Synthesis of silver and copper nanoparticles in a water-in-supercritical-carbon dioxide microemulsion

Nanometer-sized silver and copper metal particles can be synthesized by chemical reduction of Ag+ and Cu2+ ions dissolved in the water core of a water in supercritical fluid carbon dioxide microemulsion. Sodium cyanoborohydride and N,N,N',N'-tetramethyl p-phenylenediamine are effective reducing agents for synthesizing these metal nanoparticles in the microemulsion. Formation of the metal nanoparticles was monitored spectroscopically using a high-pressure fiber-optic reactor equipped with a CCD array UV-vis spectrometer. Silver and copper nanoparticles synthesized in the microemulsion showed characteristic surface plasmon resonance absorption bands centered at 400 and 557 nm, respectively. Diffusion and distribution of the oxidized form of the reducing agent between the micellar core and supercritical CO2 appeared to be the rate-determining step for the formation of the silver nanoparticles in this system.