Relationship between the bandgap and electrochemical behavior on TiO ₂ nanoparticles prepared sonochemically

Nanocristalline TiO₂ obtained by a facile and environment-friendly sonochemical method was subjected to thermal treatment in the temperature range of 400-900 °C in order to produce variable anatase-rutile phases ratio. The relationship between the optical bandgap and the electrochemical behavior was studied. All the stages of phase transformation of the as-prepared sample such as: nucleation, growth and coarsening were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was found that phase transformation mechanism stems from the redistribution of energy in the system and a critical particle size. On the other hand, the samples were characterized by UV-vis spectroscopy for the bandgap studies. The optical band gap of as-prepared sample increases to 3.31 eV with respect to 3.20 eV for bulk-anatase. This expansion could be attributed to quantum size effect. The i-E characteristics of samples with variable anatase-rutile ratio were obtained using cyclic voltammetry technique in a 0.5 M H₂SO₄ solution at room temperature. The foremost charge magnitude was obtained when anatase had a critical size of 17 nm. Analyzing both particle size for anatase and rutile, we observed that when rutile is the dominating phase and its size difference larger in 35% than anatase, the current reaches its minimum values. Based on electrochemical results, the optimal particle size and content phases control are important in order to obtain an increase in the electrochemical performance in the Hydrogen Evolution Reaction (HER) zone.