Anisotropic Overgrowth of Palladium on Gold Nanorods in the Presence of Salicylic Acid Family Additives

We explored the use of salicylic acid (SA) and its derivatives 5-formylsalicylic acid (FSA) and 5-sulfosalicylic acid (SSA) as organic additives to cetyltrimethylammonium bromide (CTAB) in synthesizing gold nanorods (AuNRs) followed by palladium (Pd) capping at the ends of AuNRs. In the AuNR synthesis step, SA family additives in the presence of low concentration of CTAB (50 mM) serve as both the prereducing agent and the cofactor in nanorod growth. At an optimum additive/CTAB ratio (0.1–0.2), AuNRs grow to the longest length. At low additive concentrations, the gold seeds do not grow. At high concentrations, the longitudinal growth of AuNRs is disrupted because the excessive additive disturbs the ligand structure, leading to more isotropic growth. In the Pd overgrowth step, Pd starts to grow from both ends for AuNRs synthesized at optimum additive/CTAB ratios. Feeding more Pd grows the particles into a core–shell structure, possibly because there lacks a tight ligand layer on Pd that favors the longitudinal growth. For AuNRs synthesized at high additive/CTAB ratios, Pd growth loses preference, showing randomized Pd nucleation on AuNR surface. Finally, the palladium-end-capped–AuNRs' catalytic activity was tested using the resazurin reduction reaction. This study shows a new way to produce controllable deposition of Pd on AuNRs.