Electric-field-directed growth of gold nanorods in aqueous surfactant solutions

The factors affecting the nucleation and growth of gold nanorods, (Jana et al., Adv. Mater. 2001, 13, 1389) have been investigated. It is shown that the size and aspect ratio can be controlled through the use of different sized seed particles. The length of the rods can be tuned from 25-170 nm, while the width remains almost constant, at 22-25 nm. The formation of rods requires the pressence of the cationic surfactant cetyltrimethylammonium bromide (CTAB). Lower temperature favors rod formation, although this reudces CTAB solubility. The addition of chloride ions or the use of dodecyltrimethylammonium bromide (DTAB) leads to shorter-aspect rods. Au-III and Au-I are shown to be quantitatively bound to the CTAB micelles. We propose an electrochemical mechanism for rod formation, whereby the flux of Au-I bound to cationic micelles to the seed surface is maximized at points of highest curvature, where the electrical double layer gradient is highest. Initial numerical solutions to the electric potential and field around an ellipsoid in a 1:1 electrolyte are provided, which indicate that the field at the particle tip scales linearly with the aspect ratio. Mean free passage times for ions are found to be shortest at the tips. The results provide a general explanation for the formation of non-equilibrium crystal habits and a mechanism for controlling crystal growth.