Sub-monolayers of carbon on alpha-iron facets: an ab-initio study

TL;DR

This study uses ab initio methods to analyze how different alpha-iron facets interact with carbon at various concentrations, revealing facet-specific behaviors relevant to carbon nanotube growth.

Contribution

It provides a detailed comparison of carbon adsorption and stability on alpha-iron facets, highlighting new insights into facet-dependent carbon behavior.

Findings

(110) facet shows carbon repulsion and dimer formation

(100) facet becomes unstable at high carbon coverage

(111) facet stability increases with carbon concentration

Abstract

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.