From graphite to graphene: direct liquid-phase exfoliation of graphite to produce single- and few-layered pristine graphene

Graphene, a two-dimensional crystalline form of carbon, has attracted wide and intense interest owing to its excellent physical properties and because its surface and edges can be chemically modified readily. Development of a method for producing high-quality graphene in large quantities is essential for further investigation of its properties and applications. The direct liquid-phase exfoliation of graphite to produce graphene is a convenient method for generating ideal graphene samples in large quantities. This direct method, which involves the use of colloidal suspensions, is based on the one-step physical transformation of graphite into graphene and has many unique advantages. A large number of liquids have been employed as exfoliation media and show a range of exfoliation efficiencies. In this review, we highlight the recent progress made on the exfoliation of bulky graphite powders or flakes into single- and few-layered graphene sheets in various liquids, including organic solvents, ionic liquids, and water/surfactant solutions. The qualities and yields of the exfoliated graphene samples, as well as their use in various applications, are also reviewed. Furthermore, future research directions for the development of novel exfoliation media and more efficient techniques for producing well-exfoliated pristine graphene are proposed.