Engineering hierarchical cubic WO3 nanostructures for sub-ppb-level NO2 gas sensor

Various WO3 nanostructures with different crystal phases have been studied as materials for gas sensors to detect various toxic gas. In this work, cubic-phase WO3 with nanoplate-assembled microspheres and ribbon-like nanosheet morphologies were successfully synthesized from a Na2WO4 precursor in different highly acidic environments (pH =-1.0, 0.0, and 0.4) via an acid precipitation process combined with spray drying and calcination at 200 degrees C. In highly acidic precursor environments (pH =-1.0 and 0.0), medium-sized WO3 center dot nH2O nanoplates were formed and then assembled into microspheres via the spray drying process. While in a lower acidic environment (pH = 0.4), large and thin WO3 center dot 2H2O sheets were formed, inhibiting their assembly into microspheres. All as-dried WO3 center dot nH2O samples were completely transformed into cubic-phase WO3 after facile calcination at a mild temperature of 200 degrees C in a closed muffle furnace in ambient air. Both nanoplate-assembled cubic WO3 microspheres and cubic WO3 ribbon-like nanosheets exhibited excellent NO2 gas sensing performance at a mild optimal working temperature of 150 degrees C, with a theoretical sub-ppb limit of detection. Among these samples, the one prepared in the highest acidic environment (sample SD-20, prepared at pH =-1.0) exhibited the best NO2 gas sensing performance. At an optimal operating temperature of 150 degrees C, the SD-20 sensor demonstrated an average response of 694-5 ppm NO2, with a theoretical detection limit of 0.004 ppb. Notably, the SD-20 sensor also demonstrated excellent selectivity and stability for NO2 gas at the optimal working temperature. These results suggest that different hierarchical morphologies of cubic WO3 nanostructures can be controllably prepared via a power-saving acid precipitation method combined with a time-efficient spray drying process for specific applications.