Zeolitic Imidazolate Framework Coated ZnO Nanorods as Molecular Sieving to Improve Selectivity of Formaldehyde Gas Sensor

Zinc oxide (ZnO) and zeolitic imidazolate framework-8 (ZIF-8) core-shell heterostructures were obtained by using the self-template strategy where ZnO nanorods not only act as the template, but also provide Zn²⁺ ions for the formation of ZIF-8 shell. The ZIF-8 shell was uniformly deposited to form ZnO@ZIF-8 nanorods with core-shell heterostructures at 70 °C for 24 h as the optimum reaction time by the hydrothermal synthesis. Transmission electron microscopy (TEM) images revealed that the ZnO@ZIF-8 heterostructures are composed of ZnO as core and ZIF-8 as shell. Nitrogen (N₂) sorption isotherms demonstrated that the as-prepared ZnO@ZIF-8 nanorods are a typical microporous material. Additionally, the ZnO@ZIF-8 nanorods sensor exhibited distinct gas response for reducing gases with different molecule sizes. The selectivity of the ZnO@ZIF-8 nanorods sensor was obviously improved for the detection of formaldehyde owing to the limitation effect of the aperture of ZIF-8 shell. This study demonstrated that semiconductor@MOF core-shell heterostructures may be a novel way to enhance the selectivity of the gas sensing materials.