Electrodeposition of crystalline silicon directly from silicon tetrachloride in ionic liquid at low temperature

Article abstract of DOI:10.1039/c5ra23085c

Crystalline silicon (Si) is widely used in modern electronics. Si is commonly produced through a series of energy-intensive reactions (>700 °C). It is thus urgent and significant to explore more economically and environmentally-benign synthetic strategies for crystalline Si at low temperature. In this contribution, we report an efficient method to prepare crystalline Si from silicon tetrachloride at the low temperature of 100 °C with an ionic liquid (IL) as electrolyte. Physicochemical characterization revealed that as-deposited crystalline Si with a diamond cubic crystal structure exhibited a dominant (111)-orientation. Moreover, in-depth insights into the growth mechanism of crystalline Si was shed light upon herein. Furthermore, the smart electrodepositing platform of crystalline Si from ILs would open up a new avenue for low-temperature metallurgy of Si.

Full text of DOI:10.1039/c5ra23085c

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Electrodeposition of crystalline silicon directly from silicon tetrachloride in
ionic liquid at low temperature
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Junling Zhang, Shimou Chen, Haitao Zhang, Suojiang Zhang*, Xue Yao and Zhaohui Shi
Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process and
Engineering, State Key Laboratory of Multiphase Complex Systems, Institute of Process
Engineering, Chinese Academy of Sciences, Beijing 100190, China
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*Corresponding author: sjzhang@ipe.ac.cn
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Abstract
Crystalline silicon (Si) is widely used in modern electronics. Si is commonly produced through a
series of energyꢀintensive reactions (> 700 °C). It is thus urgent and significant to explore more
economic and environmentꢀbenign synthetic strategies of crystalline Si at low temperature. In this
contribution, we report an efficient method to prepare crystalline Si from silicon tetrachloride at
low temperature of 100 °C with ionic liquid (IL) as electrolyte. Physicochemical
characterizations revealed that asꢀdeposited crystalline Si with diamond cubic crystal structure
exhibited a dominant (111)ꢀorientation. Moreover, inꢀdepth insights into the growth mechanism
of crystalline Si was shed light upon herein. Furthermore, the smart electrodepositing platform of
crystalline Si from ILs would open a new avenue for lowꢀtemperature metallurgy of Si .
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Keywords: Crystalline silicon; Ionic liquid; Lowꢀtemperature electrodeposition; Liquid metal
electrode
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1. Introduction
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Si is the most important semiconductor in optoelectronic technologies because of its
abundant existence, chemical stability, and excellent optical and electronic properties.1ꢀ3
Therefore, it has been the subject of many research groups to obtain the simple and economic
production process of Si. The current crystalline Si production process such as Siemens method
often involves a series of energyꢀintensive, highly polluting carbothermal reduction reactions. In
recent years, continuous increased desire for lowerꢀcost Siꢀbased solar photovoltaic devices has
motivated the researchers to explore an alternative cheaper and greener technique for Si
extraction.
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Electrodeposition has been identified as a potential alternative route for the preparation of
metals and semiconductors since it can be easily controlled, and is relatively clean and cheap.4, 5
Electrodeposition of Si has been studied in various solution media such as highꢀtemperature
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molten salts (fluoride molten salt, calcium chloride, etc.),6,
organic solvents,8,
and
roomꢀtemperature ionic liquids (ILs).10ꢀ13 Si can be well electrodeposited as crystalline products
from molten salts at high temperatures (> 700 °C), and that often results in difficult operations
and high energy consumption. Si can also be electrodeposited in organic solvents and ILs at low
temperature. However, the asꢀprepared Si from low temperature electrodeposition is always
amorphous and requiring additional high temperature thermal annealing.14 Therefore, the
incompatibility of low temperatures and a crystalline product have thus severely limited the
developing of Siꢀelectrodeposition technology. Recently, it was reported by Maldonado et al. that
crystalline Si can be electrodeposited by using a liquidꢀmetal working electrode at 80 – 100 °C.15
Liquidꢀmetal electrode can act as both the electron source for reducing dissolved species in
solution and the solvent for reꢀcrystallization. This method for Si electrodeposition overcomes the
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long standing challenge and have substantial technological impact. Organic solvent propylene
carbonate (PC) was employed as electrolyte in the research before.15 There are many advantages
in the use of conventional organic solvents such as being costꢀeffective and easy to control.
However, the system suffers from high volatility which results in quick evaporation in higher
temperatures. From the practical application point of view, it is still needed to develop novel
electrolytes with improved physicochemical features such as low volatility and good thermal
stability.
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ILs behave very differently to traditional molecular liquids when they are used as solvents.
ILs are usually nonꢀvolatile and have high thermal stability in most cases, which make them can
be used over a wide temperature range.16ꢀ22 Everything has two sides, ILs also have some main
drawbacks such as high viscosity and cost of production. The high viscosity often causes some
difficulties in their handling. Bis(trifluoromethylsulfonyl)imide anion (TFSIꢀ) is one of the most
popular anions in ILs chemistry which is widely used because it is known to produce ILs with a
low melting point and low viscosity (Table S1, ESI†). By comparing cyclic voltammetry (CV)
performance of SiCl4 in three different ILs (Fig. S1, ESI†), Triꢀ1ꢀbutylmethylammonium
bis((trifluoromethyl)sulfonyl)amide ([N4441][TFSI]) IL was adoped in this study.Viscosity ꢀ
temperature dependency of [N4441][TFSI] was shown in Table S2, ESI†. The viscosity of
[N4441][TFSI] decreases to 30.6 mPa·s with the temperature increasing to 80 ºC. Many pioneering
works have been shown that ILs are suited well for the electrodeposition of Si. Endres et al.
obtained semiconducting Si films on highly oriented pyrolytic graphite (HOPG) and Au (111)
solid electrodes in ILs such as 1ꢀbutylꢀ1ꢀmethylpyrrolidinium bis(trifluorom ethylsulfonyl)amide
([Py14] TFSI).10, 13, 23 By using EQCM measurements, Komadina et al. also studied the
electrodeposition of Si from trimethylꢀnꢀhexyl ammonium bisꢀ(trifluoromethylsulfonyl)
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amide([TMHA][TFSI]) ionic liquid, and they found that the deposition of Si occurs via a
fourꢀelectron reduction step.24 It was also reported by Katayama and coꢀworkers that a thin Si
layer can be electrodeposited in 1ꢀethylꢀ3ꢀmethylimidazolium hexafluorosilicate([EMIM][SiF6]).
However, upon exposure to air the deposits reacted completely to SiO2, which makes it difficult
to make sure whether the deposit was semiconducting or not.25 To the best of our knowledge,
there is still no report on crystalline Si directly electrodeposited from ILs at low temperature.
Herein, we reported data demonstrating the use of hydrophobic [N4441][TFSI] IL and Ga liquid
metal electrode as a platform for direct electrodeposition of crystalline Si from the dissolved
SiCl4 precursor under benign conditions.
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2. Experimental
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Materials and chemicals Lithium bis(trifluoromethylsulfonyl)imide was purchased from SCM
Industrial Chemical Co. Ltd. Butyltrimethylammonium chloride was purchased from Anhui
benma apex technology Co. Ltd. Silicon tetrachloride (SiCl4, 99.998%) was purchased from Alfa
Aesar. Gallium (Ga, 99.99%) was purchased from Aladdin. All the chemicals used in this work
were purchased commercially with analytic grade and used as received.
Preparation and characterization of [N4441][TFSI] [N4441][TFSI] was prepared by mixing the
aqueous
solutionsofbutyltrimethylammonium
chloride
(1
mol)
and
lithium
bis(trifluoromethylsulfonyl)imide (1.05 mol) at room temperature (RT) under magnetic stirring
for 3 h. The colorless IL phase at the lower layers was separated with a separatory funnel. The
separated IL was washed with ultrapure water for several times to remove the reactants. The
[N4441][TFSI] was dried under vacuum for 24 h at 100 °C to remove the volatiles and reduce
water contents to under 10 ppm (by KarlꢀFischer titration). The processed [N4441][TFSI] IL was
stored in a sealed bottle in an argonꢀfilled glove box (MIKROUNA Universal, O2 below 1 ppm,
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H2O below 1 ppm). The viscosity of [N4441][TFSI] was measured with an automated
microviscometer (Anton Paar AMVn, Anton Paar Co., Austria) with relative expanded
uncertainty Ur ( η ) = 0.005 (0.95 level of confidence). Calibration was performed using viscosity
standard oils (No. H117, Anton Paar Co.). The temperature of the sample was maintained to ±0.1
K via a builtꢀin precise Peltierthermostat.
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Electrodeposition of Si A single compartment electrochemical cell with three electrodes (Fig. S2,
ESI†) was used to probe the electrochemical window and electrodepositing behavior of Si. A
quartz beaker of 50 ml was used as the electrochemical cell. 400 ꢁL Ga (l) pool was employed as
the working electrode (ca. 0.785 cm2 active area). Pt wire (99.99%, 0.5 mm in diameter) coated
by a heat shrink tube was dipped into the liquid gallium for electrical connection. A Pt plate
(99.99%, 10 mm × 20 mm × 0.1 mm) was used as counter electrode. Ag electrode (silver wire
with 99.99% in purity and 0.5mm in diameter) was used as quasiꢀreference electrode in CV
studies. The counter electrode (Pt plate) was mechanically polished with 0.05 ꢁm alumina to a
mirror finish and ultrasonically washed in ultrapure water for 3 min, rinsed out with ethanol and
finally dried with cold N2 flow. Electrochemical measurements were performed using a CHI660
Electrochemical workstation (Shanghai Chenhua, China). All the electrochemical experiments
were carried out in the argonꢀatmosphere glove box.
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Material Characterizations Xꢀray photoelectron spectra (XPS) were recorded with an Xꢀray
photoelectron spectrometer (Thermo Fisher Scientific, Escalab 250Xi), utilizing
a
monochromatic AlꢀKα Xꢀray source, hybrid optics and a multiꢀchannel plate and delay line
detector coupled to a hemispherical analyzer. All the spectra were recorded using an aperture slot
of 300 ꢁm × 700 ꢁm, and high resolution spectra were collected with pass energy of 20 eV. The
pressure in the analysis chamber was typically 2 × 10−9 Torr during data acquisition. All binding
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energies were referenced to C 1s (284.5 eV) binding energy for data analysis. Raman spectra
were obtained on a LabRAM HR800 (514 nm wavelength, 4 mW power, Horiba JobinYVon,
France). Ramanspectroscopy was simultaneously performed in the ambient atmosphere at RT.
Bulk powder Xꢀray diffractograms (XRD) were collected with Empyrean, PANalytical B. V.
equipped with a Cu Kα source (λ = 1.5406 Å). Scanning electron micrographs (SEM) were taken
at 5 kV with a Hitachi SU8020 SEM equipped with an SE/BSE detector. Energy dispersive
spectra were taken at 20 kV with an Auger electron spectroscopy detector (AES, JAMPꢀ9500F,
accelerating voltage: 10 kV, JEOL Ltd.). Highꢀresolution transmission electron microscopy
(HRTEM) and selected area electron diffraction (SAED) were performed with a JEMꢀ2100F
UHRTEM equipped with a LaB6 source operated at 200 kV.
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3. Results and Discussion
3.1. Electrodeposition of Si
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In order to determine the cathodic limit of IL stability and to study Si reduction in SiCl4/
[N4441][TFSI], CV was performed. Fig. 1 shows the cyclic voltammetric responses for SiCl4
reduction on Ga liquid metal electrodes at the first negative scan at 100 °C. For the pristine IL,
irreversible reduction of the organic cation can be observed at the potential ꢀ2.7 V (vs. Ag QRE).
A rather weak voltammetric wave (C1) was also observed at approximately ꢀ1.39 V which
probably was caused by cleavage of the S–N bond in the anion.26 The voltammetry performed in
SiCl4 containing ionic liquid (0ꢀ500 mM) displayed two main reduction peaks (C2 and C3). The
observed cathodic wave (C2) at ꢀ1.22 V can be attributed to the underpotential deposition Si on
Ga electrode. The peak at round ꢀ1.86 V followed by a more negative wave indicated the bulk
deposition. For 250 mM SiCl4 containing IL solution, the voltammetric response showed a clear
diffusionꢀlimited wave at ꢀ2.3 V (vs. Ag QRE), which could be separated from the onset of
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cathodic solvent decomposition. The present voltammetry data differs from the results reported
for the electroreduction of SiCl4 in propylene carbonate.15 In the latter process,
diffusionꢀcontrolled peak was observed only at lower SiCl4 concentration and lower temperature,
such as 100 mM at RT. This indicates that, compared with propylene carbonate organic system,
IL [N4441][TFSI] has better electrochemical stability.
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Potentiostatic electrolysiswas used to electrodeposit Si onto liquid metal electrode at ꢀ2.3 V
(vs. Ag QRE). Fig. 2 shows currentꢀtime curves of the electrodeposition process with different
electrodepositing time. Formation and growth of the Si crystals in deposition process could be
generally divided into three sections: rapid nucleation and growth in the first ten minutes (Part I),
crystal growth in the next two hours (Part II) and a period of slowꢀgrowing (Part III). In fact, Si
electrodeposition differs from metal deposition, the deposited layer is semiconductor which has
lower conductivity than metal electrode. The reduced conductivity of working electrode surface
would complicate the electron transfer through the deposit and prevent a continuous deposition
reaction.27 This could explain that the rapidly decrease of current with time, as observed in Fig. 2.
Morphology of the deposited Si was characterized by SEM. Interestingly, two sides (see Fig. 3a)
of the deposited Si layer have completely distinct morphological properties. As shown in Fig. 3b
and Fig. 3c, side A (face to electrolyte) has a relatively smooth morphology and side B (face to
liquid metal electrode) is a polycrystalline layers composed by gathering of nanoꢀsized crystals.
TEM and SAED were performed in order to determine the structure difference of the two sides.
TEM image and SAED pattern of the side A were shown in Fig. 3d and the inset of Fig. 3d,
respectively. Diffraction pattern in Fig. 3d shows that side A, which can be quickly formed at the
beginning of the reaction, is amorphous in structure. Fig. 3e shows the TEM image of the crystal
grain. Lattice fringes could not be discernable for the large size of the grain. SAED was also used
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to determine the structure of particles on side B. The observed diffraction pattern showed in the
inset of Fig. 3e is consistent with a diamond cubic lattice as expected for a single crystal of Si.
TEM and SAED patterns of Si particles and films that electrodeposited for 2 h by this method
were also checked (Fig. S3, ESI†). The diffraction characteristics of the deposited samples are
similar to that showed in Fig. 3.
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The crystal growth on side B was investigated by electrodepositing for different lengths of
time. Fig. 4 shows a series of SEM images of Si deposits on side B. High magnification image
shows that Si deposit is spherical silicon particles range from 100 to 500 nm on the amorphous
film at the first 10 min (Fig. 4a). Coneꢀshape particles with dominant size of 0.5 – 0.8 ꢁm were
formed in the next 1 h (Fig. 4b). Every particle showed sharp facets after the electrodeposition
for 4 h (Fig. 4c), and gradually grown to polyhedrons, the size is about 2 – 3 ꢁm in 8 h (Fig. 4d).
The increasing of size and sharpness of the crystals were attributed to the using of liquidꢀmetal
electrodes which can act as both an electron source for reducing dissolved species in solution and
a solvent for reꢀcrystallization, and this process was named electrochemical liquidꢀliquidꢀsolid
(ecꢀLLS) process. 15
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3.2. Effect of working electrode disturbance on the morphology of Si deposits
SiCl4 can be electroꢀreduced to Si at the surface of solid metal electrode and producing
amorphous film has been proved previously.13, 14 Crystal Si can be electrodeposited from liquid
metal electrode by an ecꢀLLS process also was reported.15 However, deposited Si with two
different side was observed in this study and the crystal layer seems grow on the amorphous film.
In order to study the catalyst or induced effects of the amorphous film on reꢀcrystallization of Si
in the liquid metal,28, 29 a magnetic stick was adopted into the liquid metal electrode pool in the
electrodeposition process which could rupture the initially formed amorphous film on the surface
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of the liquid metal electrode. SEM images of the asꢀelectrodeposited Si layer are shown in Fig. 5.
Similar to the deposits on static working electrode, uniformly sized crystal particles were
consistently observed on an amorphous film with total widths of 1 – 2 ꢁm (Fig. 5a, b). However,
crystal particles produced on stirred working electrode surface grew along a certain line instead
of closely packed, showed meshꢀlike distributing on amorphous film (Fig. 5c, d).
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The nucleation and growth of Si requires a large amount of energy. Si likely to crystallization
and growth in the solid film on the electrode surface rather than inner of the liquid metal
electrode because the amorphous Si has nuclei effect. Furthermore, when energy of amorphous
surface distributed unevenly, Si in liquid electrode tends to crystallize along the higher energy
lines. Amorphous film deposited initially on the liquid metal electrode was cracked and wrinkled
by magnetic stirring. In the effect of external electric field, energy distribution on the wrinkled
electrodeposited films became more uneven. Crystal preferentially grew on high energy points of
the wrinkled films, consequently the Si crystals with continuous net structure were formed. SEM
and corresponding element mapping images of Si deposits prepared on the liquid metal electrode
without (Fig. S4, ESI†) and with magnetic stirring (Fig. S5,ESI†) are all showed that crystal Si
particles are grown on thin film of Si. Furthermore, both amorphous and crystal phases can be
observed in the Raman spectrum of the asꢀprepared Si (Fig. S6, ESI†). From the above
discussion, we can reach that amorphous Si film has inductive effect on Si reꢀcrystallization in
the ecꢀLLS process, then the polyhedron crystal particles with sharp facets formed. The lowest
temperature demonstrated here (100 °C) significantly favors for better electrodeposition of
crystalline Si over both of the previous reports.30, 31
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3.3. Crystal structures and composition analysis of Si deposits
Crystallographic orientation of the deposited Si in the ionic liquid was determined using
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XRD. As shown in Fig. 6a, the asꢀdeposited sample at 100 °C yielded sharp diffraction peaks at
2θ = 28.4 °, 47.2 ° and 56.0 ° which are perfectly matched with the diffraction from the (111),
(220) and (311) orientations of crystalline Si with the diamond cubic crystal structure,
respectively. The result shows that the deposited Si were polycrystalline without detectable
impurity phase and having the preferred crystallographic growth orientation of (111). This
crystallization is also confirmed by Raman spectrum of the freshly deposited Si sample. Fig. 6b
displays the Raman spectra obtained at 514 nm. High power laser can facilitate crystallization of
amorphous Si and affect the judgment of material structure. Spectra obtained by focusing the
laser spot with different power from 0.4 to 4.0 mW on the asꢀdeposited Si are compared in Fig.
6b. There is an obvious Raman peak in every curve inspired by different laser powers, which
represent the crystalline Si was formed at the electrodepositing process. Raman peaks range from
501 to 508 cmꢀ1 is slightly downꢀshifted than that of bulk Si at 520 cmꢀ1. Furthermore, the peaks
are also redꢀshifted and dissymmetrical broadened with the increasing of laser power which
demonstrated phonon confinement of the crystal Si that may arise from microꢀsize of
crystalline.32
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Energyꢀdispersive spectrometry (EDS) elemental mapping image (Fig. 7b) and EDS
spectrum (Fig. 7c) revealed that both the microꢀcrystal grain and amorphous film are mainly the
formed Si. The EDS spectrum of the Si deposit also suggests that the deposited Si is quite pure
and the amounts of other elements, such as, C and Ga, are related to the process of deposits
separation. The successful reduction of silicon tetrachloride to elemental silicon (Si0) was
confirmed by Xꢀray photoelectron spectroscopy (XPS) of the Si deposit (Fig. 7b). The presence
of SiO2 in the deposits should be due to the surface oxidization of the Si by air during the transfer
of the prepared sample.
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3.4. Growth mechanism of electrodeposited crystal Si
The mechanism for the direct electrodeposition of crystalline Si on liquid electrode from ILs
is complicate, and still unknown so far. We just can propose some possible process by referring to
the experimental results and the literatures. As illustrated in Fig. 8, the formation of crystalline Si
is divided into 3 stages. At the first stage, oxidized precursor (SiCl4) was electroꢀreduced to the
fully reduced state (Si) at the surface of liquid metal electrode and producing purely amorphous
Si film. As the deposition going, the thickness of the amorphous layer quickly increases, making
the surface of liquid electrode more resistant and deposite current dropped rapidly (see Fig. 2).
The formation of amorphous Si layer by electrodepositing from SiCl4/IL solution has been
investigated at solid electrodes previously.10, 13, 14, 23, 33, 34 The difference is that, when a liquid
metal electrode Ga (l) is used, the initially produced Si could be diffused into the liquid Ga phase.
According to the published metallurgical data,35 the solubility of Si in Ga between RT and 100 °C
ranges from 10ꢀ8 to 10ꢀ6 at.%. At stage II, Si at the interface of amorphous layer and liquid
electrode dissolved into Ga (l) and reached saturation and supersaturating conditions with
continually reduction of SiCl4 at the electrode surface. When a critical supersaturating conditions
reached, phase separation of Si(s) from Ga (l) followed by nucleation would occur. Solubility of
Si in Ga (l) is low, amorphous Siꢀinduced crystal separating is more inclined to occur on the
interface of amorphous Si and Ga (l). At stage III, as continuous crystallization and oriented
attachment going on the inner surface, the polyhedron crystal particles with sharp facets formed.
In other words, initially formed amorphous Si layer on the electrode surface might induce the
crystallization of Si. Finally, perfect large single crystals were obtained successfully by
dissolution and reꢀprecipitation of Si in the stage III. To check the universality of this method for
preparation of cystalline Si, NꢀPropylꢀNꢀmethylpiperidinium bis((trifluoromethyl)sulfonyl)imide
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([PP13] [TFSI]) was also used as electrolyte for the deposition of Si. Crystaline Si (Fig. S7,ESI†)
also can be successfully prepared in [PP13] [TFSI] at 100 °C.
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4. Conclusions
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In conclusion, we successfully prepared crystalline Si via an appealing lowꢀtemperature
electrodeposition strategy. Herein, the IL of [N4441][TFSI] and Ga (l) were utilized as electrolyte
and liquidꢀmetal electrode, respectively, which are the key factors to fabricate crystalline Si at the
mild operating conditions (atmospheric pressure and 100 °C). The [N4441][TFSI] rendered it
possible to electrodeposit at low pressure for a long time. Liquid Ga acted as a separate phase for
reꢀcrystallization of Si at low temperature. Growth mechanism of crystalline Si was tentatively
proposed including first formation of solution of Si in liquidꢀmetal Ga, and following nucleation
and growth of crystal Si on the reduced amorphous Si film. We also envision that the synthetic
approach can be easily generalized to fabricate highꢀpurity Si directly from SiCl4 or SiHCl3 just
with a single step. More significantly, it offers an appealing avenue for scalable, economic and
green fabricating process for lowꢀtemperature Si metallurgy.
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Acknowledgements
This work was financially supported by the National Basic Research Program of China (No.
2015CB251401), the National Nature Science Foundations of China (No. 51274181, 21276257),
Beijing Municipal Natural Science Foundation (No. 2132054).
†Electronic Supplementary Information (ESI) available: viscosity of ILs, CV in different ILs,
schematic illustration of the electrolytic cell, TEM and SAED of the deposits, EDS mapping
images, Raman spectrum of Si deposits and SEM images of the deposits prepared from
[PP13][TFSI] IL in this study.
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Figures and Captions
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Fig. 1 Cyclic voltammetric response of the Ga (l) electrode in the IL of
triꢀ1ꢀbutylmethylammonium bis((trifluoromethyl)sulfonyl)amide ([N4441][TFSI]) with various
concentrations of SiCl4 (a, 0 mM; b, 250 mM; c, 500 mM) in solution at 100 °C under Ar. Scan
rate is 0.05 V sꢀ1
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Fig. 2 Currentꢀtime curves of the potentiostatic electrodeposition process at ꢀ2.3 V (vs. Ag QRE)
for different time (1 h, 2 h, 4 h, 6 h and 8 h) at 100 °C. The concentration of SiCl4 is 250 mM
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Fig. 3 (a) Illustration of the electrodeposited silicon, (b) SEM micrograph of side A, (c) SEM
image of the side b, (d) TEM and SAED (the inset) of side A and (e) TEM and SAED (the inset)
of the side B. The deposits were prepared at ꢀ2.3 V (vs. Ag QRE) for 4 h and maintained at 100
°C. The concentration of SiCl4 is 250 mM
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Fig. 4 SEM images of the deposits prepared at ꢀ2.3 V (vs. Ag QRE) with different depositing
time (a, 10 min; b, 1 h; c, 4 h; d, 8 h) at 100 °C. The concentration of SiCl4 is 250 mM
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Fig. 5 SEM images of the deposits prepared on the liquid metal electrode (a, b) without and (c, d)
with magnetic stirring (200 rpm). Crystalline Si grown at ꢀ2.3 V (vs. Ag QRE) in the IL of
[N4441][TFSI] containing of 250 mM SiCl4 at 100 °C for 8h
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Fig. 6 (a) XRD pattern and (b) Raman spectra of the silicon deposit grown at ꢀ2.3 V (vs. Ag QRE)
at 100 °C in the IL of [N4441][TFSI] containing of 250 mM SiCl4
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Fig. 7 (a) SEM and (b) corresponding EDS elemental mapping images, (c) EDS and (d) XPS
spectra of the Si deposited at ꢀ2.3 V (vs. Ag QRE)
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Fig. 8 Schematic illustration of crystalline Si formed on the liquid metal electrode from ILs by
electrodeposition
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Graphical Abstract
Crystalline silicon was fabricated directly from silicon tetrachloride in ionic liquid at
low temperature of 100 °C. SEM, TEM and SEAD revealed that as-deposited
crystalline Si with diamond cubic crystal structure.