Two-Step Exfoliation of WS(2) for NO(2), H(2) and Humidity Sensing Applications

WS(2) exfoliated by a combined ball milling and sonication technique to produce few-layer WS(2) is characterized and assembled as chemo-resistive NO(2), H(2) and humidity sensors. Microstructural analyses reveal flakes with average dimensions of 110 nm, “aspect ratio” of lateral dimension to the thickness of 27. Due to spontaneous oxidation of exfoliated WS(2) to amorphous WO(3), films have been pre-annealed at 180 °C to stabilize WO(3) content at ≈58%, as determined by X-ray Photoelectron Spectroscopy (XPS), Raman and grazing incidence X-ray Diffraction (XRD) techniques. Microstructural analysis repeated after one-year conditioning highlighted that amorphous WO(3) concentration is stable, attesting the validity of the pre-annealing procedure. WS(2) films were NO(2), H(2) and humidity tested at 150 °C operating Temperature (OT), exhibiting experimental detection limits of 200 ppb and 5 ppm to NO(2) and H(2) in dry air, respectively. Long-term stability of the electrical response recorded over one year of sustained conditions at 150 °C OT and different gases demonstrated good reproducibility of the electrical signal. The role played by WO(3) and WS(2) upon gas response has been addressed and a likely reaction gas-mechanism presented. Controlling the microstructure and surface oxidation of exfoliated Transition Metal Dichalcogenides (TMDs) represents a stepping-stone to assess the reproducibility and long-term response of TMDs monolayers in gas sensing applications.