Controlled growth of gold nanoparticles in aerosol-OT/sorbitan monooleate/isooctane mixed reverse micelles

Stable anisotropic gold nanoparticles were prepared by the reduction of tetrachloroauric acid with hydrazine in mixed reverse micelles formed with anionic surfactant Aerosol-OT and nonionic surfactant sorbitan monooleate (Span80) in isooctane. It was found that the Span80 serves not only as a structure modifier but also as a stabilizer for Au particles, to prevent their further growth and precipitation. The control of particle size, shape, and degree of dispersion was achieved by varying the process variables, such as molar ratio of reduction agent to metal salt, size of water droplets (omega(o)), concentration of metal salt, and sequence of addition of metal salt into the mixed reverse micelles. When the HAuCl4 was injected directly into the mixed reversed micelles containing hydrazine, nonspherical gold nanoparticles, such as rods and cubes, were obtained at the molar ratio of hydrazine to HAuCl4 of less than 1.0. The nonspherical Au particles were preferably formed at larger omega(o) value and lower gold salt loading. By the analyses of high-resolution electron microscope, electron diffraction pattern, and energy-dispersive Xray analysis (EDX), the resultant particles have been found to be pure gold of face-centered cubic structure. (C) 2000 Academic Press.