New pathway for the synthesis of ultrafine silver nanoparticles from bulk silver substrates in aqueous solutions by sonoelectrochemical methods

Ultrafine silver nanoparticles with diameters less than 2 nm were first prepared in 0.1 M HCl aqueous solutions without addition of any stabilizer by sonoelectrochemical methods from silver substrates. First, an Ag substrate was cycled in a deoxygenated aqueous solution containing 0.1 M HCl from -0.30 to +0.30 V vs Ag/AgCl at 5 mV/s with 30 scans. After that Ag- and Cl-containing complexes were found existing in this aqueous solution. Then the Ag working electrode was immediately replaced by a Pt electrode and a cathodic overpotential of 0.2 V was applied under sonication to synthesize Ag nanoparticles. The resulting Ag nanoparticles are too small to exhibit any surface plasmon absorption, as illustrated in the UV-Vis absorption spectrum. An interesting phenomenon was also found from X-ray photoelectron spectroscopy (XPS) analyses that the binding energy of the prepared Ag nanoparticles shifts towards a higher value by ca. 0.2 eV, as compared with that of the employed Ag substrates. Furthermore, the particle sizes of the prepared Ag nanoparticles can be increased from less than 2 to 20 nm by increasing the cathodic overpotential applied from 0.2 to 0.8 V.