Metal–Organic Framework-Templated Growth of Cation-Substituted Metal Oxide Shell MO (M = Co, Ni, and Zn) on a CuO Core: An In-Depth Understanding of Methane Gas-Sensing Performance

Abstract

Significant demand for advanced sensing materials has been emerging that enable the sensitive, real-time, and continuous detection of gas molecules in gas sensors, which have increasingly proven to be effective tools for environmental monitoring. In this context, metal–organic framework (MOF)-derived metal oxide semiconductors have garnered attention as highly attractive materials for gas-sensing applications due to their high specific surface area, distinctive morphology formed by modulation of functional linkers, and plentiful metal sites. Here, three distinct types of porous hierarchical core–shell heterostructure metal oxides have been designed by shelling with a Co-based zeolitic imidazolate framework (ZIF)-67 MOF, Ni-MOF, and zinc-based ZIF-8 MOF on a core octahedron Cu-MOF followed by calcining at a very slow ramp rate. The structural advantages of core–shell CuO@Co₃O₄ having high porosity and chemisorbed oxygen result in superior sensing performance for methane gas compared to porous CuO@ZnO and CuO@NiO core–shell structures with little rigid morphology and lower defective oxygen percentage. The higher active CuO@Co₃O₄ core–shell structure has achieved room temperature methane gas sensing with quick response and recovery time (T_res = 16 s, T_rec = 11 s) by decorating on nitrogen-doped three-dimensional reduced graphene oxide (N-3DrGO) sheets (S % = 6.54 for 100 ppm). The presence of a higher number of oxygen defects supported by X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) spectra is the main reason behind the high gas-sensing performance of the nanocomposite at room temperature.