Construction Of Bi-Based Semiconductor Heterojunction And Its Performance And Mechanism Of Photoelectrocatalytic Degradation Of Tetracycline

Photoelectrocatalytic(PEC)degradation is an advanced oxidation process that combines the advantages of photocatalysis(PC)and electrocatalysis(EC),with the benefits of mild reaction conditions,the use of solar light,broad applicability,and no secondary pollution.It has been proven to be an effective method for the treatment of difficult-to-degrade organic pollutants.The efficiency of PEC degradation depends largely on the choice of light anode material.Bi-based photocatalysts(such as Bi2WO6,Bi2S3 and BiOI)are considered to have promising development prospects as light anode materials due to their visible light response,unique layered framework and adjustable electronic band structure.However,the PEC performance of single Bi-based semiconductor photocatalysts is not ideal due to factors such as low separation efficiency of photogenerated charge carriers,weak oxidationreduction ability of photogenerated electrons and holes(e-/h+),and poor light stability.In response to these issues,this study introduced modification strategies such as semiconductor heterojunctions,element doping,Bi nanoparticle(Bi NPs)loading,and preparation of surface oxygen vacancies(OVs)by introducing semiconductors with highly matched band structures to Bi-based photocatalysts to suppress the photodegradation of Bi-based semiconductors while enhancing the separation efficiency of photogenerated charge carriers and their oxidation-reduction capabilities.Thus,the PEC mineralization and degradation performance of typical difficult-to-degrade organic pollutants,tetracycline(TC),were improved.The specific research content and main results are as follows:(1)The localized surface plasmon resonance(LSPR)of Bi NPs enhances the efficient PEC degradation of TC under visible light irradiation in a Bi2O3/Bi2WO6 p-n heterojunction rich in OVs.A three-component composite material,Bi/Bi2O3Bi2WO6 p-n heterojunction,was prepared by a simple one-step hydrothermal method and a solvothermal reduction method.Its mainly mesoporous structure exhibits a large specific surface area,and the local surface plasmon resonance effect(LSPR)of Bi NPs significantly enhances the material’s absorption ability to visible light.Meanwhile,the p-n heterojunction formed by Bi2O3/Bi2WO6 generates an internal electric field(IEF)at the interface and the OVs on the material surface can act as dual functions for light-generated e-acceptors to promote the separation efficiency of photo-generated carriers.Under simulated sunlight with an external electric field(1.0 V vs Ag/AgCl),the Bi/Bi2O3/Bi2WO6 composite material exhibits excellent PEC performance.After 60 min of reaction,the degradation efficiency of TC(C0=30 mg/L)reached 89.8%,which is 2.2 times and 10.1 times that of the PC system(40%)and the EC system(8.7%),respectively.The apparent first-order rate constant of the Bi/Bi2O3/Bi2WO6 composite material is 38.1× 10-3 min-1,which is 4.2 times and 5.1 times that of single materials Bi2O3(8.99 × 10-3 min-1)and Bi2WO6(7.49 × 10-3 min-1),respectively.Through free radical capture experiments and electron paramagnetic resonance testing(EPR),it was confirmed that the main active species produced in the p-n type PEC degradation system of Bi/Bi2O3/Bi2WO6 are h+ and ·OH.(2)This passage discusses the use of a direct Z-scheme Co3O4/Bi2WO6 heterojunction for the enhanced photocatalytic degradation of TC under visible light.The authors used L-cysteine as a sulfur source and capping agent to control the morphology of Co3O4 and prepare sulfur-doped rod-shaped Co3O4.The introduction of sulfur not only controlled the microstructure of Co3O4 but also altered its intrinsic electronic structure,leading to interlayer polarization and promoting the separation of photogenerated charge carriers.A 3D flower-like Bi2WO6 was grown in situ on the S-Co3O4 to construct the heterojunction composite material.The results showed that the S-Co3O4/Bi2WO6 composite material had strong visible light response,low resistance,and low photogenerated charge carrier recombination rate,and achieved a TC degradation efficiency of 85%in 60 minutes at a concentration of 30 mg/L.By calculating the work functions(WF)of the materials,the WF of SCo3O4 and Bi2WO6 were found to be 6.18 and 8.80 eV,respectively.In the dark state,when Bi2WO6 contacts S-Co3O4,electrons spontaneously transfer from S-Co3O4 with a smaller WF to Bi2WO6 with a larger WF,so that the two semiconductor surfaces are respectively enriched with electrons and holes and form an internal electric field at the heterojunction interface.Combined with the active species produced in the degradation system,it is concluded that the S-Co3O4/Bi2WO6 heterojunction conforms to the direct Z-scheme charge transfer mechanism.(3)Direct Z-scheme heterojunction of Bi/Bi2S3/α-MoO3 enhances the photocatalytic degradation of TC under visible light.Firstly,1D nanobelt α-MoO3 was synthesized,and then,using bismuth nitrate and thiourea as raw materials,nanorod Bi2S3 and Bi NPs were in-situ grown by a one-pot hydrothermal method,to prepare a ternary composite photoanode material of Bi/Bi2S3/α-MoO3.Due to the direct Z-scheme heterojunction formed by Bi2S3 and α-MoO3,h+and e-are concentrated in the valence band(VB)of α-MoO3 and the conduction band(CB)of Bi2S3,respectively,making the composite material have stronger redox ability.At the same time,due to the LSPR effect of Bi NPs,the visible light absorption capacity of α-MoO3 can be enhanced.The PEC activity of(50%)Bi/Bi2S3/α-MoO3 composite material is the highest,with a 85.8%degradation efficiency of 30 mg/L TC within 60 min under visible light.The ·OH and ·O2-produced by the direct Z-scheme PEC degradation system of Bi/Bi2S3/α-MoO3 play a major role in the mineralization degradation of TC.(4)Direct Z-scheme heterojunction α-MnO2/BiOI with abundant oxygen vacancies enhances the photocatalytic degradation of TC under visible light.1D nanorod-shaped α-MnO2 was synthesized by a solvothermal method,and then BiOI thin-layered nanosheets were grown in situ to prepare αMnO2/BiOI composite material with abundant oxygen vacancies.As an excellent capacitor material,α-MnO2 can temporarily store photo-generated charges,and the α-MnO2/BiOI composite material conforms to the direct Z-scheme charge transfer mechanism,which greatly promotes the separation efficiency of photo-generated carriers.Therefore,the α-MnO2/BiOI photoanode exhibits excellent PEC performance under visible light irradiation,achieving a removal efficiency of 95%for 30 mg/L TC within 120 min,and has good stability and reproducibility.The WF of α-MnO2 and BiOI were calculated to be 6.72 and 6.65 eV,respectively.Due to the formation of IEF at the α-MnO2 and BiOI heterojunction interface,e-in BiOI CB directly combines with h+ in α-MnO2 VB,thus retaining h+in BiOI VB and e-in α-MnO2 CB,which has stronger redox capacity and promotes the separation and migration of photo-generated charges.Through free radical capture experiments and EPR tests,it was proved that the main free radicals generated in the degradation system were ·OH and ·O2-.The intermediate products of TC degradation process were tested by liquid chromatography-mass spectrometry(LC-MS),and two possible degradation pathways were inferred.TC was degraded into small molecule compounds under the action of ·OH and ·O2-.The toxicity of TC and its degradation intermediates was evaluated by the Toxicity Estimation Software Tool(T.E.S.T),and the toxicity of TC decreased as the PEC reaction proceeded.