The local adsorption geometry of CH3 and NH3 on Cu(111):: a density functional theory study

Total energy calculations in the generalised gradient approximation of density functional theory, using the CASTEP computer code, have been used to determine the equilibrium geometries of NH3 and CH3 on Cu(l 11). Consistent with the results of both previous calculations and scanned-energy mode photoelectron diffraction (PhD) experiments, ammonia is found to adsorb in atop sites with an adsorption energy of approximately 0.7 eV. The Cu-N nearest-neighbour bondlength shows some coverage dependence, the best agreement with experiment being achieved at lower coverage. The adsorbed methyl species, however, shows the highest adsorption energy for a threefold coordinated hollow site, as found in a PhD experimental study, with a strong preference for the C-H bonds to point towards the nearest-neighbour Cu atoms. The results, which imply involvement of both the C and the H atoms in the substrate bonding, are consistent with previous theoretical studies of methyl adsorption on Ru(0 0 0 1) and Ni(1 1 1), but quite unlike those of similar studies on Pt(1 1 1) and other metal surfaces. (C) 2001 Published by Elsevier Science B.V.