Theoretical And Experimental Study On Photoelectric Properties And Their Controls Of Two-Dimensional Composite Materials

Due to the unique physical properties of charge and heat transport in the plane,the photoelectric properties of two-dimensional materials and their regulation have become one of the hottest topics in the academic research since 2004.How to construct two dimensional composites which have outstanding photoelectrical properities has always been a big challenge.Because of their merits of direct band gap,bandwidth corresponding to visible absorption and having no dangling bands,two dimensional transition metal chalcogenides have been one of the preferred materials to build two dimensional heterostructure with another two dimensional material to improve the properites of photoelectrical properities.But this kind of two dimensional heterostructure is usually formed from two two dimensiona materials and we cannot differentiate what is primary from what is secondary.So it is needed to build sandwich heterostructures with two dimensional transition metal chalcogenides as the the mid layer and another two dimensional as the sealing coats in order to do deeper researches on regulation rules of photoelectrical properities.On the other hand,two dimensional covalent organic frameworks have good photoelectrical properities because of their high surface area,fixed active centers and so on.For example,electro-catalyzed carbon dioxide reduction by COF-366-Co has been reported.But their poor electrical conductivity hinders their further applications to electrocatalysis.However two-dimensional materials was hybridized with one dimensional carbon nano tubes to improve two-dimensional materials’ conduction of electricity and electric catalytic properties according to some reports.Therefore,that two dimensional covalent organic frameworks hybridized with one dimensional carbon nano tubes could acquire good properity of electro-catalyzed nitrogen reduction is worth hoping for.In this paper,two different series of two dimensional compound systems based on two dimensional transition metal chalcogenides（TMDCs,including MoS2,MoSe2,MoTe2,WS2,WSe2,WTe2 and so on）and two dimensional covalent organic frameworks（COFs）respectively have been constructed.The photoelectrical properities of sandwich heterostructure models（TMDCs as interlaerys and graphene,hexagonal BN（h-BN）,graphite phase C3N4（g-C3N4）as sealing coats）and hybrid system（COF-366-Ni and carbon nano tubes（CNTs））have been simulated within the density functional theory.The regulatory mechanisms have been explored in the paper.COF-366-Ni/CNTs has been pretared and applied to electrocatalytic reduction of N2.The outstanding catalyzing effectiveness is a proof of the precision of the simulations and validity of the regulatory mechanisms.The doctoral thesis consists of six chapters below.Chapter 1:The content of the chapter is a summary of two dimension materials including TMDCs and COFs.From the summary,we know that two dimension TMDCs materials have the nature of suitable direct bandgap for adorption of visible light and none of dangling bonds.So they become the preferred option for raw materials of constructing heterostructures.Two dimension COFs possess the advantage of large surface areas,regular porous channels,fixed catalytic active center and so on.But they own poor conductive ability.The poor conductive ability hinders their applications in electrocatalysis.So it is very important to construct their two dimensional compound systems,to simulate their two dimensional compound materials’ photoelectrical properities and to ivestigate their compound materials’photoelectrical regulatory mechanism.Chapter 2:In this chapter,some basic conceptions and computing methods（Born-Oppenheimer approximation,Hartree-Fock approximation,Thomas-Fermi mode,Hohenberg-Kohn theorem,Kohn-Sham equations,exchange-correlation functional,plane waves unfold,energy cut-off and pseudopotential）of first principle theory for reducing workload of solving the Schrodinger equation and ensuring the accuracy have been introduced.Vienna Ab-initio Simulation Package（VASP）as calculation software in the doctoral thesis has also been introduced.Chapter 3:In this chapter,GP（h-BN）/TMDCs/GP（h-BN）SHS modes have been constructed.The photoelectrical properities of the SHS have been simulated by VASP.Calculations show:Zero band gaps of GP have been opened by constructing Gr/（Mo,W）（Se,Te）2/GP SHS heterostructures.And their Dirac cones have not been broken.So,Gr/（Mo,W）（Se,Te2）/GP SHS hetero structures can be applied to Electronic devices such as field effect transistors.In h-BN/TMDCs/h-BN SHS syetem,band-gap widths of h-BN have been reduced to about 1.0eV and are suitable for visible spectral response after being regulated by TMDCs.Chapter 4:In this chapter,g-C3N4/TMDCs/g-C3N4 SHS modes have been constructed.The photoelectrical properities of the SHS have been simulated by VASP.In comparison with the previous two SHS in Chapter 3,the fluctuation of the g-C3N4 cover can further improve the structural stability of the two dimensional g-C3N4 composites.The band gaps of g-C3N4/TMDCs/g-C3N4 SHS（0.94～1.57 eV）are very suitable for visible spectral response.Valence electrons are contributed by N₂p orbital of g-C3N4,while conduction band are contributed by S₂p,Mo₄d or W₅d of（Mo,W）S2.The electrons excited by visible light can be separated from the holes drastically.The g-C3N4/（Mo,W）S2/g-C3N4 SHS materials could be applied in Solar cells and photocatalysis and other fields.Chapter 5:In this chapter,COF-366-Ni/CNTs compound modes have been constructed.Simulation results show that π-π stacking between COF-366-Ni and CNTs is favourable for rapid transfer of interfacial charges.The experimental results show that COF-366-Ni/CNTs has achieve good results.At 0.0 V vs RHE,12.7%of Faraday efficiency was got.And at-0.2 V（RHE）,the NH3 preparation of rate reached 8.56 μg h-1 mg-1cat.Chapter 6:In this chapter,comprehensive summary of the doctoral thesis has been made.And my outlook of scientific research has been prospected.The main results are as follows:1.In GP/TMDCs/GP SHS system,different chalcogens in TMDCs decide different electronic band structure.And different transition elements dicide different band-gap widths.2.The band gaps of h-BN/TMDCs/h-BN SHS（0.62～1.44 eV）are more suitable for visible spectral response,comparing to the ones of Gr/TMDCs/GP.3.Due to their structural stability,good thee-hole separations,and suitable band gap for visible light response,g-C3N4-SHS with MoS2 and WS2 interlayers have strong potential in applications such as photocatalysts and solar cells.4.When the two series of 2D layers constituting SHS own close band gaps and locations of VBM and CBM,CB and VB of the SHS would be contributed by the two 2D layers,respectively（for instance the g-C3N4-SHS）.Instead,when band gaps of the two series of 2D layers vary considerably from each other,then CB and VB of the SHS would be both contributed by the 2D monolayer that owning the smaller band gap（for instance the Gr-and the h-BNSHS）.5.Ni atom of COF-366-Ni is the catalyst active center in the synthesis of ammonia,π-π stacking between COF-366-Ni and CNTs is favourable for rapid transfer of interfacial charges.6.In the synthesis of ammonia experiment,COF-366-Ni/CNTs has achieved very good results.And the results of the experiment are a further proof of the validity of the computation.Theoretical and experimental study on photoelectric properties and their controls of two-dimensional composite materials has been made in the doctoral thesis.Regulatory mechanism of photoelectric properties in sandwich heterostructure has been revealed.And the regulatory mechanism can provide references in sandwich heterostructure materials’ application to Optoelectronics.Properities and mechanism of COF-366-Ni/CNTs in electrocatalytic reduction of nitrogen have been investigated.And theoretical and experimental bases of COFs composite materials’ industrial application to preparation of ammonia in the future have been provided.