Scalable Exfoliation of Bulk MoS₂ to Single- and Few-Layers Using Toroidal Taylor Vortices.

Vishakha Kaushik,Hoyoung Jang,Kyunghoon Kim,Shunhe Wu,Je Kang,Ji Suk

doi:10.3390/nano8080587

Abstract

The production of a large amount of high-quality transition metal dichalcogenides is critical for their use in industrial applications. Here, we demonstrate the scalable exfoliation of bulk molybdenum disulfide (MoS2) powders into single- or few-layer nanosheets using the Taylor-Couette flow. The toroidal Taylor vortices generated in the Taylor-Couette flow provide efficient mixing and high shear stresses on the surfaces of materials, resulting in a more efficient exfoliation of the layered materials. The bulk MoS2 powders dispersed in N-methyl-2-pyrrolidone (NMP) were exfoliated with the Taylor-Couette flow by varying the process parameters, including the initial concentration of MoS2 in the NMP, rotation speed of the reactor, reaction time, and temperature. With a batch process at an optimal condition, half of the exfoliated MoS2 nanosheets were thinner than ~3 nm, corresponding to single to ~4 layers. The spectroscopic and microscopic analysis revealed that the exfoliated MoS2 nanosheets contained the same quality as the bulk powders without any contamination or modification. Furthermore, the continuous exfoliation of MoS2 was demonstrated by the Taylor-Couette flow reactor, which produced an exfoliated MoS2 solution with a concentration of ~0.102 mg/mL. This technique is a promising way for the scalable production of single- or few-layer MoS2 nanosheets without using hazardous intercalation materials.

Highlights

Two-dimensional (2D) nanomaterials such as graphene and transition metal dichalcogenides (TMDs) have captured widespread attention in recent years [1,2,3,4,5,6]
The bulk MoS2 powders could sink on the bottom of the reactor by gravity when they were exfoliated in a batch process
The liquid could push up the bulk MoS2 powders and might induce a better dispersion and more shear interactions during the process, resulting in more exfoliation

Summary

Introduction

Two-dimensional (2D) nanomaterials such as graphene and transition metal dichalcogenides (TMDs) have captured widespread attention in recent years [1,2,3,4,5,6]. Bulk molybdenum disulfide (MoS2) has an indirect band gap of 1.2 eV, while the monolayer MoS2 has a direct band gap of 1.8~1.9 eV [7]. Their layer-dependent and tunable band gap transition make the materials special for future electronics applications [9,10,11]. The lithium-based exfoliation has several issues such as a long intercalation time, flammability of lithium in air, and tedious purifying steps, which are undesirable for an environmentally friendly and low-cost production in the industry [14]. The lithium intercalation induces the phase transformation of MoS2 from semiconducting 2H-MoS2 to metallic 1T-MoS2, requiring additional treatments to recover its semiconducting structure [15]

Methods

Results

Conclusion