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

doi:10.6023/A20100468

摘要/Abstract

现有的实验方法很难实时观测到石墨烯在液相剥离溶剂中的结构演变, 尤其是石墨烯稳定的微观机理尚不明确. 本工作通过分子动力学方法, 模拟了多层石墨烯和U型石墨烯在不同的物质的量比下的甘油/尿素溶剂中的结构变化, 研究剥离液对石墨烯稳定性的影响. 结果表明, 多层石墨烯在不同溶剂体系中的稳定性差异不显著; 而U型石墨烯在各溶剂体系的稳定性有明显差异, 且稳定能力为: 纯甘油>甘油/尿素(2/1)>甘油/尿素(3/1)>甘油/尿素(1/1). 这说明石墨烯在剥离溶剂中的稳定性与石墨烯的剥离状态有关. 通过溶剂分布发现, 尿素能够进入石墨烯层间, 增加石墨烯层间距; 同时, 甘油能够与尿素形成氢键, 随尿素进入石墨烯层间, 进一步增大层间距, 从而形成稳定的单层或多层受限二元溶剂分子层. 受限溶剂分子层对剥离的石墨烯存在排斥作用, 从而为石墨烯在甘油/尿素二元剥离液中的长期稳定提供了保障.

关键词: 分子动力学, 稳定性, 石墨烯, 甘油, 尿素

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

引用此文

刘长安, 洪士博, 李蓓. 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[J]. 化学学报, 2021, 79(4): 530-538.

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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图/表 12

图1. (a) 多层石墨烯(初始层间距分为0.6 nm和1.2 nm)和(b) U型石墨烯原子模型

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

表1. 不同甘油/尿素物质的量比下的多层石墨烯/溶剂体系模拟参数

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

表2. 不同甘油/尿素物质的量比下的U型石墨烯/溶剂体系模拟参数

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

图2. 初始层间距分别为(I) 0.6 nm和(II) 1.2 nm时, 多层石墨烯在(a) 纯甘油, (b) 甘油/尿素(3/1), (c) 甘油/尿素(2/1)和(d) 甘油/尿素(1/1)溶剂体系中的NPT和NVT系综下的最终结构. 其中灰色表示石墨烯片层, 红色表示甘油分子, 蓝色表示尿素分子, 高亮加粗部分为层间受限溶剂分子

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

图3. 初始层间距为(a) 0.6 nm和(b) 1.2 nm时, 顶层石墨烯在NVT系综下的RMSD变化曲线

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

图4. 初始层间距为(a) 0.6 nm和(b) 1.2 nm时, 石墨烯层间距离变化曲线

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

图5. 初始层间距为(a) 0.6 nm和(b) 1.2 nm时, 顶层石墨烯在NVT系综下的侧移距离曲线

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

图6. 初始层间距离为0.6 nm时, (a) 甘油/尿素(3/1), (b) 甘油/尿素(2/1)和(c) 甘油/尿素(1/1)溶剂体系中的甘油(红线)、尿素(蓝线)和所有溶剂(黑线)的分子数目变化曲线. 初始层间距离为1.2 nm时, (d) 纯甘油, (e) 甘油/尿素(3/1), (f) 甘油/尿素(2/1)和(g) 甘油/尿素(1/1)溶剂体系中的甘油(红线)、尿素(蓝线)和所有溶剂(黑线)的分子数目变化曲线, 其中插图分别为NPT和NVT系综下最终层间溶剂分布

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

图7. U型石墨烯在NPT和NVT系综下的最终结构图: (a) 纯甘油, (b) 甘油/尿素(3/1), (c) 甘油/尿素(2/1)和(d) 甘油/尿素(1/1)溶剂体系

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

图8. U型石墨烯在NVT系综下的RMSD值变化曲线

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

图9. U型石墨烯在NVT系综下的贴合面积百分比变化曲线

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

图10. U型石墨烯在NVT系综下的侧移距离变化曲线

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

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