Preparation Of Silver And Defect Co-modified Carbon Nitride For Photocatalytic Pollutants Degradation And Hydrogen Evolution

The increasingly severe energy shortage and environmental crisis have seriously threatened the sustainable development of human society.Semiconductor photocatalysis is expected to solve energy shortage problems and environmental crises as an economical and safe renewable clean technology.Graphitic carbon nitride（g-C₃N₄）has drawn broad attention as an ideal candidate for visible-light photocatalysis due to its unique features,such as environmental friendliness,high physicochemical stability,proper and tunable electronic and band structure for target reactions,and easy processability.However,the photocatalytic degradation and hydrogen production efficiency of g-C₃N₄have significantly been inhibited due to the limited visible-light utilization,the high recombination rate of photogenerated electron-hole pairs,and the small specific surface area.In order to overcome these problems,this master’s dissertation integrated two strategies of defect engineering and noble metal modification and successfully prepared silver and defect co-modified g-C₃N₄（Ag/D-UCN）.Meanwhile,the defective g-C₃N₄（D-UCN）and silver-modified g-C₃N₄（Ag/UCN）obtained by a single modification strategy were also prepared for the comparative study.Firstly,the morphology,composition,surface physicochemical properties,and band structure of the above catalysts were studied by various characterization methods.Then,the photocatalytic properties for removing refractory organic pollutants and hydrogen evolution under simulated sunlight or visible light radiation were systematically compared.Fluorescence spectroscopy characterization,photochemistry experiment,free radical scavenging test,and DMPO-/TEMP-trapping ESR test were applied to study the photocatalytic redox mechanism of Ag/D-UCN.Finally,the path of Ag/D-UCN photocatalytic degradation of representative pollutants was revealed by detecting intermediates in the degradation process.This study provides a new technology for the preparation of efficient graphitic carbon nitride-based photocatalysts,as well as new materials and corresponding primary research data for the efficient treatment of organic pollutants in water and the production of clean hydrogen energy.The specific research conclusions of this work are as follows:（1）A facile and in-situ strategy of thermal polycondensation of molten urea,by adding AgNO₃and citric acid into the molten urea,is proposed to synthesize silver and defect co-modified g-C₃N₄（Ag/D-UCN）.The characterization results show that Ag/D-UCN has a mesoporous-rich nanosheet structure with a large BET surface area(130.4 m²g^-1).The C₃N₄.Meanwhile,ultrasmall silver nanoparticles are uniformly coordinated on the N atoms neighboring the C vacancy of g-C₃N₄.The synergistic action of silver and carbon vacancy effectively regulates the band structure of g-C₃N₄,reducing the bandgap energy and introducing an intermediate energy level,thus improving the light-harvesting capacity of Ag/D-UCN.（2）The photocatalytic performance of Ag/D-UCN in oxidizing organic pollutants and reducing H₂O for H₂evolution is systematically evaluated.By selecting methylparaben（MPB）,acetaminophen（APAP）,levofloxacin（LEVO）,atrazine（ATN）,and p-nitrophenol（PNP）as the target pollutants,the photocatalytic oxidizing activity of Ag/D-UCN under visible-light irradiation（400 nm<λ<680 nm）are evaluated.The Ag/D-UCN catalyst exhibits superior performance in the photocatalytic oxidation of the above five organic pollutants than those of UCN,D-UCN,and Ag/UCN.The removal rates of MPB,APAP,LEVO,and ATN reached 100.0%after visible light irradiation for 90,30,12 and 60 min,respectively.Meanwhile,Ag/D-UCN also shows powerful mineralization ability and stability.The photocatalytic hydrogen evolution performance of Ag/D-UCN is evaluated under simulated sunlight irradiation（320 nm<λ<680 nm）.The results show that the hydrogen yield of Ag/D-UCN was up to 3500μmol g^-1,which is 8.8 times higher than that of Ag/UCN.The apparent quantum yield（AQY）of Ag/D-UCN can reach 7.30,3.38,1.64,0.67,and0.21%at 365,400,450,500,and 550 nm,respectively.Meanwhile,Ag/D-UCN shows excellent stability in photocatalytic hydrogen production.（3）Steady-state photoluminescence（PL）emission spectra,time-resolved photoluminescence decay（TRPD）spectra,and photochemistry measurements are applied to study the separation and migration efficiency of the photogenerated carrier.Then the reactive oxygen species produced in the catalytic system are detected by the free radical scavenging test,as well as DMPO-and TEMP-trapping ESR.The results reveal that the excellent photocatalytic redox activity of Ag/D-UCN is mainly due to the synergetic effect of Ag and carbon vacancies,as well as mesoporous-rich nanoparticles morphology,which effectively promotes the generation-separation-migration-transportation of the photogenerated charge carriers.Therefore,various active species,including·O₂^-anion radicals,·OH radicals,and singlet oxygen,are generated in the photocatalytic oxidation system,contributing to the degradation and mineralization of organic pollutants.In the Ag/D-UCN photocatalytic reduction system,the abundant e _CB^-with sufficient reducing power finally converts H⁺or H₂O to H₂on the surface of Ag.Finally,visible-light photocatalytic degradation pathways of representative pollutants over Ag/D-UCN were proposed by detecting intermediates in the degradation process.