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

Abstract

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.