The Cu-only crystal structure domains formed either on the surface or inside the bulk can suppress oxygen diffusion. As shown in Figure , when the top six atomic layer oxygen were removed from the initial slab, the remaining Cu atoms collapse immediately, resulting in the formation of a metallic shell on the surface with a similar configuration of face-centered cubic copper. At 300 K, no oxygen atom diffuses to the surface after 100 ns equilibrium (even after 900 ns at 400 K) as shown in Figure . This indicated that the diffusion of oxygen is highly restrained by the dense pure metallic shell.