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

Abstract

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 [AuCl x (OH)4−x ]− (x ≥ 2) at low pH but [AuCl x (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.

Building-block design

“…It is conceivable that the chloride anions are replaced slowly by hydroxide anions over time. [93] This is in line with the experimental finding that the pH value of the gold solutions decreased gradually to below pH = 3. The fact that the hydroxyl complexes (low x) are needed for the nanoparticle growth mechanism would indeed explain the necessity to work with pre-dissolved gold chloride.…”

Preparation of Electrically Conductive Au Thin Films by Colloid Sedimentation

“…Acidic and alkaline conditions were used to slow down the hydrolysis of HAuCl 4 and NaBH 4 , respectively. 18,19 Typically, 20 mL of Au colloid was prepared in a 50 mL beaker by adding 200 μL of NaBH 4 /NaOH solution to 19.60 mL of ethanol under magnetic stirring at 600 rpm. After 30 s, 200 μL of HAuCl 4 /HCl was added and the stirring was continued for 10 s. A grey, turbid suspension was immediately formed.…”

Monodisperse sub-10nm gold nanoparticles by reversing the order of addition in Turkevich method – The role of chloroauric acid

“…We found recently [43] that tannic acid, a reducing agent with a redox potential comparable to that of citrate, yields 2-10 nm gold nanoparticles with low polydispersity when chloroauric acid is added slowly to tannic acid. The presence of reactive chloroauric acid species at acidic pH [13,44,45] and the time taken to form less reactive hydroxy-chloroauric species, after a step change in pH from acidic to neutral conditions, were shown to determine the rate of reduction [43].…”