Low-frequency ultrasound induces oxygen vacancies formation and visible light absorption in TiO2 P-25 nanoparticles

Low-frequency ultrasound (LFUS) irradiation induces morphological, optical and surface changes in the commercial nano-TiO₂-based photocatalyst, Evonik-Degussa P-25. Low-temperature electron spin resonance (ESR) measurements performed on this material provided the first experimental evidence for the formation of oxygen vacancies (V_o), which were also found responsible for the visible-light absorption. The V_o surface defects might result from high-speed inter-particle collisions and shock waves generated by LFUS sonication impacting the TiO₂ particles. This is in contrast to a number of well-established technologies, where the formation of oxygen vacancies on the TiO₂ surface often requires harsh technological conditions and complicated procedures, such as annealing at high temperatures, radio-frequency-induced plasma or ion sputtering.Thus, this study reports for the first time the preparation of visible-light responsive TiO₂-based photocatalysts by using a simple LFUS-based approach to induce oxygen vacancies at the nano-TiO₂ surface. These findings might open new avenues for synthesis of novel nano-TiO₂-based photocatalysts capable of destroying water or airborne pollutants and microorganisms under visible light illumination.