Novel Two-Dimensional Photocatalysts For Water Splitting:Theoretical Design And Characterisation

The utilization of two-dimensional(2D)semiconducting photocatalysts achieving the overall water splitting to generate hydrogen is an efficient method for converting solar radiation into clean chemical energy and providing an ideal alternative to replace fossil fuels,and then to solve the energy crisis as well as the serious environmental problems.Unfortunately,most 2D semiconducting photocatalysts do not satisfy the thermodynamic conditions for overall water splitting to trigger the hydrogen evolution reaction and oxygen evolution reaction at the same time.For example,the high carrier recombination rate limits photocatalytic efficiency.Therefore,in order to realize an efficient overall water splitting process,photogenerated carriers must be effectively separated,and construction of 2D type-Ⅱ or direct Z-scheme heterostructure is a commonly adopted method to solve this issue.Nowadays,synthesis and design of 2D semiconducting photocatalysts with high performance for water splitting is a frontier basic research topic.In this dissertation for Ph.D,by performing extensive density functional theory calculation and non-adiabatic molecular dynamics simulations,based on 2D heterstructures,two semicoducting photocatalysts for overall solar-driven water splitting with different carrier transfer mechanisms(type-Ⅱ and Z-scheme)are rationally designed and theoretically characterized.In addition,a prior structure search software based on genetic algorithms is developed,and then a novel 2D metal-free monolayered photocatalysts for overall water splitting is successfully predicted.Clearly,these presented findings provide new insights and important theoretical foundation for developing high performance photocatalysts for solar-driven overall water splitting in near future.This dissertation includes the following chapters.In Chapter 1,the fundamental concepts of photocatalytic water splitting,and the current status of development of novel 2D semiconducting photocatalyst for solar-driven water splitting are briefly introduced.In Chapter 2,we briefly summarize density functional theory and non-adiabatic molecular dynamics,as well as several computational packages adopted in this dissertation.In Chapter 3,based on β-SnS/GaSe heterostructure,we rationally design and theoretically characterize an type-Ⅱ photocatalyst with high performance for solar-driven overall water splitting.Theoretical results clearly reveal that the β-SnS/GaSe heterostructure has strong optical absorption ability in the visible and ultraviolet ranges of the solar spectrum,and the sharp exciton peaks in visible-light regions are known as interlayer,intralayer,or mixed-type bright excitons.The band edge positions of the proposed heterostructure satisfies the thermodynamic conditions of overall water splitting,and the photogenerated electrons and holes have ample driving force to render the water oxidation and hydrogen reduction.The out-of-plane intrinsic dipole of the βSnS monolayer not only improves interfacial coupling with GaSe monolayer,but also induces a strong interfacial built-in electric field.Due to the interfacial electric field and strong non-adiabatic coupling,the separating process of the photogenerated carriers is accelerated within sub-picosecond,while the photogenerated electron and hole recombination is relatively slow,with a time scale of about 600 ps.Then,the separated electrons and holes with strong redox capacity could effectively participate in water oxidation and reduction reactions on the GaSe and β-SnS monolayers,respectively.Under solar light irradiation,the water oxidation and reduction reactions may spontaneously occur on the GaSe and(3-SnS surfaces,respectively,in the proposed heterojunction with rational surface modulation(i.e.introducing Ga and S atom vacancies,respectively).In Chapter 4,based on C7N6/Sc2CCl2 weak van der Waals heterostructure,we successfully design a mediator-free direct Z-scheme photocatalyst for solar overall water splitting.Theoretical results indicate that the proposed heterostructure has a small band gap of 0.17 eV,and displays a nice light-harvesting performance extending to the near-infrared region.The photogenerated electrons and holes can utilize interlayer out-of-plane vibrational modes to achieve a fast phonon-assisted recombination process,indicating the photogenerated carriers in C7N6/Sc2CCl2 heterostructure follow a direct Z-scheme transfer route.When the proposed heterostructure is radiated under solar light,the well-separated electrons and holes with a strong redox capacity make the hydrogen evolution reaction spontaneously occur on the C7N6 surface and the oxygen evolution reaction on the Se-doped Sc2CCl2 surface,respectively.In Chapter 5,we initially introduce the genetic algorithm-based priori structure prediction software package(AISP)developed by our group,including the implementation process of the genetic algorithm in AISP and some outputs.Clearly,our AISP package can be used to search and predict bulk,two-dimensional,one-dimensional,and zero-dimensional materials.Inspired by experimental synthesis of 2D boron-carbonnitrogen ternary materials,the BHO-Graphene monolayer composed of carbon four,six-,and eight-membered rings searched by our AISP package is modified to be a C6B4N4 ternary monolayered material.According to our theoretical studies,C6B4N4 is a direct semiconductor with a band gap of 3.2 eV with suitable band edge positions for overall water splitting,and the excitonic effects cause strong optical absorption from near-infrared to ultraviolet light regions.Importantly,the hydrogen and oxygen evolution reactions spontaneously can spontaneously occur on the proposed C6B4N4 monolayer.These findings imply that this novel C6B4N4 monolayer is a ideal metalfree photocatalyst without sacrificial reagents or cocatalysts for overall water splitting.A brief summary and outlook of this Ph.D dissertation are presented in the last chapter.