Facile synthesis and NO2 gas sensing of tungsten oxide nanorods assembled microspheres

Tungsten oxide nanorods assembled microspheres were synthesized by a facile hydrothermal process at 180 °C using ammonium metatungstate and oxalic acid as starting materials. The morphology and structural properties were investigated using scanning electron microscopy, powder X-ray diffraction, and transmission electron microscopy. The as-synthesized microspheres are composed of orthorhombic WO3·xH2O nanorods with diameter less than 100 nm. These microspheres lose water gradually during annealing and transfer to monoclinic WO3 when annealed at 550 °C. The gas sensing properties of the microspheres annealed at different temperatures were studied by exposing the gas sensors made from microspheres to NO2 gas. The results indicated that the crystalline phase of the microspheres has no obvious effect on the gas sensing performance. The microspheres annealed at 350 °C showed fast and the highest response to NO2 gas due to the three-dimensional network based on the nanorods and the high effective surface area.