Influence of Iodide Ions on the Growth of Gold Nanorods: Tuning Tip Curvature and Surface Plasmon Resonance

This paper describes morphological and optical changes induced by seed-mediated growth of gold nanorods in the presence of iodide ions. Addition of small amounts of iodide to the growth solution results in the growth of nanoparticles with dumbbell-like structure, meaning that gold salt reduction takes place preferentially at the rod tips. However, when excess iodide is added, homogeneous rod growth is observed, and therefore the original shape is retained. By controlling the experimental conditions, the position of the longitudinal plasmon band of grown nanorods can be shifted up to as much as 250 nm. These optical effects were also simulated by means of the boundary element method (BEM), achieving an excellent agreement with the experimental spectra. X-ray photoelectron spectroscopy (XPS) and surface enhanced Raman spectroscopy (SERS) analysis of the gold nanorods before and after iodide addition revealed the presence of AuI and AgI at the particles surface. A growth mechanism is proposed on the basis of preferential iodide adsorption at the tips {111} facets, leading to the formation of AgI, followed by reduction of gold salt precursor due to a decrease in the surface redox potential.