In the present review article, some properties of SiO₂, bulk and coatings, produced by the sol-gel process with varying amounts of metals, such as Ag, Fe, Cu and Co, are analyzed after heat treatments at temperatures in the range of 100 to 800 °C. The samples were treated in air and in some cases under a reducing atmosphere. The properties of these materials depend on the composition of the starting solution, the type and amount of metal added and the post-deposition treatments. Some of the most important preparation parameters influencing the structure are the water/alkoxide ratio and the viscosity of the precursor solution.

Using various characterization techniques, it was observed that the chemical state of the added metal depends on the annealing atmosphere and temperature. Upon annealing, under oxidizing conditions, most of the metal aggregates and forms colloidal oxide particles. If the annealing is under a reducing atmosphere (carbon monoxide), the oxide particles are reduced to form metallic particles. The diffusion of the oxidizing or reducing gas is facilitated by the opened porous structure of these materials. The optical properties of these nano-structured glasses as well as their stability under ambient conditions are analyzed. In some cases (coatings with Co), the way the metal segregates depends on the concentration and also on the viscosity of the precursor solutions from which the coatings were prepared.