Study On Photocatalytic Properties Of BiVO₄-based Heterojunctions And Oxygen-vacancy-riched Bi₂O₄

Monoclinic BiVO₄ possesses favorable visible light response capability,for its relatively small band gap,suitable position of conduction band（CB）and valence band（VB）.However,single component BiVO₄ has the fast recombination rate of photogenerated charge carriers,which could inhibit the visible light catalytic efficiency.The construction of heterojunction can effectively promote the separation efficiency of photogenerated carriers,which remarkably improved the photocatalytic performance.Therefore,this paper constructs a three-dimensional（3D）hierarchical BiVO₄/ZnCr-LDH type II heterojunction,which could separate spatially photogenerated electron-hole pairs to promote the transfer and separation of carriers.In addition,the construction of BiVO₄/ZnCr-LDH heterojunction also increases the specific surface area and broaden the spectral response range of BiVO₄,thereby improving the visible light catalytic degradation performance.Due to the narrow band gap（～2.0 e V）,Bi₂O₄ with mixed valence states(Bi³⁺and Bi⁵⁺)has powerful absorption in visible light region,which is also considered as an excellent visible light photocatalytic material.Bi₂O₄ synthesized by a typical hydrothermal method only has a relatively low concentration of oxygen vacancy.Although the construction of Bi₂O₄-based composites like Bi₂O₄/Bi₄O₇/BiO_2-x,the concentration of Bi₂O₄oxygen defect is enhanced,leading to enhancing the photocatalytic performance.Nevertheless,this strategy demands the introduction of the second component to form the composites,usually involving time-consuming and complicated preparation methods.How to increase the oxygen defect concentration of Bi₂O₄ and then enhance the photocatalytic activity of Bi₂O₄ without the help of another constituent,yet ignored.This work provides a novel strategy to enlarge the oxygen vacancy concentration of Bi₂O₄ through the pre-treatment of commercial NaBiO₃·2H₂O,so as to modulate the photocatalytic degradation performance of Bi₂O₄.This paper mainly carries out from the following aspects:Firstly,some accumulated two-dimensional（2D）ultrathin ZnCr-LDH nanosheets has been in situ distributed at the surface of 2D BiVO₄ by the hydrothermal reaction,forming a 3D hierarchical BiVO₄/ZnCr-LDH type II heterojunction.The hierarchical structure can not only increase the specific surface area but also promote light absorption through multiple scattering effects.The 1:2BiVO₄/ZnCr-LDH heterojunction has the highest degradation efficiency of tetracycline hydrochloride under visible light irradiation,which is 3.47 times and 22.4times higher than that of BiVO₄ and ZnCr-LDH,respectively.The reasons for the enhancement visible photocatalytic activity of BiVO₄/ZnCr-LDH type II heterojunction are as follows:1)the increased specific surface area provides more active sites;2)the light scattering effect of the 3D hierarchical structure enhances the light absorption efficiency and generates more carriers;3)the formation of type II heterojunction improves the separation efficiency of electron-hole pairs and prolongs the carriers’lifetime.Secondly,the commercial NaBiO₃.2H₂O was pre-treated by two methods,namely heat treatment in air and hydrothermal treatment in 10 M NaOH solution,to form NaBiO₃.xH₂O intermediate products.Then improving the oxygen vacancy of Bi₂O₄was synthesized by the typical hydrothermal method,using NaBiO₃.xH₂O intermediate products as starting materials,which exhibit the superior catalytic activity under visible light irradiation.In particular,hydrothermal pretreatment route brings about much better beneficial effects for Bi₂O₄.The best visible light photocatalytic performance of Bi₂O₄-A-3 synthesized by the hydrothermal pretreatment route,which is 2.59 times and 4.26 times higher than that of Bi₂O₄,respectively,for the degradation of methyl orange and phenol.The increase in the photocatalytic activity of the Bi₂O₄ can be attributed to:1)the relatively higher concentration of surface oxygen vacancies could cause the upshift of the VB and the downshift of the CB,namely narrowing the bandgap,and thus extends the spectral response range of the photocatalysts;2)as electron traps,the enlarged content of surface oxygen vacancy could capture more photoinduced electrons,which could improve the separation efficiency of photoinduced charge carriers,thereby prolonging the carriers’lifetime.