Highly sensitive and selective detection of ppb-level NO2 using multi-shelled WO3 yolk-shell spheres

Triple-shelled, double-shelled, and dense WO3 spheres were prepared by ultrasonic spray pyrolysis and their NO2 gas sensing characteristics were investigated. Triple-shelled WO3 yolk-shell spheres showed an unprecedentedly high response (resistance ratio = 100-49.0) to 50 ppb NO2 at 100-200 °C with negligible cross-responses to 5 ppm acetone, CO, NH3, ethanol, and toluene, whereas dense WO3 spheres showed low NO2 response (8.9-3.1). The response and selectivity to NO2 were maintained in a highly humid (relative humidity 80%) atmosphere. The ultrahigh response and selectivity of triple-shelled WO3 spheres to several tens ppb levels NO2 was attributed to the enhanced gas accessibility of the yolk-shell morphology with thin and permeable multiple shells.