Two-Stage Method for Deep Purification of Concentrated Silver Nanoparticle Sols

Owing to its simplicity and high efficiency, the citrate–sulfate method for the synthesis of silver nanoparticles could be employed for large-scale production of nanomaterials. However, the resulting silver sols are heavily contaminated with reaction by-products. To solve this problem, this study proposes a two-stage purification procedure for thorough removal of impurities from the synthesized silver sols without inducing their aggregation. At the first purification stage, silver sols are subjected to cyclic coagulation with sodium citrate solutions followed by decantation and peptization in deionized water. At the second stage, the remaining dissolved impurities are removed by deionization using a mixture of a “weak” cation exchanger in the H+ form and an anion exchanger in the OH– form. According to transmission electron microscopy, X-ray diffraction analysis, atomic emission spectroscopy, and IR spectroscopy data, the purity, stability, and size uniformity of the nanoparticles are improved progressively with each purification stage. The purified silver sols with concentrations as high as 250 g/L remain stable for a month and longer and can be concentrated by evaporation to 2145 g/L dispersions, which exhibit a low viscosity and an additive relation between their density and concentration.