Toward a Quantitative Understanding of Symmetry Reduction Involved in the Seed-Mediated Growth of Pd Nanocrystals

We report a quantitative analysis of the symmetry reduction phenomenon involved in the seed-mediated growth of Pd nanocrystals under dropwise addition of a precursor solution. In addition to the elimination of self-nucleation, the dropwise approach allows for the formation of a steady state for the number of precursor ions in the growth solution, which only fluctuates in a narrow range defined by experimental parameters such as the initial concentration of precursor solution and the injection rate. We can deterministically control the growth mode (symmetric vs asymmetric) of a seed by tuning these parameters to quantitatively manipulate the reaction kinetics and thus the lower and upper limits that define the steady state. We demonstrate that there exists a correlation between the growth mode and the lower limit of precursor ions in the steady state of a seed-mediated growth process. For the first few drops of precursor solution, the resultant atoms will only be deposited on a limited number of available sites on the seed if the lower limit of the steady state is below a critical value. Afterward, the deposition of atoms will be largely confined to these initially activated sites to induce symmetry reduction if atom deposition is kept at a faster rate than surface diffusion by controlling the lower limit of precursor ions in the steady state. Otherwise, the migration of atoms to other regions through surface diffusion can access other sites on the surface of a seed and thus lead to the switch of growth mode from asymmetric to symmetric. Our study suggests that symmetry reduction can only be initiated and retained by keeping the atom deposition at a rate slow enough to limit the number of initial nucleation sites on a seed but fast enough to beat the surface diffusion process.