First-Principle Study On Structure And Physical Properties Of Two-Dimensional Ga₂O₃ And Cr₂O₃

Oxide semiconductor materials have a wide range of applications in many fields such as photoelectric sensing,electrocatalysis,and information communication.As increasing demand poses challenges to the optical and catalytic properties of the materials.Gallium oxide and chromium oxide have wide band gaps and high surface activity,it has excellent optical and catalytic properties,but the lack of flexibility and active sites of three-dimensional materials limit the application range of gallium oxide and chromium oxide.Two-dimensional materials have the characteristics of high flexibility,large specific surface area and easy-to-control structure,which can effectively compensate for the above-mentioned shortcomings of materials.Therefore,this thesis adopts density functional theory,hybrid functional theory and DFT+U methods focusing on two-dimensional gallium oxide(2D?-Ga₂O₃,2D?-Ga₂O₃)and two-dimensional chromium oxide(2D?-Cr₂O₃)to study their stabilities,electronic structures,optical properties and catalytic performance by first principle calculations.Based on the calculations of the adsorption bond energy,dielectric function and Gibbs free energy of three materials,the regulatory effects of molecular adsorptions,strains,and dopants on the structure,optical and catalytic properties are respectively given.It provides a new idea for the performance control and preparation of the two-dimensional oxide semiconductor.Adopting first-principles calculation method,electronic structures and properties of2D?-Ga₂O₃ are studied.The calculation results show that,compared with bulk materials,2D?-Ga₂O₃ which possesses a slightly smaller band gap,has smaller optical absorption coefficient,reflectivity and refractive index,while its electron loss function increases,and the barrier of hydrogen evolution reaction decreases.The adsorption of different molecules on the surface will modify the band gap and the range of optical properties.Intrinsic defects will introduce energy levels in the band gap,increase the absorption within the visible light and infrared regions,and the enhance the intensity of refractive index and reflectivity.The band gap will decrease during the tensile process,resulting in the red shift of the optical absorption edge and decrease the hydrogen evolution reaction barrier.The compression strain has the opposite effects.The doping of group IV and VII elements will increase the Fermi level forming N-type,and lead to increase of the absorption in infrared and visible light regions,and promote the refractive index and reflectivity.Transition metal doping mainly causes the increase of electron energy loss in the low energy region.It shows a decrease in the barrier of oxygen evolution reaction and the hydrogen evolution reaction for 2D?-Ga₂O₃ by doping with Fe and Au.On the basis of density functional theory,the physical properties of 2D?-Ga₂O₃ and the effect of molecular adsorption,strain and doping on its properties are fully studied.The band gap of 2D?-Ga₂O₃ is smaller than that of bulk,the optical absorption and electron loss functions in the low-energy region are redshifted,while the reflectivity and refractive index are reduced,and the hydrogen evolution reaction barrier is lower than that of bulk materials.The adsorption of non-metallic elements will change the electronic structure of 2D?-Ga₂O₃,and the optical absorption intensity in the infrared region is increased.The increase of tensile and compressive strain will make the band gap gradually decrease,and the optical absorption edge will be red-shifted.The doping of group VII elements will make the optical absorption edge red-shifted.The doping of group IV elements will produce obvious absorption peaks in the low-energy region.Band gaps of 2D?-Ga₂O₃ increases with the concentration of Al and the optical absorption edge is blue-shifted,and the refractive index and reflectivity decrease;other metal doping will introduce new energy levels in the band gap and cause obvious absorption peaks in the low energy region,while the refractive index,reflectivity and the hydrogen evolution reaction barrier increase.Based on the first principles,the new physical properties of 2D?-Cr₂O₃ are predicted.The calculation results show that the band gap of 2D?-Cr₂O₃ is smaller than the bulk phase,and 2D?-Cr₂O₃ exhibits ferromagnetic.The optical absorption and electron loss functions in the low-energy region are red-shifted for 2D?-Cr₂O₃ compared with bulk materials,while its reflectivity and refractive index are reduced.The hydrogen evolution reaction barrier is larger than the bulk.Adsorption of non-metallic elements will change the total magnetic moment,electronic structures and optical properties.Band gaps will become smaller with uniaxial tensile and compression strains,and change the optical properties.The tensile strain will increase the barrier of hydrogen evolution reaction,while the effect of compression strain is inverse.The increase of Fe doping concentration will cause the magnetic transformation from ferromagnet to antiferromagnet in 2D?-Cr₂O₃ system,the shift of the optical absorption edge and the electron loss function edge,and the reduce of the reflectivity and refractive index in the low energy region.The barrier of hydrogen evolution reaction increases with the Fe doping.