Synthesis of atomically ordered AuCu and AuCu₃ nanocrystals from bimetallic nanoparticle precursors

A new multistep approach was developed to synthesize atomically ordered intermetallic nanocrystals, using AuCu and AuCu₃ as model systems. Bimetallic nanoparticle aggregates are used as precursors to atomically ordered nanocrystals, both to precisely define the stoichiometry of the final product and to ensure that atomic-scale diffusion distances lower the reaction temperatures to prevent sintering. In a typical synthesis, PVP-stabilized Au-Cu nanoparticle aggregates synthesized by borohydride reduction are collected by centrifugation and annealed in powder form. At temperatures below 175 °C, diffusion of Cu into Au occurs, and the atomically disordered solid solution Cu_xAu_1-x exists. For AuCu, nucleation occurs by 200 °C, and atomically ordered AuCu exists between 200 and 400 °C. For AuCu₃, an AuCu intermediate nucleates at 200 °C, and further diffusion of Cu into the AuCu intermediate at 300 °C nucleates AuCu ₃. Atomically ordered AuCu and AuCu₃ nanocrystals can be redispersed as discrete colloids in solution after annealing between 200 and 300 °C.