Site-specific Ag coating on concave Au nanoarrows by controlling the surfactant concentration

The ability to control the site-selective deposition of a noble metal with nanoscale accuracy is vital for the synthesis of well-defined heterogeneous core-shell bimetallic nanoparticles for various applications ranging from nanophotonics to catalysis. Here, precise site-specific Ag coating onto concave gold nanoarrows (GNAs) is reported by tuning the concentration of the surfactant - cetyltrimethylammonium chloride (CTAC). Three distinct nanocoating structures, namely, anisotropic coating, middle coating, and conformal coating are obtained, which are achieved under low, medium and high CTAC concentrations, respectively. The site-specific nanoscale coating on GNAs is proved by scanning transmission electron microscopy imaging in conjunction with the elemental mapping. The CTAC concentration-dependent, facet-specific passivation may be the cause for the three distinct nanoparticles obtained. The morphology differences resulted in discrete plasmonic features, and a linear relationship between the resonance peak and the CTAC concentration is found for the conformal-coated GNAs. We further fabricate free-standing monolayer nanosheets out of the three kinds of nanoparticles, which display strong shape-dependent SERS enhancements.