  In this work, we combine electron microscopy measurements of the surface
compositions in Cu-Au nanoparticles and atomistic simulations to investigate
the effect of gold segregation. While this mechanism has been extensively
investigated within Cu-Au in the bulk state, it was never studied at the atomic
level in nanoparticles. By using energy dispersive X-ray analysis across the
(100) and (111) facets of nanoparticles, we provide evidence of gold
segregation in Cu$_{3}$Au and CuAu$_{3}$ nanoparticles in the 10 nm size range
grown by epitaxy on a salt surface with high control of the nanoparticles
morphology. To get atomic-scale insights into the segregation properties in
Cu-Au nanoparticles on the whole composition range, we perform Monte Carlo
calculations employing $N$-body interatomic potentials. These simulations
confirm this effect by showing a complete segregation of Au in the (100) and
(111) faces of a truncated octahedron for gold nominal composition of the alloy
above 70\% and 60\% respectively. Furthermore, we show that there is no size
effect on the segregation behaviour since we evidence the same oscillating
concentration profile from surface to the nanoparticles core as in the bulk.
These results can shed new lights in the interpretation of the enhanced
reactivity, selectivity and stability of Cu-Au nanoparticles in various
catalytic reactions.
