Aggregation Kinetics and Stability Mechanisms of Pristine and Oxidized Nanocarbons in Polar Solvents

Owing to their high propensity for bundling and aggregation, effective and stable dispersion of nanocarbons in polar solvents is of key significance in the preparation of carbon nanotube (CNT) and graphene nanosheet (GNS)based devices and nanocomposites. Previous studies have shown that oxidation of CNT side walls and GNS surfaces ameliorates their stability in polar solvents. In this study, largescale all-atom molecular dynamics simulations were employed to shed light on the stability mechanisms of nanocarbons in polar solvents and explicate the role of surface modification in their dispersibility enhancement. The concepts of potential of mean force (PMF) and translational kinetic energy (TKE) were utilized for this purpose. Our studies disclosed the physical facts lying behind the remarkably higher stability of modified nanocarbons in polar solvents compared to the pristine ones. First, the oxidized nanocarbons are intrinsically much less motivated to form aggregates, and second, the solvent-induced repulsion is much stronger in the case of oxidized nanocarbons. It was also revealed that among the various solvents considered here, Nmethyl-2-pyrrolidone (NMP) provides the most stable solutions for the both pristine and oxidized nanocarbons, followed by dimethyl sulfoxide (DMSO), dimethylformamide (DMF), 1,2-dichlorobenzene (ODCB), and tetrahydrofuran (THF). This work provides a comprehensive understanding of the nanocarbons stability that will facilitate the handling of their aggregation issue. ■ INTRODUCTION Due to their extraordinary electrical, optical, and chemical properties, such as high electron mobility, tunable direct band gap, and high surface-to-volume ratio, carbon nanotubes (CNTs) and graphene nanosheets (GNSs) are recognized as promising candidates for future electronic, optoelectronic, and sensing applications. Furthermore, the relatively high electrical and thermal conductivity and superior mechanical flexibility and strength render them favorable fillers for reinforcing polymers to obtain functional nanocomposites, such as highly stretchable, transparent conducting films and electromagnetic interference shielding layers. One major obstacle for realizing the potentials of carbon-based nanostructures is their great inclination to aggregate and form bundles, which arises from high hydrophobicity and the strong van der Waals attractions. Functionalizing CNTs and GNSs is the widespread solution to their dispersion issue in polar solvents, which mainly includes noncovalent dispersion using surfactants and polymers and covalent modification of the surface. The latter, which is to attach functional groups to the side walls and open ends of the nanocarbons, is known as an efficient method for dispersing nanocarbons. The most common and primary surface modification approach toward solubilizing CNTs and GNSs in polar solvents is oxidizing the nanocarbons through nitric acid treatment. This treatment leaves the nanocarbons with oxygen-containing functional groups such as carboxyl (−COOH) and hydroxyl (−OH) groups and results in highly stable nanocarbons in water and other polar solvents. Yet it leads to the degradation of CNTs and GNSs intrinsic qualities by introducing structural defects. Still, some studies suggested that by carefully choosing the treatment conditions one can obtain highly dispersible nanocarbons with acceptable characteristics. Recently, molecular dynamics (MD) simulations have been broadly utilized to study the dispersion and stabilization of CNTs and GNSs in aqueous environments. Most of these studies focused on the stabilization of nanocarbons using noncovalent techniques. One useful MD concept for studying the dispersion of nanostructures in different media is that of the potential of mean force (PMF). PMF profiles provide valuable insights into the nanocarbons resistance against exfoliation, tendency to bundle after separation, and solvent-induced interactions against their agglomeration. Choudhury et al. studied the stability of pristine graphene nanosheets in water by deriving the PMF curves. By utilizing a Received: May 26, 2016 Revised: June 30, 2016 Published: July 14, 2016 Article