Cation exchange (CE) is known to be a powerful tool for the synthesis of heterogeneous nanocrystals and is currently divided into two main routes: ion solvation-driven CE reactions and thermally activated CE reactions. An international team of researchers has developed a new in-situ­ electrically-driven CE reaction to fabricate individual sulphide nanostructures inside a scanning tunnelling microscopic–transmission electron microscope (STM-TEM) system.

Cation exchange (CE) reactions occur in an ionic lattice when one interstitial cation is substituted for another. These reactions are a powerful method for the growth of heterogenous materials that are hard to obtain through direct synthetic techniques. Many principles determine the efficiency and reversibility of CE reactions, but the solubility product constant, and intervening activation barriers, are key to achieving a feasible and fast CE reaction.

CE reactions are traditionally divided into two type of reactions depending on the thermodynamic imbalance (TDI). These are ion solvation and thermally activated CE reactions. CE reactions through ion solvation cause a thermodynamic imbalance to be produced through selective binding of the liquid ligand environment (LLE) to a different set of ions. CE reactions in this instance reduce the activation barriers and can be easily tuned through the LLE and reaction temperatures, but the liquid environment is not suitable for individual nanostructures.

The second kind is through thermal activation, where the thermodynamic imbalance arises from the production of anionic lattice vacancies through evaporation methods. This is a solid-state approach that reduces the activation barriers as well as evaporating ions. It is a random and uncontrollable method that makes it difficult to selectively synthesise individual nanostructures, due to a lack of appropriate sources for the migrating ions.

Given the lack of available approaches, the researchers have developed an in-situ electrically driven process for the fabrication of sulphide-based core-shell nanostructures inside a scanning tunnelling microscopic–transmission electron microscope (STM-TEM) system.

The researchers created CdS nanowires as the building blocks through a solvothermal method and characterised them through transmission electron microscopy (TEM), powder X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS) and electron energy-loss spectroscopy (EELS).

The researchers used the CE process to transform the CdS nanowires into both Li/Li 2 S/CdS and Cu 2 S/CdS core-shell nanoparticles through migrating lithium and copper ions, respectively. The process is also driven through Ohmic heating where a sublimation of cadmium and the dissolution of the cationic electrode occurs.

The whole process was monitored at the atomic scale in real-time, where the researchers were able to control the whole CE process and could selectively fabricate individual nanocrystals by applying an electrical bias.

The electronically driven process is different to other types, as it uses a process of electromigration that is driven by an electronic wind force. The researcher attempted to replace the cadmium in the precursor nanowires with various other metals, and under certain conditions could be replaced with gold, silver copper and lithium.

Unlike other metals, upon insertion into the anionic sulphur-based lattice, both the copper and lithium ions made little to no deformation to the lattice, making them the most stable structures. A slight increase in the diameter was observed, due to the presence of the shell.

Although a similar result was observed for both, the reaction pathway was different for both nanostructures. The formation of the Cu 2 S/CdS was relatively simple and formed after exposure to the electrical bias. For the lithium core-shell nanocrystal, Li 2 S/CdS, the reaction resulted in a mixed heterostructure of Li 2 S and CdS with a thin amorphous lithium shell.

The difference between the two pathways was due to the ion migration rates and the bonding affinity between the incoming cations and the sulphur lattice. The copper ions diffused through the nanowire, which provided a faster transport path for the ions. The poor ionic diffusivity of lithium ions causes them to migrate along the nanowire and form a thin lithium shell on the nanowire surface, producing a mixed heterostructure.

Using this tuneable process, it is possible to create more complex single-entity nanostructures than the researchers have so far fabricated. Not only does this research offer a new perspective into selective CE reaction, towards single nanocrystal formation, but it also provides interesting insights into the microscopic mechanism of solid-state exchange processes.

References

"Electrically driven cation exchange for in situ fabrication of individual nanostructures" - Q. Zhang et al, Nature Comms, 2017, DOI:10.1038/ncomms14889

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