Large‐Scale Exfoliation of Inorganic Layered Compounds in Aqueous Surfactant Solutions

Abstract

We have developed methods to exfoliate MoS 2 in large quantities in surfactant-water solutions. This method can be extended to a range of other layered compounds. The layered material tends to be exfoliated as relatively defect free flakes with lateral sizes of 100s of nm. 2With high surface area and novel properties, two-dimensional (2D) materials are potentially useful for a range of applications. In addition to graphene, many 2D compounds exist with BN, MoS 2 and Bi 2 Te 3 generating renewed interest. Such materials are found stacked in layered crystals and can be metals, semiconductors or insulators.[ tend to bond via van der Waals interactions, stacking to form 3D crystals. These materials span the whole gamut of electronic structures from insulator to metal [1] and display interesting properties [6] such as superconductivity, [3] thermoelectricity [2] and topological insulator effects.[4]While micro-mechanically exfoliated [7] single flakes of materials such as MoS 2 are ideal for electronic devices, [8] large scale liquid-phase exfoliation methods will lead to a range of thin film applications such as nano-scale hybrids for use in thermoelectrics, [9] supercapacitors [10] or Li-ion batteries [11] . One advantage of such applications is that, as the electronic properties of TMDs vary relatively slowly with layer number, [12,13] full exfoliation to monolayers is not necessary; dispersed few-layer flakes are sufficient.While a number of layered compounds can be exfoliated by ion intercalation, [14][15][16][17] this method is time consuming, extremely sensitive to environmental conditions and results in structural deformations in some TMDs.[18] Furthermore, removal of the ions results in re-aggregation of the layers.[19] More promisingly, it has recently been shown that both TMDs [20] and BN can be exfoliated in organic solvents. [21][22][23][24] However, for large-scale applications, exfoliation in an aqueous environment would be hugely advantageous. While BN can be dispersed in water due to sonication-assisted hydrolysis, this method cannot be extended to other layered compounds.[25] The discovery of a facile, scalable method to exfoliate a range of layered materials in water would assist the production and 3 characterisation of a range of new materials and greatly facilitate the potential transfer of such technology to industry. In this work we show that a number of layered crystals can be exfoliated in water, resulting in thin flakes stabilised by a surfactant coating. This method is robust, can be carried out in ambient conditions, is scalable and allows the preparation of films, hybrids and composites.One possible reason why ion intercalation has been prevalent for TMDs rather than other liquid based dispersion methods is the relatively high exfoliation (surface) energy of TMDs. Computational studies have estimated this as greater than 250 mJ/m 2 for both MoS 2 and WS 2 ; [26,27] many times greater than that of graphene [28] or BN [29] . We suggest that sonication can be used to exfoliate TMDs in water,...

Solvent Exfoliation of Transition Metal Dichalcogenides: Dispersibility of Exfoliated Nanosheets Varies Only Weakly between Compounds

“…For each material, the supplier and powder particle sizes were; WS 2 background. 19,23 Such variations in background make it difficult to define an absorption coefficient which is intrinsic to each material.…”

“…18,19 As with graphene, many applications will require relatively large quantities of material suggesting that a solution processing route is required. 20 A number of possibilities exist.…”

Electronics and optoelectronics of two-dimensional transition metal dichalcogenides

“…They permit additional applications such as composites and hybrids by simple mixing of dispersions of different materials 43,44 , and thin films and coatings by filtration, inkjet printing, spray coating and doctor blading. Because solution-based graphene has been used to make highfrequency flexible electronics with a current gain cut-off frequency of 2.2 GHz (ref.…”

“…Alternatively, TMDCs can be exfoliated by ultrasonication in appropriate liquids, including organic solvents, aqueous surfactant solutions, or solutions of polymers in solvents 43,44,58,59 . Typically, ultrasonication results in the mechanical exfoliation of layered crystals to give flakes that are a few hundred nanometres in size.…”

Universal ac conduction in large area atomic layers of CVD-grown MoS2

“…Such observations, mostly on mechanically exfoliated layers of MoS 2 from single crystals, provided enough impetus to explore innovative methods for large-scale synthesis of atomically thin MoS 2 layers [3,[11][12][13][14][15]. Among these, liquid phase exfoliation [3,11], laser thinning [12], as well as the chemical vapor deposition (CVD) method [13][14][15] are now being utilized to synthesize large-scale area MoS 2 layers.…”