Abstract:

Herein, Co-doped ZIF (zeolitic imidazolate framework)-8 was synthesised via static precipitation method using methanol as solvent. The synthesised ZIF-8 was subsequently annealed under air atmosphere in a tubular furnace, yielding polyhedral bi-shell Co$_{3}$O$_{4}$-ZnO. The composition, morphology, and thermal stability of the synthesised materials were characterised via X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermal gravimetric analyzer (TGA). Results show that the pure ZIF-8 remained stable at relatively low annealing temperature, whereas at higher annealing temperature, ZIF-8 transformed to a collapsed ZnO structure. The polyhedral bi-shell Co$_{3}$O$_{4}$-ZnO (CZO-4) was obtained by annealing 4% Co-doped ZIF-8 at a low temperature, suggesting that Co provides an active site for catalytic oxidation during the oxidation of ZIF-8 and reduces the oxidation barrier of 2-methylimidazole. Meanwhile, the low annealing temperature can provide sufficient forming process for ZnO to retain the rhombic dodecahedron structure. The oxidation growth process, catalysed by Co element, from outside to inside eventually results in the growth of ZnO into the bi-shell structure. When the Co loading is increased to 8%, the formation of bi-shell layer is not obvious (CZO-8). Compared with CZO-8 and ZnO, CZO-4 has better CO sensing performance, high sensitivity ($R_\mathrm a/R_\mathrm g = 21.8$@100$\times $10$^{-6}$ CO), high selectivity (up to 8.7 times of H$_{2}$ response), and long-term stability (stable signal in 42-day tests). This is because CZO-4 has a large specific surface area, gas transport channels, and Co as an active site for CO catalytic oxidation, resulting in an enhancement in gas sensing performance.

Key words: ZIF(zeolitic imidazolate framework)-8, Co doping, catalytic oxidation, bi-shell structure, gas sensing properties, CO sensing