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 (<-0.60.3em0exeV, corresponding, e.g., to p=10.3em0exatm and T>6000.3em0exK), 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 -0.40.3em0exto0.3em0ex-0.180.3em0exeV, corresponding to, e.g., p=10.3em0exatm and T=420--2000.3em0exK), 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.