Liquid Exfoliation of Layered Materials

In addition to graphene, a wide range of layered materials, including oxides, chalcogenides, and clays are of interest because of their optical, electrical, and mechanical properties. While many methods can be used to cleave layered sheets from the bulk material, they are difficult to scale up. Liquid exfoliation routes may hold the best promise for making materials in large quantities. Nicolosi et al. (p. 1226419 ) review progress in developing exfoliation routes, both aqueous and nonaqueous for a wide range of starting materials. Not all crystals form atomic bonds in three dimensions. Layered crystals, for instance, are those that form strong chemical bonds in-plane but display weak out-of-plane bonding. This allows them to be exfoliated into so-called nanosheets, which can be micrometers wide but less than a nanometer thick. Such exfoliation leads to materials with extraordinary values of crystal surface area, in excess of 1000 square meters per gram. This can result in dramatically enhanced surface activity, leading to important applications, such as electrodes in supercapacitors or batteries. Another result of exfoliation is quantum confinement of electrons in two dimensions, transforming the electron band structure to yield new types of electronic and magnetic materials. Exfoliated materials also have a range of applications in composites as molecularly thin barriers or as reinforcing or conductive fillers. Here, we review exfoliation—especially in the liquid phase—as a transformative process in material science, yielding new and exotic materials, which are radically different from their bulk, layered counterparts.