Novel approaches towards design of metal oxide based hetero-structures for room temperature gas sensor and its sensing mechanism: A recent progress

Current research on gas sensors are focused toward developing cost-effective high-performance gas sensors at low operating temperature. Shortcomings of pristine semiconducting metal oxides (SMOs)-based high operating temperature sensors result in high power consumptions, high manufacturing cost, detonation risk for explosive gas detection and lack of long-term stability. These shortcomings could be diminished by designing suitable heterostructures to achieve superior sensing capabilities (high and faster response, high stability in environment) at room temperature. Herein, this review emphasizes on recent (last five years) advancement of SMOs-based heterostructures for room temperature gas sensors and various sensing mechanisms. The effects on different nano-junctions (p-n, n-n, p-p and Schottky), porosity, quantum dots and distinct facets have been explored for the modulation of charge depleted layers at interface, enhancing surface area, enabling more active sites etc. Diminution in sensor resistance and operating temperature can be realized by incorporating high conductive materials such as carbon nanotubes, graphene and activated carbon in SMOs. Several sensing mechanisms including heterojunction formation, Knudsen diffusion model, metal sulfuration, proton hopping, effusion effect and percolation effect have been elucidated for the correlation with high sensing performance gas sensors. Finally, a brief summary and future prospects have been addressed toward room temperature gas sensors.