Crystal Size-Dependent Pore Architecture and Surface Chemical Characteristics of Desolvated ZIF-8 Investigated Using Positron Annihilation Spectroscopy

Crystal downsizing of zeolitic imidazolate framework-8 (ZIF-8) has been shown to affect the flexibility of its framework, leading to the occurrence of “ gate-opening ” phenomenon at higher applied gas pressures compared to larger size crystals. In the present study, we have investigated the crystal size-dependent pore architecture of crystalline desolvated ZIF-8 samples which is expected to play the most deterministic role in its gas adsorption behavior. In order to avoid the pressurization or gas molecule–ZIF interaction, positron annihilation spectroscopy has been used to investigate the pores’ sizes, pores’ interconnectivity, and pores’ elemental surface characteristics of desolvated ZIF-8 samples. The studied ZIF-8 samples (size ∼14 nm −1.4 μm) have been synthesized at room temperature and systematically characterized for their phase purity, structural integrity, morphology, and thermal stability using complementary characterization techniques. Positronium annihilation from inter-crystal voids as a result of its diffusion through the crystals indicates that the pore network is highly interconnected in smaller size crystal samples. However, some interconnections from deep inside of larger crystals to the outer surface are observed to be blocked due to the imperfections produced in pore networking during crystal growth. The present study confirms that crystal downsizing results in modifications of average pore size corresponding to the aperture and central cavity. The average aperture size in nanometer size ZIF-8 crystals (size ∼14 and 45 nm) was measured to be smaller than the perfect lattice aperture (0.34 nm). We attribute this smaller aperture size in ZIF-8 nanocrystals to the occurrence of “ gate-opening ” phenomenon at higher applied gas pressure. The surface of nanocrystals is observed to be enriched by Zn compared to imidazole.