Rice University chemist Ed Billups and Yanqiu Sun, a former postdoctoral researcher in his lab, witnessed the interesting effect while working on ways to chemically reduce carbon from anthracite coal and make it soluble.
First they noticed nanodiamonds forming amid the amorphous, hydrogen-infused layers of graphite. It happened when they took close-ups of the coal with an electron microscope, which fires an electron beam at the point of interest.
Unexpectedly, the energy input congealed clusters of hydrogenated carbon atoms, some of which took on the lattice-like structure of nanodiamonds.
"The beam is very powerful. To knock hydrogen atoms off of something takes a tremendous amount of energy," Billups said.
Even without the kind of pressure needed to make macroscale diamonds, the energy knocked loose hydrogen atoms to prompt a chain reaction between layers of graphite in the coal that resulted in diamonds between 2 and 10 nanometres wide.
But the most "nano" of the nanodiamonds were seen to fade away under the power of the electron beam in a succession of images taken over 30 seconds.
"The small diamonds are not stable and they revert to the starting material, the anthracite," Billups said. The study appears in the American Chemical Society's Journal of Physical Chemistry Letters.