Sonochemical formation of gold particles in aqueous solution.

Gold(III) ions in aqueous solutions of NaAuCl4 were reduced to form gold particles by ultrasonic irradiation. The rate of formation of gold particles was accelerated in the presence of certain organic additives such as surfactants, water-soluble polymers and aliphatic alcohols and ketones. The rates of formation of gold particles from 1 mM Au(III) ions in pure water were 3 microM min-1 under argon atmosphere and approximately zero under air, and in solutions containing additive the rates were 9-133 microM min-1 under argon and 8-40 microM min-1 under air. Surfactants stabilized the particles as colloidal state for more than several months. The rates of formation of both hydrogen atoms and hydroxyl radicals were estimated to be equal to 25 microM min-1 in the sonolysis of pure water under argon. Three reaction pathways leading to the reduction of metal ions were proposed: (1) reduction by hydrogen atoms; (2) reduction by reducing radicals formed via reactions of hydroxyl radicals or hydrogen atoms with the additives; (3) reduction by radicals formed from thermal reaction of the additives at the interfacial region between cavitation bubbles and bulk solution and/or in the cavities. The order of the contribution of these three pathways to the reduction of gold ions was (3) > (2) > (1) in most cases. The number averages of the size of gold particles formed in surfactant solutions under argon atmosphere were about 10 nm with a fairly narrow size distribution.