The surface barrier phenomenon at the loading of metal-organic frameworks

Abstract

Loading with guest molecules is a crucial step for most applications of porous materials. For metal-organic frameworks, which are one of the most intensely investigated classes of porous materials, the experimentally determined rate of mass transfer into the material may vary by several orders of magnitude for different samples of the same material. This phenomenon is commonly attributed to the presence of so-called surface barriers, which appear to be omnipresent but poorly understood. Here we quantitatively study this phenomenon with a quartz crystal microbalance, using well-defined, highly crystalline, epitaxially grown thin films of metal-organic frameworks as a model system. Our results clearly demonstrate that surface barriers are not an intrinsic feature of metal-organic frameworks, as pristine films do not exhibit these limitations. However, by destroying the structure at the outer surface, for instance by exposure to air or water vapour, surface barriers are created and the molecular uptake rate is reduced.

Langmuir–Blodgett Films of the Metal–Organic Framework MIL-101(Cr): Preparation, Characterization, and CO2 Adsorption Study Using a QCM-Based Setup

“…14 QCM substrates coated with SURMOFs are very interesting systems for the development of chemical sensors and also for studying and characterizing fundamental properties of MOFs such as mass transfer processes. 15 …”

SURMOFs: Liquid-Phase Epitaxy of Metal-Organic Frameworks on Surfaces

“…When pristine HKUST-1 SURMOF samples are exposed to water vapor, for 10 or 30 s, the uptake process is strikingly longer, even though any residual water is removed by activation prior the uptake measurements. XRD patterns for such samples do also not reveal any significant change of the crystalline structure [58]. Therefore, the surface barriers are explained not as a result of water molecules adsorbed at the interface, but as a collapse of the crystal structure at the external surface due to corrosion with water.…”

“…The influence of the exposure to water or other contaminants on the kinetics can be investigated. Experiments with cyclohexane revealed that the uptake time in SURMOFs which are fabricated and activated in fully dry conditions is a quadratic function of the SURMOF thickness [58] (Figure 17.8). This is a clear indication of the diffusion limitation of the mass transfer.…”

“…All three pristine SURMOF samples have a thickness of roughly 700 nm. (Reprinted from[58]. Copyright 2014, with permission from Nature Publishing Group.…”

Thermal cis-to-trans Isomerization of Azobenzene Side Groups in Metal-Organic Frameworks investigated by Localized Surface Plasmon Resonance Spectroscopy

“…The unique properties of SURMOFs are briefly reviewed here: the layer-bylayer MOF film growth results in a precise control of the thickness, see Figure 1a [15]. Controlled lateral patterning, e.g.…”