Abstract
•WO3 yolk–shell spheres were prepared by one-pot spray pyrolysis reaction.•Triple-shelled WO3 spheres showed high response and selectivity to sub-ppm NO2.•High NO2 response was attributed to excellent gas accessibility of yolk–shell morphology.•Response and selectivity to NO2 were maintained in highly humid atmosphere.•The sensor can be used to monitor ground-level NO2 and diagnose asthma.
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 50ppb NO2 at 100–200°C with negligible cross-responses to 5ppm 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.