Synthesis of Co3O4/ZnO nano-heterojunctions by one-off processing ZIF-8@ ZIF-67 and their gas-sensing performances for trimethylamine

The formation of p-n junctions is an effective method to improve the gas sensing performances of materials. Herein, the Co3O4 quantum dots/ZnO nanocages p-n nano-heterojunctions (Co3O4/ZnO NHs) were successfully synthesized by one-off processing ZIF-8@ZIF-67 at an appropriate heat treatment condition. The prepared Co3O4ZnO NHs containing numerous heterojunctions were demonstrated as a promising sensing material for trimethylamine (TMA) gas and show high performances with fast response, high response, low detection limit, good stability and high selectivity. Moreover, based on quasi-in-situ XPS characterization and density functional theory calculations, we found that the surface of ZnO transformed to metallic Zn in the presence of TMA gas with high adsorption energy (-3.31 eV) on ZnO. In addition, the band structure of Co3O4 and ZnO also expounded that the charge transfer from ZnO to Co3O4 and contributed to form more surface adsorbed oxygen on the surface of Co3O4 /ZnO NHs. Overall, the high sensing response of Co3O4/ZnO NHs was due to both the increase of the thickness in the depletion layer induced by the p-n heterojunction and the TMA-induced surface metallization of ZnO, while the high selectivity for TMA may only come from the surface metallization induced higher response compared to other reference gases, but not the formation of heterojunctions.