Structural investigation of sol-gel derived SiO2-GeO2 noncrystalline and nanocomposite materials

摘要

Noncrystalline and nanocomposite materials of (1 -x)SiO2·xGeO2 system, with Si:Ge ratio from 8:1 to 2:1 (x = 0.111; 0.142; 0.2; 0.333), initially obtained by sol-gel method, were characterized by thermal analyses, X-ray diffraction, nuclear magnetic resonance and Raman spectroscopy. According to DTA and XRD results, the noncrystalline state of the as-prepared samples is stable up to . 1,000 °C and only after 30 min heat treatment at 1,200 °C the samples become partial crystalline, due to development of cristobalite and quartz nanocrystals. Solid-state ~(29)Si MAS-NMR was employed in order to characterize the local structure around silicon as a function of composition and thermal history of the samples. The NMR data indicate the presence of Q~2, Q~3 and Q~4 units in all samples. The fraction of the highly interconnected SiO4 tetrahedra increases both with germanium content and with annealing temperature. The Raman spectroscopy results evidence structural changes related to silicon- and germanium-oxygen units but also to their interconnection, that depend on Si:Ge ratio and annealing temperature.