Novel SnO₂@ZnO hierarchical nanostructures for highly sensitive and selective NO₂ gas sensing

The novel brush-like (B–) SnO₂@ZnO hierarchical nanostructures (HNSs) are successfully synthesized by using a simple two–step hydrothermal method. The SnO₂ nanowires (NWs) grow epitaxially on the non–polarized plane of ZnO nanorods (NRs) with a six–fold symmetry. The heterogeneous nucleation–growth processes of SnO₂ and ZnO are discussed in detail based on the dissolution–recrystallization mechanism, growth kinetics and Ostwald ripening. The excellent sensing performances of B–SnO₂@ZnO HNSs for NO₂ gas sensor are developed, including good selectivity, ultrasensitive, fast response, broad detection range and low detection limits. The detection range of the sensor is measured from 5 ppb to 10 ppm, and the detection limit of the sensor is 5 ppb at 150 °C. The response and recovery time which reach 90% of the final signal is less than 60 s, while retaining the low detection limit. The sensing mechanism is also discussed, and the unique structure of B–SnO₂@ZnO is the dominating parameter for excellent sensing performances. The improved sensing performance of the HNSs also suggests the possibilities of other 1D materials combination for further sensing applications.