Palladium-coated gold nanoparticles with a controlled shell thickness used as surface-enhanced Raman scattering substrate

We report the synthesis and characterization of gold core palladium shell (Au@Pd) nanoparticles with thickness-controlled shell as an improved transition-metal substrate for surface-enhanced Raman scattering (SERS). By changing the molar ratio of H2PdCl4 to Au, the Pd shell thickness can be precisely controlled from a few nanometers down to ca. one monolayer. A series of characterizations were performed using transmission electron microscopy (TEM), UV-vis, SERS, and electrochemical techniques. The results confirmed the core-shell structure and the uniform and pinhole-free nature of the Pd shell, ensuring the properties of Pd without possible interference from Au. Consistent with theoretical prediction, the core-shell setting borrows high SERS activity from the Au core through the long-range electromagnetic enhancement in addition to the enhancement from the Pd shell itself. Moreover, their SERS activity can be optimized by the tunable shell thickness and core size. The nm-Au@Pd/Pd electrodes allow us to obtain good quality SER spectra of various molecules on Pd that were unable to be accessed with detail in the past. We believe that this borrowing strategy will be important for in-situ extracting of detailed vibrational information for adsorbates on catalytic Pd surfaces.