Nitrogen dioxide sensing using tungsten oxide microspheres with hierarchical nanorod-assembled architectures by a complexing surfactant-mediated hydrothermal route

WO3 microspheres with hierarchical nanorod-assembled architectures were successfully synthesized by a complexing surfactant-mediated hydrothermal method in the presence of K2SO4 and H2C2O4 with a molar ratio of 1 : 1. Microstructural characterization by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy showed that WO3 microspheres with diameters ranging from 3 to 5 mm were assembled by 90 nm diameter nanorods and had a single crystal hexagonal structure. The analysis results of the elemental composition and chemical state demonstrated that the obtained WO3 microspheres were nearly stoichiometric. Based on the experimental results, a possible growth mechanism consisting of nucleation, Ostwald ripening, and self-assembly of WO3 crystals was proposed. Gas sensing properties demonstrated that WO3 microspheres exhibited not only a high response and excellent reversibility to NO2, but also a good linear relationship between the response and NO2 concentration in the range of 1 to 10 ppm. The response and recovery times significantly decreased as the operating temperature increased gradually. The highest response of 790 to 20 ppm NO2 was obtained at a relatively low operating temperature of 100 degrees C, which revealed that WO3 microspheres were very promising for fabricating low-consumption chemical gas sensors. The electron depletion theory was used for explaining the gas sensing mechanism by the chemical adsorption and reaction of NO2 gas molecules on the surface of WO3 microspheres.