Sonolytic control of rate of gold(III) reduction and size of formed gold nanoparticles: Relation between reduction rates and sizes of formed nanoparticles

The sonochemical reduction of gold(III) [tetrachloroaurate(M)] was carried out under various irradiation conditions in an aqueous solution containing only a small amount of 1-propanol. Identical starting materials and the same initial concentrations were used in each preparation. The rates of gold(Ill) reduction were strongly dependent on the atmosphere, the temperature of the bulk solution, the intensity of the ultrasound, and the distance of the reaction vessel from the oscillator. For example, the rates of reduction under several atmospheres were in the order of CH4 = CO2 < N-2 < Ne < He < Ar < Kr, where no reduction proceeded under the CH4 andCO(2) atmospheres. It was clearly found that the rates of reduction were influenced by the cavitation phenomenon. Upon irradiation, colloidal gold particles having a surface plasmon absorption were formed, although in the absence of any stabilizers for the gold particles. It was found by TEM observations that the average size of the formed gold particles changed from 30 to 120 nm by selecting the irradiation parameters. The size of the gold particles was closely correlated to the initial rate of gold(III) reduction; the higher the rate of reduction, the smaller the particles. This relationship suggested that the rate of reduction would affect the initial gold nucleation processes. In a 200 kHz standing wave system used in the present study, the strength of the generated shock waves as well as the mechanical effects could be regarded as relatively weak.