A
field of converted nanocomposites, their shape formed with self-assembly and
their composition tuned with conversion reactions. Credit: AMOLF.
Imagine if
a material would arrange itself into a shape suited for its application. It may
result in a catalyst that maximizes its own surface area for improved
efficiency or a micro-actuator that forms appendages to grab nearby objects.
This is the promise that self-assembly holds: making complex, functional
materials by letting matter shape itself. Yet, not all matter that
self-assembles into interesting forms turns out to have a useful function in
its final shape. Researchers of the Self-Organizing Matter group recently
discovered that ion exchange allows them to separate the self-assembly process
from the resulting material. Their findings were published in Advanced
Materials on November 16th and highlighted in Nature and Nature Reviews
Materials.
With their
beautiful and intricate shapes the nanocomposites studied by the
Self-Organizing Matter group look quite remarkable (see illustration). Yet, PhD
students Hans Hendrikse and Arno van der Weijden wanted more than beautiful
structures and had an itch to also utilize the functionality of the
nanocomposites. Encouraged by the shapeability and structural layout of their
nanocomposites, they started investigating the options together with
researchers from the University of Amsterdam, ARNCL, Leiden University and
Virginia Tech.
The
research team started off with nanocomposites that consisted of barium
carbonate (BaCO3) nanocrystals embedded in a silica (SiO2) matrix and converted
these to cadmium sulphide (CdS). First, they established a route to
reproducibly convert the nanocomposites to this final material, while
investigating the properties of the nanocomposites during ion exchange. Through
analysis with electron microscopy and x-ray diffraction the team learned
something fascinating: the small size of the BaCO3 nanocrystals made them
exceptionally susceptible to ion exchange reactions, while the surrounding SiO2
matrix provided mechanical stability to maintain the original nanocomposite’s
shape during conversion. Hans Hendrikse says, “it is almost like we are
changing out some of the bricks of a house while keeping the overall structure
intact.”
Based on
these insights, expanding the selection of materials was straightforward and
new routes were developed to change the nanocomposite’s composition to various
cadmium, iron, nickel and manganese salts. Moreover, the original nanocomposite
can be shaped in a large selection of pre-determined shapes. All these shapes
can be converted to any of the above mentioned compositions. So not only is it
possible to convert nanocomposites, there is also a variety of materials and
shapes to interchangeably choose from.
Finally,
the team explored the potential applications of this new approach. For
instance, they discovered that the nickel-containing nanocomposites can be used
as catalysts for the dry-reforming process, which outperforms traditional
catalysts at low temperatures. Furthermore, the team synthesized shape-controlled
magnetite (Fe3O4) nanocomposites that can be moved and reorientated using their
magnetic properties. Finally, they created e-beam activated microscopic
actuators by utilizing flexibility introduced during one of the ion exchange
reactions in conjunction with the shrinking properties of the silica matrix. In
short, they discovered shape-preserving ion-exchange reactions that open up new
routes towards self-assembled materials with various novel, functional
properties.