New insight in the impact of the crystal size on the flexibility of nanostructured materials can aid the development of ideal nanosensors.
"Crystals are like people, it is the defects in them which tend to make them interesting!" The color and corresponding price of diamonds, the performance of semiconductor devices, or even the mechanical stability of metals. In each of these almost perfectly crystalline materials, material defects and other types of spatial disorder play a crucial role in the resulting material properties. Although this has been acknowledged and consciously applied for a long time—the above quote of Sir Colin Humphreys dates back to 1979—for nanomaterials the true impact of spatial disorder on their macroscopic behavior is in many cases still poorly understood.
For instance, flexible metal-organic frameworks (MOFs)—synthetic nanoporous crystals composed of both organic and inorganic building blocks—become less flexible the smaller the crystal. New research from the Center for Molecular Modeling (CMM) at Ghent University, published in Nature Communications, now demonstrates that this phenomenon originates from a newly identified type of spatial disorder in these materials.
Finding the disordered needle in the crystalline haystack
In general, material defects and spatial disorder only impact a very limited region in the otherwise crystalline material. As a result, it is not straightforward to clearly visualize this disorder. This challenge becomes even more formidable if the spatial disorder can also dynamically propagate through the material. Experimentalists then need high resolution techniques to reveal the instantaneous nanostructure of the material. For computational researches, the opposite challenge arises. As they often work with smaller and completely ordered models in their simulations, they need to develop new models that are sufficiently large to also capture spatial disorder.
Because of these challenges to clearly visualize dynamic spatial disorder in nanomaterials, it remained unclear until now which factors influence the flexible behavior of MOFs. Flexible MOFs form a class of materials that exhibit multiple stable states. Each of these phases can exhibit different material properties, just like defects alter the material properties of diamonds, semiconductors, and metals. Depending on a MOF’s phase, the material can adsorb more or fewer gas molecules, change its color, or become a better conductor. Flexible MOFs are attractive nanosensor materials as the phase of the material can be changed by altering the pressure or the temperature, or by forcing the material to adsorb molecules.
Although researchers already succeeded in visualizing the nanostructure of the different phases of a flexible MOF, given that these phases typically show crystalline order, it remained a mystery how flexible MOFs transition between these crystalline phases. Originally, the idea was that the structure of the material remained crystalline also during the phase transition, such that the phase transition occurs cooperatively throughout the material. However, this idea was indirectly contradicted by recent experimental results that demonstrated that smaller MOF crystals are less flexible than bigger ones.
From cooperative crystals to chaotic materials
To answer this apparent contradiction, researchers at the CMM systematically increased the crystal size of different flexible MOFs in their simulations. They observed that for sufficiently large MOF crystals, with a critical size above 10 nm, phase transitions no longer occur cooperatively. Instead, their simulations demonstrated that it becomes energetically more favorable for the material not to remain crystalline during the transition, but rather to show spatial disorder by allowing two phases to coexist simultaneously in the material. This phase coexistence results in local defects at the interfaces between the two phases (see red areas in the figure).
This work not only uncovers the mechanism behind the phase transition, but also explains why these phase transitions are hard to observe experimentally. The simulations indicate that the material defects that accompany the phase transition are very dynamic, which makes them very difficult to characterize experimentally. Therefore, this study also proposed different pathways that could be used to experimentally observe the spatial disorder demonstrated in this work. The study moreover demonstrated that it becomes energetically less favorable for smaller crystals to tolerate material defects. As a result, these smaller MOF crystals are also less flexible, in accordance with the recent experimental observations. These results now allow to consciously look for spatial disorder in flexible MOFs and to exploit this disorder to develop high-performing nanosensors.
These results were published in Nature Communications:
Unraveling the thermodynamic criteria for size-dependent spontaneous phase separation in soft porous crystals
Sven M.J. Rogge, Michel Waroquier, and Veronique Van Speybroeck
Nature Communications, 10: 4842, 2019. DOI: 10.1038/s41467-019-12754-w
dr. ir. Sven M. J. Rogge, prof. em. dr. Michel Waroquier, prof. dr. ir. Veronique Van Speybroeck
Center for Molecular Modeling
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