n-Hexanol Enhances the Cetyltrimethylammonium Bromide Stabilization of Small Gold Nanoparticles and Promotes the Growth of Gold Nanorods

Gold nanorods (AuNRs) are of interest for many applications, since their absorption in the regime of visible light can easily be tuned by their exact shape. To produce these AuNRs, a two-step synthesis that starts from small seed particles is used. These seed particles are stabilized by cetyltrimethylammonium bromide (CTAB), which forms micelles at the used concentration (0.1 mol/L). In this work, the influence of the micelle morphology on the stabilization of these seed particles and the consequences on the formation of AuNRs is reported. The elongation of CTAB micelles by the addition of n-hexanol leads to much more stable seed particle dispersions and thus less polydisperse AuNRs. In contrast, a higher number of micelles compared to pure CTAB dispersions result from the addition of n-pentanol. This promotes the formation of larger seed particles and leads to lower yields of AuNRs. The gold nanoparticles are characterized by UV–vis–NIR absorption spectroscopy, transmission electron microscopy, and small-angle X-ray scattering (SAXS). The morphology of the micelles has been determined by a combination of SAXS and small-angle neutron scattering (SANS). The experimental results were used to calculate the collision kinetics of seed particles by using an improved approach of classical coagulation theory to consider the anisotropy of the micelles. The combination of these experiments with the calculations strongly supports the mechanistic model—that these gold seed particles are not stabilized by a CTAB bilayer but by the micelles itself. For the first time, the influence of the micellar size and shape on the stabilization mechanism of noble metal nanoparticles could be clarified. Theses findings contribute to the development of targeted design routes for distinct nanoparticle morphologies by the use of suitable dispersions.