Conversion of Polydisperse Au Nanoparticles into Monodisperse Au25 Nanorods and Nanospheres

We report a facile conversion of polydisperse Au nanoparticles (1-3.5 nm) into well-defined monodisperse 25-atom (Au-25) nanorods and nanospheres via one-phase and two-phase thiol etching, respectively. Our method involves two main steps: first, small Au nanoparticles (polydisperse, predominantly 1-3.5 nm) were prepared by NaBH4 reduction of Au(HI) salt in the presence of triphenylphosphine; subsequently, these polydisperse Au nanoparticles were used as a common precursor for shape-controlled synthesis of Au-25 nanorods and nanospheres following one-phase and two-phase thiol etching, respectively. Our results demonstrate that the Au-25 particle shape can be conveniently controlled by using different types of thiol ligands in the second step of thiol etching. These ultrasmall Au-25 nanoparticles do not support surface plasmons as do their larger counterparts (i.e., Au nanocrystals); instead, they exhibit molecular-like optical absorption behavior. This conversion process is striking in two features, size focusing and shape control, and may be extendable to the synthesis of other robust well-defined Au nanoclusters.