Formation of Palladium Nanostructures in a Seed-Mediated Synthesis through an Oriented-Attachment-Directed Aggregation

In the present article, we studied the structuration mechanism of palladium nanocrystals (NCs) of different shapes (cubes, pentatwinned nanorods, pyramids, and pentatwinned decahedra) prepared via the seed-mediated reduction of a palladium salt in aqueous medium, in the presence of CTAB surfactant. A detailed time-resolved TEM kinetic study of the formation of the NCs and the investigation of the influence of seeds size the characteristics of the final particles were performed. At the opposite of the gold reference synthesis reported in literature, the systematic presence of numerous very small palladium spherical nuclei generated by homogeneous nucleation was observed. A formation mechanism was proposed, taking into account an early aggregation step of the preformed seeds with some nuclei; this led to the formation of secondary objects with nonrandom shapes, the growth of which led eventually to the final well-faceted palladium NCs. An oriented attachment of seeds and nuclei, triggered by the energetically most favorable packing of spheres, was found to be responsible for the early differentiation of NCs shapes. The original formation mechanism presented here shed a new light on the seed-mediated synthesis of palladium NCs by (i) explaining shape differentiations via an oriented attachment process and (ii) explaining in detail the influence of the CTAB surfactant, the seed size on nuclei size ratio, the solvent, and the temperature onto final NCs shapes.