UV-Visible Absorption Characteristics of TEOS-Derived and Cr-Doped Silica Sonogels Aged in Different pH Solutions and Heat Treated up to 600°C

Cr-doped xerogels were obtained by sol-gel process from the acid-catalyzed and ultrasound-stimulated hydrolysis of tetraethoxysilane (TEOS) with addition of CrCl3ċ6H2O in water solution during the liquid step of the process. The gels were aged immersed in different pH solutions for about 30 days, after that they were allowed to dry. The samples were annealed at temperatures ranging from 40 to 600°C and analyzed by UV-visible absorption spectroscopy. Cr3+ is the preferable oxidation state of the chromium ion in the gels annealed up to 250–300°C, in the case of aging in solutions of pH = 5 and 11. A high UV absorption below ∼320 nm, due to the host gel, and different absorption bands, depending on the temperature, due to the chromium ion were observed in the xerogels at temperatures below ∼250°C, in the case of aging in solutions of pH = 1 and 2. These absorption bands have not been assigned. Above 300°C up to 600°C, Cr5+, and possibly Cr6+, are the preferable oxidation states of the chromium ion independent of the pH of the aging solution, so the xerogels turn to a yellowish appearance in all cases.