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Applied Materials Today 9 (2017) 10–20

Contents lists available at ScienceDirect

Applied Materials Today

j ourna l ho me page: www.elsev ier .com/ locate /apmt

ybrid electrolytes based on ionic liquids and amorphous porous siliconanoparticles: Organization and electrochemical properties

ohamed R. Tchalala a,∗, Jehad K. El-Demellawi a, Edy Abou-Hamad b, José Ramón Durán Retamal c,urushothaman Varadhan c, Jr-Hau He c, Sahraoui Chaieb d,e,∗∗

Physical Sciences and Engineering (PSE) Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi ArabiaKAUST Catalysis Center (KCC), King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi ArabiaComputer, Electrical and Mathematical Sciences and Engineering (CEMSE) Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi ArabiaBiological and Environmental Sciences and Engineering (BESE) Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi ArabiaLawrence Berkeley National Laboratory, Chemical Sciences Division, 1 Cyclotron Road, Mailstop 6R-2100, Berkeley, CA 94720, USA

r t i c l e i n f o

rticle history:eceived 5 February 2017eceived in revised form 26 April 2017ccepted 27 April 2017

eywords:onic liquid-nanoparticles hybridsmidazolium-based ionic liquidslectrochemical activityurrent transportanoparticle stability

a b s t r a c t

Ionic liquids (ILs) and ionic liquid-nanoparticle (IL-NP) hybrid electrolytes have garnered a lot of interestdue to their unique properties that stimulate their use in various applications. Herein, we investigatethe electrochemical and photo-physical properties of organic-inorganic hybrid electrolytes based onthree imidazolium-based ionic liquids, i.e., 1-buthyl-3-methylimidazolium thiocyanate ([bmim] [SCN]),1-ethyl-3-methylimidazolium tetrafluoroborate ([emim] [BF4]) and 1-buthyl-3-methylimidazoliumacetate ([bmim] [Ac]) that are covalently tethered to amorphous porous silicon nanoparticles (ap-SiNPs). We found that the addition of ap-Si NPs confer to the ILs a pronounced boost in the electrocat-alytic activity, and in mixtures of ap-Si NPs and [bmim] [SCN], the room-temperature current transportis enhanced by more than 5 times compared to bare [bmim] [SCN]. A detailed structural investigationby transmission electron microscope (TEM) showed that the ap-Si NPs were well dispersed, stabilizedand highly aggregated in [bmim] [SCN], [emim] [BF4] and [bmim] [Ac] ILs, respectively. These observa-tions correlate well with the enhanced current transport observed in ap-Si NPs/[bmim] [SCN] evidencedby electrochemical measurements. We interpreted these observations by the use of UV–vis absorbance,photoluminescence (PL), FTIR and solid-state NMR spectroscopy. We found that the ap-Si NPs/[bmim]

[SCN] hybrid stands out due to its stability and optical transparency. This behavior is attributed to theiron(III) thiocyanate complexion as per the experimental findings. Furthermore, we found that the addi-tion of NPs to [emim] [BF4] alters the equilibrium of the IL, which consequently improved the stabilityof the NPs through intermolecular interactions with the two ionic layers (anionic and cationic layers) ofthe IL. While in the case of [bmim] [Ac], the dispersion of ap-Si NPs was restrained because of the highviscosity of this IL.

. Introduction

Organic-inorganic hybrids are a class of materials where theomponents are intimately mixed at the nanometric scale [1–4].istorically, the notion of combining organic and inorganic com-onents had been an archaic challenge that dated back to the

∗ Corresponding author.∗∗ Corresponding author at: Biological and Environmental Sciences Division (BESE),ing Abdullah University of Science and Technology, Thuwal 23955-6900, Saudirabia.

E-mail addresses: mohamed.tchalala@kaust.edu.sa (M.R. Tchalala),chaib@lbl.gov (S. Chaieb).

ttp://dx.doi.org/10.1016/j.apmt.2017.04.011352-9407/© 2017 Elsevier Ltd. All rights reserved.

© 2017 Elsevier Ltd. All rights reserved.

beginning of the industrial era [5]. However, the term hybrid mate-rials became only known after the development of soft chemistrytechniques in the 1980s [1]. In recent years, hybrid materials com-posed of nanoparticles (NPs) and ionic liquids (ILs) [6–10], and alsoknown as molten salts, reaped a lot of attention. The fusion of theself-assembling property of ILs and functional inorganic nanoma-terials is anticipated to lead to novel materials that can be usedin several applications. This new class of hybrids, which is stillunder development, can offer high mechanical strength compara-

ble to that of solid polymer electrolytes [11,12]. Due to their notableelectrochemical properties and biocompatibility, they have beenrecently used in biosensing applications [13–16].

ILs are typically composed of cations and anions, and exist asliquids below 100 ◦C. In contrast to organic solvents, ILs have a lot

M.R. Tchalala et al. / Applied Materials Today 9 (2017) 10–20 11

on of

opIsdsb[

pnauapwttcAjfiftnt

bIisfesiT

Scheme 1. Schematic representati

f advantages such as high thermal and chemical stability, higholarity and they could exist with high or low volatility [17,18,21].

n addition, they are non-flammable and miscible with a variety ofolvents [17–20]. They have also shown wide electrochemical win-ows suitable for many electrochemical as well as voltammetricensing applications [21–23]. Their physicochemical properties cane tailored by judicious selection of cations/anions combinations24].

On the other hand, because of their novel characteristics com-ared to their bulk counterparts, NPs have been central forumerous applications [25–27]. Among all NPs and owing to itslready existing technological knowledge-base, we have a partic-lar interest in ap-Si NPs due to their low toxicity, availabilitynd cost-effectiveness as well as their surprising photoelectricroperties [28,29]. They exhibit a net electrodynamic attraction asell as quantum confinement effects that evolve novel optoelec-

ronic properties [30]. Furthermore, ap-Si is considered as one ofhe most outstanding photovoltaic materials for large-area solarells, especially for those operating at high temperatures [31,32].lthough p-Si nanostructures have been comprehensively sub-

ected to considerable scrutiny, they rather remain to be an activeeld of research with many aspects that are not fully covered. Apart

rom their photonic, sensing and chemical applications [33–35],heir electrochemical studies are mostly limited to p-Si films andanowires [36,37]. ap-Si NPs, on the other hand, were not subjectedo electrochemical investigations.

In this study, we report on new hybrid electrolytes formedy tethering imidazolium cations anchored onto ap-Si NPs. Our

Ls/ap-Si NPs hybrid materials exhibited surprising electrochem-cal properties with high ionic conductivity and exceptional redoxtability window. Such enhancement in the electrochemical per-

ormance hold out the eventual possibility to explore these hybridlectrolytes as additive to well-known metal-ion membrane sen-ors [38–43]. PL, UV–vis absorbance and FTIR measurements clearlyndicated the covalent interaction between the ILs and ap-Si NPs.hese findings asserted the strong relationship between the elec-

the ionic liquids used in this work.

trochemical responses and the microstructure of the resultinghybrid materials. To the best of our knowledge, there has been noreport as yet on the application of freestanding luminescent amor-phous porous Si NPs with ILs grafted, as hybrid electrolyte systemswith enhanced current transport and pronounced electrocatalyticability. Moreover, this work highlights the potential use of some ILsas a platform for efficient separation of metal ions [44,45].

2. Materials and methods

2.1. Materials synthesis

We used a modified stain etching method to make ap-Si NPs[28,29]. Laser-cut rectangular strips of p-type single crystal (100-oriented) boron doped silicon wafers (Addison Electronics) wereimmersed in an etching bath for 13 h. The etching bath composedof 4.71 g anhydrous iron(III) chloride (98%, Alfa Aesar) dissolvedin a mixture of 18 ml of hydrochloric acid (32%, Fischer Scientific)and 42 ml of hydrofluoric acid (49%, Sigma-Aldrich). The role ofthe hydrochloric acid is to acidify the iron (III) chloride in solutionto maintain its solubility. The standard oxidant potential of Fe3+

(+0.77 V) meets the required potential for stain etching of silicon(Eo > 0.7 V). The etching turned the lustrous surface of silicon stripsinto matte black. We cleaned the etched strips by rinsing themwith deionized water (Millipore) followed by absolute ethanol(Sigma-Aldrich). We then dried the strips under a nitrogen stream.After drying, a yellow film of ap-Si was formed on the surface.The etched strips were submerged in toluene (Chromasolv grade,Sigma-Aldrich), and sonicated using a VWR Ultrasonic Cleaner son-icating bath for 45 min. Sonication dispersed the tenuous structureof the ap-Si film into ap-Si NPs in toluene. Ultra-centrifugation

was used to remove large clusters (>500 nm in size) that wouldnot remain suspended. Resulting cloudy suspensions of ap-Si NPsexhibited a bright yellowish orange photoluminescence (PL) withan emission peak at 600 nm and an excitation peak at 350 nm asshown in Fig. S1 in the Supplementary information (SI).

12 M.R. Tchalala et al. / Applied Materials Today 9 (2017) 10–20

tion o

atdnt(thwS

atNDl

bme31a(csaatcai

2

GssIeaap[tmo

Fig. 1. TEM micrographs and particle size distribu

Product yields were estimated to be approximately 0.4 mg ofp-Si NPs per cm2 of silicon wafer. The photoluminescence quan-um yield of suspensions made in toluene was around 50%. Theimensions of these particles were estimated to be hundreds ofanometers in size (see Fig. S2). The selected area electron diffrac-ion (SAED) pattern of ap-Si NPs indicated an amorphous structureinset of Fig. S2b). BET measurements, as well as high-resolutionransmission electron microscopy (HRTEM) images, revealed aighly porous structure (∼60% in volume) composed of a lacey net-ork of nanometer scale bridges connecting larger aggregates (Fig.

2).Several mixtures of ap-Si NPs in ILs were carefully prepared in an

rgon-filled glove box to minimize the oxidation and the aggrega-ion of NPs. In a typical experiment, we added 36 mg of dried ap-SiPs, which were obtained by using a Lab Conco RapidVap Vacuumry Evaporation System, to 3 ml of ionic liquid. The mixtures were

eft under stirring for 12 h at room temperature and pressure.In this work, we meticulously selected three different ILs

ased on their properties, i.e., high conductivity, good ther-al stability and low melting points, in addition to sufficient

lectrochemical stability. The three used ILs are: 1-butyl--methylimidazolium thiocyanate [bmim] [SCN] (98% purity),-butyl-3-methylimidazolium acetate [bmim] [Ac] (99% purity)nd 1-ethyl-3-methylimidazolium tetrafluoroborate [emim] [BF4]99% purity). They were purchased from Sigma-Aldrich andonserved under vacuum. Scheme 1 shows the schematic repre-entation of the selected ionic liquids. They were characterized by

combination of 1H NMR, 13C NMR, halide content determinationnd water content assay techniques. Typical halide and water con-ents were always less than 0.1%. We determined the levels of waterontent and colored impurities in these ILs to be less than 20 ppmnd 25 ppm, respectively. Table 1 shows some useful physicochem-cal properties of the three selected ILs.

.2. Instrumentations

Suspensions of ILs/ap-Si NPs were handled in an argon-filled MBB 2202-C acrylic glove box (MBraun). The electrochemical mea-

urements (current density-potential J-V and electrode impedancepectra Z = Z’ + jZ”) were performed with an Autolab (Metrohmnstruments) electrochemical workstation with a standard three-lectrode cell. Two equal sized Pt wires were used as a working and

counter electrode, and a saturated silver/silver chloride (Ag/AgCl)s a reference electrode. The electrodes were immersed in highly

ure ILs electrolytes ([bmim] [SCN], [emim] [BF4], and [bmim]Ac]) with (w) and without (wo) the addition of ap-Si NPs at roomemperature and under static conditions. The linear sweep voltam-

etry (LSV) and the cyclic voltammetry (CV) was performed tobtain the J-V curves of the ILs electrolytes at a scanning rate of

f ap-Si NPs in (a) [emim] [BF4] (b) [bmim] [SCN].

5 mV s−1. The electrochemical impedance spectra (EIS) was per-formed by using a 5 mV sinusoidal potential signal at the frequencyrange from 1 MHz to 5 Hz with 10 points per decade.

Fluorescence spectra were attained using a Horiba Fluoromax-4 spectrofluorometer. Absorbance measurements were carried outusing a Genysis 10S Spectrophotometer (Thermo Scientific) usinga conventional 10 mm quartz cuvette. Fourier transform infrared(FTIR) spectra were recorded using a Thermo Nicolet iS10 FT-IRspectrometer. Samples were prepared for FT-IR measurements bydrying the suspensions in KBr salts and pressing them into pellets.HRTEM micrographs were taken using an FEI Titan Krios CT-TEMat an operating voltage of 120 kV. TEM samples were prepared bydrop casting a solution of IL-ap-Si NPs mixture onto a 200 meshcopper grid with a holey carbon film. The TEM images were takenin different regions of the grid. Scanning electron microscopy (SEM)experiments were conducted using a Nova NanoSEM 450 SEM (FEI)working in the secondary electrons mode at a voltage of 2 kV and aresolution of 2 nm.

One-dimensional 1H and 13C MAS solid-state NMR spectrawere recorded using Bruker AVANCE III spectrometers operatingat 600 MHz resonance frequencies with a conventional double-resonance 3.2 mm CP/MAS probe. The sample was packed into azirconia rotor under an inert atmosphere inside the glove box. Drynitrogen gas was utilized for sample spinning to prevent degra-dation of the samples. NMR chemical shifts are reported in theexternal references; TMS and adamantine for 1H and 13C, respec-tively. 19F MAS solid-state NMR spectrum was acquired on a600 MHZ NMR spectrometer using direct polarization with 32 scansnumber and repetition delay of 2 s.

3. Results and discussion

To elucidate the upshot of the ap-Si NPs/ILs interaction, we firstvisualized the collective behavior of the NPs by TEM (Figs. 1 and 2).

TEM measurements in Fig. 1a shows that the ap-Si NPs wereuniformly surrounded by a thin external layer in their [emim] [BF4]host. Individual NPs appear to be linked together by an over layerencapsulating several NPs, which provides a good stability for theseNPs. Normally, ILs with the general formula [emim] [X] or [bmim][X] (X = BF4•, PF6•, N(CN)2•, Tf2N•) provide stabilization to a widerange of nanoparticles, albeit some agglomeration could not some-times be avoided [46,47]. In [emim] [BF4], the imidazolium ringsare linked by hydrogen bonding with the fluoride atoms in BF4•.These interactions between [emim]+ and [BF4]• were reduced by

the addition of ap-Si NPs, leading to an increase in free ions con-centration.

These observations correlate well with those reported in otherstudies, showing that ap-Si NPs were surrounded by an ionic dou-ble layer made of anionic [BF4] and cationic [emim] compounds

M.R. Tchalala et al. / Applied Materials Today 9 (2017) 10–20 13

Table 1Physicochemical properties, sorted by the functional anion, of selected ILs at room temperature (25 ◦C/298.15 K).

Code Anion Cation MW [Da] Melting Point [◦C] Density (at 25 ◦C) [g/ml] Viscosity (at 25 ◦C) [cP]

[bmim] [SCN] Thiocynate 1-butyl-3-methylimidazolium 197.3 <−20 1.069 51.7[emim] [BF4] Tetrafluoborate 1-ethyl-3-methylimidazolium 197.8 6 1.248 66[bmim] [Ac] Acetate 1-butyl-3-methylimidazolium 198.3 <−20 1.0192 393.3

Fn

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[o[arw

tIiip

tmcw

aeioot

Table 2Electron transfer resistance (Rct) extracted from Nyquist plots of the three ILs understudy.

Ionic Liquid Rct (�) wo ap-Si NPs Rct (�) w ap-Si NPs

ig. 2. TEM image of self-assembled aggregates of amorphous porous siliconanoparticles in the presence of [bmim] [Ac].

48–50]. Consequently the [emim] [BF4] stabilizes the ap-Si NPs.his behavior will be much more prominent in the case of smallPs.

Fig. 1b clearly showS the well-defined individual spherical ap-SiPs, in [bmim] [SCN], that are uniformly distributed and homo-eneously dispersed over the TEM grid without aggregation. Evenhough ap-Si NPs have a tendency to aggregate in most solvents,he particular structure and properties of the large cations andnions in the ILs, which act as a self-assembling template like longhain surfactant, prevented the NPs aggregation [51,52]. Moreover,he pre-organized solvent structure encourages the ap-Si NPs tondergo self-organization to well-defined nanostructures.

The ap-Si NPs were well dispersed in both [emim] [BF4] andbmim] [SCN]. The size distribution of ap-Si NPs was statisticallybtained over 400 individual particles. The ap-Si NPs in [emim]BF4] had an average diameter (dav) of 40 nm with a standard devi-tion (�) of 7 nm, while the mixture of ap-Si NPs with [bmim] [SCN]esulted in the formation of homogeneously dispersed ap-Si NPsith dav of 95 nm and � of 10 nm.

On the other hand, TEM images of ap-Si NPs/[bmim] [Ac] elec-rolyte (Fig. 2) show large aggregates. Unlike with the previous twoLs, this mixture seems denser and more opaque under white-lightllumination, suggesting a high level of polydispersity, poor stabil-ty of a poorly ordered structures. This turbidity can be a sign ofoor solubility as well.

Next, we investigated the effect of ap-Si NPs on the elec-rochemical properties of these ILs by performing LSV and EIS

easurements in a standard three-cell Pt/IL/Pt vs Ag/AgCl. The J-Vurves obtained from the LSV measurements for the ILs under study/wo ap-Si NPs are depicted in Fig. 3.

The potential range under study, small currents with local peaksppear during the cathodic and anodic processes, indicating the

ffective charge transfer processes due to the electro-reduction ofmidazolium cation and the electro-oxidation of absorbed speciesn the Pt electrode surface and/or other processes. However, annset of an enhanced anodic current appears at potentials higherhan ∼0.5 V, suggesting the electrochemical decomposition of the

[bmim] [SCN] 66.84008412 44.18618495[emim] [BF4] 24.36078093 21.16364716[bmim] [Ac] 727.1647022 584.1750131

anions due to the electroactivity of the melts. We attribute sucha limited electrochemical potential stability window to the Ptcatalytic activity. It is noteworthy that alternative glassy carbonelectrodes can improve the electrochemical measurements. On theother hand, the addition of ap-Si NPs to the [bmim] [SCN] and[emim] [BF4] ILs significantly varies the current values, shift thelocal peaks, and slightly limits the electrochemical window. Thissuggests that the well-dispersed and stabilized ap-Si NPs enhancethe electrocatalytic activity due to improved charge transport.However, by adding ap-Si NPs to [bmim] [Ac] the J-V curve remainsalmost unaltered, suggesting that the highly aggregated ap-Si NPsin [bmim] [Ac] do not affect the electrical behavior of the IL.

Furthermore, we investigated the direct electrochemical redoxprocesses on the modified and unmodified ILs by cyclic voltamme-try (Fig. 4). The results showed that the redox processes are veryweak on the bare ILs, but the addition of ap-Si NPs into the ILsenhanced the peak currents and reduced the overpotential. In thecase of [bmin] [Ac] IL, the addition of ap-Si NPs did not alter sig-nificantly the cyclic voltammogram, although it introduced a highoxidation activity peak in the forward sweep suggesting therebythat the nature of the electrode reaction is irreversible. This is partlydue to the highly aggregated ap-Si NPs in [bmim] [Ac]. On the otherhand, a substantial current shift and current increase were observedwith the addition of ap-Si NPs into [bmin] [SCN] and [emin] [BF4],suggesting the catalytic ability of ap-Si NPs due to the ability topromote charge transfer when are well-dispersed or stabilized inthe [bmin] [SCN] and [emin] [BF4] ILs, respectively.

Following the electrical characterization, we depict in Fig. 5 theNyquist plots extracted from the EIS measurements of the threeILs tested w/wo ap-Si NPs. The Nyquist plots show only one high-frequency semicircle for the [bmim] [SCN] and [emim] [BF4], whichare related to one relaxation process. The equivalent circuit canbe modeled as a parallel resistance-capacitor (RC) circuit, wherethe resistance models the mobile ions and the capacitor modelsthe immobile molecular chain. In contrast, [bmim] [Ac] exhibits ahigh frequency semicircle with a low frequency inclined spike. Thesemicircle is related to a diffusion limited electron transfer processcharacteristic of a parallel bulk resistance-capacitor circuit, whilethe non-vertical spike refers to an interfacial impedance betweenthe electrolyte and the electrode surface characteristic of a constantphase element due to the formation of an insulating layer on therough electrode surface. The equivalent circuit combines a Randle’scircuit in series with a Warburg impedance. Moreover, accordingto the magnitude of the semicircles, the resistance order followsthe following trend R[bmim] [SCN] < R[emim] [BF4] < R[bmim] [Ac],

and the electron transfer resistances (Rct) of the ILs with and with-out ap-Si NPs are tabulated in Table 2. This finding is in agreementwith the current density values observed in the LSV measurementswhere J[bmim] [SCN] > J[emim] [BF4] > [bmim] [Ac]. The impact ofthe addition of ap-Si NPs on the EIS is significantly different for the

14 M.R. Tchalala et al. / Applied Materials Today 9 (2017) 10–20

Fig. 3. J-V curves of the ILs electrolytes (a) [bmim] [SCN], (b) [emim] [BF4], and (c) [bmim] [Ac] with (empty symbol) and without (filled symbols) the addition of ap-Si NPs.

Fig. 4. Cyclic voltammograms of the ILs electrolytes (a) [bmim] [SCN], (b) [emim] [BF4], and (c) [bmim] [Ac] with (empty symbol) and without (filled symbols) the additionof ap-Si NPs.

F (c) [bN

tciitrgIcrat

aep

[tT

ig. 5. Nyquist plots of the ILs electrolytes (a) [bmim] [SCN], (b) [emim] [BF4], andPs.

hree ILs under study: (i) it shifts the curvature center of the semi-ircle in [bmim] [SCN] indicating enhanced dispersion effect, (ii)t reduces the diameter of the semicircle in [emim] [BF4] suggest-ng prompter initiation of the chemical reaction, and (iii) it reduceshe semicircle angle and the inclination of the spike in [bmim] [Ac]evealing narrower relaxation time distribution. These results sug-est that the good dispersion of the ap-Si NPs into [bmim] [SCN]L increases the conductivity by increasing charge carriers and/orarrier mobility due to salt dissociation and/or lower viscosity,espectively. In contrast, the stabilized ap-Si NPs in [emim] [BF4]nd highly aggregated ap-Si NPs in [bmim] [Ac] restrict the chargeransfer.

To better understand the behavior of ap-Si NPs/ILs mixtures,nd the existing rapport between the dispersion quality and thenhanced electrocatalytic activity, we conducted extensive photo-

hysical and vibrational studies in addition to solid-state NMR.

The UV–vis absorption spectra of the blank [bmim] [SCN],bmim] [Ac] and [emim] [BF4], are depicted in Fig. 6a. Fig. S3a showshe transparency of the three ILs under UV illumination (365 nm).he absorbance spectra of blank ILs show similar absorption band

mim] [Ac] with (empty symbol) and without (filled symbols) the addition of ap-Si

between 200 and 480 nm and a noticeable long tail absorption sim-ilar to those reported for other imidazolium ILs [53,54].

On the other hand, we found that the absorption spectra of thehybrid electrolytes show an absorption band between 210 nm and480 nm, with different intensities (Fig. 6b). This band is attributed tothe imidazolium cation, whereas the weaker absorption maximumis due to ap-Si NPs. The width of this band indicates probably thebroad size distribution of our NPs [55].

The most important difference between these absorption spec-tra is the strong and broadband observed at 489 nm in the ap-SiNPs/[bmim] [SCN] UV–vis spectrum (Fig. 6b). The appearance ofthis notable peak was accompanied by a change in the color of theobtained electrolyte, from colorless to vivid red as shown in Fig. S3b.This change in color occurs because of the anionic complex formedbetween iron (III) and thiocyanate ion ([Fe (SCN) 6]3−) [56,57]. The

kinetics of this iron (III) and thiocyanate complexation are depictedin Fig. S4. SCN• is known as a highly coordinating ligand as con-firmed by the NMR studies (see SI). Furthermore, it is known thatthe action of alkali thiocyanate on ferric salts leads to the forma-tion of intensely red colored metal complexes. Initially, we did not

M.R. Tchalala et al. / Applied Materials Today 9 (2017) 10–20 15

F e) andN h leng

ewuTcrtfloeesasm

itwm

[baw

ig. 6. (a) UV–vis absorption spectra of blank [bmim] [SCN] (red), [emim] [BF4] (bluPs. The absorption spectra in green are for ILS with ap-Si NPs. We used a 1 cm pat

xpect the formation of such ferric complex because no iron sourceas added during the mixing process. However, iron chloride was

sed as a catalyst during the preparation of the ap-Si NPs [28,29].he same behavior was also reported in the study of dispersed ironarbide (Fe3C) nanoparticles in [emim] [SCN] leading to the sameed-colored solution [57]. Therefore we believe that the complexa-ion does not occur from the surface of our nanoparticles but ratherrom the traces of iron left in a solution that bond with the SCN•igand near the ap-Si NPs surface. This finding reveals the presencef traces of Fe3+ from the ap-Si NPs preparation process. It, how-ver, highlights the role played by [bmim] [SCN] as a very goodlectrolyte that can potentially replace conventional electrolyteystems for various potentiometric sensors with high selectivitynd sensitivity [58–70]. It can typically be used for Fe3+ selectiveensors [71], and also to develop new ion-selective sensors for otheretals [72–75].

In the case of [emim] [BF4], the addition of ap-Si NPs resultedn a red shift of about 20–30 nm. This behavior alludes to the facthat there is a modification in the coordination around the IL’s ionshich results in the turbid color of the ap-Si NPs/[emim] [BF4]ixture as shown in Fig. 6b.

In contrast, the ap-Si NPs do not significantly alter the [bmim]

Ac] absorption spectrum, even after prolonged stirring. This coulde attributed to the high viscosity of the IL. Thus, obviously, theggregation of NPs is highly affected by the degree of viscosity,hich in turn influences the optical properties of the ILs’ solutions.

[bmim] [Ac] (black) as obtained. (b) Comparative UV–vis spectra of ILs w/wo ap-Sith cuvette for measurements.

For further elucidation, we conducted PL and FTIR experiments.Depending on the excitation wavelength, the PL spectra for thethree bare ILs; [bmim] [Ac], [bmim] [SCN] and [emim] [BF4], wascharacterized by the presence of two components; one at shortwavelengths and the other at longer wavelengths as shown inFig. 7a. When [bmim] [Ac] is excited at 310 nm, the fluorescenceemission band maxima appeared at about 462 nm. Excitationsaround 390 nm led to a progressive decrease in the fluorescencemaxima and a shift toward longer wavelength. The other twoILs showed the same behavior with two regions; one below andanother above the excitation wavelength of 390 nm (Fig. 7a).

The spectrum at short wavelengths is the same for the threestudied ILs, whereas the emission band corresponding to the exci-tation from the tail portion of the absorption band shifts graduallytowards the red. The phenomenon is modeled in Scheme 2, whichexplains how the excitation wavelength depends on the nature ofthe species formed (i.e., ion pairs versus ion clusters).

Samanta et al. [76–81] and Obliosca et al. [82] reported that theshort wavelength emission is due to the monomeric form of imi-dazolium ion (free ion), and the long wavelength bands are due todifferent associated species. To justify their hypothesis, Samantaet al. showed that ILs with sufficient dilution induced a complete

disappearance of the long wavelength emission peak indicating thedisappearance of the different kind of associated species. The factthat the long tail absorption is not due to any impurities [83] hasalready been demonstrated by other studies [54]. The absorbancebehaviors of the present ILs are similar to the results from pre-

16 M.R. Tchalala et al. / Applied Materials Today 9 (2017) 10–20

Fig. 7. PL spectra of (a) the emission of blank ILs; [bmim] [SCN] (top), [emim] [BF4] (middle) and [bmim] [Ac] (bottom), vs. excitation wavelength (�excitation), and (b) ap-SiNPs/ILs hybrid electrolytes. In (b), �excitation corresponding to the 1st component of [bmim] [SCN] (top) is 310 nm, and the one corresponding to the 2nd component is 410 nm.Similarly in [emim] [BF4] (middle), �excitation is 300 nm for the 1st component and 430 for the 2nd component. Likewise in [bmim] [Ac] (bottom), the �excitation is 350 nm and450 nm for the 1st and 2nd components, respectively.

ht-in

vNcwetibl

Scheme 2. Cartoon depicting the response of ILs to a lig

ious reports [76]. It is worth noticing that the presence of ap-SiPs in the ILs alters the equilibrium between isolated and asso-

iated ILs species. Fig. 7b shows the PL spectra of the three ILsith ap-Si NPs. The PL spectra of the ap-Si NPs/[bmim] [SCN]

lectrolyte is significantly different from bare [bmim] [SCN], par-icularly at long wavelengths. The emission band characteristic ofsolated imidazolium ion is slightly shifted whereas the emissionand corresponding to the second component (at longer wave-

ength) is significantly red-shifted when the excitation wavelength

duced perturbation in the electronic structure of the IL.

is moved to the tail portion of the absorption band. Such phe-nomenon can be ascribed to the complexation of iron (III) withlinear ligand (SCN) [56,57], which confirms the result obtained byUV–vis spectroscopy.

Unlike [bmim] [SCN]/ap-Si NPs electrolytes, the fluorescencebehavior of the [emim] [BF4]/ap-Si NPs presents large red-shiftover the complete composition range. The fact that both compo-nents show a red-shift, particularly at longer wavelengths, couldbe explained by the fact that both emim+ and the BF4• interact

M.R. Tchalala et al. / Applied Materials Today 9 (2017) 10–20 17

F [SCN][

winari

[nbm3iata3babiaa[

pm

ig. 8. FTIR spectra of (a) [bmim] [SCN] for (1) bare [bmim] [SCN] and (2) [bmim]

BF4]/ap-Si NPs mixture.

ith the ap-Si NPs surface. The mesoscopic structure of “pure”midazolium-based ILs can be considered as three-dimensionaletworks of anions and cations, linked by weak hydrogen bondsnd Van der Waals interactions [46]. The addition of ap-Si NPs dis-upts the hydrogen bonding network, which is associated with anncrease of polydispersity and/or agglomeration of nanoparticles.

Fig. 8 shows the FT-IR spectra (520–4000 cm−1) of bare [bmim]SCN] and [emim] [BF4] and their electrolytes. Vibrational reso-ance for the [bmim]+ cation displays six peaks. The absorptionands observed above 3000 cm−1 correspond to C H vibrationalodes of the imidazolium ring. The first two bands at 3161.5 and

122 cm−1 correspond to anti-symmetric and symmetric stretch-ng vibrational modes of the HC(4)-C(5)H, respectively. The bandt the shoulder can be attributed to the C(2)-H stretching vibra-ional modes. Although the two ionic liquids have similar spectrabove 3000 cm−1, they obviously display different one below000 cm−1 due to the different C H stretching vibrations fromutyl and ethyl groups. The bands at 2964.2 cm−1, 2937.8 cm−1

nd 2877 cm−1 are assigned to CH3 stretching vibrations, and theands at 2913 and 2856.7 cm−1 are attributed to sp3 CH2 stretch-

ng vibrations of [bmim] [BF4] [84]. In contrast, weaker bandsround ∼2940–2990 cm−1 were observed for the [emim]+ and are

ttributed to the CHx stretching of the ethyl chain and methyl group85].

The FTIR spectrum (Fig. 8a) of [bmim] [SCN] showed a singleeak at ∼2056 cm−1 corresponding to the anion C N stretchingode [80], while the spectrum of [emim] [BF4] (Fig. 8b) showed a

/ap-Si NPs mixture, and (b) [emim] [BF4] for (1) bare [emim] [BF4] and (2) [emim]

peak at ∼754 cm−1 which correspond to stretching vibrations of theB-F bonds in the BF4• [86]. Adding ap-Si NPs to [bmim] [SCN] did notcause any change in the C H bands characteristic of alkyl and H C(4) C (5) H and C (2) H stretching mode of [bmim]+. However,the band characteristic of the SCN anion observed at 2050 cm−1 inbare IL appears to be shifted to 2087 cm−1 in the mixture. This highselectivity was attributed to the formation of a new adduct betweenthe Fe (III) in the center of the complex and the thiocyanate ion.

The red-shifted band of SCN• corresponds to C N stretchingvibration of the thiocyanate group within the iron-thiocyanatecomplexes, which is in agreement with UV–vis and PL measure-ment. The band centered at 2070 cm−1 could be explained by achange in electron distribution between the metal iron(III) and theligand (SCN) giving rise to the ligand-to-metal charge transfer ofthe surface iron (III)-thiocyanate complexes [87].

The band characteristic of imidazolium-based IL depends on itsenvironment. FT-IR spectrum of [emim] [BF4] (Fig. 8b) shows anoverall red-shift in the cationic and anionic range when ap-Si NPsis added. For instance, the BF4• symmetric mode was shifted by a9 cm−1, showing that the change in the anionic part is more promi-nent compared to the cationic moiety. This observation indicatesthat imidazolium ring and the anionic part experience a different

environment in the mixture as compared with that in bare IL.

Based on the previous results, unique physical adsorption struc-tures for [emim] [BF4] on ap-Si NPs are proposed (Scheme 3b). Theanionic part BF4• interacts covalently with the surface of ap-Si NPsforming a negatively charged layer. FTIR and TEM micrographs as

18 M.R. Tchalala et al. / Applied Materials Today 9 (2017) 10–20

S hous

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cheme 3. Schematic representation of nonstructural organization in binary amorpb) ap-Si NPs/[emim] [BF4].

ell as solid-state NMR (SI) showed that the cationic part appearbove this layer forms a double layer structure. However, it seemso be far from the surface which explains the smaller change in themidazolium ring behavior compared to the anionic part [47–49].

Scheme 3a shows the complexation of SCN• with iron atomsear the ap-Si NPs that leads to the strong red coloring and is alsoesponsible for the stability of the mixture.

. Conclusions

Having an objective to combine the novel properties of nanopar-icles and ionic liquids, we designed novel organic-inorganic hybridlectrolytes composed of ionic liquids tethered to amorphousorous silicon nanoparticles. All investigated electrolytes havehown adequate ionic conductivity with excellent electrochemicaltability except for the [bmim] [Ac]. Using optical and morpholog-cal characterization techniques, we found that the behavior of ourp-Si NPs/ILs hybrid electrolytes strongly depends on the chemi-al composition of the ILs. The optical inspection showed that thepectral behavior of pure ILs strongly depends on the excitationavelength; however, this drastically changed when mixed with

p-Si NPs.Bright red color was attained when the transparent [bmim]

SCN] was mixed with ap-Si NPs. This coloration was assignedo the formation of a complex between thiocyanate and iron (III)resent in the electrolyte as an impurity. The observed shoulder in

porous silicon NPs/ionic liquids mixtures: (a) ap-Si NPs/[bmim] [SCN] mixture, and

the UV–vis spectrum of the electrolyte and the shift in the vibra-tional modes as well as the unusually positive chemical shifts ofthe NMR peak (SI), all revealed an undoubtedly iron-thiocyanatecoordination. Therefore, it is likely a new way to enhance the vibra-tional mode of ILs by coordinating or bridging of the thiocyanateligand. Incidentally, this complexation leads to good dispersion ofnanoparticles.

Furthermore, both anion and cation of [emim] [BF4] were red-shifted when the IL was mixed with ap-Si NPs. The anion had thestrongest shift and enhancement in the vibrational mode assignedto the tetra fluoroborate band. While the BF4• interacts chemicallywith the surface of ap-Si NPs forming a negatively charged layer,the [emim]+ formed a second layer without interacting with theap-Si NPs surface. The above results were corroborated by detailedNMR studies showing a weak interaction between the anions andthe silicon NPs. TEM measurements also confirmed the formationof a stabilizing ionic liquid layer around the nanoparticles. This sup-ports the notion that the anion BF4• – ap-Si NPs contact can be seenas crucial for the nanoparticle stabilization.

In the case of the two previous ILs, the ap-Si NPs/ILs electrolyteswere found to be very stable even in the absence of additional stabi-

lizing agents. The quality of dispersion and particle size distributionprimarily depended on the employed IL. However, in the case of[bmim] [Ac], the presence of ap-Si NPs did not cause any change,which is probably due to the high viscosity that limited the use ofthis ionic liquid for further studies.

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Overall, the incorporation of ap-Si NPs into imidazolium-basedLs highlights tunable electrochemical properties, i.e., wider elec-rochemical window and higher conductivities. Moreover, suchonic ILs contribute to the enhancement of the physicochemicalroperties of ap-Si NPs through intermolecular interactions, andevelop the stability and reusability of the NPs for various pro-esses. Ultimately, our findings hold a great promise for gamut oflectrochemical applications, ranging from electrosynthesis, elec-rocatalysis, electrodeposition, batteries and fuel cells to a varietyf sensors.

cknowledgment

The authors would like to thank King Abdullah University ofcience and Technology (KAUST) for the financial support.

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at http://dx.doi.org/10.1016/j.apmt.2017.04.11.

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