Surfactant- and reducer-free synthesis of gold nanoparticles in aqueous solutions

We report here the surfactant- and reducer-free synthesis of gold nanoparticles from an aqueous hydrogen tetrachloroaurate (III) tetrahydrate (HAuCl4·4H2O) solution using a high-frequency (950 kHz) ultrasound (in the absence of any stabilizing, capping and reducing agents). In particular, size, shape and stability of gold nanoparticles prepared by the 950 kHz ultrasound irradiation (sonication) for 8 min in the surfactant-free aqueous solutions were examined in terms of AuCl4− concentration (in the range of 0.01–0.1 mM), additional salts (NaCl, HCl and NaOH) and temperature (in the range of 4–60 °C). We found that higher AuCl4− concentration promoted particle growth (size increase) and plate formation. In addition, the plate formation was enhanced with the addition of NaCl or HCl (but not NaOH). This is most likely due to the AuCl4− reduction on a certain crystal facets (e.g. (1 1 0) facets) caused by the adsorption of Cl− ions on specific crystal facets (e.g. (1 1 1) facets). Furthermore, we revealed that the temperature elevated above 50 °C led to the formation of spherical gold nanoparticles with the diameter of 20–60 nm from a 0.1 mM AuCl4− aqueous solution while triangular plates formed coexisting with spherical nanoparticles below 50 °C.