Unveiling and quantifying the topology-dependent pre-melting of nanoparticles

The melting of metallic nanoparticles is governed by surface pre-melting, a phenomenon traditionally modeled as the isotropic growth of a uniform liquid shell. Challenging this classical view, we report facet-dependent surface pre-melting in hexagonal close-packed Co nanoparticles, arising from the structural heterogeneity of the nanoparticle surface. Characterizing melting in molecular dynamics simulations (500 to 6000 atoms), we observe the onset of surface mobility, starting as low as $0.2\times T_{M,\infty}$ (the bulk melting point), driven by the early disordering of stepped $\{01\bar{1}1\}$ facets. We found that these facets consistently melt at temperatures nearly 200 Kelvin lower than flat $\{0001\}$ facets, regardless of particle size, and relate facets melting temperatures to the nanoparticle size via a 2D extension of the Gibbs-Thomson relation. We determine a critical liquid layer thickness that triggers the melting of the entire nanoparticle, which is found to be size-dependent. Our results confirm the recent experimental observation of the surface pre-melting effect, and extend it to anisotropic particles with different facet orientations.