Preparation of High Purity Crystalline Silicon by Electro-
Catalytic Reduction of Sodium Hexafluorosilicate with
Sodium below 180
6
C
Yuan Chen, Yang Liu, Xin Wang, Kai Li, Pu Chen*
Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada
Abstract
The growing field of silicon solar cells requires a substantial reduction in the cost of semiconductor grade silicon, which has
been mainly produced by the rod-based Siemens method. Because silicon can react with almost all of the elements and
form a number of alloys at high temperatures, it is highly desired to obtain high purity crystalline silicon at relatively low
temperatures through low cost process. Here we report a fast, complete and inexpensive reduction method for converting
sodium hexafluorosilicate into silicon at a relatively low reaction temperature (,200uC). This temperature could be further
decreased to less than 180uC in combination with an electrochemical approach. The residue sodium fluoride is dissolved
away by pure water and hydrochloric acid solution in later purifying processes below 15uC. High purity silicon in particle
form can be obtained. The relative simplicity of this method might lead to a low cost process in producing high purity
silicon.
Citation: Chen Y, Liu Y, Wang X, Li K, Chen P (2014) Preparation of High Purity Crystalline Silicon by Electro-Catalytic Reduction of Sodium Hexafluorosilicate with
Sodium below 180uC. PLoS ONE 9(8): e105537. doi:10.1371/journal.pone.0105537
Editor: Vipul Bansal, RMIT University, Australia
Received April 20, 2014; Accepted July 21, 2014; Published August 25, 2014
Copyright: ß 2014 Chen et al. This is an open-access article distri buted under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: The authors confirm that all data underlying the findings are fully available without restriction. All relevant data are within the paper and its
Supporting Information files.
Funding: This research was financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), Canadian Foundation for
Innovation (CFI) and the Canada Research Chairs (CRC) program. The funders had no role in study design, data collection and analysis, decision to publish, or
preparation of the manuscript.
Competing Interests: Co-author Pu Chen is a PLOS ONE Editorial Board member. This does not alter the authors’ adherence to PLOS ONE Editorial policies and
criteria.
* Email: p4chen@uwaterloo.ca
Introduction
In 1823, Berzelius obtained iron-free silicon by reducing SiF4
gas, which came from the heat decomposition of K2SiF6, with
red-hot potassium metal above 520uC. From then on, the
processes of producing silicon with precursor silicon tetrafluoride
or trichlorosilane have been extensively studied.
Current industrial manufacturing of high purity silicon adopts
the Siemens method using purified trichlorosilane, SiHCl3. With
hydrogen gas, SiHCl3, obtained by converting crude silicon with
hydrogen chloride, decomposes and deposits silicon onto high-
purity silicon rods and enlarges the rods at 1150uC.
The well-known Stanford Research Institute International (SRI)
reduction process involves that purified silicon tetrafluoride (SiF4)
gas through fractional distillation is reduced to silicon by metal
sodium above 500uC. SiF4 is from the heat decomposition of
sodium hexafluorosilicate (Na2SiF6) at 647uC:
Na2SiF
6
(s)~SiF
4
(g)z2NaF (s) ð1Þ
SiF
4
(g)z4Na(l)~Si(s)z4NaF (s) ð2Þ
An alternative method to transform SiF4 gas into elemental
silicon with NaAlH4, in which silane (SiH4) gas is decomposed at
727uC to generate elemental silicon, was used by Ethyl Corpo-
ration. In 2006, Renewable Energy Corporation announced
construction of a plant based on the fluidized bed method using
SiH4 gas, which was obtained by conversion of metallurgical
grade silicon into SiHCl3 and redistribution/distillation to SiH4.
The continuous flow process recycles all hydrogen and chloride
materials back to the initial reactors, while continuous distillation
steps purify the SiH4 gas.
However, there are many drawbacks with these methods,
including high deposition temperature, high cost for constructing
durable reactors, high energy consumption, operation with
explosive raw materials, and post-treatment of hazardous
exhausted gas and amorphous silicon dust waste. Much of the
recent research effort to produce solar cell grade silicon has thus
focused on electrochemical reduction of silica in molten salts [1–4]
at 850uC, or metallo-thermic reduction [5–7] of silicon com-
pounds. Among them, the magnesio-thermic reduction method
[5] above 650uC was well-known.
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The preparation of crystalline silicon using other silicon
precursors has also been reported. Among them, the synthesis of
nanometer-sized silicon crystals by reducing SiCl4 with metal
sodium in a nonpolar organic solvent at high temperature (385uC)
and high pressure (. 100 atmospheres) was reported [7].
To our knowledge, no studies have been conducted on
electrochemical sodium reduction to obtain crystalline silicon
using one step process at temperature less than 180uC and in
nitrogen atmosphere with a pressure of less than 1 atm. Moreover,
this method does not involve silicon precursor gas purification that
is necessary to all above-stated industrial processes. The silicon
preparation carried out at low temperature may effectively reduce
amounts of impurities from side reactions and containers.
Materials and Methods
Conversion method of sodium hexafluorosilicate into silicon
particles by metal sodium was investigated as follow:
Under nitrogen atmosphere, the certain amount of Sodium (a
purity of .99wt%, Aldrich) and sodium hexafluorosilicate
(Analytical reagent, Alfa Aesar) which had been dried at 120uC
for 2 hours to remove the moisture, were put into the round
bottom flask with three-necks. The Na
2
SiF
6
: Na molar ratio were
,0.25, 0.25, 0.3. One neck of round bottom flask was used as
nitrogen gas passage which was through a condenser. One glass
pipe was used as gas inlet which was put in the condenser. One
magnetic stirrer was put into the round bottom flask. A piece of
single-crystal silicon plate (CZ, Phosphorous dopant, Resistivity 1–
10 ohm/cm, orient ,100.+0.9, Virginia semiconductor) was
used as negative working electrode. Another piece of silicon plate
was used as positive counter electrode. The two electrodes were
put into the round bottom glass flask through the other two necks.
Such flask was set in oil bath that was on a fisher hotplate. All
experiments were conducted under a dry nitrogen atmosphere.
Copper wire and stainless steel clip were used to connect with Si
wafers. The silicon wafers were used to contact the Na-Na
2
SiF
6
mixture. A typical experimental example was as follows: the
amount of Sodium was 1.0 g, the amount of Na
2
SiF
6
was 2.1 g.
The mixture of sodium and sodium hexafluorosilicate were stirred
at a high speed 750 rpm and at 120uC, 150uC, 180uC, 200uC, or
250uC respectively. A direct current potential 1.4 V, or 2.4 V, or
3.6 V supplied by DC power (B&K precision) was applied to the
two electrodes. When Na-Na
2
SiF
6
mixture was heated at the
fixing temperature and stirred at a speed of 750 rpm, they turned
dark. After there were brown particle materials appearing on the
surface of the glass flask, kept holding at that temperature for half
Figure 1. XRD pattern of the produced samples in glass flask.
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an hour, then stopped the reaction and cooled the mixture to
room temperature.
The brown region materials were immersed in ultra-pure water
to selectively dissolve sodium hexafluorosilicate and sodium
fluoride. Then the samples were exposed to a 1 M hydrochloric
acid (HCl) solution to get rid of impurities contained in the
samples. The ultra-pure water treatment and the subsequent HCl
solution treatment were conducted at 10uC, 40uC, or 60uC
respectively. The treatment container was polypropylene beaker.
When filtering, whatman filter paper and buchner funnel were
used. All experimental operations were done without cleanroom.
Characterization: Scanning electron microscopy was conducted
with a field emission scanning electron microscope operating at an
accelerating voltage of 20 kV. XRD studies were carried out at
20uC in air atmosphere, using a D8 Discover X-ray diffraction
system made by Bruker Company. The purity of the silicon
powder was determined by Elan 9000 ICP-MS system (Perkin
Elmer Sciex).
Results and Discussion
The situation that nobody had carried out the preparation of
silicon under these conditions was probably due to two precon-
ceived notions: (1) Na
2
SiF
6
begins to decompose at 647uC [6] and
has no melting point; (2) silicon tetrafluoride, which is a byproduct
of producing superphosphate fertilizer from phosphate rock [6], is
not pure. This was substantiated by the consequences that (1) the
temperature for converting Na
2
SiF
6
into silicon tetrafluoride must
be above 647uC; (2) SiF
4
gas has to be purified by passing it over
iron at 797uC to remove air and SO
2
, and by subsequent
fractional distillation [6].
However, we found that metal sodium can react with solid
Na
2
SiF
6
below 200uC:
4Na(s)zNa
2
SiFP
6
(s)~Si(s)z6NaF (s) ð3Þ
The experimental set-up and reaction phenomena were
described in the Experimental section (also refer to Figure A in
File S1). The reaction happened obviously when brown materials
in the reactor became visible by eyes. The reacted samples (see
Figure B in File S1) were brown colour. The following X-ray
diffraction (XRD) analyses indicated that the brown region
contained silicon. The unreacted samples were observed to
contain three regions of different colours. The region located
nearest to the bottom of the glass flask was silver in colour, which
was metal sodium. The other two regions, where both metal
Figure 2. Silicon powder SEM micrographs recorded at different magnification after the sample was washed with pure water.
doi:10.1371/journal.pone.0105537.g002
Figure 3. XRD patterns of silicon particles prepared at different ratios of raw materials in glass flask (R is the value of Na2SiF6: Na
molar ratio) after the samples were washed with pure water.
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sodium and Na
2
SiF
6
existed, were black (containing more metal
sodium) and gray (containing more Na
2
SiF
6
), respectively. The
XRD analysis of the reacted samples in Figure 1 showed that not
only the diffraction peaks of crystalline silicon (JCPDS 27-1402)
existed in the XRD pattern, but also those of NaF (JCPDS 36-
1455) and a few unreacted Na
2
SiF
6
(JCPDS 33-1280) (see Figure
C in File S1). No peaks of metal sodium or silicon nitride Si
3
N
4
existed in the XRD pattern.
After the sample (see Figure B in File S1) was washed with pure
water, SEM results in Figure 2 showed that the size of silicon
particles ranged from several tens of nanometres to 30 microme-
tres.
Although the free energy of reaction is a quantitative measure,
and accurate data for such reactions are not readily available, it is
usual for the chemists to rely on the heats of substance formation
to estimate the heats of the actual reactions which take place above
25.15uC. The reaction (3) could be thought of as the combination
of the reactions (1) and (2), considering similar states of metal
sodium. The reaction (2) is strongly exothermic and the reaction
(1) is endothermic. Based on the data of the standard Gibbs free
energy for substance formation [8], the changes in Gibbs free
energy, standard entropy and standard enthalpy for the reaction
(3) are 2523.6 kJ/mol, 286.9 J/molNK and 2550 kJ/mol,
respectively (see Table S1 in File S1). Consequently from the
chemical thermodynamic point of view, the reaction (3) could
occur spontaneously at the temperature of 30.7uC theoretically,
and release heat. This confirms the occurrence of the reaction.
If sodium hexafluorosilicate and sodium were stirred at a speed
of 750 rpm and maintained at 180uC with applying a 3.6 V
electrical voltage to the electrodes, the molar ratio (between 0.25
and 0.35) of Na
2
SiF
6
to sodium played an important role in the
purity of silicon produced. When the sample was immersed in
water, there was no reaction of sodium metal with water. This
proved that the reaction conversion in terms of sodium was nearly
100%. In Figure 3, the XRD analysis of the produced samples
showed that when the Na
2
SiF
6
: Na molar ratio was 0.3:1, there
were fewer impurities in silicon particle samples after washing with
pure water. Because sodium metal can react with water to produce
NaOH very violently, the residual of sodium metal would affect
the purity of silicon and lead to Na
2
SiO
3
(see Table S1 in File S1).
To evaluate the electrical potential effect control experiments
were done without applying potential to the electrodes. The
reaction could take place at 200uC with a stirring magnet at a
speed of 750 rpm. Dispersing the reactants evenly was important
here, and the reaction would not occur without stirring even at
300uC. When the Na
2
SiF
6
: Na molar ratio was suitable (between
0.25 and 0.35), only the brown mixture could be observed (also
refer to Figure 4).
When sodium metal was melted, liquid sodium formed a dark
conductive dispersion with Na
2
SiF
6
. While applying 2.4 V
electrical voltage to the electrodes, the reaction could take place
at 190uC and the colour of the liquid-solid mixture changed more
quickly than the one without applying potential. Flashes were
often observed at the same time as the dark liquid-solid mixture
became brown all-solid mixture (also refer to Figure 4). When the
potential applied was 3.6 V, the reaction could proceed at below
180uC. Based on the definition of chemical catalysis and electro-
catalysis, it is the chemistry in electrochemical reactions, which are
most hampered, needing catalytic acceleration. Electrochemical
catalysis does not differ very much fundamentally and mechanis-
Figure 4. Description of the reaction mechanism.
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tically from chemical catalysis, apart from the fact that charge
transfer rates and electrosorption equilibriums depend exponen-
tially on electrode potential.
A possible electro-catalytic reduction reaction mechanism is
described in Figure 4. The whole reactive systems might involve
the exchange of charges between the electrodes and electron
donors or acceptors present in the liquid-solid mixture, with ionic
charge transfer through the liquid-solid mixture between two
electrodes. The flashes might be the result of discharge of short
circuits between sodium metal and Na
2
SiF
6
.
The actual decomposition potential should be higher than the ideal
reversible decomposition potential (1.36 V) that is estimated by the
Nernst equation DGr~DHr{TDSr~{nFE
(rev)
znFT (LE=LT)
p
(see Table S1 in File S1) when T is supposed to be 298 K. However,
the potential actually required is the sum of the theoretical reversible
decomposition voltage, overvoltage at the electrolyte-electrode
interphase boundary, and the ionic conduction resistance of the
electrolyte. The ionic conduction resistance of the electrolyte
depends on the ion diffusion speed (stirring magnet speed affects it
greatly), the distance of the electrodes from each other, and the
current density. When electrical energy was input to overcome the
activation energy threshold of each sub-reaction in the reaction (3),
the overall reaction could be accelerated. Once flash occurred, the
entire reaction was completed within minutes. However, due to the
complication of chemical kinetics in this reaction system, it is
difficult to predict the accurate reaction temperature and reaction
rate. Note that an increase in temperature might lower the
reversible decomposition potential. Na2SiF6 begins to decompose
at 920 K (647uC) and has no melting point. This further
complicates the reaction kinetics. The experimental results are
reported when certain applied potential, stirring speed, concentra-
tion of reactants and heating temperature are chosen, and the
reaction takes place. Na2SiF6 can decompose below 453 K (180uC)
with the presence of sodium metal.
To separate silicon from NaF, the samples were dissolved into
water to form two immiscible phases, for which solid–liquid
extraction (migration of impurities from silicon to NaF solution)
provided additional purification. NaF and Na
2
SiF
6
could be
dissolved into ultra-pure water easily. This process is different from
the known the SRI process [6], which comprises heating Si-NaF
mixture at a temperature slightly below the melting point of silicon
with a molten purifying agent, which does not appreciably react
with silicon to cause the impurities to separate from silicon.
Because the water washing process takes place at room temper-
Figure 5. HTEM image and SAED patterns of silicon particles washed with pure water and HCl at 10
6
C.
doi:10.1371/journal.pone.0105537.g005
Figure 6. XRD patterns of silicon particles washed with pure water and HCl at 10
6
C, 40
6
C and 60
6
C.
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ature, the possibilities to cause impurities into silicon are reduced
greatly. The polycrystalline structure of silicon was confirmed by
HTEM and SAED analysis shown in Figure 5. No Na
2
SiF
6
or
NaF (see Figure C in File S1) was found. When trying to obtain
ultra-pure silicon through pure water and HCl solution washing
under air atmosphere, by comparing the XRD patterns, we
observed that surface oxidation of silicon particles could be
inhibited below 10uC:
Si(s)zO
2
(g)~SiO
2
(s) ð4Þ
The change in Gibbs free energy, standard entropy and
standard enthalpy for the reaction (4) are 2856.3 kJ/mol, 2
182.5 J/molNK and 2910.7 kJ/mol, respectively [8]. Consequent-
ly from the chemical thermodynamic point of view, the reaction (4)
can occur spontaneously at 25uC (see Table S1 in File S1).
Silicon particles were washed by ultra-pure water and HCl
solution at 40uC and 60uC, respectively, in air atmosphere. The
results showed that silicon particles could be mildly oxidized at
60uC (see Figure D in File S1) when exposed to aqueous HCl
without removing oxygen. Figure 6 showed the XRD pattern of
the silicon particles, which were washed at 10uC through ultrapure
water and HCl solution. In this XRD pattern, there was no
obvious peak near 2h =23u, which was corresponding to a
broadened peak for amorphous SiO
2
.
The possible reasons are as follows: 1) surface oxidation of
silicon might be inhibited at below 10uC; 2) the remainder NaF in
the sample can react with aqueous HCl solution to produce HF
solution, and the HF solution can remove some silicon dioxide on
the surface of silicon particles. According to the magnesiothermic
reduction method [5] at 650uC, the silicon dioxide on the surface
of silicon particles can be removed by using the ethanol-based
hydrofluoric solution. Our results are consistent with the statement
that oxygen in water can oxidize silicon slowly at room
temperature in the magnesiothermic reduction method [5].
Table 1 showed that the total content of metallic impurities
from the silicon powder obtained by ICP-mass was less than
34.86 ppm at. Sodium in the sample was 26 ppm at. Sodium was
the major metal impurity. Aluminum and iron contents were
1.6 ppm at and 0.8 ppm at, respectively. Phosphorus in the silicon
sample was 0.9 ppm at. This means that the production of solar-
grade silicon with phosphorus content of ,1 ppm (parts per
million) is possible in this method, if the raw material Na
2
SiF
6
is of
analytical grade reagent. The detailed elemental analysis could be
seen in the Table S2 in File S1. Since all experimental operations
were done without cleanroom, with respect to metallic impurities,
the purity of the silicon was estimated to be at least 99.996 at%. In
the Siemens method, hydrogen chloride gas etching is generally
applied to remove impurities from the surface of silicon rods and
Table 1. Results of Impurities analysis in silicon powder by ICP-MS.
Metallic Impurities in ppm atom
Na 26
Mg 0.7
Al 1.6
K 0.4
Sc 0.2
Ti 0.5
V 0.1
Mn 1.9
Fe 0.8
Ni 0.5
Cu 0.1
As 0.2
Sr 0.1
Sn 0.2
Sb 0.2
Total 33.2
13impurities:Cr,Zn,Ga,In,Te,La,Pr,Nd,Dy,Hg,Pb Total ,1.3
36impurities:Li,Be,Ca,Ge,Rb,Y,Nb,Ru,Rh, Pd,Cd,Ag,Cs,Ce,Sm,Eu,Gd,Tb,Ho,Er, Tm,Yb,Lu,Hf,Ta,W,Re,Os,Ir,Pt,Au,Tl,Bi,U,Th Total ,0.36
Silicon Purity at % .99.996
*All other non-metal impurities in ppm atom. Not including H,C,N,B,O,F
I 1.1
P 0.8
Br ,0.1
S 0.5
Total Conc. ,2.5
Not including H,C,N,B,O,F, Inert gas The detection limit of the analysis was 1 ppm wt., and accuracy and precision were estimated to be on the order of 10% relative.
doi:10.1371/journal.pone.0105537.t001
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can make purity of silicon above 99.99999%. According to
Henry’s law, the concentration of a gas in a liquid is directly
proportional to the partial pressure of the gas in equilibrium with
the liquid [6]. Aqueous HCl solution can remove metallic
impurities equally well as hydrogen chloride gas in the Siemens
method. Therefore, we believe the concentration of metal
impurities could be further reduced with increasing the washing
times of HCl solution in a clean environment, which avoids
contamination of dust from atmosphere.
Compared with current industrial manufacturing of high purity
silicon, our new and relatively lower temperature preparation
method has several advantages as follows:
First, it does not involve purifying of liquid or gas silicon
compounds, and will not consume energy as high as that of the
Siemens method, which requires preparation and distillation of
trichlorosilane, or the fluidized bed method which requires pure
silane. Such purifying steps (involving conveyance, heat transfer,
separation and/or mixing operations) in the Siemens method or
the fluidized bed method also result in huge equipment cost.
Second, our method produces more silicon in per cubic meter of
reactor space at 180uC with applying 3.6 V to electrodes, and
saves large amounts of electrical energy because electro-catalytic
reduction of Na
2
SiF
6
could be carried out completely within
several minutes in liquid-solid reactive state, not in gas-solid
reactive state as used in the Siemens method at 1127uC, or the
fluidized bed method at above 647uC. The Siemens method
produces poly-silicon at about
$28/kg. The reactants in our
method are low-cost and cheaper than silicon tetrachloride and
hydrogen gas used in the Siemens method. The price of metal
sodium (99.7%) is
$2.3/kg and that of Na
2
SiF
6
(99.5%) is $0.6/kg
in the world market. There is actually an energy cost saving,
comparing with the cost of existing industrial methods that take
place above 1100uC. Sodium metal is used as raw material, but
this disadvantage could be offset greatly by recycling NaF to
prepare Na
2
SiF
6
.Na
2
SiF
6
can be obtained by adding NaF to
H
2
SiF
6
. The product solid silicon can be purified with pure water
and HCl solution at room temperature. The cost for preparing
high pure silicon is lower.
Third, no gas products or polluting effluents are produced. At
the end of the treatment, water only contains Na
2
SiF
6
, which can
be easily collected and recycled. It is an eco-friendly process, unlike
that of the Siemens method, which requires recycling toxic acid
gas, silicon tetrachloride and hydrogen gas.
Its drawback is that it is a batch process rather than a
continuous one, so that more washing times by water and HCl
solution are required to obtain high pure silicon powder.
Conclusions
In conclusion, for the first time we demonstrated that sodium
hexafluorosilicate powder can be electro-catalytically reduced to
pure crystalline silicon at below 180uC by sodium metal. High
pure crystalline silicon powder can be obtained by washing with
HCl solution at low temperature. Further work to obtain solar
grade silicon is undergoing with better preparation conditions.
Our study provides a new promising opportunity for low-cost
production of high purity crystalline silicon.
Supporting Information
File S1 This file contains Figures A, B, C, and D, and
Tables S1 and S2. Figure A. The glass reactor for preparing
silicon below 175uC. Figure B. The image of the products of
reaction Figure C. XRD pattern of NaF and Na2SiF6. Figure D.
EDX analyses obtained from silicon particles washed at
313K.(Carbon is from conductive tape). Table S1. Standard
Thermodynamic Properties of Chemical Substances. Table S2.
The ICP-Mass test results of silicon samples.
(PDF)
Author Contributions
Conceived and designed the experiments: YC PC. Performed th e
experiments: YC YL. Analyzed the data: YC XW KL PC. Contributed
reagents/materials/analysis tools: PC. Contributed to the writing of the
manuscript: YC PC.
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