FTIR studies of the thermo-reversible sol–gel transition of a titanium butoxide solution modified by nitrate ions

Abstract

A thermo-reversible sol–gel transition in solutions made of titanium n-butoxide, ethanol, water, and nitric acid is studied by in situ temperature-dependent Fourier transform infrared spectroscopy. Infrared spectra show that at temperatures departing from 45\u2009°C, the nitrate ion substitutes the partially hydrolyzed butoxide ligands, by forming bridges and mono- and bidentate chelates. The exposed oxygen atoms in the nitrate ion favor the assembly between oligomers of low molecular weight through hydrogen bonds where water molecules work as binding agents, leaving to the gel formation. When the material returns to room temperature, the nitrate ion comes back to its free state, regenerating the sol. The suspension evolves with aging time from transparent to white, indicating the formation of TiO2 particles, although the thermo-reversible effect is still observed upon heating. Infrared results suggest that gel formation proceeds through the same mechanism, however, the chelates and bridges occur through exposed Ti–OH moieties onto the particle surfaces. Additionally, after an aging period, a gel is formed at room temperature, that recovers the fluid state upon mechanical perturbation. In this case, TiO2 particles join directly by week interactions such as hydrogen bonds. The dynamic rheological studies of the temperature-dependent viscoelastic behavior indicate that the elastic component is the most important in the material’s structure. Thermo-reversible (TR) sol-gel transition observed in Ti but-EtOH-H2O in the presence of NO3−. From 45\u2009ºC NO3− form chelates and bidentate bridges with Ti atoms, forming the TR gel. Small weight oligomers are the building blocks of the TR gel in the early stages. Condensation reactions lead to slow TiO2 formation. TiO2 particles bridged thru surface groups are the building blocks of the gel in the latter stages. Thermo-reversible (TR) sol-gel transition observed in Ti but-EtOH-H2O in the presence of NO3−. From 45\u2009ºC NO3− form chelates and bidentate bridges with Ti atoms, forming the TR gel. Small weight oligomers are the building blocks of the TR gel in the early stages. Condensation reactions lead to slow TiO2 formation. TiO2 particles bridged thru surface groups are the building blocks of the gel in the latter stages.