Colloidal silver iodide: synthesis by a reverse micelle method and investigation by a small-angle neutron scattering study

Silver iodide nanoparticles were prepared by mixing two microemulsions containing the precursor salts, silver nitrate and potassium iodide. Five values of w=[water]/[surfactant] were used, namely 2, 4, 8, 16, and 32. Transmission electron microscopy (TEM) showed that the nanoparticle diameters were 4–240 nm and increased proportionally with w. The nanoparticles prepared from a microemulsion of w=2 and 8 showed brown and ocherous colors respectively, while bulk silver iodide powder is pale yellow. The crystal structure was determined by neutron powder diffraction. The composition was estimated to be 25% β-AgI and 75% γ-AgI. The super-ionic conductor phase could not be detected at room temperature. The w=8 microemulsion was examined with small-angle neutron scattering (SANS) in order to study the core-shell structure of the reverse micelles at various temperatures. Measurements were carried out at 20 and 9 °C and then at 70 °C and finally at 28 °C, in order to investigate whether the structural changes were reversible with temperature. The total radius of the micelles, surfactant shell thickness and water core radius decreased with increasing temperature and the changes were reversible with temperature. The particle diameters derived by TEM were generally larger than those estimated from the SANS measurements. This suggested that the silver iodide nuclei formed initially grew to reach a certain size, corresponding to the most thermodynamically stable species in the microemulsion, because of the fast exchange between the water cores.