Pore size dependent gas-sensing selectivity based on ZnO@ZIF nanorod arrays

Abstract

Abstract Recently, a metal-organic framework (MOF) coating method has been reported for improving the poor gas selectivity of metal oxide semiconductor (MOS) gas sensor. However, the correlation between MOF’s pore size and the size of gas molecule is not fully discussed, as well as gas-sensing properties to gases with similar molecular size. In this paper, two zeolitic imidazolate frameworks (ZIFs) with different pore sizes (∼3.4 A for ZIF-8, ∼4.8 A for ZIF-71) are synthesized on the surface of ZnO nanorod arrays to form a ZnO@ZIF core-shell structure, where ZIFs act as gas molecule sieve membrane that shields ZnO from gas molecules larger than the pore size of ZIFs. Hydrogen, ammonia, ethanol, acetone and benzene (molecular size in the range of 2.89–5.85 A) are selected as testing gases. The ZnO@ZIF-8 sensor exhibited a clear response to smaller molecules of hydrogen and ammonia rather than larger molecules of ethanol, acetone and benzene. While the ZnO@ZIF-71 sensor only exhibited inhibited response to benzene, which has the largest molecular size. These results indicate that the selectivity of ZnO gas sensor can be regulated by surface MOF coating with specific pore size. But this pore size depending selectivity regulation method is insufficient effective for targeting gases with similar molecular sizes, which may be improved by further modification on MOF materials for selective adsorption in our future work.