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

Developing a mass production method for graphene is essential for practical usage of this remarkable material. Direct exfoliation of graphite in liquid is the 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 quaity (i.e., multilayer graphene with small size). Here, we found that soluble graphite produced by mechanochemical reaction with salts overcomes the above three drawbacks. The soluble graphite was exfoliated into monolayer graphene with more than 10 % yield in five minutes of sonication. The modified graphite was easily exfoliated in low-boiling point solvent such as acetone, alcohol and water without the aid of 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 atom ion and the base molecule is kept after the reaction. Thus, the alkali metals are easily dissociated in polar solvent, leading to negative charging 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.