Construction Of Bismuth-based Heterojunction Photocatalysts And Photogenerated Charge Separation Mechanism

Along with the rapid industrialization,human beings are faced with severe environmental pollution problems,among which water pollution is a hot spot of concern,As a green scientific solution,photocatalysis technology is considered to be an effective way to deal with emerging new organic pollution.Bismuth-based materials have stable properties and suitable band gaps,but a single component is not enough to effectively separate photogenerated electron-hole pairs.In this paper,a bismuth-based heterojunction is constructed so that the photogenerated carriers can be effectively separated and migrated,thereby realizing the efficient photocatalytic degradation of organic wastewater.XRD,XPS,TEM,SEM,DRS were used to characterize the phase composition,morphology,light absorption ability,etc.Using the constructed bismuth-based heterojunction g-C₃N₄/Bi₈（Cr O₄）O₁₁,Bi₂O₂CO₃/Bi₈（Cr O₄）O₁₁,BiOBr/g-C₃N₄ to degrade the pollution of antibiotic norfloxacin,environmental hormone bisphenol A,organic dye methylene blue and tested for contaminant concentrations by UV-vis or HPLC.The photoluminescence,photoelectricity tests,carrier concentration,theoretical calculations,trapping agent experiments,and electron paramagnetic resonance experiments were used to determine the energy band structure position and the photocatalytic degradation mechanism for efficient separation and migration of photogenerated charges.1.g-C₃N₄/Bi₈（Cr O₄）O₁₁ heterojunction photocatalysts were prepared by self-assembly.10%g-C₃N₄/Bi₈（Cr O₄）O₁₁ heterojunction showed the highest degradation rates for norfloxacin（NOR）and bisphenol A（BPA）.The degradation rate of NOR by10%g-C₃N₄/Bi₈（Cr O₄）O₁₁ was 1.38 and 2.33 times higher than that of pure Bi₈（Cr O₄）O₁₁ and g-C₃N₄,respectively.In addition,the degradation rate of BPA on 10%g-C₃N₄/Bi₈（Cr O₄）O₁₁ heterojunction was 1.35 and 9.11 times higher than that of pure Bi₈（Cr O₄）O₁₁ and g-C₃N₄,respectively.Differences in Fermi energy levels and carrier concentrations form relative p-n junctions with built-in electric fields,and the formation of relative p-n junctions promotes the separation of photogenerated electron-hole pairs and reduces charge carrier complexation,which is confirmed by photocurrents,electrochemical impedance spectroscopy（EIS）,steady-state fluorescence（PL）and time-resolved transient photoluminescence（TRPL）spectroscopy.In-situ X-ray photoelectron spectroscopy（in-situ XPS）,free radical capture experiments and electron paramagnetic resonance（EPR）results together verify the mechanism of photogenerated charge separation.2.The 2D/1D Bi₂O₂CO₃/Bi₈（Cr O₄）O₁₁ heterojunction was prepared by in situ growth method,and the 20%Bi₂O₂CO₃/Bi₈（Cr O₄）O₁₁ heterojunction showed the highest degradation activity for methylene blue（MB）and BPA,and the kinetic constant of photocatalytic degradation of MB is 2.161 h^-1,which are 4.188 times and 2.567 times that of Bi₈（Cr O₄）O₁₁ and Bi₂O₂CO₃,respectively.The kinetic constants of photocatalytic degradation of BPA were 3.945 h^-1,which were 2.461 and 8.826 times higher than those of Bi₈（Cr O₄）O₁₁ and Bi₂O₂CO₃,respectively.The built-in electric field and energy band bending of Bi₂O₂CO₃/Bi₈（Cr O₄）O₁₁ heterojunction promoted the segregated migration of photogenerated charges,which overcame the drawback of photogenerated charge complexation in conventional type I heterojunction.PL,TRPL,radical trapping experiments and EPR together verified the electron transfer mechanism of Bi₂O₂CO₃/Bi₈（Cr O₄）O₁₁ heterojunctions.The possible degradation pathways and degradation intermediates of MB and BPA were analyzed by LC-MS.3.2D/2D BiOBr/g-C₃N₄ heterojunctions were synthesized by self-assembly,and the photocatalytic degradation efficiency of BiOBr/g-C₃N₄-0.03 heterojunctions for BPA was 1.64 and 5.51 times higher than that of pure phase BiOBr and g-C₃N₄,respectively.The photoexcited charge separation transfer mechanism is explained by photoluminescence,photoelectrochemistry and theoretical calculations.The Fermi energy level difference between semiconductors leads to the direction of the built-in electric field from CN to BOB,which is similar to the built-in electric field of n→p in p-n junctions,and the relative p-n junction is formed between BOB/CN heterojunctions.The possible degradation paths and intermediates of norfloxacin and bisphenol A are analyzed by LC-MS.