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Study Of Electronic Structures And Photo(electro)chemical Properties For Novel Two-dimensional Materials

Posted on:2021-01-06Degree:DoctorType:Dissertation
Country:ChinaCandidate:X S LvFull Text:PDF
GTID:1361330602983328Subject:Atomic and molecular physics
Abstract/Summary:PDF Full Text Request
Two-dimensional(2D)nanomaterials are a class of emerging nanomaterials and the focus of current researches.Because electrons are restricted in 2D environment,2D nanomaterials exhibit particular electronic,physical and chemical properties.The successful fabrication of graphene in 2004 marks the birth of 2D materials,motivating increasing interests of novel 2D materials.In recent years,the theoretical and experimental researches about 2D materials have made rapid progress,and various novel 2D materials have been reported and synthesized,including transition metal dichalcogenides(TMDs),MXenes,g-CxNy and so on.They cover almost the whole range of materials properties from insulators,semiconductors,metals and superconductors,and exhibit versatile performance,playing a vital role in the fields of energy storage and catalysis.For example,in the lithium ion batteries(LIBs)application,many 2D materials have been considered as ideal electrode materials due to the superior electrochemical properties.Some research findings have achieved industrialized applications.In addition,electrocatalysis is the heart of clean energy,and 2D materials are excellent candidates of traditional noble metal catalysts because the large specific surface area and outstanding electric conductivity of 2D materials can provide better catalytic stability and activity.In the energy conversion application of semiconductors,the strengths of 2D materials,including diversity and controllability of electronic properties,high carrier mobility and anisotropy can provide new opportunities for promoting the practical application of photocatalysis and enhancing the performance of thin-film solar cells.In this dissertation,we systematically investigated the electronic structures and photo(electro)chemical properties of 2D nanomaterials as well as their applications in LIBs,electrocatalysis,photocatalysis and solar cells,underlying physical mechanisms and providing theoretical guidance for the applications of 2D materials in energy storage and conversion.This dissertation is divided into six chapters.In the first chapter,we briefly introduce the research status and applications of 2D nanomaterials;In the second chapter,the theoretical basis and software package of first-principles calculations are introduced;In the third chapter,we identify the performance of novel 2D materials as electrode material for metal-ion batteries;In the fourth chapter,the design and study of novel electrocatalysts are introduced in detail;In the fifth chapter,we investigate the regulation of electronic properties of 2D materials and their promising applications in photocatalytic water splitting,photocatalytic nitrogen fixation and thin-film solar cells;In the sixth chapter,we summarized the main research contents and innovations of this dissertation,and provided outlooks for the future of LIBs and photocatalysis.The thesis's main research contents and conclusions are as follows:(1)We investigate the feasibility of SC2C and SiC compounds as anode material for metal ion batteries.Taking the stability,adsorption properties,capacity,charge-discharge rate,average electrode potential of electrode materials as criteria,we systematically evaluated the electrochemical proeprties of Sc2C and SiC compounds.Before and after the metal ion adsorption,the metallic characteristic of the host materials is maintained,guaranteeing the excellent electric conductivity of electrode materials.Our results show that SC2C and SiC compounds all exhibit superior electrochemical performance,including high capacity and low diffusion barrier.Therefore,Sc2C and SiC compounds can serve as excellent candidates for anodes.(2)We study the controllability of oxygen functionalization on electronic properties and HER activity of novel BiXenes.Results demonstrate that both pristine and oxygen functionalized BiXenes exhibit metallic properties,indicative of the excellent electric conductivity of electrocatalysts.The electrocatalytic HER performance of pristine BiXenes is poor,while oxygen functionalization can greatly boost the HER activity,including much lower overpotential and higher exchange current density.In addition,depositing single transition metals(TM)on the surface of oxygen-terminated BiXenes and strain engineering,both of which are promising routes to further enhance the HER activity of oxygen-terminated BiXenes.Finally,we also unravel the underlying mechanism of the regulations.(3)We design a novel buckled Pt monolayer catalyst(b-PtM),which efficiently reduces Pt-loading and production cost.Meanwhile,b-PtM is highly active towards HER,OER and oxygen reduction reaction(ORR),holding great promise in electrocatalytic water splitting and fuel cells.On the basis of energy consumption and kinetics,we investigate the electrocatalytic activity of b-PtM toward the above reactions.Results show that b-PtM present good catalytic activity,including low overpotentials and fast reaction rate,which are superior to that of the widely studied Pt(111).(4)We explore the promising applications of g-CN supported bifunctional single atom catalyst(Ti,V,Cr,Mn,Fe,Co and Ni)toward overall water splitting.Among 14 candidates,Coi/g-CN and Nii/g-CN are identified as the most efficient bifunctional single atom catalysts for overall water splitting,capable of driving HER and oxygen evolution reaction(OER)simultaneously with overpotentials being as low as 0.15/0.61 V and 0.12/0.40 V,outperforming the commercial Pt and IrO2.Remarkably,the d band centers of TM atoms can act as an efficient descriptor for evaluating the OER activity,that is,adjusting the d band center to an optimal value(-2.76 and-4.04 eV for TM1/g-CN and TM2/g-CN),which provides theoretical guidance for achieving the best catalytic activity and catalyst design.(5)We explore a new family of phosphorene-like materials,GeS and GeSe.We observe that GeS exhibits an indirect band gap of 2.29 eV,while GeSe reveals a direct band gap of 1.59 eV by HSE hybrid functional,which correspond to the visible region of solar spectrum.GeS and GeSe also possess high carrier mobility and efficient carrier separation.Upon the application of strain and different pH value,the band structure can be modulated from semicon-ductor to metal and a direct-indirect bandgap transition is observed.Most intriguingly,the band gapsand band edge alignments can be effectively tuned to meet the requirement of the redox potential in water splitting.Finally,the adsorption and decomposition of water molecules on GeS and the subsequent formation of hydrogen were explored,which unravels the mechanism of photocatalytic hydrogen production on 2D GeS.Our results are helpful for the rational design of novel photocatalysts.(6)We explore the electronic structure and photocatalytic nitrogen fixation properties of metal-free g-CN supported single B atom catalyst(B@g-CN).The sp2-bonded N atoms at the vacancy hole edge of g-CN can provide coordination sites for B atoms,leading to strong covalent B-N bonds as far as possible to prohibit B aggregation.The as-designed B@g-CN can function as transition atom(TM)atoms to bind with and activate N2 molecule,that is,on the basis of the "electron donation and acception" concept,one half occupied sp2 orbital can donate electrons into the antibonding orbitals of N2 to weaken the N?N bond,while the empty sp2 orbital of B atom can accept the lone-pair electrons of N2 to strengthen the N2 adsorption.N2 can be activated and reduced to NH3 with extremely low overpotential(0.15 V)and activation barrier(0.61 eV).The external potential provided by photogenerated electrons for NRR/HER endowing B@g-CN spontaneous NRR.Meanwhile,as the main competing reaction,HER is efficiently suppressed,achieving a high Faradaic efficiency.(7)We systematically explore for the first time the contact properties of bilayer GeSe with commonly used back electrode metals in detail,such as the geometric features,electronic properties,Schottky barrier,tunneling barrier,and band alignment.Our results unravel that Au,Pt,and Ni show great potential in forming favorable contacts with GeSe due to a low Schottky barrier and tunneling barrier.More importantly,we find that when a SnS monolayer is superimposed with GeSe layers,the combined system can be used as an effective solar cell material with type-?heterostructure alignment as the power conversion efficiency(PCE)is predicted to be as high as?18%.Our results not only provide microscopic insights into the characteristics between layered GeSe and metals,but also pave the way for further experimental improvements of GeSe thin-film solar cells.
Keywords/Search Tags:first-principles calculations, two-dimensional materials, electronic structures, photo(electro)catalysis, metal ion batteries, solar cells
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