Role of Heterojunctions of Core–Shell Heterostructures in Gas Sensing

Heterostructures made from metal oxide semiconductors (MOS) are fundamental for the development of high-performance gas sensors. Since their importance in real applications, a thorough understanding of the transduction mechanism is vital, whether it is related to a heterojunction or simply to the shell and core materials. A better understanding of the sensing response of heterostructured nanomaterials requires the engineering of heterojunctions with well-defined core and shell layers. Here, we introduce a series of prototypes CNT-nMOS, CNT-pMOS, CNT-pMOS-nMOS, and CNT-nMOS-pMOS hierarchical core–shell heterostructures (CSHS) permitting us to directly relate the sensing response to the MOS shell or to the p–n heterojunction. The carbon nanotubes are here used as highly conductive substrates permitting operation of the devices at relatively low temperature and are not involved in the sensing response. NiO and SnO2 are selected as representative p- and n-type MOS, respectively, and the response of a set of samples is studied toward hydrogen considered as model analyte. The CNT-n,pMOS CSHS exhibit response related to the n,pMOS-shell layer. On the other hand, the CNT-pMOS-nMOS and CNT-nMOS-pMOS CSHS show sensing responses, which in certain cases are governed by the heterojunctions between nMOS and pMOS and strongly depends on the thickness of the MOS layers. Due to the fundamental nature of this study, these findings are important for the development of next generation gas sensing devices.