Influence of morphology and heterostructure formation on the NO₂ gas sensing properties of the ZnO-NiO system

• The formation of heterostructures improved the gas sensing selectivity property. • ZnO/NiO donuts-based sensors show good response and high selectivity for NO 2 . • ZnO/NiO needles sensor shows lower selectivity but a higher response to O 3 . • The synthesis route for the ZnO/NiO heterostructures is a cheap and fast alternative. The detection of toxic gases has gained significant attention in recent decades. Among various gas-sensing materials, metal oxide semiconductors (MOS) have emerged as highly promising due to their exceptional physical and chemical properties. However, a major limitation of MOS-based gas sensors is their lack of specificity, as they often respond to multiple gases, complicating the identification of target gases in mixed environments. This challenge can be addressed by combining two or more MOS materials to form a heterojunction, which modifies the electronic structure and enhances selectivity. NiO, a p-type semiconductor, has demonstrated the ability to improve both selectivity and sensor response when combined with ZnO, an n-type semiconductor. In this study, ZnO particles were synthesized via a precipitation method to produce two distinct morphologies: needle- and donut-like. These ZnO particles were subsequently combined with NiO via a hydrothermal reaction to form an n-p heterojunction. The selectivity of the resulting sensors was evaluated against O₃, NO₂, NH₃, and CO gases. The results indicated that ZnO sensors with needle- and donut-like morphologies exhibited high responses to oxidizing gases but lacked adequate selectivity between them. In contrast, the ZnO/NiO donut-like heterostructure demonstrated high selectivity for NO₂ detection.