Knowledge-based approach for prediction of 2D coordination polymers exfoliation
The idea of coordination polymers (CPs) nanosheets study originates from successful delamination of graphene  and other numerous layered inorganic materials. 2D CPs nanosheets have advantage in comparison to this materials due to a variability of composition (almost infinite combinations of metals and ligands), structures (more than 2000 topological types and 20000 structures [2, 3]), and promising properties (electric, magnetic, optical, sieving, catalytic, drug delivery etc. ). Despite the fact, in one of the first reviews about 2D polymers nanosheets the coordination polymers were considered as “to be unlikely that they will be separable from one another” , the 2D coordination polymer [Zn(adc)] (adc= adamantane-1,3-dicarboxylate dianion) was exfoliated at first time in 2008 . In general, there were 18 publications about exfoliation of 33 2D CPs, which discussed in 11 reviews. However, these works concern only properties of obtained nanomaterials without formulating any generalized concept or strategy for selecting bulk materials and conditions for exfoliation. All experiments were based on serendipity of scientists.
We tried to generalize the accumulated data for exfoliated 2D CPs materials. The special attention was paid to crystallochemical characteristics of the structures with goal to reveal the main principles for finding materials and appropriate exfoliation techniques. Thus, solvent selection is not trivial. Indeed, it should correlate to structural characteristics of sample. The adjustment of sonication parameters (power and time) has influence in thickness and lateral dimension of the nanosheets. However, the main problem is to find the appropriate conditions to get either single layers or few layers, but with large enough lateral dimensions in comparison to the thickness . In our study, we used abilities of program packages for crystallochemical analysis (ToposPro , Platon  etc.) to find relations between structural descriptors of bulk materials (composition, structural units, single layer topology, charge, thickness, geometry, intra- and interlayer interactions, interlayer distance, porosity) and exfoliation parameters of nanomaterials (stability, solvent, mechanical treatment, dimensions and shape of nanoparticles).
It was found, that the structures containing polar structural groups (-CO, -SO, -PO, -NH, -OH, -Cl) on the surface can be easily exfoliated in polar solvents (H2O, EtOH), and more hydrophobic solvents (DMF, toluene) are preferable for layers with less polar surface (-CH). The size of solvent molecules plays decisive role for exfoliation: small molecules more frequently and successfully leads to exfoliation. Strong interlayer interactions prevent exfoliation with simple solvent technique, the use of surfactant is necessary. Porous structures containing intercalated solvent molecules can be exfoliated with more probability.
In general, two hypothetical models for exfoliation mechanism were proposed:
- Swelling the porous structure by solvent and subsequent separation of solvated nanosheets.
- Delamination of nanosheets starting from breaking of interlayer interactions from crystal faces and diffusion of solvents into interlayer space.
Revealed correlations were used for selection of six 2D CPs from CSD  and subsequent exfoliation in water, ethanol, acetone, and THF. These bulk materials satisfied to the following recommended criteria:
- Flat layers with large interlayer distances or pores.
- Polar groups on the layer surface are able to contact with solvent molecules.
- Only weak van der Waals' interactions between layers.
- Coordination bonds are strong enough to stabilize nanosheets.
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