Research On Regulation Of Electron At Shell TiO₂ Interface And Performance And Mechanism Of Photocatalytic Of U(?)

Recently,the photocatalytic technology could reduce the soluble U(?)to insoluble U(IV),which is considered to be an effective strategy for remediation of radioactive environmental pollution.As the photocatalyst,TiO₂ has attracted much attention in the field of photocatalytic reduction of U(?)due to its advantages of clean,efficient,stable and environmentally friendly.Nevertheless,the photocatalytic reduction of U(?)requires the protective gas(N₂ or Ar)and hole sacrifice agent,limiting its practical application.In view of the inherent defects of TiO₂-based photocatalyst,this paper intends to propose an effective strategy for constructing a high-efficiency TiO₂-based photocatalyst to achieve enhanced photocatalytic reduction activity of U(?)by precisely regulating the electron migration at the TiO₂ interface.The main research contents are as follows:(1)The nano-sized hollow titanium dioxide spheres(H-TiO₂)and reduced graphene oxide(RGO)were self-assembled into a new type of 3D composite aerogel(3D RGO@TiO₂-x)by using glucose as the cross-linking agent through high-temperature hydrothermal method,intending to improve the photocatalytic reduction efficiency of U(?).The RGO in the composite material not only provides abundant adsorption and catalytic active sites as an ideal carrier,but also generates a schottky heterojunction with H-TiO₂ to promote the separation and transfer of electron in H-TiO₂ conduction band.After visible light radiating for 140 min,3D RGO@TiO₂-3(GO:H-TiO₂=1:1,mass ratio)showed the excellent photocatalytic reduction activity for U(?)with removal rate of 99.5%,and only reduced by 7.67%after five cycles.The apparent rate constant was increased by nearly five times.More importantly,the construction of 3D RGO@TiO₂-x breaks through the limitation of protective gas,and the photogenerated electrons and superoxide radicals act as the reducing agent to reduce U(?)into(UO₂)O₂·2H₂O in the air.(2)TiO₂@CdS and CdS@TiO₂ double-shell hollow nanospheres were synthesized by combining the hard template method and the hydrothermal reaction method and applied for photocatalytic reduction of U(?).The introduction of CdS into the hollow layer of H-TiO₂can significantly improve the light utilization efficiency.The photocatalytic rate of TiO₂@CdS-2(the mass ratio of 16.8%)double-shell hollow spheres could completely remove U(?)within 10 min of illumination,and corresponding the apparent rate constant was 59 and16.5 times as much as H-TiO₂ and CdS.Meanwhile,the removal efficiency still maintained88%after five cycles.3 mmol·L^-1 of Na HCO₃ could replace the hole trapping agent(methanol),and the photocatalytic removal efficiency of U(?)reached 99.1%within 20 min.Additionally,the electron transfer at the interface of TiO₂@CdS-2 conformed to the Z-type conduction mechanism to improve the efficiency of photogenerated electrons collected on CdS surface,which cooperating with superoxide radicals to reduce U(?)to?-U₃O₈.(3)TiO₂@CdS-2 as the matrix,the ternary heterojunction hollow spheres TiO₂@CdS@Au were successfully constructed by dispersing and depositing Au NPs on the outer surface of TiO₂@CdS-2 according to the liquid-phase chemical reduction method.Compared with TiO₂@CdS-2,the light absorption wavelength and intensity of TiO₂@CdS@Au were significantly enhanced.Compared with the photocatalyst loaded with 1%Pt or 1%GOQDs,the photocatalytic reduction efficiency of TiO₂@CdS@Au for U(?)was 99.4%within 30min and still remained 93.8%after five cycles,and its apparent rate constant was 3.57 times than that of TiO₂@CdS-2.Additionally,when 3 mmol·L^-1 Na HCO₃ replacing methanol,the photocatalytic removal efficiency of U(?)by TiO₂@CdS@Au-2 was 99.5%.The synergistic effect between Z-type band structure and the surface plasmon resonance effect of Au NPs could promote the accumulation of photogenerated electrons on the surface of Au NPs.Additionally,photogenerated electrons and superoxide radical together reduce the U(?)adsorbed on the surface of the photocatalyst to?-U₃O₈.(4)A new synergy strategy based thin-layer heterojunction and space separation co-catalyst was proposed.Mn O_x@TiO₂@CdS@Au were prepared by loading Mn O_x and Au NPs on the inner and outer surfaces of TiO₂@CdS with a size about 300 nm and a shell thickness about43 nm.The light response range was expanded to the near-infrared region,and the spatial separation of photogenerated electrons-holes and the directional control of electrons were also realized to further improve the utilization of light.After the simulated sunlight irradiating 50minutes,the removal efficiency of U(?)by Mn O_x@TiO₂@CdS@Au reached 98.9%and remained 94.8%after five cycles.Meanwhile,the removal efficiency was 99.6%within 15min after adding 3 mmol·L^-1 Na HCO₃,and the apparent rate constants were 3,1.3 and 4.9times higher than that of Mn O_x@TiO₂@CdS,TiO₂@CdS@Au-2 and TiO₂@CdS-2.Significantly,the removal efficiency of U(?)by Mn O_x@TiO₂@CdS@Au was 56.67%when applied to real wastewater(C_U(?)=132 ppm).According to the analysis of the mechanism,the charge lifetime was prolonged under the drive of the triple built-in electric field.Meanwhile,the synergistic effect of photogenerated electrons and superoxide radicals reduce U(?)into?-U₃O₈.(5)In view of the feasibility of the synergistic strategy between thin-layer heterojunction and space separation co-catalyst,a hollow polyhedral heterojunction of Co₃O₄@TiO₂@CdS@Au was constructed.Co₃O₄ and Au NPs co-catalyst were respectively distributed on the inner and outer surfaces of the hollow polyhedron TiO₂@CdS.The removal efficiency of U(?)by Co₃O₄@TiO₂@CdS@Au reached 98.8%within 10 min,which almost unchanged after five cycles and showed excellent photocatalytic stability.More importantly,the apparent rate constant are respectively 31 and 3.3 times than that of Co₃O₄@TiO₂ and Co₃O₄@TiO₂@CdS.The Z type heterojunction constructed by TiO₂@CdS and spatial separation co-catalyst drive electrons and holes to flow in the opposite direction,and electrons are gathered on the surface of the Au NPs,strengthening the photocatalytic reduction activities.This synergistic strategy is suitable for a variety of hollow shapes and of different types of co-catalysts.The construction of multilayer TiO₂-based photocatalysts by thin-layer heterojunction and space separation co-catalyst breaks through the limitations of protective gas and sacrificial agents during the photocatalytic reduction of U(?)process.The results in this paper provide the feasible theoretical and practical basis for the further design and construction of clean and efficient photocatalysts,which is of great significance for the environmental remediation of uranium-containing wastewater.