Phase-switchable preparation of solution-processable WS2 mono- or bilayers

Crystal phase plays a crucial role in determining the properties of two-dimensional (2D) transition metal dichalcogenides. Here we achieve phase-switchable preparation of 2D transition metal dichalcogenides using an electrochemical lithium-ion intercalation-based exfoliation strategy by controlling the discharge current density and cutoff voltage. We discover that a small discharge current density (0.005 A g −1 , with a 0.9 V cutoff voltage) produces pure semiconducting 2H phase WS 2 bilayers. In contrast, a large discharge current density (0.02 A g −1 , with a 0.7 V cutoff voltage) leads to the dominant semimetallic 1T′ phase WS 2 monolayers. The phase-switching mechanism was clarified through cryo-electron microscopy, annular dark-field scanning transmission electron microscopy, Raman, X-ray photoelectron spectroscopy, etc. The device (humidity sensor) application of produced 2D WS 2 was then demonstrated, showing phase-dependent humidity-sensing performances confirming the potential of our produced 2D WS 2 with switchable phase in device applications.