Carboxyl-directed hydrothermal synthesis of WO3nanostructures and their morphology-dependent gas-sensing properties

Three different morphologies of tungsten oxides, nanoparticles, nanosheets and hierarchical microspheres, have been successfully synthesized by a facile carboxyl-directed hydrothermal process. The chelation of carboxylic groups with W(OH)6 nuclei is recognized to be the origin of the morphological change. Gas-sensing measurements reveal that the sensing performance varies with WO3 morphology, and the hierarchical WO3 not only exhibits high sensitivity and fast response but also has low operating temperature to toxic NO2. The response of hierarchical WO3 is nearly 2 times and 10 times higher than those of the nanosheets and nanoparticles, respectively. The maximum response of hierarchical WO3 reaches 319 to 10 ppm NO2 at 200 °C. A relationship between morphology and crystal defect is established based on photoluminescence analysis. It is demonstrated that the change in defect feature in crystalline WO3 is responsible for its morphology-dependent gas-sensing properties.