Structure and chemistry of Ag-Cu nanoclusters in a silica matrix by the sol-gel process

Mixed Ag-Cu nanoclusters with doped silica coatings were prepared by the sol-gel process and characterized by conventional transmission electron microscopy, nanodiffraction, energy-dispersive X-ray spectrometry, high-resolution transmission electron microscopy and related Fourier transforms and optical absorption spectroscopy. This study reports on the results of two samples with Cu-to-Ag molar ratios of 1 and 3 respectively. A constant (Ag + Cu)-to-SiO2 molar ratio of 0.175 was maintained in both cases. Optical absorption spectra of the coated samples exhibit surface plasmon resonance bands characteristic of both Ag and Cu metallic clusters after heat treatment in 5% H-2-95% N-2 atmosphere at 700 degrees C. The transmission electron microscopy was performed in order to correlate the Cu-to-Ag molar ratio with the modification of the optical properties. A different cluster size distribution was found for the two different molar ratios. A bimodal distribution was found for a Cu-to-Ag ratio of 1, while a more homogeneous monomodal cluster distribution was obtained for the sample with a Cu-to-Ag ratio of 3. Both materials show the presence of Ag and Cu clusters as separate crystalline phases. In case of the Cu-to-Ag ratio of 3, electron microscopy results suggest the presence of a superstructure and/or a Cu-Ag alloy in the clusters.