Abstract

The formation of gold nanoparticles has been studied via the reduction of the metal salts in different Ionic Liquids (Ils) based on the 1-alkyl-3-methyl-imidazolium cation and different anions. Particles were grown at different temperatures for 24 h. The structure and morphology of the resulting nanomaterials has been studied via wide angle x-ray scattering (WAXS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and by spectroscopic techniques. Electron microscopic measurements reveal large differences between both the samples grown at different temperatures and in different ILs. The comparison between the different temperatures, but within one IL shows that 1-ethyl-3-methyl-imidazolium methane sulfonate (Emim) (MS) is the most straightforward IL to study in the case of gold nanoparticles. In this IL, the gold particles are always spherical and have a diameter of 5 to 7 nm. The significant change that occurs with increasing temperature is the aggregation of more and more gold particles into relatively well ordered supracrystalline aggregates at 180°C after 24 h. Aggregation with increasing temperature is qualitatively also supported by the UV/Vis data which show a decrease in the surface plasmon band with increasing reaction temperature. The formation of the somewhat larger particles at 140°C could be due to Ostwald ripening. If Ostwald ripening indeed takes place in (Emim) (ES), this suggests that (Emim) (ES) is not able to completely stabilize the small particles with a diameter of only a few nanometers. This is in contrast to (Emim) (MS).

Key words: Nanoparticles, plamons, aggregation, stabilization, monoclinic.