Role of steric stabilization on the arrested growth of silver nanocrystals in supercritical carbon dioxide

Perfluorodecanethiol-stabilized silver (Ag) nanocrystals were synthesized in supercritical (s)-CO2 through arrested precipitation, by reducing silver acetylacetonate (Ag(acac)) with hydrogen in the presence of fluorinated thiol. The CO2 density used during synthesis controls the particle size and polydispersity. At high solvent densities, (P > 250 bar, T = 80 degreesC), the ligands provide a strong steric barrier that maintains small particles with a 20 A diameter. At lower solvent densities (P < 250 bar, T = 80 °C), the osmotic repulsions between capping ligands are weak, resulting in 40 A diameter nanocrystals with higher polydispersity. At early stages in the growth process, metal core coagulation competes with ligand adsorption. Conditions effective for steric stabilization, such as long ligands and high solvent density, quench nanocrystal growth at relatively low ligand binding densities, which leads to smaller nanocrystals. Under poor solvent conditions, particles grow to larger sizes before the coverage of capping ligand is sufficient to prevent coagulation of metal particles. Perfluorodecanethiol-coated silver nanocrystals, synthesized in either good or poor solvent conditions, readily redisperse in acetone, fluorinated solvents, and sc-CO2 (at high density). The precursor concentration, thiol: precursor ratio, and reaction time do not affect the nanocrystal size appreciably, although these parameters do affect the polydispersity.