Theoretical investigation on carbon nucleation on nickel carbides at initial stages of single-walled carbon nanotube formation

It is a long-standing controversy whether metal carbide clusters do exist during the nucleation and growth process of single-walled carbon nanotubes (SWCNTs). In the current work, we are trying to elucidate the carbon nucleation on nickel carbides during the initial stages based on density functional theory calculated formation energy and chemical potential for a series of Ni₅₅C_n carbides (n is the number of carbon atoms dissolved in the Ni₅₅ cluster). It is found that the formation energies of the Ni₅₅C_n carbides decrease gradually with an increase of dissolved carbon atomic numbers, meaning the Ni₅₅C_n carbides are thermodynamically stable. Meanwhile, the calculated chemical potentials indicate that not only nickel carbides are preferentially formed during the initial stage of the SWCNT nucleation, but also saturated nickel carbides may be able to exist during the nucleation and growth process of SWCNTs. In addition, the nickel carbides have a high selectivity for the formation of the carbon pentagon and carbon structures with pentagon-incorporated end-edge according to the adsorption energies. All of these findings provide opportunities in controlling the growth of the SWCNTs.