Raman Spectroscopy Characterization of Dissolved Polysilicon Byproduct SiCl 4 in Ionic Liquids

Silicon tetrachloride (SiCl4) is the main byproducts of the polysilicon industries. The dissolution behaviors of SiCl4 in imidazolium ionic liquid, including [Bmim]OTf, [Bmim]NTf2, [Hmim]NTf2, and [Omim]NTf2, were investigated for recycling Si from SiCl4. Raman spectroscopy was used to understand the combination between the ionic liquid and SiCl4, and the conductivity of ionic liquids in different conditions was determined. The results indicated that [Bmim]NTf2 exhibited a better SiCl4 dissolving capacity compared to [Bmim]OTf, longer alkyl chain lengths in the imidazolium cations of the ionic liquids exhibited a better performance in dissolving SiCl4, and [Bmim]NTf2 : PC= 1 : 2 exhibited the best conductivity. In addition, molecular bondings between C-Cl, Si-O-Si, and C-Si were formed between the ionic liquids and SiCl4.


Introduction
With the rapid development of industries around the world in recent decades, the depletion of nonrenewable fossil fuels may arrive in the near future.As a result, the development of green renewable energy has been given much attention in recent years, especially silicon-based photovoltaics (PV) due to the great progress presented by the photovoltaic industries.According to the target of the Chinese Solar Photovoltaic Industry Development Plan in 13th Five-Year, which was released in 2015, industry-leading companies have a polycrystalline silicon (poly-Si) capacity production of 50,000 tons, and the backbone enterprises had a capacity of over 10,000 tons.Silicon tetrachloride (SiCl 4 ) is one of the main by-products in the production of polycrystalline silicon (poly-Si) by the Siemens process, and it is a highly toxic substance.Polysilicon production generates at least four tons of SiCl 4 for every ton of polysilicon produced.Once SiCl 4 is released, it rapidly forms a dense gas cloud and reacts violently with water vapor in the atmosphere to form a gas cloud consisting of the mixture of silicon tetrachloride, hydrochloric acid, and silicic acid, which endangers environment and people [1].Therefore, it is essential to avoid pollution and find alternative recycling processes.
Many companies have developed technologies for recycling SiCl 4 by-products to produce resource-reusable and resource-recyclable processes that generate a great amount of SiCl 4 and simultaneously relieve their economic burdens.There are two main ways for the comprehensive utilization of SiCl 4 internationally: one uses SiCl 4 as a raw material to produce chemicals, such as white carbon black, silicate ester, optical fiber, and organic silicon [2,3] and the other directly converts SiCl 4 to SiHCl 3 for recycling [4][5][6].
In recent years, the study of the dissolution of biomass materials, such as cellulose, chitin, chitosan, and the delignification of wood [7][8][9][10] in imidazolium-based carboxylate ionic liquids (ILs), has had notable advances.ILs are salts with melting point below 100 °C [11].ILs are powerful solvents that are known for their excellent solubility and low vapor pressures.In addition, ILs are good electrolytes and have a wider electrochemical window than aqueous solutions, which allow them to electrodeposit more reactive metals beyond the potential range of aqueous solutions without hydrogen gas production.As a result, the solvent function of ILs has aroused great attention in the chemistry of electrodeposition, and certain studies on the electrodeposition of Si from SiCl 4 dissolved in ILs have been performed.Abedin et al. [12] reported the initial nanoscale electrodeposition of silicon on the room-temperature IL 1butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide saturated with SiCl 4 .Gu et al. [13] indicated the presence of crystalline silicon at low temperature in an electrochemical liquid-liquid-solid process, wherein SiCl 4 was dissolved in an electrolyte system.Zhang et al. [14] reported an efficient method to prepare crystalline Si from SiCl 4 at a low temperature of 100 °C with IL [N4441]TFSI as the electrolyte.Shah et al. [15] concluded that Si can be electrodeposited as thin film and crystals using ILs [Bmim]NTf 2 and [Bmim]PF 6 .However, these studies do not illustrate the dissolution behavior and mechanisms of SiCl 4 in ILs.
The present study aims to find the solubility of SiCl 4 dissolved in four different ILs with different anions (trifluoromethylsulfonate (TfO − ) and bis[(trifluoromethyl)sulfonyl]imide (NTf 2

−
)) and different cations (1-butyl-3methylimidazolium [Bmim] + , 1-hexyl-3-methylimidazolium [Hmim] + , and 1-octyl-3-methylimidazolium [Omim] + ) and to characterize the dissolution behavior of SiCl 4 dissolved in the given ILs.At the same time, Raman spectroscopy was employed to analyze the possible dissolution mechanism at the molecular level.The study aims to provide basic support for the further research of the electrochemical deposition of Si.  2 Journal of Spectroscopy was dripped using a pipette and added into a 10 mL glass equalizer tube that contained 5.0 g dried ILs, after which the tube was sealed with parafilm.The mixture was heated at a certain temperature and mixed for half an hour by a magnetic stirrer at certain speed, and was then left to settle for half an hour to allow the layers to separate.Additional SiCl 4 was added to the sample until a separated layer was observed.

Experimental
The mixture was then cyclically weighed until a constant mixture weight was measured.Characterizing the mass of IL as M 1 and the final balanced weight as M 2 , the solubility of SiCl 4 in the ILs can be calculated as follows: where (g) represents the amount of SiCl 4 that was dissolved in 100 g ILs.

Solubility Effects of SiCl 4 in ILs with Different Alkyl Chain
Lengths in the Imidazole.As shown in Figure 1, [Omim]NTf 2 exhibited the best efficiency for dissolving SiCl 4 at any temperature among the investigated ILs.The solubility of SiCl 4 in these ILs generally decreased in the following order at any given temperature: The results indicated the effects of the alkyl chain length in the imidazole cation on the solubility of SiCl 4 given that longer alkyl chain lengths exhibited stronger advantages in dissolving SiCl 4 .As shown in Figure 2(a), the conductivity increased following an increase in temperature given that ion activities strengthen at higher temperatures.At the same time, the pure IL [Bmim]NTf 2 exhibited a lower conductivity at any given temperature due to the lower dynamic viscosity of the pure IL.The results also indicated an ideal [Bmim]NTf 2 :PC dissolution conductivity ratio of 1 : 2. The conductivity of SiCl 4 that was dissolved in [Bmim]NTf 2 /PC at a ratio of 1 : 2 is  SiCl 4 .Based on this reason, the dissolving capacity of [Hmim]NTf 2 was better than that of [Bmim]NTf 2 at the same temperature.

Figure 2 :
Figure 2: (a) Conductivity of [Bmim]NTf 2 : PC at different ratios and under different temperatures and (b) conductivity of [Bmim]NTf 2 : PC at a ratio of 1 : 2 and following the dissolution of SiCl 4 .

( 1 )( 2 )
The ILs had a better capacity for dissolving SiCl 4 at a lower temperature.At a temperature of 0 °C, the average solubility of SiCl 4 in the ILs was measured as follows: S([Bmim]OTf) = 0.180; S([Bmim]NTf 2 ) = 0.246; S([Hmim]NTf 2 ) = 0.326; and S([Omim]NTf 2 ) = 0.455.The alkyl chain length of the imidazolium cationmarkedly affected the solubility of SiCl 4 .The solubility of SiCl 4 in the ILs generally decreased in the following order: [Omim]NTf 2 > [Hmim]NTf 2 > [Bmim]NTf 2 .The [Bmim]NTf 2 and PC exhibited the highest conductivity at a [Bmim]NTf 2 : PC ratio of 1 : 2, and the change in conductivity was not apparent following the dissolution of SiCl 4 .(3) The formation of new molecular bondings were observed between the ILs and SiCl 4 , all of which were generated by the combinations of C-Cl, Si-O-Si, and C-Si.According to the molecular structure of the ILs, [Bmim]NTf 2 exhibited a better SiCl 4 dissolving capacity compared to [Bmim]OTf due to the existence of Si-O-Si.In addition, the presence of C-Cl and C-Si, specifically the alkyl chain length, had a large effect on the dissolving capacity.As a result, the dissolving capacity of [Hmim]NTf 2 was better than that of [Bmim]NTf 2 at the same temperature.

Table 1 .
China).The ILs were produced by Shanghai Cheng Jie Chemical Co. Ltd. (Shanghai, China), and their full names, abbreviations, and chemical structures are presented in All IL samples were exposed to a moderate temperature of 80 °C for 48 h in a vacuum drying oven to remove the water content and minimize the presence of volatile compounds.2.2.Experimental Methods.The solubility of SiCl 4 in ILs was tested by the liquid-liquid phase equilibrium system.SiCl 4 2.1.Materials and Apparatus.SiCl 4 (analytical reagent) was purchased from Aike Chemical Reagent Co. Ltd. (Chengdu,

Table 1 :
Ionic liquids used in the study.