Influence of Speciation of Aqueous HAuCl4 on the Synthesis, Structure, and Property of Au Colloids

We have studied the pH-dependent speciation of aqueous HAuCl4 and its influences on the synthesis, structure, and property of Au colloids. Aqueous HAuCl4 consists of [AuClx(OH)(4-x)](-) (x >= 2) at low pH but (AuClx(OH)(4-x)](-) (x < 2) at high pH. By employment of ascorbic acid as the reducing agent and sodium benzenesulfonate (SDBS) as the protecting agent, reduction of aqueous HAuCl4 at low pH leads to the synthesis of well-dispersed and uniform fine Au colloids, whereas that at high pH forms large Au colloids and ensembles of fine Au colloids. These large Au colloids and ensembles of fine Au colloids exhibit strong surface plasmon resonance in the near-infrared region. The SDBS molecules bind to the surface of Au colloids through the S element, and the charge transfer from Au atoms to S elements occurs. The charge is localized around Au atoms directly interacting with SDBS for fine Au colloids but delocalized to the entire Au colloid for large Au colloids and ensembles of fine Au colloids.