Morphological control and optical properties of Au@Ag core-shell and urchin-like heterostructures

Noble metal nanoparticles have attracted significant attention in optics, catalysis, and biosensing applications due to their unique surface plasmon resonance (SPR) properties. This study focuses on the controlled synthesis and optical characterization of Au@Ag core-shell nanostructures through a two-step seed-mediated growth approach. We systematically investigated the effects of stabilizing agents (PVP vs. oleylamine), Au/Ag molar ratios, and Au seed sizes on the morphology and optical properties of the resulting bimetallic nanoparticles. Using PVP as the stabilizer, we successfully synthesized spherical Au@Ag core-shell nanoparticles with tunable shell thickness, exhibiting dual SPR peaks corresponding to Au core and Ag shell components. The SPR peak positions and intensities could be precisely controlled by adjusting the Ag shell thickness. Notably, we discovered that Au seed size plays a critical role in determining the structural stability of core-shell architectures. Small Au seeds (~4 nm) led to alloying due to enhanced interdiffusion, while larger seeds (~17 nm) maintained distinct core-shell interfaces. When oleylamine was used as the stabilizer instead of PVP, we obtained unique urchin-like Au-Ag heterostructures featuring radial Ag nanobranches grown on Au cores. Despite their irregular morphology, these urchin-like structures preserved the characteristic dual SPR peaks, indicating their core-shell nature. The morphological diversity achieved through stabilizer selection provides new opportunities for designing plasmonic nanomaterials with enhanced electromagnetic field enhancement for surface-enhanced Raman scattering (SERS) applications.