Thermally induced transformations of Au@Cu2O core-shell nanoparticles into Au-Cu nanoparticles from temperature-programmed in situ powder X-ray diffraction

Temperature-programmed in situ X-ray diffraction with whole-powder-pattern modeling is used to investigate the reaction of Au@Cu2O core-shell nanoparticles to form nanocrystalline bimetallic CuxAu1-x alloys (x = 0, 0.25, 0.5, 0.75, 1.0) in a reducing atmosphere. The mechanisms of the reactions are key to informed design of tailored non-equilibrium nanostructures for catalytic and plasmonic materials. The Au@Cu2O reaction is initiated by reduction of the Cu2O cuprite shell to form nanocrystalline metallic Cu at about 413K. Alloying begins immediately upon formation of metallic Cu at 413K, with the nucleation of an Au-rich alloy phase which reaches the nominal Cu content of the overall system stoichiometry by 493K. All bimetallic alloys form a transient ordered Cu3Au intermetallic compound at intermediate temperatures, with the onset of ordering and subsequent disordering varying by composition. No evidence for an ordered Au3Cu intermetallic is found for any composition. Significant crystal growth in the bimetallic phase is apparent at higher temperatures, with the onset temperature increasing with Cu concentration and initial Cu-shell thickness. The reduction of the cuprite phase is slowed by the presence of the core-shell interface, and crystal growth in the Cu shell is completely suppressed within the alloy systems.