Theoretical Study On The Structures,Properties And Stability Of Perovskite Materials And Origin Of Photocatalytic Efficiency Of BiVO₄ Heterostructures

With the revolution of human life and industry,energy crisis and environmental prob-lems become more and more serious.Solar energy,as an ideal clean energy,has attracted considerable attention due to increasingly urgent demand for clean energy.Solar energy is abundant and environmentally friendly.Sunlight can be converted into clean energy that can be used directly or stored,which not only solve the problem of energy crisis but also reduce environmental pollution.So far,except for photo thermal conversion,there are another two ways for the use of solar energy:photoelectric conversion and photocatalytic hydrogen pro-duction.In last decades,great progress has been made in experimental research in these two directions and the power conversion efficiency of solar energy has been greatly improved.For example,the power conversion efficiency(PCE)based on zinc porphyrin organic dye-sensitized solar cells can reach more than 12%;as for perovskite solar cells,its PCE has reached 22.7%.At the same time,a large number of photocatalytic semiconductors with high photocatalytic activities have been designed and synthesized for photocatalytic reac-tions,such as oxides and nitrogen oxides,exhibiting good performance for photocatalytic water splitting.However,there are still many ambiguities to be clarified,especially about the micro-scopic mechanism and structure-property relationships of related materials.For example,although the PCE of perovskite solar cell is pretty high,their stability is poor.Besides,It's also found that perovskite solar cells can degrade more quickly under light conditions with respect to dark conditions.What the nature behind this phenomenon is not clear.Hence,it is essential to explore the stability of perovskite solar cell at atomic level.Moreover,due to the unique properties of two-dimensional perovskite materials,and potential applications on optoelectronic and photodetector,people have performed a lots of studies on their struc-tures,elecronics properties and exciton-binding energy etc.However,the critical variation of structures and properties with the number of layers of perovskites is not clear.Therefore,it is quiet important to systematically study the evolution of two-dimensional perovskite geometries and properties with thickness.What's more,it is also of great significance to explore the effect of heterojunction on photogenic carrier separation and photocatalytic ef-ficiency.Based on this background,we design and construct reasonable theoretical models with appropriate theoretical methods for investigating the stability of perovskite.The vari-ation of the properties of two-dimensional perovskite with the thickness and the intrinsic mechanism that bismuth vanadate heterogeneous junction can improve the photocatalytic efficiency are systematically investigated as well.In this thesis,I present the major works that I have finished during my PhD study.The model construction,computational methods and results are also presented in detailed.The major achievements are listed in the follow.(1)By performing the first-principles theoretical calculations and ab-initio molecular dynamics simulation,we systematically investigated the effects of H20 molecules on sta-bility of perovskite.It is found that PbI2-terminated surface is the most stable while both the MAI-terminated and PbI2-defective surfaces undergo structural reconstruction,leading to the formation of hydrated compounds in a humid environment.The moisture-induced weakening of photoabsorption is closely related to the formation of hydrated species,and the hydrated crystals MAPbI3·H2O and MA4PbI6·2H2O scarcely absorb the visible light.The electronic excitation in the bare and water-absorbed MAPbI3 nanoparticles tends to weaken Pb-I bonds,especially those around water molecules,and the maximal decrease of photoexcitation-induced bond order can reach up to 20%in the excited state in which the water molecules are involved in the electronic excitation,indicating the accelerated decom-position of perovskites in the presence of sunlight and moisture.(2)Both(PEA)2PbI4(N)and Cs2PbI4(N)are direct bandgap semiconductors,their band/optical gaps and exciton-binding energies vary linearly with 1/N at N ?3,and the ef-fective masses slowly vary with N.Compared to the bulk phases,the structures of ultrathin(PEA)2PbI4(N)are more flexible and deformable than Cs2PbI4(N).The giant spin-coupling effect greatly decreases the band gaps of both 2D materials,however,it only induces the spin splitting in the bands of(PEA)2PbI4.(3)For mixed-phase BiVO4,we find that the geometrical reconstruction around the interfacial area plays a significant influence on the electronic properties of the heterojunc-tion and mismatch lattices have little effect on electronic properties and stabilities of whole heterojunctions.For M-H heterojunction,the calculated valence band maximum and con-duction band minimum of z-t BiV04 are 0.29eV and 0.32 eV above those of s-m BiVO4,respectively,indicating that the presence of interface in monoclinic/tetragonal heterophase provides a spatial condition for charge carrier separation and promotes the separation of photoinduced electron-hole pairs.Further calculations reveal that the direction of electron migration across the phase boundary is from z-t to s-m,consistent with the experimental observation.