Article pubs.acs.org/JPCC
Degradation and Gelation during Plasma Synthesis of Nanoparticles in Ionic Liquids Roya Rudd,† Camille Barthe,‡ Kamil Zuber,† Peter Murphy,† Colin Hall,† Drew Evans,† and Eric Charrault*,† †
Future Industries Institute, University of South Australia, Mawson Lakes, SA 5095, Australia Ecole Nationale Supérieure de Chimie, de Biologie et de Physique, 33607 Pessac, France
‡
ABSTRACT: Ionic liquids are considered to be versatile solvents and mediums in which certain chemical reactions and physical processes can take place. Here we report how ionic liquids acting as a collection medium for sputtered nanoparticles/quantum dots are not necessarily inert and stable during deposition and may lead to possible misinterpretation of the nanoparticle optical properties. By depositing Si nanoparticles in 1-butyl-1-methylpyrrolidinium bis(trifluoromethyl sulfonyl)imide ([C4mpyrr][Tf2N]), we demonstrated that the color change observed upon sputtering was related to the electrochemical decomposition of the [C4mpyrr] cation and not to the presence of Si nanoparticles. Furthermore, a gelation of the medium is observed and is proposed to occur through interaction between the silicon nanoparticles and the ionic liquid and is facilitated by the presence of water.
1. INTRODUCTION Room-temperature ionic liquids (RTILs) are molten salts composed completely of large and complex ions. Owing to their liquid phase at room temperature and their unique properties, such as negligible vapor pressure, thermal stability, high ionic conductivity,1,2 and a large electrochemical window,3−5 they are suitable candidates for many applications.6−10 ILs are also referred to as the “designer solvents” due to the possibility of combining different anions with different cations which lead to versatile and tunable properties such as solubility, making them hydrophilic or hydrophobic.11,12 In chemical processing ILs are excellent replacements for organic solvents used in catalytic and organic reactions, thus reducing the number of volatile compounds released into the environment13,14 and avoiding a costly pressure control apparatus. In addition to the properties listed, ILs have a broad liquidus range, are nonflammable, and are recyclable.15 The negligible vapor pressure of ILs enables their use in the vacuum systems, for characterization measurements (such as Xray photoemission spectroscopy and scanning electron spectroscopy) and for the synthesis of nanomaterials using vacuum systems (such as plasma deposition).16−19 It has been reported that ILs are appropriate candidates for the production and stabilization of nanoparticles (NPs).3,20−23 Torimoto and coworkers in 2006 reported a sputter deposition method for the synthesis of gold NPs using ILs as the collection medium. The sputtered NPs were found to be very stable without the need of additional stabilizing agents.24 A growing interest in the synthesis of nanomaterials using RTILs and plasma results from the many advantages, such as easy and clean synthesis, operating away from the thermodynamic equilibrium/reaction © XXXX American Chemical Society
rates, and fabrication of alloy NPs with binary, ternary, or even quaternary compositions in the absence of additional stabilizing agents or precursors.16,19 However, as reported in one of the author’s earlier works,25 the possible evolution of the IL properties upon exposure to a sputtering/plasma environment remains a challenge to characterize the nanoparticles’ optical properties accurately, and alternative collection media might be preferred.25,26 The interactions between conductive ionic species such as ILs with an electron-rich environment (such as a plasma) cannot be ruled out, especially if they are in intimate contact with each other. As stated above, ILs possess large electrochemical windows (from ±2 V to ±6 V), whose values are established by conducting cyclic voltammetry experiments27 which minimize the period spent under high voltage exposure (applied voltages outside this window cause the electrochemical decomposition of the IL). The voltage range applied during sputter deposition is typically beyond the electrochemical windows of any IL,28 and degradation is therefore expected to be observed. Given the potential for the degradation products to show optical absorption if an IL is used for capturing plasma-deposited nanomaterials, this could lead to a misinterpretation of the NP’s presence and properties when optical absorbance is used as the interrogation tool. In this study, we report the synthesis of silicon (Si) NPs via DC magnetron sputtering into 1-butyl-1-methylpyrrolidinium bis(trifluoromethyl sulfonyl)imide ([C4mpyrr][Tf2N]) and Received: December 19, 2016 Revised: March 9, 2017 Published: March 9, 2017 A
DOI: 10.1021/acs.jpcc.6b12726 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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performed with a Varian 450 GC coupled to a Varian 220 iontrap mass spectrometer. A BP1 25 m × 0.22 mm × 1 μm film column (SGE, Melbourne Australia) was used. The oven temperature program was as follows: hold at 35 °C for 5 min; raise to 250 °C (8 °C min−1); and hold for 1 min. The split vent was opened after 3 min. Helium carrier gas was used at a constant flow rate of 1.2 mL min−1. An 1177 injector fitted with a 0.8 mm ID glass liner was held at 260 °C. The transfer line temperature was 250 °C, and the ion-trap temperature was 200 °C. The MS was tuned to FC-43 (perfluorotributylamine). The MS was scanned from 35 to 600 amu every 0.5 s. The electron multiplier and automatic gain control were set automatically. Library matching of mass spectra was carried out using Varian MS Workstation software (Version 6.9.1). The MS database used was the NIST Mass Spectral Library (Version 2.0f). To understand the changes in electrical performance of the gelled ILs the representative samples were subjected to a cyclic voltammetry test. These samples were analyzed using a Voltalab PGZ100 “All-in-one Electrochemical Laboratory” potentiostat (Radiometer Analytical, France) in a home-built three-electrode cell made of a Teflon block with cylindrical channels: 9.5 mm long, 5 mm in diameter. The cell was closed with gold auxiliary and working electrodes on both sides, and a silver wire was used as the quasi-reference electrode. The cell was designed to limit the air−liquid interface to approximately 1 mm2 to mitigate the effect of air on the measurement.
investigate its stability as a collection medium under the influence of an Ar plasma by varying the NP sputtering conditions. This study experimentally highlights the electrochemical reduction of the [C4mpyrr] cation when exposed to a plasma environment. Interestingly the collection medium turns into a colloidal gel under certain deposition (power, pressure, time) and postdeposition (humidity, aging) conditions. Overall the degradation of the [C4mpyrr][Tf2N] by the plasma is a critical piece of knowledge that is required when using it as part of the synthesis of plasma-deposited NPs.
2. EXPERIMENTAL METHODS 2.1. Deposition of Particles. A custom-built DC magnetron sputter chamber was used to deposit Si nanoparticles. An amount of 2 mL of [C4mpyrr][Tf2N] (from Merck Millipore) was dried under vacuum for 24 h and either spread on a glass plate (over a 24 cm2 area) or poured into a container and then horizontally set in the chamber at a distance of 10.5 cm from the Si target (99.99% in purity, area of 387.096 cm2). Before deposition, the target was presputtered to remove any possible oxide layer and surface contamination; after sputtering the samples were stored in a desiccator to keep them dry. To assess the interaction between the plasma and the IL, several parameters were changed. The effects of the plasma power (from 500 to 1200 W), the Ar gas flow rate (resulting working pressures 1.23 and 3.78 × 10 −3 mbar−base pressure was 10−6 mbar), the deposition time, and the exposed IL-plasma surface area to total IL volume ratio were investigated. 2.2. Characterization. The NPs were studied by transmission electron microscopy (TEM) directly in the IL. Images were taken on a Philips CM 200 FEG apparatus operating at an acceleration voltage of 200 kV. The TEM samples were prepared by putting a drop of solution on a copper grid (200 mesh) covered with a lacey carbon film (ProSciTech Pty Ltd., Australia) and draining the excess of the IL with a paper tissue. Several techniques, such as UV−vis spectroscopy (Cary Varian 5000 spectrophotometer) and Fourier transform infrared spectroscopy (FTIR, Bruker Hyperion 1000 IR microscope operating with a Bruker Vertex 80 IR spectrometer), were used to characterize the chemical composition of the neat and altered IL. The structural comparison of neat IL and IL in contact with plasma was investigated by X-ray photoelectron spectroscopy (XPS) using a SPECS SAGE XPS system with a Phoibos 150 hemispherical analyzer and an MCD-9 detector. Spectra were acquired during Mg Kα (hν = 1253.6 eV) radiation and a source power of 200 W (20 mA and 10 kV). The base pressure in the analysis chamber was 2 × 10−8 mbar. Widescan spectra were acquired at a pass energy of 100 eV and a step size of 0.5 eV. Core-level spectra were acquired at a pass energy of 20 eV and step size of 0.1 eV. All spectra were charge corrected by setting the C 1s peak position to 285 eV. C 1s and N 1s curve fittings were carried out using Casa XPS (Neal Fairley, UK). Gas chromatography and mass spectroscopy (GC-MS, Varian 450 GC coupled to a Varian 220 ion-trap MS) appeared to be the most sensitive technique to detect the decomposition of the IL. Decomposition products present in the IL in contact with plasma were sampled by inserting a 65 μm DVB solidphase microextraction (SPME) fiber into the headspace of a 4 mL septum-capped glass vial containing 1 mL of IL to capture the volatiles. The sample was heated to 60 °C, while the fiber was exposed for 30 min. The fiber was then retracted and analyzed by GC-MS. The analysis of SPME extracts was
3. RESULTS AND DISCUSSION 3.1. Sputtering Synthesis of Si NPs into [C4mpyrr][Tf2N]. A TEM image of 20 min sputtered Si NPs into [C4mpyrr][Tf2N] is depicted in Figure 1 with an energydispersive X-ray analysis (EDX) of the sample. The EDX spectrum analysis showed the presence of Si NPs in the IL as seen with the Si peak at 1.75 kV. Due to the low gas pressure and relatively short target− sample distance, there are no considerable gas-phase collisions of the sputtered species in the space between the target and the IL medium, so condensation of the NPs into aggregates is unlikely (contrary to the aggregation process in a cluster source29). Once in the IL the Si NPs are stabilized by the formation of an electrostatic double layer, which prevents their aggregation or flocculation.30 Other mechanisms proposed for the sputter synthesis of (metal) NPs in ILs involve nucleation and growth of NPs on the surface or within the liquid. It was also reported that some properties of the IL, such as its viscosity, affect the size of the sputtered NPs. These mechanisms have been proposed because different ILs have been shown to create different sized NPs.24,31 3.2. Empirical Observations: Odor, Color, and Gelation of IL-Containing NPs. Upon sputtering Si nanoparticles into [C4mpyrr][Tf2N], specific and characteristic changes of the collection medium were observed. The main observations are depicted in Figure 2. First, the color of the medium changed from almost colorless (in a neat state) to a yellow/orange color, whose shade became darker with increasing the deposition time from 5 to 45 min (power: 1200 W, pressure 1.23 × 10−3 mbar) (Figure 2a). The absorbance of the samples in the visible region enables a quantitative classification of the color change after sputtering (Figure 2b). Second, the samples acquired a strong “fishy” smell, characteristic of volatile amine or sulfur compounds, which was more pronounced by increasing the power or sputtering time. Third, an uncontrolled gelation of the medium was observed in some instances. Figure 2c shows an B
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Figure 1. (a) EDX spectrum analysis of Si NPs, showing a Si signal at 1.75 kV. (b) Typical TEM image of sputtered Si NPs into [C4mpyrr][Tf2N] (power: 1200 W, pressure: 1.23 × 10−3 mbar, 20 min sputtering).
example of (i) neat IL compared with (ii), (iii) a gelled IL. These changes in the IL properties could be obtained by varying different parameters of the Si deposition. Hence, darker shades of yellow were obtained by either increasing the sputtering time (from 5 to 45 min), or decreasing the Ar pressure (from 3.78 × 10−3 to 1.23 × 10−3 mbar), or increasing the deposition power (from 500 to 1200 W), or also increasing the surface to volume ratio of the IL container (from 11 to 37). All these parameters define the properties of the magnetron DC plasma discharge and determine the amount of Si that is sputtered. As a consequence they can be related to the Si volume fraction in the IL (the equivalent of the thickness of the Si film if it was deposited onto a solid substrate). Hereafter, the state of the IL will be stated as either “neat” for a pristine IL, “low Si volume fraction” for an IL exposed to Si sputtering leading to a light yellow color, and “high Si volume fraction” for an IL exposed to Si sputtering leading to a dark yellow color. A similar observation of color change was reported by Vanecht et al.,32 the IL turned more brownish after sputtering of gold NPs. Upon extraction of the Au NPs from the colored IL (using a sodium cyanide solution), the IL returned to its initial colorless state. However, in several studies, changes in the IL color were reported and associated with a possible decomposition of the IL. Using plasma electrochemistry, Brettholle et al.33 and Hofft et al.34 generated different NPs that were accompanied by a change of IL color. Using the IL 1-butyl-3-methylimidazolium tetrafluoroborate [BMI]+[BF4]−, Kaneto et al. working on the synthesis of metal NP and Baba et al. in their investigation of the ion irritation effects on the interface of ILs both reported a similar yellowish color change that they attributed to a decomposition of the IL upon interaction with high-energy ions.35,36 Although the decomposition of some ILs has been reported, there is still no clear evidence of the decomposition products
Figure 2. (a) Color change of IL after various sputter times (power: 1200 W, pressure: 1.23 × 10−3 mbar). (b) Visible spectra of selected neat IL and IL exposed to 5, 10, and 45 min of Si sputtering (at 1200 W and 1.23 × 10−3 mbar). (c) Examples of gelation and flow of the (i) neat IL and IL-containing (ii) high and (iii−iv) low Si volume fractions.
nor of the decomposition mechanisms. Attempts to characterize the degraded IL using XPS, FTIR, or NMR demonstrated no difference between the untreated and plasma exposed/ treated IL.12,33,34,36,37 3.3. Degradation of the IL upon Exposure to RF and DC Plasma. During the sputtering process, in addition to the presence of NPs, the collection medium is also subjected to the plasma discharge. Environmental characteristics of an Ar plasma can be summarized as UV radiation, increasing temperature, and the presence of electrons and ions. To assess the potential role of each of these elements, we performed some quality tests on the neat IL. The pyrrolidinium cation associated with a Tf2N anion has been reported to be the most resistant IL to thermal degradation. Although the reported degradation temperature of [C4mpyrr][Tf2N] is approximately 400 °C,38 recent work by Fox et al. showed that maintaining a high temperature of approximately 200 °C for 2 weeks could lead to the degradation of the IL.12 In our study, the average temperature of the substrate was below 100 °C, regardless of the deposition conditions. To analyze the effect of temperature during the process, the neat IL was placed in an oven at 100 °C for 1 h. No physical changes in the IL were observed, hence a thermal degradation mechanism was ruled out. The UV radiation C
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The Journal of Physical Chemistry C resistance of the neat IL was also tested by placing the neat IL under an UV lamp (225 mW/cm2) for 20 min. No change of color nor odor was detected. In order to fully reproduce the plasma condition without having the influence of the NPs, the neat IL was placed in an RF argon plasma cleaning chamber. Under the conditions tested (time: 45 min, power: 250−300 W, pressure: 0.4 mbar) a change of color (orange) and a fishy smell were observed. Although the RF plasma properties were not similar to those of the DC plasma during the magnetron sputtering of Si NPs (RF vs DC magnetron, different powers, and different pressures), the change of color and the odor can be related to an interaction of the IL with the plasma discharge and not to the presence of the particles. Although our system is equipped with a magnetron, the plasma discharge, which exhibits a degree of confinement to the target, is still expanding up to the IL sample. As a consequence, both ions and electrons are in equal proportions in contact with the IL. Based on the mentioned observations the color change of IL with different sputtering conditions could be related to the different characteristics of the plasma. As stated in Section 3.2 by increasing the sputtering power, the color of IL became darker along with stronger odor. Applying higher power increases the ion flux and ion energy and consequently enhancement of electron density.39 Therefore, a higher degree of decomposition occurs. Increasing the sputtering pressure, in general, results in decreasing the energy of species due to the inelastic collisions and a smaller expansion of the plasma, explaining the observation of a brighter color (less degradation). Increasing the sputtering time results in a longer exposure to the plasma, regardless of the plasma properties, hence in more degradation. In order to assess the changes in the chemical structure of the IL during the sputtering process, XPS and GC-MS were performed on neat and IL exposed to Si sputtering. Figure 3a displays the full survey XPS spectra of [C4mpyrr][Tf2N] for (i) a neat IL, (ii) an IL with low Si volume fraction (5 min of sputtering), and (iii) IL with a high Si volume fraction (20 min at 1200 W and 1.23 × 10−3 mbar). No peak attributed to the Si particles was observed, suggesting the absence of the Si NPs in the upper 10−15 nm of the IL surface. From this, it can be concluded that Si NPs are completely wetted by the IL. The F 1s, N 1s, and C 1s regions of the spectra are identical before and after Si sputtering, indicating that XPS is not sensitive enough to detect any chemical degradation of the IL. Looking at the spectrum of the IL with the high Si volume fraction, in the N 1s region (Figure 3b), two peaks with almost equal densities at 402.8 and 399.7 eV are observed. They can be associated with Ncation and Nanion, respectively, due to the lower electron density at the N atom in the cation compared to the anion.40 In the C 1s region (Figure 3c) three peaks are identified. The peaks at 285.0 and 286.3 eV referred to as the Calkyl and Chetero, respectively, are from the [C4mpyrr] cations. Calkyl arises from three carbon atoms in the butyl group and two carbon atoms in the ring, while Chetero is related to the four carbon atoms which are directly bounded to the N atom. The Canion at 292.5 eV is assigned to C atoms in the [Tf2N] anion. Figure 4a shows the GC-MS spectra of neat IL and that of an IL with high Si volume fraction (20 min to Si sputtering, at 1200 W, and 1.23 × 10 −3 mbar). The new peaks in the spectrum of IL exposed to Si sputtering indicate the presence of decomposition products. These compounds identified by library matches of GC-MS spectra are listed in Table 1 and were indicated by annotation of the chromatogram from headspace SPME analysis of IL in contact with the plasma.
Figure 3. (a) Full XPS survey spectra of a (i) neat IL, (ii) IL with low Si volume fraction (5 min of Si sputtering), and (iii) IL with high Si volume fraction (20 min of Si sputtering, at 1200 W and 1.23 × 10−3 mbar). High-resolution and curve-fitting of sample iii for (b) N 1s and (c) C 1s peaks.
Figure 4. (a) HS-SPME GC-MS chromatograms for neat IL and IL with high Si volume fraction (exposed 20 min to Si sputtering, at 1200 W, and 1.23 × 10−3 mbar). Main new peaks for which tentative identification was possible are annotated. (b) Formation of methyl pyrrolidine and the butyl radical.
Based on the quantum chemical calculations which were performed by Kroon et al.,41 the 1,1-butylmethylpyrrolidinium radical is one of the most probable to form upon reduction of the [C4mpyrr] cation due to its low energy level (E = −61 kJ D
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The Journal of Physical Chemistry C Table 1. Decomposition Products of [C4mpyrr][Tf2N] Resolved from GC-MS Spectra retention time (min)
compound
formula
0.95 2.31 4.97 12.23 12.38 12.48 13.35 14.77 18.53
2-methylpropene butanal 1-methyl-pyrrolidine sulfamic acid trifluoromethane sulfonic anhydride 3-octanamine N-(3-methyl-3-butenyl)-pyrrolidine bis(2-chloroethyl)methylamine N,N-dibutyl-3-buten-1-amine
C4H8 C4H8O C5H11N H3NSO3 C2F6O5S2 C8H19N C9H17N C5H11Cl2N C12H25N
Figure 5. Cyclic voltammograms of neat IL (large black dash), IL with low Si volume fraction (short blue dash, at 3 × 10−3 mbar), and IL with high Si volume fraction (red, at 1 × 10−3 mbar, and 1000 W for 20 min). The lines display the electrochemical window of the ionic liquids.
mol−1). This radical then decomposes into methyl-pyrrolidine and a butyl radical as shown in Figure 4b. Such a mechanism could explain the presence of methyl-pyrrolidine as one of the decomposition products from plasma exposure of [C4mpyrr][Tf2N] as detected by GC-MS. According to the standard ab initio molecular orbital theory and density functional theory (DFT) calculations, a two-electron reduction of [C4mpyrr] was also considered as the most likely reduction route resulting in the formation of methyl-pyrrolidine and a butyl anion. Identification of those compounds in addition to the products with double bonds (Table 1) suggests that the reduction of [C4mpyrr] could be through a two-electron reduction mechanism. Judging from the electron density and the lowest unoccupied orbital (LUMO) of [Tf2N], Howlett et al.42 also showed the possibility of [Tf2N] decomposition. Based on the ab initio calculations, the addition of an electron to the [Tf2N] leads to cleavage of the S−N bond and formation of SO2CF3− and nitrogen-centered radical species (NSO2CF3−). The formed degradation products can undergo further degradation such as cleavage of the S−C bond and formation of NSO2− and SO2−; however, as the authors mentioned, in reality, a more complex range of reduction processes will occur. Apart from the decomposition of unstable radicals to more stables ones, radicals can also be involved in other reactions such as radical− radical interaction or radical−alkene interaction which lead to the formation of new decomposition products.41 Sulfur- and amine-based compounds which result from such reactions could explain the strong smell of the IL after exposure to plasma. To further probe the degradation of the IL, cyclic voltammetry tests were performed in the range of −3 V to +3 V at the scan rate of 20 mV/s on neat IL and ILs with low (exposed 5 min to Si sputtering) and high Si volume fractions (exposed 20 min to Si sputtering plasma, at 1200 W and 1.23 × 10 −3 mbar), respectively (see Figure 5). Two distinct regions were found in the recorded voltammograms, the central part with a relatively low average slope and the steep region at higher absolute potential values. The central part exhibits small current peaks related to mainly reversible redox reactions within the ionic liquid. The cyclic voltammograms of ionic liquids with low and high Si volume fractions show several additional peaks, which suggest an increase in redox activity compared to the neat IL sample. These peaks can be associated with the decomposition products of the ILs after exposure to the plasma discharge during sputtering. The extreme regions of the voltage in the voltammograms display a slope an order of magnitude greater than in the central part. At this voltage range, the tested samples undergo
irreversible changes resulting from degradation of the electrolyte. The boundaries between the reversible and irreversible voltage regions define the electrochemical window. In most applications, the electrochemical window is defined as the region in the voltammogram with the absolute current density below 1 mA/cm2.43 Based on this criterion the electrochemical window of analyzed ionic liquid samples was calculated and displayed in Figure 5. The values of the electrochemical window were [−2.647; 2.750] V for the neat IL sample, [−2.365; 2.553] V for the IL with low Si volume fraction, and [−2.037; 2.264] V for the IL with high Si volume fraction. The narrower value of the electrochemical window of plasmatreated ionic liquids is indicative of their degradation through the process, further supporting the mass spectrometry results. The IL with high Si volume fraction displayed a higher reduction of the electrochemical window, most likely being the result of its exposure to the DC magnetron plasma at higher power and lower pressure (resulting in a higher kinetic energy of the clusters and ions bombarding the ionic liquid). Also, a larger difference in the electrochemical window was observed in the negative potentials with the ILs with low and high Si volume fraction, respectively, recording 0.282 and 0.610 V differences in cathodic limit compared to the neat sample and 0.197 and 0.486 V difference in the anodic limit. Such asymmetry can be explained by a larger level of degradation of [Tf2N] anions in the ionic liquid.10 3.4. Gelation via Water Diffusion. Samples that gelled exhibited the same characteristics as the nongelled samples in terms of color and odor. Various levels of flow were observed, and the gels showed evidence of a thixotropic behavior (Figure 2c), as when subjected to a shear strain (upon agitation of its container) the gel started flowing and became completely fluid in nature in some instances, with a measurable time period needed to recover its high viscosity gelled state after being perturbed. As the gelation was nonsystematic and not directly dependent on the number of nanoparticles, it suggested that an uncontrolled environmental factor was triggering the gelation. The gelation process was related to the “Si-doped” IL exposure to ambient moisture/water. To test the impact of water vapor on the gelation process for a given deposition, three vials containing the IL with the sputtered Si NPs were submitted to different atmospheres: (a) closed and (b) open container in atmospheric conditions and open containers in (c) nitrogen, (d) humid, and (e) dry environments. Only samples b E
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the highly electronegative [Tf2N] anion. The importance of the F atoms of the anion was confirmed as no gelation was observed when replacing [C4mpyrr][Tf2N] by trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate ([P6,6,6,14][(iC8)2PO2]) and sputtering Si under the same sputtering conditions. A particle to IL mass ratio (densities of [C4mpyrr][Tf2N] and Si = 1.39 and 2.32 g/cm3, respectively) of around 0.12% was calculated using the surface area to volume of the glassware containing the IL (S/V = 38) and the deposition rate as estimated from film deposited on a glass substrate (film thickness = 200 nm). The very small amount of particles required to induce gelation could be explained by the loosely outspread fractal structure formed by interconnected aggregates of the nanoparticles.53,54 It has been reported that as the size of the silica particles decreases the mass ratio required to start the gelation decreases.50,55 From the theoretical work of Allain56 we can predict the critical volume fraction of particles for the formation of colloidal gels by the fractal spatial structure of aggregates
and d, exposed to water vapor, showed preliminary signs of gelation, while the others remained totally liquid. The critical role of water was also confirmed by the identification of a peak related to the presence of water in the FTIR spectra of gelled samples, as shown in Figure 6.
⎛ 4π ΔρgR4 ⎞(3 − d f )/(1 + d f ) ⎟ φc ≈ ⎜ ⎝ 3kT ⎠
where Δρ is the difference of density between the particles and the IL, g the gravitational acceleration, R the particle radius, k the Boltzmann constant, T the absolute temperature, and df the fractal dimension of the aggregated structure (1.78). In our case, for an estimated mean radius of 5 nm, φc is 3 × 10−4. The volume fraction of particles for a gelled IL is calculated to be around 8 × 10−4 (0.12 wt %) and confirms that the number of particles present in our sample is enough to induce gelation of the medium. From observations, the presence of both Si particles and water in the IL is required to achieve gelation. However, since the IL degrades upon exposure to the plasma during the DC magnetron sputtering, another factor in the gelation could lie in a possible radical polymerization process of the decomposition products initiated by the amine cation. Many studies reported the solidification or polymerization of IL upon long-term aging at elevated temperature. In the study by Wooster et al.57 on the thermal degradation of the cyano-containing ILs, polymerization occurred only for N-based cations and cyano-containing anions only. They suggested that thermally degraded amine products acted as an initiator of the polymerization process. A similar study by Fox et al.12 reported an increase in viscosity and in some instances the solidification of various [Tf2N] ILs after several weeks of thermal treatment (200 °C). Although the temperature of the sputtering process is much shorter and operates at a much lower temperature, this thermal polymerization cannot be fully disregarded. Here again, the presence of water is critical as many radical polymerization processes require the presence of a proton scavenger (or base) to facilitate the abstraction of a proton leading to dimers and eventually oligomers being formed. Such is the case for water as a proton scavenger in the radical polymerization of poly(3,4ethylenedioxythiophene).58 The presence of water could here help to facilitate the gelation of the degraded IL due to its ability to form H3O+ and thus abstract the necessary proton to increase the kinetics of polymerization.
Figure 6. (a) FTIR spectra of neat [C4mpyrr][Tf2N] and [C4mpyrr][Tf2N] with low (in green, S/V = 11) and high Si volume fraction (in red, S/V = 37, 20 min of Si sputtering, at 1200 W and 1.23 × 10−3 mbar). (b) and (c) Magnifications around the wavenumber ranges where water-related peaks are observed.
The typical bands associated with [C4mpyrr][Tf2N] are seen between 2969 and 2883 cm−1 (stretching modes of alkyl groups from the cation) and below 1500 cm−1.44,45 Free water molecule peaks are seen with the bands at 1630 cm−1 and around 3600 cm−1 and are also present in the neat IL.46 The large band between 3200 and 3400 cm−1 is characteristic of bonded water46,47 and demonstrates that not only particles were deposited but also that they interact with water within the IL. The high diffusivity of water molecules confined in the IL, especially in a hydrophobic one, such as [C4mpyrr][Tf2N], would lead to the reaction between the Si NPs with water to result in the formation of Si core−SiO2 shell NPs. Then a typical “ionogel” formation through solvation of the silica shell particles by the IL can happen.48−52 It also suggests that through the water splitting reaction leading to the gelation there is production of H2, which could have implications in energy applications. Similar observations were reported by Erogbogbo et al. with their study on the enhancement of H2 production using Si nanoparticles.53 In our case, we hypothesize that the stabilization of the particle network is based on hydrogen bonding between the newly formed surface silanol groups of the nanoparticles and F
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4. CONCLUSIONS The electrochemical stability of [C4mpyrr][Tf2N] was tested to assess its use under plasma environment. We demonstrated that upon exposure to a high-intensity electromagnetic field environment the [C4mpyrr][Tf2N] undergoes an electrochemical reduction of its cation and anion which could be observed by a change in color, a strong odor, and a watercontrolled gelation post process. This could have a significant impact on the development of nanomaterials, as it could lead to misinterpretations of the optical properties of the synthesized materials. We also demonstrated that upon sputterin Si nanoparticles interacted with water within the IL to create a weak gel via hydrogen bonding between the particle surface and the F groups present in the liquid. Low vapor liquids, especially RTILs, are a commonly used collection medium for the synthesis of sputtered nanomaterials. As our results demonstrate, although very stable and suitable for a large variety of applications, IL must be tested before implantation in a specific process, especially those involving exposure to a plasma glow discharge. Moreover, GC-MS proves to be an efficient characterization technique to identify the decomposition of IL.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kamil Zuber: 0000-0003-0579-1983 Drew Evans: 0000-0002-1525-2249 Eric Charrault: 0000-0003-3392-136X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This original research was proudly supported by SMR Automotive Australia and the Commonwealth of Australia, through the Automotive Australia 2020 Cooperative Research Centre (AA2020CRC).
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REFERENCES
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