New rule and growth model for the synthesis method of gold nanorods with binary surfactant CTAB and NaOL

The roles of CTAB and Ag+ have been discovered and given us a deeper understanding of the seed-mediated method in the gold nanorods synthesis. Former work used binary surfactants CTAB + NaOL (sodium oleate) to greatly improve the dimensional tunability and monodispersity of gold nanorods. However, they only used a few of the concentration combinations of the binary surfactants, and the influence of NaOL under this method has not been systematically studied. In this work, we carried out systematic experiments under the variation of NaOL and used transmission electron microscopy and UV–vis-near-infrared spectroscopy to monitor the growth process of the gold nanorods. The results showed that the NaOL contributed to the symmetry breaking process. We discovered the ideal ranges of NaOL concentration under different concentrations of CTAB (10–40 mM). Lower concentrations of NaOL produced many impurities, such as Au spheres, while higher concentrations of NaOL led to the decrease of monodispersity of the obtained gold nanorods. A growth model based on the balance of diffusion/reduction of the growth solution has been proposed in order to explain the formation of the gold nanorods.