Fabrication of Extremely Concentrated Silver Hydrosols without Additional Stabilizers

Applications of silver nanoparticles (Ag NPs) in modern technologies require environmentally friendly methods of large-scale production of the nanoparticles with controlled morphology and surface state in the form of high-concentration metal sols with minimal quantities of organic stabilizers. Herein, we report a procedure based on reduction of aqueous silver nitrate with ferrous sulfate in the presence of citrate ions. We studied the effect of various factors on the chemical reaction by applying transmission electron microscopy, ultraviolet–visible absorption spectroscopy, and dynamic light scattering (DLS) and proposed protocols with reduced quantities of the reagents allowing preparation of uniform spherical Ag NPs of 5 to 15 nm in diameter. A DLS study of sols after dilution was employed to estimate the tendency of colloidal particles to interact in order to optimize post-synthetic purification and concentration procedures. Particularly, filtration instead of centrifugation and electrolytic coagulation with trisodium citrate in place of sodium nitrate were utilized to produce extremely concentrated, more than 1000 g/L Ag, and stable silver hydrosols with no additional stabilizers. The chemical, X-ray photoelectron spectroscopy, and thermogravimetric analyses demonstrated that the Ag NPs contained citrate-derived capping ligands, and low amounts of Fe are appropriated for chemical and low-temperature sintering, surface functionalization, nanofluidics, and other applications.