Role of Steric Stabilization on the Arrested Growth of Silver Nanocrystals in Supercritical Carbon Dioxide

Abstract

Perfluorodecanethiol-stabilized silver (Ag) nanocrystals were synthesized in supercritical (sc)-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 °C), the ligands provide a strong steric barrier that maintains small particles with a 20 Å diameter. At lower solvent densities (P < 250 bar, T = 80 °C), the osmotic repulsions between capping ligands are weak, resulting in 40 Å 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.

Nanoparticle Clusters: Assembly and Control Over Internal Order, Current Capabilities, and Future Potential

“…[23] A recent discussion on the values of Hamaker constants of iron oxide NPs can be found in a paper by Faure. [24] It can be calculated as: …”

Gold nanoparticles protected by fluorinated ligands: Syntheses, properties and applications

“…Supercritical CO 2 also proved to be a good solvent for fluorous nanoparticles synthesis by using suitable organometallic precursors and hydrogen as reducing agent [21,22].…”

Final Report for Fractionation and Separation of Polydisperse Nanoparticles into Distinct Monodisperse Fractions Using CO2 Expanded Liquids

“…The van der Waals attractive force 27 , Φ vdW , between two particles increases with an increase in particle radius R or with a decrease in center to center separation distance between the particles d. Φ vdW is directly proportional to the Hamaker constant, A 131 . A 131 is a proportionality factor that accounts for the interaction between two nanoparticles of the same material (component 1) across a solvent (component 3) and depends on A 11 and A 33 , where A 11 is a constant value for metallic nanoparticles with A 11 = 2.185 eV for silver 26 and A 33 for the solvent is calculated by an equation of state based on Lifshitz theory 28 . A 33 depends on the refractive index n, and dielectric constant ε of the solvent as given in equation (4) components (in this case, CO 2 and hexane), so a new mathematical expression was developed to calculate the Hamaker constant where 3' represents one of the solvent components (in this case CO 2 ) and 3" represents the other solvent component (e.g.…”

“…There is experimental evidence that thiol tails on a metal surface are in the extended mode. For example, using IR spectroscopic and ellipsometric data, Porter et al 39 have shown that thiol tails with hydrocarbon group (-CH 2 ) greater than 9 assemble on gold surfaces in a densely packed manner with fully extended alkyl chains tilted from the surface normal by [20][21][22][23][24][25][26][27][28][29][30] o . This suggests that the Condensed Phase Model may not be the correct phenomenological model for our experimental system.…”