Optical characteristics of sol–gel silica containing copper

In this work we have evaluated the influence of concentration and temperature of reagents on structural properties of sol–gel silica doped with copper nitrate. Samples were heated at 80 °C and at 120 °C and presented molar ratios of TEOS/Cu of 1/0.0084, 1/0.042, 1/0.084 and 1/0.0104. This last sample was additived with formamide at molar ratio TEOS/formamide of 1/0.015. The molar ratio of TEOS/H2O/EtOH/HNO3 used for the first three samples was 1/3.8/3.8/0.005, respectively, and for the last sample the ratio was 1/3.8/3.8/0.009. Optical transmission was studied using visible-near infrared spectroscopy (UV–vis–NIR) and Fourier transform infrared spectroscopy (FT-IR). The true density of the samples was measured using helium picnometry and the porosity was evaluated by nitrogen gas sorption (BET). During the evolution of the drying process, between 80 and 120 °C, we observed that the color changed from blue to green, which is related to the environmental condition changes on the coordination of the Cu2+ ions.