Study on Magneto-optic Regulation and Mineral Supported Modification of Multi-doped Perovskite-type ABO3 Catalyst

Author:Wang Yue Qin

Supervisor:zhang ming xu liu yin

Database:Doctor

Degree Year:2019

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Pages:158

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The excessive exploitation and utilization of fossil fuels such as coal and petroleum has brought two major issues:energy crisis and environmental pollution.Semiconductor photocatalysis technique is considered to be the most promising measures for clean energy development and environmental management.SrTiO3(STO)and BiFeO3(BFO)are typical perovskite oxide semiconctors with special dielectric,optical and ferromagnetic properties,but the problems of wide band gap,weak magnetism,poor dispersion and difficult recovery in organic wastewater treatment have restricted their further development and application.In this thesis,to overcome these obstacles,the substitution co-doping synergistic effect,charge compensation effect,defect regulation and doping concentration are proposed to broaden the optical response and improve the ferroelectric or ferromagnetic properties of perovskite-type catalysts,and to explore their magneto-optical regulation mechanism.Zn/TM(TM=Mn,Fe,Co)co-doped STO composites supported on two minerals(zeolite molecular sieves and kaolin)are constructed,providing an experimental basis for the realization of perovskite-type catalysts with high efficiency and easy recovery.The main research contents and results are listed as follows:Firstly,the effect of A-B sites substitutional co-doping with cation-cation on the electric structure and optical absorption properties of STO is investigated by using the first-principles method.For Zn and transition metals(TM=Cr,Mn,Fe,Co)co-doped systems,it is found that there has a phase transition from symmetric cubic to nonsymmetric hexahedron phase.The 3d impurity states of transition metals lead to the band gaps are reduced by 0.68~1.09 eV.The synergistic effect of Zn and transition metals can greatly improved the photocatalytic activity of STO,the theoretical predictions show that the activities of four co-doped systems are in order of Zn/Mn-STO>Zn/Fe-STO>Zn/Cr-STO>Zn/Co-STO,and the maximum absorption edge can extended to 706 nm.The effect of alkaline earth metals(A=Ca,Ba,Mg)and transition metal(Ir)on the optical properties of STO is showed.It is revealed that the A/Ir co-doping leads to the transition from indirect band gap to direct band gap in the band structure,and thus the STO system is directly changed from catalytic inactive state to an active state.The photocatalytic activities of them are proportional to the radius of the doped alki-earth metals,which is in order of Ba/Ir-STO>Ca/Ir-STO>Mg/Ir-STO.Secondly,the effect of charge compensation on magneto-optical property is discussed.The calculated defect formation energies of Nb/Cr-STO and Nb/Fe-STO charge balance systems are relatively small,and the co-doping is easily to be realized due to the relatively high structure stability.The introduction of magnetic Cr3+and Fe3+ions leads to the Nb-STO transformation from n-type into p-type conductor.The charge compensation effect of cation(Nb)and cations Cr/Fe can improve the magnetic and visible-light absorption properties of STO,and in the order of Nb/Cr-STO>Nb/Fe-STO.The effect of charge compensation of cation(Os6+)and anion(N3-)on the electronic structure of STO is showed.The calculated band gap of STO is 3.22 eV,which can achive the experimental data by adopting a GGA+Up+Ud method.The band gap of charge non-compensated Os/N-STO system is small due to some Os 5d and O 2p impurity states excited into the forbidden gap,resulting in the band gap reduced to 1.06 and 1.39 eV,thus band edge position is not suitable for hydrogen production from water splitting.The above impurity states are eliminated in the charge compensated Os/2N-STO system,and the charge compensated defects are effectively passivated,thus the photocatalytic activity can significantly improved.The band gap and band edge position of Os/2N-STO are both suitable for criterions in solar-driven hydrogen production.Thirdly,in view of magneto-optical properties of perovskite-type STO and BFO are mainly related to doping ions at the B sites,while the doping defects are easily introduced.The electronic structure,magnetic and optical properties of Fe4+/Ni2+co-doped STO and non-magnetic ions Zn2+/Mg2+co-doped BFO are investigated by first-principles method.The Fe/Ni-STO co-doped system without oxygen vacancy maintains semiconductor character and generates a weak magnetic moment of about 0.36 μB,thus the catalytic activity is relatively high.While the electronic structure of the Fe/Ni-STO co-doped system with oxygen vacancy is transition from semiconductor to half-metal,producing a giant magnetic moment of 3 μB,and the photocatalytic activity are decreased.The existence of oxygen vacancy is benefit for improvement on the magnetism of Fe/Ni-STO,but leads to a decrease of photocatalytic activity.The electronic structure and magnetism of Zn doped Mg-BFO(BFMZO)with Bi or O vacancy defects are studied.The Zn/Mg co-doping results in the change of original G-type antiferromagnetic spin ordering structure,forming a ferromagnetic odering structure and generating a giant magnetic moment of 10.08 μB,and thus the magnetism of BFO is significantly improved.Due to the Bi vacancy has a great influence on the magnetism of BFMZO,the magnetic moment of BFMZO with Bi vacancy is decreased to 7.98 μB.The magnetic moment of BFMZO with O vacancy is similar to that of BFMZO,implying a negligible influence of the O vacancy.Fourthly,the effect of co-doping concentration on the electronic structure and magneto-optical properties of STO is demonstrated.With the increasing of doping concentration of Fe/Mn co-doped ions,there has a paramagnetic-antiferromagnetic-ferromagnetic phase transition in Fe/Mn-STO system.The Fe/Mn-STO exhibits a half-metallic character when the doping concentration is 25%.The predicted calculations show that the Fe/Mn-STO system has the best photocatalytic performance with the doping concentration of 12.5%.The results show that effect of doping concentration and doping ratio play a key role on the electronic structure of La/Ir-STO.The band gap of La/Ir-STO remains unchanged when the doping concentration is lower than 12.5%.With the increasing of doping concentration,the band gap is decreased due to the increased in-gap impurity states.The magnetism of La/Ir-STO system is found to has a great relation with the doping ratio of La/Ir.The La/Ir-STO displays no net magntic moment with the doping ratio 1:1,while the absorption edge of 1:1-La/Ir-STO system is extended to 620 nm.When the ratio of La/Ir is increased to 2:1,the magnetic moment increases significantly and the photocatalytic activity decreases.Fifthly,the samples of Zn/TM-STO(TM=Mn,Fe,Cr)supported on mineral(zeolite and kaolin)are prepared by sol-gel method,and the crystal structure,surface property and photocatalytic degradation efficiency of co-doped STO with mineral loading are investigated.Mineral carriers of both zeolite(ZSM-5)and calcined kaolin can inhibit the crystalline transformation of doped STO.After ZSM-5 zeolite and calcined kaolin loading,the specific surface area and pore volume of Zn/TM-STO are increased,which is benefit for the improvement of photocatalytic activity.The photocatalytic degradation efficiencies of Zn/TM-STO for MB solution are ZnMn-STO>ZnFe-STO>ZnCr-STO,which is consistent with the theoretical prediction in Chapter 2.The degradation efficiencies of pure STO,Zn-doped STO and Zn/TM co-doped STO samples are significantly improved when they are loaded on the two carriers.The loaded sample of STO/ZSM-5 has the highest catalytic efficiency,and the degradation rate for MB solution is 91.2%after 120 min irradiation.Under the same irradiation condition,the degradation rate of STO/kaolin loading sample for MB solution is 87.5%.The photocatalytic activities of co-doped samples are greatly improved by the ZSM-5 loading with strong adsorbability,which was slightly higher than that of calcined kaolin with excellent surface activity.Based on the mechanism of improving the magneto-optical properties of perovskite-type STO and BFO,the chemical and physical properties of perovskite-type semiconductor materials have been successfully enhanced,which provides theoretical and experimental basis for the development of novel mineral supported SrTiO3-based and BiFeO3-based visible-light photocatalysts.Figure[81]table[20]reference[208]