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 Na2PdCl4 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 Na2PdCl4 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.