The different molar compositions investigated in this work as well as the sample notations are listed in Table 1. For the cobalt-doping within the hybrid silica matrix, a solution of cobalt nitrate in ethanol was prepared. Ethanol was chosen as the solvent for the salt solution because it was the same solvent used in the sols preparation. Moreover, the nitrate salt was used because it is compatible with the silica sol which was catalyzed by nitric acid [28].