First-Principles Study On Two-Dimensional Heterostructure Used As Photocatalyst For Water Splitting

In order to alleviate the world energy shortage and environmental pollution problems,hydrogen(H₂)energy is considered as a new generation of clean energy because the product of its combustion is just water.At the same time,the utilization rate of solar energy is generally low,and the sunlight has plentiful energy,so it is necessary to use sunlight to stimulate the catalyst to decompose water to produce H₂.Compared with the bulk materials,using two-dimensional(2D)materials as photocatalyst for water splitting is more suitable,because 2D photocatalyst has a larger specific surface area,which can provide a wide surface for redox reactions.However,photogenerated electrons and holes will make a combination on the surface of catalyst easily.Furthermore,the type-II semiconductor heterostructure can further enable the oxidation and reduction reactions to proceed in two different layers by separating photogenerated electrons and holes,which will prolong the lifetime of the photogenerated charges.Therefore,it is of great significance to study the 2D semiconductor heterostructure as a photocatalyst for decomposing water.At present,a large number of 2D materials have been predicted or prepared,such as graphene,molybdenum disulfide(Mo S₂),blue phosphorus(BlueP),arsenene(Are),etc.These 2D materials also have novel physical and chemical properties,which make them very promising for applications in optoelectronic,thermoelectric,and catalytic nanodevices.In addition,computer hardware is developing rapidly,and numerical calculation methods are also constantly being improved.Many scientific researchers have developed more efficient,reliable,and accurate calculation methods.Among them,the first-principles calculation method,based on density functional theory(DFT),is widely used to investigate various properties of nanomaterials.Moreover,the first-principles calculation method shows results that are very close to the experimental values.Besides,first-principles calculation methods can be carried out to predict the properties of some 2D materials,which can provide a theoretical basis for experimental work,and also explore the potential applications of 2D materials.Based on this,we gave an introduction to the first-principle calculation method based on the density functional theory calculation method used in this article,and expounded its development and improvement up to now,including the Born–Oppenheimer approximation,Hartree–Fock approximation,Hohenberg–Kohn theorem,Kohn–Sham equations and various exchange-related functionals,etc.It also introduced how these methods are implemented in the calculation and prediction of 2D semiconductor heterostructure as a photocatalyst.In this paper,we first introduced the significance and background of this work,expounded the advantages of heterostructure compared with the single semiconductors as photocatalysts,and summarized the current domestic and foreign research on the use of semiconductor heterostructure as photocatalyst to decompose water for H₂ production.Then,we demonstrated why using 2D materials as a photocatalyst for water splitting is more suitable than bulk materials by the special physical and chemical properties of 2D materials,followed by introducing some currently popular2D materials,such as graphene,transition metal dihalides(TMDs),Are,and some heterostructure.Then,we investigated theoretical calculations on the geometric structural,electronic,interfacial and optical properties of some 2D semiconductor heterostructure based on TMDs.For example,TMDs/BP,TMDs/GeC and TMDs/Mg(OH)₂.First,the band structure of the monolayered materials are calculated,and then we decided the most stable stacking structure of the heterostructure by binding energy.Next,we studied the band structures of the heterostructures and explained how it separate photogenerated electrons and holes continuously.The charge difference and potential drop across the interface of the heterostructure are also calculated.Finally,the light absorption capacity of these TMDs-based heterostructures are addressed.The research in this chapter shows a new effective method for designing heterostructures based on TMDs,and explores their applications in photocatalysis,optoelectronics,and optical devices.Next,we mainly predicted the decomposition of water for H₂ production by some 2D semiconductor heterostructures used as photocatalysts.In the study of BlueP/GeC and BlueP/SiC van der Waals(vd W)heterostructures,we considered the potential redox capacity of BlueP/GeC and BlueP/SiC vd W heterostructures respecting to different pH values in the water,and calculated their sunlight absorption performance.Then,we also explored the tunable properties of the GaN/ZnO heterostructure under external strain.By applying biaxial strain to the heterostructure in the plane direction,we studied that influence on the binding energy of the GaN/ZnO heterostructure,especially for the band edge positions so that we explored how the applied external biaxial strain tune the redox capacity for water splitting under pH values,and the influence of the applied strain on the light absorption performance of GaN/ZnO heterostructure is also calculated.Finally,we designed a heterostructure with a direct Z-scheme catalytic mechanism based on PtS₂ and Are,and demonstrated how the work function difference of two layers induce a built-in electric field across the interface of the PtS₂/Are heterostructure,which boosts the photogenerated electrons and holes moving by the Z-scheme photocatalytic mechanism,so that the PtS₂/Are heterostructure further improves the separation rate of photogenerated charges and the efficiency of the redox reaction for water splitting.The work in this chapter provides a theoretical method for designing heterostructures based on 2D materials as photocatalysts to decompose water for H₂ production.We also studied the performance of 2D semiconductor heterostructures as a photocatalyst to decompose water,such as,Gibb's free energy,carrier mobility,and band bending mode.We first calculated the Gibb's free energy of the intermediate generated form the g-GaN/Mg(OH)₂ vd W heterostructure using as a photocatalyst for water splitting,which shows that the g-GaN/Mg(OH)₂vd W heterostructure has very novel OER and HER catalytic ability.Subsequently,Janus MoSSe/GaN and Janus MoSSe/AlN heterostructures were constructed by vd W forces,and we calculated their carrier mobility along armchair and zigzag transport directions,the results show that the MoSSe/GaN and MoSSe/AlN heterostructures can significantly increase the carrier mobility of monolayered MoSSe,which can greatly improve the photocatalytic efficiency for water splitting.Finally,we also studied the band bending mode at the interface of the ZnO/Mg(OH)₂ and g-GaN/BlueP vd W heterostructures.The band bending mechanism is analyzed in depth,which can further promote the separation of photogenerated charges.The work in this chapter provides a theoretical guide for evaluating the efficiency of the 2D semiconductor heterostructure used as a photocatalyst to decompose the water into H₂ and O₂.Finally,we summarized the conclusions and the innovations overall our work.In addition,we also look forward to future work,including calculation methods,calculation content,and experimental work.