First-principles Study On The Effects Of The Doping And Strain On The Materials For The Visible Light Photocatalytic Hydrogen Generation From Water Splitting

In the last few decades,many studies have been performed to find new energy materials and improve the properties based on the existing materials to alleviate the energy crisis and environmental pollution.We try to tune the photoelectric properties of intrinsic structures or to find new material for photocatalytic water splitting driven by visible light.The main studying content including:?1?The intrinsic?-Ga₂O₃ is a wide band-gap semiconductor.The reports are focus on their UV-light application.However,visible light applications are scarce.As far as we know,the possible properties of the N doped Ga site are not considered.Based on GGA+PBE and Meta-GGA methods,we study the stability,band structure,density of state,band edge positions,and optical properties of N doped Ga-site.The results show that it has low formation energy in the condition of O-rich.As N concentration increases,?-Ga₂O₃ is gradually turned to be an indirect band-gap semiconductor.When the concentration increases to 5 at%,the band energy gap decreases into the visible light region.The VBM and CBM gradually close to the redox potential of H₂O.Especially,the band edge positions at the concentration of 5 at%still match the condition of water splitting for hydrogen generation.the structures show significant photogenic charge separation,and the optical absorption has been improved in the visible light.All the results indicate that N doped?-Ga₂O₃ is beneficial to the structure used for visible-light driven water splitting for hydrogen generation.?2?The super-hard material Pmm2-BC₂N has been widely studied in mechanical properties.However,the electronic structure and optical properties are scare.The electronic structure has been reported based on the GGA+PBE method.As we know,the GGA+PBE method often underestimates the band energy gap.Therefore,we further investigate the band structure,electronic structure,optical and mechanical properties of pristine and O-doped Pmm2-BC₂N based on the GGA+PBE and HSE methods.The results show that Pmm2-BC₂N has a wide band energy gap and no absorption in the visible light.The results of O doped Pmm2-BC₂N indicate that O doped C-2 and N sites have low formation energy compared with O doped B and C-1 sites,implying O doped C-2 and N sites are feasibility.The AIMD shows that all the O doped structures are thermodynamic and mechanical stability.Because of the different bonding strength between O and its surrounding atoms at the different doped site,the lattice parameters show two different trends.?i?The lattice parameters decrease in a direction and increase in b direction for O doped B and C-2 sites.?ii?The lattice parameters increase in a direction and decrease in b direction for C-1 and N sites.This trends further effect on the optical properties,where the absorption has a blue shift with O doped B and C-2 sites while it has a significant increase in the visible light with O doped C-1 and N sites.The further mechanical calculations indicate that all the O-doped structures have high elasticity modulus and Vickers hardness.By comparison,we can find that O doped N site can be a potential visible-light collected and wear-resistant material due to its low formation energy,significant visible-light absorption,and high hardness.?3?perovskite-type ATaO₂N?A=Ca,Sr and Ba?have been used for water splitting for hydrogen generation with low efficiency.Optical absorption is one of the important factors for efficiency.Thus,we investigate the strain effects on the band structures,electronic structures,and optical properties based on the GGA+PBE and HSE methods.The results indicate that the current calculations of pristine structures are consistent with the reported experiment values.The further calculations of strain on ATaO₂N show that?100?crystal orientation is more sensitive to strain than?010?crystal orientation.Under?100?-strain,the band energy gap has a decrease while the absorption has a blue-shift under compressive strain,which results from the decrease of N-2p orbital at VBM.Compared with the changes under compressive strain,the visible-light absorption has an obvious improvement under tensile strain.By comparison of the three structures,it can be found that the red-shift in absorption is obvious following the order of Ca²⁺<Sr²⁺<Ba²⁺.Thus,the?100?-tensile strain is an effective method to improve the visible-light absorption of ATaO₂N.?4?ZnO monolayer has appropriate band edge positions respecting the redox potential of H₂O while it does not respond to visible light.CdO monolayer has an obvious absorption in visible light but the VBM does not match the requirement.Thus,we design a new 2D ZnCdO₂monolayer.Based on the GGA+PBE and HSE methods,we find that ZnCdO₂ has dynamic and thermodynamic stability,appropriate band edge positions,obvious visible-light absorption,and significant charge separation,indicating that it can be used for visible-light driven water splitting for hydrogen generation.Further calculations of strain effect show that ZnCdO₂ monolayer is not suitable under high compressive strain.However,the band edge positions move down,and the absorption increases under tensile strain,indicating that tensile strain is beneficial to improve the properties of ZnCdO₂ monolayer.Interestingly,the band edge positions of CdO monolayer are tuned to match the requirement by applying large tensile strain,implying that a large tensile strain is also a method to tune CdO monolayer used for water splitting for hydrogen generation.The thesis includes seven chapters as follows:the first chapter is a review section,where we briefly introduce the semiconductor photoelectric device and the mechanism of water splitting for hydrogen generation.The second chapter mainly introduces the theories and methods used in our calculations.The three to six chapters are the calculated results based on the theories and methods.In chapter three,the N doped?-Ga₂O₃ used for visible-light driven water splitting for hydrogen generation is investigated.The decreased band energy gap,appropriate band edge positions,significant charge separation,and visible-light absorption show that N-doped?-Ga₂O₃can improve the properties used for visible-light driven water splitting for hydrogen generation.In chapter four,the O-doped behavior impacts on the optical and mechanical properties of Pmm2-BC₂N is investigated.O doped N site shows excellent thermodynamic stability,low formation energy,significant visible-light absorption,and high hardness,implying that it's a potential visible-light collected and wear-resistant photoelectronic material.In chapter five,strain effect on the electronic and optical properties of ATaO₂N?A=Ca,Sr and Ba?is investigated.ATaO₂N shows an obvious anisotropy under strain.?100?crystal orientation is more sensitive to strain than?010?crystal orientation.?100?tensile strain can decrease the band energy gap and improve visible-light absorption.In chapter six,we find a new 2D ZnCdO₂ monolayer to be used for visible-light-driven water splitting for hydrogen generation.Compared with ZnO and CdO monolayers,it has an appropriate band energy gap,appropriate band edge positions,obvious visible-light absorption,and charge separation.The further calculations show that the biaxial tensile strain further tunes the band edge positions and improves visible-light absorption,indicating that ZnCdO₂ monolayer is appropriate for visible-light driven water splitting for hydrogen generation.In chapter seven,we make a summary of this thesis and present a prospect for future work.