DOI: 10.1002/chem.200601259 Continuous Tuning of Cadmium Sulfide and Zinc Sulfide Nanoparticle Size in a Water-in-Supercritical Carbon Dioxide Microemulsion CarlosA. Fernandez and ChienM. Wai* [a] Introduction Synthesis of semiconductor quantum dots, such as cadmium sulfide and zinc sulfide nanoparticles, have been largely ex- plored during the last decade due to their size-dependent optical, magnetic and electric properties. [1–3] Luminescence tagging and imaging, medical diagnostics, drug delivery, and nanoelectronics are some of the applications that are being developed with these nanomaterials. [4–8] One of the ap- proaches utilized to synthesize these nanomaterials is the microemulsion-templated method using compressed gases as solvents. Particularly, supercritical carbon dioxide (scCO 2 ) has been used as the bulk solvent in microemulsion systems to prepare semiconductor nanoparticles. [9] Odhe et al. [10] pre- pared CdS and ZnS nanoparticles in scCO 2 using a microe- mulsion approach. They showed that the size of the quan- tum dots can be adjusted with the water-to-surfactant molar ratio or W value. Supercritical fluids (SCFs) have attracted considerable in- terest as a reaction medium to synthesize nanoparticles, [11–14] because the variations in SCF solvent properties, such as density, diffusivity, viscosity, and dielectric constant, can be easily manipulated by changing the system temperature and pressure., [15,16] In addition to these advantages, scCO 2 pro- vides an attractive substitute for petroleum-based organic solvents for chemical synthesis since it is nontoxic, environ- mentally-benign, nonflammable, inexpensive, and readily available in large quantities. The unique property of super- critical fluids, including scCO 2 , is its tunable solvent strength through manipulation of its density (1), which can be easily controlled, as mentioned above, by varying temperature and pressure of the fluid phase. This property is attractive for studying nanoparticles) formation using water-in-scCO 2 mi- croemulsions as nanoreactors. There are a few reports study- ing the variation in the size of metallic nanoparticles with the density of the fluid phase. Shah et al. [17] reported the in- fluence of pressure, stabilizer, and precursor concentration in the formation and dispersion of silver nanoparticles ob- tained by arrested precipitation, where a thiol stabilizer was added together with hydrogen gas (H 2 ) to a CO 2 -soluble metal precursor, silver acetylacetone. In this study, no mi- croemulsion was used for silver nanoparticle synthesis and Abstract: 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 CO 2 microemulsions. The average size of the ZnS nanoparticles decreases lin- early 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. Monodisper- sive CdS and ZnS nanoparticles were synthesized by chemical reaction of cadmium or zinc nitrate with sodium sulfide, using two water-in-supercritical CO 2 microemulsions as nanoreactors followed by protection with a fluorinat- ed-thiol stabilizer. The stabilizer is in- troduced 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 CO 2 microemulsion method represents a simple approach to use a density-tunable solvent for synthesizing size-controlled semicon- ductor nanoparticles over a broad range of values. Keywords: microemulsions · nanoparticles · semiconductors · supercritical carbon dioxide [a] C. A. Fernandez, Prof. C. M. Wai Department of Chemistry, University of Idaho P.O. Box 442343, Moscow, ID 83844-2343 (USA) Fax: (+ 1) 208-885-6173 E-mail: cwai@uidaho.edu # 2007 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim Chem. Eur. J. 2007, 13, 5838 – 5844 5838