Seeded Rods with Ag and Pd Bimetallic Tips - Spontaneous Rearrangements of the Nanoalloys on the Atomic Scale

Deposition of metal cocatalysts is a common practice to improve the activity of photocatalysts. The use of nanoalloyed nanoparticles allows the formation of diverse nanostructures, tailored for a specific application. Nevertheless, too often the spontaneous atomic scale phenomena interfere with the initial design to produce a modified structure with undesirable properties. Here, we demonstrate such a process for Pd, Ag, or their combination as metal tips mounted on seeded rods of a CdSe dot in a CdS rod (CdSe@CdS) that serve as hydrogen evolution photocatalysts. Spontaneous radial reconstruction at the metal tip brings both Pd and Ag atoms outward even when a two-stage preparation process is applied to specifically produce a core-shell structure. The diffusion of Pd outward enables hydrogen evolution even when the initial Pd tip is covered by a Ag shell, and in the opposite case, a Pd shell shows reduced activity compared with Pd-only tips, due to the surfacing of Ag atoms. In addition, we show that the tip reconstruction occurs already during synthesis; aberration-corrected high-resolution electron microscopy also reveals other processes, such as cation exchange and small clustering around the seeded rods, all quite invisible using regular TEM techniques. In addition, we studied the size effect of Pd-tipped seeded rods and showed that the %QE of seeded rods with 2.2 Pd tips is as high as 91%. These results are significant in the understanding of the structure-function relationship, as it highlights one possible hidden reconstruction pathway of nanoalloys.