First-principles Study On Magnetic Properties Of Two-dimensional CdS And Photocatalytic Performance Of G-C₃N₄/InS Heterostructures

Two dimensional（2D）CdS is a classⅡ-Ⅵwide band gap semiconductor material with a band gap of about 2.58 eV.The application of monolayer CdS in spintronic devices is greatly limited because the intrinsic monolayer CdS is nonmagnetic.Based on the first-principles method of spin polarized density functional theory（DFT）,the electronic structure and magnetism of some non-metallic elements（B-F,P,Cl,Br,Si and As）and transition metal elements（Sc-Ni and Zn）substitutional doping monolayer CdS have been systematically studied.The results indicate that the system does not show magnetism when Cd vacancy and S vacancy are introduced into the monolayer CdS primitive cell.The monolayer CdS doped by nonmetallic C,N,P and As elements can induce magnetism,while the other nonmetallic elements（B,O,F,Cl,Br and Si）doping the monolayer CdS system does not show magnetism.The doping of different atoms showed different magnetic moments.The magnetic moments of C,N,P and As atoms doping systems are 1.55,1.00,1.00 and 1.00μ_B,respectively.It is found that new impurity energy levels appear in the forbidden band of the four doping systems,among which the system doped with C atom shows a semi-metallic property.This is due to the coupling between the p orbitals of doped atoms in the doping system and the 3p orbitals of S atom near the Fermi energy level,leading to the induction of magnetism in the system.The magnetic coupling of two-dimensional CdS doped with nonmetallic diatomic is further studied.The results show As atoms doped monolayer CdS system presents ferromagnetic state,while C,N and P atoms doped monolayer CdS system present anti-ferromagnetic state.The calculated formation energy shows that the nonmetallic doped atoms are more likely to occupy the position of S atoms under Cd-rich environment than S-rich environment.The influence of substitution doping of transition metal elements X（X=Sc-Ni and Zn）on the magnetic properties of monolayer CdS has been studied using the same calculations method.The results reveal that Co,Ni,Mn,Fe,Cr,Ti and V can successfully induce magnetism,while the monolayer CdS system doped with Sc and Zn elements does not show magnetism.The total magnetic moments of Co,Ni,Mn,Fe,Cr,Ti and V atoms doping systems are 3.00,2.00,5.00,4.02,4.00,1.33 and 2.77μ_B,separately,which is caused by the p-d hybridization formed by the 3d orbital of the doped atom X and the 3p orbital of the S atom.We have studied the magnetic coupling effect of the transition metal diatomic doping of 2D CdS.The results reveal that Co atom doped monolayer CdS system displays paramagnetism,and Cr atoms doped monolayer CdS system presents anti-ferromagnetic state and ferromagnetic state,while the CdS system doped with Mn and Fe atoms show the antiferromagnetic state.In addition,the result of formation energy calculation shows that the transition metal atoms X doping can be easily realized under Sn-rich environment.Therefore,the doping of some transition metal elements can effectively adjust the electronic and magnetic properties of the monolayer CdS.In order to solve the increasingly serious problem of energy shortage and environmental pollution,hydrogen can be produced by semiconductor photocatalysts split water.Up to now,many semiconductor photocatalysts have been developed,but the efficiency of hydrogen production is not ideal due to the high electron-hole pair recombination rate and poor charge transfer ability.Therefore,the photocatalytic efficiency can be improved by constructing heterostructure photocatalyst.In present work,electronic structure and photocatalytic mechanism of the novel 2D g-C₃N₄/InS heterostructure have been systematically investigated using first-principles calculations.It is found that the novel 2D g-C₃N₄/InS heterostructure has a good visible light absorption range and strength as well as a low electron-hole recombination rate,which can significantly improve the photocatalytic performance.The novel 2D g-C₃N₄/InS heterojunction is a direct band gap semiconductor with its lattice mismatch less than 3%and binding energy of-3.69 eV,indicating that it has stable structure.The band gap value of the heterostructure is 2.47 eV.Compared with monolayer g-C₃N₄（2.68 eV）and monolayer InS（3.16 eV）,the electron transfer from VBM to CBM will be easier,which is conducive to improving light absorption efficiency.The light absorption coefficient of the 2D g-C₃N₄/InS heterostructure is as high as 10⁵ cm^-1.The band offsets at the bottom of conduction band and the top of valence band on both sides of the heterostructure are 0.44 and 0.92 eV,respectively,exhibiting a type-II band alignment between g-C₃N₄ and InS.The photo-generated electrons are transferred from the g-C₃N₄ layer to the InS layer and vice versa for holes.Above results prove that electrons and holes are spatially separated.Moreover,InS is an indirect band gap semiconductor material,which can further reduce the recombination rate of electrons and holes.In the 2D g-C₃N₄/InS heterostructure,the combination of photo-generated electrons and holes can be well inhibited,effectively improving the utilization of solar energy.These results all demonstrate that 2D g-C₃N₄/InS heterostructure has an excellent photocatalytic performance,and also provides useful information for understanding the mechanism of enhanced photocatalysis observed in experiments.