Two-Step Exfoliation of WS₂ for NO₂, H₂ and Humidity Sensing Applications

WS₂ exfoliated by a combined ball milling and sonication technique to produce few-layer WS₂ is characterized and assembled as chemo-resistive NO₂, H₂ 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 WS₂ to amorphous WO₃, films have been pre-annealed at 180 °C to stabilize WO₃ 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 WO₃ concentration is stable, attesting the validity of the pre-annealing procedure. WS₂ films were NO₂, H₂ and humidity tested at 150 °C operating Temperature (OT), exhibiting experimental detection limits of 200 ppb and 5 ppm to NO₂ and H₂ 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 WO₃ and WS₂ 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.