Continuous Tuning of Cadmium Sulfide and Zinc Sulfide Nanoparticle Size in a Water‐in‐Supercritical Carbon Dioxide Microemulsion

The size and size dispersion of cadmium sulfide and zinc sulfide semiconductor nanoparticles can be continuously tuned over a wide range of values by adjusting the density of the fluid phase in water-in-supercritical CO2 microemulsions. The average size of the ZnS nanoparticles decreases linearly from approximately 9.1 to 1.9 nm with increasing fluid density from 0.86 to 0.99 g cm(-3) at a water-to-surfactant ratio (W value) of 10. At a W value of 6, the particle size can be tuned from 7.0 to 1.5 nm in the same density range. In the case of CdS nanocrystals, the size varied from 7.1 to 2.0 nm when the W value was 10 and from 4.0 to 1.3 nm when the W value employed was 6, in the same density range. Monodispersive CdS and ZnS nanoparticles were synthesized by chemical reaction of cadmium or zinc nitrate with sodium sulfide, using two water-in-supercritical CO2 microemulsions as nanoreactors followed by protection with a fluorinated-thiol stabilizer. The stabilizer is introduced at 6 and 16 minutes after the mixing of the two microemulsions where the intensity of the characteristic absorption peak due to the quantum confinement properties of the CdS and ZnS nanoparticles (280 and 360 nm) reaches a maximum, respectively. The supercritical CO2 microemulsion method represents a simple approach to use a density-tunable solvent for synthesizing size-controlled semiconductor nanoparticles over a broad range of values.