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 1 /Au 100 structure is more stable than the unreconstructed 1 1 /Au 100 surface and the missing-row 2 1 /Au 110 structure is more stable than the unreconstructed 1 1 /Au 110 surface, which is consistent with experimental results. For oxygen adsorption on Au 100 , the most stable structure is predicted to be a low coverage 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 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 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 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.