Earle K. Plyler Prize: Probing the Structural Evolution and Size-Dependent Reactivity of Gold Clusters by Photoelectron Spectroscopy

Gold has attracted much interest in nanoscience because of its emerging catalytical and optical properties at the nanometer scale. A prerequisite to elucidate the molecular mechanisms of the catalytic effects of nanogold is a detailed understanding of the structural and electronic properties of gold clusters as a function of size. Negatively charged gold clusters (Au$_{n}^{-})$ up to $n =$ 12 were known to be planar. Using photoelectron spectroscopy and computational chemistry, we found that Au$_{16}^{-}$ to Au$_{18}^{-}$ possess hollow cage structures, while Au$_{20}^{-}$ was found to have a high symmetry tetrahedral structure. Beyond Au$_{20}^{-}$, we have found that low symmetry core-shell type structures started to emerge at Au$_{25}^{-}$. The size-dependent reactivity of O$_{2}$ with gold clusters was further studied using photoelectron spectroscopy. Previous works showed that only even-sized Au$_{n}^{-}$ clusters react with O$_{2}$, whereas odd-sized Au$_{n}^{-}$ clusters are nonreactive. Superoxo-type Au$_{n}$(O$_{2}^{-})$ complexes were proposed for even-sized clusters. We observed van der Waals complexes of odd-sized Au$_{n}^{-}$ clusters with O$_{2}$, confirming the inertness of the odd-sized Au$_{n}^{-}$ toward O$_{2}$. This observation led to new insight into how neutral even-sized Au$_{n}$ clusters interact with O$_{2}$. Further studies revealed that there is a superoxo to peroxo chemisorption transition for the O$_{2}$ interaction with even-sized Au$_{n}^{-}$ clusters. The O$_{2}$ in the peroxo O$_{2}$Au$_{n}^{-}$ complexes is much more activated (with a longer O--O bond length), suggesting that this mode of chemisorption may play a more important role in the O$_{2}$ activation by gold nanoparticles.