Mechanisms of Liquid-Phase Exfoliation for the Production of Graphene

Abstract

Liquid phase exfoliation (LPE) is the principal method of producing two-dimensional (2D) materials such as graphene in large quantities with a good balance between quality and cost, and is now widely adopted by both the academic and industrial sectors. The fragmentation and exfoliation mechanisms involved have usually been simply attributed to the force induced by ultrasound and the interaction with the solvent molecules. Nonetheless, little is known about how they actually occur, i.e. how a thick and large graphite crystals can be exfoliated into thin and small graphene flakes. Here we demonstrate that during ultrasonic LPE, the transition from graphite flakes to graphene takes place in three distinct stages. Firstly, sonication leads to the rupture of large flakes and the formation of kink band striations on the flake surfaces, primarily along zig-zag directions. Secondly, cracks form along these striations, and together with intercalation of solvent, lead to the unzipping and peeling off of thin graphite strips that in the final stage are exfoliated into graphene. The findings will be of great value in the quest to

Graphene-Based Environmental Sensors: Electrical and Optical Devices

“…To avoid the oxidation and reduction steps, which can cause permanent structural damage to the sample, a different approach in the technique called the liquid phase exfoliation (LPE) has been under exploration. In the LPE approach bulk graphite is directly sonicated in solvents (e.g., N-methyl-2-pyrroidone [30] or dimethylformamide [31]) that have sufficient surface tension for stabilizing the graphene flakes once they are delaminated from the bulk [30,32]. While this procedure is closer to mass production than ME for its scalability, the lateral size distribution is limited to a few micrometers and the number of layers achieved varies from monolayer to multilayers.…”

Synthesis of graphene: Potential carbon precursors and approaches

“…Although this process produces good quality graphene with lower costs than the time-consuming processes such as the Scotch tape method, very little is understood about the fragmentation process. Only recently, the statistical models [76] have begun unveiling the mechanisms behind this synthesis, suggesting that erosion functions in tandem with peeling. More studies need to be performed to determine the stages by which this synthesis works to identify the sources of defects or surface roughness and whether it is feasible to functionalize the produced graphene.…”

“…The graphene produced through this technique has minimal defects and can be used in electronics, energy storage devices, and nanocomposites. The electrochemical exfoliation has lower mass production costs [76] and does not require the use of harsh chemicals, and electrochemical activation negates the need for additional purification steps. This promising process uses a "one-pot approach," and the level of oxidation of the resulting graphene is tuneable.…”

Impact of Pretreatment of the Bulk Starting Material on the Efficiency of Liquid Phase Exfoliation of WS2

“…In spite of the widespread use of LPE, the detailed mechanism of the process is topic of ongoing research [ 32 , 33 , 34 , 35 ] and not yet fully understood. For example, the role of defects in sonication-assisted LPE is still in debate.…”