Synthesis and Characterization of High Concentrations of WS₂ Monolayers Using Liquid Phase Exfoliation Method

Nanosheets of WS₂, preferably monolayers, are promising materials for sensing, catalysis, and energy storage applications as they exhibit large surface areas and novel electronic properties. A reliable and scalable method of synthesizing the nanosheets on a large scale is desirable to realize the applications. The liquid phase exfoliation method is regarded as a viable method to synthesize a high yield of nanosheets from a layered material such as WS₂. In this study, we systematically investigate the optimization of the synthesis of WS₂ nanosheets by exfoliating the WS₂ powder in isopropyl alcohol, a low boiling point solvent, using a low-power probe sonicator. The atomic force microscopy characterization reveals that the exfoliated nanosheets comprise a large proportion of monolayers and bilayers. The concentrations of monolayers and bilayers can be elevated by more than 20% if the dispersions are centrifuged at higher speeds. A prolonged sonication increases the proportion of monolayers, such as 11% more monolayers can be fabricated with a five-fold increase in sonication time. As a proof-of-concept study, we investigated if the exfoliation of WS₂ powder into nanosheets can elevate photocatalytic activity by examining the photodegradation rate of methyl orange solution. We show that the nanosheets exhibit ~ 1.3 times higher photodegradation rate than the unexfoliated powder, indicating that nanosheets are more active than the bulk powder.