In situ observation of formation of silver particles in water-in-scCO 2 emulsions

Abstract

Metal particles of silver (Ag) have been synthesized in the presence of Aerosol-OT (AOT) and a small amount of 2,2,3,3,4,4,5,5-octafluoro-1-pentanol (F-pentanol) and ethanol as co-solvents in water-in-scCO 2 microemulsions by the reduction of AgClO 4. The time evolution of the formation of Ag particles both by the photoreduction using UV-irradiation and using hydrazine as a reducing reagent has been studied by in situ UV–vis, small angle X-ray scattering (SAXS), extended X-ray absorption fine structure (EXAFS), and transmission electron micrograph (TEM) observations. The average diameter of the Ag particle in the dilute colloidal dispersions prepared by the photoreduction and by the hydrazine reduction was estimated from TEM to be 6.4 and 2.9 nm, respectively, which was consistent with that obtained from SAXS analysis using the Schultz–Zimm distribution for the q-range from 0.3 to 1.6 nm −1. By using Guinier plot in a small q-range (<0.3 nm −1), it was demonstrated that the average diameter of water droplet, which consists of aggregates of ionic precursors of AgClO 4 before reduction and Ag particles after reduction in the microemulsions, was estimated to be about 20 nm. The change of water droplet size at the dilute concentration of Ag + ions is negligible in contrast to the case of the higher concentration where the diameter of water droplet rapidly increases from 15.8 to 17.8 nm due to the formation of Ag particles in the AOT-rich phase at the beginning of hydrazine reduction. EXAFS analysis also indicates that Ag + ions were completely reduced to Ag 0 atoms in the AOT-rich phase by the hydrazine reduction, followed by the formation of larger Ag particles, while Ag + ions were partly reduced in the photoreduction by UV-irradiation. Once the Ag particles produced under high-pressure condition were released to the ambient condition, these particles were partly oxidized in which Ag–Ag metallic bonds disappeared and the formation of Ag–O bonds occurred.