
reduction of SiO2.9-11 The diameters of SiC whiskers prepared
by these traditional methods were typically >1000 nm, although
diameters as small as 100 nm have been observed. Recently,
Dai et al.12 adopted an approach to prepare silicon carbide
nanorods with diameters between 2 and 20 nm, in which the
carbon tubes were converted into silicon carbide nanorods by
reacting with Si and I2 at 1200 °C. Our research group has
successfully synthesized diamond powder on the reduction of
CCl4 by Na at 700 °C.13 In the present study, SiC nanowires
were synthesized at 700 °C by the reduction-carburization
method, in which CCl4 and silicon powder were used as source
materials and metallic Na as the reductant. The reaction was
carried out in an autoclave and can be expressed as follows:
Figure 1. X-ray powder diffraction pattern of the sample prepared by
a reduction-carburization route.
Experimental Section
In a typical experimental procedure, appropriate amounts of
CCl4 (5.0 mL), Si powders (1.44 g), and Na (4.74 g) were put
into a titanium alloy autoclave of 50 mL capacity. The autoclave
was sealed and maintained at 700 °C for 10-48 h and then
cooled to room temperature. At the beginning of the reaction,
there was a high pressure in the autoclave. As the CCl4 was
reduced by Na, the pressure deceased. The product was collected
and treated by dilute HF+HNO3 to eliminate the unreacted
silicon. The free carbon in the product was removed by heating
in air at 600 °C for 3 h. However, the product also suffered
partial oxidation during the heating, and amorphous SiO2 may
have formed on the product surface. So, the product was treated
with dilute HF again to eliminate amorphous SiO2 coating. After
washing with distilled water to remove NaCl and other
impurities, a gray-white product was obtained.
Si + CCl4 + 4Na f SiC + 4NaCl
(1)
* To whom correspondence should be addressed.
10.1021/jp000124y CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/11/2000

5252 J. Phys. Chem. B, Vol. 104, No. 22, 2000
Hu et al.
Figure 2. (a) TEM image of SiC nanowires . (b) TEM images of a straight single SiC nanowire and its ED pattern. (c) HRTEM image of the
nanowire SiC shown in (b). (d) TEM image of a hollow-structure SiC nanowire and its ED pattern. (e) HRTEM image of the hollow structure SiC
nanowire.The arrows showing the hollow nature. (f) TEM image of a carbon nanorod obtained from the reduction of CCl4 by Na at 700 °C.
The X-ray powder diffraction (XRD) pattern was recorded
on a Japan MAC SCIENCE MXP18AHF X-ray diffractometer
with graphite monochromatized Cu KR (λ ) 1.54056 Å)
radiation. Transmission electron microscopy (TEM) images and
selected-area electron diffraction (ED) patterns were taken with
an Hitachi H-800 transmission electron microscope, using an
accelerating voltage of 200 kV. High-resolution electron
microscopy (HREM) images of the SiC nanowires were taken
on a JEOL-2010 transmission electron microscope. X-ray
photoelectron spectra (XPS) were carried out on a VGES-
CALAB MKII X-ray photoelectron spectrometer, using non-
monochromatized Mg KR X-ray as the excitation source. The
Raman spectra were produced at room temperature with a Spex
1403 Raman spectrometer using an Ar-ion laser exciation with
a wavelength of 514.5 nm and with a backscattering configu-
ration. Photoluminescence (PL) spectra of the SiC nanowires
were measured in an Hitachi 850 fluorescence spectrophotom-
eter with a Xe lamp at room temperature.
literature (a ) 4.3589 Å).14 No other crystalline impurities, such
as Si and SiO2, were detected in the XRD pattern.
Figure 2 shows the TEM images and ED patterns of the
samples. It can be seen that the sample consists mainly of
nanowires. The SiC nanowires produced from present route
typically have diameters of 15-20 nm and lengths of 5-10
µm (Figure 2a, 2b). The inset ED patterns (Figure 2b) are
consistent with the high crystallinity. The HRTEM image
(Figure 2c) demonstrates that the transparent section of this
smooth nanowire is well-defined 3C-SiC single crystal and
possess a very low density of planar defects. Further studies
suggest that the nanowires have grown in a 111 direction. It
is of considerable interest that a small fraction of hollow
structures is clearly revealed by the TEM images (as shown in
Figure 2d, 2e). The ratio of the structures to nanowires was
about 5%. The structure generally has a diameter of 40-45 nm,
a length of 1 µm, and a wall thickness of 15-25 nm. The ED
pattern (Figure 2d) shows the polycrystalline rings [(111), (220),
and (311)] of the 3C-SiC, which confirms the XRD result.
However, the HRTEM image (Figure 2e) indicates that this
hollow morphology contains heavy stacking faults on the basal
planes in contrast to the well-defined 3C-SiC single crystals
nanowires. These defect planes are basically rotational microt-
wins on the (111) basal planes.15,16 The fact that carbon nanorods
Results and Discussion
Figure 1 shows the XRD pattern of the sample. All of the
strong intensity peaks could be indexed to the zinc blende
structure 3C-SiC. The refinement gave the cell constant, a )
4.3565 Å, which was consistent with the reported value in the

SiC Nanowires
J. Phys. Chem. B, Vol. 104, No. 22, 2000 5253
Figure 4. Room-temperature Raman spectra from the SiC nanowires
.
Figure 3. XPS spectra of SiC nanowires.
(Figure 2f) formed in the reduction of CCl4 by Na at 700 °C
suggests that the formation of the hollow structures may be
based on the use of carbon nanorods as templates. Several SiC
nanowires were coated on the carbon nanorods first and then
the carbon in the core was oxidized into CO2 by heating at 600
°C in air, and thus the polycrystalline hollow structure formed.
Figure 3 shows the XPS spectra of the sample. The two strong
peaks at 100.20, and 282.40 eV correspond to Si2p and C1s
binding energy, respectively, for SiC.17 Quantification of the
peak intensities gives an atomic ratio of Si to C of 1.11:1.00.
However, the spectrum also displayed the other two C1s peaks
at 284.45 and 286.10 eV, which can be attributed to a small
amount of residual carbon (284.3 eV of C1s levels for graphite)
and the adsorbed CO2 onto the sample surface. No obvious
peaks due to silicon (98.7 eV for Si2p) or silica (103.5 eV for
Si 2p in SiO2) are observed.
Figure 5. Photoluminescence spectra for the SiC nanowires at room
temperature.
Raman spectral features are strikingly different from others
reported in the literature.5,18-20 Not only is the TO (Γ) phonon
line much stronger than LO (Γ) phonon line but also the two
lines are both shifted by 19-20 cm-1 toward lower energy. The
reason for this exception may originate from the small grain
sizes of the SiC and from the growth faults of the nanowires.
Figure 5 shows the PL spectra from the SiC nanowires at
room temperature. The excitation wavelength was 385 nm and
the filter wavelength was 430 nm. It is clear that a strong peak
centered at 448 nm is observed. No sharp PL lines, which are
attributable to impurity-bound excitons and their phonon
replicas, or simple defects introduced by electron irradiation21-23
are clearly seen in this spectrum. Furthermore, compared with
the blue-green luminescence from the films 3C-SiC,24 a blue-
Figure 4 shows the Raman spectra for the SiC nanowires with
a backscattering configuration. Two broadened bands at around
776 and 954 cm-1 were recorded, which correspond to the TO
and LO phonon at the Γ point of cubic SiC. We note that the

5254 J. Phys. Chem. B, Vol. 104, No. 22, 2000
Hu et al.
violet luminescence in the SiC nanowires is indicative of a blue
shift. The reason for the results is yet unknown, and a deeper
study on this work is clearly needed.
nanowires produced from the present route typically have
diameters of 15-20 nm and lengths of 5-10 µm. The SiC
nanowires with very small diameters are expected to have
special electrical, optical, and mechanical properties, and may
be constituents of novel nanoscale materials and device struc-
tures. This process has inherent advantages, including the
availability of low cost raw materials and the simplicity of the
production procedures. This versatile method may be used to
synthesize other carbide nanowires.
The influences of reaction temperature and time on the
formation of the SiC nanowires were investigated. It was found
that a suitable reaction temperature was a key factor for the
formation of SiC nanowires . If the temperature was lower than
630 °C, the SiC nanowires could not be obtained, even at
elevated pressure; instead, amorphous C formed. If the tem-
perature was higher than 800 °C, the diameters of the SiC
nanowires increased easily. An optimum reaction temperature
for the SiC nanowires is about 700 °C. The reaction time must
be sufficient to ensure the carburization of Si powders. For a
higher yield of the SiC nanowires (about 38% according to the
amount of Si used), a treatment time longer than 10 h at 700
°C was needed. These results demonstrate that the present route
differs from the known SHS reaction mode.25,26
There are two well-accepted whisker growth mechanisms:
the screw dislocation mechanism27 and the vapor-liquid-solid
(VLS) mechanism.28 The TEM images of the SiC nanowires
showed no evidence of axial screw dislocations, suggesting that
the SiC nanowires were not grown by the screw dislocation
mechanism. The liquid globules, inherent to the VLS mecha-
nism, were found on the tip of the nanowires (not shown). This
fact suggests that the VLS mechanism was dominant in the SiC
nanowire synthesis. According to free energy calculations, the
reduction of CCl4 by Na is thermodynamically spontaneous and
highly exothermic (∆Gf0 ) - 355.7 kcalmol-1, ∆Hf0 ) -360.7
kcalmol-1).29 In our experiment, substituting Si powders with
bulk Si platelet and keeping the other reaction conditions
constant, it was found that the Si platelet could melt (Si mp of
1400 °C).29 This fact, together with the free energy calculations,
suggests that the large amount of heat generated in this process
can result in a high-temperature molten NaCl-Si flux. The
surface of the molten NaCl-Si flux may has a large accom-
modation coefficient and is therefore a preferred site for
deposition of atoms from the vapor phase reactant CCl4 (CCl4
bp of 76 °C),29 which can benefit the VLS nucleation for the
SiC. Meanwhile, the molten NaCl-Si flux may also play a role
in facilitating the growth of the SiC nanowires . Since crystal
growth involves the exchange of atoms at the growth front
planes, the high-temperature molten NaCl-Si flux could keep
the impinging atoms from being dislodged by thermal diffusions
and thermal vibrations, and thus enhance the nanowire growth.
A feature of this synthesis route is an initially high pressure in
the autoclave, coming from the vaporization of CCl4, which
may facilitate the VLS nucleation of the SiC nanowires .
Acknowledgment. This work is supported by China National
Science Research Foundation.
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In summary, the 3C-SiC nanowires were successfully
synthesized through a reduction-carburization route. The SiC