An effective liquid-phase exfoliation approach to fabricate tungsten disulfide into ultrathin two-dimensional semiconducting nanosheets

Tungsten sulfide is a promising, though less explored layered transition metal dichalcogenides material which can be exfoliated into mono and fewlayers with enhanced electronic, optoelectronics, sensing and catalytic properties. Liquid-phase exfoliation in high-boiling-point solvents (i.e., N-methyl-2-pyrrolidone), surfactant-assisted exfoliation and polymer-associated exfoliation are some of the widely used techniques for mass exfoliation of layered materials. Due to the high boiling point of these well-established solvents, surfactants and co-solvents, it is difficult to get pristine nanosheets, which are of highest importance in applications like gas sensing. The common solvents like acetone and water have been employed in this work along with low power ultrasonication waves to exfoliate bulk WS₂ which makes this process low cost and convenient. The effect of freezing on exfoliation has been effectively utilized to tailor Hansen solubility parameters of solvents. Monolayers and few layers with lateral dimension of few hundred nanometers have been synthesized with yield of 2.7% and characterized extensively using electron microscopy, atomic force microscopy and optical spectroscopies. The nanosheets have been found to be stable with no precipitation formation for months. This method uses low-boiling-point solvents, hence guaranties the pristine two-dimensional surfaces.