Manipulation of the Ti³⁺/Ti⁴⁺ ratio in colored titanium dioxide and its role in photocatalytic degradation of environmental pollutants

The role of the Ti³⁺/Ti⁴⁺ ratio in colored titanium dioxide (TiO₂) for photodegradation of environmental pollutants from real wastewater samples still remains under explored. The aim of this research was to modify the Ti³⁺/Ti⁴⁺ ratio in colored TiO₂ and investigate its role in photocatalytic degradation of phenolic compounds in real agro-industrial effluent. The Ti³⁺/Ti⁴⁺ ratio on the surface of colored TiO₂ was easily controlled by only changing glycerol content in the synthesis media and calcination temperature. X-ray diffraction, X-ray photoelectron spectroscopy, diffuse reflectance UV-Visible spectroscopy, photoluminescence spectroscopy, and electrochemical impedance spectroscopy were used to investigate changes in structural, optical, and electronic properties of colored TiO₂ with varying Ti³⁺/Ti⁴⁺ ratios. Increase in the Ti³⁺/Ti⁴⁺ ratio substantially enhance the visible light absorption and narrow the bandgap of colored TiO₂. Thus, an increase in Ti³⁺/Ti⁴⁺ratio up to 0.134 improves electrons and holes separation and reduce their recombination. Increasing the Ti³⁺/Ti⁴⁺ratio above 0.134 accelerates electron-hole pair recombination. At the rise in the Ti³⁺/Ti⁴⁺ratio up to 0.134, the degradation efficiency of phenolic compounds from actual wastewater was found to be ∼48%, which is 2.5-fold higher than that of commercial TiO₂. The photocatalytic degradation efficiency reached 83% for individual model phenol in an aqueous solution. Exceeding the Ti³⁺/Ti⁴⁺ratio over 0.134 diminishes the photocatalytic performance down to 21% phenolic compounds degradation at the highest Ti³⁺/Ti⁴⁺ratio of 1.154.