Synthesis of Tungsten Oxide Nanofibers by Electrospinning Method and Their Application to Gas Sensors

Abstract

Tungsten oxide nanofibers were successfully prepared via thermal treatment of electrospun composite nanofibers consisting of polyvinylpyrrolidone (PVP) and tungstic acid at 500?aC 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?aC. These results were interpreted by applying the space-charge layer model used in the semiconducting metal-oxide sensor systems. Mesoporous tungsten oxide nanofibers were synthesized via a 500?aC thermal treatment of composite nanofibers prepared by electrospinning an ethanol solution of tungsten ethoxide, P123 triblock copolymer, and polyvinylpyrrolidone. The as-electrospun composites exhibited unwoven nanofibers with an average diameter of 233 nm and smooth surface morphology. During the calcination process, they were shrunk to be 85 nm in diameter and converted into rough, wormhole-like nanofibers formed by agglomerating polycrystalline WO3 particles of 10 ?V 30 nm along the axial direction. Furthermore a measured pore size distribution indicated that this nanofiber mat had different types of meso-sized porosities, maybe resulting from the wormhole-like structure and inter-fiber void, in addition to the intra-grain porosity with the diameter of about 1.0 nm.