Low-index Surface Geometries And Electrical Properties Of BiPO₄ And Cu₂O

Among three crystal types of wide band gap semiconductor BiPO₄,the monoclinic monazite phase has the best photocatalytic activity.As a narrow bandgap semiconductor that responds to visible light,Cu₂O also has a wide range of applications in the field of photocatalysis.The semiconductor surface is an important place for photocatalytic reactions,but experimental characterization methods alone cannot accurately reflect the surface structure characteristics and microscopic mechanism at the atomic level,which limits people's deep understanding of photocatalytic reactions.Therefore,it is particularly important to study the structural properties of semiconductor crystals through theoretical calculations.The thesis discusses the geometries and electrical properties of the low-index stoichiometric surfaces of BiPO₄ and Cu₂O through first-principles method of density functional theory,which provides theoretical guidance for their practical application.The thesis takes BiPO₄ and Cu₂O as the research objects,and proposes a new method for calculating the crystal surface energy—the corrected surface energy calculation formula based on the average surface energy.The corrected surface energy calculation method effectively takes into account the fact that the upper and lower surfaces of the unit cell have different relaxation states,and corrects the mode of the original average surface energy calculation method which considers the relaxation of the upper and lower surface of the unit cell as the same.The scheme makes the surface energy calculation results more realistic,and provides a reference for accurately predicting the crystal surface energy.Four low-index stoichiometric surfaces of monoclinic monazite BiPO₄ were established,and the geometric and electronic structures of?001?,?010?,?011?and?100?surfaces were systematically studied by density functional theory.Firstly,based on the principle of lowest total energy,the most stable termination of each surface is determined.Next,both the average surface energy and corrected surface energy schemes show that the order of surface energy is?100?<?010?<?011?<?001?,which is not completely consistent with the order of their surface dangling bond density.The main reason is that the PO₄ tetrahedron is incomplete,i.e.there are dangling P–O bonds in the?001?surface,but only dangling Bi–O bonds are found in the other three low-index surfaces.Therefore,the dangling bond density of Bi atoms and the complete PO₄ tetrahedron are the two key factors for surface stability.Then,the charge distribution difference,electrostatic potential and total charge density are used to describe the charge distribution near the surface.Finally,the balance crystal model of BiPO₄ was determined by the Wulff shape:?100?,?010?,?011?and?001?surfaces accounted for 15%,33%,16%and 36%of the total crystal area,respectively.The two low-energy?100?and?010?surfaces occupy the main part of the Wulff structure.The high surface energy and obvious relaxation of?001?surface indicate its high photocatalytic activity.Three low-index stoichiometric surfaces of Cu₂O were established,and the geometric and electronic structures of?001?,?100?,and?111?surfaces were systematically studied by density functional theory.Firstly,based on the principle of lowest total energy,the most stable termination of each surface is determined.Subsequently,both the average surface energy and the corrected surface energy calculation scheme show that the order of the surface energy is?111?<?100?<?001?.The order of the surface energy is completely consistent with the density of dangling bonds on their surface.It can be seen that the density of dangling bonds on the surface is an important factor affecting the surface energy.Then,the surface energy band structures calculated by the GGA+U method are also presented respectively,and the band gap size order is:E_g?100?<E_g?111?<E_g?001?,where the?001?surface has the most suitable conduction band and valence band potential.Next,the charge distribution near the surface is represented by the difference in charge density and electrostatic potential.Finally,the Wulff shape shows that the?001?,?100?,and?111?surfaces account for 25%,25%,and 50%of the total crystal area,respectively,and the low-energy?111?surface accounts for the major part of the Wulff shape.The high surface energy and suitable edge potential of?001?surface indicate its high photocatalytic activity.