Journal of Applied Physics 110 073506 (2011)
We simulate the experimentally observed graphitization of nanodiamonds into multi-shell onion like carbon nano-structures, also called carbon onions, at different temperatures, using reactive force-fields. The simulations include long-range Coulomb and van der Waals interactions. Our results suggest that long-range interactions play a crucial role in the phase-stability and the graphitization process. Graphitization is both enthalpically and entropically driven and can hence be controlled with temperature. The outer layers of the nanodiamond have a lower kinetic barrier towards graphitization irrespective of the size of the nanodiamond and graphitize within a few-hundred picoseconds, with a large volume increase. The inner core of the nanodiamonds display a large size-dependent kinetic barrier, and graphitizes much more slowly with abrupt jumps in the internal energy. It eventually graphitizes by releasing pressure and expands once the outer shells have graphitized. The degree of transformation at a particular temperature is thereby determined by a delicate balance between the thermal-energy, long-range interactions and the entropic/enthalpic free-energy gained by graphitization. Upon full graphitization, a multi-shell carbon nano-structure appears, with a shell-shell spacing of about ∼3.4 ̊A for all sizes. The shells are highly defective with predominantly five and seven membered rings to curve space. Larger nanodiamonds with a diameter of 4 nm can graphitize into spiral structures with a large (∼29 atom carbon-ring) pore-opening on the outer-most shell. Such a large one-way channel is most attractive for a controlled insertion of molecules/ions such as Li-ion, water or ionic-liquids, for increased electrochemical capacitor or battery-electrode applications.
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