Nano-sized Ag metal particles can be synthesized by the addition of sodium borohydride as a reducing agent into an aqueous solution of AgNO3 mixed with 3-mercapto-propionic acid as a protective agent. Using this method the synthesis of an aqueous Ag colloid system with a high density became possible, because the surfaces of synthesized nano-sized Ag metal particles were covered and protected by the adsorbed 3-mercapto-propionic acid. The strong electrostatic repulsion of the negative charge of the carboxylate anion of the adsorbed 3-mercapto-propionic acid disturbed the aggregation of metal particles in the solution with a high pH value (pH > 5), but the aggregation occurred in a solution of low pH value (pH < 2) because of the neutral surface of particles. Using simple coating and baking processes with this aqueous Ag colloid system, an Ag membrane can be formed on glass plate by the partial welding between the nano-size Ag particles. This Ag membrane exhibited a very low electrical resistivity even after baking at a low temperature such as 423 K.

Keywords: nano-sized Ag particle; Ag colloid; mercapto-propionic acid; Ag membrane