Study On Carrier Dynamics And Photocatalytic Properties Of BiVO₄ Photocatalyst Modified By Low-dimensional Carbon Materials

As a new advanced oxidation technology,photocatalytic technology has become a research hotspot in solving environmental and energy problems due to its environmental protection,good stability and reusability.As a new type of visible light catalyst,bismuth vanadate?BiVO₄?has attracted the attention of researchers because of its low cost and easy availability.Low-dimensional carbon materials have good electrical conductivity,and there are many researches on composite photocatalytic systems modified with low-dimensional carbon materials.At present,it is generally believed that the modification of photocatalytic materials by carbon materials is beneficial to the separation of electrons and holes,and thus is beneficial to improving photocatalytic degradation performance.In the photocatalytic system,the specific mechanism for improving the photocatalytic performance of low-dimensional carbon materials is less explored,that is,there are few studies on the motion process of photogenerated carriers,and there is no direct evidence to support the relevant theory.In this paper,BiVO₄ was selected as the main catalyst,and low-grade carbon-based materials?zero-dimensional carbon quantum dots,one-dimensional carbon nanotubes,two-dimensional reduced graphene oxide?were used to form a modified composite photocatalyst.For the prepared modified composite photocatalysts?CQDs@BiVO₄,MWCNTs@BiVO₄,rGO@BiVO₄?,the basic crystal structure,morphology and optical properties analysis were carried out.Based on this,the specific mechanism of improving the photocatalytic performance of the photocatalytic composite system modified by low-dimensional carbon materials is mainly studied from the study of photo-generated carrier dynamics,free radical generation and degradation mechanism.The main conclusions are as follows:?1?The photocatalyst BiVO₄,CQDs@BiVO₄,MWCNTs@BiVO₄ and rGO@BiVO₄composites were prepared by hydrothermal method.XRD data showed that the prepared BiVO₄ was a monoclinic scheelite crystal form,and its crystal structure did not change with the addition of carbon-based materials?CQDs,MWCNTs and rGO?.It can be seen from the SEM image that the prepared bismuth vanadate is a relatively regular sphere with a uniform size and a particle size of about 5-7?m.From the SEM and HRTEM images,the CQDs are successfully prepared and successfully compounded with BiVO₄.The prepared MWCNTs@BiVO₄ has a serious agglomeration phenomenon,which has a certain relationship with the preparation method using high temperature calcination method.But the MWCNTs are still successfully compounded.At the same time,the SEM results and XRD results show the successful compounding of rGO and BiVO₄;?2?From the results of UV-visible diffuse reflectance analysis,it is concluded that BiVO₄ and the other three complexes have no migration change in the absorption wavelength range of visible light,but their absorption properties of light are obviously improved,especially carbon nanotubes and reduced graphene oxide.The addition of it significantly increased its absorbance.This is advantageous for the generation of photo-generated carriers,thereby improving the efficiency of photocatalytic degradation.XPS results showed that the addition of CQDs,MWCNTs and rGO did not change the valence state of the original elements in BiVO₄.The Bi and V elements were still present in the composite system in the valence state of Bi³⁺and V⁵⁺ions,respectively.And there are certain differences in the carbon group of the material;?3?Through PL and transient fluorescence spectroscopy,it is found that in photocatalytic degradation,the combination of photo-generated electrons and holes is an ultra-fast kinetic process,generally required to be below the nanosecond level.There are three kinds of fluorescence lifetimes for the photo-generated carriers of the composite material formed by the base material.One is the life of the conduction band electrons returning to the ground state and the hole recombination,and the other is the lifetime of the conduction electrons to the surface and the holes.One is the lifetime resulting from the migration of defects between defects.The fluorescence lifetime of MWCNTs@BiVO₄?average fluorescence lifetime 6.27 ns?and rGO@BiVO₄?mean fluorescence lifetime 6.00 ns?composite photocatalytic materials were significantly higher than CQDs@BiVO₄?average fluorescence lifetime 3.57 ns?;?4?The carbon material has good electrical conductivity,and its compounding with BiVO₄ can obviously reduce the recombination rate of photo-generated carriers,showing the growth of fluorescence lifetime,the weakening of photoluminescence spectrum and the obvious enhancement of photocurrent.Among the three carbon materials,MWCNTs exhibit optimal performance;?5?The experimental results of photocatalytic degradation factors show that when the experimental conditions in the degradation process are 0.15g of catalyst,the concentration is 4mg/L,and the light intensity is 800W,the degradation effect is best.At the same time,the photocatalyst has better stability.In the photocatalytic degradation process,photo-generated electrons and holes undergo subsequent reaction to form active radicals ·O₂^-and·OH involved in photocatalytic degradation.The free radicals that play a major role in the whole degradation process are·OH and h⁺.The effect of ·O₂^-reactant is relatively weak.