Study On Photocatalysis Properties Of CQDs/TiO₂/Bi₂O₄ Composites

Bi₂O₄,a new member of the bismuth based oxides family,which is a simple oxide with mixed valent states(Bi³⁺and Bi⁵⁺).This simple bismuth based oxide has a band gap energy of 1.94-2.1eV and shows much higher visible light degradation activity than some of the well-investigated highly efficient photocatalysts,such as CdS and Bi₂O₃.However,owing to the fast recombination of photoinduced e^--h⁺pairs,the pure Bi₂O₄ exhibits relatively low visible light photocatalytic activity.Thus,it is highly desirable to take measures to improve the separation efficiency of photoinduced charge carriers and enhance the visible light photocatalytic ability of Bi₂O₄.The construction of a semiconductor heterojunction,especially type II heterojunction has been proven to be an effective and simple strategy to improve the charges separation efficiency.The type II heterojunction can be constructed by compounding TiO₂ with Bi₂O₄ because they have a staggered gap offset,thereby improving the separation efficiency of photogenerated charge and visible light catalytic performance of Bi₂O₄.Carbon Quantum Dots?CQDs?,a new carbon nanomaterial,possesses inexpensive,non-toxicity,and good chemical stability.The excellent electrical conductivity properties of CQDs can separate and transfer photogenerated electrons and improve the separation efficiency of electron-hole pairs.In addition,the upconversion effect of CQDs can convert near-infrared light into visible light,opening up a new way for the efficient use of near-infrared light energy in the solar spectrum.Therefore,depositing CQDs on Bi₂O₄-TiO₂ heterojunctions can not only improve the separation efficiency of photogenerated charge,but make use of near-infrared light in the solar spectrum to increase the utilization for sunlight.In order to improve the photocatalytic performance of Bi₂O₄,this work carried from the following three aspects:Firstly,TiO₂ nanobelt-Bi₂O₄ type II heterojunction was prepared by a hydrothermal method.In TiO₂ nanobelt-Bi₂O₄ heterojunction,photoinduced electrons of Bi₂O₄ move to the conduction band of TiO₂,while the photogenerated holes of Bi₂O₄ will be still reserved in the valence band of Bi₂O₄,resulting in highly efficient separation of electron-hole pairs,thereby increasing visible light photocatalytic activity of Bi₂O₄ for methyl orange and phenol.For the TiO₂ nanobelt-Bi₂O₄heterojunction,h⁺and·O₂^-were identified as the main active species in the photocatalytic degradation process for methyl orange?MO?in visible light.This work shows that the novel heterojunction can improve the photocatalytic performance effectively.Senondly,TiO₂ microsphere-Bi₂O₄ heterojunction was constructed using porous hierarchical structure TiO₂ microspheres as the matrix,Bi₂O₄ was grown in the pore structure of TiO₂ microspheres.Due to the limited role of the unique pore structure in TiO₂ microspheres,the morphology of Bi₂O₄ changes from the original submicron rod-like structure to nanoparticles.In TiO₂ microsphere-Bi₂O₄ heterojunction,the Bi₂O₄ nanoparticle shortens the time during which the photogenerated charge migrates from the interior to the surface,promoting the separation of the photogenerated charge.In addition,TiO₂ nanosheets contact with Bi₂O₄ nanoparticles tightly,forming a close interface.The well-contacted interface increase the contact area of the heterojunction and improve the separation of electron-hole pairs between two phases.At last,TiO₂ microsphere-Bi₂O₄ heterojunction with higher specific surface area could provide more active sites for the adsorption of reactant molecules and accelerate the photocatalytic reaction.Through the collective effect of the above three aspects,the visible light catalytic activity of TiO₂ microsphere-Bi₂O₄ type II heterojunction for MO and phenol is significantly improved.Thirdly,The CQDs-TiO₂ microsphere-Bi₂O₄ ternary complexes were prepared by depositing CQDs on TiO₂ microsphere-Bi₂O₄ heterojunctions using hydrothermal method.In the ternary complex,the type II heterojunction effect in TiO₂microsphere-Bi₂O₄ promotes the separation of photogenerated charge.The electrical conductivity properties of CQDs can separate and transfer photogenerated electrons in Bi₂O₄ and TiO₂,thereby improving visible and simulated solar photocatalytic activity of CQDs-TiO₂ microsphere-Bi₂O₄ ternary complexes.Furthermore,the upconversion effect of CQDs convert near-infrared light into visible light to excite Bi₂O₄,so the ternary complexes generate near-infrared photocatalytic activity and increase the utilization for sunlight.