Opposite Sensing Response of Heterojunction Gas Sensors Based on SnO₂–Cr₂O₃ Nanocomposites to H₂ against CO and Its Selectivity Mechanism

Abstract

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 <i>x</i>SnO₂-<i>y</i>Cr₂O₃ nanocomposites (NCs) are prepared and used as sensing materials (<i>x</i>/<i>y</i> shows the Sn/Cr molar ratio in the SnO₂-Cr₂O₃ composite and is marked as Sn_<i>x</i>Cr_<i>y</i> 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 Sn_<i>x</i>Cr_<i>y</i> NCs with <i>x</i>/<i>y</i> greater than 0.07 demonstrate a p-type behavior to both CO and H₂, whereas the Sn_<i>x</i>Cr_<i>y</i> NCs with <i>x</i>/<i>y</i> < 0.07 illustrate an n-type behavior to the aforementioned reduced gases. Interestingly, the Sn_<i>x</i>Cr_<i>y</i> NCs with <i>x</i>/<i>y</i> = 0.07 show an n-type behavior to H₂ 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.