Shape-Controlled Syntheses of Gold Nanoprisms and Nanorods Influenced by Specific Adsorption of Halide Ions

This paper describes the effect of halide ions during the seed-mediated growth of gold nanoparticles employing cetyltrimethylammonium bromide (CTAB) as a cationic surfactant system. With the addition of a small amount of iodide ion (∼20 μM) in a growth solution, the major product of the gold nanostructures formed were notably changed into triangular nanoprisms in the presence of excessive bromide ion (∼0.1 M); otherwise, in its absence, nanorods with an aspect ratio of ∼11 were the main products. The major role of the iodide ion was in retarding the overall rate of crystal growth, and the iodide adsorption appeared to repress the crystal growth along Au(111) direction, resulting in Au(111)-faced triangular nanoprisms. When the counteranions of the surfactant were replaced with chloride ions, a novel nanostructure (i.e., nanorice) was manufactured, which demonstrates the effectiveness of the adsorption of halide ions. However, this finding is quite contrasted with the work of the Sastry group (J. Nanosci. Nanotechnol. 2005, 5, 1721−1727), wherein iodide ions strongly suppress the formation of nanoprism. The distinctive results are attributed to different experimental conditions for reducing gold precursors. Nonetheless, overall these observations suggest that the specific adsorption of halide ions is an important factor for a complete control over the shape developments in the seed-mediated growth of gold crystals.