Synthesis of core-shell flower-like WO3@ZIF-71 with enhanced response and selectivity to H2S gas

Core-shell heterostructures of metal oxide semiconductor (MOS) and metal organic framework (MOF) improve gas-sensing properties due to the MOF's molecular sieving effects. However, the selection of MOF is limited by the requirement of identical metal element in MOF to that in MOS, which is crucial for valid interface adhesion. In this paper, WO3@ZIF-71 (zeolitic imidazolate framework) structure with different metal elemental components (W and Zn) was fabricated by a step-by-step (SBS) approach. As a result, ZIF-71 was uniformly coated on the surface of WO3. In order to explore the influence of ZIF-71 (pore size: 4.8 angstrom), the gas sensitivity of WO3 and WO3@ZIF-71 were tested. Different kinetic diameters gases: H2S (3.62 angstrom), CH3COCH3 (4.6 angstrom), CH3CH2OH (4.53 angstrom), NO2 (5.8 angstrom) were selected as testing gas. Gases with small molecules (H2S, CH3COCH3 and CH3CH2OH) can pass through the membrane of ZIF-71 and contact with WO3, while larger ones (NO2) were blocked outside the membrane. Excitingly, the response of WO3@ZIF-71 toward H2S gas was significantly improved, which soars from 2.24 of 20 ppm for pure WO3 to 19.12 for H2S at 250 degrees C, increased about 9 times. These results indicate that the MOS@MOF core-shell structure can be synthesized by the SBS method without the restriction of the same metal elements in MOS and MOF, which is also an effective approach to regulate the selectivity and sensitivity of the MOS@MOF gas sensor.