The size dependent properties of metallic and semi-conductor nanoscale materials allow them to be engineered for specific applications such as in catalysis and quantum dots with unique optical properties. Solution based nanoparticle synthesis techniques are among the most simple, but, they often result in particles with a wide size range (e.g. 1 to 10 nm). To address this limitation, post synthesis processing is required to further refine the size distribution to the desired monodisperse range. Currently employed post-synthesis techniques are time intensive, solvent intensive, and inefficient.

This paper presents an environmentally friendly and efficient process for size selective fractionation of polydisperse metal and semiconductor nanoparticle dispersions into multiple narrow size populations (± 0.5 nm) using the pressure tunable physico-chemical properties of CO₂ gas expanded liquid (GEL) solutions. Our previous work on this nanoparticle separation technique has revealed that ligand stabilized nanoparticles can be size selectively precipitated by controlling the addition of CO₂(pressurization), which acts as an antisolvent, to an organic dispersion of nanoparticles. (J. Phys. Chem.B. (2005), 109(48), 22852). In this presentation, the efficiency of nanoparticle size fractionation was investigated on several types of metallic (Ag, Au, Pd, Pt) and semiconductor (CdSe/ZnS) nanoparticles to both illustrate the general applicability of the process and to provide fundamental information on the effects of several processing parameters. In the case of the semiconductor nanoparticle separations, a mixture of different sized TOPO (tri-n-octylphosphine oxide) capped ZnS coated CdSe core shell quantum dots in hexane solution were fractionated into monodisperse size fractions allowing separation of the dispersion into distinctly different colored solutions.

In this antisolvent nanoparticle precipitation technique, ligand capped particles are first dispersed in solution where the interaction between the solvent and the ligand tails provides enough repulsive force to overcome the inherent van der Waals attraction between the particles that would otherwise result in agglomeration and precipitation. Through the addition of an antisolvent, the resultant poorer solvent mixture interacts less with the ligand tails than did the pure solvent, thereby reducing the ability of the solvent/antisolvent mixture to disperse the particles. Larger particles possess greater interparticle van der Waals attractions and therefore precipitate first upon worsening solvent conditions followed by subsequent precipitation of the smaller sized particles with further addition of antisolvent. To achieve these separations using CO₂ as an antisolvent, a novel high pressure apparatus has been designed that allows the controlled nanoparticle precipitations to occur from a liquid droplet situated at a specific location on a surface by simply tuning the CO₂ pressure applied above the liquid dispersion. Compared to current techniques, this CO₂ expanded liquid approach provides for faster and more efficient particle size separation, reduction in organic solvent usage, and pressure tunable size selection in a single process.

To improve our fundamental understanding and to further refine the size separation process, detailed studies were performed on the size separation process using various types of stabilizing compounds and solvents. The effects of temperature, time and recursive fractionation (repeating a set of size separation experiments on one of the recovered fractions) on this process was also studied in order to identify the key parameters enabling size separation of various nanoparticle populations. Issues regarding the scale-up of this size-separation technique will also be discussed.