Polycrystalline tungsten oxide nanofibers for gas-sensing applications

Tungsten oxide nanofibers were successfully prepared via thermal treatment of electrospun composite nanofibers consisting of polyvinylpyrrolidone (PVP) and tungstic acid at 500 °C in air. The morphology, crystal structure, and chemical composition of the nanofibers were characterized by SEM, EDX, TEM, XRD, and FT-IR before and after the thermal treatment. It was confirmed that the calcination process was responsible for the removal of PVP component and the growth of crystalline WO3. The resulting tungsten oxide nanofibers, which had a rough surface morphology and an average diameter of around 40 nm, were found to be formed by the axial agglomeration of prolate spheroid-like WO3 nanoparticles with monoclinic crystalline phases. Gas-sensing measurements of the polycrystalline WO3 nanofiber mats were performed upon exposure to ammonia gas. They demonstrated n-type sensing response and sensitive NH3 detection up to 10 ppm with a well-defined relationship between the concentration and detection response at an operating temperature of 300 °C. These results were interpreted by applying the space-charge layer model used in the semiconducting metal-oxide sensor systems.