Silicon carbide hollow nanospheres, nanowires and coaxial nanowires

Article abstract of DOI:10.1016/S0009-2614(03)00877-7

The synthesis and characterization of hollow nanospheres, nanowires and coaxial nanowires of cubic phase silicon carbide (β-SiC) were reported. The reaction between SiCl₄ (or Si powders), C₆Cl₆ and sodium was used to prepa

Full text of DOI:10.1016/S0009-2614(03)00877-7

Chemical Physics Letters 375 (2003) 177–184
www.elsevier.com/locate/cplett
Silicon carbide hollow nanospheres, nanowires
and coaxial nanowires
*
Guozhen Shen, Di Chen, Kaibin Tang , Yitai Qian, Shuyuan Zhang
Department of Chemistry and Structure Research Laboratory, University of Science and Technology of China,
Hefei 230026, People’s Republic of China
Received 28 February 2003; in final form 9 May 2003
Abstract
The synthesis and characterization of hollow nanospheres, nanowires and coaxial nanowires of cubic phase silicon
carbide (b-SiC) were reported. The reaction between SiCl4 (or Si powders), C6Cl6 and sodium was used to prepare SiC
nanostructures at different temperature. The samples were characterized by X-ray powder diffraction (XRD), trans-
mission electron microscopy (TEM), electron diffraction (ED), high-resolution transmission electron microscopy
(HRTEM), Raman, photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS) procedures. Studies found
that temperature and reagents were key factors for the samplesÕ morphologies.
Ó 2003 Published by Elsevier Science B.V.
1. Introduction
cules or large-sized guests within the empty core
domain, are being attracted considerable attention
because of their specific structure and potential
applications. And one-dimensional nanorods,
which can be used as building blocks for many
novel functional materials, are also of great inter-
ests to materials scientists.
Since cubic silicon carbide (b-SiC) possesses
unique physical and electronic properties [7], it is
therefore a suitable material for the fabrication of
electronic devices operating at high-temperature,
high power, and high frequency, and in harsh
environment [8]. The conventional process for the
preparation of SiC powders is the carbonthermal
reduction of silica [9], the self-propagating high-
temperature synthesis (SHS) [10], polysilane
polymer precursors [11], sol–gel [12], plasma
[13], and microwave radiation [14] technologies
In recent years, one of the important goals of
material Scientists is to develop ways of tailoring
the structure of materials on specific nanomorph-
ologies [1]. The shape and size of inorganic
nanomaterials are well known to have an impor-
tant influence on their widely varying electrical
and optical properties [2], which is important in
various application, such as catalysis [3], solar cells
[4], light-emitting diodes [5], and biological label-
ing [6]. In the specific nanomorphologies, the
hollow sphere structures which have potential for
encapsulation of large quantities of guest mole-
* Corresponding author. Fax: +86-551-3601-600.
E-mail address: gzsh@mail.ustc.edu.cn (K. Tang).
0009-2614/03/$ - see front matter Ó 2003 Published by Elsevier Science B.V.
doi:10.1016/S0009-2614(03)00877-7

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G. Shen et al. / Chemical Physics Letters 375 (2003) 177–184
and so on. Recently, many other convenient
methods have also been used to prepare SiC
crystals [15–21].
for 5–12 h. The product was treated as above.
After being dried in vacuum, a gray-white sample
was obtained.
In the present study, SiC hollow nanospheres
and nanowires were synthesized at 600 and 700 °C,
respectively, by using SiCl4 and C6Cl6 as source
materials, and metallic sodium as the reductant.
And the coaxial SiC/C nanowires were synthesized
at 700 °C by the reaction between Si, C6Cl6 and
sodium. Studies revealed that temperature and
reagents were key factors for the morphologies of
the products. To the best of our knowledge, there
is no report on the synthesis of SiC hollow nano-
sphere structure.
2.4. Preparation of coaxial SiC/C nanowires
In a typical procedure, appropriate amounts of
Si powders (0.6 g), C6Cl6 (1.0 g) and metal Na
(1.5 g) were put into an autoclave. After be-
ing sealed, the autoclave was maintained at
650–700 °C for 10 h and then cooled to room
temperature naturally. The product was collected
and treated also as the above. After dried in vac-
uum, a gray-white product was obtained.
2.5. Characterization
2. Experimental
X-ray powder diffraction (XRD) patterns were
carried out on a Rigaku D/max cA X-ray diffrac-
tometer equipped with a graphite monochroma-
2.1. General procedures
ꢀ
All of the manipulations were carried out in a
dry glove box filled with flowing argon. All re-
agents were analytically pure and purchased from
Shanghai Chemistry and used without further
purification. Before use, the Si powders were
grounded.
tized Cu Ka radiation (k ¼ 1:541 A). The samples
were scanned at a scanning rate of 0.05°/s in the
2h range of 20–80°.
Transmission electron microscopy (TEM) im-
ages were taken with a Hitachi H-800 transmission
electron microscope, using an accelerating voltage
of 200 kV.
2.2. Preparation of SiC hollow nanospheres
The high-resolution transmission electron mi-
croscopy (HRTEM) images and energy dispersive
spectrum (EDS) pattern were taken on a JEOL-
2010 transmission electron microscope with an
attached EDS system.
X-ray photoelectron spectroscopy (XPS) was
recorded on a VGESCALAB MKII X-ray photo-
electron spectrometer, using nonmonochromatized
Mg Ka X-ray as the excitation source.
In a typical experimental procedure, appropri-
ate amounts of SiCl4 (3.0 ml), C6Cl6 (1.2 g), and
Na (3.5 g) were put into a stainless steel autoclave
of 30-ml capacity. After being sealed, the auto-
clave was maintained in a furnace at 600 °C for
5–12 h and then cooled to room temperature in the
furnace naturally. The product was collected and
washed with absolute alcohol. And then treated by
H2SO4 and HF to remove amorphous silicon and
other impurities, then washed with distilled water
to eliminate NaCl and other impurities. After be-
ing dried in vacuum at 70 °C for 3 h, a gray-white
sample was obtained.
The Raman spectra were produced at room
temperature with
a LABRAM-HR Confocal
Laser MicroRaman spectrometer. Photolumines-
cence (PL) spectra of the samples were measured
in a Hitachi 850 fluorescence spectrophotometer
with a Xe lamp at room temperature.
2.3. Preparation of SiC nanowires
3. Results and discussion
The reaction was conducted analogously to
that above, but the autoclave with the same
amounts of reactants was maintained at 700 °C
Phase identification of as-prepared samples was
carried out using the XRD patterns. Fig. 1 shows

G. Shen et al. / Chemical Physics Letters 375 (2003) 177–184
179
peaks of other impurities, such as Si, SiO2, could
be detected in the pattern.
Fig. 2 shows the TEM images and electron
diffraction (ED) pattern of the samples. Fig. 2a is
the image of SiC hollow nanospheres prepared at
600 °C. It can be seen that typical SiC hollow
nanospheres are 50–100 nm in diameters and 10 nm
in the shell thickness. The corresponding ED pat-
tern is shown in Fig. 2b. The three polycrystalline
rings in accordance with b-SiC [(1 1 1), (2 2 0) and
(3 1 1)] confirm the XRD result. Besides the hollow
nanospheres, some nanowires and nanoparticles
are also observed. The yield of SiC hollow nano-
spheres to the total product is about 75%.
Fig. 1. XRD pattern of SiC synthesized at 600 °C. (ÔsÕ: stacking
faults).
The morphology of SiC nanowires synthesized
at 700 °C is shown in Figs. 2c,d. It can be seen that
the sample consists mainly of nanowires. The SiC
nanowires produced from present route typically
have diameters of 20 nm and lengths up to 10 lm.
Figs. 2e,f show the TEM images of the samples
prepared from Si, C6Cl6 and sodium at 700 °C.
Fig. 2e reveals the general view of morphology of
SiC nanowires. Further insight into the structure
the typical XRD pattern of the sample prepared at
600 °C by the reaction between SiCl4, C6Cl6 and
sodium. All of the strong intensity peaks could be
indexed to the b-SiC structure with a low-intensity
ꢀ
peak at d ¼ 2:675 A which may be due to stacking
faults [22] (marked with s). The refinement gave
ꢀ
the cell constant, a ¼ 4:3606 A, which was con-
sistent with the reported value in the literature
ꢀ
(a ¼ 4:3589 A, JCPDS card, No. 29-1129). No
Fig. 2. (a) TEM image of SiC hollow nanospheres. (b) ED pattern of SiC hollow nanospheres. (c) TEM image of SiC nanowires. (d)
TEM image of a single SiC nanowire. (e) TEM image of coaxial SiC/C nanowires. (f) TEM image of a single coaxial SiC/C nanowire.

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G. Shen et al. / Chemical Physics Letters 375 (2003) 177–184
of the nanowires shows that each individual
nanowire is actually a hierarchical nanowires, in
which a center thinner nanowire is wrapped in a
uniform outer layer. The result is shown in Fig. 2f.
It can be seen that the center thinner nanowires are
about 20 nm in diameter and several micrometers
in length. The outer layer is with a thickness of
about 4 nm.
ED and HRTEM are performed in order to
obtain information about the structure of the SiC
nanostructures. The results are shown in Fig. 3.
The HRTEM image of SiC hollow nanospheres is
shown in Fig. 3a, which suggests that the SiC
hollow nanospheres are composed of well-crystal-
lized little SiC nanoparticles. The lattice fringes are
0.25 nm corresponding to (1 1 1) of SiC. ED pat-
tern inset shows its polycrystalline nature, it also
confirms the XRD pattern. The inner zone of SiC
hollow nanosphere is marked with arrow on the
image.
The ED pattern inset is the corresponding pattern
recorded along the [1 1 0] zone axis. There are
some stacking faults in this SiC nanorod. The
stacking fault planes are parallel to the {1 1 1}
crystal planes and the angle between the two
groups of stacking fault plane is 70.5°.
Fig. 3c is the representative lattice-resolved
image of a coaxial SiC/C nanowire. The spacing of
the crystallographic planes measured from the
HRTEM image is 0.25 nm. It corresponds to the
{1 1 1} lattice planes of the crystalline SiC, which
indicates that the center thinner nanowires is
b-SiC. The HRTEM analysis also reveals that the
outer layer of the coaxial nanowire is amorphous
phase. EDS analysis of the outer layer shows the
presence of carbon and absence of Si, which re-
veals that the outer layer is composed of amor-
phous carbon instead of amorphous SiO2. The
result is in consistent with [20]. There is a high
density of stacking faults and a few twins (so-
called microtwins) on the image. The inset ED
pattern is recorded along the [1 1 0] zone axis.
X-ray photoelectron spectra (XPS) are mea-
sured to derive composition information about the
samples. Fig. 4a depicts high-resolution XPS
Fig. 3b demonstrates that the transparent sec-
tion of the SiC nanowires is well-defined 3C-SiC
single crystal. The lattice fringes along the orien-
tation of the nanorod are 0.25 nm, indicating that
the nanowires have grown in a h111i direction.
Fig. 3. (a) HRTEM image of SiC hollow nanospheres. (b) HRTEM image of SiC nanowires. (c) HRTEM image of coaxial SiC/C
nanowires.

G. Shen et al. / Chemical Physics Letters 375 (2003) 177–184
181
Fig. 4. XPS spectrum of coaxial SiC/C nanowires.
spectrum taken for the Si regions of the coaxial
SiC/C nanowires. It displays a peak at 101.05 eV
corresponding to the Si 2p binding energy of SiC.
No obvious peak of Si 2p binding energy of SiO2
could be detected. Fig. 4b is the C regions of the
sample. It displays three peaks. The peak at
283.1 eV corresponds to C 1s binding energy of
SiC. The other peaks can be attributed to the
carbon (284.3 eV for C 1s in graphite) of the outer
carbon layer and the adsorbed CO2 at the pow-
dersÕ surface. All the analyses (EDS and XPS) in-
dicate that the outer layer of the coaxial nanowires
is carbon instead of amorphous SiO2.
The as-obtained SiC nanostructures were also
characterized by Raman procedure (figures were
not shown here). The Raman spectrum of as-pre-
pared SiC nanowires shows the presence of a sharp
peak at 780 cmÀ1, which confirms that the nano-
rods are well crystalline b-SiC [23].
Fig. 5. Photoluminescence spectra for the SiC hollow nano-
spheres at room temperature.
The PL spectra from the SiC hollow nano-
spheres at room temperature were shown in Fig. 5.
The excitation wavelength was 385 nm and the
filter wavelength was 430 nm. It was clear that a
strong peak centered at 435 nm is observed. The
results were in good agreement with the reported
value [24].
In present studies, it was found that tempera-
ture was important on the formation of SiC
nanostructures. In our experiments of preparation
of SiC hollow nanospheres and nanowires, we
found that if the temperature was lower than 550
°C, no SiC crystallites could be detected on the
XRD pattern. Temperature higher than 550 °C
resulted in the appearance of SiC crystallite. But
the sample was composed of a large amount of
amorphous product. If the reaction temperature
was 600 °C, XRD pattern showed the peaks of
only SiC crystalline. High-resolution electron mi-
croscopy image revealed that this sample was
hollow nanospheres consisted of high crystallized
SiC nanoparticles. But when the temperature was
as high as 700 °C, almost pure SiC nanowires
would be obtained instead of SiC hollow nano-
spheres. Temperature between 600 and 700 °C
resulted in the co-existence of SiC hollow nano-
spheres and nanowires. If the temperature was
higher than 750 °C, the diameters of SiC nano-
wires increased easily. So the suitable temperature

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G. Shen et al. / Chemical Physics Letters 375 (2003) 177–184
6SiCl4 þ C6Cl6 þ 30Na ! 6SiC þ 30NaCl
6Si þ C6Cl6 þ 6Na ! 6SiC þ 6NaCl
ð1Þ
ð2Þ
we selected was 600 °C for SiC hollow nanospheres
and 700 °C for SiC nanowires. For the preparation
of the coaxial SiC/C nanowires, temperature lower
than 600 °C resulted in no desired sample but
amorphous product. The optimum reaction con-
dition for the coaxial SiC/C nanowires was at the
temperature of 700 °C for 10 h. The whole results
are shown in Table 1.
In our synthetic route, selecting C6Cl6 as reac-
tant is a key factor to the formation of SiC hollow
nanospheres and SiC/C coaxial nanowires, when
other carbon source, such as CCl4, activated car-
bon substituted C6Cl6, only SiC nanowires or
nanoparticles could be obtained.
From Table 1, one can see that the selected
reactants also result in different reaction temper-
ature to the formation of cubic phase SiC. For
example, crystalline SiC can be obtained by co-
reduction of CCl4 and SiCl4 at 400 °C, the re-
action temperature will be raised to 600 °C by
substitution of C6Cl6 for CCl4, these results could
be attributed to the different molecular configu-
ration of CCl4 and C6Cl6. C (Si) atom in CCl4
(SiCl4) molecule is sp3 hybridization which is
consistent with the C (Si) atom configuration in
SiC crystal, crystalline SiC would be obtained at
relatively low temperature by reaction of SiCl4
and CCl4. Unlike CCl4, C atoms in C6Cl6 mole-
cule are sp2 hybridization and a quite stable
conjugated p system is formed, it is expected that
extra energy is needed to activate carbons in
C6Cl6. Therefore, the reaction temperature of
C6Cl6 and SiCl4 will be raised in order to obtain
crystalline SiC. Similar results occur by substitu-
tion of Si for SiCl4.
In previous work, we found that if C6Cl6 was
reduced by sodium at 600 °C, the product we got
was amorphous carbon hollow nanospheres with
diameters of 50–120 nm, carbon hollow nano-
spheres were observed even though the reduction
temperature was as high as 700 °C. The TEM
images show that the SiC hollow nanospheres
prepared by co-reduction of SiCl4 and C6Cl6 at
600 °C have diameters of 50–100 nm, which almost
hold the shape and the diameter size of carbon
hollow nanospheres skeleton. So it is reasonable to
expect that carbon hollow nanospheres may be the
reaction intermediate and may act as an in situ
template in the formation of SiC hollow nano-
spheres [16,25,26].
On the basis of the above analyses, the forma-
tion mechanism of SiC nanostructures could be
proposed although the exact formation mecha-
nism is not completely understood. From the for-
mation enthalpies of SiCl4 (Hf ¼ À627:71 kJ/mol)
and C6Cl6 (Hf ¼ À127:6 kJ/mol), it is desirable for
C6Cl6 to be reduced by sodium firstly through
deleting chloro to form planar carbon clusters at
relatively low temperature and then carbon clus-
ters will form metastable sphere aggregate, subse-
quently, SiCl4 will be reduced to form activate Si
particles. Once the reaction is initiated, the heat
generated in the process is sufficient to melt the
byproduct NaCl (bp ¼ 801 °C) and vaporize the
reductant Na (bp ¼ 889 °C); the newly formed Si
particles will be dispersed in the NaCl molten flux.
On the other hand, most of the carbon interme-
diates may be still hold sphere aggregates under
such a condition and they are also dispersed in the
In our present work, crystalline SiC can be
obtained based on the following reactions:
Table 1
Relationship between the sample morphologies and reagents temperature
Reagents
Temperature
(°C)
Morphology
SiCl4 + C6Cl6 + Na
600
Hollow nanospheres
600–700
700
700
Hollow nanospheres and nanowires
Nanowires with very little amounts of hollow nanospheres
Coaxial SiC/C nanowires
Nanocrystallines
Si + C6Cl6+Na
SiCl4 + C + Na [19]
Si + CCl4 + Na [20]
SiCl4 + CCl4 + Na [21]
600
700
Nanowires
Nanowires
400

G. Shen et al. / Chemical Physics Letters 375 (2003) 177–184
183
NaCl molten flux. The molten NaCl would be an
interface of solid–solid or solid–gas reaction and
may serve as a medium for the crystallization of
SiC. Thus, the newly formed Si and gaseous
SiCl4 would surround the carbon sphere aggre-
gates and react with them to form SiC polycrys-
talline hollow nanospheres. The schematic
formation process of SiC hollow nanospheres
could be formulated as followed:
similar to the route of [20]. Noting that the en-
thalpy of the reaction is much lower than that of
reaction (1) and the whole heat capacities of the
products (Cp ¼ 631:5 kJ/mol) are higher than that
of the reactants (Cp ¼ 499:06 kJ/mol). It is ex-
pected that, along with the proceeding of the re-
action, the heat generated in the reaction would be
insufficient to break C–C bond in the C6Cl6 mol-
ecules. Therefore, in the later stage of the reaction,
the amorphous carbon reduced by Na would be
created and deposited on the surface of SiC
nanowires to form an amorphous sheath, thus
coaxial SiC/C nanowires could be obtained.
The feature of the present route is an initially
high pressure in the autoclave, which comes from
the vaporization of C6Cl6 (bp ¼ 325 °C) and/or
SiCl4. It facilitates the nucleation of SiC nano-
structures, similar to [20], and makes the crystal-
line SiC form at relatively low temperature.
nC6Cl6 þ 6nNa ! 6nC ðclusterÞ þ 6nNaCl
! C ðsphere aggregateÞ
ð3Þ
ð4Þ
ð5Þ
ð6Þ
SiCl4 þ 4Na ! Si þ 4NaCl
Si þ C ! SiC
SiCl4 þ 4Na þ C ! SiC þ 4NaCl
When the reaction temperature is raised to
700 °C, in the early time, the procedure is similar
to that at 600 °C. Once the reaction is initiated, the
energy produced in reaction is so large that it may
be sufficient to break the C–C bonds in C6Cl6
molecules due to the higher reaction temperature.
In this case, the newly formed Si particles in the
molten NaCl may act as the energetically favored
site and directly react with gas-phase reactants
C6Cl6 (bp ¼ 325 °C) and Na. Thus a vapor–li-
quid–solid (VLS) process is established to the
growth of the SiC nanowires. Such a process is
similar to the route of [21]. In our experiment,
liquid globules were found on the tip of the
nanowires on the TEM image, which suggested the
VLS mechanism was dominant in the SiC nano-
wires growth. Therefore, the products prepared by
co-reduction of SiCl4 and C6Cl6 at 700 °C are
mainly SiC nanowires with a small amount of SiC
hollow nanospheres.
As for the reaction of Si and C6Cl6, the initial
reaction temperature is 700 °C. The thermody-
namics parameter calculation shows that the re-
action is thermodynamically spontaneous (DG0f ¼
À2682:5 kJ/mol) and highly exothermic (DHf0 ¼
À2731:4 kJ/mol), suggesting that a large amount
of heat also generated in the process, which is
sufficient to activate Si and break C–C bond in the
C6Cl6 molecule. Thus a VLS process is also em-
ployed to the growth of SiC nanowires, which is
4. Conclusion
In conclusion, we succeeded in synthesizing
b-SiC hollow nanospheres, nanowires and coaxial
nanowires via a simple reaction at mild conditions.
Studies showed that the temperature and reagents
were key factors of the formation of SiC nano-
structures. Given the generality of this method, it
can be in principle used to synthesize other car-
bides, such as TiC, ZrC and VC, etc. The intensive
studies are in progress.
Acknowledgements
This work is supported by Chinese National
Science Research Foundation and the 973 Projects
of China.
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