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

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 ZIFs based membranes/thin films using conventional techniques becomes even more difficult due to limited amount of material, and existence of pores that are not connected to exterior surface. ZIFs based membranes are proposed as an advanced alternative for size selective separation of gases mixtures, which is primarily governed by intraand inter-crystalline pore-structure of these membranes. We present a systematic investigation of depth dependent pore-structure of highly crystalline bimetallic (Zn/Co) ZIF-8 based films with varying metal ratio deposited on silicon substrate using a fast current driven synthesis method. Using depth dependent positronium annihilation lifetime spectroscopy, pore sizes corresponding to intra-crystalline pore network (aperture and central cavity) and inter-crystalline voids have been determined. Pore aperture (window for size selective separation) at near surface region is observed to be constrained compared to 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 comparatively more rigid CoN bond. Inter-crystalline void size of the films is reduced on ~ 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 ~ 30- 60 %. Pore network of bimetallic films is observed to be highly stable up to 200 oC under vacuum annealing, whereas under atmospheric condition pore network begins collapsing from 100 oC, which is much earlier than the decomposition temperature of ZIF-8 and ZIF-67. The study also reveals, first time, a novel temperature dependent aperture pore opening in bimetallic films due to enhanced ligand swinging at higher temperatures.