Study On The Design And Properties Of BiPO₄/g-C₃N₄ Heterojunction Photocatalytic Materials

The main work of this thesis is to design and synthesize a new kind of photocatalytic material with high stability and high catalytic activity.Bismuth phosphate is a kind of catalyst with high photocatalytic activity,which with the unique oxygen acid salt structure causes internal electric field and thus promotes the separation efficiency of the carrier.However,bismuth phosphate is a wide band gap photocatalyst and defects of visible light absorption,which will greatly limit its application.g-C₃N₄ is a new kind of photocatalyst with a narrow band gap,good conductivity and good stability.This thesis considered and calculated the electronic structure and optical properties of the two photocatalysts,seeking to design a new type of composite photocatalyst of bismuth phosphate with high catalytic activity based on the first principles calculations.The main research results and innovative work are summarized as follows.Geometric and electronic structures of three polymorphs of BiPO₄?mMBIP,nMBIP and HBIP?have been investigated by first-principles calculations.The results show that PO₄-tetrahedron in nMBIP is distorted most,and mMBIP possesses minimum effective mass of carriers in three polymorphs of BiPO₄.Further,the leading role of inductive effect of dipole moment or effective mass of carries in the separation of electron-hole pairs is analyzed.Based on the fact that nMBIP has higher photocatalytic activity than mMBIP,it can be inferred that the inductive effect of dipole moment deriving from distorted PO₄ tetrahedron is the dominant factor for affecting separation efficiency of carries.The calculated results represent that nMBIP has more appropriate redox potential and narrower band gap than others.These findings may provide meaningful guidance for further understanding on the relationship between unique crystal structure and photocatalytic activity of BiPO₄.The effects of Li,Na,K alkali metal ions on the band structures and carrier transferring of g-C₃N₄ have been investigated using the plane-wave ultrasoft pseudopotential method.The generalized gradient approximation and local density approximation are used to calculate total energies of the six adsorption configurations.It is found that three alkali ions all tend to adsorb on large cavity central position?F position?in layer.The calculated band stuctures and work function values indicate that the interface charge balance of n-type Schottky junction which formed by alkali metal and g-C₃N₄ induces the whole band edge potential of g-C₃N₄ to shift down 1.52?Li?,1.07?Na?,0.86 V?K?.It is noteworthy that the incorporation of K ion adjusts the valence bands and conduction bands to more appropriate redox potentials than pure g-C₃N₄,and increases the distribution of HOMO and LOMO orbitals of g-C₃N₄,which is beneficial for improving the mobility of carriers,meanwhile,the noncoplanar HOMO and LOMO favor the separation of electrons and holes.The structural stability,electronic structure,energy level potential and optical properties of g-C₃N₄/BiPO₄ heterostructures were studied by the first principles method.According to the relative positions of g-C₃N₄ and BiPO₄,the four structure models of g-C₃N₄/BiPO₄ heterostructures are constructed,and the results show that the B configuration is the most stable.It can be inferred that the formation of heterojunction narrowed band gap,and caused the charge redistribution in the interface,enhanced the charge separation.The energy modulation results show that the formation of heterojunction caused the change of band edge potential,and obtained a more appropriate redox potential.The optical properties are calculated and indicating the formation of heterojunction extends the visible light absorption.In conclusion,the formation of g-C₃N₄/BiPO₄ heterojunction is helpful to improve the photocatalytic efficiency.