Calcination effect on the properties of sol–gel synthesized TiO2 nanomaterials

The titanium dioxide (TiO2) nanoparticles were synthesized by the sol–gel method using a TTIP precursor. The synthesized TiO2 nanoparticles were divided into several parts and subjected to calcination for 2 h at temperatures ranging from 300 °C to 900 °C. The effect of calcination temperature on the structural properties was investigated using powder X-ray diffraction (XRD), Raman spectroscopy, and selected area electron diffraction (SAED). The morphological properties were examined using field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The optical band gap was evaluated using UV–visible spectroscopy. The TiO2 nanoparticles calcined at 300 °C exhibited XRD patterns corresponding to the anatase phase. The presence of low-intensity and broad diffraction peaks indicates the poor crystallinity of the nanostructures. Samples calcined up to 500 °C showed only anatase phase XRD patterns with improved crystallinity. The samples calcined at 600 °C exhibited predominantly anatase phase along with a very low-intensity (110) diffraction peak corresponding to the rutile phase. Samples calcined between 700 °C and 900 °C showed a gradual transformation from anatase TiO2 to rutile TiO2. However, the presence of a very low-intensity anatase (101) diffraction peak indicates that the applied calcination temperature and duration were not sufficient to achieve complete transformation to the pure rutile phase. Raman spectroscopy revealed only the anatase phase in samples calcined up to 600 °C, characterized by the intense Eg mode at approximately 144 cm⁻1. The Raman spectrum of TiO2 nanoparticles calcined at 700 °C still showed the intense anatase Eg band along with high-intensity rutile Eg and A1g modes, as well as features associated with multi-phonon processes. The Raman spectra of samples calcined at 800 °C and 900 °C exhibited characteristic rutile phase bands. The SAED patterns further confirmed the structural phase transformation from anatase to rutile in TiO2 nanoparticles calcined at 800 °C and 900 °C. The optical band gap energy of the annealed TiO2 nanostructures was found to decrease with increasing annealing temperature. .