Microstructure characterization and NO2-sensing properties of tungsten oxide nanostructures

Nanowires and nanosheets of tungsten oxide were synthesized by solvothermal method with different tungsten hexachloride (WCl6) concentrations in 1-propanol solvent. The morphology and crystal structure of the tungsten oxide nanostructures were investigated by means of field emission scanning electron microscope, X-ray diffraction and transmission electron microscope. The specific surface area and pore size distribution were characterized by Brunauer–Emmett–Teller gas-sorption measurements. One-dimensional W18O49 nanowire bundles were synthesized at the WCl6 concentration of 0.01 M. With the concentration increasing to 0.02 M, the structure of the pure two-dimensional WO3 nanosheets was formed. The NO2 gas sensing properties of W18O49 nanowires and WO3 nanosheets were investigated at 100 °C up to 250 °C over NO2 concentration ranging from 1 to 20 ppm. Both nanowires and nanosheets exhibit reversible response to NO2 gas at different concentrations. In comparison to WO3 nanosheets, W18O49 nanowire bundles showed a much higher response value and faster response–recovery characteristics to NO2 gas, especially a much quicker response characteristic with response time of 19 s at the concentration of 5 ppm.