Abstract
Thin films of crystalline and porous metal-organic frameworks (MOFs) have great potential in membranes, sensors, and microelectronic chips. While the morphology and crystallinity of MOF films can be evaluated using widely available techniques, characterizing their pore size, pore volume, and specific surface area is challenging due to the low amount of material and substrate effects. Positron annihilation lifetime spectroscopy (PALS) is introduced as a powerful method to obtain pore size information and depth profiling in MOF films. The complementarity of this approach to established physisorption-based methods such as quartz crystal microbalance (QCM) gravimetry, ellipsometric porosimetry (EP), and Kr physisorption (KrP) is illustrated. This comprehensive discussion on MOF thin film porosimetry is supported by experimental data for thin films of ZIF-8.
Highlights
Introduction instrumentsoften only qualitative porosity characterization has been performed, e.g., through intercalation of Crystalline and porous metal–organic frameworks (MOFs) fluorescent dyes or other labels.[8,9]are built up from metal ion nodes interconnected by organic far, quantitative porosimetry of MOF films has relied on linkers
MOFs display record-breaking specific surface areas physisorption, by measuring the adsorbed quantity of a probe and unique capacities for selective mole- molecule through manometric/volumetric (e.g., Kr physisorpcular uptake based on a uniform pore size combined with a tion, Kr physisorption (KrP)), gravimetric
Characteristic lifetimes should be considered as a “MOF fingerprint”. Their values should be established from Positron annihilation lifetime spectroscopy (PALS) measurements of well-characterized reference samples, and the corresponding pore sizes compared with values obtained with other methods (e.g., Ar physisorption on powder at 87 K), or calculations based on the crystal structure (e.g., Monte Carlo, molecular dynamics, or density functional theory).[10,51,54]
Summary
Positrons (e+) are the anti-particle of electrons (e−); they have the same mass but opposite charge. With E = implantation energy (keV), sample density ρ (g cm−3), A = 2.81 μg cm−2 keV−n and n = 1.71 for polymers This value has only been determined for polymers, and is assumed to hold for MOFs. PALS spectra are analyzed through fitting a continuous range of lifetimes (e.g., with the software packages MELT,[32] LT,[33] or CONTIN[34]), or a set of discrete lifetimes (e.g., with POSWIN,[35] PALSfit3[36]). Since the C coefficient is generally unknown, FFV’s are preferably compared against a reference sample of the same composition.[40,41,42] The material composition (e.g., high-electron-density functional groups) can directly affect both the o-Ps lifetime and intensity through quenching (faster annihilation) and inhibition (reduced o-Ps formation probability), respectively.[43,44] It is recommended to verify if the results are physically meaningful and to cross-check them with complementary methods. The best-fitted value equals 0.166 nm.[12,20]
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