In-situ Synthesis And Photocatalytic CO₂ Reduction Performance Of Ag/BiPO₄/BiOBr Heterojunction Thin Films

Fossil fuel is a non-renewable resource,and our uncontrolled exploitation and utilization of it makes human society face an unprecedented energy crisis challenge.In addition,the use of fossil fuels will inevitably bring about many environmental problems.At present,the world emits a large amount of greenhouse gases such as CO₂ every year,which leads to frequent environmental problems such as global warming,sea level rise,land area reduction,and ozone hole in today’s society.Therefore,how to convert and benefit CO₂,especially how to efficiently and greenly convert it into gaseous fuels to achieve carbon recycling,and ultimately achieve the"dual carbon"strategic goal,has become a research hotspot.At present,the core problem of solar light-driven CO₂ reduction to solar fuel is how to construct a photocatalyst system with high light response,high activity,high selectivity,high stability and easy recovery.Among the numerous photocatalytic materials,BiOBr with a narrow bandgap of 2.7–2.9e V has attracted increasing attention as an alternative photocatalyst due to its superior visible light absorption,low toxicity,and high photocatalytic activity.In addition,BiPO₄ is also a promising photocatalyst,and its strongly negatively charged PO₄^3-has an inductive effect,which tends to attract holes and repel electrons,contributing to the separation of e^--h⁺.However,the single nano-sized photocatalyst powder has many disadvantages such as high photogenerated e^--h⁺pair recombination rate,easy agglomeration,and difficulty in recycling,which greatly limits its wide application.Therefore,its core lies in how to construct an efficient solar photocatalytic system and realize its immobilization.In response to the above problems,we prepared BiOBr-based photocatalytic materials in situ using Bi plate as Bi source,and constructed BiPO4/BiOBr composite thin film heterojunction and modified it with precious metals,and finally realized the perfect fixation of ternary components.It can solve the problems faced by a single nanometer-sized photocatalyst powder to the greatest extent.The main research contents and conclusions of this paper are as follows:（1）Electrochemical preparation of BiPO₄/BiOBr composite film and its photocatalytic CO₂ reduction performance.A two-layer heterojunction film with BiPO₄ nanorods as the upper layer and BiOBr nanosheets as the lower layer was successfully synthesized by a one-step electrochemical ion exchange method.The effects of different electrolyte concentrations and p H values on the composite films were investigated,and the optimum electrolyte concentration was 0.1 mol/L and the optimum p H value was 6.The photocatalytic CO₂ reduction and cycling experiments show that the prepared BiOBr/BiPO₄ composite films have better photocatalytic CO₂reduction activity and stability than pure BiOBr films under simulated sunlight irradiation,and are approximately 1.92 times that of pure BiOBr films.In addition,the in-situ formation mechanism of the film was discussed,and its possible photocatalytic CO₂ reduction mechanism（S-Scheme mechanism）was proposed based on the surface morphology and in-situ XPS characterization of the BiOBr/BiPO₄ composite film.This work is of great significance for realizing the immobilization of powder catalysts and developing an in-situ preparation method for double-based membranes.（2）The chemical preparation of Ag/BiPO₄/BiOBr composite film and its photocatalytic CO₂ reduction performance.On the basis of the first work,the noble metal Ag was deposited on the BiPO₄/BiOBr composite film by in-situ photoreduction,and the effect of different Ag mass fractions（wt.%:1.0,1.5,2.0）on the ternary system was investigated.Activity effect of photocatalytic reduction of CO₂.The results show that the Ag/BiPO₄/BiOBr ternary composite film does not destroy the structure and morphology of the original composite film,and its photocatalytic CO₂ reduction to CO activity increases first and then decreases with the increase of Ag loading.And both are larger than the BiPO₄/BiOBr composite films.Among them,Ag loading of 1.0 wt.%has the highest activity,which is 1.24 times that of the BiPO₄/BiOBr composite film and 2.38 times that of the pure BiOBr film.The reason why the activity first increases and then decreases is that when the Ag loading is small,it can capture electrons,and when the Ag nanoparticles increase to a certain extent,they become the recombination centers of photogenerated e^-and h⁺.The reason for the enhanced activity is that the noble metal Ag can induce the LSPR effect and form a Schottky barrier,which broadens the photoresponse range and effectively prevents the recombination of photogenerated e^--h⁺pairs.Finally,an S-Scheme photocatalytic CO₂ reduction mechanism is proposed.To summarize,both heterojunction films and ternary composite films have excellent photocatalytic CO₂ reduction activity and stability,and also have the advantages of easy recovery,no agglomeration,and no secondary pollution.This work is of great significance for realizing the immobilization of powder catalysts and developing the in-situ preparation of multi-component composite films.