Tunable nanofibril heterojunctions for controlling interfacial charge transfer in chemiresistive gas sensors

Chemiresistive sensors, particularly those based on nanostructures, have drawn increasing attention for application in security and environmental monitoring, healthcare, biomedicine and others due to their high selectivity and sensitivity in detection of gaseous chemicals. Nanofibers possess large surface area, and exhibit unique electronic and optical properties that arise from their one-dimensional (1D) structures. They are an ideal candidate for development as sensors, even when constructed into heterojunction structures between n-type (electron acceptor) and p-type (electron donor) materials. Nanofibril heterojunctions created are highly tunable for enhancing the interfacial charge separation and transfer by modifying and optimizing both the material electronic structures and interface configuration spacing. This review aims to provide a comprehensive overview of the current state of the art of chemiresistive gas sensors based on nanofibril heterojunctions, with special focus on the control of interfacial charge transfer which is critical to the sensor performances. Various nanofibril heterojunction structures, including inorganic metal oxides, carbon materials, conjugated organic molecules, and functional polymers, are summarized. The properties of precisely tunable interfaces are discussed, in conjunction with the sensor mechanisms. The potential limitations and challenges of these exciting materials and heterojunction structures for further sensor enhancement and real-world application are also discussed. Lastly, an outlook is given on the future directions of developing nanofibril heterojunction sensors. This review will not only provide deep understanding of the structural design of nanofibril heterojunctions, and the interfacial charge transfer and chemiresistive sensor mechanisms, but also lay out more potential for extending them to other electronic and optoelectronic applications.