▎ 摘　　要

Understanding of interfacial structure and stabilization mechanism of graphene sheets in green solvents during liquid-phase exfoliation is of great importance in advancing preparation, characterization and synthesis of graphene-based materials. However, it is difficult to monitor structural evolution of graphene in solvents using current available experimental techniques, and the resulting graphene stabilization mechanism has not been fully understood. In this work, molecular dynamics simulations are performed to investigate the structural evolution and stability behavior of the graphene sheets with different states in the glycerol/urea green solvents with varying concentrations. The results show that only the pristine graphene sheet at an initial interlayer spacing of 0.6 nm in the glycerol solvent restacks back and stays close to each other at a separation close to the intrinsic thickness of graphene. This is due to the fact that the glycerol molecules fail to diffuse into the graphene interlayer, and they could not afford a sufficient repulsive barrier to hinder the graphene aggregation. While in other pristine cases, a single- or double-layer solvent structure is formed and the interlayer separation is maintained at 0.65 nm or 1.12 similar to 1.18 nm, offering an efficient dispersion medium to stabilize the graphene sheets. Although the pristine multilayer graphene sheets present similar stability in different glycerol/urea solvents, the U- type graphene experiences distinct levels of stabilization in solvents in the order of pure glycerol>glycerol/urea(2/1)>glycerol/urea(3/1)>glycerol/urea(1/1), signifying that the shifted and exfoliated state of the graphene sheets plays an important role in the stabilization during liquid-phase exfoliation. Moreover, in the glycerol/urea binary solvents, the small urea molecules firstly diffuse into the graphene interlayer due to their strong pi-pi interaction with graphene, acting as a "dispersion initiator". And then the glycerol molecules could have the chance to diffuse around or insert into the graphene interlayer to assist in stabilizing the graphene due to the hydrogen bonding between urea and glycerol. In this way, the glycerol helps to further increase the interlayer separation leading to a more stable dispersion, acting as a "dispersion co-stabilizer". The formation of the confined urea and glycerol solvents thus provides a stable molecular layer structure between the graphene interlayer, enabling to stabilize the exfoliated graphene sheets effectively. As such, the findings in this work are believed to provide the atomic/molecular scale understanding of the stability behavior of the graphene sheets in glycerol/urea binary solvents during liquid-phase exfoliation.