Oxidation-Induced Internal Chemical Patterning in Copper-Gold Nanoalloys.

Chemical segregation governs the structural evolution and properties of nanoalloys and, under vacuum or reducing conditions, typically leads to core-shell structures that minimize surface free energy. Here, we show that surface oxidation fundamentally alters this behavior in Cu-Au nanoalloys by suppressing conventional surface segregation and driving twin-boundary (TB)-mediated chemical separation within the particle interior. Combining in situ electron microscopy, ambient-pressure X-ray photoelectron spectroscopy, and density functional theory, we reveal that oxidation redirects Au transport from the surface to internal TBs through uphill diffusion from Au-poor to Au-rich regions, producing alternating Au-rich and Cu-rich lamellae with local Au enrichment of up to ∼50 at.%. This behavior is driven by the negative Cu-Au mixing enthalpy, which favors heteroatomic bonding, while the CuOx overlayer inhibits Au surface segregation that would otherwise dominate. Consequently, surface oxidation acts as a chemical pump, redirecting atomic flux into the particle interior, where defect-mediated segregation lowers the free energy by maximizing Cu-Au bonding. These findings demonstrate how surface chemistry, alloy thermodynamics, and lattice defects collectively govern mass transport and internal chemical patterning, providing atomistic insight into the structural evolution and stability of nanoalloys under reactive environments.