Opposite Sensing Response of Heterojunction Gas Sensors Based on SnO-CrO Nanocomposites to H against CO and Its Selectivity Mechanism.

Metal oxide semiconductor (MOS) gas sensors show poor selectivity when exposed to mixed gases. This is a challenge in gas sensors and limits their wide applications. There is no efficient way to detect a specific gas when two homogeneous gases are concurrently exposed to sensing materials. The p-n nanojunction of SnO-CrO nanocomposites (NCs) are prepared and used as sensing materials (/ shows the Sn/Cr molar ratio in the SnO-CrO composite and is marked as SnCr for simplicity). The gas sensing properties, crystal structure, morphology, and chemical states are characterized by employing an electrochemical workstation, an X-ray diffractometer, a transmission electron microscope, and an X-ray photoelectron spectrometer, respectively. The gas sensing results indicate that SnCr NCs with / greater than 0.07 demonstrate a p-type behavior to both CO and H, whereas the SnCr NCs with / < 0.07 illustrate an n-type behavior to the aforementioned reduced gases. Interestingly, the SnCr NCs with / = 0.07 show an n-type behavior to H but a p-type to CO. The effect of the operating temperature on the opposite sensing response of the fabricated sensors has been investigated. Most importantly, the mechanism of selectivity opposite sensing response is proposed using the aforementioned characterization techniques. This paper proposes a promising strategy to overcome the drawback of low selectivity of this type of sensor.