Role of ZnO morphology in promoting NiO heterojunction formation and enhanced gas sensing

Zinc oxide samples with plate and rod morphologies were synthesized via the microwave-assisted hydrothermal (MAH) method and decorated with NiO to form heterostructures. Rietveld refinement revealed NiO contents of ∼25 wt% and ∼40 wt% for each morphology. The ZnO plates demonstrated superior response to 1 ppm ethanol compared to ZnO rods, attributed to their more reactive exposed crystal facets and stronger depletion layer effects. Upon decoration with 25 wt% NiO (ZnO-P-20Ni), the ethanol response increased by 275% and 410% relative to the pure ZnO plates and rods, respectively. Conductance measurements confirmed the p–n heterojunction formation only in ZnO-P-20Ni, with conductance decreasing from 1.7 × 10⁻7 to 7.6 × 10⁻8 S at 350°C compared to the ZnO plates. In contrast, 40 wt% NiO and all rod-based heterostructures showed no improvements, likely due to parallel conduction through NiO. The plate-like ZnO/NiO heterostructures displayed improved sensing properties across all testing conditions in dry air and better performance for CO sensing under humid conditions. The plates retained 75% of the ethanol response under 40% relative humidity. These findings highlight the importance of ZnO morphology in enhancing gas sensing performance and enabling heterojunction formation, providing a valuable strategy for designing efficient chemoresistive sensors.