Sol–gel synthesis and structural characterization of niobium-silicon mixed-oxide nanocomposites

(Nb2O5) x ·(SiO2)1−x gels of four different compositions with x = 0.025 (2.5Nb), 0.050 (5Nb), 0,10 (10Nb) and 0.20 (20Nb) were synthesized at room temperature from niobium penta-chloride and tetra-ethoxysilane and their structural evolution with the temperature was examined by X-ray diffraction, thermogravimetry/differential thermal analysis, Raman and IR spectroscopy (Fourier transform). The synthesis procedure tuned in this work allowed to obtain for each studied composition transparent chemical gels in which the niobium dispersion resulted to be strongly dependent on the Nb2O5 loading: it was on the atomic scale for the 2.5Nb and 5Nb gel samples whereas the gel structure of the 10Nb and 20Nb appears formed by phase separated niobia-silica nanodomains. All dried gels keep their amorphous nature up to 873 K, while at higher temperatures crystallization of T- and H-Nb2O5 polymorphs were observed according to the Nb2O5 loading: at low loading T-Nb2O5 was the main crystallising phase, whereas at higher one the H-Nb2O5 prevails. Particularly, T-Nb2O5 was the sole crystallising phase in the whole explored temperature range for the 2.5Nb, keeping its nanosize up to 1273 K for all samples except for the 20Nb.