Credit: University of Manchester
The University of Manchester's research has shown that ions can diffuse 10,000 times faster in atomically thin clays, than in bulk clay crystals. Clays are used in many different membrane applications. This result suggests that ultra-thin clays could be used to produce membranes with greatly improved performance.
Clays are made up of layers of crystals that have been stacked on top each other. They can be separated chemically or mechanically to make ultra-thin materials. While the layers are only a few atoms thick each, the space between them is molecularly narrower and contains ions. Different ion species can penetrate between layers to alter the interlayer ions.
This property is known as ion-exchange and allows for the control of the physical properties these crystals in membrane applications. Despite its importance in these emerging technologies however, the ion-exchange process in atomically fine clays remains largely unknown.
A team of researchers led by Dr. Marcelo Lozada Hidalgo and Professor Sarah Haigh published in Nature Materials that they were able to capture snapshots of ions inside clay crystals with scanning transmission electron microscopy. This allows for atomic resolution study of the ion-exchange process. Researchers were thrilled to discover that ions diffuse extremely fast in atomically thin ceramics, which is 10,000 times faster than bulk crystals.
There is space to move
Complementary atomic force microscope measurements revealed that fast migration is caused by the van der Waals forces (long-range) that bind the 2D clay layers. This allows them to swell further, effectively the ions have greater space and move faster.
The researchers discovered that they could manipulate the interlayer space of substitute ions by twisting or misaligning clay layers. The size of the ions was determined by the angle between layers. These arrangements are called 2D moire superlattices. However, they have not been seen before in 2D ion lattices.
Dr. Yichao Zou is a postdoctoral researcher who was the first author of this paper. This opens up the possibility of studying electronic and optical behavior of these new structures. This could be important for quantum technologies where twisted lattices have been intensively studied.
New insights in diffusion
Researchers are excited by the possibility of using clays as well as other 2D materials for understanding ion transport in low dimension. Marcelo Lozada Hidalgo said: "Our observation of ion exchange being accelerated by four orders-of-magnitude in atomically thin ceramics is a demonstration of the potential for 2D materials to control or enhance ion transportation." This provides fundamentally new insights on diffusion in molecularly narrow spaces and suggests new strategies for designing materials for wide-ranging applications.
Researchers also believe their "snapshots” technique is more widely applicable. Professor Haigh said that clays are very difficult to study using atomic resolution in an electron microscope because they can damage very quickly. These difficulties can be overcome with some tricks and patience by a dedicated group of researchers. The methodology presented here should provide new insights into confined water systems and applications of clays to create novel membrane materials.
Learn more about how ions get their electrons back
Information: Yi-Chao Zou and colleagues, Ion exchanges in atomically thin micas and clays, Nature Materials (2021). Information from Nature Materials Yi-Chao Zou et al., Ion exchanges in atomically fine clays and micas. (2021). DOI: 10.1038/s41563-021-01072-6
