Researchers made a surprising discovery when they looked at treated coal under an electron microscope, which fires an electron beam at the point of interest. Apparently the beam is powerful enough to form microscopic diamonds.
The energy input congealed clusters of hydrogenated carbon atoms, some of which took on the lattice-like structure of nanodiamonds. They made the discovery while working on ways to chemically reduce carbon from anthracite coal and make it soluble.
“The beam is very powerful,” says Ed Billups, a chemist at Rice University. “To knock hydrogen atoms off of something takes a tremendous amount of energy.”
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 nanometers wide.
But the most “nano” of the nanodiamonds faded away under the power of the electron beam in a succession of images taken over 30 seconds.
Tiny, unstable diamonds
“The small diamonds are not stable and they revert to the starting material, the anthracite,” Billups says.
To explain what he saw, Billups turned to Rice theoretical physicist Boris Yakobson and his colleagues at the Technological Institute for Superhard and Novel Carbon Materials in. Yakobson, Pavel Sorokin and Alexander Kvashnin had already come up with a phase diagram that demonstrates how thin diamond films might be made without massive pressure.
They used similar calculations to show how nanodiamonds could form in treated anthracite and subbituminous coal. In this case, the electron microscope’s beam knocks hydrogen atoms loose from carbon layers. Then the dangling bonds compensate by connecting to an adjacent carbon layer, which is prompted to connect to the next layer. The reaction zips the atoms into a matrix characteristic of diamond until pressure forces the process to halt.
Natural, macroscale diamonds require extreme pressures and temperatures to form, but the phase diagram should be reconsidered for nanodiamonds, the researchers say.
Window of stability
“There is a window of stability for diamonds within the range of 19 to 52 angstroms (tenths of a nanometer), beyond which graphite is more stable,” Billups says. Stable nanodiamonds up to 20 nanometers in size can be formed in hydrogenated anthracite, though the smallest nanodiamonds were unstable under continued electron-beam radiation.
Subsequent electron-beam experiments with pristine anthracite formed no diamonds, while tests with less-robust infusions of hydrogen led to regions with “onion-like fringes” of graphitic carbon, but no fully formed diamonds. Both experiments lent support to the need for sufficient hydrogen to form nanodiamonds.
The research is published in Journal of Physical Chemistry Letters. The Robert A. Welch Foundation, the Ministry of Education and Science of the Russian Federation, and the Russian Foundation for Basic Research supported the research.
Source: Rice University