A novel thermal electrochemical synthesis method for production of stable colloids of “naked” metal (Ag) nanocrystals

This paper describes a novel thermal electrochemical synthesis (TECS) method for producing aqueous solutions (or sols) that contain metal silver nanocrystals as small as a few nanometers. The TECS method requires mild conditions (25 to 100 °C), low voltage (1 to 50 V DC) on silver electrodes, and water or simple aqueous solutions as the reaction medium. Furthermore, a tubular dialysis membrane that surrounds the electrodes provides favorable conditions for producing nanosized (less than 10 nm) silver nanocrystals. Unlike nanocrystals reported in the literature, our nanocrystals have several unique features: (1) small nanometer-scale size, (2) “nakedness” (i.e., surfaces of metal nanocrystals are free of organic ligands or capping molecules and need no dispersant in synthesis solutions), and (3) colloidal stability in water solutions. It was discovered that silver nanoparticles with initially large size distribution can be homogenized into near-monodispersed colloidal sol by a low-power (less than 15 mW) He–Ne laser exposure treatment. The combination of the TECS technique and the laser treatment could lead to a new technology that produces metal nanoparticles that are naked, colloidally stable, and uniformly sized. In the presence of a stabilizing agent (also a supporting electrolyte) such as polyvinyl alcohol, high yields of silver nanoparticles (less than 100 nm) in the form of thick milky sols are produced.