Conductometric methane gas sensors based on ZnO/Pd@ZIF-8: Effect of dual filtering of ZIF-8 to increase the selectivity

Selectivity remains the biggest challenge for metal oxide semiconductor (MOS)-based gas sensors, especially for inactive gases such as CH4. In this work, the core-shell structured ZnO/Pd@ZIF-8 was fabricated via the self-templated method and the sensor based on the ternary hybrid showed excellent CH4 selectivity against NH3, CO and NO2. ZIF-8 was critical for the selectivity enhancement via dual filtering effects: a) size-induced sieving, where NO2 with a larger kinetic diameter of 4.5 Å can be hindered by ZIF-8 with an effective aperture size of 4.0–4.2 Å; b) polarity-induced sieving, where NH3 (2.6 Å) and CO (3.76 Å) with larger polarity showed stronger interaction with ZIF-8 and their diffusion is therefore retarded. Therefore, the selectivity of nonpolar CH4 (3.8 Å) is improved. The present insight about the dual filtering effects of ZIF-8 may provide a new insight to improve MOS sensing selectivity by rational designing MOF structures.