Growth Kinetic of a Rod-Shaped Metal Nanocrystal

The crystallization dynamics in wet-chemical synthesis is the key parameter controlling nanoparticle morphology, which determines the frequency of the optical plasmon resonance in metal nanoparticles. Despite several in situ (dark-field microscopy) and ex situ (electron microscopy) studies of metal nanoparticle growth, the anisotropic growth kinetics are not well-understood mainly because the crystallization is a nonequilibrium process. Using simultaneous optical spectroscopy and time-resolved small-angle X-ray scattering at a synchrotron X-ray source, we directly monitor the anisotropic growth kinetics of gold and gold-copper nanorods and extract the growth parameters for both crystal directions (along the rod's long and short axes) independently. We find a crossover from 1D to 3D growth modes at 8 and 12 min, respectively, where the nanorods attain their maximum aspect ratio. The growth model explains and predicts this crossover point without the need of a switch for the growth mode and allows for the fine-tuning of the particle shapes.