Oxygen Vacancy-Mediated Photocatalytic Reduction of Perrhenate and Pertechnetate on Ultrathin TiO2 Nanosheets

Technetium-99 (99Tc) predominately exists in the form of oxygenated anions (99TcO4–) under oxic conditions, which are highly soluble and non/weakly complexing; converting it to a sparingly soluble 99TcO2·nH2O precipitate for further disposal is a well-established tactic. Oxygen vacancy (OV) engineering paves a shortcut for modulating the kinetics, energetics, and mechanisms of photocatalytic reduction synergistically. However, the mechanism that OVs contribute to the photocatalytic reduction of 99TcO4– remains unclear. Herein, OV-enriched ultrathin two-dimensional TiO2 is fabricated via the solvent-free confined synthesis strategy. Based on the characterization analysis with experiments and density functional theory, it is demonstrated that the apparent elevated ReO4– (a nonradioactive surrogate for 99TcO4–) conversion efficiency is directly dependent on the content of OVs, which allows for the rapid photocatalytic reduction of Re(VII). Significantly, under simulated sunlight irradiation at pH = 2, 2D TiO2-500 °C reduced to 97.0% Re(VII) (20 mg/L) within 180 min. A reducing radical (·CO2–)-mediated pathway was proposed to account for the high-efficiency reduction of Re(VII) to Re(IV). During Re(VII) photoreduction, OVs can not only promote the adsorption of reactants but also effectively reduce the conversion barrier for the reducing radicals (·CO2–), providing a convenient channel for the rapid reduction of Re(VII). This study reveals the mechanism of OVs during the photoreductive transformation of Tc(VII)/Re(VII) comprehensively.