Large-scale exfoliation of inorganic layered compounds in aqueous surfactant solutions.

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. in layered crystals and can be metals, semiconductors or insulators. [1] In addition, some are thermoelectric materials, [2] superconductors [3] or topological insulators [4]. In order to unlock these valuable properties, these materials must be easily exfoliated to individual layers. Here we show a simple, large-scale method to exfoliate BN, transition metal dichalcogenides and transition metal oxides down to very thin platelets in water. The platelets are stabilised against re-aggregation by a surfactant coating. Liquid phase exfoliation allows the formation of films and the production of hybrids and composites. For example, hybrid nanotube-MoS 2 films show promise for thermoelectric and battery applications while NbSe 2 films show potential as transparent conductors. In addition to graphite, a wide range of inorganic layered compounds exist. A good example are the transition metal dichalcogenides (TMDs). [1] These form 2-dimensional layers of covalently bonded transition metal atoms, M, and chalcogen atoms, X (X=S, Se, Te), in the stoichiometry MX 2 , with examples such as MoS 2 , NbSe 2 and NiTe 2. [1] In addition, some transition metal oxides [5] form 2D structures as do other materials such as BN, Bi 2 Te 3 , Sb 2 Se 3 , FeTe etc. In general these 2D layers 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. 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, [14-17] this method is time consuming, extremely sensitive to environmental conditions and results in structural deformations in …