Seed-Mediated Synthesis of Pd Nanocrystals: The Effect of Surface Capping on the Heterogeneous Nucleation and Growth

Seed-mediated growth has emerged as an effective approach to the synthesis of noble-metal nanocrystals with well-controlled sizes, shapes, compositions, and structures. Although surface capping is known to affect the growth pattern of a seed, its explicit role remains to be fully understood. In this article, we applied the collision model established for surface science to seed-mediated growth of nanocrystals in an effort to account for the heterogeneous nucleation of atoms on the surface of a seed and thus the growth pattern in the presence or absence of a surface capping agent. Using Pd cubic seeds as a model system, we demonstrated that the heterogeneous nucleation of Pd atoms only occurred at the corner and edge sites when the {100} side faces were selectively passivated by chemisorbed Br- ions. In comparison, the Pd atoms were found to randomly nucleate on the entire surface of similar Pd seeds if the Br- ions were removed from the surface in advance. For a 1:1 mixture of Br--capped and Br--free Pd cubic seeds, more atoms were added onto the Br--free seeds due to the involvement of a much larger bare surface with a higher sticking coefficient. In addition, we found that the growth mode (island vs layer-by-layer) of a seed was also highly sensitive to the surface condition and could be altered by manipulating the rate of surface diffusion. We further extended the collision model to account for the growth behavior of other types of seeds whose surfaces were enclosed by a mix of {111} and {100} facets in different proportions or characterized by different internal structures, including Pd cuboctahedra and pentatwinned nanowires. The mechanistic insights from this study clearly demonstrate the role played by a surface capping agent in determining the sticking coefficient of atoms and the morphology taken by nanocrystals in a seed-mediated synthesis and should be extendable to other systems involving different types or combinations of metals.