When a drop of the solution is dried, the still dissolving nanosheets connect into different tiled shapes. When left to fully dissolve, only single layer sheets are found.
When a drop of the solution is dried, the still dissolving nanosheets connect into different tiled shapes. When left to fully dissolve, only single layer sheets are found.

A simple strategy for creating solutions of two-dimensional nanomaterials could make large-scale production of future devices easier and cheaper, according to researchers from the UK and Switzerland [Cullen et al., Nature Chemistry (2016), doi: 10.1038/ nchem.2650].

Two-dimensional nanomaterials suspended in liquids could be used to print, assemble, or incorporate these materials into coatings, membranes, thin films, or composites. The usual approach to liquid exfoliation – as this process is known – is to use physical force to break apart layered nanomaterials and then add chemical groups, ligands, surfactants, or charge to functionalize their surface to prevent reaggregation.

Instead, Christopher A. Howard of University College London and his colleagues at the University of Bristol, Cambridge Graphene Centre, and École Polytechnique Fédérale de Lausanne are taking a different approach – one that enables layered materials to dissolve spontaneously in certain liquids without any need of force.

“Two-dimensional nanomaterials have outstanding properties and a unique size, which suggests they could be used in everything from computer displays to batteries to smart textiles, but many methods for making and applying two-dimensional nanomaterials are difficult to scale or can damage the material,” explains Howard.

The key to the researchers’ simple approach is the insertion of positively charged Li and K ions in between the layers of two-dimensional nanomaterials such as transition metal dichalocogenides (WS2, MoSe2, MoS2, and TiS2), metallic superconductors (FeSe), graphite, layered III–VI and V–VI semiconductors (GaTe, Bi2Te3, Sb2Te3), and transition metal oxides (V2O5). This creates alternating layers of negatively charged sheets and positively charged ions. When these layered material salts are added to specially selected solvents, they dissolve spontaneously – just like salt dissolves in water – to create an ionic solution.

“There are previous examples of thermodynamically-driven, liquid-based delamination of layered materials,” says Howard. “But the technologically important materials we wanted to exfoliate are naturally charge neutral. So we had to add charge to the layers in a way that would preserve the pristine structure of the two-dimensional layers, which is not trivial.”

The new approach preserves the morphology of the original nanomaterial, which is undamaged by the process, and protects against subsequent reaggregation. In an inert environment, the nanomaterial solutions are stable but rapidly precipitate in air.

Nicholas A. Kotov of the University of Michigan believes the work represents a conceptual step forward in the chemistry and physics of colloids.

“The authors convincingly justify what many of us suspected already – that dispersions of nanoscale materials can be described in terms of true solutions” he says.

The researchers believe that their simple approach will help realize the potential of two-dimensional nanomaterials in the future.

“The fact that they form a liquid, along with their negative charge, makes them easy to manipulate and use on a large scale, which is scientifically intriguing but also relevant to many industries,” explains first author of the study Patrick L. Cullen. “We’ve shown they can be painted onto surfaces and, when left to dry, arrange themselves into different tiled shapes [and] they can also be electroplated onto surfaces.”

A key advantage of the process is that it is intrinsically scalable – greater yields can be achieved by simply increasing the amount of starting material and using larger vessels.

“Our process is also unique in that the solutions contain only the most highly prized monolayers and, because they do not flocculate with time, have a very long shelf life,” adds Howard.

The researchers believe the approach holds numerous possibilities from applications in protective coatings and surface deposition techniques to enabling the formation of materials that have not been exfoliated into single layers before.

This article was originally published in Nano Today (2016), doi: 10.1016/j.nantod.2016.12.008