ZnO seed-mediated hydrothermal growth of advanced 1-D ZnO and 2-D CuO nanostructured oxide ceramics for gas sensing applications

ZnO and CuO nanostructured oxide ceramics were synthesized on ZnO-seeded micro-glass substrates (denoted as ZOZO and ZOCO, respectively), and their surface and gas-sensing characteristics were explored. The thin film seed layers were deposited using dip-coating and both ZnO and CuO nanostructures were produced through hydrothermal reactions under similar circumstances. X-ray diffraction patterns showed high crystallinity and texturization of ZOZO (in ZnO hexagonal wurtzite system) compared to ZOCO (in CuO monoclinic system). X-ray photoelectron measurements demonstrated the proper oxidation of Zn into Zn2+ in ZOZO and Cu into Cu2+ in ZOCO. Scanning electron microscope images revealed one-dimensional bundled nanorod-like structures in ZOZO and two-dimensional nano-coffee bean-like structures in ZOCO. The different morphologies induced different wettability features as indicated by water contact angles where the hydrophilic surface nature of ZOZO was more advantageous for NH3 detection compared to ZOCO. Gas sensing measurements in 100 ppm NH3 medium showed initially better results by the ZOCO sensor at a low operating temperature (70 degrees C, S = 9%) compared to the ZOZO sensor which did not show any credible sensitivity below 90 degrees C. However, as temperature increased, the ZOZO sensor exhibited an ultra-sensitivity of S = 275% at an optimal operating temperature of 150 degrees C with a limit of detection of 7 ppb whereas the ZOCO sensor recorded a maximum sensitivity of S = 19% at an operating temperature of 170 degrees C with a limit of detection of 40 ppb. Both sensors showed good repeatability, long-term stability, and selectivity towards ammonia gas. The variation in the properties of the produced films was discussed based on a seed-mediated approach that is consequently suggested to control the growth and alignment of nanoscale structures and their efficiency in subsequent device applications.