Ethylenediamine Assisted High Yield Exfoliation of MoS2 for Flexible

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Ethylenediamine Assisted High Yield Exfoliation of MoS2 for Flexible Solid State Supercapacitor Application Arup Ghorai, Samit K. Ray, and Anupam Midya ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02002 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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ACS Applied Nano Materials

Ethylenediamine Assisted High Yield Exfoliation of MoS 2 for Flexible Solid State Supercapacitor Application Arup Ghorai1, Samit K Ray2,3and Anupam Midya1*, 1

School of Nanoscience and Technology, Indian Institute of Technology Kharagpur, Kharagpur 721302, India S N Bose National Centre for Basic Sciences, Kolkata 700106, India 3 Dept of Physics, Indian Institute of Technology Kharagpur, Kharagpur 721302, India 2

*Corresponding email: [email protected]; [email protected]

Abstract A high yield synthesis technique of few layers MoS 2 by ethylenediamine (EDA) assisted liquid phase exfoliation is reported. In the cyclic two steps process, EDA is adsorbed initially followed by sonication with propylene carbonate. The combination of organic solvents results in the exfoliation of micron sized few-layer two dimensional (2D) MoS 2 nanosheets within a short duration. Controlled gradual centrifugations separate 2D MoS 2 of different sizes ranging from nanocrystals to micron size flakes. The method differs from other sonication induced fragmentation process and results in larger sized nanosheets up to ~2.0 micron of lateral dimension having a thickness of ~ 2 to 3 nm with high yield. Optical properties of the 2D nanosheets are studied by UV-visible absorption and Raman spectroscopy. Finally, the exfoliated MoS 2 is employed to fabricate flexible solid-state supercapacitors, which shows superior performance compared to bulk MoS 2 -based devices, indicating its potential for energy storage applications. Keywords: Ethylenediamine (EDA), MoS 2 exfoliation, Supercapacitor, Sonication, Cyclic voltammetry, Charge discharge, Adsorption

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Introduction: MoS 2 has emerged as a rising star among two dimensional (2D) transition metal dichalcogenides having S-Mo-S tri-atomic layers attached to each other via a weak van der Waals attraction force in the bulk. Mo atoms are sandwiched between two layers of S atoms forming semiconducting 2H (trigonal prismatic D3h) and metallic 1T (octahedral Oh) phases. Mechanically exfoliated MoS 2 has shown great promise for electronic and photonic devices such as photodetectors,1 field effect transistors2 etc., whereas chemically or liquid phase exfoliated MoS 2 is found to be suitable for energy storage,3,4 catalysis5,6 plasmonics,7 acoustics modulations,8 and sensing9 applications. However, existing synthesis methods suffer from low yield production, poor control of dimensionality and the use of toxic chemicals. A successful realization of MoS 2 exfoliation by lithium intercalation method was reported almost thirty years back by Joenson et al.10 using n-BuLi, a highly toxic and pyrophoric chemical to yield metallic 1T MoS 2 . Only few efforts are reported to replace n-BuLi by hydrazine, followed by intercalation with sodium naphthalimide.11 Following the advent of graphene,12 single layer dispersion of MoS 2 in 2H phase is obtained recently. Coleman’s group pioneered the liquid phase exfoliation of 3-12 nm thick MoS 2 layer13 in 2H phase using several group of solvents. Two poor solvents can be effective for exfoliation and dispersion of TMDC, when they are mixed in an appropriate proportion (viz., 45% mixture of ethanol and water).14 A combined process of N-methyl-2-pyrrolidone (NMP) assisted grinding and sonication (in mixed solvent, ethanol/water) for the direct exfoliation of bulk MoS 2 to monoand few-layer nanosheets was developed to fabricate ammonia gas sensors.15 Nguyen et al. synthesized single as well as a few layer thick flakes of MoS 2 by grinding-assisted sonication and exfoliation using NMP and ethanol.16 Common organic solvents such as N,N-di2


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methylformaide, NMP and dichlorobenzene are proven to be efficient exfoliating solvents to obtain few layer (3 to 12 nm thick) 2H MoS 2 but the yields (below 1mg /ml) of these methods are still very low. In addition, liquid phase exfoliation by sonication suffers from lower lateral dimension (20 to 500 nm), and a smaller fraction of thinner sheets. The addition of NaOH to NMP solvent3 is found to improve the exfoliation efficiency of MoS 2 . It has been reported that a combination of electric field and mechanical shear force facilitates MoS 2 exfoliation.17 For successful exfoliation, the solvation energy should circumvent the van der Waals attraction force between the layers in the bulk. In addition, the exfoliating solvent should have optimum material dispersity, polarity and H bonding components. In this paper, a new solvent, propylene carbonate (PC) in combination with ethylenediamine is proposed for efficient exfoliation of MoS 2 in 2H phase. Ethylenediamine (EDA), a Lewis base, chelating agent and H-bond donor is chosen to bind the surface of S-Mo-S layer as shown in the Figure 1. Non-covalent interactions between MoS 2 and EDA weaken the van-der-Waal attraction between the layers and facilitate the exfoliation process using organic solvent through bath sonication. Here, shorter sonication time does not reduce the lateral dimension of the nanosheets resulting in micron size few-layer 2D sheets of MoS 2 . In this generic process, the use of pyrophoric, toxic, and hygroscopic solvents like n-BuLi or hydrazine could be avoided. The efficacy of the method is demonstrated by fabricating an efficient solid-state flexible supercapacitor using exfoliated MoS 2 flakes with superior properties as compared to bulk MoS 2 .

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Experimental: Material: MoS 2 (cataloge no 69860) bulk powders, ethylenediamine, propylene carbonate, N,NDimethylformamide (DMF) and organic solvents were purchased from Sigma Aldrich and used without any further purification. Synthesis: The use of a new chemical method of exfoliation of 2D MoS 2 in 2H phase is schematically depicted in Figure 1. Concisely, 0.5 g of bulk MoS 2 powder was taken in a round bottom flask equipped with a magnetic bar and 25 ml of ethylenediamine (EDA) was added into it. The dispersion was stirred using a magnetic stirrer for different durations (0, 1, 2, 4, 8, 24 and 48 hr) at room temperature. After adsorbing EDA on the surface of the bulk MoS 2 powder, the dispersion was centrifuged to remove the excess un-adsorbed EDA. The process was repeated once after dispersing the precipitate in an organic solvent (DMF or propylene carbonate) and discarded the supernatant. Finally, 50 ml of EDA adsorbed MoS 2 dispersion in organic solvent (PC or DMF etc.) was prepared and sonicated for 2 hr at room temperature to yield greenish colloidal dispersion of MoS 2 in PC using bath sonication. A short duration (typically 10 min) centrifugation using a high speed benchtop centrifuge (Model: Z36HK Make (Hermle Labortenik GmbH, Germany) was carried out at different speeds from 1000 to 10000 rpm (RCF90 g to 9390 g) to collect thin layers of MoS 2 with different lateral dimensions. Precipitates of 1000 rpm (RCF-90 g) centrifugation were discarded to remove partially exfoliated particles. Supernatant samples of 10000 (RCF-9390 g), 8000 (RCF-6010 g) and 3000 (RCF-845 g) rpm were termed as MoS 2 at 10K, 8K and 3K, respectively.

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Concentration measurements: For the estimation of concentration, 100 mg of EDA adsorbed MoS 2 was exfoliated by sonication for 2 hr with 5 ml of different solvents. Following sonication, bigger particles were removed by centrifugation at 1000 rpm (RCF-90 g). The stable MoS 2 dispersion in different solvents DMF, 45% ethanol water, DMSO, NMP and PC were precipitated by adding absolute ethanol. Thereafter, the precipitate was centrifuged at 1000 rpm (RCF-90 g) and few layers MoS 2 powders were obtained after drying at 50 °C under vacuum. By measuring the weight of the centrifuge tube before and after centrifugation, the yields of the exfoliation process were calculated. Characterization: The surface morphology and thickness of MoS 2 flakes were investigated using an atomic force microscope (AFM), (Model No Veeco Nanoscope-III). The chemical composition of MoS 2 was studied using X-ray photo electron spectroscopy (PHI 5000 Versa Probe II, ULVAC−PHI, INC, Japan) with incident AlKα X-ray of energy hν = 1486.6 eV. The UV-visible absorption spectra of MoS 2 dispersions were recorded using a fiber-probe-based UVVis-NIR CCD spectrometer. High resolution transmission electron microscope HRTEM (Model No JEM–2100) equipped with a charge couple device (CCD) camera from Gatan, Inc. USA, at an operating voltage 200 kV from JEOL, Japan was used to image the layer structure of MoS 2 . Fourier transform infrared spectroscopy (FTIR), (Model No Nexus-870) from Thermo Nicolet Corporation was used to analyze the vibrational bonds of MoS 2 and MoS 2 -EDA intermediate. High-resolution X-ray diffractometer (Model No Philips X’ Pert MRD) was employed to study the crystallinity of synthesized MoS 2 flakes.

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Results and Discussion: Sonication assisted liquid phase exfoliation followed by stepwise centrifugation results in a mixture of nanosheets and nanocrystals with a poor yield (0.01-0.9 mg/ml). Moreover, extensive sonication of TMDC suspension in DMF solvent leads to the fragmentation of the flakes in all directions resulting in smaller particles.18 Therefore, to optimize the method, ethylenediamine as a bidentate chelating ligand is employed to bind MoS 2 sheets through S atoms followed by sonication using propylene carbonate (PC) or DMF to yield monolayer to few-layer MoS 2 sheets of larger size flakes (~2 micron). The schematic diagram of the two-step process is depicted in the Figure 1. For a control experiment, only PC solvent is employed to exfoliate MoS 2 under same sonication condition. Though PC is a good solvent for CNT and graphene, no exfoliation is observed using only EDA or only PC through short sonication (below 2 hr) or stirring condition at room temperature. Generally, solvents having surface tension value close to 40 mN/m are found to be good for the exfoliation of TMDCs. More specifically, the interfacial surface tension of the solvents and solid plays a vital role for the exfoliation. For a good solvent, the interfacial surface tension between solvent and solid should be low.19 We have calculated the interfacial surface tension for different solvents taking the ratios of polar to dispersive component of MoS 2 as ~0.449 and surface tension around 40 mN/m.20 The interfacial surface tension value for PC and DMF is smaller as compared to other solvents resulting in a high yield (a maximum yield of 23.2 % (w/w) in PC) of exfoliation of MoS 2 compared to those of IPA, ethanol and acetone (see Table S1, Supporting Information). The soaking of MoS 2 in EDA at room temperature for 24 hr maximizes the adsorption of EDA on the surface MoS 2 layer through ligand interactions, as shown in Figure 1. Following the removal of excess EDA at room temperature, several groups of solvents are 6


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examined to exfoliate MoS 2 but only a few solvents enabled the exfoliation of MoS 2 in 2H phase (see Figure S1 in the Supporting Information). The role of EDA is clearly noticeable by performing a control experiment by sonication for 2 hr of MoS 2 powder dispersed in DMF or PC in the absence of EDA, where no color change or exfoliation is observed after 12 hr sedimentation of the reaction mixture as shown in the Figure S2(a) (Supporting Information). In addition, in the first step, MoS 2 is adsorbed with EDA at different durations (1, 2, 4, and 8 hr) (Figure S2 (b)-(e), Supporting Information) and even the precipitate of the first cycle is adsorbed with EDA for 1 day and 2 days (Figure S3, Supporting Information). In the next step, after removal of excess EDA, 2 hr sonication with PC produces different concentrations of MoS 2 dispersion. These phenomena ascertain the crucial role of bidentate ligand EDA in the exfoliation process. A substantially high yield of 23.2 % is obtained in the first cycle of exfoliation using PC solvent. MoS 2 exfoliation and separation process are precisely characterized by high resolution transmission electron microscopy (HRTEM). Exfoliated MoS 2 sheets collected by gradient centrifugation from high to low speed 10000 rpm to 3000 rpm (RCF-9390 g to 840 g) are examined using a HRTEM, as shown in Figure 2. High transparency of the MoS 2 nanosheets under electron beam confirms the ultrathin layered structure of exfoliated MoS 2 . High speed 10000 rpm (RCF-9390 g) suspension contains quantum dots of size below 5 nm, interspersed with MoS 2 sheets of lateral dimension of 100 nm and smaller (Figure 2(a)). Whereas, supernatant of 8000 rpm (RCF- 6010 g) contains nanosheets from 50 to 300 nm in lateral dimension (Figure 2(b)). Typical MoS 2 sheets with lateral dimension of ~2 micron is observed in the supernatant of 3000 rpm (RCF- 840 g), as shown in Figure 2(c). High crystallinity and hexagonal symmetry of the exfoliated MoS 2 sheets are observed in selected area electron 7



diffraction (SAED) pattern (Figure 2(d)). Visually, a clear difference in color can be observed with different dimensions of the nanosheets, as shown in the insets of Figure 2(a), (b) and (c). Thickness and dimensions of the MoS 2 flakes are also examined by atomic force microscopy in tapping mode. As-prepared MoS 2 dispersion is spin casted on hydrophilic Si substrates at 1800 rpm. Figure 3(a) shows that typical lateral dimension of the as-exfoliated MoS 2 layer is around 2 µm, which agrees well with the TEM results. As shown in the height profile in Figure 3(b), the thickness of the sheets is found to be 2-2.5 nm, which is equivalent to 3 to 4 (0.61 nm×n) S-MoS tri-atomic layers of MoS 2 .

Figure 1: Schematic diagram of high yield synthesis of MoS 2 using EDA as adsorbing and intercalating agent.

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Photophysical properties of the exfoliated MoS 2 are investigated by UV-visible absorption spectroscopy. Absorption spectra of MoS 2 dispersions collected after gradual centrifugations at different speeds of 3000, 8000 and 10000 rpm (RCF- 840, 6010 and 9390 g) are plotted in Figure 4. The spectrum of 3000 rpm (RCF- 840 g) supernatant contains three distinct absorption peaks (675, 621 and 457 nm) marked as A, B, and C. The strong absorption peaks at 675 nm (1.83 eV) and 621 nm (1.99 eV) originated from the transition of spin-split valence bands to the conduction band at the K-point of the Brillouin zone reveal the direct band gap nature of MoS 2 and successful exfoliation of 2D sheets.21 The peak C at 457 nm is attributed to the transition from the deep valence to the conduction band. The peak position of this transition is found to vary with the size of nanosheets, which is presented in Table 1. The variation of peak C (λ max of 457 nm to 431 nm) position of MoS 2 absorption spectrum is ascribed to the quantum size effect in nanosheets.22,23 With decreasing layer thickness, the excitonic peak A at 675 nm is blue shifted to 670 and 665 nm. In addition, the peak intensity also decreases with the decrease in size in agreement with reported results.22

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(a)

(b)

100 nm

100 nm

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(d) (c)

(100)

0.2 µm 10 1/nm Figure 2: Typical TEM images of as-exfoliated MoS 2 collected at different centrifugation speeds (a) 10000 (RCF- 9390 g) rpm, (b) 8000 (RCF-6010 g) rpm, and (c) 3000 (RCF- 840 g) rpm, (d) SAED pattern of MoS 2 sample.

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Figure 3: Typical AFM image of as-exfoliated MoS 2 nanosheets showing the (a) surface topography, and (b) height profile. 11


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

Absorbance (a.u.)


1.0

Exfoliated MoS2-3K Exfoliated MoS2-8K Exfoliated MoS2-10K

C

0.8 B

0.6 0.4

A

Lateral Size decreases

0.2 0.0

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Thickness decreases

400 500 600 700 800 900 1000

Wavelength (nm) Figure 4: Typical UV-visible absorption spectra of as-exfoliated MoS 2 collected at different centrifugation speeds.

Table 1: Centrifugation

A (nm)

B (nm)

C (nm)

3000 rpm (RCF-840 g)

675

621

457

8000 rpm (RCF-6010 g)

670

617

445

10000 rpm (RCF-9390 g)

665

612

431

12


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The surface chemical composition as well as the oxidation state of the MoS 2 flakes is analyzed by X-Ray photoelectron spectroscopy. The signature peaks of Mo4+ 3d 5/2 at 229.3 eV and Mo4+ 3d 3/2 at 232.6 eV of exfoliated MoS 2 sheets displayed in Figure 5(a) confirm the formation of stoichiometric MoS 2 . The corresponding S 2p peak shown in Figure 5(b) consists of a doublet due to S 2p 1/2 (163.4 eV) and S 2p 3/2 (162.3 eV) electrons. The S 2p spectrum is deconvoluted and fitted using a Gaussian-Lorentzian function after baseline correction. Interestingly, XPS analysis of MoS 2 -EDA and exfoliated MoS 2 nanosheets in the narrow scan area of C and N confirms the presence of C and N atoms of EDA, as shown in the Figure S4 (Supporting information).

6000

Mo 3d5/2 228.9

(a)

10000

Mo 3d3/2 232.1

8000 6000 4000 2000

S 2s 226.1

Intensity (a.u.)

12000

Intensity (a.u.)

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


225

230

5000

235

S2p1/2 163.4

4000 3000 2000 1000 0 156

0

S2p3/2 162.2

(b)

158

160

162

164

166

168

Binding Energy (eV)

Binding Energy (eV)

Figure 5: High resolution X-Ray photoelectron spectra of exfoliated MoS 2 . High resolution XRD is employed to study the crystal structure of the exfoliated MoS 2 samples. Figure 6(a) shows the diffraction patterns of MoS 2 samples obtained from centrifugation at 10000 (RCF- 9390 g), 8000 (RCF-6010 g) and 3000 (RCF- 840 g) rpm. The formation of 2H semiconducting phase of exfoliated MoS 2 (JCPDS no.37-1492) is confirmed from the well resolved peaks, as observed in the XRD patterns. 13



MoS2-Bulk

(002) (004)

(100) (103) (102)

(b)

(105) (110) (008)

Transmittance (a.u.)

(a)

MoS2-EDA

Intensity (a.u.)

Exfoliated MoS2-3K

Exfoliated MoS2-8K

Bulk MoS2 Mo-S strech

EDA N-H Bend

N-H strech

MoS2-EDA

Exfoliated MoS2-10K

0

10

20

30

40

50

2θ (degree)

(c)

60

2800

0

70

1000

2000

3000

Wavenumber (cm-1)

4000

-Bulk MoS2

-Exfoliated MoS2

Intensity (a.u.)

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

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2100 1400 700 0

360

390

420

450

480

Raman Shift (cm-1) Figure 6: (a) X-ray diffraction patterns of MoS 2 at different stages of exfoliation, (b) FTIR spectra of MoS 2 -EDA, bare EDA and bulk MoS 2 , (c) Raman spectra of bulk and exfoliated MoS 2 samples. Interestingly, after EDA adsorption in MoS 2 -EDA and exfoliation, the peak at 28.58° due to the (004) plane disappeared completely. As the (004) peak indicates the basal plane parallel to the (002) plane directed along the c-axis, the absence of the former indicates the expansion of S-MoS---S-Mo-S bond length due to the interaction with the amine group of the EDA and S-atoms of the MoS 2 crystal. Others peaks due to (102), (103), (105), (110), (008) planes of bulk MoS 2 are absent, indicating successful exfoliation of 2D sheets . 24 14


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To investigate the bonding and interactions between organic EDA and the surface of the exfoliated nanosheets, FTIR spectra are acquired. FTIR spectra of bare EDA, bulk MoS 2 and EDA-MoS 2 intermediate are shown in Figure 6(b). The spectrum of bare EDA shows a strong doublet N-H stretching vibration peaks at 3350 (for symmetric stretching) and 3280 cm-1 (for anti-symmetric stretching). It also exhibits two peaks, one at 2930 cm-1 for C-H stretching (symmetric) and another at 2860 cm-1 for anti-symmetric stretching. In addition, the peaks for NH bending at 1590 cm-1 and N-H out-of-plane bending at 931 cm-1 are also observed.25 Characteristic vibrational peaks of 2D MoS 2 layers are observed at 400-500 cm-1 region.26 All the peaks responsible for C-H and N-H bonds are also present in the MoS 2 -EDA hybrid except the one for N-H bending at 1590 cm-1. This observation indicates that there may be strong C-NH---S-Mo-S interactions between EDA and the MoS 2 bulk. Raman spectroscopy is employed to investigate the structure of as-exfoliated MoS 2 by irradiating with 514 nm laser. Raman spectrum of bulk MoS 2 in the Figure 6(c) shows two characteristic peaks at 377 cm-1 and 404 cm-1 which are attributed to in-plane E1 2g and out-of- plane A 1g vibration, respectively. Because of exfoliation to thinners sheets, the E1 2g band is shifted to 384 cm-1 in as-exfoliated MoS 2 . This phenomenon also supports the exfoliation process where E1 2g phonon bands stiffen with decreasing number of layers and a blue shift of the peak is occurred.27 In contrast to the reported A 1g vibration band at 403 cm-1 for monolayer MoS 2 , it is red shifted to 408.5 cm-1 in as-exfoliated MoS 2 . Finally, flexible solid-state supercapacitor is fabricated using MoS 2 flakes collected at 3000 rpm (RCF- 840 g) centrifugation speed and the charge storage performance is compared with that of bulk MoS 2 . Detailed description of the fabrication process is given in the Supporting Information. To improve the conductivity of MoS 2 , it is blended with the acetylene black (15% 15



w/w), which does not have any capacitive behaviour.28,29 Synergistic effect between exfoliated MoS 2 and acetylene black increases the charge storage ability of the devices. PVA-H 3 PO 4 is used as a gel electrolyte and a very thin (0.1 mm width) steel plate is employed as a flexible electrode. The electrochemical charge storage property of the as-synthesized material is evaluated by cyclic voltammetry (CV) in both two and three electrode systems. The CV behavior of MoS 2 and acetylene black (85:15) coated working electrode is measured in three electrode system using 1M H 3 PO 4 as an electrolyte, platinum wire as a counter and Ag/AgCl, 2M KCl as a reference electrode. However, the absence of Faradic current in the CV curve at low scan rate (2mV/sec) (Figure S5, Supporting Information) indicates the non-existence of pseudocapacitance behavior and the formation of an electric double layer (EDL) at the interface between gel electrolyte and electrode is solely responsible for the capacitive behavior of the material. The CV curve of the solid-state device is performed from 0 to 0.8 Vat different scan rates from 300 to 20 mV/s, as shown in Figure 7(a). The specific capacitance of devices from their CV curves under different scan rates is found to be 1040, 1060, 940, 720, and 620 mF/gm for the scan rate of 300, 200, 100, 50, 20 mV/s, respectively. The quasi-rectangular shape of the CV curves of fabricated devices indicates nearly ideal supercapacitive nature due to the formation of an electric double layer. Due to the large surface area of 2D nanosheets, adsorption and desorption of ions readily could take place resulting in an increase of charge storage ability.30 The mechanism of capacitive behavior can be explained by the encapsulation of H+ ions in between the interlayer position of MoS 2 . The large surface area and nanoscale size of MoS 2 greatly reduce the diffusion path length of ions and facilitate electron transfer during charge discharge process. The cyclic charge-discharge curves shown in Figure 7(b) at different current 16


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densities exhibit nearly triangular shape, indicating good capacitive characteristics and superior reversible adsorption desorption of ions in the 2D composite.31 The exfoliated MoS 2 based supercapacitor also shows good (~91%) retention of areal specific capacitance after continuous 1000 charge discharge cycles (Figure 7(c)). To correlate the charge storage ability with the surface properties of exfoliated MoS 2 , we have also compared the capacitance behavior of MoS 2 flakes and bulk under an identical condition. MoS 2 flakes with higher surface area can store higher amount of charges, showing enhanced capacitance behavior over that of bulk MoS 2 , which is clearly observed in the CV curve at a scan rate of 300 mV/s, as displayed in Figure 7(d). The areal specific capacitance of the device is also calculated from the charge discharge curve using the following equation. C sp =𝐼𝐼∆𝑡𝑡/(𝑚𝑚∆𝑉𝑉)

(1)

Where I(A), t(s), m(gm) and ΔV(V) are the discharge current, discharge time, mass of the active device, and the potential windows, respectively. An areal specific capacitance of maximum 6.74 mF/cm2 (specific capacitance of 1348 mF/gm) at a current density of 0.1 mA/cm2 is estimated in accordance with our previous report.28 The performance is comparatively better than the capacitance value reported by Krishnamoorthy et al.32 With the increase of current density, the specific capacitance (see Table S2 in Supporting Information) of the active material decreases to 1072 mF/gm at a current density of 1 mA/cm2. We have carried out a statistical analysis of the supercapacitor device performance of 5 devices at a scan rate of 1mA/cm2 (See the Table S3, Supporting Information) to demonstrate the reliability of device characteristics. Moreover, a negligible Internal Resistance (IR) drop is observed in the charge-discharge curves either at a low or a high current density, indicating that 17



the devices have very low internal resistance between the solid electrolyte and electrode.33 The columbic efficiency has also been calculated using the following equation. 0.0018 0.0012

Current (A)

(b)

20 mV/s 50 mV/s 100 mV/s 200 mV/s 300 mV/s

0.0006

Voltage Window (V)

(a)

0.0000

-0.0006 -0.0012 0.0

0.2

0.4

0.6

0.1 mA 0.2 mA 0.4 mA 0.6 mA 0.8 mA 1 mA

0.6 0.4 0.2 0.0 0

(d) 0.0015

Exfoliated MoS2

Current (A)

8 7 6 5 4 3 2 1 0

0.8

0.8

Voltage Window (V)

(c)

Specific Capacitance (mF/cm2)

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

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0.0010

25

50

75

100

Time (s) Exfoliated MoS2 Bulk MoS2

0.0005 0.0000

-0.0005 -0.0010 300 mV/s

-0.0015 0

200

400

600

800 1000

0.0

No of Cycles

0.2

0.4

0.6

0.8

Voltage (V)

Figure 7: (a) Typical CV curves at different scan rates; (b) Charge discharge curves at different current densities; (c) Cyclic stability of the fabricated devices; (d) Comparison of charge storage behaviour of bulk MoS 2 and exfoliated MoS 2 at 300 mV/s scan rate.

�𝜂𝜂 =

𝑡𝑡 𝑑𝑑 𝑡𝑡 𝑐𝑐

𝑋𝑋100�

(2)

where t d and t c are the discharge and charging time, respectively. A high Columbic efficiency means a higher rate of H+ ions transportation through the electrode material. The flexibility of the fabricated device is demonstrated by measuring CV characteristics at different bending angles, as

18


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shown in Figure 8(a). There is negligible change in the CV behaviour under different bending angles, indicating it’s suitability for flexible applications.

(a)

(b)

0.0005 0.0004 0.0003 0.0002 0.0001 0.0000 -0.0001 -0.0002

400

Experimental curve Fitted Curve

C Rs

30° 45° 60° Twisted

Ф CPE

300 Rct

W

200

20

-ImgZ (Ω)

-ImgZ (Ω)

Current (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 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


100

10 0

-10

0 0.0

0.2

0.4

0.6

0.8

-20

0

Voltage Window (V)

100

200

0

5

10

RelZ(Ω)

300

15

400

RelZ (Ω)

Figure 8: (a) CV of MoS 2 nanosheet based flexible devices at different bending angles at a scan rate of 100 mV/s; (b) Nyquist plot of the MoS 2 nanosheet based device (inset showing the equivalent circuit diagram of fitting) Electrochemical impedance spectroscopy (EIS) is performed within the frequency range of 0.1 Hz to 10 kHz of the solid-state device to study the electrochemical capacitive behavior of the device. We have fitted the Nyquist plot using the equivalent circuit diagram shown in the inset of Figure 8(b). In the high frequency region of the Nyquist plot, as shown in Figure 8(b), there is a small semicircle and then it becomes stepper in the low frequency region, indicating superior capacitive nature of the device. From the fitting, we have found a low R ct (~1.57Ω) value of the fabricated device. The ESR (Rs) value is also found to be very low (~2.8Ω), revealing a good ionic conductivity between electrode material and the electrolyte.

19



Conclusions: In this paper, we have exfoliated MoS 2 nanosheets using ethylenediammine as an exfoliating agent in combination with propylene carbonate. In the two steps process, a few layer nanosheets of micron size as well as nanocrystals of MoS 2 are obtained within a short duration. We have obtained two dimensional micron size MoS 2 nanosheets which is an uncommon product in a liquid phase exfoliation method. The as-synthesized MoS 2 nanosheets and nanocrystals were extensively characterized by TEM, XRD, XPS, UV-visible, and Raman spectroscopy and confirmed the formation of 2H phase of 2D MoS 2 . Finally, we have fabricated flexible solidstate supercapacitors using as-exfoliated MoS 2 nanosheets and bulk MoS 2 under identical conditions compared their charge storage property. MoS 2 nanosheets based solid-state supercapacitors exhibit a higher specific capacitance of 1348 mF/gm, indicating it as a potential candidate for solid-state energy storage technology. Supporting Information: Calculations of interfacial surface tension and the yield of the MoS 2 dispersion, digital image of MoS 2 dispersions, supercapacitor device fabrication, specific capacitance with the variation of current density, CV curve of three electrode cell, statistics on the supercapacitor device performances, high resolution X-Ray photoelectron spectra for N and C are described in the Supporting Information Acknowledgments: The work is supported by Department of Science and Technology, Govt. of India (Grant No IFA 13-MS-09) 20


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TOC

MoS2 Flakes Bulk MoS2

Current (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 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 24 of 24

300 mV/S

0.0

0.2

0.4

0.6

Voltage (V)

24


0.8

Ethylenediamine Assisted High Yield Exfoliation of MoS2 for Flexible

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