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— 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 M_n⁰ 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.