Bimetallic colloids of silver and copper in thin films: sol–gel synthesis and characterization

Abstract

A sol–gel route to synthesize thin films containing alloy- and phase separated mixed-colloids of silver (Ag) and copper (Cu) has been developed. Ag–Cu alloy colloids with Ag:Cu molar ratios of 4:1–1:2 in the coatings were obtained by the addition of copper ions stabilized in a pre-hydrolyzed SiO 2-sol in the presence of hydroxylamine hydrochloride, to a colloidal silver sol stabilized by a polymer, followed by the reduction of Cu + ions under reducing atmosphere. After a thermal treatment at temperatures ranging from 150 to 550 °C, the coatings were characterized by ultraviolet–visible spectroscopy, X-ray diffraction and high-resolution transmission electron microscopy. Mixing of silver and copper as ions always led to the formation of Ag–Cu phase separated mixed colloids. For Ag–Cu alloy colloids absorption peak remains as a single peak as long as the molar ratio of Ag and Cu is less than 4:1, whereas two absorption peaks were obtained on increasing the copper concentration. This behavior was attributed to the limited miscibility of Ag and Cu. On the other hand, for Ag–Cu mixed colloids two plasmon peaks were obtained irrespective to their molar ratios.