Atomistic mechanisms of the initial oxidation of stepped Cu3Au(100)

Alloy oxidation is complex and involves several critical processes that lack understanding on the atomic level. Here, we report an atomistic picture of the initial-stage oxidation of stepped Cu3Au(100) using a combination of surface science tools and modeling to illuminate the microscopic processes underlying oxygen-adsorptioninduced structural and compositional changes. Pristine Cu3Au(100) consists of wide CuAu-terminated terraces and narrow Cu-terminated terraces separated by monatomic steps. Counterintuitive to the common expectations of the adsorbate-induced surface segregation of the more reactive alloy component, our observations demonstrate that the oxygen adsorption leads to the exfoliation of the outermost CuAu layer, thereby exposing the inner Cu plane to O attack. This occurs via the oxygen-assisted abstraction of Au and Cu atoms from step edges and CuAu terraces, which generates many Cu adatoms aggregating into Cu clusters and Au adatoms dissolving into the bulk. The oxygen adsorption onto fourfold hollow sites of the exposed Cu plane results in nucleation and growth of the c(2 x 2)-O superstructure, which can be fit well by the Johnson-Mehl-Avrami-Kolmogorov theory with a site-saturated nucleation mechanism.