Comparative study of monoclinic and cubic WO₃ nanoplates on NO₂ gas-sensing properties

Monoclinic and cubic WO₃ nanoplates were controllably prepared from orthorhombic WO₃·H₂O (o-WO₃·H₂O) nanoplates via a facile calcination method at 200 °C for 2 hours in ambient air using tubular and muffle furnaces, respectively. The o-WO₃·H₂O nanoplates were previously prepared from Na₂WO₄·2H₂O via an acid precipitation method at room temperature. Calcination stimulated the dehydration and phase transformation from hydrated WO₃·H₂O nanoplates to WO₃ nanoplates, and different crystal structures were observed under different air environments. In an open-air environment (tubular furnace), a stable monoclinic WO₃ (m-WO₃) phase was obtained, while in a closed-air environment (muffle furnace), a high-entropy cubic WO₃ (c-WO₃) phase was obtained. The difference in the phase transformation was confirmed using various physicochemical analyses, such as X-ray diffraction, field emission scanning electron microscopy, Brunauer-Emmett-Teller measurement, diffuse reflectance spectroscopy, and Raman scattering spectroscopy. Both m-WO₃ and c-WO₃ exhibited excellent NO₂ gas-sensing performance, with ultra-high sensitivity, exceptional selectivity, and ultra-low theoretical limit of detection, at a mild optimal-working temperature of 150 °C. In particular, chemiresistive sensors based on m-WO₃ and c-WO₃ nanomaterials exhibited responses of 1322 and 780 to 2.5 ppm NO₂ and theoretical limits of detection of 0.10 and 0.05 ppb to NO₂ at 150 °C, respectively. These results imply that the phase transformation of WO₃ nanostructures or even phase junctions could be achieved via a facile calcination process in different controlled environments (in closed or open ambient air) for various designed applications such as gas sensors.