Ordering disordered materials

When we look around the world, we see order and symmetry. It’s evident in snowflakes, flowers, and beehives, just to name a few. Going beyond what the plain eye can see, we also know that several chemical structures consist of ordered atoms. For example, think of sodium chloride (more plainly known as table salt). Its patterned structure consists of alternating atoms of sodium and chlorine. Nature seems to have a good handle on producing materials which are ordered and symmetric, but as humans, how do we control the order in the materials we make?

Materials can undergo a disorder-order transformation. A disorder-order transformation involves a rearrangement of the atoms in a crystal to form a different symmetry. The Skrabalak Lab in the Chemistry Department at Indiana University works to create new methods for making materials. One method which they have used to successfully make ordered materials is “seed-mediated co-reduction.” In this example, they started with nanoparticles where copper and palladium atoms are randomly distributed. These nanoparticles are then heated in a solution containing the elements from which the nanoparticles are made of. As the solution is heated to 270 °C over the course of half an hour, these nanoparticles nearly double in size and transform into a chemically ordered material.

This route is more advantageous than the traditional annealing approach, which involves heating the small nanoparticles up to 650 °C for several hours. Not only does annealing take more time and energy but it also can cause particles to coalesce creating un-uniform particles in many cases.

Now you may be wondering, does order really matter? One of the palladium-copper materials of interest to the Skrabalak Group has the potential to replace platinum-based catalysts–which are very costly–in fuel cells. In an experiment, the catalytic activity of palladium, disordered palladium-copper, and ordered palladium-copper nanoparticles were compared for the oxygen reduction reaction. The oxygen reduction reaction has important applications in fuel cells for energy conversion. From their studies, they found that the ordered palladium-copper catalyst outperformed the other two [1]. In fact, its activity was more than double than that of the disordered catalyst! After further testing, it was determined that the disordered material suffered aggregation and deformation during the reaction while the ordered material only suffered minor aggregation. The disordered material was, consequently, deemed to be less durable and stable than its ordered counterpart. Thus, the order of the material was found to be directly related its ability to catalyze the oxygen reduction reaction.

Humankind receives constant inspiration from nature to expand technology. For instance, Velcro was inspired by burdock burrs, strong adhesive tape by the adhesive of gecko’s feet, and solar power by photosynthesis. The use of the “seed-mediated co-reduction” method for creating ordered materials allows us to further mimic the ordered materials that already exist in nature. These materials then have the potential to help us address some of the many problems facing the world today.

Edited by Lana Ruck and Noah Zarr

[1] C. Wang, D.P. Chen, X. Sang, R.R. Unocic, S.E. Skrabalak, Size-Dependent Disorder–Order Transformation in the Synthesis of Monodisperse Intermetallic PdCu Nanocatalysts, ACS Nano 10(6) (2016) 6345-6353.

[2] Courtesy: National Science Foundation

[3] FreeImages.com/- –

[4]FreeImages.com/YiannisPapadimitriou