Optimizing Seed Aging for Single Crystal Gold Nanorod Growth: The Critical Role of Gold Nanocluster Crystal Structure

Controlled synthesis of single crystal gold nanorods (Au NRs) with desired dimensions, narrow polydispersity, and minimal byproducts depends critically on the state of the seed solution. Herein, we clarify the growth of cetyltrimethylammonium bromide stabilized Au seeds and their impact on conventional seed-mediated growth of Au NRs. UV vis spectroscopy, TEM microscopy, and small-angle X-ray scattering reveal that during aging of the seed solution, constituents evolve from solubilized Au-salt precursors (Au[III]) to Au nanoclusters (Au-n) to multitwinned Au nanocrystals. The most consistent single crystal Au NR growth, with minimal byproducts, occurs when the seed solution contains the maximum concentration of small (<1 nm) nanoclusters (n < 55, 1 min < t(age) < 10 min at T = 25 degrees C). This point can be identified spectroscopically between 300 and 480 nm. Additionally, the optimal clusters coincide approximately with a relative shift in structural equilibrium of closed-shell Au nanoclusters from predominately cuboctahedral to icosahedral. Geometrically, a cuboctahedral cluster can evolve into a single crystal fcc lattice, whereas an icosahedral cluster must adopt a twinned structure upon growth. This shift is consistent with the increased number fraction of polycrystalline byproducts (e.g., twinned spherical particles and rods) if the seeds are aged beyond this point. Therefore, these observations suggest that the unique structural properties of Au-n nanoclusters are paramount to the initial formation of single crystal seeds and their subsequent anisotropic growth.