Strain/Doping Tuning Optoelectronic And Catalytic Properties Of Semiconductor Heterojunctions

With the continuous development of society and industry,energy shortages and environmental pollution have become the main problems restricting economic development.As a kind of green energy,hydrogen can be produced by photocatalytic decomposition of water under sunlight.This can not only recycle energy,but also avoid pollution caused by fossil fuel combustion.However,the high recombination rate of photo-generated carriers in the photocatalyst greatly limits its photocatalytic performance.The study found that the strong built-in electric field at the interface of the heterostructure formed by the "face-to-face" stacking of different semiconductors can accelerate the effective separation of carriers.Recently,two-dimensional materials have become ideal candidates for constructing heterostructures because of their large specific surface area,easy-to-control electronic states,and weak van der Waals forces between layers.However,the photocatalytic activity of the heterostructure is still low,and its interlayer charge transfer and redox capabilities need to be further improved.Therefore,this paper chooses bismuth oxyhalide heterostructure,binary and ternary heterostructure based on SnS2,g-C3N/MX2(M=Mo and W,X=S and Se)heterostructure,g-C6N6/Gr heterostructure and WS2/BlueP heterostructure are typical examples.Based on the density functional theory,the influence of the interface on the photoelectric and photocatalytic properties of the heterostructure is discussed in depth.For g-C6N6/Gr and WS2/BlueP heterostructures,the regulation of in-plane uniaxial and biaxial strain on their electronic state,interlayer charge transfer and photocatalytic performance was studied.In addition,the(IV,V,VI)group atoms were respectively doped in the WS2/BlueP heterostructure to study the influence of interface defects on electron transport and photocatalytic performance.The main results were achieved as follows:(1)The band of bismuth oxyhalide heterostructure BiOI/BiOX(X=F,Cl,Br)introduces I-5p orbitals into VB,which leads to the reduction of the band gap and produces more photo generated carriers,which also corresponds to the red shift of the absorption edge.The built-in electric field is generated between the layers,which can realize the effective separation of photogenerated carriers,which is beneficial to the photocatalytic performance under visible light.The band gaps of SnS2-based binary and ternary vdW heterostructures are in the range of 1.44 eV?1.91 eV and suitable to water decompostion.The heterostructures are sorted into Z-scheme,Type-?,Type-?/Z-scheme,Z-scheme/Type-I and Z-scheme/Z-scheme taking into account the synergistic effects of band edges and built-in electric field.Except for ZrS2/SnS2,redox centers of all the other heterostructures are located in different layers,and their can be used to conduct spontaneous full water decom-position.Moreover,the ternary heterostructures have stronger absorption intensities with red shift absorption edges compared with the isolated nanosheets and binary heterostructures,which could be a better choice for enhanced photocatalytic performances under visible light.The directional movement of charge of g-C3N/MX2(M=Mo and W,X=S and Se)will form a polarization field directed by g-C3N to MX2.The charge transfer of g-C3N/MoSe2,g-C3N/WS2 and g-C3N/WSe2 follows the direct Z scheme,and the CB of g-C3N becomes the reduction center,and the VB of MX2 becomes the oxidation center.The g-C3N/MX2 has the same order of absorption strength as the isolated nanosheet,and the absorption edge is red-shifted.Therefore,the hetero structure can effectively improve the photoelectric performance of the material and improve photocatalytic activity.(2)The in-plane-8%?8%uniaxial and biaxial strain was introduced to control the electronic state of g-C6N6/Gr.The uniaxial strain will significantly affect the band gap,while the biaxial strain will affect the interface distance,strain energy and work function.The-4%strain make VBM and CBM of g-C6N6/Gr locate in the Gr and g-C6N6 layer,respectively,achieving effective separation of carriers.The Gibbs free energy of conducting hydrogen evolution reaction(HER)is 0.522 eV,1.185 eV,1.671 eV and 1.845 eV for-6%,-4%,-2%biaxial strained and ideal g-C6N6/Gr,respectively.For the tungsten disulfide/blue phosphene(WS2/BlueP),strain can affect the band arrangement from Type ? to Type ?(Z-scheme).At PH=0 and PH=7,-2%uniaxial and biaxial strain WS2/BlueP is complete water splitting.Therefore,the in-plane uniaxial and biaxial strains can effectively adjust the electronic and photocatalytic performances of the heterostructures.(3)The inherent type-I band arrangement of WS2/BlueP can also be adjusted by the interfacial defects.The band alignment of M-WS2/BlueP(M=C,Si,Ge,Sn,Sb,S,Bi)and M-WS2/BlueP(M=N,P)are changed from type ? to type?.WS2/C-B1ueP,WS2/S-BlueP,N-WS2/BlueP and P-WS2/BlueP can conduct the full water decomposition.More charge transfer occurs in WS2/S-BlueP,N-WS2/BlueP and P-WS2/BlueP,which will produce a greater interface potential difference resulting in the existence of a greater built-in field,and realizing the effective separation of photogenerated carriers and enhancing the photocatalytic activity.Therefore,the electronic and photocatalytic properties of the heterostructures can be effectively modulated by interfacial defects.In summary,type ? and direct Z scheme heterostructures charge transport paths can effectively separate photogenerated carriers,and redox centers are located in different layers.This type of heterostructure is an ideal candidate for high-efficiency photocatalysts.The redox centers of type ? heterostructures are located in the same layer,and the interatomic interactions can be controlled by in-plane uniaxial and biaxial strains to improve the electronic state occupancy and transform them into type ? and direct Z scheme heterostructures.The introduction of interface defects in the heterostructures,the electronic state and the charge transfer between layers are regulated by the defect energy level,and it can also cause the band arrangement of the heterostructure to change from type ? to type ? or direct Z scheme.The transition of the band arrangement changes the transmission path of the photo-generated carriers,thereby realizing the effective separation of the photo-generated carriers.Therefore,type ? heterostructures and the direct Z scheme heterostructure of BiOI/BiOX(X=Cl,Br),SnS2 based binary and ternary heterostructures,g-C3N/MX2(M=Mo and W,X=S and Se)can improve the photocatalytic performance of the corresponding nanosheets,and type ? energy band arrangement of WS2/BlueP heterostructure can be transformed into type ? and direct Z scheme through strain and interface defects,regulating its charge transport path and improving photocatalytic activity.