Discriminable Sensing Response Behavior to Homogeneous Gases Based on n-ZnO/p-NiO Composites

Metal oxide semiconductor (MOS) gas sensors have the advantages of high sensitivity, short response-recovery time and long-term stability. However, the shortcoming of poor discriminability of homogeneous gases limits their applications in gas sensors. It is well-known that the MOS materials have similar gas sensing responses to homogeneous gases such as CO and H₂, so it is difficult for these gas sensors to distinguish the two gases. In this paper, simple sol-gel method was employed to obtain the ZnO-xNiO composites. Gas sensing performance results illustrated that the gas sensing properties of composites with x > 0.425 showed a p-type response to both CO and H₂, while the gas sensing properties of composites with x < 0.425 showed an n-type response to both CO and H₂. However, it was interesting that ZnO-0.425NiO showed a p-type response to CO but an discriminable response (n-type) to H₂, which indicated that modulating the p-type or n-type semiconductor concentration in p-n composites could be an effective method with which to improve the discriminability of this type of gas sensor regarding CO and H₂. The phenomenon of the special gas sensing behavior of ZnO-0.425NiO was explained based on the experimental observations and a range of characterization techniques, including XRD, HRTEM and XPS, in detail.