Growth Behavior of Gold Nanorods Synthesized by the Seed-Mediated Method: Tracking of Reaction Progress by Time-Resolved X‑ray Absorption Near-Edge Structure, Small-Angle X‑ray Scattering, and Ultraviolet–Visible Spectroscopy

Gold nanorods (GNRs) are attractive and important nanomaterials that show enormous promise for a wide range of applications. To investigate the formation process of GNRs generated by the seed-mediated method, we tracked the growth of GNRs with aspect ratios of 2 (GNR2s), 4 (GNR4s), and 6 (GNR6s) using time-resolved X-ray absorption near-edge structure (XANES). Moreover, for GNR6s, additional measurements by small-angle X-ray scattering and ultraviolet–visible spectroscopy were carried out for longer reaction times of up to 20 000 s. Cetyltrimethylammonium bromide (CTAB) was used as the surfactant in the generation of GNR2s and GNR4s, whereas benzyldimethylhexadecylammonium chloride (BDAC) and CTAB were used for GNR6s. The three analysis methods used provided consistent and compensatory results. It was found that GNR2s and GNR4s finish growing by 2000 s, consuming all the Au in the solutions, and that GNR6s keeps growing for more than 20 000 s by a different formation process. From these comprehensive results, it was revealed that severe competition for existence among the GNRs occurs in all solutions. The seed particles added to the solutions of GNR2s and GNR4s start growing but the whole seed particles cannot mature into GNRs. Conversely, in the solution for GNR6s, some of the already-grown GNRs release Au atoms, allowing the growth of further GNRs, making the formation process slow, unique, and complex. The growth of GNR6s coordinated by CTAB and BDAC in appropriate proportions continues more than 20 000 s. In particular, back-and-forth growth of GNR6s is first observed by tracking the growth solution using time-resolved XANES.