Hollow cylinder ZnO/SnO₂ nanostructures synthesized by ultrasonic spray pyrolysis and their gas-sensing performance

Hierarchical and hollow cylinder comb-like nanostructures with ZnO backbones and SnO₂ branches were successfully prepared by an ultrasonic spray pyrolysis process. The observation of field scanning electron microscopy and transmission electron microscopy showed that a high density of SnO₂ nanowires grew epitaxially on as-synthesized ZnO nanorod arrays. The diameter and length of the SnO₂ nanowires were about 50 nm and 300 nm. The morphologies of ZnO/SnO₂ hierarchical nanostructures could be tailored by changing the growth time of SnO₂ nanowires. A possible formation process and growth mechanism are proposed from the viewpoint of inside-out Ostwald ripening. In addition, gas sensors based on the hierarchical ZnO/SnO₂ nanostructures were fabricated and exhibited good response to ethanol. At an ethanol concentration of 100 ppm, the response was about 11 and the response time was about 1 s at the operating temperature of 275 °C.