Adsorption of hydrocarbons on a diamond (111) surface: Anab initioquantum-mechanical study

The adsorption of H⋅, ${\mathrm{CH}}_3$⋅, ${\mathrm{CH}}_2$, CH⋅, ${\mathrm{C}}_2$H⋅, and ${\mathrm{C}}_2$ ${\mathrm{H}}_2$ on hydrogen-terminated (111) surfaces of diamond has been investigated theoretically. Ab initio molecular-orbital theory was used in order to calculate the relative adsorption energies of these different hydrocarbon species. The effects of electron correlation, included by means of second-order Mo/ller-Plesset theory, on the relative adsorption energies, as well as the choice of basis sets, were investigated. The effects of different sizes of model clusters and different geometry optimizations were also studied. Counterpoise corrections were used to estimate the magnitude of the basis-set superposition errors. The adsorption energies were found to be in the order ${\mathrm{C}}_2$H⋅>H⋅≊${\mathrm{CH}}_2$(singlet)>CH⋅>${\mathrm{CH}}_2$⋅(triplet)>${\mathrm{CH}}_3$⋅>${\mathrm{C}}_2$ ${\mathrm{H}}_2$. The ${\mathrm{C}}_2$H⋅ species was predicted to adsorb stronger to the H-terminated diamond (111) surface than H⋅ does. ${\mathrm{C}}_2$ ${\mathrm{H}}_2$ with one site binding to the surface was predicted to yield a significantly weaker adsorption than the other hydrocarbons.