Preparation of Liquid-exfoliated Transition Metal Dichalcogenide Nanosheets with Controlled Size and Thickness: A State of the Art Protocol

Summary

A protocol for the liquid exfoliation of layered materials to nanosheets, their size selection and size measurement by microscopic and spectroscopic techniques is presented.

Abstract

We summarize recent advances in the production of liquid-exfoliated transition metal dichalcogenide (TMD) nanosheets with controlled size and thickness. Layered crystals of molybdenum disulphide (MoS₂) and tungsten disulphide (WS₂) are exfoliated in aqueous surfactant solution by sonication. This yields highly polydisperse mixtures containing nanosheets with broad size and thickness distributions. However, for most purposes, specific sizes (in terms of both lateral dimension and thickness) are required. For example, large and thin nanosheets are desired for (opto) electronic applications, while laterally small nanosheets are interesting for catalytic applications. Therefore, post-exfoliation size selection is an important step that we address here. We provide a detailed protocol on the efficient size selection in large quantities by liquid cascade centrifugation and the size and thickness quantification by statistical microscopic analysis (atomic force microscopy and transmission electron microscopy). The comparison of MoS₂ and WS₂ shows that both materials are size-selected in a similar way by the same procedure. Importantly, the dispersions of size-selected nanosheets show systematic changes in their optical extinction spectra with size due to edge and confinement effects. We show how these optical changes are related quantitatively to the nanosheets dimensions and describe how mean nanosheets length and layer number can be extracted reliably from the extinction spectra. The exfoliation and size selection protocol can be applied to a broad range of layered crystals as we have previously demonstrated for graphene, gallium sulphide (GaS) and black phosphorus.

Introduction

The possibility to produce and process graphene, related two-dimensional (2D) crystals in the liquid phase makes them promising materials for an ever growing range of applications as composite materials, sensors, in energy storage and conversion and flexible (opto) electronics.^1-6 To exploit 2D nanomaterials within applications such as these will require inexpensive and reliable functional inks with on-demand lateral size and thickness of the nanoscale constituents, as well as controlled rheological and morphological properties amenable to industrial-scale printing/coating processes.⁷ In this regard, liquid phase exfoliation has become an importa....

Protocol

1. Liquid Exfoliation — Preparation of Suitable Stock Dispersions

1. Mount a metal cup underneath a sonotrode in an ice bath.
2. Immerse 1.6 g of the TMD powder in 80 mL aqueous solution of sodium cholate (SC) surfactant (sodium cholate concentration, C_SC= 6 g L^-1) in the metal cup.
3. Move the sonic tip to the bottom of the metal cup and then up by ~ 1 cm. Wrap aluminum foil around the sonic probe to avoid spillage.
4. Sonicate the mixture under ice-cooling by probe sonication to avoid heating using a solid flathead tip (750 W processor) for 1 h at 60% amplitude (pulse of 6 s on and 2 s off).
5. Centrifuge th....

Results

Liquid cascade centrifugation (Figure 1) is a powerful technique to sort liquid-exfoliated nanosheets by size and thickness as illustrated in Figure 2 for both MoS₂ and WS₂. Nanosheet lateral sizes and thicknesses can be characterized by statistical TEM and AFM, respectively. A typical AFM image is shown in Figure 2A. The apparent nanosheet thickness is converted to layer number using step height analysis (Fi.......

Discussion

Sample preparation

The samples described here are produced by tip sonication. Alternative exfoliation procedures can be used, but will lead to different concentrations, lateral sizes and degrees of exfoliation. Higher amplitudes and longer on pulses during the sonication should be avoided to prevent damaging of the sonicator. Similar results were obtained using 500 W processors. However, sonication time and amplitude has an impact on the nanosheet exfoliation and variations from this protocol may.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Sodium cholate hydrate, from ox and/or sheep bile	Sigma Aldrich	C1254-100G	Surfactant used as stabilizer in the form of an aqueous solution (i.e., after dissolving the powder in millipore water)
MoS2 powder	Sigma Aldrich	69860-100G	Other distributors available, but exfoliation and outcome of size selection can vary
WS2, powder 2 μm	Sigma Aldrich	243639-50G	Other distributors available, but exfoliation and outcome of size selection can vary
ImageJ Software	Developer: National Insitutes of Health	64-bit Java version 2.45 1.6.0_24	Image processing software used for TEM analysis, free download
Gwyddion Software	Developer: Czech Metrology Institute	64-bit Java version 2.45	Image processing software used for AFM analysis, free download
Origin Pro Software	OriginLab	Version 2016	Software used for data analysis such as differntiation and fitting of the extinction spectra
Centrifuge	HettichLab	Mikro 220R	any other benchtop centrifuge is suitable
Rotor 1	Hettich	Rotor 1016	for centrifugation <5,000 x g
Rotor 2	Hettich	Rotor 1195-A	for centrifugation >5,000 x g