Shape and surface structure of gold nanoparticles under oxidizing conditions

We perform density- functional theory calculations to investigate the adsorption of oxygen at the Au ( 100 ) and Au ( 110 ) surfaces. For the clean surfaces, we find that the added- row ( 5 X 1 ) / Au ( 100 ) structure is more stable than the unreconstructed ( 1 X 1 ) / Au ( 100 ) surface and the missing- row ( 2 X 1 ) / Au ( 110 ) structure is more stable than the unreconstructed ( 1 X 1 ) / Au ( 110 ) surface, which is consistent with experimental results. For oxygen adsorption on Au ( 100 0, the most stable structure is predicted to be a low coverage ( similar to 0.1 ML ) on the added- row reconstructed surface, while for adsorption on Au ( 110 ), the most stable configuration of those considered is a ( 2 X 1 ) missing- row structure with 1 ML coverage of oxygen. From these results, together with those of our previous investigations into the O/ Au ( 111 ) system, we use the Wulff construction to predict the nanoparticle shape as a function of oxygen chemical potential, which we correlate with pressure ( p ) and temperature ( T ). For low values of the oxygen chemical potential ( <- 0.6 eV, corresponding, e. g., to p = 1 atm and T > 600 K ), the nanoparticle consists of clean ( 111 ) facets. For slightly higher values, clean ( 111 ) facets still dominate but there are small regions of ( 110 ) facets, which are covered with the ( 2 X 1 )- 2O reconstruction. With progressively increasing values of the chemical potential ( e. g., from - 0.4 to - 0.18 eV, corresponding to, e. g., p= 1 atm and T= 420 - 200 K ), the ( 111 ) facets become covered with a thin oxide- like structure, and the ( 110 ) regions with the ( 2 X 1 )- 2O/ ( 110 ) surface reconstruction become larger and finally dominate. These findings indicate that for low temperature oxidation reactions, where gold nanoparticles have been reported to be surprisingly active, such thin surface- oxide- like structures on the ( 111 ) and ( 110 ) surfaces could possibly play a role in the behavior of the nanogold catalysts.