Thermal stability of WS2 flakes and gas sensing properties of WS2/WO3 composite to H2, NH3 and NO2

We report on the fabrication and on the morphological, structural, chemical and the electrical characterization of WS2 thin films sensors prepared by drop casting a commercial solution of dispersed few-layers WS2 flakes on Si3N4 interdigitated substrates and annealing the films in air at 150 °C, 250 °C and 350 °C. Thermal stability of WS2 in air at different annealing temperatures has been investigated by X-ray photoemission spectroscopy, scanning electron microscopy, X-ray diffraction and by simultaneous thermal analysis techniques. We found that WS2 is not stable in air and partially oxidizes to amorphous WO3 in the annealing temperature range 25 °C–150 °C. The oxidation of WS2 in air at 250 °C and 350 °C yields a composite crystalline WS2/WO3 hierarchical structure characterized by the presence of surface oxygen and sulphur vacancies. The contribution of each phase of the WS2/WO3 composite to the overall chemoresistive gas response utilizing H2 (1–10 ppm), NH3 (1–10 ppm) and NO2 (40 ppb–1 ppm) gases in dry air carrier is presented and discussed. WS2/WO3 composite films show excellent gas sensing properties to reducing (H2, NH3) as respect to oxidizing (NO2) gases at 150 °C operating temperature. In this work we found low detection limits of 1 ppm H2, 1 ppm NH3 and 100 ppm NO2 in dry air carrier, among the smallest so far ever reported for transition metal dichalcogenides. Furthermore, the sensor doesn't show any cross sensitivity effects to both H2 and NH3 when exposed to water vapor. Outstanding reproducibility responses, by exposing the 150 °C annealed film to dynamic and cumulative gas pulses where obtained utilizing H2 gas.