Theoretical Study On Photocatalytic Water Splitting Based On Layered Materials Such As G-C₃N₄

Global warming causes a serious threat to human survival and development,and governments of all countries are committed to preventing the trend of global warming.To reduce greenhouse gas emissions,the world's major energy-consuming countries all regard the development and utilization of renewable energy as an important energy strategy.Hydrogen energy is generally regarded as the most ideal and most important renewable energy because it does not emit greenhouse gases and is not harmful to the environment.Using solar energy to split water into hydrogen and oxygen is the most environmentally friendly and cheap way to produce hydrogen.The most direct way to convert solar energy into hydrogen energy is to use photocatalysts to split water.For many years,finding a photocatalyst that can achieve an energy conversion efficiency of 10%or higher has been regarded as a"Holy Grail"problem in materials and chemistry science.The difficulty in achieving high energy conversion efficiency is to find a catalyst that can simultaneously meet the requirements of good visible light absorption,complete splitting of water,and good photogenerated charge carrier separation.This dissertation mainly explores the performance improvement and internal mechanism of Zr S₂,g-C₃N₄,BCN,and other two-dimensional semiconductor materials in photocatalytic water splitting.Including van der Waals(vd W)heterostructures to improve the photogenerated carrier separation,transition dipole moment(TDM)analysis to analyze and improve visible light absorption and strain to control the size and position of the bandgap.The main contents are as follows:Chapter 1:First,the basic principle of photocatalytic water splitting on semiconductor materials is briefly introduced.From a theoretical point of view,the factors affecting the performance of the material for photocatalysis of water to hydrogen are elaborated.Then a brief introduction to several types of two-dimensional materials related to this paper,including graphene,porous graphene,transition metal disulfide,graphitic carbon nitride,hexagonal boron nitride,and hexagonal boron carbon nitride.Finally,several special properties related to the photocatalytic performance of the layered two-dimensional vd W heterostructures are introduced in detail.Chapter 2:A brief introduction of the theoretical basis and calculation methods used in this dissertation.The first-principles density functional theory method(DFT)is mainly used to study the ground state physical properties of materials.The calculation of TDM and the theory of point group are mainly used to study the mechanism of light absorption properties of materials.Based on first principles,non-adiabatic molecular dynamics(NAMD)combined with time-dependent density functional,fewest-switches surface hopping and classical path approximation methods are mainly used to study the dynamic process of photogenerated charge carriers after light absorption.Finally,several software packages used in the calculation of this dissertation are introduced.Chapter 3:Four kinds of vd W heterostructures are designed based on two-dimensional single-layer Zr S₂,and their structures,electronic properties,and optical properties are theoretically studied.These four vd W heterostructures all exhibit excellent visible light absorption capabilities.By forming a vd W heterostructure with type II energy band arrangement,charge redistribution,and built-in electric field appear between these two-dimensional material layers,and good charge separation is achieved.Furthermore,we adjusted the band edge position of the Zr S₂ heterostructures by applying strain to make it meet the basic requirements of catalytic water splitting.Through theoretical calculations of several key factors,we elaborated on the role and mechanism of vd W heterostructures in improving and regulating the performance of two-dimensional materials for photocatalytic water splitting.Chapter 4:We studied the visible light absorption enhancement and its internal mechanism brought about by the spatial symmetry changes of two-dimensional material.We use TDM analysis and point group irreducible representation theory to explain the internal mechanism of oxygen-modified g-C₃N₄'s visible light absorption enhancement,that is,oxygen modification leads to a change in the symmetry of the material,making the original symmetry forbidden transitions in low-energy range allowed again,which in turn improves the material's visible light absorption.Then,we found templated polyheptazine imide(PHI)similar to g-C₃N₄.From the perspective of spatial symmetry,through oxygen modification,the spatial symmetry of PHI was changed,and the visible light absorption capacity was greatly improved.Finally,we used BCN and PHI to form a vd W heterostructure to improve the photogenerated charge carrier separation performance of the PHI material and analyzed the possibility of BCN/PHI vd W heterostructure to achieve Z-scheme photocatalysis using TDM analysis.Chapter 5:We studied the effect of rotation on the photocatalytic performance of the bilayer g-C₃N₄ system.The two-layer rotated system will produce moirépatterns and interlayer moirépotentials.By theoretically simulating the electronic properties,optical properties,reaction energy barriers,and exciton dynamic properties of two kinds of rotated g-C₃N₄ bilayer structures,we understand the effect of moirépotential on the material and explain the improvement of photo/electrocatalytic performance in a rotated bilayer system in the experiment.The interlayer moirépotential can cause localization of the spatial distribution of the valence band and conduction band orbitals,which further affects the adsorption energy of molecules on the surface of the material,thereby tuning the reaction energy barrier on the material.At the same time,the change in the spatial symmetry of the orbital wave function affects the TDM and improves the visible light absorption capacity.Finally,NAMD simulations show that there is an ultrafast interlayer transfer of electrons in the rotated bilayer g-C₃N₄ system.This ultrafast transfer of electrons between layers will boost charge carrier separation and extend the lifetime of excitons.