We report here naked gold nanoparticles (Au-NPs) synthesized through the sonochemical reduction of [AuCl_4]^- using a high-frequency (950 kHz) ultrasound in aqueous solutions (in the absence of any amphiphiles as stabilizing and capping agents and reductants). In order to evaluate the mechanism on the size and shape determination of naked Au-NPs formed in solutions, we have investigated the effect of sonication time, ultrasound frequency, HAuCl_4 concentration and temperature on the size and shape of the naked Au-NPs. In this work, in particular, the effect of byproducts such as hydrogen (H_2), hydrogen peroxide (H_2O_2), hydrogen chloride (HCl) and chloride ions (Cl^-) produced during ultrasonication on the size and shape of naked Au-NPs were examined. We found that spherical naked Au-NPs with the diameter of 〜20 nm were selectively synthesized from a H_2-purged HAuCl_4 (0.1 mM) aqueous solution while the naked Au-NPs synthesized from an Ar-purged HAuCl_4 (0.1 mM) aqueous solution were multi-shaped, for example, spherical particles, triangular and hexagonal plates and rods. Furthermore, formation of the triangular and hexagonal plates was enhanced with the addition of hydrogen chloride (HCl) and sodium chloride (NaCl) in HAuCl_4 aqueous solutions. This is most likely due to the affinity of chloride ions with gold surface and the resulting specific adsorption of chloride ions on the particular crystal face (e.g., {111} face) of Au-NPs.
