Morphological and crystal structural control of tungsten trioxide for highly sensitive NO2 gas sensors

We discuss the synthesis of cuboid-monoclinic tungsten oxide (C-m-WO3) particles and hexagonal-plate-like-hexagonal tungsten oxide (H-h-WO3) particles by acidification and hydrothermal treatment with (NH4)10W12O41·5H2O as a starting material. From the WO3 particles, thin- and thick-film gas sensors were fabricated for NO2 sensing applications and the effect of the morphology and crystal structure of the WO3 particles on the NO2 sensing properties was investigated. The extremely high sensor responses of 160 to 20 ppb NO2 and 10 120 to 500 ppb NO2 were obtained for a C-m-WO3 thin-film sensor at an operating temperature of 200 °C. The crystal structure features of as-prepared particles were studied by XRD, XPS, SEM, TEM, and solid state 1H and 13C NMR measurements. The presence of residual impurities, cations, and high density of surface OH− in the H-h-WO3 particles was demonstrated, which could be one of the key reasons for the crystal structure dependence of the sensing properties in C-m-WO3 and H-h-WO3 gas sensors.