Optimising the crystallinity of anatase TiO2 nanospheres for the degradation of Congo red dye

Anatase TiO₂ nanospheres having a high degradation efficiency are prepared through hydrothermal method employing titanium (IV) isopropoxide as precursor. The effects of hydrothermal temperature on structural, morphological and optical properties of TiO₂ nanospheres are investigated. X-ray diffraction (XRD) results show that, the TiO₂ nanospheres crystallise in the anatase phase, the tetragonal crystal system. The crystallite size increases with the increase in hydrothermal temperature and the values are in the range of 10–19 nm. On the other hand, the spherical particles evenly distributed on the surface are observed in the scanning electron microscope (SEM) images. The band gap of the nanospheres is determined from the reflectance spectra using Kubelka–Munk function. The band gap energy decreases from 3.189 (2) to 3.023 (5) eV with the increase in hydrothermal temperature. The decrease in band gap values well agree with the XRD results of higher crystallite size and enhanced crystallinity. The removal of Congo red dye under dark condition varies with the hydrothermal temperature and it is maximum for the higher crystallinity anatase TiO₂ synthesised at 160 °C having a lower band gap of 3.023 (5) eV. Maximum efficiency of 96.9% and rate constant value of 0.01413 g mg^?1 min^?1 are observed for the sample hydrothermally synthesised at 160 °C.