UV-activated hybrid hierarchical ZnO/ZIF-8 gas sensor

Metal-organic frameworks (MOFs) are new crystalline hybrid materials that have aroused great interest because of their growing applications in drug delivery, gas storage and catalysis. The key limitation of pure MOFs for use in gas sensors is their low conductivity. Therefore, composites are often used. MOF ZIF-8 improves gas-sensitive characteristics of composites due to its hydrophobity, thermal stability and high specific surface area. The purpose of this study was to develop a hierarchical structure of ZnO nanorods within a porous ZIF-8 shell and to investigate its characteristics in detecting volatile organic compounds.

For synthesis, 2-methylimidazole was dissolved in distilled water and dimethylformamide. The resulting solution was placed in an autoclave along with substrates coating with ZnO nanorods. The autoclave was kept at 70°C for 20 hours, then the samples were annealed at 320°C for one hour. Scanning electron microscopy revealed a net-like structure on the surface of ZnO nanorods.

The gas analytical response was studied at 250°C under the influence of acetone and ethanol vapors. The response and recovery times for ZnO/ZIF-8 structure were found to be much longer in the presence of these gases compared to pure zinc oxide nanorods. So, ZnO/ZIF-8 was studied under UV irradiation. The response increased and the response and recovery times decreased significantly. UV irradiation accelerates the diffusion of gas molecules to the surface of ZnO through ZIF-8 porous structure. This process leads to faster gas adsorption and desorption. Additional factors are photoinduced charge transfer and water desorption at ZnO/ZIF-8 interface, contributing to the observed changes in the response.

The work was supported by the Russian Science Foundation (project number 26-29-20111) and the Saint Petersburg Science Foundation.