Atomically Localized Ordered Phase and Segregation at Grain Boundaries in Au–Ag–Cu Ternary Alloy Nanoparticles

Metallic nanoparticles have been widely used and investigated because of their unique electrical and optical properties as well as their catalytic functions. The degree of freedom for tuning the material properties, such as electrical conductivity, permittivity, and mechanical strength, is significantly extended by alloying multiple metal elements, for example, in high-entropy alloys. For catalysis applications, the interfaces between the different phases and grain boundaries in the nanoparticles play an important role because the catalytic reactions occur most efficiently at the interfaces. The interfaces are also crucial for the chemical stability of the alloy. In the present work, we synthesized Au–Ag–Cu ternary alloy nanoparticles and investigated their atomically localized structures, particularly at the grain boundaries. To investigate the interface structure in the nanoparticles, we used scanning transmission electron microscopy with a spherical aberration corrector and energy dispersive X-ray spectroscopy. In the solid solution particles, L10 ordered phases with sizes of a few lattice sites, around 2 nm in diameter, were found at the defect sites between the twin boundaries. In the phase-separated alloy particles, segregated Ag atoms, which are not soluble in the Cu-rich phase, were found specifically at the grain boundaries with finite thicknesses.