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

Automated summary

The research on the mechanism of nanoparticle formation reveals that many common moderate reduction agents are unable to reduce metallic cations to zero-valent atoms, leading to a need for reconsideration of the mechanism of nanoparticle production. Both AgNP and AuNP formations were found to be multi-step processes starting with the construction of clusters from M+-M+ bonds, eventually forming NPs.