Seed-mediated synthesis of gold nanorods: Role of the size and nature of the seed

We report studies on the synthesis of gold nanorods by a three-step seeding protocol method using a variety of different gold seeds. The synthetic method is adapted from one we published earlier (Jana et al. J. Phys. Chem. B 2001, 105, 4065). The seeds chosen for these studies have average diameters in the range from 4 to 18 nm, with positively charged as well as negatively charged surface groups. In all the cases, along with a large concentration of long rods, a small number of different shapes such as triangles, hexagons, and small rods are observed. The proportion of small rods increases with an increase in the seed size used for nanorod synthesis. For long nanorods synthesized by different seeds a comparison of various parameters such as length, width, and aspect ratio has been made. A dependence of the nanorod aspect ratio on the size of the seed is observed. Increasing the seed size results in lowering of the gold nanorod aspect ratios for a constant concentration of reagents. The charge on the seed also plays a role in determining the nanorod aspect ratio. For positively charged seeds variation in the aspect ratio is not as pronounced as that for negatively charged seeds. The gold nanorods synthesized were characterized by transmission electron microscopy (TEM), UV-vis spectroscopy, and Fourier transform infrared spectroscopy. The role of seed size in the size and shape evolution of the nanocrystal, at different growth stages, has been studied by TEM.