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

Abstract

(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 ethanolcompared to ZnO rods, attributed to their more reactive exposed crystal facets and stronger depletion layereffects. 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 heterojunctionformation only in ZnO-P-20Ni, with conductance decreasing from 1.7 × 10⁻7 to 7.6 × 10⁻8 S at 350◦Ccompared 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 displayedimproved sensing properties across all testing conditions in dry air and better performance for CO sensing underhumid conditions. The plates retained 75% of the ethanol response under 40% relative humidity. These findingshighlight the importance of ZnO morphology in enhancing gas sensing performance and enabling heterojunctionformation, providing a valuable strategy for designing efficient chemoresistive sensors.