Why only solubility parameters? Liquid-phase exfoliation from the point of view of colloidal chemistry

Abstract

Liquid-phase exfoliation has been widely used to obtain 2D nanomaterials from layered or non-layered solids. Most reports have focused on exfoliation procedures using aqueous solutions of passivating agents, functionalized materials in water, or pristine materials in 'good' organic solvents. Herein, we highlight the third method applied to carbon nanomaterials because it results in stable and concentrated systems without any other additives, and the deposited materials form conductive patterns after solvent drying without the need for post-treatments like (electro)chemical reduction or removal of passivating agents. Good solvents for exfoliating pristine materials have been ascribed as having the 'correct' match of Hansen solubility parameters. Although this approach successfully enhances the yield in terms of mass concentration, it does not explain the high stability of dispersions prepared in liquids ascribed as 'bad' solvents or some properties observed for exfoliated materials in both 'good' and 'bad' ones, like sedimentation. In this review, we have reinterpreted organic dispersions of pristine carbon materials using concepts of Colloidal Chemistry, which can explain many poorly understood properties and guide new improvements in this area.