Prediction Of 2D Semiconductors And The Applications In Catalysis

Since the separation of monolayer graphene in 2004,two-dimensional（2D）materials,such as hexagonal boron nitride（h-BN）,transition metal dichalcogenides（TMDS）,silicon,phosphorene,have gained increasing attention due to their unique and exceptional physical properties.In the preceding decades,these 2D materials have been successfully prepared in the laboratory,and both advantages and limitations of them have been studied intensively.2D materials calculated or prepared in the laboratory have many advantages,but also some disadvantages that are difficult to overcome.Therefore,the electronic properties of 2D materials can be regulated and their defects can be improved through doping,adsorption,defect,heterojunction construction or other operations.At the same time,its practical application value in low-dimensional piezoelectric,ferroelectric,thermoelectric,photoelectric and catalysis and other fields is explored,and its application prospect is expanded.Especially,in catalytic reactions,such as nitrogen reduction reaction（NRR）and CO₂ reduction reactions,the preparation of efficient and stable catalysts is the key to the reduction reaction,and efficient electron transfer is the key factor to regulate the catalytic reduction activity.Ultra-thin 2D nanomaterials also exhibit excellent catalytic activity in many catalytic reactions due to their high specific surface area and exposed surface atoms.In recent years,as the isoelectric counterpart of phosphorene,two-dimensionalⅣ-Ⅵcompound structure,including the Ge S,Ge Se,Sn S and Sn Se,has higher chemical stability and relatively low geometric symmetry,causing the extensive concern of academic materials.The non-hexagonal Haeckelite structure consisting of various polygons,including quadrilateral,pentagon,hexagon,heptagon,octagonal and dodecagon,may also be stable.The unique geometry of the Haeckelite structure may exhibit different electronic properties from the corresponding hexagonal allotrope.In this article,corresponding to 2D hexagonalⅣ-Ⅵcompounds,we explore three previously unknown Haeckelite allotropes of 2D Ge S,and explore their new physical properties.The catalyst with heterogeneous structure interface can provide a large number of reactive sites,which can improve the conductivity of the catalyst and optimize the performance of synthesis and reduction reaction catalyst.Therefore,in this work,the structural characteristics and physicochemical properties of La-doped g-C₃N₄ photocatalyst（named LGCN）were studied by DFT theory calculation,and the effects of La on the structure,morphology and photocatalytic activity of g-C₃N₄ in CO₂ reduction reaction were discussed.In addition,Ni₂P-black phosphorus（Ni₂P-BP）electrocatalyst with rich Ni-P bonds on the interface was synthesized by selectively growing Ni₂P on the edge of BP nanosheets.The catalytic performance of its NRR was investigated.In this work,the research contents of the physical properties and catalytic performance of2D materials include the following three aspects:（1）Based on first-principles density functional theory,this work explores three new allotropes of square/octagonal Haeckelite structures Ge S,namely h₁-、h₂-and h₃-Ge S.All predicted allotropes are wide（2.5～2.7 e V at HSE06 level）,adjustable band gap semiconductors.Phonon spectrum calculation shows that h₁-、h₂-and h₃-Ge S have good dynamic stability.Through calculation of electronic structure and carrier mobility,it is found that h₁-Ge S are quasi-direct bandgap semiconductors,h₂-、h₃-Ge S are indirect bandgap semiconductors,and h₁-和h₂-Ge S have very high electron mobility,as high asμ_x/y=24.94×10³ cm²V^-1s^-1andμ_y=9.75×10³cm²V^-1s^-1,respectively.At the same time,we found that the size of Haeckelite semiconductor band gap can be adjusted by changing the strain and atomic layer number.The strain energy?E is less than or equal to 6 me V/atom for all three allotropes within the compression or tensile strain range of 3%,which indicates that all allotropes have good flexibility and their band gap values show an approximate linear relationship with the reciprocal of atomic layer number N.More interestingly,due to the unique topological configurations,the Haeckelite Ge S allotropes exhibit more exotic electric polarization orders than the ferroelectric hexagonal allotropes.h₁-Ge S displays an unusually noncollinear polarization order with atomic-scale dipole vortices.h₂-and h₃-Ge S allotropes are found exhibiting an antiferroelectric character and can be transformed to the corresponding ferroelectric hexagonal allotropes through the polarization switching process.In addition,the Haeckelite Ge S are found able to form in-plane connections with selected hexagonal allotropes.The extremely low phase boundary energies indicate the possibility for the coexistence of the Haeckelite and the hexagonal phases and the formation of heterostructures in the same Ge S layer.（2）We studied the structural characteristics and physicochemical properties of La-doped g-C₃N₄ photocatalyst（named LGCN）by DFT theoretical calculation,and discussed the effects of La on the structure,morphology and photocatalytic activity of g-C₃N₄.The doped La atoms enter g-C₃N₄ and chemically coordinate with g-C₃N₄ in the form of La-N bonds.The most stable configuration for the La is within a large C-N ring of g-C₃N₄.We designed two adsorption methods of carbon dioxide,in which the adsorption energy E_ad of CO₂ on the surface of LGCN is-0.81 e V and-0.56 e V,respectively,through and independent of La atom.In the case of La doping,the length of La-O bond is about 2.759?.However,the shortest distance from CO₂ to CN-La layer is about 3.223?without La.E_ad in the presence of La is larger and the distance is closer,indicating that CO₂ adsorption prefers LGCN near La atoms.At the same time,the measurement results of bond length in CO₂ show that the elongation of the C-O bond,indicating that the charge redistribution weakens the C-O bond and enhances the CO₂ adsorption activity of La.The additional electron flow to the O atom caused by the La-O bond can effectively change the chemical activity of the adsorbed CO₂ molecule.These results indicate that doped La can provide a good channel for charge transfer between LGCN and CO₂,and promote the transfer and separation of charge generated by light.The change of charge behavior in the transport path is conducive to breaking the C=O bond at the active site of La and generating methyl radicals on Ru,thus generating C2 hydrocarbons.（3）In this work,two kinds of Ni₂P-BP nano-heterojunctions were constructed by selectively growing Ni₂P on the edge of BP nanosheets in the laboratory.It was found that when there is abundant Ni-P bond between Ni₂P and BP,the Ni₂P-BP nano-heterojunctions can be used as an effective non-noble metal photocatalyst for photocatalytic NRR.When the heterojunction Ni₂P-BP was used as the catalyst,Ni₂P and BP were used as the active center and conductive material,respectively.The Ni-P bond between the interface of Ni₂P and BP layers was used as atomic electron transfer channel,which reduced the potential electron transfer barrier at the interface and improved the activity of NH₃ formation.Therefore,the photocatalytic performance of Ni₂P-BP catalyst for NRR was significantly enhanced.Based on the calculated photocatalytic results,it can be concluded that the Ni-P bond in the Ni₂P-BP catalyst plays a key role in improving the photocatalytic performance of NH₃ generation,and the Ni₂P growing at the BP edge passivates the activity of the atom at the P edge,inhibits the degradation of black phosphorus,and significantly improves the stability of the Ni₂P-BP photocatalytic NH₃generation.These advantages make Ni₂P-BP an photocatalyst with high reactivity and durability.