Site Stability on Cobalt Nanoparticles: A Molecular Dynamics ReaxFF Reactive Force Field Study

The stability of step-edge-type surface sites on cobalt nanoparticles is investigated for particles of increasing size of 1.8, 2.2, and 2.9 nm, that contain 321, 603, and 1157 atoms, respectively. The stability of surface configurations is probed by analyzing the kinetics of the disappearance of step-edge sites as a function of temperature using ReaxFF reactive force field molecular dynamics (MD) simulations. The MD simulations are based on a newly designed reactive force field. Two different activation energy regimes are identified. A low activation barrier of the order of 7 kJ/mol corresponds to single atom movement, which is independent of Co nanoparticle size. Higher activation energies (28, 37, and 22 kJ/mol for the three clusters, respectively) correspond to the shift of overlayer terraces. These concerted shifts appear to be sensitive to particle size, terrace size, and the structure of the facet. Step edges are more stable on larger particles. Shifting of the (111) surface layers leads to transformation of a thin surface layer from the initially face-centered cubic structure to hexagonal close-packed structure.