Metal–organic framework-derived ZnO/ZnCo2O4 microspheres modified by catalytic PdO nanoparticles for sub-ppm-level formaldehyde detection

Designing novel structures and selecting the right catalyst play significant roles to overcome the inherent limitations of chemical sensors. The transition metal-organic framework is one of the most promising precursors for the development of advanced gas sensors with high porosity. In this work, novel ZnO/ZnCo 2 O 4 hetero-structured microspheres synthesized from a Zn–Co-Prussian blue analog (PBA) precursor is rationally designed by changing the temperature of thermal treatment and the decoration of PdO nanoparticles is carried out by a water bath strategy. Formaldehyde-sensing properties of the sensors based on pure ZnCo 2 O 4 NPs, ZZCO-500, ZZCO-700, and PdO-ZZCO microspheres are studied in detail. The experiment results show that the PdO-ZnO/ZnCo 2 O 4 microsphere-based sensor has a higher response value (about 26.9), better selectivity (S formaldehyde /S ethanol  = 5) and rapid response and recovery (9/14 s) towards 100 ppm formaldehyde as well as a low detection limit (0.2 ppm). The improvement in formaldehyde-sensing properties could be attributed to the larger specific surface areas (SSA), abundant active sites, the heterojunctions and the catalytic effect of PdO nanoparticles. This work provides a feasible method for synthesizing MOF-derived hetero-structured nanomaterials, demonstrating the potential application for monitoring indoor air pollutants.