Pore architecture evolution and OER catalytic activity of hollow Co/Zn Zeolitic Imidazolate Frameworks

Tunable porosity through hollow features at nanoscale in metal organic frameworks (MOFs) can lead to enormous applications especially in catalysis. Hollow ZIF-67/ZIF-8 (core/shell) frameworks have been synthesized using solvothermal synthesis in methanol, wherein preformed ZIF-67 nanocrystals have been used as core. Using this method, hollow core/shell frameworks have been synthesized by excavation of ZIF-67 core and concurrent deposition of ZIF-8 shell for varying time. The crystal structure and chemical bonding of the frameworks have been characterized using X-ray diffraction, FTIR and Raman spectroscopy. Morphology of the hollow frameworks has been investigated using FE-SEM and EDX. Positron annihilation lifetime spectroscopy in combination with small angle X-ray scattering have been used to characterize the multiscale pore architecture (ultramicropores to mesopores) of these hollow frameworks. The sodalite topology and associated pore architecture of ZIFs are retained during the synthesis of hollow framework, however the excavation of core leads to enhancement in mesopores. Hollow frameworks have been tested for their performance towards oxygen evolution reaction. The hollow frameworks show improvement in the catalytic activity as compared to pure ZIF-67 nanocrystals. It is proposed that in order to further improve their catalytic performance, post-synthesis modifications for enhancement in stability of pore network of shell and loading of active species are required.