Reduction of TiO2 by a low-temperature degradation of adsorbed organic solvents

We discuss how employing a refluxing solvothermal treatment of TiO2 in organic solvents at mild temperature (120 °C) allows producing blue-colored and reduced titanium dioxide (TiO2-x). The treatment, tested for different organic solvents, increases the density of Ti(III) species and the appearance of two optical absorption features: a broad absorption band responsible for the blue coloration and a subgap absorption tail close to the band gap energy. Experimental analyses based on X-ray photoemission spectroscopy and excitation-resolved photoluminescence suggests that the solvothermal process at 120°C leads to formation of surface and subsurface oxygen vacancies (VO). Different solvents have been tested, where ethanol is found to be the most effective. Improved photocatalytic degradation by the processed TiO2 under VIS illumination is demonstrated, and the possible mechanism involved in the formation of surface VO is discussed. The method outlines a very simple, low-cost, and fast procedure to target the formation of VO in the TiO2 surface region.