Surface Reconstruction Of Two-dimensional Semiconductor Materials And Study On Photocatalytic Reduction Of Uranium

Uranium,as the main nuclide produced in nuclear fuel production,nuclear power plant operation and nuclear facility decommissioning,has characteristics of long half-life and high toxicity.Uranium mainly exists in the form of dissolved hexavalent uranium（U（VI））in the environment,which easily migrates to the biosphere with surface water and groundwater,causing great harm to the ecological environment and human activities.Therefore,how to treat the wastewater containing U（VI）efficiently,safely and cheaply is still a difficult problem faced by researchers at home and abroad.Photocatalytic reduction treatment of radioactive wastewater containing U（VI）has attracted much attention because of the advantages such as good selectivity,non-toxicity,fast reaction speed and mild reaction conditions.Photocatalytic technology can transform U（VI）into tetravalent uranium（U（IV））to form precipitates so as to realize the separation and extraction of U（VI）.Currently,the catalysts used for photocatalytic reduction of uranium are mainly inert metal sulfur/oxide semiconductors,while the surface of these semiconductor materials lacks binding sites to uranium.In order to build a large number of binding sites to uranium,a large specific surface area needs to be constructed on the semiconductor material,that is,a two-dimensional semiconductor material.Additionally,it is necessary to reconstruct the surface of the two-dimensional semiconductor materials and introduce confinement sites to uranium.Therefore,the surface reconstruction and modification of traditional two-dimensional semiconductor materials and the development of new two-dimensional semiconductor materials with high efficiency,environmental protection,stability and high utilization to solar energy play an important role in promoting the development of photocatalytic technology in the field of radiochemistry.In this paper,the highly efficient two-dimensional semiconductor materials were designed by three different kinds of surface reconstruction methods,including the surface vacancies introducing coordination sites,the in-situ generation and dynamic regeneration of vacancies through atomic-level doping achieving sustainable coordination sites,as well as surface dangling bonds providing sites and plasmons adjusting the electrical and organic combination of the surface.This paper realized the separation of uranium in uranium-containing wastewater by highly efficient two-dimensional semiconductor materials through photocatalytic reduction method,studied the effect of surface reconstruction of two-dimensional semiconductor materials on its performance,and revealed the mechanism of photocatalytic reduction of U（VI）.The main content and results of this thesis are as follows:1.Using the gas reduction method,oxygen vacancies were introduced into the WO₃nanosheets for surface reconstruction,which achieved the efficient removal of U（VI）and organic matter.The WO_2.78 nanosheets showed a U（VI）removal ratio of 95.6%with the reduction ratio of 84.5%in 8 mg/L of U（VI）.Additionally,the maximum extraction capacity of U（VI）on WO_2.78 nanosheets reached 507.2 mg/g at the U（VI）concentration of 200 mg/L.Meanwhile,the WO_2.78 nanosheets exhibited a superior anti-interference ion and recycling performance.The mechanistic study demonstrated that the introduction of oxygen vacancies optimized the band structure and broadened the visible light response range of the WO₃nanosheets,thus improving the extraction efficiency of uranium-containing wastewater.This work provides a successful example for improving the photocatalytic reduction of U（VI）by introducing oxygen vacancies for surface reconstruction of two-dimensional semiconductor materials.2.To further promote the coordination and confiment effect of vacancies on uranium,the in-situ generation and dynamic regeneration of vacancies were realized by introducing uranium single-atom doping.This work selected common TiO₂ semiconductor and introduced uranium single-atom doping in TiO₂ nanosheets（TiO₂-U）via high-temperature annealing.The oxygen vacancies were produced in-situ in a photocatalytic environment to achieve high-capacity single-atom confinement of uranium,improving the removal and reduction ability of U（VI）in radioactive wastewater.In the 8 mg/L U（VI）solution,the removal rate of U（VI）by TiO₂-U was 92.6%,and it showed good anti-interference ion and recycling performance.The mechanism studies showed that uranium single atoms achieved electronic localization through the formation of defect energy levels,creating oxygen vacancies in situ in a photocatalytic environment.The oxygen vacancies created by uranium single atoms greatly reduced the adsorption energy of uranyl ions,resulting in more uranium single atoms.The continuously generated oxygen vacancies became the reaction sites of confined uranyl ions,and the initial uranium single atoms played the role of initiating the reaction,forming a chain reaction.These results indicated that we can design a highly efficient semiconductor photocatalyst that continuously removes U（VI）in radioactive wastewater by using atomic-level doping to promote the in-situ generation and dynamic regeneration of vacancies in two-dimensional semiconductors to realize the sustainable coordination sites.3.To enhance the coordination and confinement effect of the reconstruction sites over the semiconductor surface on uranium,the semiconductor and Te nanowires were combined and the plasmonic effect was introduced.Partially oxidized Sn S₂ nanosheets and Te nanowires form a heterojunction（Te@O-Sn S₂）,which not only built coordination uranium sites on the surface of Sn S₂,but also used plasmonic effect to inject hot electrons to promote the Coulomb interaction between the surface and uranium,improving the bonding strength of uranium and the photocatalytic reduction efficiency.Under simulated light irradiation,the removal efficiency of U（VI）by Te@O-Sn S₂ was as high as 97.3%within 60 minutes,and the extraction capacity was 704.8 mg/g.Te@O-Sn S₂also showed good anti-interference ion characteristics.In addition,in a wide p H range,Te@O-Sn S₂ maintained a removal efficiency higher than88.4%,with a high removal efficiency of 92.4%towards uranium after 5 cycles of use.In this thesis,based on the study of surface reconstruction to construct coordination sites for uranium confinement,the in-situ formation of coordination sites and plasmonic-enhanced coordination and confinement effects were realized,and three high-efficiency photocatalytic materials were constructed and applied for the photoreduction of uranium.This research was expected to provide a method reference for the surface reconstruction of two-dimensional semiconductors,as well as a theoretical reference for the photocatalytic reduction of U（VI）in radioactive wastewater and the treatment of organic compound-containing radioactive wastewater.