High-yield synthesis and fine-tuning aspect ratio of (200) faceted gold nanorods by the pH-adjusting method

Tight-controlling of the aspect ratios (ARs) and fine-tailoring of the crystallographic facets of gold nanorods (GNRs) are critical for their further applications in material, biological, and medical fields. However, the high-yield synthesis of GNRs with certain facets and ARs is still challenging because it requires grasping of various components and complicated interactions in the reaction system. Herein, we first report the pH-adjusting method to synthesize uniform and monodispersed GNRs in high yield with the (200) facet and fine-tuned their ARs within an optimal pH range. Through optimizing the experimental parameters, we found that the formation of GNRs was significantly influenced by the concentrations of silver ions and cetyltrimethylammonium bromide (CTAB). Based on the precise control over the quantity of silver ions and CTAB additives, we further optimized the pH in the buffer system. Under the optimized weakly acidic conditions (pH 5.5–7.0), HAuCl4 was reduced by NH2OH to form single-crystalline GNRs with a preferential growth along the [200] facet (i.e. [100] direction), which was supported by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Interestingly, there was a linear relationship between the ARs and pH values over a narrow pH range of 5.5–6.5. The pH-dependent reducing ability of NH2OH enhanced with the increasing pH.