Nanoscale metal oxide heterojunction chemiresistive gas sensor: a review

Abstract Nano-material based resistive gas sensors are gaining in popularity because of their small size, low cost, and simple integration with analogue interface electronics. In gas sensors, semiconducting metal oxides are most widely used as the sensing layer, because they offer high sensitivity to gases and have a low detection limit (ppb to ppm). However, metal oxides suffer from a poor selectivity, because they generally respond to both oxidising and reducing gas molecules. In addition, they can suffer from baseline drift/stability and in some cases degradation under humid conditions. Some of these drawbacks can be ameliorated using metal oxide heterojunctions. A metal oxide heterojunction is the formation of a junction between two materials with different work functions. The metal oxide heterojunction can improve sensor performance through a controlled depletion region (band bending), charge transfer, catalytic effects, and improved gas adsorption kinetics. They can also permit a lower operating temperature (hence lower power), improved sensitivity, faster response and better stability. This review paper discusses in detail the different techniques to synthesise metal oxide heterojunctions, the sensing mechanisms, and how they can be a generation of improved gas sensors. Finally, we discuss the emergence of artificial intelligence to enable the identification of gas type and concentrations from multi-component environments.