Fabrication of two-dimensional nanosheets via water freezing expansion exfoliation.

Layered materials, if exfoliated effectively, will exhibit several unique properties, offering great potential for diverse applications. To this end, in this study, we develop a novel, universal, and environmentally friendly method named as 'water freezing expansion exfoliation' for producing two-dimensional nanosheets. This method exploits the expansion in the volume of water upon freezing. When the water freezing expansion condition is reproduced in layered materials, the layers exfoliate to overcome the van der Waals force between them. The expansion process is performed by repeated cycling between 4 °C and -20 °C to effectively exfoliate layered materials of graphite, hexagonal boron nitride (h-BN), MoS2 and WS2. Systematic characterization of the samples thus obtained using electron microscopy and optical studies substantiate the formation of thin flakes (graphene, h-BN, MoS2, and WS2 nanosheets). The method demonstrated in this study is cost-effective and does not demand sophisticated equipment and stringent high temperature conditions. Given this general applicability, this method holds great promise for exfoliating layered materials that are sensitive to elevated temperature.