Understanding the Role of AgNO3 Concentration and Seed Morphology to Achieve Tunable Shape Control in Gold Nanostars

Gold nanostars are one of the most fascinating anisotropic nanoparticles. Nanostar morphology can be controlled by changing various synthetic parameters; however, the detailedgrowth mechanisms are not fully understood. Herein, we investigate this process in six-branched nanostars, focusing first on the properties of the single crystalline seed, which evolves to include penta-twinned defects as the gateway to anisotropic growth into 6-branched nanostars. In particular, we report on a high-yield seed-mediated protocol for the synthesis of these particles with high monodispersity in the presence of Triton-X, ascorbic acid, and AgNO3. Detailedspectroscopic and microscopic analyses have allowed the identification of several key intermediates in the growth process, revealing that it proceeds via penta-twinned intermediate seeds. Importantly, we report the first experimental evidence tracking the location of silver withsub-nanometer resolution and prove its role as stabilizing agent in these highly branched nanostructures. Our results indicate that metallic silver on the spikes stabilizes the nanostar morphology, and that the remaining silver, present when AgNO3 is added at high concentration, deposits on the core and between the base of neighboring spikes. Importantly, we also demonstrate the possibility to achieve monodispersity, reproducibility, and tunability in colloidal gold nanostars that are substantially higher than previously reported, which could be leveraged to carry out holistic computational-experimental studies to understand, predict, and tailor their plasmonic response.