Size and shape-dependent melting mechanism of Pd nanoparticles

Molecular dynamics simulation is employed to understand the thermodynamic behavior of cuboctahedron (cub) and icosahedron (ico) nanoparticles with 2-20 number of full shells. The original embedded atom method (EAM) was compared to the more recent highly optimized version as inter-atomic potential. The thermal stability of clusters were probed using potential energy and specific heat capacity as well as structure analysis by radial distribution function, G(r), and common neighbor analysis (CNA), simultaneously, to make a comprehensive picture of the solid state and melting transitions. The result shows ico is the only stable shape of small clusters (Pd55-Pd309 using original EAM and Pd55 using optimized version) those are melting uniformly due to their small diameter. An exception is cub Pd309 modeled via optimized EAM that transforms to ico at elevated temperatures. A similar cub to ico transition was predicted by original EAM for Pd923-Pd2075 clusters while for the larger clusters both cub and ico are stable up to the melting point. As detected by G(r) and CNA, moderate and large cub clusters were showing surface melting by nucleation of the liquid phase at (100) planes and growth of liquid phase at the surface before inward growth. While diagonal (one corner to another) melting was dominating over ico clusters owing to their partitioned structure which retarded the growth of the liquid phase. The large ico cluster, using optimized EAM, presented a combination of surface and diagonal melting due to the simultaneous diagonal melting started from different corners. Finally, the melting temperature as well as latent heat of fusion were calculated and compared with available models and previous studies which showed, unlike the present result, the models failed to predict size-dependent motif crossover.