Mechanism of Producing Metallic Nanoparticles, with an Emphasis on Silver and Gold Nanoparticles, Using Bottom-Up Methods

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—E(0)(M(n+)(aq)/M(0)(aq))—is significantly lower than the usual standard reduction potential for reducing metallic ions M(n+) in aqueous solution to a metal in solid state. E(0)(M(n+)(aq)/M(0)(solid)). E(0)(M(n+)(aq)/M(0)(aq)) values are negative for many typical metals, including Ag and Au, for which E(0)(M(n+)(aq)/M(0)(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 BH(4)(−)), 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 M(0), to form M(n)(0) 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.