Complementary Reducing Agents are Responsible for Temporally Distinct Nucleation and Growth Phases During the Polyol Synthesis of Ag Nanocubes

Ethylene glycol or one of its oxidation products are believed to serve as reducing agents in the shape-controlled synthesis of Ag nanocubes (NCs) by the polyol process. The identity of end-groups of polyvinylpyrrolidone (PVP) impacts shape control with alcohol and aldehyde moieties serving as a primary Ag reducing agent. We explored the role of PVP end-groups in the polyol process by measuring the dependence of particle number density of Ag NCs produced on the initial concentration(s) of Ag and PVP using small angle x-ray scattering and statistically large particle size distributions analyzed by scanning electron microscopy. The number density of Ag NCs is strongly dependent on the starting concentration of PVP chains demonstrating PVP end-groups play an important role in the nucleation of NCs. The concentration of Ag⁺ is 2 orders of magnitude higher than the end-groups suggesting ethylene glycol must participate in the reduction of Ag⁺ during growth. Perturbation experiments and analysis of resultant particle size distribution reveal nucleation is fast relative to growth of NCs, reinforcing the synergy between PVP end-groups and ethylene glycol. The evidence demonstrates PVP end-groups and ethylene glycol are tandem reducing agents operative in temporally distinct phases of the polyol synthesis of Ag NCs.