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

Abstract

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 μm 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 °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.