Radiation-induced and chemical formation of gold clusters

The kinetics of the gamma-radiolytical or chemical reduction of Au-III Cl-4(-), or of the combination of both methods, is followed as a function of the experimental conditions through the time evolution of the surface plasmon spectrum of the gold nanoparticles formed or of their sizes as observed by AFM imaging. It appears from the discussion on the mechanism that even with the strongly reducing radiolytic radicals, the low valency Au-I ions are somewhat protected by the more concentrated Au-III ions from reduction, up to a ratio of Au-I/Au-III = 1, and are stabilized for hours, unless clusters or 2-propanol (or PVA, but more slowly) catalyze their disproportionation. The cluster concentration increases correlatively with the dose. 2-Propanol or PVA are mild reducing agents and are unable to reduce Au-III directly except at the surface of clusters, previously formed, for instance, by partial radiolytic reduction. In this case, the cluster concentration remains the same but the size obtained after reduction by the alcohol increases slowly with time up to 100-500 nm, as in a development process. In order to avoid the relative extent of this development, associated with chemical reduction and even with the direct gamma-reduction of Au-III, in particular the Au-I disproportionation and reduction steps, high dose rate radiolysis has been used up to total reduction of the same solutions. The mechanism of reduction and growth, step-by-step, is discussed.