Effect of annealing on microstructure and NO₂-sensing properties of tungsten oxide nanowires synthesized by solvothermal method

Quasi-orienting W₁₈O₄₉ nanowire bundles were synthesized by solvothermal method. The microstructures and the NO₂-sensing properties of the as-synthesized nanowires annealed at different temperatures were studied. To characterize the morphology and crystalline structure of the annealed tungsten oxide, field emission scanning electron microscope, X-ray diffraction and transmission electron microscope were employed. It was found that with annealing temperature increasing, the nanowire bundles became straighter and slightly thicker, and eventually a nonobelt-like structure was formed via the nanowires coalescence at 450°C. Meanwhile, the monoclinic W₁₈O₄₉ was transformed to monoclinic WO₃ when annealing temperature rising from 350 to 450°C. In comparison to the W₁₈O₄₉ nanowire bundles-based sensor, the sensor based on the 450°C annealing-induced WO₃ nanobelt-like structure exhibited markedly higher response value and gas selectivity, as well as much better response-recovery characteristics to NO₂ gas due to its favorable microstructure feature to gas-sensing.