Theoretical Investigation Of Several Proposed Two-dimensional Heterostructures For Solar Water Splitting

At present,the problem of environmental pollution and energy shortage is becoming more and more serious.One of the effective methods to solve the problem is to use solar energy to split water based on various semiconductor photocatalysts,so as to obtain environmental friendly,green and efficient hydrogen energy.The semiconductor photocatalyst used for overall water splitting must meet several basic conditions:the candidate has an appropriate band gap,being able to effectively capture sunlight,and generating photogenerated carriers;its conduction band minimum(CBM)and valence band maximum(VBM)must match with water redox potentials;photogenerated electrons and holes can be effectively separated and transferred,and then participates in hydrogen production and oxygen production respectively,and so on..In recent years,due to the large specifific area,more reaction sites,short carrier migration distances,abundant and easily tunable physical and chemical properties,to construct high-performance photocatalysts based on two-dimensional(2D)materials is attracting great research attention.One of the advanced and fundamental projects is to improve the photocatalysis of the 2D materials by forming 2D heterojunctions via interface coupling.In this dissertation for the master degree,we examine several 2D heterojunctions by performing extensive first-principles calculations based on density functional theory(DFT).The photocatalytic performance is characterized by the interfacial polarized electric field,and the alignment between the VBM and CBM positions and the water redox potentials.The master dissertation is composed of the following chapters.In Chapter 1,we briefly introduce the basic concepts of DFT,and the Vienna Ab initio Simulation Package(VASP),which is adopted in this dissertation.In Chapter 2,the basic principle of photocatalytic overall water splitting,the research status of 2D photocatalysts for solar water splitting,and several effective strategies to improve their photocatalytic performance are concisely summaried.The motivation of this dissertation is given at end of this chaper.In Chapter 3,we propose three 2D MX/Bi2Se3(MX=GaTe,InTe,and InSe)heterostrucures and explore their photocatalytic performance for solar water splitting.According to the calculated total energies,band structures,charge transfer,interface polarization electric field,electrostatic potential distribution,optical absorption,the band alignments of the heterojunction with the requirement of photocatalytic water splitting,we find several following conclusions:(i)The isolated MX monolayer can only be used for hydrogen production,while the free Bi2Se3 nanosheets may be used for oxygen production.(ii)Via introducing the interface coupling of MX-Bi2Se3,we observe the transition from indirect band gap to quasi-direct band gap.The band gap of the proposed 2D heterostructure is effectively reduced,which is beneficial to harvest sunlight.(iii)In these 2D MX/Bi2Se3 heterostructures,due to the difference of two work functions,the charge transfers from the MX monolayer to the Bi2Se3 nanosheet,resulting in forming a relative strong built-in interfacial polarized electric fields,which causes the valence band edge of MX sheet and the conduction band edge of Bi2Se3 monolayer to bend upward and downward slightly,respectively.(iv)The photogenerated holes on the valence band of the MX monolayer are easily combined with the photogenerated electrons on the conduction band of the Bi2Se3 nanosheet,so that the photogenerated electrons and holes remain on the MX monolayer and the Bi2Se3 nanosheet in these 2D heterojunction,respectively.(v)The VBM of the MX monolayer in the heterojunction is higher than the water reduction potential,while the CBM of Bi2Se3 sheet is lower than the water oxidation potential,which ensures that the photogenerated electrons and holes on them participate in the hydrogen-production reaction and oxygen-production reaction,respectively.These theoretical findings obtain in this chapter show that the topological insulator Bi2Se3 can be used for water splitting photocatalysis via the interface coupling,and the proposed 2D MX/Bi2Se3 heterojunctions are direct Z-scheme photocatalysts for solar water splitting.In Chapter 4,we preliminarily investigate the photocatalytic performance of the 2D ZrS2/MNI(M=Hf,Zr)heterojunctions.Theoretical results show that the isolated ZrS2 sheet and MNI monolayer can not be used to split water into hygrogen and oxygen at the same time.Interestingly,as they are coupled together to form 2D vdW heterojunctions,based on the similar calculations and analysis in chapter 3,we find that ZrS2/MNI heterojunctions are also promising Z-scheme photocatalysts for solar water splitting.