Mass production of low-boiling point solvent- and water-soluble graphene by simple salt-assisted ball milling

Abstract

Developing a mass production method for graphene is essential for practical usage of this remarkable material. Direct exfoliation of graphite in a liquid is a promising approach for production of high quality graphene. However, this technique has three huge obstacles to be solved; limitation of solvent, low yield and low quality (i.e., multilayer graphene with a small size). Here, we found that soluble graphite produced by mechanochemical reaction with salts overcomes the above three drawbacks. Soluble graphite was exfoliated into monolayer graphene with more than 10% yield in five minutes of sonication.The modified graphite was easily exfoliated in a low-boiling point solvent such as acetone, alcohol and water without the aid of a surfactant. Molecular simulation revealed that the salt is adsorbed to the active carbon at the graphite edge. In the case of weak acid salts, the original bonding nature between the alkali ion and the base molecule is retained after the reaction. Thus, alkali metals are easily dissociated in a polar solvent, leading to negative charge of graphene, enabling the exfoliation of graphite in low boiling point solvents. The approach proposed here opens up a new door to practical usage of the attractive 2D material.

Exfoliation Behavior of Large Anionic Graphite Flakes in Liquid Produced by Salt-Assisted Ball Milling

“…However, small graphene was ultimately produced from large anionic graphite due to the edge fragmentation mechanism described (Figure 12). In our previous research, we ball-milled graphite for 20 min and the average size of graphene was 366 nm, which is two times higher than this study [33]. We also found that the adsorbed salt increased as the milling time increased, suggesting that a higher amount of adsorbed salts and a smaller graphite size possibly hinders edge fragmentation.…”

“…Graphene concentration in acetone increased approximately 100 times when we used anionic graphite instead of natural graphite, and with an increase of at least 40 times for the other solvents as well. Concentrated graphene dispersions of anionic graphene can be obtained due to the enhanced negative charging caused by adsorbed salts [33]. The high electrical repulsion between anionic graphene sheets restricts the typical van-der-Waals-induced restacking observed in polar solvents.…”

Mechanochemistry: New Tools to Navigate the Uncharted Territory of “Impossible” Reactions

“…Because of this, milling of graphite is performed in the presence of agents that can prevent such restacking. A wide range of different agents can be employed; for example, graphite can be milled in the presence of sugars, [78] organic solvents, [79] salts, [80] small aromatic molecules [81] dry‐ice, [82] or melamine or other triazine derivatives [83] . Another option is to mill graphite in water in the presence of surface‐active agents such as proteins, [84] resulting in aqueous graphene dispersions.…”

Recent trends in covalent functionalization of 2D materials

“…Moreover, a ball mill with some additives such as acid, base, or salt, can accelerate exfoliation, breakage, and hydroxyl functionalization of 2D materials, even for chemically stable materials such as hBN [464][465][466][467][468][469][470] and graphene. 471…”