Formation of Bimetallic Copper-Gold Alloy Nanoparticles Probed by in Situ X-ray Absorption Fine Structure Spectroscopy

Advanced studies of the formation and growth processes of metal alloy nanoparticles are of great importance and provide a better understanding for the fine tuning of their properties for the desired applications. In this work, we followed the synthesis of Au1-xCux colloidal nanoparticles by in situ time-resolved X-ray absorption fine structure (XAFS) spectroscopy. The particles were synthesized by a one-pot approach with a homemade reactor adapted to the XAFS measurements. The colloidal nanoparticles were also characterized by complementary ex situ techniques. Analyses at the Cu-K edge and Au-L-3 edge confirmed that the initial reduction of the Au species occurs at low temperatures (<100 degrees C) to form Au seeds that are subsequently enriched with Cu to form Au0.75Cu0.25 alloy nanoparticles at 225 degrees C. An important finding was the temporary formation of a Cu2O-type phase during the colloidal synthesis. This phase was consumed with the concomitant formation of the Au1-xCux alloy nanoparticles. These results directly confirm the previous proposal that the alloy formation occurs through a digestive-ripening-like process of an amorphous Cu-enriched phase.