Hydrothermal-hydrolysis synthesis and photocatalytic properties of nano-TiO2 with an adjustable crystalline structure

Tri-phase (anatase, rutile, and brookite), bi-phase (anatase and rutile), and mono-phase (rutile) TiO2 nanomaterials with different morphologies were successively synthesized using a hydrothermal-hydrolysis method and adjusting the Ti4+/Ti3+ molar ratio in a precursor solution. The properties of the fabricated nanomaterials were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photocatalytic reaction, and other techniques. It has been shown that TiO2 nanorods can be obtained by increasing the Ti4+/Ti3+ molar ratio in a precursor solution from 1:0 to 0.3:0.7. TiO2 nanoparticles are formed if the Ti3+ fraction in the solution is further increased. The selective synthesis of TiO2 nanomaterials is explained by a decrease in the reaction rate and by changes in acidity with increasing Ti3+ content. The tri-phase nanorods and bi-phase nanoparticles synthesized with Ti4+/Ti3+ molar ratios from 1:0 to 0.8:0.2 and 0.2:0.8 to 0:1, respectively, have a higher degradation ability with respect to methylene blue aqueous solutions under UV irradiation at ambient temperature compared to purely rutile TiO2 nanorods synthesized with Ti4+/Ti3+ molar ratios from 0.7:0.3 to 0.3:0.7. The high photocatalytic activity of the multi-phase TiO2 samples is primarily attributed to their larger band gap and suppressed recombination of photo-generated electron-hole pairs.