Pore-Structure Investigation of Bimetallic Zeolitic Imidazolate Framework-8 Films with Varying Co/Zn Nodes Ratio Using Depth Sensitive Positronium Annihilation Lifetime Spectroscopy

The gas-framework interaction induced gate-opening phenomenon occurring in zeolitic imidazolate frameworks (ZIFs) has restricted the ability of conventional experimental techniques involving gas or liquid molecules intrusion in ZIFs for determination of their inherent pore structure. The pore structure investigation of ZIF-based membranes/thin films using conventional techniques becomes even more difficult due to the limited amount of material, and the existence of pores that are not connected to the exterior surface. ZIF-based membranes are proposed as an advanced alternative for size selective separation of gases mixtures, which is primarily governed by the intra- and intercrystalline pore structure of these membranes. We present a systematic investigation of the depth-dependent pore structure of highly crystalline bimetallic (Zn/Co) ZIF-8-based films with varying metal ratio deposited on a silicon substrate using a fast current driven synthesis method. Using depth-dependent positronium annihilation lifetime spectroscopy, pore sizes corresponding to the intracrystalline pore network (aperture and central cavity) and intercrystalline voids have been determined. Pore aperture (window for size selective separation) at the near surface region is observed to be constrained compared to the bulk of the films due to surface energy minimization by linker twisting. Co loading in ZIF-8 reduces the aperture and cavity pore sizes due to the comparatively more rigid Co-N bond. The intercrystalline void size of the films is reduced on similar to 13.5% Co loading in ZIF-8, which is consistent with the high gas separation selectivity of membranes reported with this loading. Positronium intensity variation corresponding to different pores indicates that pore interconnectivity is reduced in the films with Co loading similar to 30-60%. The pore network of bimetallic films is observed to be highly stable up to 200 degrees C under vacuum annealing, whereas under atmospheric conditions the pore network begins collapsing from 100 degrees C, which is much earlier than the decomposition temperature of ZIF-8 and ZIF-67. The study also reveals, for the first time, a novel temperature-dependent aperture pore opening in bimetallic films due to enhanced ligand swinging at higher temperatures.