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

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