Effect Of Al,Mn,Mg Combinatorial Doping On Photocatalytic Activity Of α-Fe₂O₃/ZnFe₂O₄

Al,Mn,Mg combinatorially dopedα-Fe₂O₃,ZnFe₂O₄ andα-Fe₂O₃/ZnFe₂O₄（doping amounts of 0,1%,3%and 5%,molar ratio）was prepared by sol-gel method using chemical reagents as starting reactants,and the effects of combinatorial doping and dopants dosage on microstructure,optical properties,photoelectric response and photocatalytic performances were systematically investigated by XRD,SEM,TEM,EDS,Uv-vis DRS,Mott-Schottky and photoelectrochemical tests.The mechanism of the combinatorial doping of Al,Mn and Mg on the photocatalytic performance ofα-Fe₂O₃/ZnFe₂O₄ was preliminarily found,which provided the necessary theoretical basis and data support for the preparation of multi-dopedα-Fe₂O₃/ZnFe₂O₄photocatalytic materials from metallurgical zinc-bearing dust.Al,Mn,Mg combinatorially dopedα-Fe₂O₃ samples with single phase were prepared by the sol-gel method are single-phase products at the doping level of 1%,3%and 5%,respectively.XRD results showed that the doping elements are embedded in the lattice structure ofα-Fe₂O₃ without obvious second phase.SEM results showed that the micro-morphology of all samples was in the form of plates formed by aggregation of nanoparticles,and the doping of each combination had no significant effect on the micro-morphology ofα-Fe₂O₃.The results of Uv-vis DRS,photoelectrochemical teats（unbiased transient photocurrent response,EIS and Mott-Schottky）and photocatalytic degradation MB test were performed.The degradation rate of MB(D_MB)over undopedα-Fe₂O₃ is 85%.The effect of Al doping onα-Fe₂O₃ photocatalytic activity is not significant.Mn doping suppresses the photocatalytic activity ofα-Fe₂O₃,and although the band gap ofα-Fe₂O₃ narrows down with the increase of the doping amount,the flat band potential and the photocurrent density decreases and the carrier transport impedance increases,resulting in a decrease of the photocatalytic activity of the product(D_MB<65.7%).Mg doping significantly improves the photocatalytic activity ofα-Fe₂O₃,and the narrowing of band gap,increase in the flat band potentialand the photocurrent density and decrease in the carrier transport impedance occur toα-Fe₂O₃ with increase of the Mg dosage,leading to significant improvement on photocatalytic activity of the product(D_MB>90.0%).Al-Mn binary doping inhibits the photocatalytic activity ofα-Fe₂O₃.As the amount of doping increases,Narrowing in band gap,decrease in flat band potential and photocurrent density and increase in carrier transport impedance occurred toα-Fe₂O₃,leading to decrease in photocatalytic activity(D_MB<69.5%),which indicates that Mn plays a major role.With the increase of the doping amount,the binary doping of Mn-Mg makes the band gap ofα-Fe₂O₃ narrow,the flat band potential decreases first and then increases,the photocurrent density decreases first and then increases,and the carrier transport impedance first increases and then decreases,which makes the photocatalytic activity the product first decrease and then increase,reflecting the synergistic effect of Mn and Mg doping.Al-Mg binary doping is beneficial to the photocatalytic activity ofα-Fe₂O₃.As the amount of doping increases,the band gap ofα-Fe₂O₃ narrows,the flat band potential increases,the photocurrent density increases and the carrier transport impedance decreases,which makes the photocatalytic activity ofα-Fe₂O₃ increase(D_MB>85.9%),and the degradation rate of MB over 3%Al-Mg-dopedα-Fe₂O₃ reaches 92.8%.Al-Mn-Mg co-doping decreases the photocatalytic activity ofα-Fe₂O₃.As the amount of doping increases,the flat band potential and photocurrent density ofα-Fe₂O₃ decreases and the carrier transport impedance increases,leading to a decrease in the photocatalytic activity of the product(D_MB<82.2%).At a doping level of 1%,3%and 5%,respectively,series of Al,Mn,Mg combinatorially doped ZnFe₂O₄ samples prepared by the sol-gel method are all single-phase products.XRD results shows that the doping elements are embedded in the lattice structure of ZnFe₂O₄,no obvious second phase is found.SEM characterization showed that the micro-morphology of all samples was vesicle-like strcture formed by aggregation of nanoparticles,and the doping of each combination had no significant effect on the micro-morphology of ZnFe₂O₄.Uv-vis DRS,photoelectrochemical tests（unbiased transient photocurrent response,EIS,Mott-Schottky）and photocatalytic degradation of MB were performed to characterize and analyze the photoelectrochemical performance and photocatalytic activity of the product.Compared with the photocatalytic activity of undoped ZnFe₂O₄(D_MB=73.1%),Al doping has no significant effect on ZnFe₂O₄,Mn doping weakens the photocatalytic activity of ZnFe₂O₄(D_MB<63.7%),and Mg doping can slightly improve the photocatalytic activity of ZnFe₂O₄(D_MB>79.6%),of which 5%Mg doped ZnFe₂O₄ has the best photocatalytic activity(D_MB=85.4%).Both Al-Mn and Mn-Mg binary doping inhibit the photocatalytic activity of ZnFe₂O₄(D_MB<69.9%and D_MB<69.1%,respectively).The photocatalytic activity of Al-Mg binary doped ZnFe₂O₄ showed a gradual decrease trend with increasing doping amount(D_MB<80.2%).Al-Mn-Mg ternary co-doping can inhibit the photocatalytic activity of ZnFe₂O₄(D_MB<71.1%).At a doping level of 1%,3%and 5%,respectively,all the Al,Mn and Mg combinatorially dopedα-Fe₂O₃/ZnFe₂O₄ prepared by the sol-gel method are the composite ofα-Fe₂O₃ and ZnFe₂O₄,XRD shows doping elements embedded in the lattice structure ofα-Fe₂O₃/ZnFe₂O₄,no obvious second phase exists.SEM images showed that the micromorphology of all samples was some ruptured vesicle formed by aggregation of nanoparticles,and the doping of each combination had no significant effect on the micromorphology ofα-Fe₂O₃/ZnFe₂O₄.Uv-vis DRS,photoelectrochemical tests and photocatalytic degradation of MB were used to characterize and analyze the photoelectrochemical performance and photocatalytic activity of the products.Compared with the photocatalytic activity of undopedα-Fe₂O₃/ZnFe₂O₄(D_MB=89.0%),Al doping has no significant effect onα-Fe₂O₃/ZnFe₂O₄,and Mn doping weakens the photocatalysis ofα-Fe₂O₃/ZnFe₂O₄activity(D_MB<66.3%),and Mg doping can significantly improve the photocatalytic activity ofα-Fe₂O₃/ZnFe₂O₄(D_MB>95.2%),of which 5%Mg doped sample achieves the best photocatalytic activity(D_MB),showing the synergistic effect of Mg doping and heterojunction.Al-Mn and Mn-Mg binary doping both reduce the photocatalytic activity ofα-Fe₂O₃/ZnFe₂O₄(D_MB<71.6 and D_MB<69.8%,respectively),the binary doping of Al-Mg makes the photocatalytic activity ofα-Fe₂O₃/ZnFe₂O₄ increase first and then decrease,and the best photocatalytic activity occurred to the 3%Al-Mg dopedα-Fe₂O₃/ZnFe₂O₄(D_MB=95.5%).Al-Mn-Mg co-doping inhibits the photocatalytic activity ofα-Fe₂O₃/ZnFe₂O₄(D_MB<82.2%).The experimental results show that compared with single-phaseα-Fe₂O₃ and ZnFe₂O₄,the existence of heterojunction inα-Fe₂O₃/ZnFe₂O₄ endow the composite better photocatalytic activity than eitherα-Fe₂O₃ or ZnFe₂O₄,and the synergistic effect of Mg doping and heterojunction further improved the photocatalytic activity ofα-Fe₂O₃/ZnFe₂O₄.Al doping has little effect on the photocatalytic activity ofα-Fe₂O₃and ZnFe₂O₄,while Mn doping significantly reduces their photocatalytic activity.Therefore,when regulating the preparation ofα-Fe₂O₃/ZnFe₂O₄ composite photocatalyst from metallurgical zinc-bearing dust,the Mn content in the product should be reduced as much as possible,and the Mg content should be increased appropriately.The Al content can not be controlled intentionally.