Poly(ionic liquid)-Promoted Solvent-Borne Efficient Exfoliation of

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Poly(ionic Liquid)-Promoted Solventborne Efficient Exfoliation of MoS/MoSe Nanosheets for Dual-Responsive Dispersion and Polymer Nanocomposite Yajnaseni Biswas, Madhab Dule, and Tarun Kumar Mandal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00952 • Publication Date (Web): 02 Feb 2017 Downloaded from http://pubs.acs.org on February 3, 2017

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The Journal of Physical Chemistry

Poly(ionic liquid)-Promoted Solventborne Efficient Exfoliation of MoS2/MoSe2 Nanosheets for Dual-Responsive Dispersion and Polymer Nanocomposite Yajnaseni Biswas, Madhab Dule and Tarun K. Mandal* Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India

ABSTRACT.

Considering the great potential of layered transition-metal dichalcogenides

in thin film photovoltaic, advanced composite materials and biomedical applications, it is of high importance to have a highly efficient method for their generation in both aqueous and nonaqueous media. Here, we demonstrate a simple one-pot efficient exfoliation approach to prepare dispersion of single or few-layers MoS2 nanosheets by quick sonication in presence of cationic poly(ionic liquids)s (PILs) in both aqueous and nonaqueous media at room temperature. These PILs are synthesized by simple conventional free radical polymerization from designed ionic liquid monomers. This method is extendable for efficient generation of MoSe2 nanosheets’ dispersion in these solvents. Owing to the solubility in both water and organic solvents, cationic PIL molecules serve the dual purpose of an exfoliating-cum-stabilizing agent. PIL-stabilized nanosheets’ dispersions are stable for more than two months at ambient temperature. The adsorption of PIL to the surface of MoS2 nanosheet converts them to responsive towards ions and temperature in aqueous medium. Additionally, MoS2-PIL nanosheets can easily be dispersed in water-soluble poly(vinyl alcohol) and nonaqueous-soluble poly(methyl methacrylate) matrices for making well-dispersed homogeneous nanocomposites and their dielectric properties are studied. 1

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INTRODUCTION Inspired by the huge applications of graphene, recently, materials scientists have shown

immense interest on transition-metal dichalcogenide (TMD) materials due to their unique properties (electrical, photonic, mechanical, catalytic, etc.) that originates because of their layered two-dimensional (2D) structure and may not have observed in the three-dimensional (3D) bulk solid.1-9 Among TMDs, single and few-layers molybdenum disulfide (MoS2) has certainly attracted the major share of attention in terms of study of its physical properties and applications in diverse areas such as biosensors,10-11 photovoltaics,12 thermoelectric materials,13 energy storage,10 supercapacitor,14 water desalination,15 phototransistor,16 and hydrogen evolution catalysis.17-18 Bulk MoS2 is an indirect bandgap semiconductor with an energy gap of ∼1.2 eV and showed tunable bandgap with layer thickness and can increase up to 1.9 eV for a single monolayer MoS2.1,19 This makes single or few-layers MoS2 a very important direct bandgap semiconductor with wide range of applications.10-13,18,20 Beside MoS2, other layered TMDs (MoSe2, WS2, WSe2, etc) are also important because of their similar properties.6,9,21-24 Thus, controlling layer thickness of TMDs has been a longstanding challenge and this problem has been addressed in several ways. For example, chemical vapor deposition (CVD)13 and epitaxial growth25-26 techniques produce high quality layered 2D MoS2 nanosheets, but requires sophisticated instrumentations and harsh process conditions and the size are limited. Alternatively, the shear-assisted grinding27 and tape peeling28 methods generate single or fewlayers MoS2 nanosheets but not of good quality and the scaling-up is not easy. Recently, another interesting ion-intercalation chemical exfoliation strategy for scaling up of layered MoS2 nanosheet using butyllithium or sodium naphthenate has been introduced by many research 2


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groups.1-3,29

But, this method is a multistep, time-consuming, extremely sensitive to

environment, limited in water media and sometimes introduces additional structural deformation in MoS2. In addition, the method often results in the transformation of MoS2/WS2 from its pristine semiconducting hexagonal structure (2H phase) to the metallic trigonal structure (1T phase).1-3,29 Direct ultrasonic exfoliation of bulk MoS2 in different solvents such as NMP, acetonitrile, NMP-H2O2 mixture, water etc

without any chemical agent is an interesting

approach to prepare single or few-layers MoS2, but, suffers from poor yields, requires very long sonication time.30-36

Many research groups have utilized different chemical reagents in

conjugation with ultrasonic vibration for exfoliation of MoS2 in aqueous solvent.27,37-39 For example, Kapil et al. have used self-assembled cross-β-amyloid nanotubes for the exfoliation of MoS2 crystal to make biocompatible single or few-layers nanosheets dispersion in water.38 Zhang et al. described a single-step ionic liquid (IL) assisted grinding method to prepare functionalized MoS2 nanosheets in water.27 In another method, thiol-based organic ligands were used to chemically exfoliate MoS2 nanosheets.37 However, none of these reports really discussed about the stability of dispersed MoS2 nanosheets. Also, none of these methods are able to generate MoS2 nanosheets both in aqueous and nonaqueous media. In this connection, several research groups also employed surfactant-based exfoliation method to increase the stability of dispersed MoS2 and other TMDs nanosheets in aqueous environment,9,40-42 which have the limited applicability in the preparation of nonaquoeus-based polymer nanocomposites for device applications. In this context, except the polymer-stabilized WS2 dispersion,23 no one used any polymeric dispersants to directly exfoliate and stabilized MoS2 nanosheet in dispersion. Therefore, it would be really interesting to explore the possibility of use of different polymers as efficient exfoliating as well as dispersing agent for MoS2 and other TMDs in both water and in 3



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nonaqueous solvents for their applications in biology as well as in polymer nanocomposite preparation. In this exploring process, we find that poly(ionic liquid)s (PILs) is an important new class of ionic polymeric material, containing ionic liquid (IL) species in each of its repeating unit bearing unique properties. Thus, we use two different cationic PILs, namely poly(triphenyl-4vinylbenzylphosphonium chloride) (P[VBTP][Cl]) and poly(3-n-butyl-1-vinylimidazolium bromide) (P[VimBu][Br]) for the prepare of stable dispersion of single or few-layer nanosheets by an easy, one-step and efficient exfoliation of bulk MoS2 in both water and in nonaqueous solvents at room temperature. This PIL-assisted method also efficiently generates MoSe2 nanosheets in both aqueous and nonaqueous solvents. It observed that these PIL-adsorbed nanosheets’ dispersions are highly stable even after several weeks resting at room temperature. The efficiency of this PIL-assisted exfoliation of TMD layers is established from various instrumentation techniques. As a proof of concept, it is shown that PIL-stabilized MoS2 dispersion can lead to the formation well-dispersed nanocomposites upon mixing with water soluble poly(vinyl alcohol) (PVA) and nonaqueous soluble poly(methyl methacrylate) (PMMA) and their dielectric properties are compared with that of neat polymers. Some ILs/PILs are known to be good debundling/exfoliating agents for carbon nanotubes (CNTs)/graphenes in different solvents under ultrasonic vibration.43-45 However, the uses of PILs for debundling/exfoliating CNT/graphene in solvents are very few except our recent work of dispersion of multiwalled carbon nanotube (MWCNT) using phosphonium PILs.43 This prompted us to utilize the PIL-assisted exfoliation as new method to prepare dispersion of MoS2/MoSe2 nanosheets in different solvents, which to the best of our knowledge has never been investigated. An additional benefit is the ability to prepare stimuli-responsive single or few4


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layers MoS2 nanosheets as the adsorbed PILs exhibits responsiveness towards different anions and temperature in aqueous solution.

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EXPERIMENTAL SECTION

Materials.

Molybdenum(IV) sulphide (MoS2, 98%), molybdenum(IV) selenide (MoSe2,

99.9%) were purchased from Alfa Aesar. 4-Vinylbenzyl chloride (VBC) (Aldrich, 90%) was obtained from Sigma-Aldrich and was purified by passing through a neutral alumina column to remove inhibitors before use. Triphenylphosphine (TPP, ≥95%) and 2,2′-azobis(2methylpropionitrile) (AIBN, 98%) were purchased from Sigma-Aldrich and were recrystallized twice from ethanol prior to use. 1-Vinylimidazole (Vim) (Aldrich, ≥95%) was purified by vacuum distillation prior to use. 1-Bromobutane was used as received from Spectrochem, India. Poly(methyl methacrylate) (PMMA, Mw = 1,50,000-1,90,00 gmol-1 ) and poly(vinyl alcohol) (PVA, Mw = 74,885-79,290 gmol-1) were purchased from Sigma-Aldrich. Dimethyl formamide (DMF) was vacuum distilled over calcium hydride just before use. Methanol (MeOH) and dimethyl sulfoxide (DMSO) were used as received from E. Merck, India. Milli-Q Water was used throughout the study. Synthesis of 3-n-Butyl-1-vinylimidazolium bromide ([VimBu][Br]) Ionic Liquid (IL) Monomer.

In a typical procedure of synthesis of imidazolium-based IL monomer, a 100 mL

round-bottom flask was charged with 1-bromobutane (4.3 mL; 39.74 mmol), Vim (3 mL; 33.12 mmol) and 6 mL of dry MeOH and the reaction mixture was placed in a preheated oil bath at 50 °C and continued stirring for 24 h. The synthesis of [VimBu][Br] monomer was schematically shown in Scheme S1. After cooling down, the reaction mixture was added dropwise into 300 mL of ethyl acetate. The process was repeated twice for purification. Finally, an oily liquid was 5



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collected and was kept in a refrigerator under argon atmosphere for subsequent use in polymerization. Yield: 78%. Synthesis

of

Poly(3-n-butyl-1-vinylimidazolium

bromide)

(P[VimBu][Br])

PIL.

Conventional radical polymerization technique was used to polymerized [VimBu][Br] monomer as also shown in Scheme S1. Typically, 1.27 g (5.521 mmol) of [VimBu][Br], 9.06 mg (0.055 mmol) of AIBN were taken in a long-necked 25 mL RB flask followed by the addition of 12.7 mL of dry DMF and then purged with argon gas for 45 min. Finally, the rubber septum sealed flask was placed in an oil bath preheated at 65 °C and stirred magnetically for 24 h. After cooling down to room temperature, the formed polymer was obtained by precipitation in diethyl ether. It was further purified by extensive dialysis (using a dialysis bag of 14 kDa MW cut off) in water followed by isolation through freeze drying. The purified PIL was then dried to a constant weight in a vacuum oven at 50°C. Yield: 75%. Synthesis of Poly(triphenyl-4-vinylbenzylphosphonium chloride) (P[VBTP][Cl]) PIL. P[VBTP][Cl] was also synthesized by conventional free radical polymerization of a phosphonium IL monomer [VBTP][Cl], prepared by following our protocol reported earlier.43 Typical procedure included addition of 1.5 g (3.953 mmol) of [VBTP][Cl] and 13 mg (0.079 mmol) of AIBN in a long necked 25 mL RB flask followed by the addition of 10 mL dry DMF. Finally, the whole reaction mixture was purged with argon gas for 45 min and was sealed with a silicone rubber septum and placed in a preheated oil bath at 65 °C and continued stirring for 24 h. The obtained P[VBTP][Cl] was isolated by its precipitation in acetone after cooling down the mixture to room temperature. The obtained white precipitate was dissolved again in water and reprecipitated in acetone. The whole process was repeated twice, after that the white solid was collected and dried under vacuum at 50 °C for 24 h. Yield: 78%. 6


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Exfoliation of MoS2 by PILs in Aqueous and Nonaqueous Solvents.

To

obtained

PIL-

stabilized dispersion of MoS2 nanosheets, the following experimental protocol was used. First a stock solution of 1wt% PIL (either P[VBTP][Cl] or P[VimBu][Br]) was prepared in water or in other nonaqueous solvents such as DMF, DMSO and MeOH. MoS2 powder (6 mg) was then added separately in 2 mL solution of two different PILs in different solvents. The mixtures were sonicated continuously for 60 min using Cole Parmer 8891 ultrasonic bath (Frequency ~ 42 KHz). The temperature was maintained at 25 °C throughout the sonication period using icecooled water. Finally, the dark green dispersions were centrifuged for 30 min at 1500 rpm to remove unexfoliated bulk MoS2 flakes. After centrifugation, the greenish supernatant containing dispersed MoS2-PIL nanosheets was taken in a vial and was analyzed by various instruments for characterization as well as used for making polymer composites. Furthermore, to check whether this PIL-assisted liquid phase exfoliation (LPE) method is applicable in the exfoliation of other TMDs, we also exfoliated bulk MoSe2 to make its dispersion by following the similar procedure as describe above. Estimation of Amount of MoS2 in Dispersed State.

In

order

to

calculate

the

concentration of MoS2 nanosheets in dispersion of different solvents, after centrifugation at 1500 rpm, typically a known volume (10 mL) of MoS2-P[VBTP][Cl] dispersion in DMF (greenish in color) was carefully filtered under high vacuum using a hydrophilic poly(vinylidene fluoride) (PVDF) membrane (Pore diameter = 0.22 micron) of known mass (124.00 mg). The thick residue was collected and washed thoroughly with neat DMF to remove the excess unadsorbed PIL. Note that the filtrate was colorless. Finally, the membrane containing the residue was dried in a vacuum oven at room temperature overnight till constant weight to ensure complete removal of solvents. The mass of membrane containing the residue material was found to be 134.33 mg 7



from which the final concentration of MoS2 nanosheet was calculated to be 1.03 mg mL-1. Similarly, the final concentrations of dispersed MoS2-P[VBTP][Cl] were also calculated in water and in other nonaqueous solvents and also the concentrations of dispersed MoS2- P[VimBu][Br] in water and in DMSO (Figure 1 and Table S1). Error ranged from 5-15% in triplicates in all the cases.

1.2

-1

)

1.0

A m o u n t (m g m L

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0.8

0.6

0.4

Mo S -P [V B T P ][C l] 2 Mo S -P [V im B u ][B r] 2

0.2

0.0

Water

Figure 1.

D MF

D MS O

MeO H

Amount of dispersed MoS2-PILs nanosheets in different solvents, Inset shows

photographs of dispersed MoS2-P[VBTP][Cl] in H2O (a), DMSO (b), DMF (c); MeOH (d) and MoS2-P[VimBu][Br] dispersion in DMF (e), H2O (f).

Cloud Point Measurement. The cloud point (Tcp) of the aqueous solution of P[VimBu][Br] was determined by following our earlier reported procedure.43 Tcp was measured from an UV-vis spectrophotometer equipped with a temperature controller in the transmittance mode at a fixed wavelength of 600 nm. Typically, a transparent aqueous solution of 0.5 wt% P[VimBu][Br] was first filtered through a membrane filter (Pore diameter 0.45 µm). The transparent solution was then titrated with NaI to obtain a turbid solution at 25 ºC. The Tcp of this resultant turbid solution was then measured by recording the % transmittance (% T) in the temperature window of 10-90 ºC with increasing/decreasing the temperature at scan rate of 1 ºC/min after equilibration for 2 min at the experimental temperature using a UV-vis spectrophotometer. Similarly, we also 8


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measured the UCST-type Tcps of the aqueous solution of P[VimBu][Br] in presence of varying concentrations of NaI. Tcp was determined to be the point at which the solution transmittance reduced to half of its original value. Ion- and Temperature-Responsiveness of MoS2-PIL Nanosheets in Water.

The

responsiveness of MoS2-P[VBTP][Cl] and MoS2-P[VimBu][Br] nanosheets’ dispersion in water was investigated by adding 475 mM NaCl and 35 mM NaI into 2 mL of their aqueous dispersion respectively. The addition of salts (NaCl or NaI) transformed the greenish transparent solution to cloudy greenish white suspension instantly, resulting in restacking and aggregation of the exfoliated MoS2 nanosheets. Further, the agglomerated MoS2 nanosheets can again be redispersed in water upon sonication for 10 min with heating at 40 °C. Polymer-MoS2 Nanocomposite Preparation.

Polymer-MoS2

nanocomposites

were

prepared by solution intercalation film casting method. In a typical procedure, 30 mg MoS2 and 100 mg P[VBTP][Cl] were added into 10 mL DMF taken in 30 mL flat vial and sonicated for 60 min in an ice-cooled water-bath at 25 °C. After sonication, this MoS2-PIL dispersion was centrifuged for 30 min at 1500 rpm to remove bulk unexfoliated MoS2 flakes. After centrifugation, the supernatant solution containing single or few-layer MoS2 nanosheets (greenish in color) was further centrifuged at higher speed at 12500 rpm. The supernatant was removed and the residue was then collected to obtain as-exfoliated MoS2-PIL nanosheets with no loosely bound PIL molecules. Finally, fresh DMF was added to it and subsequently stirred for 5 min which resulted in retrieval of the dispersed MoS2-P[VBTP][Cl] nanosheets. Intriguingly, this process of centrifugation and redispersion were carried out for at least five consecutive times to remove excess unadsorbed P[VBTP][Cl] molecules present in the dispersion. After that, MoS2P[VBTP][Cl] nanosheets’ dispersion was lyophilized and 40mg of the dried sample was added 9



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into 60 mL of 10 wt% PMMA solution in DMF. The solution was then sonicated for 15 min at 25 °C, poured into a clean glass mould to allow homogeneous thin film composite formation and then dried at 50 °C for 48 h. The prepared greenish film was free from air bubbles with uniformly dispersed MoS2 nanosheets in the PMMA matrix. PVA-MoS2 nanocomposite was prepared from MoS2-P[VBTP][Cl] nanosheets’ dispersion in water following the similar experimental protocol as described above. Characterization NMR Spectroscopy.

1

H-NMR spectra of [VimBu][Br] monomer and P[VimBu][Br] were

recorded by using a Bruker DPX 400 MHz spectrometer. CDCl3 was used as a solvent for acquiring the spectrum of [VimBu][Br]. But, the spectrum of P[VimBu][Br] was acquired from D2O. ESI-Mass Spectrometry.

The ESI-Mass spectrum of [VimBu][Br] monomer was recorded

by using a quadrupole time-of-flight (Q-TOF) Micro YA263 mass spectrometer. The sample was prepared at a concentration of 1 mg mL−1 in methanol. Fourier Transform Infrared (FTIR) Spectroscopy. FTIR

spectra

of

[VimBu][Br],

P[VimBu][Br], bulk MoS2, MoS2-P[VBTP][Cl] and MoS2-P[VimBu][Br] composites were recorded from pellets prepared by mixing with KBr in a 1:100 (w/w) ratio in a Spectrum 400 spectrometer (Perkin Elmer). MoS2-P[VBTP][Cl] and MoS2-P[VimBu][Br] composite samples were thoroughly washed several times with solvents to ensure complete removal of unabsorbed PILs prior to acquire the FTIR spectra. Size Exclusion Chromatography (SEC).

The number average molecular weights (Mn) and

dispersities (Đs) of P[VimBu][Br] and P[VBTP][Cl] were measured by SEC technique using a Waters 1515 isocratic HPLC pump connected to three Waters Styragel HR1, HR3 and HR4 10


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columns at 45 °C and a Waters 2414 RI detector at 35 °C. The eluent was DMF with 50 mM LiBr, with a flow rate of 1 mL min−1. The columns were calibrated against seven PMMA standards with peak molecular weights (Mp) of 6780, 10100, 18700, 31600, 56600, 87650 and 102 000. Transmission Electron Microscopy (TEM). The

samples

for

TEM

were

prepared

by

centrifuging the greenish dispersion of MoS2-PIL in aqueous and other nonaqueous solvent at speed of 12500 rpm. The residue was then collected to obtain as-exfoliated single or few-layer MoS2 nanosheets. The excess PIL was removed through redispersal of the sediments in the respective solvents followed by sonication/stirring. Finally, the redispersed MoS2-PIL in appropriate solvent were diluted to a concentration of 0.01 wt% and dropcasted onto carboncoated copper grid and allowed to dry in air at room temperature for 24 h. TEM micrographs were then taken using a JEOL JEM-2010 electron microscope operated at an accelerating voltage of 200 kV. HRTEM image, selected-area electron diffraction (SAED) pattern and energydispersive X-ray (EDX) spectra were also acquired for each sample. Field-Emission Scanning Electron Microscopy (FESEM). For FESEM study, the redispersed MoS2-PIL in appropriate solvent (0.01 wt%) was dropcasted on small pieces of glass and allowed it to dry overnight in air. The piece of glass containing the sample was then mounted on a metal stub followed by platinum coating to minimize charging. FESEM images were then acquired by placing the sample under a ZEISS JSM-6700F electron microscope operating at an accelerating voltage of 5 kV. Atomic Force Microscopy (AFM).

AFM samples were prepared by casting the dilute

redispersed MoS2-PIL/MoSe2-PIL nanosheets’ suspension (0.01 wt%) on freshly cleaved mica followed by drying in air for 24 h. AFM images were then recorded in a MFP-3D origin 11



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microscope from Oxford Instruments (Asylum Research) operating in tapping mode at room temperature. UV-Vis Spectroscopy. UV-Vis absorption spectral measurements of layered MoS2 dispersion in aqueous and in nonaqueous solvents were carried out on a Hewlett-Packard 8453 diode array UV−Vis spectrophotometer. Furthermore, Tcps of the aqueous solution of P[VimBu][Br] containing different concentration of NaI salts were determined in the same UV-Vis spectrophotometer equipped with a temperature controller in the transmittance mode at a fixed wavelength of λ = 600 nm. Photoluminescence (PL) Spectroscopy.

The PL spectra of the aqueous and nonaqueous

dispersions of MoS2-PIL nanosheet were recorded using a Jobin-Yvon Fluoromax-3 photoluminescence spectrophotometer. For PL measurements, the dispersed MoS2-PIL nanosheets were excited at a wavelength depending on their UV–Vis absorption maxima. λ

ex

=

610 nm. Raman Spectroscopy. The samples for Raman measurements were prepared by drop casting the MoS2-PIL/MoSe2-PIL nansheets’ dispersion on glass slide and air-dried. Raman spectra of exfoliated/bulk MoS2/MoSe2 were acquired at room temperature in a triple Raman spectrometer (Model T64000) using a 532 nm Ar+ laser at a power of 1 mW. Dynamic Light Scattering (DLS).

Hydrodynamic diameter (Dh) and zeta potential (ξ) of

exfoliated MoS2-PIL nanosheets were measured from a DLS instrument (Malvern Zetasizer NANO ZS 90, Model 3690) using He-Ne gas laser of wavelength 632.8 nm. X-ray Diffraction (XRD) Study.

The crystallinity of MoS2 was analyzed using a Bruker D8

X-ray diffractometer operated at an accelerating voltage of 40 kV using a CuKα (l = 1.5405 Å) as the X-ray radiation source with a current intensity of 40 mA. The dispersion of MoS212


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P[VBTP][Cl] and MoS2-P[VimBu][Br] nanosheets in aqueous and nonaqueous solvents were dropcasted onto a glass slide and dried in vacuum at 25 °C. X-ray Photoelectron Spectroscopy (XPS) Study.

The

dispersed

MoS2-P[VBTP][Cl]

and

MoS2-P[VimBu][Br] samples were casted onto a glass slide (1 cm × 1 cm) and dried under vacuum. Subsequently, XPS spectra were acquired in an Omicron instrument with an Al Kα radiation source under 15 kV voltages and 5 mA current. Differential Scanning Calorimetry (DSC).

The glass transition temperature (Tg) and other

thermal histories of neat PVA, PVA-MoS2 nanocomposite, neat PMMA and PMMA-MoS2 nanocomposite were measured using a Perkin–Elmer diamond differential scanning calorimeter (DSC) equipped with an intra-cooler. For Tg measurement, samples were heated at a scan rate of 20 °C/min and Tg values were taken as the onset of the transition in the second scan. Electrical Measurements.

Electrical properties, such as conductance and capacitance of neat

PVA, PVA-MoS2 nanocomposite, neat PMMA and PMMA-MoS2 nanocomposite films were carried out using a RLC meter (Quad Tech, Model 7600) in the frequency range from 10 Hz – 2 MHz in an anhydrous environment. The films were kept between two stainless steel blocking electrodes of a conductivity cell for the electrical measurements.

§

RESULTS AND DISCUSSION

Synthesis and Characterization of IL Monomers and PILs. To begin with, an imidazoliumbased IL ([VimBu][Br]) monomer was first synthesized by nucleophilic substitution reaction as shown in Scheme S1. Additionally, a phosphonium-based IL ([VBTP][Cl]) monomer was also synthesized following the exactly similar protocol as mentioned in our earlier report.43 The ESIMS spectrum (Figure S1) showed molecular ion peak at m/z = 151.09, which is exactly equal to 13



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the mass of [VimBu][Br] monomer. The 1HNMR spectrum showed characteristic signals at δ 5.2, 5.9 and 7.4 ppm for three vinyl protons along with characteristic signals of imidazole ring protons and butyl protons of [VimBu][Br] monomer (Figure S2). The detail characterization of [VBTP][Cl] monomer has already been described in our earlier report.43 The as-synthesized IL monomers were then polymerized by conventional free radical polymerization to obtain P[VBTP][Cl] and P[VimBu][Br] PILs as schematically shown in Scheme S1. FTIR spectra (Figure S3) showed bands at 1650 and 3100 cm-1, corresponding to the C-N stretching and C=N deformation of imidazole ring of P[VimBu][Br] PIL. Furthermore, the spectrum of P[VBTP][Cl] also showed the characteristics bands at 1437 and 1113 cm-1 for its P–CH2–Ph of [VBTP]+ deformation and three P–Ph stretching vibrations (Figure S3). The 1HNMR spectrum (Figure S4) showed all the characteristic signals of P[VimBu][Br] PIL. It should be noted that the detail NMR characterization of P[VBTP][Cl] has already been described in our earlier paper.43 P[VimBu][Br] and P[VBTP][Cl] PILs were structurally characterized by SEC and the molecular weights (Mns) were found to be 26089 and 54980 Da with dispersities (Đs) of 1.42 and 1.45 respectively (Figure S5). Exfoliation of MoS2. For exfoliation, typically, 6 mg of bulk MoS2 powder was simply separately mixed with 2 mL solution of these two PILs (1 wt%) in water, DMF, DMSO and MeOH followed by sonication for 60 min in an ultrasonicator as schematically shown in Scheme 1. This process yielded homogeneous greenish dispersions indicating primarily the exfoliation of MoS2 crystals to single or few-layers nanosheets in these solvents (Inset of Figure 1). The concentrations of MoS2 nanosheets in these dispersions were estimated by filtration and weighing method as used by other researchers reported elesewhere.9,32,38-39 The concentration of MoS2-P[VBTP][Cl] and MoS2-P[VimBu][Br] nanosheets in water were found to be 0.55 and 14


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Exfoliated MoS 2/ MoSe2 nanosheets

Scheme 1.

Schematic representation of PIL-assisted exfoliation and stabilization of

MoS2/MoSe2 nanosheets in aqueous and nonaqueous media.

0.68 mg mL-1 respectively (Table S1 and Figure 1), those values were nearly 3 times higher than that (0.18 mg mL-1) so far highest reported38 and were also higher than that reported for dispersed MoS2 in NMP.31 Note that concentrations of MoS2-P[VBTP][Cl] and MoS2P[VimBu][Br] in organic solvents (e.g., DMF, DMSO and MeOH) were higher than that of water and highest (1.03 and 0. 98 mg mL-1) in the case of DMF (Figure 1 and Table S1). The reported viscosities of these solvents is in the order of ηDMF>ηDMSO>ηMeOH>ηwater. Thus, the higher viscosity of PIL solution exerted higher shear force in the process of exfoliation of bulk MoS2 which resulted in the generation of more nanosheets with concentrations in the order of [MoS2]DMF> [MoS2]DMSO>[MoS2]MeOH> [MoS2]water (Figure 1).46 UV–Vis spectra of all the greenish dispersions obtained after centrifugation at 1500 rpm exhibited typical characteristic peaks at 672, 610, 454 and 397 nm, indicating generation of single or few-layers of MoS2 nanosheets (Figure 2A).31,36 Peaks at 672 and 610 nm were assigning to A- and B- excitons respectively arising from direct excitonic transitions at the K point with the energy difference originating from spin–orbital splitting of the valence band.36 15



Whereas peaks at 454 and 397 nm were the C- and D-excitons arising from direct excitonic transitions at M point between higher densities of state regions of the band structure.36 The positions of these four exciton peaks were almost independent of solvent used indicating easy and efficient PIL-assisted exfoliation of bulk MoS2 into single or few-layers nanosheets (Figure 2A). However, it was observed that in the cases of MoS2-P[VBTP][Cl] and MoS2-P[VimBu][Br] nanosheets’ dispersion in water these two peaks at lower wavelength (454 and 397 nm) somehow overlapped with each other. We are unable to explain this discrepancy at this point of time. But, these two typical characteristic peaks (454 and 397 nm) for MoS2-P[VimBu][Br] nanosheets’ dispersion could be easily indentified upon deconvolution of the overlapped peak (Inset of Figure 2A). A

B

379 454 1.2

610

379 454

672

Normalized Abs

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Page 16 of 39

D

1000 rpm 2000 rpm 3000 rpm 4000 rpm 5000 rpm 6000 rpm

C

0.8

B A 0.4

668

0 300

Figure 2.

672

400

500

600

Wavelength (nm)

700

800

(A) UV−Vis absorption spectra of MoS2-P[VBTP][Cl] dispersion in DMF (a),

DMSO (b), MeOH (c), H2O (d) and MoS2-P[VimBu][Br] dispersion in DMF (e), H2O (f). Inset in A showed the deconvoluted spectrum (f) of MoS2-P[VimBu][Br] dispersion in water. (B) The normalized UV−Vis spectra (with respect to the intensity at 348 nm) of MoS2-P[VBTP][Cl] dispersions in DMF acquired after centrifugation at varying speed from 1000−6000 rpm for 30 min. 16


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In order to have an idea of average number of MoS2 layer present in one single MoS2 nanosheet, as per the literature report, the normalized spectra of dispersed MoS2-P[VBTP][Cl] fractions in DMF, obtained by centrifugation at a varying speed from 1000 to 6000 rpm for 30 min, were recorded (Figure 2B). The figure clearly revealed a continuous blue-shift of the Aexciton (λA) peak from 672 to 668 nm with the increase of centrifugal force (Figure 2B). Similar blue-shift of the A-exciton (λA) peak was also observed for dispersed MoS2-P[VBTP][Cl] nanosheets in water and for dispersed MoS2-P[VimBu][Br] nanosheets in DMF (Figure S6). Such blue-shift of λA peak for different fractions can be ascribed to the presence of nanosheet containing less number of MoS2 layers per nanosheet, which is a very common for MoS2 and other TMDs due to confinement effects.1,21 Furthermore, the A-exciton absorbance-based empirical equation gave the values ranging from 7.7 to 4.7 for average layer numbers (Navg) present in one dispersed 2H-MoS2 nanosheet of the fractions, obtained after centrifugation at 1000 to 6000 rpm respectively in both aqueous and nonaqueous solvents.21 In order to study the stability, the dispersions of MoS2-PIL nanosheets in different solvents were kept undisturbed for different times up to 60 days and the UV-vis spectra were recorded after the time intervals of 15, 30, 45 and 60 days. Photographs (Figure 3A) of stabilized MoS2-P[VBTP][Cl] nanosheets’ dispersions in different solvents after 1 day and 60 days clearly revealed that there was no visible color change of these dispersions after such long time resting at room temperature. The UV-vis spectra of MoS2-P[VBTP][Cl] and MoS2-P[VimBu][Br] nanosheets’ dispersions (Figure 3B and Figure S7) in water and in different organic solvents showed feeble absorbance decay even after 60 days of standing implying excellent colloidal stability in all these solvents. The comparison of the intensities of A- and B-exciton peaks

17



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Page 18 of 39

revealed that the stability of MoS2-P[VBTP][Cl] nanosheets’ dispersion was higher in organic solvents compared to that in water. A H2O

DMSO

H2O

DMF

DMSO

DMF

Day 60

Day 1

B

a

Figure 3.

c

b

(A) Photographs of P[VBTP][Cl] stabilized MoS2 nanosheets’ dispersions in

different solvents after 1 day and 60 days. (B) UV−Vis absorption spectra of MoS2-P[VBTP][Cl] nanosheets’ dispersion in different solvents: (a) H2O; (b) DMSO; (c) DMF upon standing for different times from 1 day to 60 days.

The direct liquid-phase exfoliation (LPE) of bulk MoS2 to dispersed nanosheets is generally achieved in neat solvents like NMP or acetonitrile with poor yield.36 Furthermore, the direct exfoliation of MoS2 in neat water was not achievable and did not even try in polar organic solvents like DMF, DMSO and MeOH owing to the hydrophobic nature and high surface energy of MoS2.27,35 In fact, our initial attempts of exfoliation of bulk MoS2 to nanosheets’ dispersion in neat DMF, DMSO and MeOH were stable for only 60 min. Even the stability of MoS2 nanosheets’ dispersion using neat [VBTP][Cl] monomer was also very poor. However, as mentioned above, these two PILs efficiently exfoliated bulk MoS2 to nanosheets and drastically increased their stability in dispersions for more than 60 days without any sign of precipitation. 18


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Thus, it is assumed that cationic PIL molecules enters inside the galleries of MoS2 layers under ultrasonic vibration and can effectively overcome the inherent van der Waals forces existing between the layers, promoting its exfoliation into single or few-layers nanosheets and hindering the detached MoS2 layers from restacking as schematically shown in Scheme 1. In this case, the adsorption of PIL molecules on MoS2 nanosheets’ surface through a week physical interactions actually hinder restacking of MoS2 layers as observed in the cases of debundling of graphene44 or MWCNTs43. The FTIR results indeed confirmed the adsorption of PIL (P[VBTP][Cl] and P[VimBu][Br]) molecules on nanosheet’s surface (Figure S3). Furthermore, the analysis of STEM image and the corresponding elemental mapping and the positive zeta potential (ξ) value of the MoS2 dispersions further confirmed the PIL adsorption which will be discussed in more detail later in this section. TEM images exhibited a typical laminar morphology of the dispersed MoS2P[VBTP][Cl] nanosheets of average lateral sizes of 150 nm in water (Figure 4A), 100 nm in DMSO (Figure 4B), 100 nm in DMF (Figure 4C) and 170 nm in MeOH (Figure 4D). As can also be seen from TEM images that the P[VimBu][Br] can also generated dispersed MoS2 nanosheets of laminar morphologies of average lateral sizes of 100 nm both in water (Figure 4E) and in DMF (Figure 4F) respectively. FESEM images also exhibited a laminar morphology of the dispersed MoS2-P[VBTP][Cl] nanosheets obtained from water (Figure S8 ). As usual due to PIL adsorption, hydrodynamic diameters (Dhs) of MoS2-PIL nanosheets in these solvents, as measured from DLS (Figure S9), were much larger compared to that obtained from TEM measurement (Table S2). The presence of both single layer MoS2 nanosheets as well as Moiré fringes originating from few-layers MoS2 nanosheet were confirmed from HRTEM images (Figures 5A and Figure 5B). 19



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A

D

Figure 4.

Page 20 of 39

C

B

E

F

TEM images of MoS2-P[VBTP][Cl] nanosheets in water (A), DMSO (B), DMF

(C) and MeOH (D) respectively. Inset in A showed SAED pattern of MoS2 nanosheet. TEM images of MoS2-P[VimBu][Br] nanosheets in water (E) and DMF (F) respectively. Inset in E showed SAED pattern of MoS2 nanosheet.

B

A

Figure 5.

(A) HRTEM image showing crystalline fringes of single-crystalline MoS2

nanosheet. (B) HRTEM image of a multilayered MoS2 flake exhibiting Moiré patterns from rotated stacked crystallographic planes. Moreover, the enlarged view of HRTEM image (Inset of Figure 5A) of MoS2 nanosheets clearly showed crystalline fringes with a spacing of 0.28 nm, which exactly matched with that reported 20


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hexagonal spot pattern,18,35 confirming the formation of only semiconducting 2H-MoS2 sheets by this PIL-assisted LPE. The STEM image and corresponding elemental mapping from energydispersive X-ray (EDX) (Figure S10) revealed the presence of P along with Mo and S atoms throughout the layer which again confirmed PIL adsorption. The positive zeta potential (ξ) value of the MoS2 dispersions in aqueous and nonaqueous solvents (Figure S11), after discarding the loosely-bound PIL also revealed phosphonium PIL adsorption. Moreover, elemental proportion of Mo:S estimated from EDX analysis (Figure S10) was found to be 1: 2.2, essentially signifies the unaffected chemical composition of MoS2 nanosheets.32 The thicknesses of the exfoliated nanosheets were systematically monitored through AFM examination of the exfoliated samples. The typical tapping mode AFM images of exfoliated MoS2 from different solvents exhibited numerous thin polydispersed wrinkled sheets rather than large-sized

B

A

80nm

410nm

D

C

E

F

60nm

Figure 6.

(A) AFM image of MoS2-P[VBTP][Cl] nanosheets in DMF. (B) Enlarge view of

MoS2 nanosheets in panel A. Inset of Figure B showed the height profiles of nanosheets. (C) The thickness distribution of MoS2-P[VBTP][Cl] nanosheets in DMF is based on 100 nanosheets with average lateral sizes over 80 nm. (D) AFM image of MoS2-P[VBTP][Cl] nanosheets in 21



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Page 22 of 39

H2O. (E) Enlarge view of MoS2 nanosheets in panel D. Inset of Figure E showed the height profiles of nanosheets. (F) AFM image of MoS2-P[VimBu][Br] nanosheets in H2O. Inset of Figure F showed the height profiles of nanosheets.

aggregated bulks (Figure 6 and Figures S12A-S12B). The height analysis (Figure 6C and Figures S12C-S12D) showed a distribution with highest populations of sheets with thickness in the range from 1.5-2.5 nm, corresponding to single-layer or few layer nanosheets. Although, it was higher than the typical thickness of a single-layer MoS2 (between 0.6 and 1.0 nm) sheet,47 but, this is because of adsorption of PIL molecules on both surface of nanosheet.39 Raman spectra of dispersed nanosheets and bulk powder showed only two typical characteristic E12g and A1g peaks corresponding to in-plane and out-of-plane S atoms’ vibration modes, respectively with no additional peaks due to 1T phase, regardless of exfoliation conditions, which again proved the existence of only 2H-MoS2 (Figure 7A). Furthermore, there was a blue-shift of E12g peak from 380 to 382 cm-1 and a red-shift of A1g peak from 406 to 404 cm-1 due to transformation of bulk MoS2 into nanosheets, which further confirmed the efficient exfoliation of MoS2 in to single or few-layers nanosheets. XPS spectra of nanosheets after deconvolution showed the signature of Mo 3d orbitals consisted of peaks at 232.4 and 229.3 eV for Mo4+ 3d3/2 and 3d5/2, respectively along with the S 2s peak at 226.3 eV (Figure 7B). Similarly, the deconvoluted peaks at 163.1 and 162.5 eV were attributable to 2p1/2 and 2p3/2 orbitals of S atom respectively (Figure 7C). These results directly evidenced the formation of 2H-MoS2 polyphase1,48. Note that there was no signal (around 236 eV) due to oxidized Mo6+ 3d5/2 arising for 1T-MoS2 phase, revealing generation of only 2HMoS2 nanosheets in dispersion without any damage. Further, the deconvoluated peaks at 135.9, 134.5, 398.8, 396.9 eV (Figure S13) corresponding to P 2p1/2, P 2p3/2, N 1s1/2 and Mo 3p3/2 22


Page 23 of 39

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orbitals were indicative of adsorption of phosphonium and imidazolium PILs at the surface of MoS2 nanosheet. XRD patterns of both MoS2-PIL nanosheets and bulk powder exhibited a major peak at 2θ value of 14.4 (Figure 7D) corresponding to (002) plane of crystalline 2H-MoS2 (ICDD card no. 77-1716).2-3 The photoluminescence (PL) spectra (Figure 7E) of dispersed

A

E12g

A1g

B Mo 3d5/2

Mo 3d3/2

S 2s

C

S 2p 3/2

S 2p 1/2

E

D

(002)

2θ (degree)

Figure 7.

(A) Raman spectra of bulk and MoS2-PIL nanosheets. XPS spectra of Mo 3d (B),

S 2p (C) of MoS2-P[VBTP][Cl] nanosheets. (D) XRD patterns of bulk and MoS2-PIL nanosheets.

(E) PL spectra of MoS2-P[VBTP][Cl] and MoS2-P[VimBu][Br] nanosheets in

various solvents. 23



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Page 24 of 39

MoS2-PIL nanosheets in different solvents exhibited only one major emission centered at 688nm corresponding to the energy of A-exiton of direct band gap single or few-layers MoS2 nanosheet, which was absent for bulk MoS2, which matched well with that reported by other groups.1,49 But we were unable to detect the emission due to the energy of B-exiton. Exfoliation of MoSe2 using P[VBTP][Cl]. To check the versatility of this method for other TMDs, we further exfoliated bulk MoSe2 using P[VBTP][Cl] in water and DMF. The efficient exfoliation of MoSe2 was clearly evident from the appearance of absorption peaks at 413, 689 and 802 nm (Figure 8A) as well as A1g Raman band at 241.4 cm−1 in the highly stable nanosheets’ dispersion (Figure 8B).9,24 The out-of-plane A1g peak was red shifted from 241.4 to 240 cm−1 due to transformation of the bulk to nanosheets with ∼3–4 layers MoSe2 nanosheets in water and in DMF (Figure 8B).50

A

B

413

240 cm-1

689

Figure 8.

A1g

802

241.4 cm-1

(A) UV-Vis spectra of MoSe2-P[VBTP][Cl] nanosheets’ dispersions, (Inset)

photograph of MoSe2-P[VBTP][Cl] dispersion in water. (B) Raman spectra of bulk and MoSe2P[VBTP][Cl] nanosheets. TEM image also showed numerous thin flakes (Figure 9A), indicating the presence of single or few-layers of MoSe2 nanosheets with lateral size distribution from 35 to 137nm, with 24


Page 25 of 39

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mean = 75nm. HRTEM image clearly revealed the crystalline fringes with distance of 0.32 nm (Figure 9B). The ED pattern (Inset of Figure 9A) suggested an undistorted lattice with hexagonal symmetry indicating the formation of semiconducting 2H-MoSe2 polyphase.9 The AFM images of MoSe2-P[VBTP][Cl] nanosheets’ dispersion in DMF and in water (Figures 9C9E) revealed numerous thin nanosheets with an average lateral size of 75 nm. The histogram of thickness distribution of MoSe2-P[VBTP][Cl] nanosheets’ dispersion in DMF (Figure 9F) clearly revealed the average thickness of nanosheets between 1.2-2.5 nm.

A

B

C 0.32 nm

D

Figure 9.

E

F

(A) TEM image of MoSe2-P[VBTP][Cl] nanosheets in water with typical ED

pattern in the Inset. (B) HRTEM image of crystalline fringes of single-crystalline MoSe2P[VBTP][Cl] nanosheet. (C) AFM image of MoSe2-P[VBTP][Cl] nanosheets in DMF along with its height profile. (D) AFM image of MoSe2-P[VBTP][Cl] nanosheets in H2O along with its height profile. (E) Enlarge view of MoS2 nanosheets in panel D. (F) The thickness distribution of MoSe2-P[VBTP][Cl] nanosheets in DMF is based on 100 nanosheets with an average lateral size of 75 nm. 25



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Page 26 of 39

Ion- and Temperature-Responsive MoS2-PIL Nanosheets’ Dispersion in Water. Recently, we have reported that the P[VBTP][Cl] PIL exhibited dual ion-responsiveness and upper critical solution temperature (UCST)–type thermo-responsiveness in water.43 It has been discussed that the aqueous solution of P[VBTP][Cl] was responsive to different anions such as Cl-, Br- and I-. Furthermore, the newly designed P[VimBu][Br] PIL also showed ion responsiveness in water, but, only with the addition of I- ion. We observed a clear change from transparent one-phase to a turbid two-phase system in presence of addition of minimum of 30 mM NaI, which upon heating above 40 °C became a transparent single-phase solution as shown pictorially in Figure 10A. The exact cloud point (Tcp) of such UCST-type transition of P[VimBu][Br] in water (0.5 wt%) was measured from the turbidity experiment and was found to be 38 °C (Figure 10B). As explained in our earlier report, in the case of P[VimBu][Br] PIL, the addition of I- effectively screened the cationically charged pendent imidazolium groups of this PIL and eventually made the PIL chains became hydrophobic in nature.43 The addition of foreign I- anion formed bridge between the neighbouring cationic P[VimBu] chains through the intra+

and/or inter-molecular ionic crosslinking which resulted in the formation of

insoluble

aggregated PIL chains and the solution became turbid. Furthermore, the weak ionic interaction between the foreign I- anions and the imidazolium cations of PIL can be disrupted by increasing the temperature, which converts the aggregated PIL into soluble uncoiled PIL chains indicating an UCST-type transition. The increase of I- concentration increased the extent of ionic interaction among the PIL chains. Thus, high energy is required to disrupt the ionic bridging that resulted in the increase of Tcp of P[VimBu][Br] PIL in aqueous solution with increasing I- concentration (Figure 10B).

26


Page 27 of 39

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B

A Heating

NaI Cooling

Figure 10.

(A) Photographs of aqueous P[VimBu][Br] (0.5 wt%) solution without NaI at 25

°C (left), turbid solution below Tcp (middle) and transparent solution above Tcp (right). (B) Turbidity curves of 0.5 wt% aqueous solution of P[VimBu][Br] in presence of NaI of varying concentrations during heating/cooling cycles. Such stimuli-responsive properties of PIL were employed in making responsive MoS2PIL nanosheets’ dispersion due to its adsorption on MoS2 surface. Indeed, the transparent greenish aqueous dispersions of MoS2-P[VBTP][Cl] and MoS2-P[VimBu][Br] transformed to cloudy dispersion up on addition of 475 mM NaCl or 30 mM NaI respectively at 25 °C as can be seen clearly from the photographs of the dispersions at these conditions (Scheme 2A). As mentioned above, this is because the halide ion induces the aggregation of the adsorbed PIL that eventually forced to restack the dispersed PIL-adsorbed MoS2 nanosheets (Scheme 2B). It was very clear from the photographs of MoS2 dispersion at different temperatures that the agglomerated/stacked MoS2-PIL nanosheets can again be back into dispersion in water upon ultrasonication for 10 min and heating at above 40 °C (Scheme 2A). As mentioned above, the heating provide sufficient energy to disrupt the ionic interactions among PIL chains on MoS2 surface and PIL molecules become soluble which results in a redispersion of the staked MoS2PIL in water (Scheme 2B). This observation was totally reversible for several cycles. 27



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Page 28 of 39

A

NaCl

Aggregated MoS2-PIL

MoS2-PIL dispersion

Powder MoS2 + H2O

NaI

MoS2-PIL dispersion

B

Scheme 2.

(A) Photographs of dual ion- and temperature-responsive behaviors of MoS2-

P[VBTP][Cl] and MoS2-P[VimBu][Br] nanosheets’ dispersion in water. (B) Schematic representation of dual halide ion- and thermo-responsive behaviour of stable MoS2-PIL nanosheets’ dispersion in water. Preparation of Polymer-MoS2 Nanocomposites and Their Electrical Properties.

To

make homogenously dispersed polymer nanocomposites, MoS2 dispersions were mixed with different polymers. As PIL molecules can easily exfoliate MoS2 crystal into layered nanosheets in both aqueous and nonaqueous solvents (DMF, DMSO and MeOH), we have chosen both poly(methyl methacrylate) (PMMA) and poly(vinyl alcohol) (PVA) as model polymers to make MoS2 based nanocomposites in nonaqueous and aqueous media respectively. For preparation, MoS2-P[VBTP][Cl] nanosheets’ dispersion were separately mixed with PVA in water and PMMA in DMF. The photographs indeed showed the formation of greenish homogeneous PMMA-MoS2 and PVA-MoS2 nanocomposites’ thin films (Insets of Figure 11A and Figure 11B), containing 0.6 and 0.5 wt% of MoS2 respectively. DSC thermograms of PMMA-MoS2 and 28


Page 29 of 39

PVA-MoS2 nanocomposites showed a single glass transition temperature (Tg) with an onset of 110.4 and 71.6 °C, those were bit higher than the Tg (onsets) 108 °C and 67.9 °C of neat PMMA and PVA respectively (Figure S14). These results indicated that the incorporation of very small amount MoS2 raised the Tg of both the nanocomposites compared to that of neat polymers. It was also clear that the adsorbed P[VBTP][Cl] on MoS2 surface was miscible with both PMMA and PVA, which intern helped to disperse MoS2 nanosheets in these polymer matrices forming homogeneous nanocomposites. N ea t P V A

14

P V A -MoS 2 compos ite

D ie le c tric lo s s

D ie le c tric c o n s ta n t

P V A -MoS 2 compos ite Electrical properties measurements revealed100that the incorporation of only 0.6 wt% MoS2 N ea t P V A 12 10

10 increased the dielectric constants (ξ') of the PMMA-MoS 2 film throughout the frequency range 8 6

1 from 10 Hz to 2 MHz in comparison to that of neat PMMA, because of addition of MoS2 of 4 2

0.1

51 10 10 10 nanocomposite 10 10 10 10 10 10 10 Whereas 10 higher reported ξ' value (Figure 11A). ξ' of10 the10 PVA-MoS was found 2 10 2

3

4

5

6

1

7

2

3

4

ω (rad/s )

10

5

ω (rad/s )

10

6

10

P MMA -MoS 2 compos ite

N eat P MMA

3.2

A 10

10

10

10

10

5

ω (rad/s )

2

10

3

3

10

10

4

4

10

5

10

5

10

ω (rad/s ω (rad/s ) )

10

6

10

6

7

10

7

10

12 10 8 6

8

N ea t P MMA

0.1

2 10

2

10

3

N eat P V A

N eat P MMA

10

0.1

6

C

4

10

6

10

7

10

3

2

3 10 10

4 1010

5

10 10

5

6

10 10

ω (rad/s ) ω (rad/s )

Figure 11.

4

6

10

7

10

7

10 5 10

6 1010

5

10 10 7

6

10

ω (rad/s )ω (rad/s )

4.4

P MMA -MoS compos ite

2 P V A -MoS 2 compos ite

12

4

1010 4

3

4.2

100

7

10

1

0.1 1 10

10

2

10

3

10

4

10

5

10

P V A -MoS 2 compos ite P MMA -MoS 2 c ompos ite N eat P V A N ea t P MMA


4

3.8

3.8

10

3.6 3.41

0.1

D

0.1 1 10

3 2

1010

3

4 10 10

4

10 10 5 10

5

6 6

ω (rad/s ) ) ω (rad/s

1010

7

7 10 10

10

8

10

3

10

4

10

5

10

6

ω (rad/s )


MoS2/neat PVA (B) films against frequency. Variation of dielectric loss of PMMA-MoS2/neat 3.6

0.1

PMMA (C) and PVA-MoS2/neat PVA (D) films against frequency. Insets of A and B showed 3.4 3.2

photographs of PMMA-MoS2 and PVA-MoS2 nanocomposites’ films respectively. ω (rad/s ) ω (rad/s ) 29 10

3

10

4

10

5

10

6

10

7

7

N ea t P MMA

MMA N eat P MMA Variation ofN eat P dielectric constants of PMMA-MoS 2/neat PMMA (A) and PVA

10

P MMA -MoS 2 c ompos ite

4.4 4.2

6

ω (rad/s )

4

3.2

N ea t P V A

100

B

4

P V A -MoS 2 c ompos ite

P MMA -MoS compos ite

2 P V A -MoS 2 c ompos ite

D ielec tric lo s s

3.4


1

14

8

8

N ea t P V A

14

4

N ea t P MMA

3.6

2 3

N ea t P V A compos ite P MMA -MoS 2

10 3.8

0.1 1 10

7

7


4

4

tric c lo DDieielelecctric o ns s ta n t

10

6

ω (rad/s )


3

100 4.2


10


2

5


4.4

D ielec tric c o n s ta n t


D le iecletric c tric D ie c lo o nsssta n t

N ea t P V A


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 14 29 30 12 31 10 32 8 33 6 34 35 4 36 2 37 10 38 4.439 4.240 441 3.842 3.643 44 3.4 45 3.2 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

3

10

4

10

5

10


6

10

7

10

7

10

8


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

Page 30 of 39

to be lower than that of neat PVA in the same frequency range (Figure 11B). Here, the reported ξ' value of MoS2 was actually lower than that of neat PVA.51 Furthermore, the dielectric loss (ξ") (Figure 11C and Figure 11D) and conductivity values obtained from complex impedance plot (Figure S15) also exhibited the similar trend like dielectric constant. Hence the addition of high ξ' component help to increase the conductivity and that of low ξ' component decrease the conductivity than that of the base polymer.52 Therefore by incorporating MoS2 nanosheets into polymer matrices, we were able to prepare homogeneous nanocomposites of both high and low dielectric constants.

§

CONCLUSIONS In summary, we have demonstrated an one-step efficient approach for making highly

stable single or few-layers MoS2/MoSe2 nanosheets’ dispersion in aqueous and nonaquous media using cationic PILs as the exfoliating-cum-stabilizing agent. This PIL-assisted method produced single or few-layers MoS2 nanosheets of mainly 2H polyphase as established from several instrumental techniques. The PIL stabilized TMD’s dispersion was highly stable and showed no precipitation upon resting for more than 60 days. The dual ion- and temperature-responsive properties of the PILs were exploited to achieve waterborne responsive MoS2 nanosheets with possibilities of their potential as sensory materials. In addition, the ease of dispersibility of MoS2-PIL nanosheets in various solvents opened up possibilities of making various polymer nanocomposites based on water-soluble and nonaqueous-soluble polymers having both high and low dielectric constants featuring many materialistic applications. ■

ASSOCIATED CONTENT

Supporting Information (SI): SEC traces of PIL, 1HNMR spectra of IL monomer and PIL, ESI-MS spectra of IL, FTIR spectra of MoS2-PIL, UV-Vis spectra of MoS2-PIL dispersion, 30


Page 31 of 39

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


AFM images of MoS2-P[VBTP][Cl] nanosheets in various solvents, DLS data of dispersed MoS2-PIL, STEM image and EDX spectrum of MoS2-P[VBTP][Cl], zeta potential of MoS2P[VBTP][Cl] dispersions, XPS spectra of P 2p (A) and N 1s (B) of MoS2 nanosheets, DSC thermograms of PMMA-MoS2 and PVA-MoS2 nanocomposites, Cole Cole plot and variation of conductivity with frequency of nanocomposites. The Supporting Information is available free of charge on the ACS Publications website. ■

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Fax: 91-33-2473 2805 The authors declare no competing financial interest.

■

ACKNOWLEDGEMENTS

Y.B. thanks IACS for providing fellowship. M.D. thanks CSIR for fellowship. This research was partially supported by the grants from SERB, India. Authors thank S. Das and Professor A. Ghosh of IACS, Kolkata, for their help in measuring electrical properties.

■

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Poly(ionic liquid)-Promoted Solvent-Borne Efficient Exfoliation of

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