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.