Modified side-chain COFs construct built-in electric fields with low exciton binding energy for photo-reduced uranium

Abstract

Photoreduction of uranium(U) is recognized as an effective U-enrichment method that converts soluble U(VI) into insoluble U(IV). However, deficient exciton dissociation and low charge mobility in COFs limit their photocatalytic activity. Excitonic dissociation and charge transfer in COFs could be regulated by grafting different polar functional groups, but the relevant studies are still insufficient. In this work, we constructed a series of donor–acceptor (D-A) COFs (TpBD-X, X = –OH, NH2, –OCH3, –NO2, -SO3H) materials with electric built-in fields by different functional groups for U(VI) adsorption-photoreduction. The research results showed that, due to the introduction of functional groups, the exciton dissociation energy was decreased to 22.91 m·eV, the carrier lifetime was prolonged to 3.109 ns, light-absorption capacity was significantly improved, the forbidden bandwidth was reduced and the carrier transfer impedance was lowered. Thereby, the U(VI) reduction efficiency was enhanced from TpBD (33.34 %) to TpBD-(NO2)2 (73.11 %), and the corresponding reduction reaction rate constants were 1.40–5.58 times higher than that of TpBD, among which TpBD-(OH)2 and TpBD-(NO2)2 exhibited the strongest photocatalytic performance. In addition, DFT calculations revealed that the calculated Eads of TpBD-X was much lower than that of TpBD, and the chelating sites were increased from the single benzidine cluster to multi-coordination sites (internal H-bonds, keto-O, and bridged-N), which improved the adsorption affinity for UO22+ and stabilized the adsorption conformation, as well as these sites were platforms for electron-transfer contributing to the U(VI) reduction. More importantly, the generation of H2O2 during the photocatalytic process facilitated the U-extraction (in the form of UO2 and (UO2)O2·2H2O) from water, wherein TpBD-(OH)2 and TpBD-(NO2)2 exhibited the strongest H2O2 generating capacity of 166.14 and 223.31 mM, respectively. This study provided a valuable example of rationally designed and constructed COFs photocatalysts for light-assisted U-extraction with the purpose of regulating carrier behavior.