Conversion of Polydisperse Au Nanoparticles into Monodisperse Au₂₅ Nanorods and Nanospheres

Abstract

We report a facile conversion of polydisperse Au nanoparticles (1−3.5 nm) into well-defined monodisperse 25-atom (Au25) 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(III) salt in the presence of triphenylphosphine; subsequently, these polydisperse Au nanoparticles were used as a common precursor for shape-controlled synthesis of Au25 nanorods and nanospheres following one-phase and two-phase thiol etching, respectively. Our results demonstrate that the Au25 particle shape can be conveniently controlled by using different types of thiol ligands in the second step of thiol etching. These ultrasmall Au25 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.