Scientists Mix the Unmixable to Create 'Shocking' Nanoparticles

Making a giant leap in the 'tiny' field of nanoscience, a multi-institutional team of researchers is the first to create nanoscale particles composed of up to eight distinct elements generally known to be incapable of being mixed together. The blending of multiple, generally unmixable elements into a unified, homogenous nanostructure, called a high entropy alloy nanoparticle, greatly expands the landscape of nanomaterials, and what we can do with them.

This research trumps previous efforts that have typically produced nanoparticles limited to only three different elements and to structures that do not mix evenly. Indeed, it’s extremely difficult to squeeze and blend different elements into individual particles at the nanoscale.

The collaborative team of engineers – including UMD ChBE professor Michael Zachariah and Zachariah Group graduate students, Rohit Jacob and Miles Rehwoldt – responsible for this study published a peer-reviewed paper based on the research featured on the March 30 cover of Science.

"Imagine the elements that combine to make nanoparticles as Lego building blocks. If you have only one to three colors and sizes, then you are limited by what combinations you can use and what structures you can assemble," said Liangbing Hu (PI), associate professor of Materials Science and Engineering (MSE) at UMD and one of the corresponding authors of the paper. "What our team has done is essentially enlarged the toy chest in nanoparticle synthesis; now, we are able to build nanomaterials with nearly all metallic and semiconductor elements."

According to the research team, this advance in nanoscience opens vast opportunities for a wide range of applications that includes catalysis (the acceleration of a chemical reaction by a catalyst), energy storage (batteries or supercapacitors), and bio/plasmonic imaging, among others.

To create the high entropy alloy nanoparticles, the researchers employed a two-step method of flash heating followed by flash cooling. Metallic elements such as platinum, nickel, iron, cobalt, gold, copper, and others were exposed to a rapid thermal shock of approximately 3,000 degrees Fahrenheit, or about half the temperature of the sun, for 0.055 seconds. The extremely high temperature resulted in uniform mixtures of the multiple elements. The subsequent rapid cooling (more than 100,000 degrees Fahrenheit per second) stabilized the newly mixed elements into the uniform nanomaterial.

To demonstrate one potential use of the nanoparticles, the research team used them as advanced catalysts for ammonia oxidation, which is a key step in the production of nitric acid (a liquid acid that is used in the production of ammonium nitrate for fertilizers, making plastics, and in the manufacturing of dyes). They were able to achieve 100 percent oxidation of ammonia and 99 percent selectivity toward desired products with the high entropy alloy nanoparticles, proving their ability as highly efficient catalysts.

Yonggang Yao, a UMD graduate student and first author on the paper, says another potential use of the nanoparticles as catalysts could be the generation of chemicals or fuels from carbon dioxide.

"The potential applications for high entropy alloy nanoparticles are not limited to the field of catalysis. With cross-discipline curiosity, the demonstrated applications of these particles will become even more widespread," says Steven Lacey, a Ph.D. student at UMD and a lead author of the paper.

This research was performed through a multi-institutional collaboration of ChBE Prof. Michael Zachariah's group at UMD; MSE Prof. Liangbing Hu's group(UMD); Prof. Reza Shahbazian-Yassar's group at the University of Illinois at Chicago; Prof. Ju Li's group at MIT, and Prof. Chao Wang's group at Johns Hopkins University.

What outside experts say about this research:

"This discovery opens many new directions. There are simulation opportunities to understand the electronic structure of the various compositions and phases that are important for the next generation of catalyst design. Also, finding correlations among synthesis routes, composition, and phase structure and performance enables a paradigm shift toward guided synthesis," says George Crabtree, Argonne Distinguished Fellow and director of the Joint Center for Energy Storage Research at Argonne National Laboratory.

More from the research coauthors:

"Understanding the atomic order and crystalline structure in these multi-element nanoparticles reveals how the synthesis can be tuned to optimize their performance. It would be quite interesting to further explore the underlying atomistic mechanisms of the nucleation and growth of high entropy alloy nanoparticle," says Reza Shahbazian-Yassar, associate professor at the University of Illinois at Chicago and a corresponding author of the paper.

"Carbon metabolism drives 'living' metal catalysts that frequently move around, split, or merge, resulting in a nanoparticle size distribution that’s far from the ordinary, and highly tunable," says Ju Li, professor at the Massachusetts Institute of Technology and a corresponding author of the paper.

Published April 20, 2018