Motivated by recent in situ studies of carbon nanotube growth from large
transition-metal nanoparticles, we study various alpha-iron (ferrite) facets at
different carbon concentrations using ab initio methods. The studied (110),
(100) and (111) facets show qualitatively different behaviour when carbon
concentration changes. In particular, adsorbed carbon atoms repel each other on
the (110) facet, resulting in carbon dimer and graphitic material formation.
Carbon on the (100) facet forms stable structures at concentrations of about
0.5 monolayer and at 1.0 monolayer this facet becomes unstable due to a
frustration of the top layer iron atoms. The stability of the (111) facet is
weakly affected by the amount of adsorbed carbon and its stability increases
further with respect to the (100) facet with increasing carbon concentration.
The exchange of carbon atoms between the surface and sub-surface regions on the
(111) facet is easier than on the other facets and the formation of carbon
dimers is exothermic. These findings are in accordance with a recent in situ
experimental study where the existence of graphene decorated (111) facets is
related to increased carbon concentration
