Preparation And Photocatalytic Performance Of G-C₃N₄-Based Nanomaterials Modified By Up-Conversion Fluoresence

With the rapid development of the national economy,the unscientific consumption of energy has caused a large amount of pollution emissions,which is increasingly beyond the capacity of the environment.At the same time,the rapid consumption of energy also leads to the rapid depletion of traditional energy.At present,the double crisis of energy and environment is changing the earth’s ecosystem at an unprecedented speed and amplitude.Therefore,it is very important to promote the implementation of green sustainable energy development and improve the environmental ecosystem.Photocatalysis technology driven by solar energy not only can decompose water to obtain hydrogen to alleviate energy crisis,but also can reduce carbon dioxide and degrade pollutants to improve environmental ecological system,thus attracting wide attention.Among them,graphitic phase carbon nitride（g-C₃N₄）,as a new photocatalyst,has the advantages of easy preparation,good stability,green economy,visible light response and so on,and is favored by researchers.However,pure g-C₃N₄ has some problems such as small specific surface area,low conductivity and low photogenerated carrier separation rate,which seriously affect its application in the field of photocatalysis.To solve the above problems,this paper prepared g-C₃N₄-based composite photocatalytic materials by means of heterojunction construction,morphology and size regulation,up-conversion fluorescence modification,etc.While realizing efficient photocatalytic performance,combined with photochemical testing,the mechanism of enhancing its performance was explained as follows:（1）Er:BiOBr/g-C₃N₄ QDs heterojunction was prepared by solvothermal method,and the effect of different loading loads of g-C₃N₄ QDs on photocatalytic degradation of tetracycline hydrochloride was studied.The results show that Er:BiOBr has better performance than single BiOBr,and Er:BiOBr/g-C₃N₄ QDs has the best photodegradation effect,and has good stability in the cycle test.This is mainly attributed to the introduction of rare earth Er³⁺doping,which can effectively use its upconversion fluorescence characteristics to expand the range of photoresponse,and the formation of heterojunction can promote the separation of photogenerated carrier and accelerate the photodegradation reaction.Meanwhile,g-C₃N₄ QDs are uniform and highly dispersed on the surface of Er:BiOBr,providing a large number of reactive sites,further enhancing the photodegradation performance.（2）In order to expand the application of photocatalysis in other fields,g-C₃N₄/Er:Cd Se/Si O₂ terre core-shell heterojunction was prepared by continuous chemical-calcination method,which not only has excellent effect on pollutant degradation,but also shows good photocatalytic hydrogen evolution performance.The results show that the performance of the prepared g-C₃N₄/Er:Cd Se/Si O₂ core-shell heterojunction is twice that of Cd Se and twice that of g-C₃N₄ when used for photodegradation.The photocatalytic hydrogen production reaches 894.27μmol·g^-1·h^-1,which is 35 times of Cd Se and 70 times of g-C₃N₄,and has good stability in continuous cycle experiment.It can be attributed to Si O₂ spheres as optical lenses,which can increase the reflection to improve the efficiency of sunlight and maintain the stability of the structure.Er³⁺doping brings unique upconversion fluorescence properties,further improving the utilization rate of solar energy.In addition,the construction of heterojunction can quickly separate the photoelectron from the hole,and promote the further improvement of the photocatalytic performance.On the other hand,the ternary core-shell structure also provides sufficient active sites for the reaction,and finally realizes the efficient degradation and hydrogen evolution of g-C₃N₄/Er:Cd Se/Si O₂.（3）In order to further optimize the hydrogen evolution performance of g-C₃N₄-based materials,Er:WO₃/Cd S/g-C₃N₄ QDs nanorods heterojunction was prepared by a three-step wet chemical method combined with the above work.Photocatalytic test results show that Er:WO₃/Cd S/g-C₃N₄ QDs produced 1912.12μmol·g^-1·h^-1,and photocatalytic degradation rate was up to 93.1%（16 min）,which was 20 times of the hydrogen evolution capacity of single Cd S,and the photodegradation efficiency was 3.5 times of that of single Cd S.The mechanism of the enhanced photocatalytic performance of the system was quantitatively analyzed by means of crystal structure characterization and photoelectrochemical measurement.On the one hand,the upconversion performance of Er³⁺doping can increase the optical response range.On the other hand,the modification of g-C₃N₄ QDs increases the specific surface area and carrier injection of the composite sample,which is conducive to the forward reaction.More importantly,the Z-scheme structure constructed by Er:WO₃/Cd S can realize the efficient separation of photogenerated electron-holes in space,which greatly enhances the photocatalytic performance.In addition,the structure of nanorods can also improve the diffusion path of charge carriers,inhibit the photogenerated electron-hole pair recombination to a certain extent,and maintain the stability of the material,so as to achieve efficient and stable photocatalytic degradation and photocatalytic hydrogen production.