Revealing the details of how these structures formed, including their chemical makeup, was no simple task. The scientists used chemical-sensitive electron tomography, which is a nanoscale version of a CAT scan, to track what was happening structurally and chemically on the surface and inside the particles in 3D as they were oxidizing. This process occurs as the sample is heated to 500 degrees Celsius.
"We custom-designed a sample holder that could withstand that change in temperature, while also letting us tilt the sample to scan it from every angle-all within a transmission electron microscope," Xin said.
These capabilities are unique to the CFN, a DOE Office of Science User Facility that offers both state-of-the-art instruments and the expertise of scientists like Xin to the entire scientific community through its user program.
Xin's team tracked precisely where metal ions were reacting with oxygen to become metal oxides-and discovered that the process takes place in two stages.
"In the first stage, oxidation occurs only on the surface, with metal ions moving out of the particles to react with the oxygen forming an oxide shell," Xin said. "In the second stage, however, oxidation starts to happen on the inside of the particles as well, suggesting that oxygen moves in."
The scientists suspected that tiny pinholes were created on the particles' surface as the oxide shell was forming, providing a pathway for the influx of oxygen. A closer look at one partially oxidized particle confirmed this suspicion, showing that as the oxide formed on the surface, it beaded up like droplets on a water-repellent surface, leaving tiny spaces in between.
The scientists also used "electron energy loss spectroscopy" and the distinct "chemical fingerprints" of nickel and cobalt to track where the individual elements were located within the particles as the oxidation process progressed. This gave them another way to see whether oxygen was finding a way into the particles.
"We found that cobalt moves preferentially to where the oxygen is," Xin explained. "This is because cobalt reacts more easily with oxygen than nickel does."
During early oxidation, cobalt preferentially moved to the exterior of the particles to engage in the formation of the oxide shell. But later-stage scans revealed that the internal surfaces of the Swiss cheese pores were rich in cobalt as well.
"This supports our previous idea that oxygen is getting inside and pulling the cobalt out to the surface of the internal pores to react," Xin said.
This ability to monitor the surface chemistry of nanoparticles, both externally and along the internal curved surfaces of pores, could result in a more rational approach to catalyst design, Xin said.
"People usually try to just mix particles and create a better catalyst by trial and error. But what really matters is the surface structure. This imaging technology gives us an accurate way to determine the composition of naturally curved surfaces and interfaces to understand why one catalyst will perform better than another."