Tuning the structural and optical properties in cobalt oxide-doped borosilicate glasses

Borosilicate glass systems with composition xSiO2 - (50-x) B2O3 - 30Na2O - 19.5ZnO - 0.5CoO (x = 0, 5, 10 and 20 mol. %) have been successfully prepared following the traditional melt quenching technique. The incorporation of SiO2 into borate glass network was found to tune its properties at different aspects, as probed by a set of complementary experimental techniques. The density of such boraosilicate glasses was decreased by SiO2 addition, thus, rendering the glassy network less tightly packed. In addition, the BO4 structural units were found to increase with SiO2 content as obtained from comparative FTIR spectra. Interestingly, the optical properties of such borosilicate glasses could be tuned through SiO2 addition. In particular, the optical band gaps attain larger values together with noticeable suppression of the band tails associated with defective structures upon SiO2 addition. Further insights into the crystal field interaction and d-levels splitting were obtained by a detailed deconvolution procedure applied to the characteristic absorption bands of Cobalt. The outcome of such a process revealed noticeable reductions in the ligand field strength and the associated d-levels splitting with SiO2 additives. Furthermore, the octahedral symmetry contribution was increased in borosilicate compared to the pure borate glasses as inferred from the oxidation states of Co ions probed by ESR. The present system combines the advantageous of the pure borate and silicate glasses and, therefore, offers an alternative candidate for optical applications.