Silicon electrodeposition in molten fluorides

Article abstract of DOI:10.1016/j.electacta.2011.12.039

Silicon nucleation process was investigated in molten NaF-KF (40-60 mol%) on silver electrodes in the 820-950 °C temperature range in order to optimize silicon coating operating conditions. Chronoamperometric measurements evidenced that silicon electrodeposition process involved an instantaneous nucleation with diffusion-controlled nuclei growth whatever temperature and Si(IV) ions concentration in the mixture. The overpotential and temperature influence on nucleation sites number was also studied. Silicon deposits were obtained using the same temperature range as nucleation study, for different current densities on substrates: Ni, Ag, C_graphite and C _vitreous. A sensitive influence of the cathodic substrate on the deposit adherence and roughness was observed and discussed.

Full text of DOI:10.1016/j.electacta.2011.12.039

Electrochimica Acta 62 (2012) 282–289
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Silicon electrodeposition in molten fluorides
∗
A.L. Bieber, L. Massot , M. Gibilaro, L. Cassayre, P. Taxil, P. Chamelot
Université de Toulouse; INPT, UPS, CNRS; Laboratoire de Génie Chimique; 118 Route de Narbonne, F-31062 Toulouse, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 May 2011
Received in revised form 8 December 2011
Accepted 9 December 2011
Available online 19 December 2011
Silicon nucleation process was investigated in molten NaF–KF (40–60 mol%) on silver electrodes in the
◦
8
20–950 C temperature range in order to optimize silicon coating operating conditions. Chronoam-
perometric measurements evidenced that silicon electrodeposition process involved an instantaneous
nucleation with diffusion-controlled nuclei growth whatever temperature and Si(IV) ions concentration
in the mixture. The overpotential and temperature influence on nucleation sites number was also studied.
Silicon deposits were obtained using the same temperature range as nucleation study, for different
current densities on substrates: Ni, Ag, Cgraphite and Cvitreous. A sensitive influence of the cathodic substrate
on the deposit adherence and roughness was observed and discussed.
Keywords:
Instantaneous nucleation
Silicon
Electrodeposition
Molten fluorides
Chronoamperometry
©
2011 Elsevier Ltd. All rights reserved.
1
. Introduction
point of the study, the purity of the obtained deposits was not
considered as an issue.
Photovoltaic technology is emerging as a major source of electri-
It was shown, in a previous work [6], that the volatility of Si(IV)
compounds greatly depends on the nature of the fluoride mixture.
The NaF–KF eutectic mixture, which highly stabilizes Si(IV) com-
pounds, was thus selected as the electrolyte for the present study.
The article describes a next stage in the preparation of silicon
layers for solar energy which is the demonstration of the electrol-
ysis technique validity in molten salts technique to recover pure
silicon on a cathode.
For this purpose, the nucleation process and the silicon nuclei
growth on silver are investigated using cyclic voltammetry and
chronoamperometry and these results are compared with suitable
models [4,7] in order to state whether the nucleation is instanta-
neous or progressive. Meanwhile, the overvoltage and temperature
influence on the nucleation mode and nucleation sites number
were examined. Then, silicon deposition runs were carried with
cal energy [1], and the most common base material for photovoltaic
cell is solar grade silicon (SoG-Si). Due to a constantly growing
demand for SoG-Si, the development of new processes allowing
the production of cheap SoG-Si is a main issue for the solar energy
industry.
In the 1980s, silicon electrodeposition in molten salts has been
considered as an attractive option for SoG-Si production [2]. Indeed,
silicon metal is more reductive than hydrogen meaning that silicon
electrodeposition has to be carried out in non-aqueous electrolytes
such as alkali metal halides. However, no electrolytic Si production
process has found commercial application up to now. Consider-
ing more recent concerns about carbon-free energy production
and growing needs for SoG-Si, such kind of process is nevertheless
being reassessed, and, for that purpose, basic investigations on the
salts properties, Si(IV) reduction mechanism and silicon deposition
strategies are needed.
various electrode substrates (Ni, Ag, C
and Cvitreous), cur-
range) and temperatures
graphite
−
2
rent densities (−20 to −200 mA cm
◦
In the frame of a production route based on electrorefining
either from MG–Si or from Si cutting remains [3]), it was decided to
focus on the electrodeposition process of dissolved Si(IV) in molten
fluoride. The main objective was to evaluate the feasibility of such
reaction and to provide an academic study of the Si(IV) reduction
reaction, thanks to specific electrochemical techniques [4,5]. At this
(820–900 C range). The influence of each parameter on the coat-
ing adherence and roughness was observed by scanning electron
microscopy (SEM).
(
2. Experimental
2.1. Cell
The cell was a vitreous carbon crucible placed in a cylindrical
∗ Corresponding author. Tel.: +33 561558194; fax: +33 561556139.
E-mail address: massot@chimie.ups-tlse.fr (L. Massot).
vessel made of refractory steel and closed by a stainless steel lid
cooled by circulating water. The description of this cell has been
0
013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.12.039

A.L. Bieber et al. / Electrochimica Acta 62 (2012) 282–289
283
detailed in previous work [8]. The experiments were performed
3. Results and discussion
under an inert argon atmosphere (Linde). The cell was heated using
a programmable furnace and the temperature was measured using
a chromel–alumel thermocouple.
3.1. Si(IV) ions reduction mechanism
The electrolytic bath consisted of the NaF–KF (40–60 mol%)
eutectic mixture (Carlo Erba 99.995%). As mentioned earlier,
the choice of this specific solvent was based on its stabilizing
properties regarding to the volatility of Si(IV) compounds [6].
The mixture was initially dehydrated by heating under vacuum
The silicon ions reduction in fluorides media was found to be a
single step process exchanging 4 electrons (as already presented in
Ref. [6]):
Si(IV) + 4e− = Si
(1)
−
2
(
7 × 10 mbar) from ambient temperature up to its melting point
Cyclic voltammetry was carried out on a silver electrode
during one week. Silicon ions were introduced into the bath in
◦
in NaF–KF–Na SiF6 system in the 820–950 C temperature
2
the form of sodium hexafluorosilicate Na SiF6 (Alfa Aesar 99.99%)
2
range. Fig. 1 presents a typical cyclic voltammogram of the
powder.
−
1
◦
NaF–KF–Na SiF6 (0.24 mol kg ) system at 850 C on silicon elec-
2
trode. This cyclic voltammogram exhibits only one reduction peak
at −0.15 V vs Si and its associated reoxidation peak at 0.15 V vs Si.
The linear relationship between Si(IV) reduction peak intensity and
the square root of the scan rate has been verified (see the inset of
Fig. 1), proving that the electrochemical reduction process is con-
trolled by the Si(IV) ions diffusion in the bath (Berzins–Delahay [9]
valid for a reversible soluble/insoluble system):
2
.2. Electrodes
Silver wires (1 mm diameter), vitreous and graphite car-
bon rods (3 mm diameter–Mersen spectroscopic quality) and
nickel plates (50 mm × 10 mm × 1 mm) were used as working
electrode. The surface area of the working electrode was deter-
mined after each experiment by measuring the immersion depth
ꢀ
ꢁ
1/2
nFD
RT
in the bath. The auxiliary electrode was a silicon thick plate
0
v1/2
Ip = −0.61nFSc
(2)
2
(
4 mm × 8 mm × 11 mm) with a large surface area (3 cm ). The
potentials were measured with reference to a silicon plate
immersed in the molten electrolyte.
where n is the number of exchanged electrons, F is the Fara-
−1
2
0
day’s constant (C mol ), S is the electrode surface area (m ), c
is the solute concentration (mol m ), D is the diffusion coefficient
m s ), T is the temperature (K) and v is the potential scan rate
−
3
2
−1
(
2.3. Techniques
(
V s−1).
Furthermore, the diffusion coefficient D can be calculated using
The electrochemical techniques used for the silicon nucle-
Eq. (2) and the results obtained from the inset of Fig. 1. At the
ation and deposition investigation were cyclic voltammetry and
chronoamperometry. All the electrochemical measurements were
performed with an AutolabPGStat 30 potentiostat/galvanostat con-
trolled by a computer using the research software GPES.
◦
−9
2
−1
temperature of 850 C, its value is 2.9 × 10 m s . The temper-
◦
ature dependence of the diffusion coefficient, in the 820–950 C
temperature range, is given in Eq. (3):
ꢂ
ꢃ
The silicon deposits were observed by SEM coupled with EDX
3
D = 1.13 × 10−5 exp − 7
7.2 × 10
(3)
◦
probe, after cleaning in a HCl–AlCl –H O mixture at 50 C with
3
2
RT
ultrasonic waves.
v1/2/V1/2 s-1/2
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
-
100
200
300
400
500
600
700
800
900
1
000
-
-
8
6
4
2
00
00
00
00
0
-
-
-
-
-
-
-
0.5
0
0.5
1
1.5
2
E vs Si/V
-
-
-
200
400
600
Fig. 1. Cyclic voltammogram of NaF–KF–Na2SiF6 (c0 = 0.24 mol kg−1) at 850 C; working electrode: Ag; auxiliary electrode: Si; reference electrode: Si; Scan
◦
−1
rate = 100 mV s /Inset: Linear relationship of Si(IV) reduction peak current density versus the square root of the scanning potential rate.

2
84
A.L. Bieber et al. / Electrochimica Acta 62 (2012) 282–289
Fig. 2. Cyclic voltammogram highlighting the nucleation cross-over for silicon deposition in NaF–KF–Na2SiF6 (c0 = 0.24 mol kg−1) at 850 ◦C; working electrode: Ag
2
−1
.
(
S = 0.31 cm ); auxiliary electrode: Si; reference electrode: Si; Scan rate = 50 mV s
2
−1
−2
where N is the nuclei density (cm ), N0 the total number of
nucleation sites (cm ), A the nucleation constant for a site (s
and t the time (s).
where D is the diffusion coefficient (m s ) and T the temperature
K).
−
2
−1
)
(
-
If the value of A is high, thus N = N and the nucleation is instan-
3.2. Silicon electrocrystallisation process
0
taneous meaning that all the nuclei appear at the beginning of
the polarization.
3
.2.1. Cyclic voltammetry
Fig. 2 shows a cyclic voltammogram where the cathodic limit
-
For low values of A, N = AN t, meaning that the nucleation is
0
progressive.
potential is higher than the Si(IV) ions reduction peak potential
observed on Fig. 1. This figure highlights the main features of an
electrocrystallisation process:
•
Part II: The current rises, due to an active surface area increase,
owing to the nuclei growth. Correlatively, the electrolyte diffu-
sion promotes the current decrease. The competition between
these two effects leads to a maximum peak intensity and its coor-
dinates (tm; Im) are used for the nucleation modeling. For short
times, the current in this second part allows the nucleation mode
determination and obeys the following relation according to [7]:
•
•
•
The observation of a cross-over of direct and reverse scanning
curves, which is the consequence of the irreversible nucleation
step during the reduction scan [9,10].
The asymmetrical shape of the reoxidation peak, typical of the
redissolution of a solid phase deposited during the cathodic run
x
j = ˛t
(5)
(
stripping peak).
The low cathodic background current observed between 0.02 V/Si
and −0.03 V/Si can be attributed to the formation of dissolved
silicon into the cathodic substrate (Ag).
where ˛ and x depend on the growth control, the nuclei geometry
and the nucleation mode. In the case of hemispherical 3D nucle-
ation limited by diffusion, x is equal to 1/2 or 3/2 for instantaneous
or progressive nucleation respectively. Eq. (5) has been plotted in
Fig. 4 and the proportionality between j and t1 evidences that
an instantaneous silicon nucleation occurs.
/2
3.2.2. Chronoamperometry
According to previous works in molten salts [11,12], chronoam-
•
Part III: Si(IV) ions diffusion control is predominant, and a
slow current decrease is observed following the Cottrell’s law
perometry is
a suitable technique to investigate nucleation
phenomena. The transient curves shape (I = f(t)) at the beginning of
the cathodic polarization allows to determine the nuclei formation
mode, growth and geometry [7,13,14]. A series of chronoamper-
ograms are presented in Fig. 3, at different overvoltages on silver
[
t
8,9], which implies a linear relationship between current and
−
1/2
.Scharifker and Hills [7] established a model based on a
2
dimensional intensity and time: (I/Im) = f(t/tm). For an instan-
taneous nucleation, the curve parameters are described by the
Eqs. (6)–(9) as:
−
1
◦
electrode, in NaF–KF–Na SiF6 (c = 0.24 mol kg ), at 850 C. This
2
0
technique was not found suitable for nucleation studies on carbon
substrates, since this element alloys with silicon at the first stage
of the Si electrodeposition according to the binary diagram [15].
The curves are divided into three parts (cf. inset of Fig. 3):
ꢀ
ꢁ
2
ꢄ
ꢅ
ꢀ
ꢁꢆ ꢇ
2
I
1.9542
t/tm
t
=
1 − exp −1.2564
(6)
Im
tm
with
tm =
and
•
Part I: During this part, corresponding to the double-layer charg-
ing, the first nuclei form on the electrode. The time dependence
on the number of nuclei generated is given by the Poisson’s law
ꢀ
ꢁ
1/2
1
.2564
8c Mꢀ
0
k =
(7)
(8)
N ꢀkD
ꢁ
0
[
7]:
N = N (1 − e−At)
(4)
Im = 0.6382 nFDc (kN )
1/2
0
0
0

A.L. Bieber et al. / Electrochimica Acta 62 (2012) 282–289
285
Fig. 3. Chronoamperograms on silver electrode at various overvoltages in NaF–KF–Na2SiF6 (c0 = 0.24 mol kg−1) at 850 ◦C; auxiliary electrode: Si; reference electrode: Si/Inset:
−
1
◦
Chronoamperogram on silver electrode at ꢂ = −0.04 V in NaF–KF–Na2SiF6 (c0 = 0.24 mol kg ) at 850 C.
2
2
Using the chronoamperograms at different temperatures, the
experimental curves (I/Im) = f(t/tm) are compared to the theoretical
I
tm = 0.1629(nFc ) D
(9)
m
0
2
ones in Fig. 5.
where n is the number of exchanged electrons, F the Faraday’s
The experimental data are in agreement with Eq. (6), prov-
ing that the instantaneous nucleation mode is predominant at
the considered temperature, overvoltage and concentration range.
Based on SEM observations, similar conclusions were obtained by
−1
−3
constant (C mol ), c the solute concentration (mol m ), M the
0
−
1
−3
molar mass (kg mol ), ꢁ the density (kg m ) and D the diffusion
2
−1
coefficient (m s ).
Fig. 4. Plottings of j = f(t1/2) of the chronoamperogram second part at various overvoltages in NaF–KF–Na2SiF6 (c0 = 0.24 mol kg ) at 850 C; working electrode: Ag; auxiliary
−1
◦
electrode: Si; reference electrode: Si.

2
86
A.L. Bieber et al. / Electrochimica Acta 62 (2012) 282–289
Fig. 5. Comparison of the dimensionless experimental data at various temperatures with the theoretical models in NaF–KF–Na2SiF6; working electrode: Ag; auxiliary
electrode: Si; reference electrode: Si.
Table 1
Carleton et al. for the silicon nucleation mode on vitreous carbon
◦
Overvoltage influence on the nuclei favourable sites number in NaF–KF–Na2SiF6
in LiF–KF–K SiF at 750 C [16]. The SEM micrograph of nuclei, pre-
2
6
−1 ◦
(
c0 = 0.24 mol kg ) at 850 C; working electrode: Ag; auxiliary electrode: Si; refer-
sented in Fig. 6 (t = 1.1 s and ꢂ = −0.04 V vs Si), seems to confirm the
instantaneous nucleation mode and the hemispherical geometry of
germs.
ence electrode: Si.
im (A cm−2)
tm (s)
−2
10 im2tm
−11
10 N0
ꢂ (V)
The nucleation sites number N0 were calculated using Eqs. (6)
and (8) at various overvoltages and temperatures; the values are
gathered in Table 1.
−0.030
−0.098
−0.158
−0.236
−0.274
−0.316
−0.393
−0.469
1.55
1.49
1.70
1.66
1.64
1.66
1.79
1.67
2.10
4.79
10.9
14.9
19.5
28.1
42.8
−0.040
−0.050
−0.055
−0.060
0.680
0.298
0.218
0.167
0.116
0.076
An increase in the nucleation sites number with the overvoltage
2
is observed while the product Im tm remains constant, as foreseen
−0.070
−
0.080
in Eq. (9). As previously demonstrated in Section 3.1, the nuclei
growth is limited by Si(IV) ions diffusion, which is either hemi-
spherical or linear. In linear diffusion, the slope ˛ (defined in Eq.
(
5)) follows Eq. (10) whereas the Eq. (11) describes a hemispherical
diffusion:
2
3
−2 −1/2
˛
= −8nFM c0 N D3/2ꢁ
ꢀ
(10)
(11)
0
ꢀ
ꢁ
1/2
M
ꢁ
3/2
a = −nFꢀN (2Dc )
0
0
The N0 values, reported in Table 1, are compared in Table 2
with the one calculated from the slopes of j = f(t
−
1/2
) (Fig. 4) in each
different diffusion conditions, derived from Eqs. (10) and (11).
For the values mentioned in Table 2, this comparison seems to
state that the nuclei’s growth is governed by a linear diffusion and
thus should lead to a dendritic growth.
Table 3 shows the temperature influence for silicon nucleation
on a silver electrode at various overvoltages. It indicates that
the nucleation sites number N0 increases significantly with both
Table 2
Comparison of the nuclei favourable sites number in NaF–KF–Na2SiF6
−
1
◦
(
c0 = 0.24 mol kg ) at 850 C for different diffusion models; working electrode: Ag;
auxiliary electrode: Si; reference electrode: Si.
ꢂ (V)
10−11N0
Linear diffusion
Eq. (10)
Hemispherical diffusion
Eq. (11)
10 N0
−
11
−8
10
N0
−0.03
−0.04
−0.05
2.10
4.79
10.9
2.14
4.44
6.49
0.47
0.98
1.44
Fig. 6. SEM micrograph of silicon nuclei on silver electrode in NaF–KF–Na2SiF6
−1
◦
(c0 = 0.24 mol kg ) at 850 C; ꢂ = −0.04 V vs Si; t = 1.1 s.

A.L. Bieber et al. / Electrochimica Acta 62 (2012) 282–289
287
η/V
-0 .06 -0.055
-0.08 -0.07
-0.05
-0 .045
0- .04
30
29
28
27
26
25
0
100
200
300
400
500
600
700
η-²/V-2
Fig. 7. Linear relationship between the logarithm of the nucleation favourable sites number and square overvoltage inverse in NaF–KF–Na2SiF6 (c0 = 0.24 mol kg−1) at 850 C;
◦
working electrode: Ag; auxiliary electrode: Si; reference electrode: Si.
temperature and overvoltage. In Fig. 7, a linear relationship is
efficiencies are higher than those obtained by Elwell and Rao [20]
◦
evidenced between ln(N ) and the inverse of square overvoltage
in LiF-KF-K SiF at 750 C: 50% as an average current efficiency and
0
2
6
−
2
and obeys Eq. (12) [13,17]:
80% for −25 mA cm current density.
B
ꢂ2
ln(N ) = A −
(12)
0
3.3.1. Operating conditions dependence
Several silicon coatings were obtained in NaF–KF–Na SiF
c0 = 0.47 mol kg ) in the 820–900 C temperature range on carbon
2
6
Knowing the nucleation mode, Fig. 7 allows optimized elec-
trodeposition conditions to be proposed. Indeed, to obtain a smooth
coating structure, the creation of a high number of nuclei is essen-
tial, which is favoured by both high temperatures and overvoltages.
Nevertheless for high overvoltage, a too high electrolyte diffusion
limitation can promote dendritic growth phenomena. Taking into
account these conflicting effects, it was decided to realise silicon
coatings at different overvoltages and temperatures.
−1
◦
(
−2
rod, for applied current densities between −20 and −200 mA cm
.
After frozen salt removal, the coating microstructures were
observed by SEM. The micrographs, presented in Fig. 8, suggest
first that higher temperatures favour somewhat the surface coat-
ing regularity but also that high current densities are detrimental
for this property, which is in agreement with studies from Elwell in
NaF–CaF –SiO2 [19], Boen and Bouteillon in LiF–NaF–Na SiF6 [18]
2
2
and also for other metals deposition, e.g. on tantalum in molten flu-
orides [13]. For applications where high plating rates are required,
the use of pulsed current could help preventing the diffusion limita-
tion and thus lead to more regular coatings at high current densities
3
.3. Silicon electrodeposition
Various runs of silicon electrodeposition on different cathodic
substrates were performed taking into account the conditions
suggested in the previous section. The electrolyses were con-
ducted under intentiostatic conditions as mentioned in references
[
21–23].
EDX analysis of the deposits did not reveal any other compound
than Si that means that the deposit purity is assumed to be at least
9.9%. As for the purity of the obtained Si, which is a key issue for
[
16,18,19].
9
The current efficiency was determined on graphite and silver
the solar industry, recent work from Nohira et al. [24,25] showed
that, in the case of processes involving molten salts solvents at high
temperatures, a careful design of the electrorefiner has to be made
in order to avoid pollution from the cell elements.
electrodes using the mass difference between the cathode before
and after the electrolysis run. It was found to be between 88 and 96%
for low current densities (-20 and -100 mA cm ) resulting in dense
deposits; at high current densities, the deposits were very dendritic
and brittle, which caused silicon losses during the washing treat-
ment: the current efficiency could not be evaluated. These current
−2
3.3.2. Substrate influence
Due to the strong effect of the current density on the regularity
of the coating surface state and the moderate influence of the tem-
−
2
perature, electrodeposition runs were carried out at −20 mA cm
Table 3
◦
at 820 C for 5 h. Fig. 9 shows SEM micrographs of silicon deposits
Temperature influence on the nuclei favourable sites number at different over-
voltages in NaF–KF–Na2SiF6 (c0 = 0.24 mol kg ); working electrode: Ag; auxiliary
−1
on three electrode substrates (Ni, Ag and C ).
v
electrode: Si; reference electrode: Si.
On nickel (a), silver substrate (b) and vitreous carbon (c), a
columnar dendritic coating, a silicon microwire structure and a
sponge-like structure were observed respectively. The smooth and
ꢂ (V)
tm (s)
10−11N0
◦
◦
◦
◦
900 C
850
C
900
C
850
C
◦
−2
dense deposit observed on graphite carbon at 820 C, −20 mA cm
−0.03
−0.04
−0.05
1.55
0.680
0.298
0.568
0.336
0.203
1.46
3.41
12.4
5.15
8.70
14.41
(Fig. 8) is in agreement with Rao et al. work [26] in LiF–KF–K SiF
2 6
◦
at 750 C. According to Carleton et al. [16], the differences between
vitreous and graphite carbon are due to their different structural

2
88
A.L. Bieber et al. / Electrochimica Acta 62 (2012) 282–289
Fig. 8. SEM observations of silicon coatings on carbon at various current densities and temperatures.
properties leading to significant differences of nucleation active
sites.
contrary to other substrates. Our conclusion is that Ni substrate
is not adapted to silicon electrodeposition due to high interdiffu-
sion rate between the compounds which promotes the formation
of intermetallic compounds instead of pure Si.
Fig. 10 presents SEM observations of silicon coatings cross sec-
−
2
tion on various substrates (Ni, Ag, C and Cv) at −20 mA cm
at
◦
8
20 C.
• Silver electrode (b): no alloy formation is observed, in accordance
with the Ag–Si phase diagram [29]. The silicon layer, covering the
Ag surface, becomes progressively more porous and filamentous
and tends to break during the washing treatment.
•
Nickel substrate (a): Ni–Si alloys layers are observed at the
substrate-coating boundary, which seems to promote a very high
adherence of the silicon deposit on the substrate. SEM-EDX anal-
ysis shows two silicide layers under the columnar layer and
XRD measurement on the Ni–Si surface deposit indicates the
presence of Ni Si , NiSi and Si formation. Previous works on
•
•
Graphite carbon (c): a dense SiC layer (∼10 ␮m) was formed by
interdiffusion between silicon deposit and carbon electrode. On
this thin layer, a dense and adherent silicon layer of ∼100 ␮m
thickness was obtained. Similar results on graphite were obtained
by Rao et al. who attributed the deposit compactness to the coa-
lescence of silicon nodules in a three-dimensional growth [30].
Vitreous carbon (d): a rough and poorly adherent deposit was
obtained. This brittle deposit was most probably broken during
the hot pressing carried out for the sample embedding.
3
2
intermetallic compounds Ni–Si preparation by high tempera-
ture solid diffusion evidenced four nickel silicides: Ni5Si, Ni Si,
2
Ni Si , NiSi [27,28]. Borivent et al. [27] showed that the growth
3
2
of Ni Si proceeds by needles: it could explain that in the present
3
2
work, the silicon coating growth over Ni Si needles is columnar,
3
2
Fig. 9. SEM observations of silicon deposit surfaces at −20 mA cm− at 820 C for 5 h on various substrates: (a) nickel; (b) silver; (c) vitreous carbon.
2
◦

A.L. Bieber et al. / Electrochimica Acta 62 (2012) 282–289
289
−
2
◦
Fig. 10. Cross section SEM observations of silicon deposits at −20 mA cm at 820 C for 5 h on various substrates: (a) nickel; (b) silver; (c) graphite carbon; (d) vitreous
carbon.
4
. Conclusions
[4] P. Chamelot, P. Taxil, B. Lafage, Electrochim. Acta 39 (17) (1994) 2571.
[
5] P. Taxil, P. Chamelot, L. Massot, C. Hamel, J. Min. Metall. B 39 (1–2) (2003)
77.
1
The silicon nucleation mechanism was studied in
a
[
6] A.L. Bieber, L. Massot, M. Gibilaro, L. Cassayre, P. Chamelot, P. Taxil, Electrochim.
Acta 56 (2011) 5022.
NaF–KF–Na SiF salt mixture on silver electrode, using mainly
2
6
[7] B. Scharifker, G. Hills, Electrochim. Acta 28 (1983) 879.
[8] L. Massot, P. Chamelot, F. Bouyer, P. Taxil, Electrochim. Acta 47 (2002) 1949.
[9] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applica-
tions, John Wiley and Sons, New York, 2001.
chronoamperometry: the silicon nuclei are generated instanta-
neously with a linear diffusion-controlled hemispherical growth.
Moreover, the nucleation sites number N0 increases with the
temperature and the overvoltage. Then, galvanostatic electrolyses
were performed by varying temperature and current density on
a carbon rod. SEM observations showed that high temperature
and low current density lead to a smoother deposit, with a purity
[10] X.H. Xu, C.L. Hussey, J. Electrochem. Soc. 139 (1992) 1295.
11] L. Massot, P. Chamelot, P. Palau, P. Taxil, Electrochim. Acta 50 (2005) 5408.
12] L. Massot, P. Chamelot, F. Bouyer, P. Taxil, Electrochim. Acta 48 (2003) 465.
[13] P. Chamelot, B. Lafage, P. Taxil, J. Electrochem. Soc. 143 (5) (1996) 1570.
[14] L. Legrand, A. Tranchand, R. Messina, J. Electrochem. Soc. 141 (2) (1994)
[
[
378.
(
(
measured by EDX) higher than 99.9%. The cathodic substrate tests
Ni, Ag, C and Cv) allowed concluding that graphite carbon is a
[
[
15] R.W. Olesinski, G.J. Abbaschian, Bull. Alloy Phase Diagrams 5 (1984) 486.
16] K.L. Carleton, J.M. Olson, A. Kibbler, J. Electrochem. Soc. 130 (4) (1983) 782.
well-adapted cathodic material due to the deposit adherence and
structure; on vitreous carbon, the deposits are dendritic; on silver
substrate, the deposits are poorly adherent while on nickel, many
alloys were formed, hindering pure silicon deposition. When the
current density increases, the coating becomes rougher. At high
current densities, the silicon ions diffusion limitation strongly
influences the silicon growth, leading to a more dendritic deposit.
[17] T. Erdey-Gruz, M. Volmer, Z. Phys. Chem. 157 (1931) 165.
[
[
[
18] R. Boen, J. Bouteillon, J. Appl. Electrochem. 13 (1983) 277.
19] D. Elwell, Solar Energy Mater. 5 (1981) 205.
20] D. Elwell, G.M. Rao, J. Appl. Electrochem. 18 (1) (1988) 15.
[21] N. Ibl, J.C. Puippe, H. Angerer, Surf. Technol. 6 (4) (1978) 287.
[
22] J.C. Puippe, Dissertation, ETH, Zürich, Switzerland (1978).
[
23] P. Chamelot, P. Taxil, D. Oquab, J. Serp, B. Lafage, J. Electrochem. Soc. 147 (2000)
4131.
[24] K. Yasuda, T. Nohira, R. Hagiwara, Y. Ogata, Electrochim. Acta 53 (2007) 106.
[
25] K. Kobayashi, T. Nohira, R. Hagiwara, K. Ichitsubo, K. Yamada, ECS Trans. 33 (7)
2010) 239.
(
References
[
[
26] G.M. Rao, D. Elwell, R.S. Feigelson, J. Electrochem. Soc. 128 (8) (1981) 1708.
27] D. Borivent, J. Paret, B. Billia, J. Phase Equilib. Diff. 27 (6) (2006) 561.
[
[
[
1] M.A. Green, Solar Energy 76 (2004) 3.
[28] K.N. Tu, G. Ottaviana, H. Föll, J. Appl. Phys. 54 (2) (1983) 758.
[29] P.Y. Chevalier, Thermochim. Acta 130 (1988) 33.
[30] G.M. Rao, D. Elwell, R.S. Feigelson, Surf. Technol. 13 (1981) 331.
2] D. Elwell, R.S. Feigelson, Solar Energy Mater. 6 (1982) 123.
3] T. Ulset, S. Julsrud, L. Cassayre, P. Chamelot, L. Massot, P. Taxil, T.L. Naas, PCT
Int. Appl. WO 2008156372 (2008).