Recent Study of Growth Kinetic for Gold Nanorods

For the first time, scientists at Mainz University, Germany have  reported the growth kinetic model for the formation of gold nanorods by combining Small-angle X-ray scattering(SAXS) and optical extinction spectroscopy.

Despite several in situ (dark-field microscopy) and ex situ (electron microscopy) studies of metal nanoparticle growth, the anisotropic growth kinetics was not well-understood until now 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, Researchers directly monitor the anisotropic growth kinetics of gold  nanorods and extract the growth parameters for both crystal directions (along the rod’s long and short axes) independently. They have found that 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.

From their study, The Researchers concluded that the time evolution of nanorod formation extracted in parallel with SAXS and optical spectroscopy shows a simple exponentially decreasing growth rate. The initial growth rate in the long nanorod direction is 5 times larger than that in the short axis direction. Both rates decrease exponentially with a slightly faster time constant for the long axis, which leads to a switch from 1D to 3D growth after about 8−12 min and reduces the aspect ratio of the final products to about 3. 

Researcher believe that the experimentally determined time constants and growth models discussed in their work will aid the development of more detailed molecular simulations for gold nanorod growth.