Synchrotron Radiation-Assisted Oxidation of Gold Nanoparticles

We explored the oxidation of solid gold nanoparticles under ambient conditions by X-ray nanodiffraction at a synchrotron radiation source. A droplet of carbohydrate-ligand-functionalized gold nanoclusters was evaporated on a superhydrophobic surface. The resulting layer of core-shell nanoparticles with minimum substrate interactions was repeatedly raster-scanned through a nanoscale X-ray focal spot. We observed radiation-induced liberation of ∼1.3 nm diameter disordered nanoparticles, composed of, on average, 71 gold atoms, and their transformation into ∼2.1 nm diameter face-centered cubic gold nanocrystallites by a nucleation/growth process. Lattice metrology and previously reported simulations supported the formation of a disordered gold oxide surface layer, transforming with increasing particle size into epitaxially stabilized Au₂O nanocrystallites with the theoretically predicted cuprite lattice structure. The nonstoichiometric phase showed lattice expansion, which was attributed to oxygen uptake from the interface. Lattice expansion continued beyond the stoichiometry limit, associated with a loss in crystallinity and the emergence of short-range order.