| With the rapid development of modern industry,human needs for energy are increasingly intensified.How to get rid of oil dependence and vigorously develop clean energy represented by new energy has become an important consensus in the process of realizing sustainable development all over the world.Among them,solar energy has been paid more attention because of its wide distribution range,stable energy input,cleanness and safety.Hence,scientific researchers are committed to developing environmentally friendly,sustainable,and economical industrial technologies by applying solar energy.Solar energy can be converted to thermal,electric,and chemical energy.In the field of photothermal and photoelectric conversion,it has been achieved considerable development based on representative technologies including seawater desalination,solar water heater,and photovoltaic power generation.Up to now,the conversion process from solar energy to chemical energy has become the most vital study hotspot.This chemical energy conversion process is realized by utilizing the absorption of solar radiation.The main applications of photocatalyst for the conversion of solar energy to chemical energy include photocatalytic degradation of organic pollutants,photocatalytic water splitting to produce hydrogen,and photocatalytic CO2 reduction and nitrogen fixation.These applications provide effective solutions to the energy demand crisis,serious environmental pollution,and greenhouse effect.It is undeniable that although photocatalysis technology has achieved a huge technological breakthrough,however,the photogenerated charge carriers would recombine rapidly after light excitation,which limits the photocatalytic efficiency.The main effecting factors on photocatalytic activity are as follows:(1)light absorption.It can ensure that more photogenerated charge carriers are excited by absorbing solar energy as much as possible.(2)The separation of photogenerated charge carrier,and transfer to the surface catalytic sites.(3)Photogenerated charge carriers react with reactants on the surface of the photocatalyst.Hence,the design of high-efficiency photocatalysts mainly revolves around the above three factors.Photoinduced charge carriers are non-equilibrium carriers formed by the transition of the valence band electrons to conduction band after absorption of energy,which can be easily compounded to return to the equilibrium state.In addition,the impurities and vacancies in the material have a strong ability in capturing the charge carriers,forming the recombination center.Therefore,the material intrinsic photoinduced charge carrier separation efficiency is still insufficiency,which is the key to limit the improvement of photocatalytic efficiency and the difficulty of material regulation.Over the past few decades,many solutions have been proposed,but the most widely used are mainly the following two approaches:(1)combining the different photocatalytic reaction systems,to develop the energy band matching photocatalytic heterostructures,such as p-n junctions;type-? heterostructures;z-scheme heterostructures and Schottky junctions,which are considered to play a vital factor to improve the separation of photogenerated charge carriers at the material interfaces due to the generation of built-in electric fields.(2)The other approach for increasing photoinduced charge carrier separation is to apply an external field,such as microwave;mechanical force;thermal field;electric field and magnetic field,to the photocatalyst.It provides the necessary external driving force to overcome the rapid recombination of photogenerated charge carriers,improving the photocatalytic performance.However,most external field application,such as electric field,requires the catalytic material to be loaded on the electrode.Although it can improve the photocatalytic efficiency of the photocatalyst per unit mass,which also has two adverse effects:1.The three-dimensional catalytic system of the suspension is decreased to the two-dimensional catalytic system on the electrode material,and the overall photocatalytic efficiency is reduced.(2)When the material is loaded on the electrode,the catalytic site of the photocatalyst will be covered,which affects the full utilization of the photocatalytic active site of the powder material.In addition,Applying the external electric field in photocatalysis also need the electrodes,conductors,and power supplies,which will bring about greater consumption of resources and energy.It is a new breakthrough and a difficult point in photocatalytic technology to realize the non-contact field effect on the powder photocatalytic material.The magnetic field is beneficial to act on nanoparticles in suspension systems,which has significant advantages for the regulation of nanoparticle properties.Based on the discussion above,the research thought of this work is to introduce the role of magnetic field,which is rarely paid attention to in photoinduced charge carrier regulation,into the research system.Conducting the following studies:1.Our work firstly revolves around the basic physical principle that the moving charged will change the direction of motion under the action of Lorentz force in magnetic field,the regulation of photogenerated charge carriers in non-magnetic photocatalytic materials was studied.2.With the assistance of electron spin polarization induced by magnetic field,the regulation of photogenerated charge carriers in magnetic photocatalytic materials was studied by introducing the electron spin polarization to change the charge carrier separation characteristics.The main contents are as follows:firstly,the direct effect of Lorentz force on photoinduced charge carrier in magnetic field was investigated.Based on the opposite direction of Lorentz forces acting on the moving photoinduced electrons and holes in magnetic field,the recombination of photoinduced charge carrier was suppressed.Secondly,taking advantage of the micro-electric potential generated by electric polarization of nano-conductor induced through Lorentz force,the regulation of photogenerated charge carrier of surface assembled nano-semiconductor was realized.Then,according to the various spin directions in magnetic semiconductor materials,the spin polarization state can be regulated.Therefore,by adjusting the spin polarization state of the electron,the spin polarization state of the photogenerated electron and hole can be tuned to affect their recombination rate,so that the magnetic field can regulate the separation of the photogenerated charge carrier in the magnetic semiconductor photocatalytic material.The main research contents are as follows:Therefore,the main research contents are as follows:(1)Lorentz force regulates the separation of photoinduced charge carrier and magnetic field-assisted photocatalysis:through Lorentz force-drived electrons and holes to move along the opposite direction that suppresses the recombination of photogenerated charge carriers.this work selected the TiO2 nanobelts with complete crystal structure as the photocatalyst,the photocatalytic efficiency can be improved by 40%just by placing a permanent magnet beneath the normal photocatalytic system without any additional power supply.Electrochemical Mott-Schottky polts were used to analyze the concentration of photogenerated charge carriers,and it was found that the concentration of photogenerated charge carrier increased by about 30%under the action of magnetic field.Therefore,the mechanism by which the Lorentz force acts oppositely on the photogenerated electrons and holes is introduced,resulting in the suppression of the photoinduced charge recombination.This opens up a new strategy for the design of materials and systems that enhance the separation of photogenerated charge carriers.Moreover,on the basis of the Lorentz force-driven improved separation of photoinduced charge carrier in TiO2 nanobelts,further to explore the Lorentz force effect on the separation of photogenerated charge carrier for the heterostructure photocatalysts through constructing a reduced graphene oxide(rGO)/TiO2 nanobelts(NBs)with complete crystal structure.The photocatalytic efficiency of the rGO/TiO2 NBs heterostructure improved by 34%compared with pure TiO2 NBs under the same magnetic field conditions,which demonstrates that the built-in electric field of heterostructures has a significant effect on the separation of photogenerated charge carrier.Combining with the Lorentz force and heterogeneous structure interface built-in electric field synergy action,the mechanism of photocatalytic enhancement is discussed in two stages:separation and transport of photogenerated charge carrier.On the one hand,during the separation of photoinduced charge carrier,Lorentz force suppresses the recombination of photogenerated charge carrier at the initial stage of production and increases the number of charge carrier that can participate in transfer process;for photoinduced charge carrier transfer process,The build-in electric field formed by the RGO-TiO2 heterostructure provides a spontaneous transport path for the charge carriers,which realizes the transfer of more photogenerated charge carriers.This work provides a new approach that combining the "build-in electric field and magnetic field" to improve the photocatalytic activity.(2)Construction of on-situ micro-electric field by electromagnetic induction and the regulation of magnetic field on photoinduced charge carrier of nanocomposite photocatalysts:Through constructing the composite structure material with the nanoconductor as the core,the electromotive force generated by electromagnetic induction in metal conductor,this in-situ micro-electric field is provided for the composite material to enhance the separation of photogenerated charge carriers.According to the electromagnetic induction principle,electrons are acted on by Lorentz force to form a charge polarization distribution when the metal conductor moving in the magnetic field,which is macroscopically represented as the electromotive force.In the composite photocatalysts constructed with the nanoconductor as the core,the electromotive force of the nano-conductors acts as an in-situ micro-electric field to regulate the separation of photogenerated charge carrier,improving the photocatalytic performance.On the basis of above material design ideas,the electromagnetic induction derived micro-electric field in metal-semiconductor(Au nanorods CdS nanoparticles)core-shell hybrid nanostructure was used to enhance photoinduced charge separation in the shell semiconductor photocatalysts.Applying the self-designed and modified moving magnetic field photocatalytic test device,the change of the photocatalytic properties of the composite in magnetic field was studied.The results show that the efficiency of photocatalytic hydrogen production can be improved around 110%by utilizing an Au@CdS core-shell nanostructure.It is proved that the micro-electric field provided by electromagnetic induction can effectively enhance the separation of photogenerated charge carriers.This electromagnetic induction derived electric field via the metal-semiconductor core-shell structure shows efficient conversions from relative motion to electric potential that provides a new opportunity to enhance photocatalytic performance with non-contacted interaction.On the other hand,because the localized surface plasmon resonance(LSPR)is an electromagnetic oscillation formed by the interaction of free electrons and photons in the metal surface region,it is the interaction between the surface charge oscillation and the electromagnetic field of light wave.The hot electrons formed also have the characteristics of photogenerated charge carriers.Therefore,based on the electromagnetic induction,the micro-electric field has possible to regulate the hot electrons generated by LSPR effect on the surface of the metal conductor.Through constructing Pd-tipped Au nanorod heterostructures to achieve plasmon-enhanced formic acid dehydrogenation,the magnetic-field-derived electromagnetic induction effect is utilized to further boost the generation and transfer of plasmonic hot charges in Au nanorods.By exposing the plasmonic catalytic system to a rotating permanent magnet at 28?,formic acid dehydrogenation efficiency was improved by approximately 60%.The improvement rate in the same system can exceed 150%at 45?.Since the formic acid dehydrogenation catalytic reaction by Pd is closely related to the electron density of Pd surface,the significant improvement of catalytic performance under the action of magnetic field indicates the enrichment of electrons on Pd surface.This is attributed to more hot electrons produced by Au nanorods LSPR were transferred to the surface of Pd nanoparticles.It is verified that the electromotive force can regulate the hot electrons of LSPR,and the regulation effect of the in-situ micro-electric field on itself(Au)hot electrons self-carrier is formed.This work further confirms the feasibility and universality of the design of composite structure photocatalysts with functional materials mediated in situ micro-electric field enhanced photogenerated charge carrier separation.(3)Magnetic field regulation of electron spin polarization in ferromagnetic semiconductors enhances the separation of photogenerated charge carriers:The magnetic semiconductor materials have different electron spin directions and do not show the effect on photogenerated charge carriers.But the direction of its electron spin can be adjusted by applying a magnetic field.When the electron spin polarization,the spin polarization states of photoexcited electrons and holes in magnetic semiconductor materials will be affected,and then the fate of photogenerated charge carrier will be regulated.Here,we synthesize a ferromagnetic ZnFe2O4(ZFO)photoelectrode with an improved ferromagnetic property by introducing cation disorder and oxygen vacancies.Ferromagnetic ZnFe2O4 with an improved ferromagnetic property enables significant enhancement of the oxygen evolution reaction(OER)simply by placing a permanent magnet to provide the photoelectrode with a magnetic field.Phosphorus-doped ZnFe2O4 can achieve 150%and 125%enhancement of the OER performance at 1.23 and 1.57 V vs.RHE,respectively.The improvement is assigned to electron spin polarization of the ferromagnetic ZFO photoelectrode regulated by the magnetic field.The mechanism can be explained as:the introduction of cation disorder and oxygen vacancy increases the spin-electron concentration,and more electron spin polarization will be realized under the action of magnetic field.Spin state loss of excited electrons due to the hyperfine interaction,spin-orbit coupling effect,there are many photoinduced electrons in the same spin state as holes,which is unfavorable for their recombination.Thus,the recombination of photoinduced charges is remarkably suppressed during the light excitation process.Combined with the resistance decrease based on the magnetoresistance effect,more photoinduced charge carriers are generated and transfer to the active surface,resulting in a significant enhancement of the OER process.This work extends the theory and material system of magnetic field enhanced photocatalysis by regulating electron spin polarization to enhance the separation of photogenerated charge carriers.On the whole,this paper studies the mechanism of magnetic field regulating photocatalytic performance,proposes the material design principle,and builds the material system of magnetic field enhanced photocatalysis.The Lorentz force of magnetic field on charge and the regulation of electron spin polarization are applied to the separation process of photogenerated charge carrier,which can suppress the recombination of photogenerated charge carrier and enhance their transfer,thus improving the photocatalytic performance.As a non-contact external field regulation system,it provides a certain guiding significance for the construction and development of high-performance photocatalytic material system. |
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