TEM turns back time
Image: Hydrogen-induced reduction of iron oxide nanostructure bicrystals [W Zhu et al/ACS Nano and K Irvine/NIST]
Using an environmental TEM, researchers at the National Institute of Standards and Technology, and colleagues, have imaged slow-motion, atomic-scale transformation of iron oxide to pure iron metal.
Working at NIST’s NanoLab facility, the researchers documented the step-by-step transformation of nanocrystals of hematite (Fe2O3) to magnetite (Fe3O4), and finally to iron metal.
“Even though people have studied iron oxide for many years, there have been no dynamic studies at the atomic scale,” highlights Wenhui Zhu, now at the State University of New York at Binghamton. “We are seeing what’s actually happening during the entire reduction process instead of studying just the initial steps.”
By lowering the temperature of the reaction and decreasing the pressure of the hydrogen gas that acted as the reducing agent, the scientists slowed down the reduction process so that it could be captured with their E-TEM.
“This is the most powerful tool I’ve used in my research and one of the very few in the United States,” says Zhu.
The researchers examined the reduction process in a bicrystal of iron oxide, consisting of two identical iron oxide crystals rotated at 21.8 degrees with respect to each other.
The bicrystal structure also served to slow down the reduction process, making it easier to follow with the E-TEM.
a) Colourised SEM images of iron oxide nanoblades used in the experiment. b) Colourised cross-section of SEM image of the nanoblades. c) Colourised SEM image of nanoblades after 1 hour of reduction reaction at 500 °C in molecular hydrogen, showing the sawtooth shape along the edges (square). d) Colourised SEM image showing the formation of holes after 2 hours of reduction. The scale bar is 1 micron. [W Zhu et al/ACS Nano and K Irvine/NIST]
In studying the reduction reaction, the researchers identified a previously unknown intermediate state in the transformation from magnetite to hematite.
In the middle stage, the iron oxide retained its original chemical structure, Fe2O3, but changed the crystallographic arrangement of its atoms from rhombohedral (a diagonally stretched cube) to cubic.
This intermediate state featured a defect in which oxygen atoms fail to populate some of the sites in the crystal that they normally would.
This so-called oxygen vacancy defect is not uncommon and is known to strongly influence the electrical and catalytic properties of oxides.
But the researchers were surprised to find that the defects occurred in an ordered pattern, which had never been found before in the reduction of Fe2O3 to Fe3O4.
By manipulating the microstructure, researchers may be able to enhance the catalytic activity of iron oxides.
“The more we understand, the better we can manipulate the microstructure of these oxides,” says Zhu.
Research is published in ACS Nano.