Movement Of Energized Charges Of Functional Composite Materials:A First-principles Study

Materials are greatly important to people's daily life and promote the development of human society.However,the number of available materials in nature is too small to meet the ever-growing demand of people.It is necessary to research and develop new materials with unique features and new function based on the existing materials by various physical or chemical means.The actual experiences of design and synthesis of new functional materials has been proved that the most effective method is to play experiment combined with theory,which can avoid consuming much time,energy and experimental resources.In recent years,thanks to the rapid development of the computer,as well as the electronic structure calculation of density functional theory?DFT?,the first principle calculation based on DFT has become a powerful tool to investigate the nature of materials at atomic level and design new functional materials.Specially,the theoretical research of DFT has huge advantage in terms of screening of suitable materials and mechanism investigation.Catalytic heterogeneous reaction are always complicated.Importantly,we realize that the kinetic of energized electron-hole charges is the root cause of catalytic heterogeneous reaction.The more information of the kinetic of energized electron-hole charges we get,the deeper we understand how catalysts work.The aim of the dissertation is to study the kinetic of energized electron-hole charges by DFT.The kinetic of energized charges mainly include three processes:the migration of energized charges,the energy loss of energized charges?energy transfer?,and the chemical reaction involved energized charges at activity sites on the surface of catalysts.We have a few specific examples to illustrate the kinetic of energized charges in the dissertation.The dissertation includes four chapters.In chapter 1,we briefly introduce the research status of the important chemical reaction of catalysis,which is closely related to the energy issues.The photocatalysts based on semiconductors can carry out the conversion from solar energy into chemical energy in the form of chemical bond.As an example,the productions of photocatalytic water splitting are clear energy,which has been considered as a promising solution to the energy issues.The influence factors of photocatalytic water splitting have been systematically investigated,as well as the hybrid system of photocatalysts.Specially,the photocatalytic hyrid systems consisting of metal and semiconductor have attracted wide interest of researchers,because the Schottky junction at the interface of metal and semiconductor in these systems can drive the migration of energized charges,resulting in effective charge separation and improved photocatalytic performance.The plasmonic effect of metal can be used to enhance the light absorption and improve photocatalytic performance of metal-semiconductor systems.Electrocatalysts based on metal materials or non-metallic materials can catalyze the electrical water splitting or oxygen reduction reaction in fuel cell,which are also promising solutions to energy issues.Overall,tremendous efforts have been made to develop catalysts with low cast,high stability and excellent catalytic activity.In chapter 2,we briefly introduce the DFT and transition state search methods.The theoretical framework,application and development of DFT are included.DFT is based on quantum mechanics,and particle density is the basic variable instead of the wave function.As a result,all properties of the system can be included in the unique functionals of the ground state density.Practically,the implementation of DFT is through solving Kohn-Sham equation,which convert the problem of interacting multiparticle into non-interacting multiparticle problem.The Nudged Elastic Band Method?NEB?method of transition state search,based on the known reactant and product,is used in the dissertation.In chapter 3,there are two works,which are about the magriation of energized charges?charge separation?.?1?The central idea of the first work is the reasonable synergism of two Schottky junctions for the realization of effective energized charge separation.In this work,we analyze and contrast the photocatalytic mechanism of the hybrid systems of Z-scheme and p-n junction,and we theoretically design the pmn hybrid system,which combines the advantages of the hybrid systems of Z-scheme and p-n junction.The pmn hybrid system can absorb visible light and the synergism of two formed Schottky junctions at the metal-semiconductor interface derive the migration of energized charges and guarantee the effective charge separation.Moreover,the energized charges of pmn system can maintain the strong redox ability.These features mentioned above indict that pmn system has improved photocatalytic performance.The prerequisite of the pmn construction is that the work function?W?of component should be in the order of Wp<Wm<Wn.Based on this prerequisite,we have got two pmn systems of Cu₂O-Pd-CuO and CoO-Pd-WO₃.?2?The central idea of the second work is the synergism of plamonic effect and Schottky junctions for the energized hole extraction.As a proof-of-concept demonstration,we have achieved broad-spectrum photocatalysis using BiOCl?a p-type semiconductor harvesting UV light?,Ag?a metal with visible plasmonic band?and Pd?a metal for the Schottky junction?as model materials.This system is designed by leveraging the charge spatial separation in BiOCl where photogenerated holes and electrons preferentially migrate to?110?and?001?facets.To achieve hole extraction,Pd nanocrystals are assembled on the BiOCl?110?facets to establish the Schottky barrier.Meanwhile,Ag nanocrystals are placed on the BiOCl?001?facets for one-way injection of plasmonic hot holes,as the Schottky barrier cannot be formed between BiOCl?001?and Ag as revealed by our data.As demonstrated by photocurrent,photoluminescence and photocatalysis measurements,the plasmonic effect thus can deliberately synergize with the Schottky junction in our Ag-BiOCl-Pd hybrid structure,well harnessing the charge flow.Chapter 4 is mainly about the energy loss of energized charges?energy transfer?,of which the central idea of the this work is the decrease of non-harmonic phonon modes?the detrimental energy depletion channels?for effective energy transfer.As upconversion materials,hexagonal NaYF₄ nanocrystals typically exhibit significantly higher upconversion efficiency than their cubic counterparts.Based on the experimental result that a cubic phase can offer high upconversion luminescence efficiency.We proposed a "smart" mechanism of phase transition:During the phase transition,non-harmonic phonon coupling within the lanthanide-doped hexagonal lattice converts intense NIR excitation energy into lattice heat which drives lattice atoms to relocate towards a new stable cubic structure.Compared with traditional cubic NaYF₄,this new cubic phase has Na+ and Y3+ ions well-ordered distributed in the lattice.As a result,the newly formed lattice contains minimal non-harmonic phonon modes-the detrimental energy depletion channels,limiting the upconversion efficiency.In chapter 5,there are three works:the first work is about the OER catalytic activity of TM@CN system,the second work is about the O₂ catalytic activity of TM@C₂N and TM₂@C₂N system,and the third work is about the oxide defect engineering for coupling solar energy into oxygen activation.?1?The central idea of the first work is the polarization charge?local chemical coordination?of activity site of single-atom catalysts for OER catalytic activity.Using density functional theory calculations,we propose TM@CN hybrid structure,in which single-atom transition metal?TM=Pt,Pd,Co,Ni,Cu?is supported by graphitic carbon nitride?g-CN?,as promising high-performance OER catalyst.The TM singe-atom forms strong covalent bond with Nitrogen on g-CN,leading to two advantages of prohibiting metal atom aggregation and inducing effective polarized charges.The former bestows TM@CN high durability in comparison to other few-atom based catalysts.The latter creates active electrocatalytic sites with exceptionally low OER overpotential?0.16 V and 0.49 V for non-noble Co@CN and Ni@CN catalysts,respectively?.Moreover,the under-coordination feature of TM@CN offers the opportunity to form other useful metal coordination complexes such as TM-Ox@CN or TM-?OH?x@CN.Co-O@CN exhibits excellent OER activity and low overpotential of 0.41 V,and Pd-?OH?₂@CN can achieve efficient solar-driven OER as it produces well-separated energized electron-hole charges after absorbing visible light.?2?The central idea of the second work is the polarization charge?local chemical coordination?of activity site of single-atom and double-atom catalysts for 02 activity.Here we use first-principles simulations to design oxygen reduction reaction?ORR?catalyst based on double transition metal?TM?atoms stably supported by two-dimensional crystal C₂N.It not only holds characters of low cost and high durability,but also effectively accumulates surface polarization charges on TMs and later deliveries to adsorbed O₂ molecule.The Co-Co,Ni-Ni,and Cu-Cu catalysts,exhibit high adsorption energies and extremely low dissociation barriers for 02,as compared with their single-atom counterparts.Co-Co on C₂N presents less than the half value of the reaction barrier of bulk Pt catalysts in the ORR rate-determining steps.These catalytic improvements are well explained by the dependences of charge polarization on various systems.?3?The central idea of the second work is the O defect on tungsten oxide for O₂ activity where the energized electron triggers organic oxidation reaction.Cooperated with experimental group,we found that defect engineering on oxide catalyst can serve as a versatile approach to bridge light harvesting with surface reactions by ensuring species chemisorption.The chemisorption not only spatially enables the transfer of photoexcited electrons to reaction species,but also alters the form of active species to lower the photon energy requirement for reactions.In a proof of concept,oxygen molecules are activated into superoxide radicals on defect-rich tungsten oxide through visible-near-infrared illumination to trigger organic aerobic couplings of amines to corresponding imines.