Thermal Behavior and Phase Transformation of TiO2 Nanocrystallites Prepared by a Coprecipitation Route

TiO2 freeze-dried precursor powders were synthesized using a coprecipitation route that includes titanium tetrachloride (TiCl4) as initial material prepared at 348 K (75 °C) and pH 7. Differential scanning calorimetry/thermogravimetry (DSC/TG), X-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) and high resolution TEM were utilized to characterize the thermal behavior and phase transformation of the TiO2 freeze-dried precursor powders after calcination. The main compound of the TiO2 freeze-dried precursor powders was TiO2·H2O based on a TG analysis conducted at a heating rate of 20 K (20 °C)/min. The anatase TiO2 (a-TiO2) first appeared at 473 K (200 °C), then from a-TiO2 transformed to rutile TiO2 (r-TiO2) at 773 K (500 °C). The activation energy of a-TiO2 formation from TiO2 freeze-dried precursor powders was 242.4 ± 33.9 kJ/mol, whereas, the activation energy of phase transformation from a-TiO2 to r-TiO2 was 267.5 ± 19.1 kJ/mol. The crystallite size of a-TiO2 grew from 3.5 to 23.2 nm when raising the calcination temperature from 473 K to 873 K (200 °C to 600 °C). In addition, the crystallite size of r-TiO2 increased from 17.4 to 48.1 nm when calcination temperature increased from 773 K to 1073 K (500 °C to 800 °C).