1 High -yield exfoliation of MoS 2 nanosheets by a novel spray technique and the importance of soaking and surfactants

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High-yield exfoliation of MoS2 nanosheets by a novel spray technique and the

importance of soaking and surfactants

Suvigya Kaushik1#, Siva Sankar Nemala1#, Mukesh Kumar2, Devesh Negi3, Biswabhusan Dhal1, Lalita

Saini1, Ramu Banavath4, Surajit Saha3, Sudhanshu Sharma2 & Gopinadhan Kalon1,5 *

1Discipline of Physics, Indian Institute of Technology Gandhinagar, Gujarat 382355, India

2Discipline of Chemistry, Indian Institute of Technology Gandhinagar, Gujarat 382355, India

3Department of Physics, Indian Institute of Science Education and Research Bhopal, Bhopal, 462066,

India

4Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology-

Bombay, Mumbai 400076, India

5Discipline of Materials Engineering, Indian Institute of Technology Gandhinagar, Gujarat 382355,

India

(*E-mail: gopinadhan.kalon@iitgn.ac.in)

(#-Equally contributed)

Liquid-phase exfoliation of two-dimensional materials is very attractive for large-scale applications.

Although used extensively, isolating MoS2 layers (<10) with high efficiency is reported to be

extremely difficult. Further, the importance of soaking has not yet been studied, and the

surfactants' role in stabilizing MoS2 nanosheets is poorly understood1. Herein, we report a novel

approach to exfoliating large quantities of MoS2 via high-pressure (HP) liquid-phase exfoliation (LPE)

in deionized (DI) water. 4 to 7 layers of MoS2 nanosheets were obtained from 60 days-soaked

samples and they were found to be stable in solvents for periods of up to six months. Studies on the

effect of three surfactants, namely sodium dodecyl benzenesulfonate (SDBS), sodium cholate (SC),

and tetra-butyl ammonium bromide (TBAB), indicate that exfoliation of MoS2 nanosheets in SDBS

is highly efficient than the other two surfactants. The estimated yield reaches up to 7.25%, with a

nanosheet concentration of 1.45 mg/ml, which is one of the highest ever reported. Our studies also

suggest that the nanosheets' concentration and the lateral size depend on exfoliation cycles, applied

pressure and surfactant concentration. Hydrogen evolution reaction (HER) and ion-transport study

show that the nanosheets prepared by our method are stable in an acidic medium and free from

surfactants. A high hydrogen evolution rate of 30.13 mmol g-1 h-1 was estimated under ambient

laboratory conditions.

1. Introduction

Transition metal dichalcogenides (TMDs) have been increasingly explored due to several useful

characteristics, such as atomically small thickness and large spin-orbit coupling, which promises a wide

range of applications2. The TMDs are represented by the chemical formula MX2, where M is a

transition metal (Mo, W, etc.), X is a chalcogen (S, Se, Te, etc.), and they have a hexagonal structure.

In MX2, one M layer is sandwiched between two X layers with strong covalent bonds within one layer

but weak van der Waals interaction between adjacent layers. This makes it feasible to exfoliate into

mono- or few-layered nanosheets.

Among TMDs, MoS2 crystals are abundant, which prompted us to focus on MoS2 nanosheets. MoS2

sheets can be arranged into nanocapillary channels exhibiting ion sieving effects, thus useful for

desalination and nanofiltration applications3. Due to its layered structure, MoS2 is an excellent

lubricant in space applications4. Monolayer MoS2 is a direct bandgap semiconductor ( ̴1.8 eV) in

contrast to bulk ( ̴1.23 eV), making it useful for transistors5 and solar energy harvesting6. Few-layered

MoS2 is equally interesting for applications such as flexible and wearable electronics7, sensors8–11, drug

delivery12, supercapacitor13, and photo-catalysis14. MoS2 nanosheets have more surface-active sites

than the bulk, which is expected to enhance its electrocatalytic activity. These nanosheets have a high

surface area, thermal stability, electrical conductivity, and adsorption capability15. Several studies

indicate their potential for immediate applications as a catalyst in HER15–22.

The large-scale synthesis of MoS2 has been attempted several times, but it remains a challenge to

produce good quality and large quantities commercially5,14,23,24. The commonly used exfoliation

method of ultra-sonication can accommodate only a limited volume of parent dispersion. High-shear

methods25,26 employ rotor-stator shear mixers or kitchen blenders. They have been reported to

produce nanosheets similar in quality and size to those obtained from sonication, with high production

rates. Although the high-shear method is better than sonication in volume, it is inefficient. A huge high

shear mixer can compensate for the volume but leaves a large amount of material (away from the

rotor blade) unexfoliated. It also requires high power for operation. The attempts in this field are

directed towards increasing the exfoliation efficiency but have not been as successful as exploiting the

best out of the technique of liquid-phase exfoliation. Wang et al. exfoliated using sonication and

achieved the final nanosheets concentration of 0.2 mg/ml27. Using a solvothermal treatment with

formamide, Huang et al. found the concentration of the MoS2 nanosheets to be 0.21 mg/ml28. Liu et

al. achieved a 0.24 mg/ml by a high-pressure method, wherein they improved the exfoliation

efficiency of MoS2 nanosheets in isopropanol using inexpensive salts23. The high shear mixing

technique applied by Varrla et al. led to a better concentration of 0.4 mg/ml25. Recently, Janica et al.

exfoliated MoS2 nanosheets using lithium compound and achieved 0.5 mg/ml concentration12. Hai et

al. employed a surfactant-free method to increase MoS2 monolayers' yield and achieved a

concentration of 0.45 mg/ml in isopropanol29. Earlier, Lin et al. produced MoS2 nanosheets with a yield

of 52% in NaNO3/HCl solution, but the method utilized hazardous acids30.

In this paper, we explore an eco-friendly and safe high-pressure liquid-phase exfoliation approach to

overcome issues associated with large-scale and high-yield production of stable MoS2 nanosheets. A

large quantity of MoS2 (>1000 ml) was exfoliated with this method using deionized (DI) water as the

solvent, which has been deemed more complicated than exfoliation in NMP28 or DMF. This is due to

the difference in surface energy of DI water (72.2 mJ m-2) and MoS2 (46.5 mJ m-2)31, whereas the

surface energy of NMP is 40 mJ m-2 32. Molybdenum disulfide, being difficult to exfoliate with other

techniques, has been successfully exfoliated and characterized in this paper. We achieved 7.25%

exfoliation efficiency and 1.45 mg/ml nanosheets concentration, considerably higher than the

previous methods.

2. Experimental section

A. Liquid-phase exfoliation of MoS2

MoS2 exfoliation was done using a novel airless spray exfoliation technique. The details of materials

used for the exfoliation are provided in the Supporting Information (Section 1). This method consisted

of several steps: intercalation, expansion, exfoliation, and separation. In the initial phase, 1 mg/ml of

SDBS was added to water and stirred for 15 minutes at room temperature to get a transparent

solution. Then, 20 mg/ml of MoS2 bulk powder was taken in a large container (1.5 L), and water was

added to it, to which the prepared SDBS transparent solution was mixed. The solution was sonicated

for 30 minutes to get a homogeneous solution. The prepared solution was kept for soaking for a long

duration of 15 & 60 days at room temperature under dark conditions for the proper intercalation. This

process weakened the van der Waals force between the individual MoS2 layers. It is expected that the

expanded MoS2 flakes can then be easily exfoliated into nanosheets at low pressure/force.

Finally, the prepared intercalated MoS2 solution was passed through the nozzle of an airless paint

sprayer (AEROPRO Painter R450, Flow rate 2.2 L min-1) (Fig. 1 (a)) and collected in another container

(Fig. 1 (b)). The shear force created at the nozzle (Fig. 1 (a)) helped to separate individual MoS2

nanosheets. The collected solution was again passed through the nozzle, repeated several times, and

hereafter referred to as cycles. The final solution was collected and centrifuged at 5000 rotations per

minute (rpm) for 30 minutes to separate the smaller and larger flakes. The finer MoS2 nanosheets

were collected from the top two-thirds of the centrifuge tube. Surfactant removal is a crucial step and

is not much explained in the literature1. We tried to remove the surfactant to obtain pure nanosheets.

The nanosheets, after centrifugation, were filtered on PVDF (poly (vinylidene fluoride)) or AAO (anodic

aluminium oxide) substrate (pore size 0.1 µm) and washed first with acetone and then isopropanol.

Without letting them dry, the nanosheets were scooped from the surface of AAO using a spatula and

dispersed in isopropanol again for further characterizations. We found this method very efficient for

obtaining pure nanosheets devoid of surfactant.

B. Electrochemical Studies

The electrochemical analysis of MoS2 nanosheets was done using a three-electrode system connected

to Metrohm Autolab electrochemical workstation with NOVA software. Platinum (Pt) wire was used

as the counter electrode (CE), Ag/AgCl as the reference electrode (RE), and glassy carbon electrode

(GCE) as the working electrode (WE). The surface of GCE was polished with alumina powder and

cleaned with DI water several times before the preparation of the electrode. A homogeneous

dispersion of the MoS2 nanosheets (slurry) was prepared in Nafion (1:1 w/w) and isopropanol and

ultrasonicated for 30 minutes. 3 μl of this slurry was then drop-casted on GCE (diameter = 3 mm,

surface area = 0.071 cm2) to prepare the WE. 0.5 M sulphuric acid (H2SO4) served as the electrolyte.

Linear sweep voltammetry (LSV) was performed at a slow scan rate of 5 mV s-1 in the range of 0 V to

-0.8 V and used to determine the Tafel slopes. Cyclic voltammetry (CV) of MoS2 nanosheets was

performed at a narrow voltage window (from 0 V to -0.1 V) at different scan rates (20-100 mV s-1) to

obtain the double layer capacitance. To check the stability of the nanosheets, chronoamperometry (i-

t) studies were conducted for 24 hours under constant stirring. Prior to all the studies, the LSV/CV

response of the blank GCE (without the nanosheets coating) was recorded. Then, the GCE was coated

with nanosheets and inserted into the system. Nitrogen gas was purged for 20 minutes to create an

inert atmosphere and eliminate the dissolved oxygen. Gas chromatography was conducted to

investigate the gaseous products formed during the reaction. The system was sealed with parafilm

before the experiment. The gaseous products from the sealed system were collected in an air-tight

syringe (Hamilton, 2ml) and analyzed with a gas chromatography instrument (CIC Dhruva, Baroda). It

is equipped with a thermal conductivity detector (TCD) for identifying hydrogen, which was calibrated

using a standard gas mixture. The number of moles of hydrogen produced by 1 ml of collected gaseous

products was calculated from the area under the peak of the gas chromatogram.

C. Ion-transport Study

A 9 mm x 3 mm rectangular strip was cut from an MoS2 membrane prepared on PVDF (pore size 0.1

µm). It was encapsulated with the PDMS in such a way that the only transport path for ions was

through the planes of the membranes. This was then heated at 100 0C for 10 minutes in the convection

oven till the PDMS was set. It was ensured that out-of-plane transport was prohibited by compact

encapsulation. The thickness of the PDMS was ̴ 3 mm.

The voltage was varied from -200 mV to +200 mV using a Keithley 2614B source meter. The resulting

current was measured using Ag/AgCl electrodes through a LabVIEW program connected with the

source meter. Aqueous KCl solutions with concentrations from 10-5 M to 1 M were used as

electrolytes.

3. Results and Discussion

The dispersion of the nanosheets can be probed by a laser beam. The path of the laser beam is visible

through the MoS2 solution (Fig. 1 (c)), which indicates that the sample is a colloidal dispersion of MoS2

nanosheets; this phenomenon is called Tyndall effect. Different concentrations of MoS2 nanosheets

prepared by varying the exfoliation parameters are shown in Fig. 1 (d).

The Supporting Information (Section S2) describes the details of the techniques used to characterize

the exfoliated nanosheets. The atomic force microscopy (AFM) image in Fig. 2 (a) with the height

profile provides a flake thickness of 4-5 nm. Based on the interlayer spacing of 0.67 nm for MoS2, our

exfoliated sheets primarily consist of 6-7 layers. Transmission electron microscopy (TEM) analysis of

MoS2 was done to confirm its exfoliation and the size of nanosheets. Fig. S1 (a) shows the high-

resolution TEM (HR-TEM) image of exfoliated MoS2 flakes confirming that the high-pressure

exfoliation process is efficient in exfoliating bulk MoS2 to few layers. These nanosheets are a few

hundred nanometers in size. The fringe spacing of 0.27 nm is calculated from the HR-TEM image (Fig.

S1 (b)). As reported previously33, it corresponds to the (100) plane of MoS2, which is shown as the inset

of Fig. 2 (b). A Moiré pattern is clearly visible in our TEM images (Fig. 2 (b)), very similar to the

previously reported works31,32. These fringes arise in few-layered MoS231 or from the boundary

between two grains32. The appearance of the pattern is due to the rotation of the top flake by a slight

angle with respect to the bottom flake. It is further evidence of the thinness of our MoS2 nanosheets.

Fig. 1: Liquid-phase exfoliation of MoS2 using high-pressure spray technique. (a) The airless paint

sprayer and the side view of the spray gun with nozzle used for the high-pressure liquid phase

exfoliation. (b) A large amount of MoS2 solution collected after exfoliation and centrifugation and

stored in a glass bottle. (c) Tyndall effect in the MoS2 nanosheets shows that the path of the red laser

is visible through the colloidal suspension of nanosheets (water on the left side shows no such

property). (d) Different concentrations of MoS2 nanosheets prepared by varying exfoliation cycles and

applied pressure.

Raman spectroscopy is a non-destructive characterization technique that provides structural and

electronic information about materials. The Raman spectra of MoS2 nanosheets and bulk MoS2 are

shown in Fig. 2 (c). The two sets of peaks correspond to E2g

1 and A1g modes, indicating the in-plane

and out-of-plane vibrational modes of the MoS2 layer34. The peak position and the frequency

difference (the peak-to-peak separation) of these two modes are sensitive to the layer thickness

of MoS2. In bulk MoS2, the peak-to-peak separation of these modes is 25.2 cm-1. After exfoliation, A1g

and E2g

1modes shifted towards the higher wavenumber. The peak-to-peak separation is estimated to

be 24.6 cm-1 for MoS2 sheets that are exfoliated after 60 days of soaking. This indicates that four or

fewer layers are present23 in the exfoliated MoS2. Moreover, the shift of A1g mode is more remarkable

than that of the E2g

1 , which indicates that the number of layers has been reduced to a few layers of

MoS2 by weakening the van der Waal forces between the layers during the exfoliation process. These

results verified the successful exfoliation of the material via the novel high-pressure liquid-phase

exfoliation technique and the existence of nanosheets. Fig. S1 (c) shows that the Raman shift for the

15 days nanosheets is more than that for 60 days, suggesting that layer number reduces significantly

upon increasing the soaking duration.

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1High-yieldexfoliationofMoS2nanosheetsbyanovelspraytechniqueandtheimportanceofsoakingandsurfactantsSuvigyaKaushik1#,SivaSankarNemala1#,MukeshKumar2,DeveshNegi3,BiswabhusanDhal1,LalitaSaini1,RamuBanavath4,SurajitSaha3,SudhanshuSharma2&GopinadhanKalon1,5*1DisciplineofPhysics,IndianInstituteofTechnolog...

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1 High -yield exfoliation of MoS 2 nanosheets by a novel spray technique and the importance of soaking and surfactants

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