In situ temperature-dependent properties of metal–organic framework ZIF-Coated ZnO hybrid structures: structural and advanced spectroscopic insights

Abstract

Advancements in material design, specifically the development of hybrids combining metal oxides (MOx) and zeolitic imidazolate frameworks (MOF/ZIFs), have revolutionized modern materials science by employing their synergistic effects and creating the hybrid interface MOF/MOx. This work systematically examines the structural evolution, defect engineering, and thermal stability of zeolitic imidazolate framework (ZIF-7, ZIF-8, ZIF-67, and ZIF-71)-coated ZnO and Cd-doped ZnO columnar structures. Comprehensive characterization was performed on all four studied ZIFs, including scanning electron microscopy revealing dodecahedral morphology of ZIF particles, and in situ temperature-dependent X-ray diffraction showing smaller coherently scattering regions compared to the sizes of the particles. SEM images indicate that these particles are polycrystalline and show a large contribution of surface-related domains. Thermolysis behavior and bond energy analysis reveal the role of Co–N bonds in thermal degradation of ZIF-67 at 250 °C compared to Zn–N bond of ZIF-8, which degrades at 325 °C, although Co–N has a higher bond energy compared to Zn–N, attributed to unsaturated coordination of Co with N, which leads to easy oxygenation of a Co–N bond. In ZIF-71 and ZIF-8, peak shifts are observed at higher angles by increasing the temperature, which would point to a negative thermal expansion. Using current–voltage characteristics, an inverted hysteresis was observed by employing forward and reverse voltage sweep, which may be characterized as capacitive hysteresis attributed to charge traps that slow down the return path. Gas-sensing studies revealed functional implications that ZIF-67-coated ZnO exhibited selective VOC sensing at 250 °C and enhanced hydrogen sensing at 300 °C, with structure–property correlations elucidated through defect analysis. Other hybrid materials, such as ZIF-7-coated ZnO, ZIF-8-coated ZnO, and ZIF-71-coated ZnO, elucidate better hydrogen, n-butanol, and 2-propanol sensing compared to ethanol and acetone. Our findings highlight the critical role of ZIF type and Cd doping in tuning the structural, thermal, and functional properties of ZnO-based composites, offering new perspectives for the rational design of advanced sensing materials based on the hybrid interface MOF/MOx.