Exploring the Role of Metal Oxide Heterostructures for Next-Generation Gas Sensors: A Focus on NH3, H2S and NO2 gases

Metal oxide semiconductor-based gas sensors have attracted widespread attention for the detection of toxic gases such as ammonia, hydrogen sulfide and nitrogen dioxide due to their simplicity, cost-effectiveness and sensitivity. This review presents a comprehensive analysis of recent advancements in SnO 2 , WO 3 and ZnO based nanocomposites, emphasizing their structural modifications, heterojunction engineering, synthesis strategies and gas sensing mechanisms. Particular focus is given to heterojunction formation (like n-n, p–n, and p-p) which improves charge separation and modulates resistance, thereby enhancing sensor response. The integration of hierarchical nanostructures such as nanoflowers, nanotubes and hollow microspheres significantly improve surface-to-volume ratio, gas diffusion and active site availability. Doping with noble metals (such as Ag, Pt) and mixed-valence oxides (e.g., CeO 2 , FeCo 2 O 4 ) further enhances sensitivity and environmental stability. Finally, this review identifies the most effective material combinations for the selective detection of the studied gases. This review also discusses the critical role of fabrication techniques such as sol-gel, hydrothermal, and electrospinning in tailoring morphology and performance. Challenges related to selectivity, humidity interference, long-term stability and scalability are addressed. This work aims to guide the design and optimization of next-generation gas sensors with improved sensitivity, selectivity and reliability for environmental and industrial applications.