Molecular simulation perspective of liquid-phase exfoliation, dispersion, and stabilization for graphene

Effects of size and shape of hole defects on mechanical properties of biphenylene: a molecular dynamics study

The distinctive multi-ring structure and remarkable electrical characteristics of biphenylene render it a material of considerable interest, notably for its prospective utilization as an anode material in lithium-ion batteries. However, understanding the mechanical traits of biphenylene is essential for its application, particularly due to the volumetric fluctuations resulting from lithium ion insertion and extraction during charging and discharging cycles. In this regard, this study investigates the performance of pristine biphenylene and materials embedded with various types of hole defects under uniaxial tension utilizing molecular dynamics simulations. Specifically, from the stress‒strain curves, we obtained key mechanical properties, including toughness, strength, Young's modulus and fracture strain. It was observed that various near-circular hole (including circular, square, hexagonal, and octagonal) defects result in remarkably similar properties. A more quantitative scaling analysis revealed that, in comparison with the exact shape of the defect, the area of the defect is more critical for determining the mechanical properties of biphenylene. Our finding might be beneficial to the defect engineering of two-dimensional materials.

Graphene Exfoliation in Binary NMP/Water Mixtures by Molecular Dynamics Simulations

We investigate the molecular mechanism underlying the liquid-phase exfoliation of graphene in aqueous/N-methyl-2-pyrrolidone (NMP) solvent mixtures and calculate the associated free energies, considering different NMP concentrations and exfoliation temperatures. We employ steered molecular dynamics to establish a path for the exfoliation of a graphene sheet from graphite within each solvent environment. Then, we conduct umbrella sampling simulations throughout the created paths to compute the potential of mean force (PMF) of the graphene sheet. As the exfoliated nanosheet disperses into the liquid, it becomes fully covered by an adsorbed solvent monolayer. We analyze the composition of the monolayer by measuring the direct contacts of either NMP or water molecules with the carbon surface. The carbon surface exhibits a preference for adsorbing NMP over water. The NMP molecules form a hydrophobic compact monolayer structure, effectively protecting the carbon interface from unfavorable interactions with water. The creation of the hydrophobic monolayer is a key factor in the exfoliation process, as it effectively inhibits the restacking of exfoliated nanosheets. An adequate level of graphene solubility is achieved through the addition of 20 % to 30 % water by weight to the NMP solvent. This finding holds significant importance for improving production efficiency and reducing dependence on organic solvents in the industrial manufacturing of graphene.

Solvent-Assisted Graphene Exfoliation from Graphite Using Umbrella Sampling Simulations

We employed molecular dynamics (MD) simulations coupled with umbrella sampling to explore the thermodynamics governing the exfoliation of a single graphene layer from a graphitic substrate in five different solvents such as dimethylacetamide (DMA), N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), cyclohexane (CHX), and water. The substrate was modeled as a stack of three identical graphene layers with the graphene sheet undergoing exfoliation positioned on top of this stack. The initial configurations for each umbrella simulation were generated through steered MD simulations carried out along two distinct coordinates: one parallel and the other perpendicular to the graphene layers. Our analyses revealed a uniform wetting behavior for both the nanosheet and the graphitic substrate in all of the tested solvents. Consistent with experimental observations, the steered simulations confirmed that exfoliation is more favorable along the parallel direction than along the perpendicular one. All non-water solvents exhibit comparable effectiveness in the exfoliation of graphene. The calculated free energies of these solvents in parallel exfoliation consistently fell within the range of 90-100 kJ/mol/nm2. In perpendicular exfoliation, however, the corresponding energies converge to lower values. This difference is attributed to the nonequilibrium nature of the perpendicular exfoliation, primarily caused by the great steering velocity of the graphene sheet immediately after detachment from the substrate. This rapid motion of the nanosheet along the perpendicular coordinate results in an elevated system energy and heating.

Adsorption Affinities of Small Volatile Organic Molecules on Graphene Surfaces for Novel Nanofiller Design: A DFT Study

The adsorption of organic molecules on graphene surfaces is a crucial process in many different research areas. Nano-sized carbon allotropes, such as graphene and carbon nanotubes, have shown promise as fillers due to their exceptional properties, including their large surface area, thermal and electrical conductivity, and potential for weight reduction. Surface modification methods, such as the "pyrrole methodology", have been explored to tailor the properties of carbon allotropes. In this theoretical work, an ab initio study based on Density Functional Theory is performed to investigate the adsorption process of small volatile organic molecules (such as pyrrole derivatives) on graphene surface. The effects of substituents, and different molecular species are examined to determine the influence of the aromatic ring or the substituent of pyrrole's aromatic ring on the adsorption energy. The number of atoms and presence of π electrons significantly influence the corresponding adsorption energy. Interestingly, pyrroles and cyclopentadienes are 10 kJ mol-1 more stable than the corresponding unsaturated ones. Pyrrole oxidized derivatives display more favorable supramolecular interactions with graphene surface. Intermolecular interactions affect the first step of the adsorption process and are important to better understand possible surface modifications for carbon allotropes and to design novel nanofillers in polymer composites.

Aggregation behavior of partially contacted graphene sheets in six-carbon alkanes: all-atom molecular dynamics simulation

Molecular dynamics simulations are used to investigate the aggregation of the cross-contacted and non-cross-contacted graphene sheets in n-hexane, 2,3-dimethylbutane, and cyclohexane solvents. The results show that the main driving force of the graphene aggregation is the interaction between the graphene sheets, and the interaction between solvent molecules also contributes to the aggregation slightly. The initial graphene configurations and the solvent molecule structures both have effects on the graphene aggregation speed. Specifically, the cross-contacted graphene sheets aggregate faster than the non-cross-contacted configuration, since the interaction between the graphene sheets is larger and the direction of this interaction is conducive to pushing away the solvent molecules adsorbed on the graphene surface. The graphene aggregation speed is larger in n-hexane mainly since the mobility of the solvent molecules is higher than the other two solvents, while the interaction between graphenes/solvents has little influence for the systems used in this work. This work provides useful insights into the graphene aggregation in the solvents with different initial graphene configurations and solvent molecule structures.

Adsorption of Organic Molecules and Surfactants on Graphene: A Coarse-Grained Study

The research studies on the adsorption of surfactants on graphene help us to know how to use surfactants to exfoliate graphene from graphite or functionalize the graphene surface. Among them, molecular dynamics (MD) simulation has been widely used to investigate the adsorption of organic molecules and surfactants on graphene. In particular, coarse-grained (CG) MD simulation greatly improves the computational efficiency by simplifying the complexity of the studied systems, allowing us to explore the structure and dynamics of complex systems on larger spatial scales and longer time scales. However, an accurate prediction of the adsorption of surfactants on graphene is required by optimizing the interaction between surfactants and graphene, which is often overlooked by some CG models. In this work, we found that an accurate prediction of the adsorption enthalpies of organic molecules on graphene can be achieved by optimizing the interactions between organic molecules and benzene. Meanwhile, we simulated the adsorption of a surfactant on single-layer and double-layer graphene nanosheets, respectively. Our results revealed that increasing the temperature would favor the interactions between hydrophilic groups of surfactants. In addition, we discovered that the surfactant prefers to be adsorbed on the inner surfaces of double-layer graphene compared with the outer surfaces, and this is owing to the dehydration in the middle of double-layer graphene, which is beneficial to the hydrophilic interactions between surfactant molecules inside the double-layer graphene.

Disentangling the liquid phase exfoliation of two-dimensional materials: an "in silico" perspective

Liquid Phase Exfoliation (LPE) is one of the most successful synthetic roots for the preparation of two-dimensional (2D) materials from their bulk counterparts. In recent years, significant progress has been accomplished for the development and modification of LPE techniques. However, precise identification of the hierarchical steps of the molecular mechanism of LPE remains to some extent elusive. Additionally, the a priori choice of suitable solvents for successful exfoliation and dispersion of various layered materials poses a challenge for both academia and industry. Computational methods, particularly Molecular Dynamics (MD) simulations with classical force-fields have contributed a great deal towards the understanding of the underlying mechanism of LPE, providing remarkable insights into the molecular-level details of the solvent-material interactions at the nanoscale and predicting "good" and "bad" solvents for exfoliation as well as stabilization of the dispersed state. With an intention to build up a unified understanding, in this perspective article, we summarize the recent advancements of molecular simulation techniques employed to decipher the mechanism of LPE, pointing out the key features of molecular interactions and identifying several thermodynamic parameters governing the phenomena. In addition, we outline the necessary characteristics of solvent molecules, essential for their use as "good" solvents towards LPE. Also, we highlight the limitations of simulation methods for the modelling of LPE. We believe that this article will be beneficial for the selection of solvents for the synthesis of novel 2D materials via LPE and will also provide a comprehensive view to computational material scientists towards the development of novel simulation protocols for investigating and analysing such complex molecular events.

Exfoliation of Two-Dimensional Materials: The Role of Entropy

Liquid-phase exfoliation (LPE) is the best-known method for the synthesis of two-dimensional (2D) nanosheets. Compared to enthalpy, entropy is hardly considered to be a factor in choosing energy-efficient solvents and has not even been verified to be negligible. In this Letter, we explore the entropy contribution in LPE by performing molecular dynamics (MD) simulation of the structural flexibility effect in graphene, hexagonal boron nitride (hBN), and molybdenum disulfide (MoS₂). Our results show that surface vibration favors the exfoliation of graphene and hBN and destabilizes the reaggregation of nanosheets in water at 300 K, whereas the opposite is found for MoS₂. The entropy change is found to be 41%, 48%, and 4% of the enthalpy gain for graphene, hBN, and MoS₂ in LPE, respectively, and 64%, 32%, and 56% in reaggregation, which amounts to a step advancement for solvent screening in LPE of 2D materials.

Interfacial complexation driven three-dimensional assembly of cationic phosphorus dendrimers and graphene oxide sheets

High content nitrogen, sulfur and phosphorus heteroatoms assembled in tree-like dendrimers (DG _n ) are confined within the galleries of two-dimensional graphene oxide (GO). The presence of the ternary diethyl-N-ethyl-ammonium groups on the dendrimer peripheries ensures the exfoliation of graphene sheets thereby affording interfacially bridged, three-dimensional heteroatom-enriched graphene-based hybrid nanostructures (DG _n -GO). Dendrimer generation (from 1 to 4) that reflects the bulkiness of these conceived nano-trees impacts increasingly the degree of dispersion-exfoliation and sheet desordering. The long-term stability of these aqueous suspensions associated with their handling flexibility allows uniform accommodation of the resulting hybrid materials as flame-retardants in bioplastics.

Molecular Dynamics Investigation of Graphene Nanoplate Diffusion Behavior in Poly-α-Olefin Lubricating Oil

Graphene as a type of novel additive significantly enhanced the tribological performance of blended lubricating oil. However, the dispersibility of graphene with long-term stability in lubricating oil is still a challenge. Chemical modification for graphene, rather than using surfactants, provided a better method to improve the dispersibility of graphene in lubricants. In this study, the equilibrium molecular dynamics (EMD) simulations were carried out to investigate the diffusion behavior of graphene nanoplates in poly-α-olefin (PAO) lubricating oil. The effects of graphene-size, edge-functionalization, temperature, and pressure on the diffusion coefficient were studied. In order to understand the influence of edge-functionalization, three different functional groups were grafted to the edge of graphene nanoplates: COOH, COON(CH3)2, CONH(CH2)8CH3 (termed GO, MG, and AG, respectively). The EMD simulations results demonstrated that the relationships between diffusion coefficient and graphene-size and number of functional groups were linear while the temperature and pressure had a nonlinear influence on the diffusion coefficient. It was found that the larger dimension and more functional groups provided the lower diffusion coefficient. AG with eight CONH(CH2)8CH3 groups exhibited the lowest diffusion coefficient. Furthermore, the experimental results and radial distribution function for graphene-PAO illustrated that the diffusion coefficient reflected the dispersibility of nanoparticles in nanofluids to some degree. To our best knowledge, this study is the first time the diffusion behavior of graphene in PAO lubricating oil was investigated using EMD simulations.

Scalable exfoliation and dispersion of two-dimensional materials - an update

The preparation of dispersions of single- and few-sheet 2D materials in various solvents, as well as the characterization methods applied to such dispersions, is critically reviewed. Motivating factors for producing single- and few-sheet dispersions of 2D materials in liquids are briefly discussed. Many practical applications are expected for such materials that do not require high purity formulations and tight control of donor and acceptor concentrations, as required in conventional Fab processing of semiconductor chips. Approaches and challenges encountered in exfoliating 2D materials in liquids are reviewed. Ultrasonication, mechanical shearing, and electrochemical processing approaches are discussed, and their respective limitations and promising features are critiqued. Supercritical and more conventional liquid and solvent processing are then discussed in detail. The effects of various types of stabilizers, including surfactants and other amphiphiles, as well as polymers, including homopolymeric electrolytes, nonionic polymers, and nanolatexes, are discussed. Consideration of apparent successes of stabilizer-free dispersions indicates that extensive exfoliation in the absence of dispersing aids results from processing-induced surface modifications that promote stabilization of 2D material/solvent interactions. Also apparent paradoxes in "pristineness" and optical extinctions in dispersions suggest that there is much we do not yet quantitatively understand about the surface chemistry of these materials. Another paradox, emanating from modeling dilute solvent-only exfoliation by sonication using polar components of solubility parameters and surface tension for pristine graphene with no polar structural component, is addressed. This apparent paradox appears to be resolved by realizing that the reactivity of graphene to addition reactions of solvent radicals produced by sonolysis is accompanied by unintended polar surface modifications that promote attractive interactions with solvent. This hypothesis serves to define important theoretical and experimental studies that are needed. We conclude that the greatest promise for high volume and high concentration processing lies in applying methods that have not yet been extensively reported, particularly wet comminution processing using small grinding media of various types.

Surfactant Assemblies on Selected Nanostructured Surfaces: Evidence, Driving Forces, and Applications

Surfactant adsorption at solid-liquid interfaces is critical for a number of applications of vast industrial interest and can also be used to seed surface-modification processes. Many of the surfaces of interest are nanostructured, as they might present surface roughness at the molecular scale, chemical heterogeneity, as well as a combination of both surface roughness and chemical heterogeneity. These effects provide lateral confinement on the surfactant aggregates. It is of interest to quantify how much surfactant adsorbs on such nanostructured surfaces and how the surfactant aggregates vary as the degree of lateral confinement changes. This review focuses on experimental evidence on selected substrates, including gold- and carbon-based substrates, suggesting that lateral confinement can have pronounced effects both on the amount adsorbed and on the morphology of the aggregates as well as on a systematic study, via diverse simulation approaches, on the effect of lateral confinement on the structure of the surfactant aggregates. Atomistic and coarse-grained simulations conducted for surfactants on graphene sheets and carbon nanotubes are reviewed, as well as coarse-grained simulations for surfactant adsorption on nanostructured surfaces. Finally, we suggest a few possible extensions of these studies that could positively impact a few practical applications. In particular, the simultaneous effect of lateral confinement and of the coadsorption of molecular compounds within the surface aggregates is expected to yield interesting fundamental results with long-lasting consequences in applications ranging from drug delivery to the design of advanced materials.