Etching and aging effects in nanosize Au clusters investigated using high-resolution size-exclusion chromatography

We report experiments using high-resolution size exclusion chromatography (HRSEC), dynamic light scattering, and transmission electron microscopy to investigate the effects of aging of Au nanoclusters in the presence of surfactant ligands. We first describe our observations of the role of thiols as etchants to produce clusters in a micelle-free synthesis by reduction of a metal-organic precursor. Clusters with large abundances are identified using HRSEC, and their sizes and optical properties are reported. The smallest, D-c similar to 1 nm molecular sized An clusters, with approximately I closed atomic shell, N similar to 13 atoms, have nonclassical features in their room temperature absorbance spectra. The other dominant sub-populations also correspond closely to closed-shell structural stabilities. We show that, contrary to the expectation that aging in solution will always broaden the size dispersion and increase the average size (Ostwald ripening), a narrowing of the size dispersion and change in average size can occur with time under ambient conditions. In the presence of various chain length alkanethiols, an etching and size decrease usually occurs; in the case of weakly bound alkylated poly(ethylene oxide) surfactants, an increase in size with time is observed.