Separation Of Light Induced Carrier And Research Of Photocatalytic Efficiency Manipulated By The Magnetic Field

With the continuous development of human industrialization,water pollution has gradually become one of the serious problems facing mankind.During the industrialization process,a large number of antibiotics,such as tetracycline,are randomly discharged into natural water without treatment,affecting the health of natural organisms and human beings.In order to repair water pollution,many methods have been proposed.Among them,photocatalysis technology is a green and environmentally friendly technology because it can effectively use solar energy,which has attracted widespread attention.Photocatalysis technology can finally decompose tetracycline into non-polluting products such as carbon dioxide and water by converting solar energy into chemical energy and using active species such as electronic holes produced by photocatalysts.However,at present,photocatalysis technology is mainly limited by two problems,the narrow light absorption range,and the combination of photoinduced carriers.Therefore,in order to improve the efficiency of photocatalytic reactions,the catalyst is usually modified by various means to improve the light absorption capacity or inhibit the composition of carriers.At present,commonly used modification methods to construct heterogeneous,such as p-n knots,S-type heterojunction,or Z-type heterojunction,to achieve efficient carrier separation through built-in electric fields;or introducing external driving fields to improve the reaction rate of photocatalysis through electric fields,piezoelectric,thermoelectric and photothermal effects.However,these methods need to provide additional energy input during the reaction process,which is contrary to the green environmental protection concept of photocatalysis technology.Therefore,it is urgent to find an external field that does not require additional energy input to improve the efficiency of photocatalytic reaction.Magnetic field,as an external driving field,also has the potential to regulate the separation of photogenerated carriers.The charged particles moving in the magnetic field will be affected by the Lorentz force,thus changing the track of motion.Based on this feature,a magnet can be placed outside the photocatalytic reaction vessel to the magnetic field control and Lorentz force are used to suppress carrier separation.At the same time,because Lorentz force cannot play the role of directional separation of carriers,boron nitride quantum dots are introduced to attract holes,and the two combine to achieve the effect of directional separation.In addition,the magnetic field can also change the spin polarization state of the electron,thus achieving the effect of inhibiting the rapid recombination of carriers.Specifically,in the electron hole pair,the spin direction of the electron and the hole is opposite.Under the light excitation,the electron will transition from the valence band to the conduction band,and then the electron will recombine with the hole,during which the spin state of the electron will not change.However,if the spin state of the electron is deflected during this process,and the spin state of the electron is consistent with the previously matched hole spin state,according to the Pauli exclusion principle,then the electron will not be able to successfully recombine with the hole,thus achieving the goal of restraining carrier recombination.At the same time,combining spin polarization with traditional Z-type heterojunction can further separate photogenerated electrons and holes,thus improving the efficiency of photocatalysis.The specific research contents are as follows:（1）The influence of magnetic field Lorentz force on photogenerated carriers was studied.A series of zero-dimensional（0D）/two-dimensional（2D）-boron nitride quantum dots（BNQDs）/bismuth oxobromide（Bi OBr）photocatalysts with visible light response were prepared by hydrothermal method.The samples were characterized by XRD,SEM,TEM,UV-vis,and other methods.At the same time,the degradation of tetracycline（TC）was studied under the irradiation of xenon lamp with 420 nm filter.The influence of Lorentz force of magnetic field on photocatalytic efficiency was explored by controlling the presence or absence of magnets.The experiment shows that the removal rate of TC by Bi OBr has been significantly improved（41%）within 60 minutes after placing magnets compared with the experimental conditions without placing magnets.Further research found that the removal rate of TC was further improved by 21%after adding BNQDs to Bi OBr.This work shows that the Lorentz force of magnetic field can effectively inhibit the recombination of photogenerated carriers and improve the photocatalytic efficiency.At the same time,introducing BNQDs to trap holes can further improve the degradation efficiency.（2）The deeper interaction between magnetic field and photocatalyst was studied.The magnetic field regulated the electron spin state and inhibited the electron hole recombination.A series of Mn_xZn_1-xFe₂O₄（Mn_xZFO）photocatalysts with different Mn doping amounts were prepared by hydrothermal method.In order to explore the effect of electron spin polarization on the separation of electron-hole pairs,a suitable semiconductor material is needed for experimental verification.Zn Fe₂O₄（zinc ferrite）,as a semiconductor catalytic material with weak magnetism,is very suitable for exploring this experiment.However,according to the first-principle DFT calculation,since zinc ferrite itself does not have electron spin polarization,its upper and lower spin state electrons are in equilibrium symmetry,and cannot produce spin polarization.Therefore,in order to destroy the spin symmetry,it is necessary to add the magnetic element manganese（Mn）into it.Through DFT calculation,Mn_xZFO shows obvious electron spin polarization characteristics,and the strongest polarization effect is shown when the content of Mn is 0.6.（3）In order to shorten the degradation time,a series of Mn_0.6ZFO/Bi₁₂O₁₇Cl₂photocatalysts with different composite ratios were prepared by hydrothermal stirring method.In order to explore the synergistic effect of electron spin polarization and heterojunction,a photocatalyst with suitable valence band is required to be compounded with Mn_0.6ZFO.Among them,Bi₁₂O₁₇Cl₂ has a wide band gap and a higher conduction band position than Mn_0.6ZFO,which is suitable for combining with Mn_0.6ZFO to explore the role of heterojunction and electron spin polarization.The experiment shows that when the composite mass ratio of the two is 1:2,that is,Mn_0.6ZFO:Bi₁₂O₁₇Cl₂=1:2（1M:2B）,the degradation effect of TC is the best.At the same time,through XPS analysis,we found that the type of heterojunction formed by the two is Z-type heterojunction.