Theoretical Study On The Fine-Tuning Of Magnetic And Photocatalytic Water-Splitting Properties Of Several Layered Materials

Since the discovery of graphene,great interest has been aroused in the research of low-dimensional layered materials.When the thickness of materials is reduced down to a single layer or few layers,the materials will present novel properties,due to quantum confinement effect,which are different from their bulk counterparts.With more and more research on layered materials,a number of new materials with special properties have been found,leading to a broad range of potential applications in spintronics,magnetic materials,catalytic materials,new energy techniques and so on.To better meet the requirements for practical applications,the electronic structure of layered materials usually should be tailored.Currently,the fine-tuning of electronic structure has become a hot topic for new materals design.In this dissertation,first-principles method based on density functional theory is used to investigate several layered materials for applications in the fields of nano-spintronics and photocatalytic water splitting by means of doping and surface modification.The main conclusions are summarized as follows:(1)Tuning electronic structure and magnetism of an AIN monolayer doped with first-row elements.AIN monolayer is a nonmagnetic semiconductor.In order to achieve its applications in spintronics,we propose a method to modify its magnetic properties by doping Be,B and C,where Be will be located substitutionally at the Al sites while B and C atoms will be at the N sites.This method can avoid the interference of magnetic clusters or magnetic secondary phases,which is formed by the doping of magnetic elements.The calculated results reveal that Be and C dopants can induce local spin moments,while B dopant cannot lead to spin polarization in the AIN monolayer.Furthermore,the calculated results show that when the distance between dopants increases,C doping system transfers into paramagnetic state,while Be doping system always maintains a stable ferromagnetic state.The long-range ferromagnetic coupling between doped Be atoms can be explained by a hole-mediated p-p interaction.(2)Tuning electronic structure and magnetism of two-dimensional layered K2CoF4 by defect engineering.By theoretical modeling,we find that a stable two-dimensional layered K2CoF4 system could be exfoliated from its layered bulk while its ground state is antiferromagnetic semiconductor.When the K vacancy exists,the ground state remains antiferromagnetic.However,under suitable tensile strain,the K vacancy system can transform into a ferromagnetic half-metal state,and the ferromagnetic state becomes more stable with the increase of tensile strain.The ferromagnetism may originate from the p-d exchange interaction.Monte Carlo simulations based on Heisenberg model predict that the Curie temperature for the K vacancy system at a tensile strain of 2%is higher than room temperature,suggesting that the two-dimensional layered K2CoF4 with K vacancy system has potential applications in nano-spintronic devices.(3)Tuning photocatalytic water-splitting property for two-dimensional layered GaS by isovalent anion and cation doping.Due to the large band gap(3.22eV),two-dimensional layered GaS only absorbs a small portion of the solar spectrum in the ultraviolet region.In order to reduce the band gap and improve the visible-light activity,we propose an isovalent anion and cation doping method,i.e.the O,Se and Te atoms will be introduced at the S sites,while Al,In,Sc and Y atoms will be introduced at the Ga sites.The calculations reveal that Se,Al and In dopants change the electronic structure of the system only scarcely,while Te dopant gives rise to slight upward shift of the valence band maximum.In contrast,O dopant reduces the band gap of the two-dimensional layered GaS significantly,but the conduction band minimum of the system is shifted downward below the water reduction potential.In the case of Sc and Y dopants,not only the band gap of the system can be effectively narrowed,but also the band edge positions still straddle the water redox potentials.The calculated results further show that the band gap could be narrowed even more and the band edge positions match better with the water redox potentials by codoping(Sc,Te)or(Y,Te),in which the(Y,Te)-codoped system is believed to be an ideal visible-light-driven photocatalyst for water splitting due to stronger light absorption and better match of band edge position with the water redox potentials.Then we investigate the photocatalytic water splitting property of GaS1-xTex(x=0,0.125,0.25,0.5,0.75,0.875,1)systems.The results indicate that two-dimensional layered GaS0.5Te0.5 is a promising visible-light water splitting photocatalyst,since this system is not only a direct band gap(2.09eV)semiconductor,but also its band edge positions match with the water redox potentials in both acidic and neutral environments.Besides,the effective mass of charge carriers is smaller in GaS0.5Te0.5 system than in GaS and GaTe systems along the direction of Γ-K,which facilitates the migration of carriers to catalyst surface to participate in the catalytic reaction.(4)Band structure engineering of two-dimensional layered IVB group nitride halides MNX and MNX/GaS heterostructure.Bulk Group-IVB nitride halides MNX(M=Zr,Hf;X=Cl,Br,I)in the β-form belong to layered crystals,in which the interlayer distance are all larger than 3.0 A.Based on density functional theory,we examine the possibility to realize stable two-dimensional layered MNX by exfoliating layered Group-IVB nitride halides bulks.The calculated electronic property suggests that two-dimensional layered MNX systems are all indirect band gap semiconductors with band gaps in the range of 1.55-3.37 eV.Among them,MNI systems have moderate band gaps and better charge separation,which is required for photocatalytic water splitting.In order to achieve the separation of carriers for the rest two-dimensional layered systems,we further study the heterostructures formed by MNX(M=Zr,Hf;X=Cl,Br)and GaS.Theoretical calculation shows that the MNX/GaS heterostructures belong to type-Ⅱ band structure,which is beneficial to the separation of electrons and holes.Furthermore,the band gap and band edge position of MNX/GaS heterostructures can be effectively tuned by biaxial strain.These results indicate that two-dimensional layered MNI systems and MNX/GaS heterostructures have great potential application in optoelectronic devices.