Oxygen Vacancy-Mediated Photocatalytic Reduction of Perrhenate and Pertechnetate on Ultrathin TiO 2 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.