Chemistry controlled on tiniest scale can create hollow nanoparticles

Blame oxidation for rusted bridges and browned avocados. But this fundamental process can be harnessed for good, too — and now scientists have scored front-row seats that could show them how.

Researchers watched at near-atomic resolution as iron nanoparticles transformed into iron oxide — not rust in this case, but related compounds. That closeup view could help scientists better control oxidation and design corrosion-resistant materials or new kinds of catalysts, the researchers report in the April 21 Science.
This is the first time the oxidation process has been observed in such detail, says Andreu Cabot, a physicist at the Catalonia Institute for Energy Research in Barcelona who wasn’t part of the study.

When a metal oxidizes, its atoms mix and mingle with oxygen atoms to create a new material. That process is perhaps most famous for creating rust, which flakes and corrodes. But iron can oxidize in a variety of ways, some of which are useful.

For instance, chemist Yugang Sun and his colleagues at Temple University in Philadelphia are trying to create hollow iron oxide nanoparticles that could serve as catalysts to speed up chemical reactions or as vessels to deliver drugs or store energy in chemical form. But making these “nanoshells” from iron nanoparticles requires precise control over the oxidation process.

If oxygen atoms work their way into an iron nanoparticle faster than the iron atoms can diffuse out, that nanoparticle becomes a tight, solid ball, Sun says. If the iron diffuses out faster than the oxygen comes in, on the other hand, it becomes the hollow sphere that Sun’s lab wants.

Controlling that process is difficult because it has been unclear exactly how these shells form on an atomic level, Sun says. Scientists haven’t been able to watch it happen, because high-powered microscopy techniques can disrupt the reaction or show the action in only two dimensions.
Sun’s team tried a different approach to observe the reaction, by shooting X-rays at many identical iron nanoparticles suspended in a liquid. Each time the X-rays hit a different material — moving from the liquid to the solid, for instance — they scattered.
By tracking how the X-rays bounced off many small, uniform iron nanoparticles, the researchers were able to reconstruct where individual atoms were going as the particles oxidized into hollow shells over the course of several hours.

The researchers watched as the iron moved out of the center of the nanoparticle to react with the oxygen, initially forming many small holes inside the nanoparticle. Eventually, those empty spaces merged together to form one big void in the middle of the nanoparticle.

“The impact of this paper is more than just the hollow [nanoparticles],” says Yadong Yin, a chemist at the University of California, Riverside who wasn’t involved in the research. The imaging technique itself will be a useful way to study how other types of nanoparticles form — something scientists still don’t understand well, he says. It can be used to gain insight into other types of oxidation, too.