Impact of ZnO nanorods vs nanotrees morphology on NO2 gas sensor performance

This study presents the synthesis and characterization of ZnO nanorods (NRs) and ZnO nanotrees (NTs) thin films. The gas sensing properties, particularly for NO2 detection, were evaluated. ZnO NRs and NTs thin films were synthesized using an open aqueous solution method. The XRD patterns of the ZnO NRs and ZnO NTs thin films confirm the hexagonal wurtzite crystal structure with preferred orientation along the (002) plane. The SEM micrographs show vertically aligned ZnO NRs grown perpendicular to the surface of the substrate. However, in the case of ZnO NTs, SEM shows the secondary ZnO NRs grown on the surface of the primary ZnO NR structure forming a tree-like nanostructure. The water contact angle was 83.27 degrees and 109.58 degrees for the ZnO NRs and ZnO NTs thin films indicating that the ZnO NTs thin film had a higher surface roughness than the ZnO NRs thin film. The gas responses (%) of ZnO NRs and ZnO NTs sensor devices are 167%, 352%, 576%, 595%, 858%, & 1213% and 199%, 448%, 658%, 880%, 1153%, & 1380% measured at an operating temperature of 200 degrees C for 10, 20, 40, 60, 80, & 100 ppm of NO2 gas concentration, respectively. The NO2 gas sensing performance of the ZnO NTs thin film was significantly higher compared to the ZnO NRs thin film. This study underscores the importance of morphology in achieving efficient NO2 detection, suggesting that optimizing the hierarchical nanostructure based-topography of ZnO can outperform NO2 gas sensing.