Critical Concentrations and Role of Ascorbic Acid (Vitamin C) in the Crystallization of Gold Nanorods within Hexadecyltrimethyl Ammonium Bromide (CTAB)/Tetraoctyl Ammonium Bromide (TOAB) Micelles

Growth of gold nanorods is investigated in the presence of ascorbic acid in an aqueous mixed micelle solution of hexadecyltrimethyl ammonium bromide (CTAB) and tetraoctyl ammonium bromide (TOAB) through photochemical reduction of HAuCl4·3H2O using 300 nm Hg lamps. The role of the ascorbic acid concentration, [H2A], in the formation of gold nanorods was studied using UV−vis absorption and transmission electron microscopy. Gold nanorods were produced only if [H2A]/[Au3+] ≥ 0.75 in the reaction cell. For 0.75 < [H2A]/[Au3+] < 6.2, the aspect ratio of nanorods increased as [H2A] increased. For [H2A]/[Au3+] > 6.2, the aspect ratio decreased, and shorter nanorods formed as [H2A] increased. These results are indicative of a lower and upper limit in the concentrations of ascorbic acid and its conjugate base (ascorbate anion) for the formation of gold nanorods. The roles of ascorbic acid and ascorbate ions are discussed, and these observations support the existence of soft templates formed by CTAB/TOAB micelles where Au nanorods grow. Evidence for parallel nucleation and growth processes is also presented.