In Situ Atomic Force Microscopy and Electrochemical Quartz Crystal

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

In Situ Atomic Force Microscopy and Electrochemical Quartz Crystal Microbalance Studies on the Electrodeposition and Oxidation of Silicon Abhishek Lahiri, Giridhar Pulletikurthi, Niklas Behrens, Tong Cui, and Frank Endres J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02462 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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The Journal of Physical Chemistry

In situ Atomic Force Microscopy and Electrochemical Quartz Crystal Microbalance Studies on the Electrodeposition and Oxidation of Silicon Abhishek Lahiri*, Giridhar Pulletikurthi*, Niklas Behrens, Tong Cui, and Frank Endres* Institute of Electrochemistry, Clausthal University of Technology, Arnold-Sommerfeld-Str. 6, 38678 Clausthal-Zellerfeld, Germany Abstract In this paper, we investigated how the anion of an ionic liquid affects nanostructure of electrode/electrolyte interface and subsequently the deposition process of silicon. For this purpose, ILs with the same cation 1-butyl-1-methylpyrrolidinium and various anions, namely, tris(pentafluoroethyl) trifluorophosphate ([Py1,4]FAP), bis(trifluoromethylsulfonyl)amide ([Py1,4]TFSA) and bis(fluorosulfonyl)amide ([Py1,4]FSA) were chosen. Cyclic voltammetry and

electrochemical Quartz Crystal Microbalance (EQCM) were used to study the

electrochemical processes. Raman spectroscopy and in situ Atomic Force Microscopy (AFM) were used to evaluate the changes in the ILs on addition of SiCl4 and to probe the changes in the Au (111)/electrolyte interface, respectively. From CV and EQCM measurements, it was found that the electrochemical processes changed significantly on changing the anion. However, from Raman spectroscopy a few spectral changes related to vibrational modes of the employed anions were observed on addition of SiCl4 to the ILs. In situ AFM studies revealed that on changing the anions and on applying a negative electrode potential, the number of solvation layers and their widths changed in the presence of SiCl4. Although silicon thin films could be electrodeposited from all the three ILs at room temperature, the best deposit was obtained from [Py1,4]TFSA. Introduction Silicon is an important semiconductor widely used in electronics and optoelectronics industries. The properties of silicon can be tuned by controlling its particle size, doping level (~ 1019 cm-3) 1, and roughness, which can be used in thermoelectric devices, light emitting diodes (LEDs) and in lasers.1-3 Electroluminescence in silicon nanocrystals was achieved by plasma-enhanced chemical vapour deposition of SiOx (x 99.9%) from Io-Li-Tec (Germany) and Solvionic, respectively, and were used after drying under vacuum at 100 oC to achieve a water content of below 10 ppm. [Py1,4]FAP ionic liquid was purchased in the highest available quality (custom made) from Merck (Germany). SiCl4 (99.9999%) was purchased from Alfa Aesar. The working electrode in the experiments was gold on mica or gold on glass. Platinum wires were used as counter and a quasi reference electrodes which gave good stability in the ionic liquid. The electrochemical cell was made of Teflon and clamped over a Teflon-covered Viton O-ring onto the substrate yielding a geometric surface area of 0.3 cm2. Prior to the experiments, the Teflon cell and the O-ring were cleaned in a mixture of 50:50 vol% of concentrated H2SO4 and H2O2 (35%) followed by refluxing in distilled water.

The electrochemical measurements were performed in an argon-filled glove-box with water and oxygen contents of below 2 ppm (OMNI-LAB from Vacuum Atmospheres) by using a VersaStat II (Princeton Applied Research) potentiostat/galvanostat controlled by powerCV software. The scan rate during cyclic voltammetry was 10 mV sec-1. The deposits were obtained by constant potential electrolysis and were characterised using scanning electron microscopy (SEM, Carl Zeiss DSM 982 Gemini). Force-distance curves were collected using a Molecular Imaging PicoPlus AFM in contact mode. The substrate for AFM experiments was Au(111) (gold on mica), purchased from Molecular Imaging. A silicon SPM-sensor from NanoWorld was employed for all the experiments. The spring constant was 6 N/m. Two thin Pt wires were used as the CE and RE, 3 ACS Paragon Plus Environment

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respectively. Before each experiment, the electrochemical cell and Pt wires were cleaned with isopropanol in an ultrasonic bath for 5 min and then the Pt wires were annealed in hydrogen flame to red glow in order to remove any possible contaminations. All force curves were acquired at room temperature in an argon-filled glove-box. Raman spectra were recorded with a Vertex 70 V (Nd: YAG 1064 nm) attached to a RAM II module. For Raman spectra, the electrolyte was enclosed in a glass tube and wrapped with parafilm© inside of the glove-box. The spectra were obtained at an average of 250 scans with a resolution of 2 cm-1. Electrochemical quartz crystal microbalance (EQCM) measurements were carried out simultaneously

along

with

the

classical

electrochemical

measurements,

such

as

potentiodynamic and chronoamperometric experiments. Gold sputtered qurtz crystals (AT cut 10 MHz, procured from KVG GmbH) were as used working electrodes and two Pt wires were used as the counter and reference electrodes, respectively. A network analyzer (Agilent E5100A) connected to a PC via a GBIP card was used for recording the resonance frequency of quartz crystals and the full width at half maximum of the resonance curves. An in-house made EQCM cell was used, which can be heated up to 150 °C with a resistive element. Results and Discussion

Figure 1a compares the CVs of 0.1M SiCl4 in [Py1,4]FSA and in [Py1,4]TFSA on Au. Two reduction peaks at -1.4 V and -2.2 V are seen for 0.1M SiCl4 in [Py1,4]TFSA (red line, Fig. 1a). The more negative peak corresponds to the deposition of Si whereas the less negative one can be allocated to the formation of subvalent SiClx species. In comparison, the CV of 0.1M SiCl4 in [Py1,4]FSA shows three reduction processes at -1.1 V, -1.5 V and -2.0 V. The first peak could be due to adsorption of IL, followed by SiClx formation at -1.5 V before silicon is deposited at -2.0 V.28 Comparing the two CVs in Fig.1a, it is evident that the current obtained in [Py1,4]FSA is considerably higher which can mainly be related to the lower viscosity of [Py1,4]FSA. Figure 1b compares the CVs of 0.1 M SiCl4 and 0.25 M SiCl4 in [Py1,4]FAP on Au. For 0.1 M SiCl4 in [Py1,4]FAP, a reduction peak at ~ -2 V is observed which is consistent with literature.29 A current loop was observed in the forward scan of the CV. Beyond -2.8 V, an irreversible decomposition of the [Py1,4]+ cation takes place.20 In these two CVs we mainly see the bulk deposition of Si below -2 V. The current loop at -2.65 V can be associated with a nucleation phenomenon. In the anodic scan, a rise in current is observed above +0.2 V which can be assigned to the oxidation of silicon along with possible gold oxidation. On addition of 0.25 M SiCl4 to [Py1,4]FAP, the CV shows two difficult to allocate broad waves at -0.75 and 1.4 V (red line, Fig 1b) before the reduction current starts to increase at ~ -1.8 V with a peak 4 ACS Paragon Plus Environment

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at -2.5 V. This increase in current below -1.8 V is due to the deposition of Si. In the anodic direction, an increase in the oxidation current takes place above -0.25 V where Si and also Au oxidation take place. In all CVs, the peak currents for the reduction process are larger than the oxidation process, suggesting that the oxidation of Si is kinetically hindered.

Figure 1: (a) Comparison of CVs of 0.1M SiCl4 in [Py1,4]FSA and [Py1,4]TFSA (b) CVs of 0.1M and 0.25M SiCl4 in [Py1,4]FAP at room temperature on polycrystalline Au. Changes in mass of the gold coated quartz crystal with electrode potentials for (c) [Py1,4]TFSA, (d) [Py1,4]FSA, (e) 0.1 M SiCl4/ [Py1,4]TFSA, and (f) 0.1 M SiCl4/[Py1,4]FSA at 50 °C. 5 ACS Paragon Plus Environment


In order to get more information on the processes in the CV, EQCM experiments were carried out in the presence and in absence of SiCl4 in the ILs. As the damping is too high at room temperature, EQCM experiments had to be carried out at 50 °C. To interpret the EQCM results, we have recorded the cyclic voltammograms of neat ILs (Fig S1) in the EQCM cell. The cyclic voltammogram of [Py1,4]TFSA on gold at 50 °C shows a prominent reduction process at ~ -2.7 V (Fig S1a) in the forward scan, which is due to the irreversible reduction of the IL. An oxidation process was seen at + 0.5 V in the backward scan, which seems to be related to either oxidation of the anion or to a certain dissolution of gold. A similar electrochemical behavior was observed for [Py1,4]FSA (Fig S1b), which shows a reduction process with an onset at -2.4 V in the forward scan. The peak at +0.5 V can be related either to oxidation of the anion or to a certain dissolution of gold. Owing to the high viscosity of [Py1,4]FAP at room temperature and at 50 °C, we could not carry out EQCM measurements for [Py1,4]FAP and for SiCl4/[Py1,4]FAP. Using EQCM, a quantitative determination of mass can be obtained from a shift in the resonance frequency using Sauerbrey’s equation. Together with the charge, the apparent molar mass can be calculated. For Si, the theoretical molar mass from such a measurement should be 7.02 g/mol of electrons by assuming a four electron transfer reduction process. The change in the mass vs. potential for [Py1,4]TFSA and [Py1,4]FSA at 50 °C is shown in Fig. 1c and 1d. Both of the liquids show a similar response over the potential scan, wherein a slight apparent mass change with respect to the electrode potential can be observed. This apparent mass change is less than 0.25 µg (limit of the EQCM device for measurements) and can be attributed to the high damping of the resonance frequency of the quartz crystal caused by the viscosity of the IL. Therefore, we could not exactly allocate the observed oxidation process to a definite process. Figures 1e and 1f show the changes in mass of the quartz crystal vs. electrode potential for 0.1 M SiCl4/[Py1,4]TFSA and for 0.1 M SiCl4/[Py1,4]FSA. The addition of 0.1 M SiCl4 to [Py1,4]TFSA changes the electrochemical response, where a reduction peak can be observed at -2.3 V in the CV (Fig. 1a). The EQCM data also shows a corresponding increase in mass approximately at the same potential (Fig. 1e). The increase in mass can be observed between -2.3 and -3 V, which continues until -2.8 V in the backward scan. The reduction process at -2.3 V can be related to the silicon deposition, which increases the mass of the quartz crystal. Thereafter, a decrease in mass can be observed until +0.2 V and a sharp decrease in mass occurs at more positive potentials (+0.3 to 1 V) which is related to the dissolution of the deposit during the anodic scan. 6 ACS Paragon Plus Environment

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In 0.1 M SiCl4/[Py1,4]FSA three reduction processes can be observed in the CV. No mass change can be observed until -1 V (Fig. 1f) and therefore the reduction process can be associated with a solution reduction process, i.e. the formation of subvalent SiClx-IL species. The reduction peak at around -1.4 V shows a tiny increase in mass that can be related to a surface reduction process (e.g. an underpotential deposition, UPD), after which the bulk deposition starts at -2.3 V and lasts until -3 V. However, in the backward scan, only a slight decrease in mass is observed from -2.0 V to 0 V.

Figure 2. Comparison of CVs of 0.25M SiCl4 in ILs on polycrystalline Au at 50 °C (a) 0.25M SiCl4 in [Py1,4]TFSA and (b) 0.25M SiCl4 in [Py1,4]FSA. Comparison of change in apparent mass of quartz crystal with electrode potentials for SiCl4/ILs at 50 °C, (c) 0.25 M SiCl4 in 7 ACS Paragon Plus Environment


[Py1,4]TFSA (d) 0.25M SiCl4 in [Py1,4]FSA. Comparison of mass vs. charge plots for the electrodeposition of Si in 0.25 M SiCl4/ILs at 50 °C, (e) [Py1,4]TFSA and (f) [Py1,4]FSA, the red line is the slope of the curves. The CVs of 0.25 M SiCl4 in [Py1,4]TFSA and [Py1,4]FSA are shown in Figures 2a and b . Two clear reduction processes (C2 and C3) along with a broad reduction process were observed for [Py1,4]TFSA upon adding 0.25 M SiCl4 (Fig. 2a). The process C1 cannot be allocated without doubt. The reduction processes C2 and C3 are related to the deposition of Si and the decomposition of the [Py1,4]+ cation, respectively. However, 0.25 M SiCl4 in [Py1,4]FSA shows a different electrochemical behaviour in comparison to 0.25 M SiCl4 in [Py1,4]TFSA. Four reduction processes were noticed for 0.25 M SiCl4 in [Py1,4]FSA in the cathodic scan. The reduction processes C2 and C3 can be related to the deposition of Si and to the irreversible reduction of the [Py1,4]+ cation, respectively. The reduction process C1 seems to be related to the reduction of SiCl4-IL species to a different SiClx-IL species. Furthermore, the reduction processes C1 and C2 have nearly similar magnitude of current. The reduction process C* can be related to a surface reduction process. The changes in the EQCM measurements were monitored during CV. On addition of 0.25 M SiCl4 to [Py1,4]TFSA, a reduction peak is observed at -2.3 V in the CV (Fig. 2a). The corresponding EQCM measurement shows an increase in mass at the same potential (Fig. 2c) until -3 V. An increase in the mass of the crystal was observed between - 2 and -3 V (Fig. 2c). A change in slope of the mass of the quartz electrode can be seen at ~ -2.5 V, which can be related to the combined processes of silicon deposition and partial decomposition of [Py1,4]TFSA. The EQCM results support the CV results (Fig. 2a and Fig. 2c). The reduction process at -2.3 V is related to the electrodeposition of silicon, which increases the mass of the quartz crystal. Thereafter, a gradual decrease in mass was observed until -0.2 V followed by another gradual increase in mass at more positive potentials (-0.2 to 1 V). For 0.25 M SiCl4/[Py1,4]FSA, three reduction processes were observed in the CV (Fig. 2b). The first two peaks at -1.0 V (C1) and -1.4V (C*) in Fig. 2b do not relate to a mass change in the EQCM response as shown in Fig. 2d. Therefore, these two processes can be related to the formation of subvalent SiClx species. Si bulk deposition starts at -2.3 V (peak at C2 in Fig.2d) wherein a mass change can be observed until -3 V in the EQCM as shown in Fig. 2d. Furthermore, an increase in mass can be observed even in the backward scan until -2 V, which indicates that the reduction processes at C2 and C3 are responsible for an increase in the mass of the electrode. Subsequently, a gradual decrease in the mass was observed from -2 8 ACS Paragon Plus Environment

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to 0 V, which indicates a partial dissolution of the deposited material in the backward scan. In any case, SiCl4/[Py1,4]TFSA and SiCl4/[Py1,4]FSA do not behave the same.

Figure 3: QCM measurement of electrodeposited Si on exposure to air In order to investigate the electrodeposition process, the change in mass was plotted against the charge passed (Q) during constant potential electrodeposition. The slope of the curve is calculated, which gives a ratio of molar mass to the number of transferred electrons (M/z, which is the product of Faraday constant and ratio of ‘dm to dq’). Such plots for 0.25 M SiCl4/[Py1,4]TFSA and for 0.25 M SiCl4/[Py1,4]FSA are shown in Fig. 2e and 2f, respectively. The ideal value for a four electron reduction process for Si is 7.02 g/mol of electrons. Constant-potential electrolysis of 0.25 M SiCl4/[Py1,4]TFSA was carried out at -2.4 V for 2 h where a constant increase in mass over the entire deposition time can be observed (see Fig. 2e). A rise in the mass for the first seconds can be related to roughness and viscoelastic effects. The ratio of the molar mass to the number of transferred electrons was calculated from the slope, which was found to be 6.3 g mol-1. Komadina et al. reported a value of 13.2 g mol-1, which is 188 % of the theoretical value. In our case, the discrepancy was found to be around 10 % and is close to the theoretical value indicating a four-electron transfer mechanism for silicon electrodeposition. However, we could not observe such a close theoretical value for 0.25 M SiCl4/[Py1,4]FSA (Fig. 2f). In the case of 0.25 M SiCl4/[Py1,4]FSA, a relatively high amount of mass is deposited on the electrode, which gives a slope of 18.33 g/mol of electrons correponding to 261% of the theoretical value. Such a high value can be regarded as a strong hint for the entrapment of electrolyte. Other possible side reactions have been mentioned in literature.27 Tsuyuki et al. analysed the cathodic reaction process of SiCl4 in ionic liquids by X-ray Reflectivity (XRR) and by Density Functional Theory calculations. They suggested a sequential reduction of SiCl4 through formation of 9 ACS Paragon Plus Environment


polymer-like Si structures containing dimers such as Si2Cl6. Therefore, it appears that several factors play a role in silicon electrodeposition from [Py1,4]FSA, leading to large errors in the EQCM measurements.30

Figure 4: (a) Raman spectra of neat [Py1,4]FSA, 0.1 M SiCl4 and 0.25 M SiCl4 in [Py1,4]FSA between (a) 250 and 600 cm-1 and (b) 680 and 800 cm-1 (c) Raman spectra of [Py1,4]TFSA, 0.1 M SiCl4 and 0.25 M SiCl4 in [Py1,4]TFSA between 250 and 600 cm-1 and (d) 700 and 800 cm-1 The oxidation of silicon by air was also evaluated as silicon is of interest to silicon-air batteries. To investigate the reactivity of the deposited silicon layer, QCM was performed under air. The electrodeposited silicon on the quartz crystal was first cleaned inside of the glove-box with toluene and isopropanol. After cleaning the cell was sealed with adhesive tape to prevent any oxidation. The cell was then put outside of the glovebox and the QCM measurement was started. Figure 3 shows the mass change on exposure of the electrodeposited Si to air. Due to cutting a hole into the tape during QCM measurement, the measurement is slightly disturbed in the beginning. After about 350 s, the mass starts to increase rapidly due to oxidation of silicon. After 1000 s the mass only slowly increases indicating that most of the silicon has been oxidized. The oxidation rate was found to be ~4 ng/s.

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We also performed in situ AFM for the electrolytes. As the interfacial structures should be related to the species formed in the bulk, Raman spectroscopy of SiCl4 in different ILs was performed. Figure 4 compares the Raman spectra of the two ionic liquids and their respective solutions with 0.1 M and 0.25 M SiCl4. The Raman spectra of pure [Py1,4]FSA with the addition of 0.1 M and 0.25 M SiCl4 are shown in Fig. 4a. Some changes in the torsional bending modes of SO2F are evident on addition of SiCl4 to the IL31. Furthermore, a new peak at 424 cm-1 occurs which is related to SiCl4. In the Raman spectra, the most significant changes occur between 700 and 800 cm-1 (Fig. 4b), which relates to the CF bending mode of FSA-. It is clear from Fig. 6b that on addition of SiCl4 to the IL, there is a decrease in the intensity of the peak at 729 cm-1 which indicates that SiCl4 interacts with the FSA- anion. However, on increasing the SiCl4 concentration, no change in the CF bending mode occurs. Figure 4c compares the Raman spectra of 0.1 M and 0.25 M SiCl4 dissolved in [Py1,4]TFSA. Unlike in [Py1,4]FSA, between 200 and 600 cm-1 (Fig. 4a), almost insignificant changes in the Raman spectra are observed. Also, no changes are observed in the CF bending mode as seen in Figure 4d.

Figure 5: (a) Raman spectra of neat [Py1,4]FAP, 0.1 M SiCl4/[Py1,4]FAP, and 0.25 M SiCl4/[Py1,4]FAP between 400 and 650 cm-1 and (b) between 410 and 470 cm-1 The Raman spectra of 0.1 M SiCl4 and 0.25 M SiCl4 in [Py1,4]FAP are shown in Figure 5a. It is evident that only on addition of 0.25 M SiCl4 changes in the C-F bending mode take place, which indicates that SiCl4 interacts with FAP-. This is further evidenced by comparing the Raman spectra in Figure 5b. On addition of 0.1 M SiCl4 to [Py1,4]FAP, two peaks at 423 and 431 cm-1 are observed, which are related to SiCl4 and P-F asymmetric stretching modes, respectively.32 On increasing the concentration of SiCl4 to 0.25 M, only one peak is observed which shifts to 425 cm-1. Such changes suggest either a possible formation of new species or a 11 ACS Paragon Plus Environment


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stronger interaction between SiCl4 and FAP-. Thus, from Figures 4 and 5 it is evident that SiCl4 reacts somehow with [Py1,4]FSA and [Py1,4]FAP, whereas for [Py1,4]TFSA, no clear changes can be observed. Also, with an increase in concentration of SiCl4, spectral changes in [Py1,4]FAP take place whereas not much change occurs in [Py1,4]FSA and [Py1,4]TFSA. The question arises if these IL-SiCl4 complexes also affect the interfacial structure.

Figure 6: Force-separation profiles of 0.1 M SiCl4 in [Py1,4]TFSA on Au(111) (a) at OCP (b) at -0.5 V, and (c) at -1.0 V (d) force-separation profiles of 0.25 M SiCl4 in [Py1,4]TFSA on Au (111) at OCP (e) at -0.5 V and (f) -1.0 V Thus, in situ AFM was performed to understand the effect of addition of SiCl4 on the nanostructure of the IL/Au(111) interface. Figure 6 compares the force-separation profiles of 0.1 M SiCl4 and 0.25M SiCl4 in [Py1,4]TFSA. At OCP, as the AFM cantilever approaches the Au(111) surface, no detectable force is encountered until 5 nm. Below 5 nm, it experiences an interaction with the ionic liquid and two ‘push-through’ forces are observed (Fig. 6a) at distances of 1.5 nm and 0.7 nm from the electrode surface. The force-separation profiles of neat [Py1,4]TFSA on Au(111) consist of at least four discrete solvation layers at OCP

23

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Compared to [Py1,4]TFSA on Au(111), only 2 solvation layers are observed in the presence of 0.1 M SiCl4 in [Py1,4]TFSA on Au(111) at OCP, which indicates that the interfacial structure has changed due to the addition of SiCl4 to it. However, the innermost layer shows a distance of 0.7 nm which is consistent with the presence of an ion-pair of the ionic liquid.23 The second step shows a separation distance of 12 ACS Paragon Plus Environment

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0.8 nm and is also consistent with the presence of an IL ion-pair. On changing the potential to -0.5 V (Fig. 6b), again two solvation layers are observed at 0.6 nm and 1.7 nm. The decrease in the innermost layer can be related to a slight increase in attraction strength between the ionpair and the Au(111) surface which also results in an increase in force of about 8 nN to rupture the innermost layer compared to ~ 4 nN as observed at OCP. On changing the potential to -1.0 V (Fig. 6c), only a double layer is observed with a separation distance of 0.7 nm which can again be related to the presence of an IL ion-pair. However, comparing the results with pure [Py1,4]TFSA on Au(111)

23

, it is evident that the

presence of SiCl4 influences the interfacial structure in this ionic liquid. Also, from the Raman spectra no clear interaction of SiCl4 was observed with the TFSA anion and therefore it appears that the ionic liquid dominates the interfacial structure which is disturbed by the addition of SiCl4. On increasing the concentration of SiCl4 to 0.25 M, a change in the interfacial structure can be observed at OCP in Fig. 6d. The innermost layer shows a separation of 0.5 nm with the second and third layer occurring at a separation of 0.9±0.1 nm. This suggests that SiCl4 species are present at the innermost layer and the second and third layer is comprised mainly of IL ion-pairs. On changing the potential to -0.5 V (Fig. 6e), only two solvation layers are observed. No change in the innermost layer width is observed compared to OCP, whereas the second layer occurs at a separation of 0.6 nm which implies that at negative potentials SiCl4 species also affect the layers adjacent to the innermost layer. At -1.0 V (Fig. 6f), a slight decrease in the innermost layer width is observed, which can be related to a higher attraction force between the electrode and SiCl4 species. The second layer occurs at a separation of 0.9 nm which relates to the presence of an IL ion pair. Thus, from Fig. 6, it appears that the higher concentration of SiCl4 has a pronounced effect on the solvation structure of [Py1,4]TFSA. Figure 7 compares the force-separation profiles of 0.1 M SiCl4 and of 0.25 M SiCl4 in [Py1,4]FSA. On changing the anion of the ionic liquid from [Py1,4]TFSA to [Py1,4]FSA, changes in the interfacial nanostructure can be clearly seen. Three solvation layers are observed for 0.1M SiCl4 in [Py1,4]FSA (Fig. 7a), whereas neat [Py1,4]FSA at OCP showed five solvation layers.31 The change in the interfacial structure can be related to the presence of SiCl4 in [Py1,4]FSA. The innermost layer shows a distance of 0.4 nm which can be related to SiCl4-[Py1,4]FSA species. The second layer occurs at a distance of about 0.8 nm which could be related to the presence of an IL ion-pair. However, interestingly, on changing the potential to -0.5 V (Fig. 7b), the number of solvation layers increases to four. This is in contrast to 13 ACS Paragon Plus Environment


previous observations in the presence of metal salts23,31 The increase in the width of the innermost layer from 0.4 nm at OCP to 0.5 nm at -0.5 V suggests that a change in the orientation of the adsorbed species must have taken place. At an applied potential of -0.7 V (Fig. 7c), three interfacial layers are observed with the innermost layer at a distance of 0.4 nm followed by two layers at a separation of 0.8 nm. This suggests that SiCl4-IL species are present at the innermost layer with the IL-ion pairs in the subsequent layers. On going to more negative potentials, no interfacial layers were observed. On increasing the concentration of SiCl4 to 0.25 M, again two solvation layers (Fig. 7d) occur with a separation of 0.4 and 1.0 nm which is the same as seen for 0.1 M SiCl4. With the change in potential to -0.5 V (Fig. 7e) and -1.0 V (Fig. 7f), no changes in the interfacial layers are observed which indicates that the ion arrangement of SiCl4-IL species is not affected by the electrode potential. It was observed that at -1.2 V, only one solvation structure existed at a separation of 0.4 nm and on further changing of negative potential, no defined solvation structure was observed.

Figure 7: Force-separation profiles of 0.1 M SiCl4 in [Py1,4]FSA on Au(111) (a) at OCP (b) at -0.5 V, and (c) at -0.7 V (d) force-separation profiles of 0.25 M SiCl4 in [Py1,4]FSA on Au (111) at OCP (e) at -0.5 V and (f) -1.0 V


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Figure 8: Force-separation profiles of 0.1 M SiCl4 in [Py1,4]FAP on Au(111) (a) at OCP (b) at -0.5 V (c) at -1.0 V, and (d) -1.4 V

Figure 9: 0.25 M SiCl4 in [Py1,4]FAP on Au(111) (a) at OCP (b) at -0.5 V (c) at -1.0 V (d) 1.4 V 15 ACS Paragon Plus Environment


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Figure 8 shows the force-separation profiles of 0.1 M SiCl4 in [Py1,4]FAP. At OCP, five prominent solvation layers are observed at 0.5, 1.5, 2.6, 3.6 and 4.6 nm (Fig. 8a) from the electrode surface. The interfacial nanostructure of Au(111)/[Py1,4]FAP was reported wherein the width of the innermost layer was found to be 0.31 nm

33, 34

, which was related to the

presence of the [Py1,4]+ cation. Here, in the case of 0.1 M SiCl4/[Py1,4]FAP a slight increase in the width of the innermost layer is detected, which suggests the presence of SiCl4 species together with [Py1,4]FAP at the interface. The second layer occurs at a distance of about 1 nm which also indicates the presence of SiCl4-[Py1,4]FAP species. On changing the potential to 0.5 V, four solvation layers are observed (Fig. 8b) with the innermost layer showing a distance of 0.5 nm. This slight increase in separation distance can be due to reorientation of the SiCl4-[Py1,4]FAP species. On changing the electrode potential to -1.0 V (Fig. 8c) and to -1.4 V, a further decrease in number of solvation layers is observed. At -1.4 V (Fig. 8d), only two layers are seen at 0.2 nm and 0.7 nm. The decrease in the innermost layer can be correlated to the presence of SiCl4 species at the Au(111) surface. On increasing the concentration of SiCl4 in the ionic liquid at OCP, four interfacial layers are observed (Fig. 9a) at distances of 0.7 nm, 1.7 nm, 2.7 nm and 3.8 nm. Compared to 0.1 M SiCl4-[Py1,4]FAP, an increased innermost layer distance of 0.7 nm is observed on addition of 0.25 M SiCl4. This indicates the presence of SiCl4-[Py1,4]FAP species at the interface which affects the width of the layers as evident from the Raman spectroscopy where the bulk liquid structure changed. The second, third, and fourth steps occur at 1.0 nm in force-separation profiles, which suggests the presence of SiCl4 -[Py1,4]FAP species. On changing the potential to -0.5 V (Fig. 9b), three solvation layers are observed. The innermost layer and the second layer show widths of 0.2 nm and 0.8 nm, respectively. The decrease in the innermost layer can either be related to a change in orientation of the species present at the electrode surface or due to potential induced structural changes of the species. On changing the potential to -1.0 V (Fig. 9c), two solvation layers are observed at distances of 0.7 nm and 1.8 nm. The change in the innermost layer can be related to the rearrangement of the SiCl4-[Py1,4]FAP species. Similar phenomena have been shown previously for both pure [Py1,4]FAP and for TaF5/[Py1,4]FAP wherein the change in the innermost layer was related to the reorientation of the anion.25 On further changing the potential to -1.4 V (Fig. 9d), only a double layer is seen at 0.6 nm. This change can be related to the presence of SiCl4-FAP species prior to the deposition of Si. On changing to more negative potentials, the force-distance profiles become distorted due to the 16 ACS Paragon Plus Environment

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onset of Si deposition. Thus, from the force-distance measurements in Fig. 9 as well as from previous results for the deposition of Li and Ta from ILs,

20, 23, 25

it appears that the IL

solvation layers must be modified considerably in order to reduce metal/semiconductor ion species/complexes for their deposition.

Figure 10: SEM micrographs of Si electrodeposits on Au at RT at -2.4 V in (a) 0.25 M SiCl4/[Py1,4]FSA, (b) 0.25 M SiCl4/[Py1,4]TFSA, inset: high-resolution image of Fig. 10 b, (c) 0.1 M SiCl4/[Py1,4]FAP, and (d) 0.25 M SiCl4/[Py1,4]FAP In order to evaluate if there is an influence of the IL anion on the morphology of Si deposits, constant potential electrodeposition was performed from the electrolytes. Figure 10a and 10b shows the electrodeposited Si from 0.25 M SiCl4/[Py1,4]FSA and 0.25 M SiCl4/[Py1,4]TFSA at -2.4 V for 1 hour. Agglomerated and inhomogensou Si deposit was obtained from 0.25 M SiCl4/[Py1,4]FSA (Fig. 10a). Si nanoparticles of about 50 nm in size form a relatively homogenous deposit from 0.25 M SiCl4/[Py1,4]TFSA (as seen from the inset of Fig. 10b) . On changing the ionic liquid to [Py1,4]FAP containing 0.1 M SiCl4, small islands of silicon are seen on the Au surface (Fig.



10c). However, on increasing the concentration of SiCl4 to 0.25 M, a porous network structure of Si is formed (Fig. 10d). Based on the electrolysis experiments we conclude that Si can be electrodeposited from 0.25 M SiCl4 in [Py1,4]TFSA and from 0.25 M SiCl4 in [Py1,4]FAP. However, the electrodeposition of Si from [Py1,4]FSA is complicated as we could obtain only thin layers of Si containing trapped ionic liquid. This is consistent with the EQCM analysis shown in Figure 2d where the mass increase continued even in the anodic regime. Furthermore, quite thin Si films were obtained from 0.1 M SiCl4 in [Py1,4]FAP which can be related to the influence of the solvation structure at the Au(111)/[Py1,4]FAP interface. This study signifies that changing the constituent ions and the concentration of SiCl4 influence the structure of the electrode/electrolyte interface prior to the deposition processes in ILs and subsequently affect the Si electrodeposits. At a minimum we can conclude that the different interfacial layering has an influence on the deposit morphology. Conclusions In this paper, we have studied the electrochemical behaviour of SiCl4 in three different ionic liquids composed of the same cation and three different anions. The electrochemical behaviour of SiCl4 in the employed three ILs was found to be different. EQCM studies showed that deposition of Si in [Py1,4]TFSA occurs via a four electron transfer process. However, an erroneous value was obtained for [Py1,4]FSA at 50 °C. Raman spectroscopy showed few changes in the spectral features of ILs and their solutions with addition of 0.1M SiCl4. No clear spectral changes occurred on increasing the concentration of SiCl4 (e.g. 0.25 M) in [Py1,4]TFSA and in [Py1,4]FSA. Moreover, clear changes can be observed upon increasing the SiCl4 concentration for [Py1,4]FAP. From in situ AFM studies, it was observed that the change in the electrode potential affects the interfacial nanostructure in all the three ionic liquids. A classical double layer structure was observed prior to silicon electrodeposition. From constant potential electrolysis, Si thin films having silicon particle sizes of about 50 nm were obtained from [Py1,4]TFSA and [Py1,4]FAP, whereas Si thin films along with entrapped ionic liquid were obtained from [Py1,4]FSA. Supporting information available: Supporting information contains cyclic voltammetry


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Acknowledgement Financial support by the BMBF project AlSiBat (BMBF:03SF0486A) is gratefully acknowledged References 1.

Hochbaum A.I.; Chen R.; Delgado R.D.; Liang W.; Garnett E.C.; Najarian M.; Majumdar A.; Yang P. Enhanced Thermoelectric Performance of Rough Silicon Nanowires. Nature. 2008, 451, 163-167.

2.

Franzò G.; Irrera A.; Moreira E.C.; Miritello M.; Iacona F.; Sanfilippo D.; Di Stefano G.; Fallica P.G.; Priolo F. Electroluminescence of Silicon Nanocrystals in MOS Structures. Appl. Phys. A. 2002, 74, 1-5.

3.

Jaiswal S.L.; Simpson J.T.; Withrow S.P.; White C.W.; Norris P.M. Design of a Nanoscale Silicon Laser. Appl. Phys. A. 2003, 77, 57-61.

4.

Obrovac M.N.; Christensen L. Structural Changes in Silicon Anodes during Lithium Insertion/Extraction. Electrochem. Solid-State. Lett. 2004, 7, A93-A96.

5.

Chan C. K.; Peng H.; Liu G.; McIlwrath K.; Zhang X. F.; Huggins R. A.; Cui Y. HighPerformance Lithium Battery Anodes using Silicon Nanowires. Nature Nanotechnol. 2008, 3, 31-35.

6.

Esmanski A.; Ozin G. A. Silicon Inverse-Opal-Based Macroporous Materials as Negative Electrodes for Lithium Ion Batteries. Adv. Funct. Mater. 2009, 19, 1999-2010.

7.

Song T.; Xia J.; Lee J.-H.; Lee D. H.; Kwon M.-S.; Choi J.-M.; Wu J.; Doo S. K.; Chang H.; Park W. I.; Zang D. S.; Kim H.; Huang Y.; Hwang K.-C.; Rogers J. A.; Paik U. Arrays of Sealed Silicon Nanotubes as Anodes for Lithium Ion Batteries. Nano Lett. 2010, 10, 1710-1716.

8.

Teki R.; Datta M. K.; Krishnan R.; Parker T. C.; Lu T.-M.; Kumta P. N.; Koratkar N. Nanostructured Silicon Anodes for Lithium Ion Rechargeable Batteries. Small. 2009, 5, 2236-2242.

9.

Agrawal A. K.; Austin A. E. Electrodeposition of Silicon from Solutions of Silicon Halides in Aprotic Solvents. J. Electrochem. Soc. 1981, 128, 2292-2296.

10. Gobet J.; Tannenberger H. Electrodeposition of Silicon from a Nonaqueous Solvent. J. Electrochem. Soc. 1988, 135, 109-112. 11. Munisamy T.; Bard A. J. Electrodeposition of Si from Organic Solvents and Studies Related to initial Stages of Si Growth. Electrochim. Acta. 2010, 55, 3797-3803.



12. Pulletikurthi G.; Lahiri A.; Carstens T.; Borisenko N.; Zein El Abedin S.; Endres F. Electrodeposition of Silicon from Three Different Iionic Liquids: Possible Influence of the Anion on the Deposition Process. J. Solid State Electrochem. 2013, 17, 2823-2832. 13. Endres F.; Höfft O.; Borisenko N.; Gasparotto L. H.; Prowald A.; Al-Salman R.; Carstens T.; Atkin R.; Bund A.; Zein El Abedin S. Do Solvation Layers of Ionic Liquids Influence Electrochemical Reactions? Phys. Chem. Chem. Phys. 2010, 12, 1724-1732. 14. Atkin R.; Zein El Abedin S.; Hayes R.; Gasparotto L. H. S.; Borisenko N.; Endres F. AFM and STM Studies on the Surface Interaction of [BMP]TFSA and [EMIm]TFSA Ionic Liquids with Au(111). J. Phys. Chem. C. 2009, 113, 13266-13272. 15. Hayes R.; Warr G. G.; Atkin R. At the interface: Solvation and Designing Ionic Liquids. Phys. Chem. Chem. Phys. 2010, 12, 1709-1723. 16. Mezger M.; Schröder H.; Reichert H.; Schramm S.; Okasinski J. S.; Schöder S.; Honkimäki V.; Deutsch M.; Ocko B. M.; Ralston J.; Rohwerder M.; Stratmann M.; Dosch H. Molecular Layering of Fluorinated Ionic Liquids at a Charged Sapphire (0001) Surface. Science. 2008, 322, 424-428. 17. Perkin S. Ionic Liquids in Confined Geometries. Phys. Chem. Chem. Phys. 2012, 14, 5052-5062. 18. Lovelock K. R. J.; Villar-Garcia I. J.; Maier F.; Steinrück H-P.; Licence P. Photoelectron Spectroscopy of Ionic Liquid-Based Interfaces. Chem. Rev. 2010, 110, 5158-5190. 19. Baldelli S. Interfacial Structure of Room-Temperature Ionic Liquids at the Solid–Liquid Interface as Probed by Sum Frequency Generation Spectroscopy. J. Phys. Chem. Lett. 2013, 4, 244-252. 20. Gasparotto L. H. S.; Borisenko N.; Bocchi N.; Zein El Abedin S.; Endres F. In situ STM Investigation of the Lithium Underpotential Deposition on Au(111) in the air- and waterStable Ionic Liquid 1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide. Phys. Chem. Chem. Phys. 2009, 11, 11140-11145. 21. Hayes R.; Borisenko N.; Corr B.; Webber G. B.; Endres F.; Atkin R. Effect of Dissolved LiCl on the Ionic Liquid-Au(111) Electrical Double Layer Structure. Chem. Commun. 2012, 48, 10246-10248. 22. Borisenko N.; Atkin R.; Lahiri A.; Zein El Abedin S.; Endres F. Effect of Dissolved LiCl on the Ionic Liquid–Au(111) Interface: An In Situ STM Study. J. Phys: Condens Matter. 2014, 26, 284111-284115. 23. Lahiri A.; Carstens T.; Atkin R.; Borisenko N.; Endres F. In Situ Atomic Force Microscopic Studies of the Interfacial Multilayer Nanostructure of LiTFSI-[Py1,4]TFSI on 20 ACS Paragon Plus Environment

Page 20 of 24

Page 21 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


Au(111): Influence of Li+ Ion Concentration on the Au(111)/IL Interface. J. Phys. Chem. C. 2015, 119, 16734-16742. 24. 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. J. Phys. Chem. B. 2006, 110, 6250-6256. 25. Carstens T.; Ispas A.; Borisenko N.; Atkin R.; Bund A.; Endres F. In situ Scanning Tunneling Microscopy (STM), Atomic Force Microscopy (AFM) and Quartz Crystal Microbalance (EQCM) Studies of the Electrochemical Deposition of Tantalum in Two Different Ionic Liquids with the 1-Butyl-1-methylpyrrolidinium Cation. Electrochim Acta. 2016, 197, 374-387. 26. Komadina J.; Akiyoshi T.; Ishibashi Y.; Fukunaka Y.; Homma T. Electrochemical Quartz Crystal Microbalance Study of Si Electrodeposition in Ionic Liquid. Electrochim Acta. 2013, 100, 236-241. 27. 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. Electrochim Acta. 2015, 168, 403-413. 28. Nishimura Y.; Fukunaka Y.; Miranda C. R.; Nishida T.; Nohira T.; Hagiwara R. In situ Raman

Spectroscopy

Studies

of

the

Electrolyte-Substrate

Interface

During

Electrodeposition of Silicon in a Room-Temperature Ionic Liquid. ECS Trans. 2009, 16, 1-6. 29. Borisenko N.; Lahiri A.; Pulletikurthi G.; Cui T.; Carstens T.; Zahlbach J.; Atkin R.; Endres F. The Au(111)/IL Interfacial Nanostructure in the Presence of Precursors and its Influence on the Electrodeposition Process. Faraday Discuss. 2018, 206, 459-473. 30. Tsuyuki Y.; Fujimura T.; Kunimoto M.; Fukunaka Y.; Pianetta P. A.; Homma T. Analysis of Cathodic Reaction Process of SiCl4 in Ionic Liquids. 231st ECS meeting, New Orleans, May 28-June 1st, 2017. 31. Carstens T.; Lahiri A.; Borisenko N.; Endres F. [Py1,4]FSI-NaFSI-Based Ionic Liquid Electrolyte for Sodium Batteries: Na+ Solvation and Interfacial Nanostructure on Au(111). J. Phys. Chem. C. 2016, 120, 14736-14741. 32. Mao J. X.; Damodaran K. Spectroscopic and Computational Analysis of the Molecular Interactions in the Ionic Liquid [Emim]+[FAP]−. Ionics. 2015, 21, 1605-1613.



33. Hayes R.; Borisenko N.; Tam M. K.; Howlett P. C.; Endres F.; Atkin R. Double Layer Structure of Ionic Liquids at the Au(111) Electrode Interface: An Atomic Force Microscopy Investigation. J. Phys. Chem. C. 2011, 115, 6855-6863. 34. Atkin R.; Borisenko N.; Drüschler M.; Zein El Abedin S.; Endres F.; Hayes R.; Huber B.; Roling B. An In Situ STM/AFM and Impedance Spectroscopy Study of the Extremely Pure

1-Butyl-1-methylpyrrolidinium

tris(pentafluoroethyl)trifluorophosphate/Au(111)

interface: Potential Dependent Solvation Layers and the Herringbone Reconstruction. Phys. Chem. Chem. Phys. 2011, 13, 6849-6857.


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190x142mm (300 x 300 DPI)

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In Situ Atomic Force Microscopy and Electrochemical Quartz Crystal

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