Shape-controlled synthesis of palladium nanocrystals: A mechanistic understanding of the evolution from octahedrons to tetrahedrons

Palladium octahedrons and tetrahedrons enclosed by eight and four {111} facets have been synthesized from cuboctahedral Pd seeds by using Na ₂PdCl₄ and Pd(acac)₂, respectively, as the precursors. Our mechanistic studies indicate that the cuboctahedral seeds were directed to grow into octahedrons, truncated tetrahedrons, and then tetrahedrons when Pd(acac)₂ was used as a precursor. In contrast, the same batch of seeds only evolved into octahedrons with increasing sizes when the precursor was switched to Na₂PdCl₄. The difference in growth pattern could be attributed to the different reduction rates of these two precursors. The fast reduction of Pd(acac)₂ led to a quick drop in concentration for the precursor in the very early stage of a synthesis, forcing the growth into a kinetically controlled mode. In comparison, the slow reduction of Na ₂PdCl₄ could maintain this precursor at a relatively high concentration to ensure thermodynamically controlled growth. This work not only advances our understanding of the growth mechanism of tetrahedrons but also offers a new approach to controlling the shape of metal nanocrystals.