Nickel Oxide Nanostructures for Gas Sensing: Recent Advances, Challenges, and Future Perspectives.

The need for efficient and reliable gas sensors has grown significantly due to increasing industrial activities, transportation, and environmental pollution, posing serious risks to human health and the environment. Advanced sensor technologies are crucial for detecting these harmful gases at low concentrations with a high accuracy. Nickel oxide, a p-type metal oxide semiconductor, has emerged as a promising candidate for gas sensing applications owing to its unique and excellent structural, electronic, and catalytic properties along with its high chemical stability. Interestingly, the possibility to synthesize NiO in versatile nanostructure forms: nanowires, nanoflowers, and nanospheres, helps to enhance surface area and porosity, which are critical factors to improve gas adsorption and diffusion. This review presents a comprehensive and critical assessment of the latest advancements in the synthesis, characterization, and gas-sensing performance of NiO nanostructures. We explore how structural modifications, such as decoration with noble metal nanoparticles, formation of different composites, and surface functionalization with self-assembly enhance the sensitivity, selectivity, and operational temperature of NiO sensors. Particular focus is given to the integration of NiO in novel nanoheterostructures, where the formation of p-n and p-p junctions significantly improves charge transport and overall sensor response. Finally, we identify current challenges in reproducibility, stability, and operating conditions, while offering directions for future research on tailoring NiO nanostructures for more effective, scalable, and robust sensor technologies.