First-principles Studies On The Electronic Structures And Photocatalytic Properties Of Direct Z-scheme GeC/BSe And GeC/Ga₂SO Van Der Waals Heterostructures

With the shortage of fossil fuels and the increasingly serious environmental problems,the development of green and sustainable energy has become the focus of current energy research.Among them,hydrogen energy has attracted extensive attention because of its renewable,high calorific value and green and clean characteristics.Among the existing hydrogen production methods,the technology of semiconductor photocatalytic water splitting based on solar energy is expected to become the most effective way to solve the current energy crisis and environmental problems,and has a far-reaching application prospect.In recent decades,researchers have made great efforts to find and manufacture advanced and efficient semiconductor photocatalysts.Most of the early studies focused on the traditional bulk photocatalyst,most of the bulk semiconductor due to the shortcomings of high photocarrier recombination rate and low visible light utilization rate,their hydrogen production rate is not high.In recent years,the rapid development of two-dimensional materials has prompted researchers to turn their attention to two-dimensional materials.Two-dimensional materials have the unique advantages of large specific surface area,high carrier mobility and easy electronic structure regulation,which provides a new idea for the design of novel efficient photocatalysts.The recent experimental and theoretical approach of"constructing a direct Z-scheme two-dimensional van der Waals heterojunction"is a novel modification strategy.The results show that the Z-sheme van der Waals heterojunction can not only effectively inhibit the electron hole pair recombination,but also ensure the strong redox ability,so as to improve the photocatalytic performance of the materials.Therefore,the construction of two-dimensional van der Waals heterojunction has become a hot research topic in the field of photocatalytic water splitting.In this paper,based on first-principles calculations,the electronic structures and photocatalytic properties of Ge C/BSe and Ge C/Ga₂SO heterostructure are studied in detail.The main research contents are summarized as follows:In the first chapter,the research background is introduced.Firstly,basic principle and the research status of photocatalytic water splitting were introduced,and then the research status and modification of two-dimensional photocatalytic materials were introduced.Meanwhile,research progress of Z-scheme photocatalyst are summarized emphatically.Finally,the research motivation of this paper is proposed.In the second chapter,the theoretical basic knowledges involved in this thesis are introduced.The first principle calculation method based on density functional theory is introduced.It includes the development of density functional theory,exchange correlation functional and the theoretical basis of van der Waals correction.In the third chapter,the electronic structures,optical properties and photocatalytic mechanism of the two-dimensional Ge C/BSe van der Waals heterostructure are investigated by first-principles calculations.It is demonstrated that the Ge C/BSe heterostructure forms Z-type charge transfer mode under the action of the built-in electric field and its band edges exactly straddle the water redox potentials,meeting the basic requirements of the photocatalytic water splitting.Ge C/BSe heterostructures exhibit good dynamical-thermodynamic stability and light absorption capacity,and their light absorption intensity in the visible light range can be further improved by using tensile strain.Finally,the excellent abilities of H₂ desorption are shown in the Ge C/BSe heterostructure.These features make the Ge C/BSe heterostructure promising for application in photocatalysis for water splitting.In the fourth chapter,the thermal stability,electronic structure,optical properties and photocatalytic properties of van der Waals Ge C/Ga₂SO heterostructure（Model-I and Model-II）were investigated based on first-principle calculations.Ab initio molecular dynamic simulations results showed that model-I and model-II could stably exist at room temperature（300K）,and further analysis of electronic structure and charge density showed that model-II was a Z-scheme heterostructure.Meanwhile,the band position of Model-II can still cross the standard redox level to carry out spontaneous redox reaction.The light absorption property and band edge position of Ge C/Ga₂SO heterostructure are well regulated and improved by biaxial strain.Our results indicate that Ge C/Ga₂SO heterostructure has an important application prospect in the photocatalytic water splitting.The fifth chapter is devoted to the conclusion and perspective.