Enhanced Gas Sensing Mechanisms of Metal Oxide Heterojunction Gas Sensors

The metal oxide heterojunction has often been used to improve the gas sensing properties of resistive metal oxide semiconductor gas sensors. Metal oxide heterojunctions have been demonstrated to have many unique properties such as Fermi-level mediated charge transfer effects as well as synergistic behavior of different components. In this short review, we summarize the fundamental types of metal oxide heterojunction materials reported in domestic and foreign research in recent years. Metal oxide heterojunctions are mainly divided into five categories of mixed composite structures; multi-layer films, structure modified with a second phase, 1 D nanostructure and core-shell structure. We review the enhanced gas sensing mechanisms of metal oxide heterojunctions. These mechanisms are discussed in detail, including the role of the heterojunction, synergistic effects, the spill-over effect, response-type inversion, separation of charge carriers, and microstructure manipulation. We also analyze the remaining challenges of metal oxide heterojunction gas sensors. Finally, we provide an outlook for future development of metal oxide heterojunction gas sensors. The future research directions of metal oxide heterojunction gas sensors can be developed from the definition of heterojunction interface mechanisms. It is hoped that determining the heterojunction interface mechanisms will provide some reference for the design of needed gas sensors in a bottom-up route.