Pore-Structure
Investigation of Bimetallic Zeolitic
Imidazolate Framework‑8 Films with Varying Co/Zn Nodes Ratio
Using Depth Sensitive Positronium Annihilation Lifetime Spectroscopy
posted on 2024-01-30, 10:47authored byRenjith
B. Nelliyil, Jaideep Mor, Manoj Thota, Maciej Oskar Liedke, Maik Butterling, Ahmed G. Attallah, Eric Hirschmann, Andreas Wagner, Sandeep Kumar Sharma
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 ∼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%. The pore network of bimetallic
films is observed to be highly stable up to 200 °C under vacuum
annealing, whereas under atmospheric conditions the pore network begins
collapsing from 100 °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.