Design and Optimization of Metal Oxide-Based Humidity Sensors: A Review on Mechanisms and Material Engineering

Humidity sensors based on metal oxides have garnered significant attention due to their excellent physicochemical properties, including high surface area, thermal stability, and tuneable electrical conductivity. However, conventional humidity sensors—particularly those based on polymers and electrolytes—often suffer from drawbacks such as poor long-term stability, high hysteresis, and limited sensitivity under extreme environmental conditions. This review comprehensively examines the sensing mechanisms, synthesis methods, and performance optimization strategies of various metal oxides (e.g., Al2O3, TiO2, ZnO, and SiO2) for humidity sensing applications. The fundamental adsorption processes of water molecules, including chemisorption and physisorption, are discussed, along with the impact of ionic and electronic conduction mechanisms on sensor performance. Advances in material design—such as nano structuring, doping, and composite formation—are highlighted as effective strategies to enhance sensitivity, reduce response time, and improve stability. Furthermore, the emerging role of ferroelectric materials in humidity sensing is introduced, emphasizing their spontaneous polarization and potential for next-generation sensor technologies. This review aims to provide a solid foundation for developing efficient, reliable, and cost-effective humidity sensors suitable for applications in environmental monitoring, healthcare, and industrial automation.