Effects of aging and drying on structure of V₂O₅ gels

Semiconducting V₂O₅ gels with and without addition of GeO₂ were prepared by hydrolysis of metal alkoxides in alcoholic solutions. Wet gels, xerogels dried at room temperature and supercritically dried aerogels, were characterized by transmission electron microscopy, X-ray diffraction and small-angle X-ray scattering. Crystalline vanadium oxides were not observed in xerogels, but V₃O₇ diffraction peaks were found for the aerogels kept at 250°C and 210 atm for 2 h or longer before drying. In the Porod region, in the range of scattering vector between about 10⁻² and 10⁻¹ Å⁻¹, the power law was valid for wet and dry gels. Wet gels and aerogels are mass fractals. The fractal dimension of the wet gels increased from 2.1 to 3 during aging, but scarcely changed with supercritical drying. Xerogels are surface fractals with a surface fractal dimension of 2.1–2.5.