Separation of Nanoparticles in a Density Gradient: FeCo@C and Gold Nanocrystals

Size and geometric control of nanomaterials are important to the discovery of intrinsic size/shape dependent properties and bottom up approaches for the fabrication of functional nanodevices.[1–10] Two general strategies have been employed to create size-uniform nanocrystals. One method is direct particle size control during synthesis by adjusting growth parameters;[1–3,5, 7–9] the other is post-synthesis separation.[11–19] Much capacity exists to improve size separation efficacy in the latter case. Differential centrifugation can remove large and unstable particles from colloidal systems, but lacks precise control over particle size.[9, 11] Addition of adjustable amounts of “anti-solvent”[19] (including CO2)[12] into colloidal systems may make precipitation processes more controllable. Other methods include filtration[14] (including diafiltration[15]), electrophoresis, [16, 17] and chromatographic methods[11, 18] that can produce particle fractions with narrow shape and size distributions...