Gas-sensing selectivity of n-ZnO/p-Co3O4 sensors for homogeneous reducing gas

Poor selectivity has always been the most severe challenge in the practical applications of gas sensors based on metal-oxide semiconductors (MOSs). It is difficult to distinguish CO and H2 because of the cross-sensitivity phenomenon. Herein, a series of n-ZnO/p-xCo3O4 NPs (1:x represents the molar ratio of Zn:Co) was prepared through a simple sol-gel method. Structural and material characterization were performed via X-ray diffraction (XRD) and transmission electron microscopy (TEM), while other techniques were used to assess the gas-sensing properties. The characterization results indicated that the oxides in the n-ZnO/p-0.61Co3O4 NPs formed p-n heterojunction instead of being dissolved together. The gas sensing results confirmed that the Zn:Co molar ratio influenced the gas-sensing selectivity of the n-ZnO/p-xCo3O4 NPs. Interestingly, regarding the gas-sensing behavior of n-ZnO/p-0.61Co3O4 NPs, the overall conductance of the film decreased in the presence of CO and increased in the presence of H2 at the testing temperature of 350 °C. This paper provides an approach to address the problem of poor selectivity of MOS-based gas sensors. •Interestingly, n-ZnO/p-0.61Co3O4 NPs shows p-type response to CO but n-type response to H2.•The characterization results demonstrate that the n-ZnO/p-Co3O4 NPs formed a p-n heterojunction.