Tight-binding and evolutionary search approach for nanoscale CoRh alloys

The dependence of the structural and magnetic properties of Co–Rh alloys at the nanoscale was investigated by exhaustively searching the minimum energy via a symbiotic algorithm on a Gupta potential for particle sizes of 13, 19, and 23 atoms. An unrestricted spd tight-binding Hamiltonian was then used to model the electronic properties. Our results underscore the importance of determining both the geometrical and the chemical configuration. A central result points toward a surface segregation that qualitatively and quantitatively depends on system size, with size effects dominating the surface segregation for small Co–Rh clusters (Rh atoms preferentially occupy surface sites), whereas surface energy dictates the segregation for large Co–Rh nanoparticles (Co segregates to the surface). This might have important consequences for heterogeneous catalysis, where the catalytic activity strongly depends on surface composition.