*2.5.1.3. pH*

The pH of the sol-gel system for the preparation of uniform nanoparticles of anatase titania from condensed TiO2 gel is a key factor for controlling the final particle size and shape of the product [67]. The grain size of the TiO2 particles generally increases with increase in pH of the sol [68]. When the hydrogen ion concentration is high, the particles grow rapidly to form large grains because the hydrogen ions interfere in the reaction and decrease the nucleation rate. The new nucleus has enough time to grow and aggregate into large TiO2 particles [69]. Matijevic et al. [70] reported the synthesis of TiO2 spherical particles of a narrow size by aging a highly acidic solution of titanium tetrachloride at elevated temperatures for 6–47 h. The amount of acid (pH) determines not only the size of the nanoparticles but also the stability of the sol [71].
