Synergistic Effect Of S/O Atomic Vacancies And Schottky Heterojunctions For Photocatalytic Reduction Of U(Ⅵ)

The depletion of fossil resources has led to the rapid development of nuclear energy,but has inevitably led to a series of environmental problems.Among them,hexavalent uranium in radioactive nuclear wastewater poses a serious threat to human health due to its unique chemical toxicity and radioactivity.The efficient removal/enrichment of uranium from radioactive nuclear wastewater is a major challenge to address uranium contamination and uranium mineral shortage for sustainable development.In recent years,the conversion of highly soluble hexavalent uranium[U（VI）]to relatively low-soluble tetravalent uranium[U（IV）]using photocatalysis has been considered as one of the effective strategies for the removal and immobilization of U（VI）in view of some problems of conventional treatment methods for uranium contamination.Unfortunately,conventional photocatalytic materials lack effective carrier separation mechanism and directional charge transfer mechanism leading to high photogenerated carrier recombination rate,which inhibits the catalytic activity of photocatalytic materials to some extent.In addition,most conventional photocatalysts can be driven by only a single light energy,ignoring other energy fields present in the radioactive nuclear wastewater,which makes the photocatalyst underutilized for energy in the liquid phase environment.Therefore,there is an urgent need to investigate and design an efficient carrier separation mechanism and a multi-field driven photocatalyst for the efficient reduction of U（VI）.This work is based on polarized electric field,molecular orbital hybridization and density generalization theory,and the mechanism of vacancy-induced asymmetric distortion of the crystal structure to enhance photocatalytic activity has been investigated in depth from a phonon or electron perspective using phonon spectra,energy bands,bader topological calculation,HOMO/LUMO orbitals and electron/hole effective masses.The synergistic effect of vacancy and Schottky heterojunctions and the mechanism of atomic-vacancy enhance thermoelectricity are systematically revealed through extensive thermodynamic and kinetic experiments.It will provide the theoretical basis and technical support for the design of multi-field-driven photocatalytic materials with both catalytic activity and practicality.The main contents include the following:（1）A heterojunction photocatalyst（CA@Cd S-SV）was designed consisting of carbon aerogel and Cd S nanoflowers with S vacancies.The removal ratio of U（VI）by this catalyst exceeds 97%within 40 min without adding any vacancy sacrificial agent,in which more than 83.2%of U（IV）can be effectively immobilized.The mechanism of the synergistic effect of S vacancies and Schottky heterojunctions on the immobilization of U（VI）by the photocatalyst was revealed by thermodynamic and kinetic studies.（2）TiO_2-x/1T-MoS₂photocatalysts that can be driven by both light and thermal energy were prepared by exploiting the difference in the work function between 1T-Mo S₂and Ti O_2-x.It was demonstrated that the presence of O vacancies reduced the electron affinity in Ti O_2-xand led to a reduction of the potential barrier height at the heterojunction contact surface to0.25 e V.The Seebeck coefficient was further enhanced to 82.3μV/K,enabling more efficient energy filtration.The photocatalyst can remove 98.2%of U（VI）from the liquid-phase system within 60 min and maintain good U（VI）removal ratio even under strong acidic and alkaline environments.（3）TiO_2-x/1T-MoS₂/RGO dual co-photocatalyst with dual charge-transfer channels were successfully synthesized by utilizing the difference in Fermi energy levels.It was found that the formation of local lattice defects resulted in an ohmic contact between Ti O_2-xand the co-catalyst 1T-Mo S₂with a very small contact potential barrier.The presence of the electron buffer layer promoted the efficient migration of photogenerated electrons between the dual charge-transfer channels.Kinetic experiments showed that this photocatalyst achieved97.4%photoreduction of U（VI）in the liquid phase system within 80 min.（4）Through extensive experimental and theoretical calculations,it was revealed at the atomic orbital level that the main reason why the confinement of Co single atom by S-vacancy in Cd S will inhibits the activity of the catalyst.This leads to the extension of the general adaptation principle for vacancy confinement single atoms.In summary,this work investigates the mechanism of the synergistic effect of vacancies and Schottky heterojunctions to coordinate the spatial separation of photogenerated carriers from the perspective of thermodynamics and dynamics based on polarized electric field,molecular orbital theory and density functional theory,etc.in depth.The mechanism of enhanced thermoelectric effect due to asymmetric distortion of crystal structure caused by atomic vacancies was revealed.A dual charge-transfer channels was constructed for the directional separation of photogenerated carriers.The main reason why the confinement of Co single atom by S-vacancy in Cd S will inhibits the activity of the catalyst itself is revealed at the atomic orbital level.This work will provide a practical reference for the design of non-homogeneous catalysts with both catalytic activity and practicality for photocatalytic reduction of U（Ⅵ）.