Design And Optical Characteristic Research Of Two-dimensional Polar Heterojunction Photocatalytic Materials

Photocatalytic technology for hydrogen production from water molecule and CO₂reduction（CO₂RR）is of great significance to alleviate energy crisis and solve environmental problems.The low efficiency of carrier separation has become a bottleneck in the design and development of high-efficiency photocatalytic materials,especially in the two-dimensional materials with thin thickness and strong exciton effect.Normally,semiconductors must have a band gap of at least 1.23 e V for their potential application as photocatalytic materials for overall water splitting.The wide band gap limits the absorption of solar spectrum to ultraviolet light region,and the narrow band gap results in the insufficient driving forces for redox reactions of photocatalytic systems,which is contradictory to the high utilization of solar spectrum.Based on this,this thesis starts with the research on two-dimensional single-layer polar semiconductors,focuses on the optimization of interfacial charge transfer mechanism by constructing two-dimensional polar heterojunctions,and achieves the modulation of band gap and optical absorption characteristic of single-layer polar materials,which provides the scheme of improving photocatalytic efficiency of semiconductors from the perspectives of electronic band structure and optical property.The intrinsic internal electric field of polar materials is conducive to the spatial separation of photogenerated charges.Two-dimensional single-layer polar W,Pt and Zr-based dichalcogenides with mirror asymmetric structures are first constructed in this thesis.The relatively large difference in the charge amount（electronegativity）between the upper and lower surfaces induces the formation of the relatively strong intrinsic internal electric field,which is favorable for the spatial separation of charges.The aforementioned materials exhibit semiconducting characteristics with indirect bandgaps,and have the potential to be used for photocatalysis.By further analyzing the band alignment,carrier mobility,optical absorption spectrum and reaction free energy,the potential application for photocatalytic hydrogen evolution reaction,oxygen evolution reaction and CO₂RR is researched,where the impact of intrinsic internal electric field,lattice self-strain,surface atomic radius and crystal structure on the reaction Gibbs free energy is also explored.The appropriate heterojunctions can push different photogenerated charges located in different components,which is helpful to realize the efficient spatial separation.In this thesis,the two-dimensional polar heterojunctions are constructed based on two-dimensional single-layer polar materials,namely Dipole-scheme（D-scheme）heterojunctions,which are compared with other types of heterojunctions.In D-scheme heterojunctions,the strong spontaneous polarization can deflect the redox potentials of the material bearing reduction reaction,realizing the effective enhancement of driving forces for reduction reaction of the photocatalytic systems.The synergy between the interfacial depolarization field and the polar materials’polarization field in D-scheme heterojunctions is explored,as well as their contributions to the total internal electric field.A method to make a quantitative analysis about the relationship of internal electric field at the interfaces between D-scheme and traditional type-II heterojunctions is developed,which can be utilized to guide how to select materials to obtain the stronger internal electric field in the opposite direction as compared to type-II heterojunctions.Additionally,in D-scheme heterojunctions,it is of great probabilities to avoid the problem of small number of carriers that exists in the Z-scheme heterojunctions.In addition to the spatial separation of photogenerated charges and driving forces for redox reactions,the band gap and optical absorption characteristic have a great impact on the photocatalytic application of heterojunctions.In this thesis,utilizing the two-dimensional single-layer polar MX（M=Ge,Sn,X=S,Se）materials,the two-dimensional polar heterojunctions Ge Se/Ge S（Sn Se/Sn S）with wide band gaps（close to2.0 e V）are constructed,where the band gap,optical absorption spectrum range and exciton effect of the composite systems are effectively modulated through the interfacial coupling.The potential application for visible-light-driven photocatalytic water splitting of the D-scheme heterojunctions Ge Se/Ge S（Sn Se/Sn S）with MX stacked in the same polarization direction is also explored.The proportion of more than 40%in the solar spectrum means that the absorption of infrared light is favorable to improving the solar utilization efficiency of semiconductors.In this thesis,using the two-dimensional single-layer polar transition metal dichalcogenides,two D-scheme heterojunctions Pt OSe/Pt SSe and Pt OSe/Zr OS with narrow band gaps（close to 1.23 e V）are designed,which exhibit relatively excellent response capability of infrared light.The strong intrinsic polarizations of two components composing the heterojunctions can further strengthen the total internal electric field at the heterojunction interfaces,which is beneficial for the realization of efficient interfacial charge transfer,and the driving forces for redox reactions under light illumination are effectively enhanced at the same time.By calculating the reaction free energies,the reason for the relatively excellent CO₂RR activity and selectivity of D-scheme heterojunction Pt OSe/Zr OS is emphatically analyzed.Combined with the adsorption energies of different reduction products,the potential optimal path of CO₂RR is screened.In virtue of these,the promising application of aforementioned two narrow-band-gap D-scheme heterojunctions with strong polarity in the field of infrared-light-driven photocatalytic water splitting and CO₂RR is explored in this thesis.