Semiconducting Metal–Organic Frameworks Decorated with Spatially Separated Dual Cocatalysts for Efficient Uranium(VI) Photoreduction

Reduction of soluble hexavalent uranium (U(VI)) to sparingly soluble tetravalent uranium (U(IV)) with semiconductor photocatalysts is recognized as a novel, green, and simple U-extraction method. Furthermore, effective charge separation and utilization are critical factors to achieve high-efficiency U(VI) photoreduction. Herein, a UiO-66-based heterostructured photocatalyst (MnOx/UiO-66/Ti3C2Tx) with spatially separated dual cocatalysts (MnOx nanoparticles and Ti3C2Tx MXene nanosheets) is successfully developed for efficient U(VI) photoreduction without sacrificial agents. As co-catalysts, MnOx nanoparticles favor the trapping of holes, while Ti3C2Tx MXene nanosheets tend to collect electrons. Consequently, the photogenerated holes and electrons flow into and out of the photocatalyst, respectively, achieving efficient charge separation required by MnOx/UiO-66/Ti3C2Tx to remove U(VI). Impressively, the U(VI) removal ratio via MnOx/UiO-66/Ti3C2Tx reaches to 98.4% in the U(VI) solution after 60 min, with a photoreaction rate constant of 0.0948 min−1. Moreover, MnOx/UiO-66/Ti3C2Tx exhibits brilliant U(VI) extraction capacity in various U(VI) wastewater and U(VI)-spiked real seawater. Further mechanistic studies indicates that the photogenerated electrons are transferred from the conduction band of UiO-66 to Ti3C2Tx MXene to reduce U(VI) and generate ·O2–, further leading to a stable crystal phase of (UO2)O2·2H2O. Furthermore, the photogenerated holes are extracted by MnOx nanoparticles in MnOx/UiO-66/Ti3C2Tx to oxidize water.