Molecular Dynamics Simulation of the Stability Behavior of Graphene in Glycerol/Urea Solvents in Liquid-Phase Exfoliation

doi:10.6023/A20100468

Abstract

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 1.12~1.18 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 π-π 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.

Key words: molecular dynamics, stability, graphene, glycerol, urea

Cite this article

Chang-An Liu, Shi-Bo Hong, Bei Li. Molecular Dynamics Simulation of the Stability Behavior of Graphene in Glycerol/Urea Solvents in Liquid-Phase Exfoliation[J]. Acta Chimica Sinica, 2021, 79(4): 530-538.

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Fig. & Tab. 12

Figure 1. Atomic models of the (a) pristine multilayer graphene with the initial interlayer spacing of 0.6 nm and 1.2 nm, and (b) U-type graphene

石墨烯尺寸/nm²	初始层间距/nm	溶剂	甘油分子数	尿素分子数	模拟盒子初始尺寸/nm³
2.944×2.996	0.6	纯甘油	1432	0	6×6×6
		甘油/尿素(3/1)	1251	417
		甘油/尿素(2/1)	1142	571
		甘油/尿素(1/1)	935	935
		纯甘油	1442	0
	1.2	甘油/尿素(3/1)	1257	419
		甘油/尿素(2/1)	1154	577
		甘油/尿素(1/1)	939	939

Table 1. Parameters for pristine multilayer graphene/solvent systems with different glycerol/urea concentrations

石墨烯尺寸/nm²	溶剂体系	甘油分子数	尿素分子数	模拟盒子初始尺寸/nm³
4.912×4.963	纯甘油	4227	0	9×9×9
	甘油/尿素(3/1)	3471	1157
	甘油/尿素(2/1)	3188	1594
	甘油/尿素(1/1)	2558	2558

Table 2. Parameters for U-type graphene/solvent systems with different glycerol/urea concentrations

Figure 2. The final structure of the pristine multilayer graphene in solvents of (a) pure glycerol, (b) glycerol/urea(3/1), (c) glycerol/urea(2/1) and (d) glycerol/urea(1/1) with the initial interlayer spacing of (I) 0.6 nm and (II) 1.2 nm using NPT and NVT ensemble, respectively. The graphene sheets are colored as gray, and the glycerol and urea molecules are red and blue. In particular, the solvent molecules between the top and second graphene sheets are highlighted

Figure 3. Evolution of the RMSD of the top graphene sheet under NVT with the initial interlayer spacing of (a) 0.6 nm and (b) 1.2 nm

Figure 4. Evolution of the interlayer distance between the top and second layers of the graphene with the initial spacing of (a) 0.6 nm and (b) 1.2 nm

Figure 5. Evolution of the lateral shift distance of the top graphene sheet under NVT with the initial interlayer spacing of (a) 0.6 nm and (b) 1.2 nm

Figure 6. Evolution of the number of glycerol (red line), urea (blue line) and all organic (black line) molecules in solvents of (a) glycerol/urea(3/1), (b) glycerol/urea(2/1) and (c) glycerol/urea(1/1) with the initial interlayer spacing of 0.6 nm, and those in (d) pure glycerol, (e) glycerol/urea(3/1), (f) glycerol/urea(2/1) and (g) glycerol/urea(1/1) with the initial interlayer spacing of 1.2 nm under NPT and NVT ensemble, respectively. The insets indicate the final distribution of the constrained solvent molecules

Figure 7. The final structure of U-type graphene sheets in solvents of (a) pure glycerol, (b) glycerol/urea(3/1), (c) glycerol/urea(2/1), and (d) glycerol/urea(1/1) under NPT and NVT ensemble, respectively

Figure 8. Evolution of the RMSD of the U-type graphene under NVT

Figure 9. Evolution of the percentage of the contact area of the U-type graphene sheet under NVT

Figure 10. The evolution of the lateral shift distance of the U-type graphene sheet under NVT

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石墨烯在甘油/尿素剥离液中的稳定行为的分子动力学模拟研究