Effects of subsurface Na, H and C on the bonding of carbon to nickel surfaces

This paper reports the results of a theoretical study of Na, H and C subsurface atomic species in nickel and demonstrates how these interstitial atoms influence the reactivity of the Ni(111) surface and the structure of carbon species adsorbed on the surface. The benzene molecule, C₆H₆, in planar and nonplanar geometries, is used to probe bonding at the surface. Adsorption energies are calculated by ab initio configuration techniques modelling the surface as an embedded cluster. Adsorption energies of planar C₆H₆ at the most stable, three-fold, adsorption site are 18 kcal/mol for the Ni(111) surface, and 10, 19 and 44 kcal/mol in the presence of the Na, H and C interstitials, respectively. The energies required for the planar to puckered distortion are 99 kcal/mol on Ni(111), 69 kcal/mol with the Na interstitial, 83 kcal/mol with H, and 134 kcal/mol with C compared to 198 kcal/mol for distortion of C₆H₆ in the gas phase. The possible relevance of these results to the nucleation of diamond on nickel are discussed. The results indicate that subsurface Na stabilizes tetrahedrally bonded carbon subunits of the diamond structure while subsurface C may make it easier for the overlayer to revert to a planar graphite structure.