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Direct exfoliation of layered materials in low-boiling point solvents using weak acid salts

Arao, Yoshihiko, Tanks, Jonathon D., Kubouchi, Masatoshi, Ito, Akira, Hosoi, Atsushi, Kawada, Hiroyuki
Carbon 2019 v.142 pp. 261-268
boron nitride, coatings, crystals, dispersions, forces, graphene, isopropyl alcohol, liquids, molybdenum disulfide, nanocomposites, nanosheets, solvents, sonication, toxicity
The method for mass production of nanosheets is essential for fully realizing their commercial use. Direct exfoliation of layered materials in liquid is the simplest and low-cost technique for exfoliation without introducing defects in basal plane. However, the technique requires toxic and high-boiling point solvents, which makes it difficult for further processing. In addition, solvent exchange involves time-consuming processes such as filtering and redispersion. A breakthrough is necessary to make 2D-crystal-based functional dispersions. Here, we find a new method to directly exfoliate layered materials in low-boiling point solvents. Addition of small amounts of salt prevents reaggregation of exfoliated nanosheets and improves electrical repulsion. In this process, the mechanical force applied by sonication generates active carbon species at the fractured surfaces, leading to a reaction between the active carbon and the salt in liquid. High concentration graphene dispersions (1 g/L) can be obtained in isopropanol with only 5 min of sonication. We also confirm salt-assisted exfoliation is effective for the other 2D crystals such as MoS2 and boron nitride in a wide variety of polar solvents. Direct exfoliation in processable solution opens up a range of large-area applications such as high performance nanocomposites and coatings.