Electrodeposition of Si from an Ionic Liquid Bath at Room Temperature

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Electrodeposition of Si from Ionic Liquid Bath at Room Temperature in Presence of Water Nisarg K. Shah, Ranjan Kumar Pati, Abhijit Ray, and Indrajit Mukhopadhyay Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03621 • Publication Date (Web): 30 Jan 2017 Downloaded from http://pubs.acs.org on January 31, 2017

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Electrodeposition of Si from Ionic Liquid Bath at Room Temperature in Presence of Water Nisarg K. Shah, Ranjan Kumar Pati, Abhijit Ray and Indrajit Mukhopadhyay* Department of Solar Energy, Pandit Deendayal Petroleum University, Raisan, Gandhinagar382007, Gujarat, India

ABSTRACT: Electrochemical deposition of Si has been carried out in ionic liquid medium in the presence of water in a limited dry nitrogen environment on highly oriented pyrolytic graphite (HOPG) at room temperature. It has been found that the presence of water in ionic liquids does not affect the available effective potential window to any large extent.

Silicon has been

successfully deposited electrochemically in the over potential regime in two different ionic liquids, namely BMImTf2N and BMImPF6 in presence of water. While Si thin film has been obtained from BMImTf2N, only distinguished Si crystals protected in ionic liquid droplets have been observed from BMImPF6. The most important observation of the present investigation is that, the Si precursor, SiCl4, instead of undergoing hydrolysis, even in the presence of water, coexisted with ionic liquids and elemental Si has been successfully electrodeposited.

KEYWORDS: Electrodeposition, silicon, ionic liquid, thin film, XPS

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INTRODUCTION Silicon is one of the most important semiconductors as it has been widely used in varieties of applications such as fabricating various electronic devices, Solar PV cells and negative electrode material for Li on battery.1-3 At room temperature it has in-direct band gap of 1.1 eV. It has higher optical efficiency than other commercially used semiconductors such as GaAs and CdTe. However, the quantum efficiency of silicon (Si) is comparatively low.4 Currently, silicon thin films are deposited by methods such as chemical vapor deposition (CVD) and plasma enhanced chemical vapor deposition (PECVD).5 The major disadvantages of above mentioned techniques are the economic viability, complexity, material consumption and slow rate of deposition at large scale. Electrodeposition is an elegant technique compared to all other above mentioned methods because it is simple, versatile and offers less complex setup at very low cost. The target structure and size can be easily influenced by known parameters such as voltage, current density and concentration of the solute.

The consumption of target material is less in case of

electrodeposition compared to other techniques.6 deposit silicon electrochemically since 1980

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Several studies have been carried out to

because it is easily scalable and in principle it

allows to deposit any semiconductor on any conducting substrate.8 Unlike many other semiconductors, electrodeposition of Si in aqueous medium is very difficult due to large cathodic potential of Si precursors compare to water (-0.83 V vs NHE),

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which

results in the hydrolysis of water and formation of H2. Also due to hydrolysis of water, Si immediately reacts with oxygen (O2) and forms silicon dioxide (SiO2). Recently, in one of the studies, silicon was electrodeposited using aqueous medium, however, it was not clear whether


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Si was oxidized or not.10

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Due to hydrolysis of water in aqueous medium, many studies on

electrodeposition of Si have been carried out either using organic solvent or high temperature molten salt.

Silicon was successfully electrodeposited using molten salt at very high

temperature.11-14

The main disadvantage, in all such studies, was the requirement of high

temperature. Recently, electrodepostion of silicon using room temperature molten salt has also been reported.

The resulted Si was in the form of SiO2.15 Similarly, several studies on

electrodepostion of silicon using organic solvent have been reported 16-22 and in all these studies, the resulted Si was oxidized. Due to oxidation of Si in organic media and requirement of high temperature in molten salt, in recent years, focus has been shifted to deposit Si electrochemically using room temperature ionic liquid (RTIL). Ionic Liquid (IL) is defined as ionic material having melting point below 100 oC.23 Ionic liquids have many advantages such as very low vapor pressure, large electrochemical window as compared to water, avoidance of hydrolysis of water and formation of hydrogen gas, and stability in air and water. Due to these advantages, in recent years, there has been remarkable progress in synthesis of ILs and are easily available from market.24 In recent times, ILs have been investigated thoroughly for their effectiveness as solvent for various electrochemical deposition processes. Very few studies have been carried out to deposit Si electrochemically in air and water stable IL.25-30 However, to avoid effect of water, all the experiments are carried out using pure ionic liquid in a dry and inert atmosphere where oxygen and moisture level is in the range of 1-2 ppm. Due to unwanted interference of water content in IL during electrochemical investigation, not a single attempt was made, to the best of our knowledge, to investigate effect of water content (with permissible limit) in IL while depositing Si electrochemically. Hence, in the present investigation, we focus on the particular interest of


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looking at the possibility of electrodeposition of Si from ionic liquids containing substantial amount of water (in the range of 3000-8000 ppm) on highly oriented pyrolytic graphite (HOPG) substrate in an N2 filled glove box having moisture and oxygen content of 20 ppm. The choice of two ionic liquids was based on their water content at a relative humidity of 90 %. EXPERIMENTAL SECTION The

electrodepostion

of

silicon

was

performed

methylimidazolium-bis(trifluoromethylsulfonyl)

imide

using

ionic

([BMIm]Tf2N)

liquids

1-bulyl-3

and

1-bulyl-

3methylimidazolium-hexafluoro phosphate, (BMImPF6), purchased from Sigma Aldrich, without further purification. The ionic liquids were kept in a nitrogen-filled glove box having oxygen and moisture of 20 ppm. The water content of both the ionic liquids was determined by Karl Fisher titration using absolute methanol (99.99%) as solvent. A teflon electrochemical cell with three electrode configuration was used for electrochemical experiments. The working electrode, highly oriented pyrolytic graphite (HOPG, ZYH Ceramics) of 12x12 mm2 area and 2 mm thickness was sealed to a specially designed teflon cell31 with a teflon coated silicon O ring (effective geometric area of 3.6 x 10-5 m2).

HOPG was cleaved freshly just before each

experiment. A Pt ring and Pt wire were used as counter and reference electrode, respectively. Ptwires were thoroughly cleaned using hydrochloric acid (HCl) and deionized water (DI) before the

experiment.

Electrochemical

experiments

namely

cyclic

voltammetry

and

chronoampereometry were performed using potentiostat / galvanostat electrochemical workstation (PGSTAT 302N, AUTOLAB).

All electrochemical experimental studies were

carried out in indigenously made N2 filled glove box having water and oxygen concentration of 20 ppm.


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The ionic liquids, BMImTf2N and BMImPF6 were saturated with silicon tetrachloride (SiCl4) precursor at an optimum concentration of 1 M and 100 mM, respectively. The saturated ionic liquid stirred at 1200 rpm for 5-6 hrs for thorough mixing prior to electrochemical deposition of Si in the glove box.

The solution, BMImTf2N with SiCl4, was clear throughout the

electrochemical studies (for a length of 72 hrs in limited dry-box environment), is shown in Figure S1. The surface morphology of the electrodeposited Si has been characterized by FESEM (ZEISS ULTRA 55). The thickness of the Si film has been determined by applying Faraday’s law as well as from the capacitance measurement using an LCR meter (AGILENT 4285A). X-ray diffraction (XRD) study of the samples has been carried out by PanAlytical (Xpert Pro) X-ray diffractometer using CuKα (1.54178 Å). The Si deposits over HOPG substrate has been rinsed with iso-propanol for ten times and then dried before placing it on to the XRD sample holder. The X-ray photoelectron spectroscopic (XPS) analyses of the electrodeposits have been carried out by PHI 5000 Versa Probe II analyzer using Mg Kα X-ray source. The surface of the Si film has been etched up to a depth of 5 nm using Ar ion at a rate of 0.25 nm per second. The diffuse reflectance measurement were carried out by using UV-visible spectrophotometer (Shimadzu UV-2600). RESULTS AND DISCUSSION Cyclic voltammetry. The electrochemical behavior of BMImTf2N, having water content to the extent of 0.86 wt.%, (determined by Karl Fisher titration, results shown in Table S1 in supplementary information) is shown in Figure 1. The water content of the ionic liquid enhances the overall redox activity on the substrate used. The pH of the ionic liquid containing such large


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quantity of water was measured as 4.6. A relatively stable potential window of -3.0 V (i.e. from 1 to -2.0 V vs. Pt) has been observed on HOPG (Figure 1). In the cathodic sweep, BMImTf2N shows two humps at -1.3 and -1.6 V, respectively. At the extreme cathodic side a huge current ensures the decomposition of the ionic liquid. The cathodic humps at -1.3 and -1.6 V (Figure 1) are attributed to the decomposition of water molecules in ionic liquid. In the anodic scan, the major current peak at -0.45 V is associated with the oxidation of the decomposed product of the ionic liquid. However, two anodic current humps at -0.60 and 0.82 V (see in the inset of Figure 1.) are attributed to the oxidation of water in ionic liquid.

Figure 1. Cyclic voltammogram of BMImTf2N on HOPG at room temperature at a sweep rate of 50 mV/s. The in-set shows the CV in the potential range of -1.7 to 1.2 V.


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The electrochemical behavior of water contaminated (~0.33 wt.% as determined by Karl Fischer titration) BMImPF6 is shown in Figure 2.

It can be observed that BMImPF6 has stable

electrochemical window of -2.30 V (-1.3 to 1.0 V vs. Pt) on HOPG substrate. The pH of the water content ionic liquid was measured as 5.1. In cathodic sweep, BMImPF6 shows two humps at -0.40 V and -0.85V (see small inset of Figure 2).

Both humps are attributed to the

decomposition of water molecules. Similarly, in anodic sweep two current humps at -0.66 (see small inset of Figure 2) and 0.77 V (see large inset of Figure 2) are attributed to the oxidation of water molecules in ionic liquid.

In the extreme cathodic direction, the ionic liquid starts

decomposing at -1.3 V.

Figure 2. CV of BMImPF6 at room temperature at a sweep rate of 50 mV/s. The large inset shows the anodic decomposition potential of the ionic liquid in a different potential window while the small inset indicating the zoom portion of the potential range as indicated by the arrows.


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It is thus observed that the water content of both the ionic liquids introduces some limitation to the stable potential window available from corresponding dry system. Further it can be noticed from Figures 1 and 2 that the decomposition of water results in increasing the current density to few hundreds of microampere only leaving clear indication of the decomposition of ionic liquid, which involves huge increase in the current density. Decomposition of BMImTf2N is rather prominent over the HOPG substrate (Figure 1). Since the overall electrochemical stability of ionic liquids in the presence of water is found to be good, the next point of our investigation was to see the nature of CV of these ionic liquids, which contains water, in presence of SiCl4 precursor. CV for the electrodeposition of Si from BMImTf2N saturated with 1 M SiCl4 solution at room temperature is shown in Figure 3. Two nearly reversible couples are observed at -0.2 and -0.8 V, respectively. Since we observed deposits over the substrate (Figure S2) only at minimum applied potential of -0.9 V, the couple at -0.2 V is attributed to the adsorption and desorption of Si precursor in the presence of water in ionic liquid. The cathodic peak at -0.8 V is attributed to the reduction of Si4+ species to Si2+ followed by further electroreduction to Si at an applied potential of ≤ -0.9 V). The above peak assignments are based on the independent ex-situ FESEM analysis of the HOPG substrate after electrodeposition at various applied potentials. It is important to note that the net current density due to reduction increased when the potential is scanned to -2.2 V in 1 M SiCl4 in BMImTf2N. The anodic peak at -0.8 V may be assigned to the leaching of Si as Si2+. At the extreme anodic side the current peak is attributed to the oxidation of Si2+ to Si4+ species. It was observed that the sticking coefficient of the electrodeposited Si over HOPG had been very low when the deposition was carried out at the applied potential range of -0.9 to -1.3 V from BMImTf2N.


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Therefore, in the present investigation electrodeposition of Si was carried out at an applied potential of -1.35 V from BMImTf2N medium.

Figure 3. Cyclic voltammogram of 1 M SiCl4 in BMImTf2N on HOPG at a sweep rate of 50 mV/s at room temperature. The in-set shows the zoomed part of the CV in the potential range of -1.15 to 1.2 V. The CV for the electrochemical deposition of Si from 100 mM SiCl4 in BMImPF6 on HOPG at room temperature is shown in Figure 4. Two current inflations at cathodic potential of -0.16 and -1.28 V are observed. The inflations are continuous over a wide range of potential. The cathodic broad peak at -0.16 V can be attributed to the reduction of Si4+ to Si2+ species. Reduction of Si2+ to Si takes place at -1.28 V. Hence, a wide range of potential is available for under potential deposition (UPD) of Si. However, the sticking coefficient of the deposits over HOPG substrate in the UPD region is found to be very low.

Hence, in the present investigation, Si was


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electrodeposited from BMImPF6 at an applied potential of -1.2 V at room temperature.

The

current density for the over potential deposition enhanced to about four times when the cathodic potential set to -2.0 V. The anodic scan shows a major hump for the oxidation of Si to Si2+ at 0.70 V. A small hump at 0.10 V due to oxidation of Si2+ to Si4+ have also been recorded. The most interesting observation in Figure 4 is the current hysteresis at the extreme cathodic end which is a signature of the three dimensional nucleation and growth. The nature of CV remains almost identical when it was recorded from 200 mM SiCl4 solution in water contaminated BMImPf6 in the potential region of -2.0 to 1.2 V. The current density in the extreme cathodic direction remains approximately same as that of Figure 4 (see Figure S3).

Figure 4. Cyclic voltammogram of 100 mM SiCl4 in BMImPF6 on HOPG at a sweep rate of 50 mV/s at room temperature. The in-set shows the zoomed part in the potential range of -0.9 to 1.1 V.


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It has been observed through several independent experiments that the sticking coefficients of the electrodeposited Si on HOPG substrate from BMImTf2N and BMImPF6 medium become adequate only after 45 min of deposition. So, efforts have been made to characterize the film obtained by electrodeposition for 45 mins by FESEM. However, the effect of longer deposition time on the nature of electrodeposits has also been explored. FESEM Studies. Visual observation indicates reduction of shiny luster of the HOPG substrate after the deposition process, which can be attributed to the presence of electrodeposits. In order to further confirm the nature of deposits, results of ex-situ characterization of the working electrode surface by FESEM is shown in Figure 5.

It can be seen from Figure 5a that

electrodeposition at -1.35 V in BMImTf2N results in continuous layer of Si on the HOPG

(a)

(b)

100 µm

100 µm

Figure 5. The FESEM image showing (a) the full coverage and (b) the center of the Si film obtained by electrodeposition from 1 M SiCl4 in BMImTf2N at -1.35 V at room temperature for 45 min. working electrode. The coverage is found to be more than 90 % by the electrodeposited Si for a deposition time of 45 min.

The sticking coefficient has been found adequate as the


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electrodeposits remain intact even after washing with alcohol followed by drying prior to characterization by FESEM. A close look at the center part of the deposits (Figure 5b) indicates the presence of very compact film with overgrowth at few locations. One of the important observations from Figure 5 is the absence of large number of pit holes. Decomposition of water molecules in ionic liquid medium is expected to evolve hydrogen through decomposition at the applied negative bias. The evolved hydrogen gas underneath the Si deposits is thus expected to generate bubbles and consequently pit holes. The FESEM in Figure 5 thus supports the CV data and confirms the minimum interference of the water content of ionic liquid on the electrodeposition of Si.

However, the FESEM image in Figure 5 also indicates that the

electrodeposited Si film is amorphous in nature. The coverage enhanced to almost 100 % with increase in deposition time to 90 min. Thick deposit having several domain cracks was observed (see Figure S4). The nature of the electrodeposited Si changed drastically as the ionic liquid medium, BMImTf2N is replaced by BMImPF6. Figure 6 shows the FESEM image of such electrodeposited Si on the HOPG substrate obtained from 100 mM SiCl4 solution in BMImPF6. It can be seen from Figure 6a that the whole substrate is covered with small crystals of Si embedded in the droplets of ionic liquid. It seems that the wetting of HOPG substrate in water contaminated BMImPF6 is very poor and consequently, the sticking ability of the electrodeposited Si over the HOPG substrate was very poor. However, the average coverage after 45 minutes of electrodeposition is found to be less than 40 %. The HOPG substrate was mildly cleaned with ethanol followed by drying before SEM analysis. Taking account of the less sticking coefficient, the cleaning was carried out with extra care so that the Si crystals remain intact on the HOPG substrate. A close view of the deposits inside the ionic liquid droplets


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(Figure 6b) clearly shows the facets of the electrodeposited crystalline Si. It has also been confirmed from independent experiments that increase in concentration of the precursor in BMImPf6 does not change the nature of the deposit to any observable extent on the HOPG substrate at room temperature. It has been observed that the maximum solubility of SiCl4 in water contaminated BMImPF6 is 200 mM, under the present experimental conditions. Hence, we tried to deposit Si from an electrolytic bath containing 200 mM SiCl4 in BMImPF6. Increase in concentration of the active electrolyte enhances the number density of deposits confined in the BMImPF6 droplets. Formation of thin film is not observed (see Figure S5). Further, the formation of crystals in the case of BMImPF6 may be attributed to its low wetting character on HOPG substrate, which facilitates three dimensional nucleation and growth in a limited volume as supported by the CV results shown in Figure 4.

(b)

(a)

10 µm

2 µm

Figure 6. The FESEM image showing (a) the full coverage and (b) the center of the Si deposits obtained by electrodeposition from100 mM SiCl4 in BMImPF6 at -1.2 V at room temperature for 45 min. Thickness Measurement.

The thickness of the electrodeposited film obtained from 1 M SiCl4

in water contaminated BMImTf2N medium on HOPG substrate as shown in Figure 5b was thus calculated from two different methods. The first approach was by using Faraday’s law assuming


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100% current efficiency for the electrodeposition process and considering the geometric area of the exposed working electrode as the active area. The thickness was found to be 36 nm assuming four electron transfer process for reduction of SiCl4 to Si. As a complementary support, we measured the capacitance of the electrodeposited Si using a sandwich configuration: HOPG (substrate)|Si|Au(0.03 cm2) at a frequency of 100 kHz. The thickness thus calculated was found to be 31 nm, which is in good agreement with the first technique. The thickness of the film was also obtained from FESEM cross section analysis. Si film deposited over freshly cleaved HOPG substrate was utilized for this purpose. As it can be seen from Figure S6, electrodeposition for 45 min. from 1 M SiCl4 in BMImTf2N results a film of thickness 21 nm while the thickness increases to 127 nm with increase in deposition time to 90 min. The FESEM results thus supports the thickness data obtained from other two techniques fairly well, considering the approximation involved in the formal techniques. XRD analysis. The observation of clear crystal facet of the electrodeposited Si from BMImPF6 leads us to examine the crystals by X-ray diffraction (XRD) technique. The XRD pattern (recorded in thin film mode) of the electrodeposited crystals on HOPG substrate is shown in Figure 7. The clear peak at 2θ = 28.42o besides the highly intense peak of HOPG, indicates the presence of well crystalline Si with preferential orientation along the [111] direction. This result supports the FESEM data of Figure 6b. However, the Si film obtained on HOPG substrate from BMImTf2N does not show any characteristic peak indicating amorphous behavior.


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XPS analysis. X-ray photoelectron spectra (XPS) of electrodeposited Si film on HOPG surface are shown in Figure 8. The survey spectrum shown in Figure 8a indicates the presence of all the possible elements such as C, O, Cl, F and Si in the film. All the elements other than Si are due to

Figure 7. XRD profile of the electrodeposited Si crystals from 100 mM SiCl4 in water contaminated BMImPF6

the contamination, which are originated from ionic liquid, Si precursor, water (Figure S7 and S8 ) and the atmosphere inside the glove box. The high resolution spectrum of Si 2p is shown in Figure 8b. A single major peak is observed at 101.3 eV. This peak may be attributed to the


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presence of Si with surface oxygen species like SiOx

28, 32

. Moreover, the peak at 101.3 eV has

also been attributed to the electrodeposited Simet from ionic liquid in few literatures 25, 33 Similar to the earlier observation

33

, it may be believed that the surface oxygen contamination of Si

occurred during the ex-situ analysis of the electrodeposited Si by XPS.

To justify this

speculation, FTIR has been carried out for pure BMImTf2N and the solution containing BMImTf2N and SiCl4 to see the nature of bonding. It has been found that the water present in BMImTf2N is free in nature (Figure S7).

It is also found that there was no Si-O bond formed

after the addition of SiCl4 in BMImTf2N even though bound water was present in the solution (Figure S8). This may be because of the strong interaction between BMImTf2N and water molecule in the presence of SiCl4, which prevents the reaction between Si4+ and H2O (hydrolysis of SiCl4). The Si 2p peak appeared at 101.0 eV when the surface of the electrodeposited Si was etched by Ar ion to a depth of 5 nm (Figure is not shown here). The shift towards lower binding energy value indicates that the core of the electrodeposit is elemental Si. In order to further ascertain the presence of Si, we have determined its band gap energy using diffuse reflectance measurement. The plot of transformed Kubelka-Monk function against the incident photon energy (see Figure S9) indicates a band gap energy of 1.15 eV, which confirms the presence of Si.

Intense investigation is in progress to deposit thick film by both

potentiostatic and galvanotatic mode to get insight of the mechanism of the electrodeposition process.


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Figure 8. X-ray photoelectron spectra of as synthesized electrodeposited Si thin film from SiCl4 in BMImTf2N: (a) survey spectrum and (b) Si 2p.


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CONCLUSION It is concluded that Si can be electrodeposited as thin film and crystals using the ionic liquids, BMImTf2N and BMImPF6 (with substantial water content), respectively, on HOPG substrate in limited dry environment. Cyclic voltammetry studies show that the working potential window of ionic liquids is not affected to any large extent in the presence of water. Instead of undergoing hydrolysis, the silicon precursor, SiCl4 can readily go into ionic liquid and form active electrolyte for Si electrodeposition.

Si thin film of 21 nm thickness has been obtained by the

electrodeposition from water containing BMImTf2N (0.86 wt.% of H2O) on HOPG for 45 min at an applied potential of -1.35V. However, only Si crystals are observed on HOPG when the electrodeposition has been carried out from water contaminated BMImPF6 (0.33 wt.% of H2O) at an applied bias of -1.2 V for same duration at room temperature. The observation of Si(111) diffraction peak in the XRD pattern confirms the formation of Si crystals from BMImPF6 medium. The observation of core electron binding energy of Si 2p at 101.3 eV in the XPS analysis indicates the presence of some SiOx species on the electrodeposited Si surface. The presence of Si further confirmed from its indirect band gap energy of 1.15 eV, determined from diffuse reflectance measurement. Thorough investigation of the mechanism (i.e. the nucleation and growth) of Si electrodeposition process from water contaminated ionic liquids on HOPG and Au(111) substrate is in progress. ACKNOWLEDGEMENT The authors would like to thank SRDC, Pandit Deendayal Petroleum University (PDPU) for providing necessary facilities to carry out the work. Financial support from Department of


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Science and Technology (DST), Government of India, (Project number SR/S1/PC-44/2011) is deeply acknowledged to carry out this whole investigation.

ETRC at Incheon National

University, South Korea is highly appreciated for providing the XPS facility. Supporting Information Available: Photograph of clear solution of SiCl4 in IL, Optical photograph of the bare and electrodeposited HOPG substrate, CV, FESEM for cross section analysis, FTIR of the ionic liquid BMImTf2N containing water, FTIR of 1 M SiCl4 in water contaminated BMImTf2N, Transformed Kubelka Monk plot for determination of band gap energy and Table showing the results of Karl Fischer Titration for determining water content of the ionic liquids.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes: The authors declare no competing financial interest.


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REFERENCES 1.

Johnson, R. A.; De la Houssaye, P. R.; Chang, C. E.; Chen, P.-F.; Wood, M. E.; Garcia,

G. A.; Lagnado, I.; Asbeck, P. M. Advanced thin-film silicon-on-sapphire technology: microwave circuit applications. IEEE Transactions on Electron Devices 1998, 45 (5), 10471054. 2.

Chen, L.; Wang, K.; Xie, X.; Xie, J. Enhancing electrochemical performance of silicon

film anode by vinylene carbonate electrolyte additive. Electrochemical and solid-state letters 2006, 9 (11), A512-A515. 3.

Kulova, T. L.; Skundin, A. M.; Pleskov, Y. V.; Terukov, E. I.; Kon’Kov, O. I. Lithium

insertion into amorphous silicon thin-film electrodes. Journal of Electroanalytical Chemistry 2007, 600 (1), 217-225. 4.

Collins, R. T.; Fauchet, P.; Tischler, M. A. Porous silicon: from luminescence to LEDs.

Physics today 1997, 50, 24-31. 5.

MD Archer, R. H. Clean Electricity from Photovotaic. Imperial College Press, London,

2001. 6.

Pandey, R. K.; Sahu, S. N.; Chandra, S. Handbook of semiconductor electrodeposition. .

New York: Marcel Dekker, Inc, 1996. 7.

Zhang, X. G. Electrochemistry of Silicon and its Oxide; Springer2001.

8.

Colletti, L. P.; Flowers, B. H.; Stickney, J. L. Formation of Thin Films of CdTe, CdSe,

and CdS by Electrochemical Atomic Layer Epitaxy. Journal of The Electrochemical Society 1998, 145 (5), 1442-1449. 9.

Bard, A. J.; Parsons, R.; Jordan, J. Standard potentials in aqueous solution; CRC

press1985; Vol. 6.


20

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

10.

Page 22 of 24

Krishnamurthy, A.; Rasmussen, D. H.; Suni, I. I. Aqueous, Room Temperature

Electrochemical Deposition of Compact Si Films. Electrochemical and Solid-State Letters 2011, 14 (9), D99-D101. 11.

De Mattei, R. C.; Elwell, D.; Feigelson, R. S. Electrodeposition of molten silicon US

4292145A., 1981. 12.

Cho, S. K.; Bard, A. J.; Fan, F.-R. F. Production of thin film solar grade silicon on metals

by electrodeposition from silicon dioxide in a molten salt US20150107657A1, 2013. 13.

Yasuda, K.; Maeda, K.; Nohira, T.; Hagiwara, R.; Homma, T. Silicon Electrodeposition

in Water-Soluble KF–KCl Molten Salt: Optimization of Electrolysis Conditions at 923 K. Journal of The Electrochemical Society 2016, 163 (3), D95-D99. 14.

Zhao, J.; Yin, H.; Lim, T.; Xie, H.; Hsu, H.-Y.; Forouzan, F.; Bard, A. J.

Electrodeposition of Photoactive Silicon Films for Low-Cost Solar Cells. Journal of The Electrochemical Society 2016, 163 (9), D506-D514. 15.

Nishimura, Y.; Fukunaka, Y.; Nishida, T.; Nohira, T.; Hagiwara, R. Electrodeposition of

Si Thin Film in a Hydrophobic Room-Temperature Molten Salt. Electrochemical and Solid-State Letters 2008, 11 (9), D75-D79. 16.

Agrawal, A. K.; Austin, A. E. Electrodeposition of Silicon from Solutions of Silicon

Halides in Aprotic Solvents. Journal of The Electrochemical Society 1981, 128 (11), 2292-2296. 17.

Gobet, J.; Tannenberger, H. Electrodeposition of Silicon from a Nonaqueous Solvent.

Journal of The Electrochemical Society 1988, 135 (1), 109-112. 18.

Nicholson, J. P. Electrodeposition of Silicon from Nonaqueous Solvents. Journal of The

Electrochemical Society 2005, 152 (12), C795-C802.


21

Page 23 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

19.

Nishimura, Y.; Fukunaka, Y. Electrochemical reduction of silicon chloride in a non-

aqueous solvent. Electrochimica Acta 2007, 53 (1), 111-116. 20.

Munisamy, T.; Bard, A. J. Electrodeposition of Si from organic solvents and studies

related to initial stages of Si growth. Electrochimica Acta 2010, 55 (11), 3797-3803. 21.

Vlaic, C. A.; Ivanov, S.; Peipmann, R.; Eisenhardt, A.; Himmerlich, M.; Krischok, S.;

Bund, A. Electrochemical lithiation of thin silicon based layers potentiostatically deposited from ionic liquid. Electrochimica Acta 2015, 168, 403-413. 22.

Zhang, J.; Chen, S.; Zhang, H.; Zhang, S.; Yao, X.; Shi, Z. Electrodeposition of

crystalline silicon directly from silicon tetrachloride in ionic liquid at low temperature. RSC Advances 2016, 6 (15), 12061-12067. 23.

Wasserscheid, P.; Welton, T. Ionic liquids in synthesis; Wiley Online Library2008; Vol.

1. 24.

Endres, F.; MacFarlane, D.; Abbott, A. Electrodeposition from ionic liquids; John Wiley

& Sons2008. 25.

Lahiri, A.; Olschewski, M.; Oliver Höfft, O.; Zein El Abedin, S.; Endres, F. In Situ

Spectroelectrochemical Investigation of Ge, Si, and SixGe1–x Electrodeposition from an Ionic Liquid. J. Phys. Chem. C 2013, 117 (4), 1722-1727. 26.

Mallet, J.; Martineau, F.; Namura, K.; Molinari, M. Electrodeposition of silicon

nanotubes at room temperature using ionic liquid. Physical Chemistry Chemical Physics 2013, 5 (39), 16446-16449. 27.

Pulletikurthi, G.; Lahiri, A.; Carstens, T.; Borisenko, N.; Zein El Abedin, S.; Endres, F.

Electrodeposition of silicon from three different ionic liquids: possible influence of the anion on the deposition process. Journal of Solid State Electrochemistry 2013, 17 (11), 2823–2832.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

28.

Page 24 of 24

Tsuyuki, Y.; Huynh, T. A. P.; Komadina, J.; Fukunaka, Y.; Homma, T. Electrochemical

quartz crystal microbalance, X-ray photoelectron spectroscopy, and Raman spectroscopy analysis of SiCl4 reduction in ionic liquids. Electrochimica Acta 2015, 183, 49-55. 29.

Zein El Abedin, S.; Borissenko, N.; Endres, F. Electrodeposition of nanoscale silicon in a

room temperature ionic liquid. Electrochemistry Communications 2004, 6 (5), 510-514. 30.

Borisenko, N.; Zein El Abedin, S.; Endres, F. In situ STM investigation of gold

reconstruction and of silicon electrodeposition on Au (111) in the room temperature ionic liquid 1-butyl-1-methylpyrrolidinium bis (trifluoromethylsulfonyl) imide. The Journal of Physical Chemistry B 2006, 110 (12), 6250-6256. 31.

Mukhopadhyay, I.; Freyland, W. Electrodeposition of Ti Nanowires on Highly Oriented

Pyrolytic Graphite from an Ionic Liquid at Room Temperature. Langmuir 2003, 19 (6), 19511953. 32.

Alfonsetti, R.; Lozzi, L.; Passacantando, M.; Picozzi, P.; Santucci, S. XPS studies on

SiOx thin films. Applied Surface Science 1993, 70/71 (2), 222-225. 33.

Bebensee, F.; Borissenko, N.; Frerichs, M.; Höfft, O.; Maus-Friedrichs, W.; Zein El

Abedin, S.; Endres, F. Surface analysis of nanoscale aluminium and silicon films made by electrodeposition in ionic liquids. Zeitschrift für Physikalische Chemie 2008, 222, 671-686.


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Electrodeposition of Si from an Ionic Liquid Bath at Room Temperature

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