Syntheses of Au@PdAg and Au@PdAg@Ag core-shell nanorods through distortion-induced alloying between Pd shells and Ag atoms over Au nanorods

Noble Au@PdAg and Au@PdAg@Ag core-shell nanorods (NRs) having PdAg alloy shells were synthesized using Au@Pd NRs as seeds. Their crystal structures and growth mechanisms were examined using data from TEM, TEM-EDS, XRD, and UV-Vis-NIR extinction spectroscopy. First, cuboid or dumbbell-type Au@Pd NR seeds were prepared using Au NRs as seeds. In the second step, when Ag⁺ ions were reduced over these seeds, cuboid and dumbbell-type Au core PdAg alloy shell NRs were formed with maximum Ag content of about 16% and 24%, respectively, after heating at 60 °C. The driving force of alloying between Pd shells and Ag atoms was attributed to the distortion of Pd layers over Au NRs based on peak shifts and broadening of XRD data. The higher Ag content in dumbbell-type Au@PdAg NRs was explained by the greater distortion of Pd shells over Au NRs. When the Ag content exceeds its maximum solubility in Pd shells, the third Ag shells were grown over Au@PdAg NRs. For cuboid Au@PdAg NRs, the crystal growth of Ag shells over cuboid Au@PdAg cores begins with the formation of single nuclei on a wide side PdAg alloy facet, followed by growth to one cuboid rod shell and further growth of one or two neighboring cuboid rod shells having {100} facets. On the other hand, for dumbbell-type Au@Pd NRs, Ag shells were deposited over concave curvature of the side facets of Au@PdAg and cuboid Au@PdAg@Ag NRs having one large cuboid Ag block was finally formed in each particle. Our study provides a new simple technique for the epitaxial growth of uniform PdAg alloy layers using spontaneous alloying between Pd shells and Ag atoms over Au NRs.