Carbon-induced surface transformations of cobalt

A reactive force field has been developed that is used in molecular dynamics (MD) studies of the surface transformation of the Cobalt (0001) surface induced by an over layer of adsorbed carbon atoms. Significant surface reconstruction is observed with movement of the Co atoms upwards and part of the C atoms to positions below the surface. In a particular C ad atom coverage regime step edge type surface sites are formed, which can dissociate adsorbed CO with a low activation energy barrier. A driving force for the surface transformation is the preference of C adatoms to adsorb in five- or six-fold coordinated sites and the increasing strain in the surface because of the changes in surface metal atom-metal atom bond distances with the increasing surface overlayer concentration. The process is found to depend on the nanosize dimension of the surface covered with carbon. When this surface is an overlayer ontop of a vacant Co surface it can reduce stress by displacement of the Co atoms to unoccupied surface positions and the popping up process of Co atom does not occur. It explains why small nanoparticles will not reconstruct by popping up of Co atoms and do not create CO dissociation active sites even when covered with a substantial overlayer of C atoms.