Gold colloids from cationic surfactant solutions. 1. Mechanisms that control particle morphology

The mechanism of formation of gold particles by reduction of Au-III in solutions of alcyltrimethylammonium chloride surfactants was studied in the absence and in the presence of NaCl. AuIII anions interact strongly with trimethylammonium cations forming insoluble ion pairs (Torigoe et al. Langmuir 1992,8, 59). Above the surfactant critical micelle concentration, the ion pairs are solubilized in the micelles returning to the solution. Gold particles were produced by photochemical reduction of the clear micellar solutions. The coupling between surfactant aggregation and inorganic crystallization phenomena in these systems was investigated using transmission electron microscopy (TEM), UV-vis, and time-resolved fluorescence spectroscopy. At concentrations close to the phase boundary of the L-1 phase with the lyotropic liquid crystalline phases many gold particles have a threadlike morphology, as previously noted by Esumi et al. (Langmuir 1995,11, 3285). The presence of NaCl modifies the micellar size and affects the gold crystallization process in surprising and unexpected ways, as evidenced by intermediate structures observed by TEM. Our observations support the idea that the formation of threadlike gold particles occurs primarily through a combination of crystal aggregation and specific crystal face stabilization and not through templating mechanisms.