Study On Photocatalytic Properties Of Bi₂O₄-based Heterojunctions

Bi₂O₄,a trivalent and pentavalent mixed valence state bismuth-based oxide,is a promising visible light catalyst due to its strong visible light responsiveness.However,the Bi₂O₄ prepared via the hydrothermal method shows a submicron rod-like structure with small specific surface area and little surface active sites.In addition,single-component Bi₂O₄ also possesses the problem of rapid recombination of photogenerated carriers.These shortcomings together limit its photocatalytic performance.Therefore,this paper takes Bi₂O₄ as the research object,by constructing Bi₂O₂CO₃/Bi₂O₄ type Ⅱ heterojunction,hierarchical BiOBr/Bi₂O₄p-n junction and BiO_2-x/Bi₂O₄ type Ⅱ heterojunction to enhance the efficiency of photogenerated electron-hole separation and broaden the spectral range,thereby improve Bi₂O₄photocatalytic activity.This paper mainly carrys out from the following aspects:Firstly,using NaBiO₃·2H₂O and nitrogen-doped carbon quantum dots（N-CQDs,3.17 mg/m L）solution as raw materials to prepare Bi₂O₂CO₃/Bi₂O₄ type Ⅱ heterojunction via the one-pot in-suit hydrothermal method.The obtained Bi₂O₂CO₃/Bi₂O₄ heterojunction was denoted as BOC/BO-x,where x represented the volume of the N-CQDs（x=2,6,14,20 and 30 m L,respectively）.It is indicated that changing the amount of N-CQDs can adjust the content of the Bi₂O₂CO₃/Bi₂O₄heterojunction component.Furthermore,strong interface interaction exists between layered Bi₂O₂CO₃ and short rod-shaped Bi₂O₄ due to the in situ preparation,forming a large number of Bi₂O₂CO₃/Bi₂O₄ type Ⅱ heterojunctions,which benefit to promote the migration and separation of electron-hole pairs.Photocatalytic degradation activity results show that BOC/BO-20 has the best simulated-solar photocatalytic degradation activity,and its MO degradation rate constant is 2.73 times that of Bi₂O₄.Therefore,the enhanced photocatalytic activity of the Bi₂O₂CO₃/Bi₂O₄ heterojunction can be attributed to the increase of the contact area between the short rod-shaped Bi₂O₄ and the layered Bi₂O₂CO₃,which builds a great of type Ⅱ heterojunctions and be favorable to the separation of charges.Secondly,using the submicron rod-shaped Bi₂O₄ as the framework,hierarchical BiOBr/Bi₂O₄ p-n heterojunction were prepared by the co-precipitation method,which BiOBr nanosheets grown on the rod-shaped Bi₂O₄ surface to form initamate contact.The hierarchical structre can not only increase the specific surface area but also promote light absorption through multiple scattering effects.The photocatalytic results showed that when the molar ratio of BiOBr to Bi₂O₄ is 0.1,the rate constant of heterojunction degradation of 4-chlorophenol（4-CP）under visible light was 2.7 times that of Bi₂O₄.The reasons for the enhancement of BiOBr/Bi₂O₄ p-n junction photocatalytic activity are as follows:1)the improved specific surface area increase the concentration of surface active sites;2)the light scattering effect of the three-dimensional hierarchical structure enhances the light absorption efficiency and generates more carriers;3)built-in electric field produced by p-n junction promotes the migration and separation of photogenerated carriers and prolongs the carriers lifetime.Thirdly,using NaBiO₃·2H₂O as the bismuth source,prepare the BiO_2-x/NaBiO₃·x H₂O intermediate product via adjust the concentration of the Na OH aqueous solution under hydrothermal process;then use intermediate product as the raw material prepare BiO_2-x/Bi₂O₄ heterojunction with adjustable content by convert NaBiO₃·x H₂O to Bi₂O₄ via hydrothermal method.Due to the introduction of the near-infrared light material BiO_2-x,the light absorption range of the BiO_2-x/Bi₂O₄heterojunction can extend to the near-infrared region.Photocatalytic experiments showed that when the concentration of Na OH is 3.5 mol/L,the prepared BiO_2-x/Bi₂O₄heterojunction had the best photocatalytic activity,and the degradation rate constant for MO under visible light and near-infrared light was 2.68 and 11.1 times of Bi₂O₄,respectively.The increase in the photocatalytic activity of the BiO_2-x/Bi₂O₄heterojunction can be attributed to:1)the light absorption range of the heterojunction extends to the near-infrared region,which broadens the response range of the solar spectrum,and generates more carriers;2)the construction of the type Ⅱ heterojunction at the interface of BiO_2-x and Bi₂O₄ promotes the separation of electron-hole pairs and inhibits the recombination of carriers.