Surface- and Structure-Dependent Catalytic Activity of Au Nanoparticles for Oxygen Reduction Reaction†

Polycrystalline Au nanoparticles (NPs) of 3, 6, and 8 nm were synthesized via solution phase reaction of HAuCl4·3H2O with tert-butylamine borane and oleylamine in tetrahydronaphthalene. The sizes of the Au NPs were tuned by varying the reaction temperatures (40−3 °C). For polycrystalline Au NPs made at room temperature or above, smaller NPs showed more positive onset potential in catalyzing oxygen reduction reaction (ORR) in 0.5 M KOH media. However the most active Au NPs were the 8 nm ones that were synthesized at 3 °C. We rationalized the ORR activity of these Au NPs by the ease of oleylamine surfactant removal and the degree of disorder in the polycrystalline structure. This was further confirmed by the low activity observed from the same Au NPs passivated with hexadecanethiol, or from the Au NPs with higher degree of crystallinity made from the etching of the composite Au−Fe3O4 NPs.