Atomically thin clays and micas offer super speedy ion exchange

Fig. 1. Top panel: a schematic atomic model showing the ion exchange process, where the yellows balls denote the ions from electrolyte (Cs+ in this case) and the green balls denotes the pristine ions (K+) in a biotite crystal.

Bottom panel: an experimental cross-sectional ADF-STEM image showing the ‘snap shot’ taken during the Cs+ exchange process in a biotite crystal.

Clays and micas are stacks of crystal layers a few atoms thick, with ions such as K+ or Mg2+ in the interlayer space. Ion exchange allows different ions to penetrate the interlayer space in a controllable way (Fig. 1).  

It is possible to separate the crystal layers - mechanically or chemically - to create ultra-thin materials. These 2D materials can then be layered into laminate membranes and composites in a similar way to graphene oxide. In these fabricated materials, the crystal surface charge controls water and ion transport through the interlayer channels. Advances in fabrication methods mean we can now produce model structures with precise control of the number and composition of layers and the stacking geometry.  

However, although this offers great potential for applications such as osmotic power generation, ion selective transport or solvent filtration, ion exchange in these atomically thin clays and micas is not well understood. 

A team led by Professor Sarah Haigh and Dr Marcelo Lozada-Hidalgo used advanced aberration-corrected scanning transmission electron microscopy (STEM) to investigate the dynamics of ion exchange in atomically thin (Fig. 1) and artificially restacked clays and micas (Fig. 2). The advantage of using STEM over more common techniques such as atomic force microscopy or X-ray measurements is that it reveals the local binding environment of exchanged ions with sub-ångström resolution. Using STEM to observe ion exchange processes in atomically thin layered and restacked clays allowed the team to see substantially larger ion diffusion constants and moiré effects on ion dynamics. 

The team chose to use caesium (Cs+) for their ion exchange experiments for two reasons. Firstly, caesium's large atomic number makes it easily visible in annular dark field (ADF) STEM images. And an improved understanding of the diffusion of radiocaesium (caesium-137) through clays and micas will inform environmental clean-up after nuclear contamination.    

Professor Sarah Haigh, explains:

The team found that the ion diffusion coefficient for the interlayer space of atomically thin samples was up to 104 times larger than in bulk crystals, allowing ions to diffuse 10,000 times faster. Using complementary atomic force microscopy measurements, they showed that this effect likely arises because the long-range (van der Waals) forces that bind the 2D clay layers are weaker in thin layers. This allows more swelling, effectively giving the ions more space to move quickly. 

Fig. 2. An experimental plan-view ADF-STEM image showing Cs+ (bright dots) islands observed from a Cs+ exchanged twisted bilayer biotite.

The research team found, unexpectedly, that when they added a rotational offset or misalignment between layers, the caesium ions arranged themselves in clusters (or islands), the size of which depends on the angle between the layers. The presence of these moiré superlattices will allow us to use stacked 2D materials to create unique nanoscale interlayer environments that can accommodate ions or molecules. 

The technique the team developed to investigate these clays and micas is likely to have much broader application. The team are now working to develop it from taking "snapshots" to producing "movies".

Prof Haigh, added:

The electron Physical Science Imaging Centre (ePSIC) is a national facility for aberration-corrected electron microscopy. To find out more about ePSIC or discuss potential applications, please contact Principal Electron Microscopist Chris Allen: [email protected].