Opposite gas-sensing behavior of n-xZnO/p-yCr2O3 nanocomposites to H2 against CO and its selectivity mechanism

Abstract

There are several advantages to using metal-oxide semiconductor (MOS) gas sensors, including their high sensitivity, short response–recovery time, and long-term stability. However, the poor selectivity is a severe challenge in the applications of MOS gas sensors. For instance, it can be difficult for MOS sensors to differentiate between carbon monoxide (CO) and hydrogen (H₂) due to their similar gas-sensing behaviors. In this paper, a series of n-xZnO/p-yCr₂O₃ nanocomposites (Zn_xCr_y NPs, x:y represents the molar ratio of Zn:Cr) were prepared using a simple sol–gel method. The structural, composition, and the surface physicochemical states of the Zn_xCr_y material were studied via X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The characterization results demonstrate that p-n heterojunctions of Zn_xCr_y NPs have been prepared. Gas-sensing results showed that the Zn_xCr_y NPs' gas-sensing behavior was influenced by the Zn:Cr molar ratio. Interestingly, Zn₈Cr₂ NPs sensors showed n-type responses to H₂ but opposite responses (p-type) to CO, demonstrating that the low selectivity of MOS gas sensors can be addressed by modulating the n- or p-type semiconductor concentration in ZnxCry NPs. This paper offered an effective way to address the problem of poor selectivity of MOS sensors to CO and H₂. The gas-sensing mechanism of Zn_xCr_y NPs-based sensors is experimentally studied in details.