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.
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