Mechanism of producing metallic nanoparticles, with an emphasis on silver and gold nanoparticles, using bottom-up methods

Abstract

Bottom-up nanoparticle (NP) formation is assumed to begin with the reduction of the precursor metallic ions to form zero-valent atoms. Studies in which this assumption was made are reviewed. The standard reduction potential for the formation of aqueous metallic atoms— E0 (Mn+ aq/M0 aq)—is significantly lower than the usual standard reduction potential for reducing metallic ions Mn+ in aqueous solution to a metal in solid state. E0 (Mn+ aq/M0 solid). E0 (Mn+ aq/M0 aq) values are negative for many typical metals, including Ag and Au, for which E0 (Mn+ aq/M0 solid) is positive. Therefore, many common moderate reduction agents that do not have significantly high negative reduction standard potentials (e.g., hydrogen, carbon monoxide, citrate, hydroxylamine, formaldehyde, ascorbate, squartic acid, and BH4− ), and cannot reduce the metallic cations to zero-valent atoms, indicating that the mechanism of NP production should be reconsidered. Both AgNP and AuNP formations were found to be multi-step processes that begin with the formation of clusters constructed from a skeleton of M+-M+ (M = Ag or Au) bonds that is followed by the reduction of a cation M+ in the cluster to M0, to form Mn0 via the formation of NPs. The plausibility of M+-M+ formation is reviewed. Studies that suggest a revised mechanism for the formation of AgNPs and AuNPs are also reviewed.