HCl-Retarded Gold Nanorod Growth for Aspect Ratio and Shape Tuning

Traditionally, gold nanorods (GNRs) are mostly prepared via a seed-mediated growth approach, which usually produce an initial burst growth followed by slower kinetics and generate dogboneshaped GNRs with relatively small aspect ratios (ARs). We demonstrate here that the growth of GNRs can be effectively retarded by addition of hydrochloric acid (HCl), leading to the formation of symmetric, long GNRs with high ARs. Furthermore, time-dependent kinetic analysis of GNR growth reveals two completely different growth modalities: in the absence of HCl, the GNR growth is rapid, reaching a concentration of 0.64 nM in 10 min. Its AR also rapidly reaches a maximum value of 4.1 at 2 min and then gradually decreases to 3.3 over 5 h. In sharp contrast, in the presence of HCl additive, the GNR growth happens very slowly, reaching a concentration of 0.33 nM at 7 h, and the AR also grows slowly to reach a maximum value of 5.3 at 3 h. These growth modalities endow different formation mechanisms of GNRs, which determine their aspect ratios and shape.