Pd–Au Asymmetric Nanopyramids: Lateral vs Vertical Growth of Au on Pd Decahedral Seeds

Seed-mediated growth in a site-selected and asymmetric fashion provides a versatile route to the fabrication of noble-metal nanocrystals with unique optical and catalytic properties. Herein, we report the synthesis of Pd–Au asymmetric nanopyramids through the site-selected growth of Au from penta-twinned Pd decahedral seeds. As a result of symmetry breaking arising from slow reduction kinetics, we observed two asymmetric growth modes: lateral growth initiated from one of the five edge vertices along the twin boundary and vertical growth from one of the two axial vertices along the fivefold axis, with the former being more favorable. The slow reduction kinetics was achieved by switching from AuCl₄^– to AuBr₄^– as a precursor, together with the use of l-ascorbic acid 2-phosphate trisodium salt (Asc-2P) for its relatively weaker reduction power than ascorbic acid. The aspect ratio and tip sharpness of the nanopyramids could be manipulated by varying the amount of Ag⁺ ions introduced into the growth solution. The nanopyramids exhibited two surface plasmon resonance peaks, corresponding to the transverse and longitudinal modes, and the extinction spectra could be used to track the growth and evolution of morphology. This work not only enriches our understanding of seed-mediated growth on multiply-twinned decahedra but also provides an effective strategy for preparing bimetallic nanocrystals with asymmetric morphology and tapered dimensions for the creation of high-index facets on the surface.