Often, model systems can provide insight that would not otherwise be observable into the workings of actual systems. For metal-organic frameworks (MOFs), this can constitute creation of thin film systems that offer nearly 2D analogs to the particle systems that many of us deal with. In their recent paper, Heinke et al. (Nat. Commun., DOI: 10.1038/ncomms5562) take this one step further, constructing a film of HKUST-1 in situ on a quartz crystal microbalance without exposure to atmospheric conditions, creating a pristine surface MOF (“SURMOF”). By varying the thickness of the SURMOF, they are able to show that mass transfer is essentially limited by intracrystalline diffusion – that is, there are no external barriers to mass transfer. However, the rate of mass uptake was significantly decreased after exposure to humid air or water vapor without any change in bulk crystallinity, indicating that exposure to these conditions primarily affected the outer surface of the crystal.
This suggests the formation of surface defects which are certainly well known to the EFRC. The paper doesn’t offer suggestions for what the nature of the surface defects might be, but instead only suggests that the defects are created upon exposure to water vapor that would be present under ambient processing conditions. Importantly, these defects are not necessarily an intrinsic property of MOFs, offering the possibility that modification prior to exposure to water vapor could help prevent these defects.
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