A review on liquid-phase exfoliation for scalable production of pure graphene, wrinkled, crumpled and functionalized graphene and challenges

Graphene, the 2D form of carbon-based material existing as a single layer of atoms arranged in a honeycomb lattice has set the science and technology sectors alight with interest in the last decade in view of its astounding electrical and thermal properties, combined with its mechanical stiffness, strength and elasticity. Mass production of graphene is prerequisite for its viability and wide applications. The liquid-phase exfoliation of graphite into graphene is one of the most promising ways to achieve large-scale production at an extremely low cost. Another related main challenge is to find an economical procedure for large scale production of functionalized single-and few-layered graphene sheets. This review focuses on discussing different liquid-phase exfoliation methods based on a common mechanical mechanism. It is our premise that a deep understanding of the exfoliation mechanism can provide fruitful information on how to efficiently achieve high-quality graphene by optimizing exfoliation techniques. We highlight the recent progress on liquid-phase exfoliation for graphene production during the last decade. The emphasis is set on the widely used direct ultrasonic exfoliation methods, stabilizer-based exfoliation procedures, the newly explored wet-ball milling methods, shear exfoliation approaches, functionalization-assisted exfoliation techniques, electrochemical exfoliation routes, and the innovative supercritical fluid approaches. Various synthetic approaches are categorized and the advantages and disadvantages of different liquid-phase exfoliation methods are described. In addition, the main concerns regarding the quality and structure of the graphene sheet produced from aforementioned methods and the importance of a comprehensive evaluation of the final bulk graphene materials will also be discussed. We hope this review will point towards a rational direction for the scalable production of graphene.