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

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 degreesC, 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. (C) 2002 Elsevier Science B.V. All rights reserved.