Synthesis of nanosized silicon particles by a rapid metathesis reaction

Article abstract of DOI:10.1016/j.jssc.2009.09.007

A solid-state rapid metathesis reaction was performed in a bed of sodium silicofluoride (Na₂SiF₆) and sodium azide (NaN₃) powders diluted with sodium fluoride (NaF), to produce silicon nanoparticles. After a local ignition

Full text of DOI:10.1016/j.jssc.2009.09.007

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Journal of Solid State Chemistry 182 (2009) 3201–3206
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Journal of Solid State Chemistry
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Synthesis of nanosized silicon particles by a rapid metathesis reaction
a
a,
Ã
a
b
C.W. Won , H.H. Nersisyan , H.I. Won , H.H. Lee
a
RASOM, Chungnam National University, Yuseong, Daejeon 305-764, South Korea
b
Korea Research Institute of Chemical Technology, Daejeon 305-600, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
2 6
A solid-state rapid metathesis reaction was performed in a bed of sodium silicofluoride (Na SiF ) and
Received 2 July 2009
Received in revised form
3
sodium azide (NaN ) powders diluted with sodium fluoride (NaF), to produce silicon nanoparticles.
2 6 3
After a local ignition of Na SiF +4NaN +kNaF mixture (here k is mole number of NaF), the reaction
3
September 2009
proceeded in a self-sustaining combustion mode developing high temperatures (950–1000 1C) on very
short time scales (a few seconds). Silicon nanoparticles prepared by the combustion process was easily
separated from the salt byproducts by simple washing with distilled water. The structural and
morphological studies on the nanoparticles were carried out using X-ray diffractometer (XRD) and field
emission scanning electron microscope (FESEM). The mean size of silicon particles calculated from the
FESEM image was about 37.75 nm. FESEM analysis also shows that the final purified product contains a
noticeable amount of silicon fibers, dendrites and blocks, along with nanoparticles. The mechanism of Si
nanostructures formation is discussed and a simple model for interpretation of experimental results is
proposed.
Accepted 12 September 2009
Available online 19 September 2009
Keywords:
Silicon nanoparticles
Combustion synthesis
Sodium silicon tetrafluoride
Sodium azide
Combustion temperature
Sodium fluoride
&
2009 Elsevier Inc. All rights reserved.
1
. Introduction
produce homogenous size Si powder. Moreover, silane as a precursor
material is expensive and explosive when contacts with air. Electron
beam evaporation of silicon usually allows to obtained nanopowders
of average particle size 3–4 nm [20]. Similar to previous cases, this
method also suffers from low production rate.
In these points of view rapid metathesis reactions (RMR) looks
promising for preparation of silicon nanopowders. Rapid metathesis
reactions producing nanoscale ceramic materials are well-known in
the literature [21,22]. Gillan et al. reported the synthesis of refractory
ceramics using exothermic metathesis reaction between metals
Silicon nanopowder is an exciting and relatively new material
with potential to revolutionize the electro-optic semiconductor
industry, which comprises solid state lighting, lasers, microelec-
tronics and biological tags, etc. [1–3]. Furthermore, nanosized Si
which is non-toxic would be an ideal candidate for replacing
fluorescent dyes for labeling in vivo cells and may serve as an
alternative to other semiconductor-based luminescent tags, like
CdSe [4,5]. Although nanostructured silicon displays very high
chemical reactivity and is also a promising fuel for highly
energetic materials [6,7].
chloride (TiCl
nitride and boride ceramics (TiN, ZrN, BN, NbB
synthesis of Si nanoparticles and nanorods via chemical metath-
esis route using SiCl and NaN precursor materials were reported in
[22]. In this process, well-crystallized Si with an average size of
3 4 5 3 2
, ZrCl , MoCl , etc.) and Li N and/or MgB to produce
2
, TaB , etc.) [21]. The
2
Currently considerable research effort is devoted to the produc-
tion and engineering of Si nanopowders and numerous synthesis
techniques have been developed for Si nanopowders. Among them
3 4
N
4
3
3 4
N
pulsed laser ablation (PLA) technique [8–15], CO
2
laser pyrolysis of
150 nm was obtained at 480 1C in stainless autoclave. To the best of
our knowledge, the synthesis of nanostructured silicon via RMR
route has not been reported yet.
In this paper, the processing route for the synthesis of
nanostructured silicon via rapid metathesis reaction is demonstrated
silane [16–18], electron beam evaporation of silicon ingot [19,20]
become attractive to produce Si nanopowders. Generally, the
synthesis of silicon nanoparticles by PLA technique is made in an
ambient gas as argon, helium and thus produced nanoparticles are
collected on a filter, substrate, cold plate, etc. Usually this method is
appropriate to produce small amount of powder, but the technique
2 6 3
on Na SiF +4NaN precursor mixture. The effect of NaF mole
fraction and argon gas pressure on the combustion parameters and
the morphology of silicon powder is examined and discussed as well.
2
of CO laser pyrolysis of silane gas appears as a flexible tool for the
production of Si nanoparticles in development quantities [16].
However, this method required precise control of reaction para-
meters, such as pressure of silane, reaction temperature, etc. to
2. Experimental
Ã
Na
(o200
SiF
powder (o100
m
m, purity 98%), NaN
powder
Corresponding author. Fax: +82 42 822 9401.
2
6
3
E-mail address: haykrasom@hotmail.com (H.H. Nersisyan).
mm, purity 99%) and NaF (ꢀ50 m
m, purity 97%) were the
0
022-4596/$ - see front matter & 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2009.09.007

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C.W. Won et al. / Journal of Solid State Chemistry 182 (2009) 3201–3206
raw materials used in this study. As shown in Fig. 1, 37.6 g Na
powder mixed with 52 g NaN was hand compacted into a
2
SiF
6
nickel–chromium ignition wire (c). The reactor was evacuated to
10 MPa followed by backfilling with Ar gas from 0.5 to 5.0 MPa.
À2
3
metallic cup 3.0 cm in diameter and 10 cm in height (a) and then
the cup was placed into a combustion reactor (b) under the
The combustion reaction was induced by electrically heated
nickel–chromium wire 7 mm in diameter. The temperature
profiles during
a combustion reaction were monitored by
chromel-alumel thermocouples inserted directly into the
reaction mixture. The signals of thermocouples were collected
by data logger system and recorded on personal computer (d). The
combustion parameters examined were the combustion
c
temperature (T ) and combustion wave propagation velocity (U
c
)
The combustion velocity was calculated as U
c
=x/t (t is the time
interval between temperature profiles and x is the distance
between thermocouples).
The entire combustion process was accomplished within 10 s.
The combustion product was hand grinded, washed by distillated
water and dried at 70–80 1C. The mass of the recovered powder
Fig. 1. Schematic representation of the experimental apparatus.
Fig. 4. Adiabatic combustion temperature (Tad) and equilibrium composition of
products as a function of NaF mole fraction.
2 6 3
Fig. 2. Temperature profiles of Na SiF +4NaN system vs inert gas pressure.
c c
Fig. 3. Combustion temperature (T ) and velocity (U ) as a function of NaF mole
fraction.
Fig. 5. XRD patterns of combustion products before (a) and after purification (b).

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3203
was about 5.5 g/sample, which corresponds to the theoretical
yield of silicon. Phase purity of silicon was identified by X-ray
diffraction (XRD) (Siemens, Karlsruhe, Germany) using CuK
3. Results and discussion
a
3.1. Combustion parameters
radiation, and the microstructure was examined by a field
emission scanning electron microscopy (FESEM; JSM 6330F).
Oxygen and nitrogen concentration were analyzed using ELTRA
ON-900 Oxygen/Nitrogen Determinator.
The effects of two variables, namely argon pressure (PAr) and
NaF concentration (flux) on combustion parameters and the phase
composition of the final product were investigated. Temperature
2 6 3
profiles recorded in the combustion wave of Na SiF +4NaN
mixture upon inert gas pressure are shown in Fig. 2. In accordance
with the results shown in Fig. 2, argon pressure has negligible
effect on the combustion temperature. The measured values of T
c
are 980720 1C, in the 1.0–5.0 MPa pressure interval of argon. The
visual inspection of the samples derived under different argon
pressure has revealed that all combusted samples have a white
brown color. The experimental measurement showed that the
height and the width of the final sample were not changed
significantly compared with the initial values of these parameters.
Meanwhile the weight loss of the sample calculated by the
equation:
D
m=(min. Àmfin./min.)100% (where, min. and mfin. are the
initial and final sample weights, respectively) was ꢀ40 wt%. This
value corresponds to the theoretical yield of nitrogen gas from
3
NaN that had left the sample during the combustion process.
A well known approach to affect on the combustion tempera-
ture is dilution of green mixture by alkali metal halide [22,23]. For
2 6 3
the Na SiF +4NaN system NaF would be an effective inert
diluent, as NaF is an expected by-product of this reaction. The
influence of sodium fluoride mole fraction on the combustion
temperature and velocity is shown in Fig. 3. Combustion
c
temperature, T , maintains almost constant (980 1C) when mole
Fig. 6. Typical FESEM images of as-combusted (a) and water purified (b) samples.
fraction of NaF is between 0 and 3 mol. Above this range (k43) a
Fig. 7. Typical FESEM images of as-combusted (a) and water purified samples: (b) k=0; (c) k=2 and (d) k=3.

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point of NaF and the formation of Si powder is expected in the
medium of solid NaF.
3.2. Reaction product characterization
A typical XRD pattern of the combustion product obtained
2 6 3
from the Na SiF +4NaN is shown in Fig. 5(a). The diffraction
peaks were identified as sodium fluoride and silicon. In order to
separate the silicon from residual reaction product (NaF), the as-
synthesized powder was treated with warm distillated water.
The X-ray diffraction diagram presented in (Fig. 5(b)) shows
single-phase silicon. Despite NaN
nitrogen gas, when was heated to 400 1C, no noticeable peaks of
Si can be seen in the XRD pattern. This result can be explained
3
generates large amount of
3 4
N
in the light of the combustion temperature, which is sufficiently
low (980 1C) to support the nitridation process of silicon. It is
known, that nitridation process of silicon starts at the
temperature above 1200 1C. More particularly, Leal Cruz et al.
reported [26] that thermal decomposition of Na
2 6
SiF in nitrogen
atmosphere can produce Si phase above 1200 1C. XRD patterns
3 4
N
of combustion products prepared with 2, 4 and 6 mol NaF are
shown in Fig. 6. No visible compositional change in the final
products was found by XRD analysis, when different amount of
NaF was added to the green mixture (Fig. 6). A small peak at 401
was identified as other modification of silicon. Also, the noticeable
pattern line broadening from
k arises from the decreased
crystallite sizes as well as from lattice distortions by internal
stress caused by NaF.
FESEM images of as-synthesized and water purified samples
are shown in Fig. 7. The microstructure of as-synthesized sample
(
k=0) is nonuniform and porosity, exhibiting different molten
fragments of NaF with impregnated Si particles Fig. 7(a). Water
purification has resulted silicon samples with different
a
morphology. The as-prepared sample (k=0) is composed of large
porosity fragments with a wide distribution in size (Fig. 7(b))
whereas the samples prepared with NaF (k=2 and k=3) have a
uniform and fine particles in the nanometer range (Fig. 7(c, d)).
These particles have nearly around shape, homogenous size
and soft agglomeration degree. Therefore, after short time
ball-milling (1–2 hr) in alcohol media silicon particles (k=3)
becomes well-dispersed as shown in Fig. 8(a). The particle
size distribution obtained from the FESEM image by particle
diameter measurements shows good uniformity and corresponds
to the normal Gaussian distribution (mean value ꢀ37.75 nm)
which is superimposed on a histogram (Fig. 8(b)). The role of
NaF is twofold: on one hand it dilutes the reactive mixture
and limits the particle growth by decreasing the collisions
between primary particles, on the other hand it contributes to
decrease flame temperature. Consequently NaF concentration in
the reactive mixture is on of the important parameters to
obtained fine particles of Si in the nanometer range. It must be
noted, however, that a high dilution of mixture with NaF
Fig. 8. FESEM image of Si powder after the short-time grinding (a), and particle
size distribution (b) calculated from the micrograph.
noticeable decrease in combustion temperature is occurred.
c
Combustion velocity, U , drops monotonously with increasing
diluent concentration. Generally, with increasing concentration of
the diluent, combustion temperature decrease linearly for many
of exothermic mixtures [24]. Stability of T
would be explained by analyzing Na SiF
c
in 0rkr3 interval of k
+4NaN +kNaF system
2
6
3
(k44 mol) has a detrimental effect on the nanoparticle
using software ‘‘THERMO’’ designed for the combustion pro-
cesses [25]. This analysis allows predicting adiabatic combustion
temperature, Tad, and equilibrium composition of final pro-
ducts and the results of calculation are shown in Fig. 4. The
second ordinate (C) on Fig. 4 displays mole number of the
reaction products. As can be seen, the tendency of adiabatic
characteristics. According to experimental results, the optimum
concentration of NaF is found to by between 2 and 4 mol. It must
be also noted, that FESEM analysis did not reveal any noticeable
change in Si powder morphology depending on argon gas
pressure.
It is worthy to notice that along with nanoparticles other
nanostructural formations of silicon, such as blocks, dendrites and
fibers were also found by FESEM analysis (Fig. 9(a–d)). A close
inspection of micrographs shows that silicon blocks (Fig. 9(b)) and
dendrites (Fig. 9(c)) have a nanogranular structure. It seems that
these newly formed structures were obtained by rejoining of Si
nanoparticles caused by the melting, spreading and cooling
combustion temperature, Tad, is identical to that of T
c
and
this tendency is caused by consumption of reaction heat for
heating and subsequence melting of NaF. Therefore, maximum
values of combustion temperature become limited by the
melting point of NaF (990 1C). With a relatively large amount of
c
diluent (more than 3 mol), T becomes lower than the melting

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3205
Fig. 9. FESEM photomicrographs showing Si particles, blocks, dendrites and fibers formed in specimen during the combustion processes.
3.3. Silicon formation mechanism
Two main salts of sodium, Na
2
SiF
6
and NaN
3
are involved in
the rapid metathesis reaction to produce Si nanoparticles in the
medium of molten NaF. These salts become thermally decom-
posed in the combustion wave above 400 1C. Decomposition
process can be presented as follow:
Na
2
SiF
6
-2NaF+SiF
4
(gas)
(1)
(2)
NaN
3
-Na(liq)+1/5N
2
Formed by reaction (1) SiF
produce Si element:
4
easily reacts with liquid sodium to
SiF
4
(gas)+4Na(liq)-4NaF+Si
(3)
As can be seen from the Eq. (3) the formation of Si
nanoparticles occurs by SiF (gas)+Na(liq) mechanism. However,
a partial vaporization of Na (Tvap.Na=883 1C) in the combustion
wave is also possible and SiF (gas)+Na(gas) mechanism of
4
Fig. 10. A model of growth mechanism of Si nanostructures.
4
reaction is not excluded. Large amount of NaF serves as a liquid
and/or solid reaction medium for Si nanoparticles formation and
the mechanism of Si nanoparticles formation in the presence of
sodium fluoride is identical to refractory metal nanopowders
synthesis reported in our previous investigations [22,23].
The formation of silicon blocks and dendrites shown in Fig. 9(b–
c) could be associated with partial rejoining of Si nanoparticles in
NaF melt. At that, two persuadable causes can be noted for this
phenomenon: moving of liquid phase under the gravity forces and
intensive gas release during the combustion process. A simple design
shown in Fig. 10 illustrates how silicon nanoparticles, blocks and
dendrite structures may form during the combustion processes. The
growth mechanism of nanofibers shown in Fig. 9(d), may be
explained within two well-known models: vapor–liquid–solid
(VLS) process and vapor–solid (VS) process [26]. In Fig. 9(d)
processes of NaF. Also we suppose that the role of gas phase
reactions with participation of SiF
formation process.
4
could be crucial in the fibers
Si nanoparticles produced by RMR have a relatively high
degree of oxygen incorporation. According to analysis data the
content of oxygen was between 1.5 and 5.0 wt%, when k changed
from 0 to 6 mol. In the optimized conditions (k=2–4) the
concentration of oxygen was between 1.5% and 3.0%. Despite
2
relatively high oxygen content, no silicon oxide peaks (SiO, SiO )
were detected in the XRD patterns (Fig. 7). It appears that either
oxygen is not chemically bonded or that the silicon oxide phases
are amorphous and can not be detected by the X-ray technique.
The amount of nitrogen in silicon nanopowders was ꢀ0.5 wt%
level according to analysis data.

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droplets can be easily seen at the end of the nanofibers. Therefore,
the growth of the nanofibers can be considered in term of the VLS
mechanism. For example it could be initiated from liquid sodium–
silicon system by rapid decomposition of sodium silicide. Further
work is warranted in order to understand full reaction mechanism,
which will make it possible to control the process of diverse
nanostructures formation.
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The utility of RMR reaction in the synthesis of silicon nanos-
[
[
[
[
tructures from the bed of sodium silicofluoride (Na
azide (NaN ) powders is described. Rapid exothermic reaction in
Na SiF +4NaN +kNaF mixture develops high temperatures (950–
000 1C) and produces Si nanoparticles incorporated into molten
2 6
SiF ) and sodium
3
2
6
3
1
NaF. The effect of NaF on the combustion parameters and Si
morphology was examined and discussed herein. It was shown, that
silicon nanoparticles 37.75 nm in mean diameter can be obtained
from the reaction mixture diluted with 3 mol of NaF. FESEM analysis
also revealed a noticeable amount of silicon fibers, dendrites and
blocks in the final purified products. The mechanism of Si
nanostructures formation is discussed and a simple model for
interpretation of experimental results is proposed.
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