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Knowledge-based approach for prediction of 2D coordination polymers exfoliation

Name
Alexandrov
Surname
Eugeny
Scientific organization
Samara Center for Theoretical Materials Science, Samara National Research University, Moskovskoye shosse 34, 443086 Samara, Russia
Academic degree
PhD
Position
Research fellow
Scientific discipline
Chemistry & Chemical technologies
Topic
Knowledge-based approach for prediction of 2D coordination polymers exfoliation
Abstract
We generalized the accumulated data for exfoliated 2D coordination polymers to reveal the main structural features leading to easy exfoliating materials, and to elaborate recommendations for using this technique. In our study, we used abilities of programs for crystallochemical analysis to find correlations between structural descriptors of bulk materials, conditions for exfoliation, and parameters of nanomaterials. Using knowledge-based approach, we selected six coordination polymers for experimental check of revealed recommendations.
Keywords
Knowledge-based approach, 2D coordination polymers, exfoliation, nanosheets
Summary

The idea of coordination polymers (CPs) nanosheets study originates from successful delamination of graphene [1] 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. [4]). 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” [5], the 2D coordination polymer [Zn(adc)] (adc= adamantane-1,3-dicarboxylate dianion) was exfoliated at first time in 2008 [6]. 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 [7]. In our study, we used abilities of program packages for crystallochemical analysis (ToposPro [8], Platon [9] 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:

  1. Swelling the porous structure by solvent and subsequent separation of solvated nanosheets.
  2. 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 [10] and subsequent exfoliation in water, ethanol, acetone, and THF. These bulk materials satisfied to the following recommended criteria:

  1. Flat layers with large interlayer distances or pores.
  2. Polar groups on the layer surface are able to contact with solvent molecules.
  3. Only weak van der Waals' interactions between layers.
  4. Coordination bonds are strong enough to stabilize nanosheets.

 

References

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[2] T.G. Mitina, V.A. Blatov, Cryst.Growth Des. 2013, 13, 1655.

[3] L. Carlucci, G. Ciani, D. M. Proserpio, T. G. Mitina, V. A. Blatov Chem. Rev. 2014, 114(15), 7557.

[4] Special Issue about Metal-Organic Frameworks Chem. Rev. 112(2), 673.

[5] J. Sakamoto, J. Heijst, O. Lukin, A. D. Schlüter Angew. Chem. Int. Ed. 2009, 48, 1030.

[6] R. B. Nielsen, K. O. Kongshaug, H. Fjellvag J. Mater. Chem. 2008, 18, 1002.

[7] D. Rodrıguez-San-Miguel, P. Amo-Ochoa, F. Zamora Chem. Commun. 2016, 52, 4113.

[8] V.A. Blatov, A.P. Shevchenko, D.M. Proserpio Cryst. Growth Des. 2014, 14, 3576; http://topospro.com.

[9] A.L. Spek J. Appl. Cryst. 2003, 36, 7.

[10] F.H. Allen Acta Crystallogr. 2002, B58, 380.