Synthesis And Catalytic Mechanism Of Composite Photocatalyst With Magnetic Molybdenum Tungsten Heterojunction

The rapid development of global industrialization has led to frequent incidents of environmental pollution.Among them,water contamination is the most serious,which directly threatens the safety and health of mankind and triggers an energy crisis.In order to deal with such safety and environmental protection problems,semiconductor photocatalysis technology,as an emerging and green water treatment method,can use sunlight to achieve energy conversion and environmental governance.It is deemed to have important prospect,particularly for refractory organic wastewater.Currently,most semiconductor photocatalysts have a wide band gap,a narrow photoresponse range,and easy recombination of photogenerated carriers,resulting in low photocatalytic activity,and poor degradation effects.Especially,it is hard to be popularized after participating in the reaction because of its difficulty in recovery,high cost,and secondary pollution.Therefore,designing a photocatalyst with safety,efficiency,energy saving and environmental protection to solve these problems has important academic significance and application value.Molybdenum disulfide(MoS₂)and tungsten trioxide(WO₃)have attracted attention as two types of representatives of visible light photocatalysts with the appropriate band gap and stable physicochemical properties.However,a single semiconductor photocatalyst is still unable to avoid recombination of photogenerated electron-hole pairs.Based on this,we will take MoS₂ and WO₃ as research objects,introduce a hard-magnetic phase,strontium ferrite(SrFe₁₂O₁₉)as the magnetic substrate,and construct the composite photocatalysts with different heterojunction to improve photocatalytic activity.Meanwhile,these catalysts can be separated from the aqueous solution rapidly under the action of external magnetic field,which are convenient for recycling to meet the needs of practical application.A novel composite magnetic photocatalyst MoS₂-SrFe₁₂O₁₉ with p-n heterojunction was prepared by the facile two-step hydrothermal method,which was applied to the degradation of Rhodamine B(RhB)and Methylene Blue(MB).The results showed that the degradation rate of organic dye by the MoS₂-SrFe₁₂O₁₉(10 wt%,MS-10)was 96.5%(RhB-10 mg·L^-1)and 95.8%(MB-50 mg·L^-1)under the simulated sunlight for 120 min,respectively.The rate constant was about 3.4 times(RhB)and 1.4times(MB)higher than that of the pristine MoS₂.Charge transfer resistance(R_ct)of the electrode surface modified by the composite was 173?,and the maximum photocurrent density was 316?A·cm^-2,both of which exhibited better electrochemical stability than MoS₂ and SrFe₁₂O₁₉.PL confirmed the low recombination rate of photogenerated electron-hole pairs.The main reason for the improvement of the photocatalytic activity of MoS₂-SrFe₁₂O₁₉ is that its unique p-n heterojunction can continuously use the light source to inhibit the recombination of photogenerated electron-hole pairs and extend their life.Moreover,the specific surface area of MS-10 was 6.2 m² g^-1,and the most approximate pore size was 1.1 nm.With more micropores and slit micropores,it can provide a large number of active sites,thereby promoting interface electron transport and exciting photocatalytic activity.Importantly,MoS₂-SrFe₁₂O₁₉ has excellent saturation magnetization of 10.1 emu·g^-1 and coercivity of 1407.7 Oe,which can be quickly recycled under an external magnetic field.Composite magnetic photocatalyst WO₃-SrFe₁₂O₁₉ was synthesized by hydrothermal-roasting method,and a new n-n heterojunction has been successfully formed.SEM results displayed that flake nanospheres WO₃ was compactly decorated with hexagon nanoplate SrFe₁₂O₁₉.This unique multilayer frame is convenient to make full use of the sunlight.UV-vis,EIS and TPR techniques were used to verify the response of the composite to visible light and the superior separation efficiency of photogenerated charge.The magnetic properties of WO₃-SrFe₁₂O₁₉(Ms=3.4 emu·g^-1,Mr=1.0 emu·g^-1,Hci=951.1 Oe),proving that it is easy to recycle.In addition,the photocatalytic activity of WO₃-SrFe₁₂O₁₉ was significantly higher than that of monomer.Among them,WO₃-SrFe₁₂O₁₉(5 wt%)used H₂O₂ as the electron trapping agent,the degradation rate of RhB reached 98.4%at 80 min,and the k_app was 0.039 min^-1.The efficient photocatalytic reaction was due to the semiconductor coupling between WO₃-SrFe₁₂O₁₉,which effectively stimulated the separation of photogenerated electron-hole pairs.What's more,H₂O₂ acted as an electron acceptor to assist the reaction for the electron accumulation layer.It interacted with homotype n-n heterojunction of WO₃-SrFe₁₂O₁₉ to produce strong oxidizing radical·OH.MoS₂-WO₃,a kind of Z-scheme heterojunction photocatalyst,was prepared by one-step hydrothermal method.Flower-like MoS₂ was coated on the skeleton structure of WO₃ nano-microspheres.The diameter of spherical particles was between 4-10?m,and the two grew in a“lamella-bent slice”mode.BET proclaimed that the composite had porous structure and a large specific surface area(30.0 m²·g^-1).The pore diameter was mainly concentrated in the range of 1-30 nm and the most probable pore size was3.2 nm.It provided a convenient transport pathway for photocatalytic reaction,helping to transfer reactants to the surface of the hierarchical structure or the interior of the porous structure at most,while also amplifying the effective contact with dye molecules.The band gap E_g of MoS₂-WO₃ was 1.79 e V,which had good response ability to visible light.In the photocatalytic activity experiment,the degradation rate of RhB was 95%(60 min)for the composite MoS₂-WO₃(50 wt%).And it had good stability,the degradation rate was 91.4%after three cycles.Besides,the active group capture experiment demonstrated that the charge transfer mechanism of MoS₂-WO₃heterojunction satisfied the Z-scheme migration.It made the photocarriers realize the maximum separation in space,improved the redox ability of the reaction system,and effectively promoted the photocatalytic activity.Ternary composite magnetic photocatalyst MoS₂-WO₃-SrFe₁₂O₁₉ was synthesized by stepwise method.XRD,FTIR,XPS,and TEM characterization revealed that SrFe₁₂O₁₉ was successfully loaded on the surface of MoS₂-WO₃ nanospheres.Furthermore,the most probable pore size of the composite was 2.5 nm and the specific surface area was 9.0 m²·g^-1.Therefore,the catalyst could be considered as a mesoporous magnetic material.MoS₂-WO₃-SrFe₁₂O₁₉(5 wt%)possessed the best photocatalytic activity,and the degradation rate of RhB was 93.0%at 80 min.The COD removal rate was 47.2%(60 min)and the chroma of the wastewater was reduced when it was applied to the treatment of the aged landfill leachate.It was believed to have the potential to treat refractory organic wastewater.Saturation magnetization(Ms)and coercivity(Hci)of the ternary composite were 3.6 emu·g^-1 and 1037.2 Oe,respectively.The promising magnetic property was beneficial to separate and resist demagnetization for this catalyst.According to the mechanism analysis,the charge transfer of MoS₂-WO₃-SrFe₁₂O₁₉followed the dual Z-scheme,which accelerated the separation of photogenerated carriers and improved the photocatalytic activity.The active groups·OH and·O₂^-generated in the reaction both played a role in the degradation of dyes.In summary,MoS₂-WO₃-SrFe₁₂O₁₉ was considered to be safe,environmental friendly and efficient photocatalyst,which would be used as a potential candidate material for the degradation of polluted wastewater in the future.