Research led by The University of Manchester has found that ions diffuse
10,000 times faster inside atomically thin clays than in bulk clay crystals.
Clays are used in a wide variety of membrane applications, so this result
offers the potential to achieve vastly improved desalination or fuel cell
performance simply by switching to ultra-thin clays when producing the
membranes.
Clays, like graphite, consist of crystal layers stacked on top of each other
and can be mechanically or chemically separated to produce ultra-thin
materials. The layers themselves are just a few atoms thick, while the space
between layers is molecularly narrow and contains ions. The interlayer ions
can be altered in a controllable way by allowing different ion species to
penetrate between the layers.
This property, known as ion exchange, allows for control of the physical
properties of these crystals in membrane applications. However, despite its
relevance in these emerging technologies, the ion exchange process in
atomically thin clays has remained largely unexplored.
Writing in Nature Materials, a team led by Professor Sarah Haigh and Dr.
Marcelo Lozada-Hidalgo shows that it is possible to take snapshots of ions
as they diffuse inside the interlayer space of clay crystals using scanning
transmission electron microscopy. This allows study of the ion exchange
process with atomic resolution. The researchers were excited to find that
ions diffuse exceptionally fast in atomically thin clays—10,000 times faster
than in bulk crystals.
Space to move
Complementary atomic force microscopy measurements showed that the fast
migration arises because the long-range (van der Waals) forces that bind
together the 2D clay layers are weaker than in their bulk counterparts,
which allows them to swell more; effectively the ions have more space so
move faster.
Unexpectedly, the researchers also found that by misaligning or twisting two
clay layers, they could control the arrangements of the substituted ions
within the interlayer space. The ions were observed to arrange in clusters
or islands, whose size depends on the twist angle between the layers. These
arrangements are known as 2D moire superlattices, but had not been observed
before for 2D ion lattices—only for twisted crystals without ions.
Dr. Yichao Zou, postdoctoral researcher and first author of the paper, said:
"Our work shows that clays and micas enable the fabrication of 2D metal ion
superlattices. This suggests the possibility of studying the optical and
electronic behavior of these new structures, which may have importance for
quantum technologies, where twisted lattices are being intensively
investigated."
New insights in diffusion
The researchers are also excited about the possibility of using clays and
other 2D materials to understand ion transport in low dimensions. Marcelo
Lozada-Hidalgo added: "Our observation that ion exchange can be accelerated
by four orders of magnitude in atomically thin clays demonstrates the
potential of 2D materials to control and enhance ion transport. This not
only provides fundamentally new insights into diffusion in
molecularly-narrow spaces, but suggests new strategies to design materials
for a wide range of applications."
The researchers also believe that their "snapshots" technique has much wider
application. Professor Haigh added: "Clays are really challenging to study
with atomic resolution in the electron microscope as they damage very
quickly. This work demonstrates that with a few tricks and a lot of patience
from a dedicated team of researchers, we can overcome these difficulties to
study ion diffusion at the atomic scale. We hope the methodology
demonstrated here will further allow for new insights into confined water
systems as well as in applications of clays as novel membrane materials."
Reference:
Yi-Chao Zou et al, Ion exchange in atomically thin clays and micas, Nature
Materials (2021).
DOI: 10.1038/s41563-021-01072-6
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