ZnO/Pd@ZIF-7-Based Gas Sensors for Selective Methane Sensing

Selective methane detection is essential for process safety in industries such as coal mining, where CO, NH3, and NO2 serve as interfering gases. A promising approach is to use metal oxide semiconductor (MOS)-based sensors, which are low-cost, highly sensitive, and easy to fabricate. However, the poor selectivity of MOS sensors due to nonselective surface reactions remains a significant challenge. In this study, we fabricated a ZnO/Pd@ZIF-7 core-shell structure-based gas sensor using a self-sacrificial method. The ZnO/Pd layer served as the sensitive layer to generate sensing signals, while the ZIF-7 shell acted as a filter. By manipulating gas diffusion, ZIF-7 significantly improved CH4 sensing selectivity against CO, NH3, and NO2. For NO2, which strongly interacts with ZIF-7, the diffusion through ZIF-7 was significantly hindered, resulting in a decreased response across all temperature ranges (110-250 degrees C). For CH4, CO, and NH3, which weakly interact with ZIF-7, the influence of ZIF-7 depended on temperature, as competition occurred between surface reactions and diffusion through ZIF-7. At low temperatures, ZIF-7 enriched gases and promoted the response of the three gases. At elevated temperatures, ZIF-7 separated gases according to their molecular polarity, where the diffusion of polar CO and NH3 was more hindered than nonpolar CH4. The excellent CH4 selectivity against CO, NH3, and NO2 was achieved at 210 degrees C, with fast response/recovery, good repeatability, and long-term stability. Our study not only provides a possible solution to enable sensing selectivity of MOS to CH4, but the insights into the effect of the ZIF-7 filter may also inspire the development of highly selective gas sensors.