Study On The Photogenerated Charge Separation Modulating Strategies Of G-C₃N₄ Based Composite Photocatalyst And Its Catalytic Performance

With the development of science and technology and the growing needs of the community,antibiotics,hormones,pesticides,dyes and other non-biodegradable organic matters have been widely used in human life,pharmaceutical industry and animal husbandry.The prevention and treatment of these refractory organic pollutants have become one of the most important tasks in water pollution control.Photocatalysis technology have attracted much attention due to its high efficiency and strong sustainability.Graphitic carbon nitride(g-C₃N₄),a kind of non-metal semiconductor photocatalysts,have been widely studied in the field of photocatalysis because of its cheap and readily available raw materials,strong physical and chemical stability,and intense response to visible light.However,the specific surface area of g-C₃N₄ prepared by solid-state method is lower and the photogenerated electrons and holes are easily recombinated,which severely restricts the application of g-C₃N₄ photocatalysts in the pollutant treatment.In this paper,the modified g-C₃N₄ based materials with efficient separation of photoelectrons and holes were prepared to improve the photocatalytic performance,and the mechanism of the effect of the modification on the photocarriers was also investigated.In order to improve the specific surface area of g-C₃N₄ and the separation rate of photoinduced electrons and holes,g-C₃N₄ nanosheets with thinner layer were prepared at high temperature with NH₄HCO₃ and urea as raw materials.The relationship between the phocatlytic property and specific surface area of thinner layer g-C₃N₄ nanosheets were investigated through specific surface area,photoresponse and photocatalytic performance tests.The result showed that when the ratio of NH₄HCO₃ to urea was 0.5,the specific surface area of N/U0.5 was the biggest,and it was 105.40m²/g,the pore structure of N/U0.5 was the most abundant,and the total pore volume was 0.81cm³/g.Compared with the unmodified g-C₃N₄ material,the specific surface area and the total pore volume of N/U 0.5 increased by 56.47% and 78.98%,respectively.The abundant pores could promote the photoelectron transfer and inhibit the recombination of electron and hole pairs,the PL signal intensity of N/U 0.5 photocatalyst was the lowest and the transient photocurrent density was 0.14mA/m²,it was 71.43% higher than that of unmodified g-C₃N₄,therefore the photocatalytic performance of g-C₃N₄ was enhanced.When the initial concentration of SMX was 500?g/L and the dosage of the catalyst was 50mg/L,the removal rate of SMX reached 82.22% within half an hour,which was15.82% higher than that of the unmodified g-C₃N₄.The stability of photocatalyst N/U0.5 was strong,the removal rate of SMX decreased only 0.57%after 5 repeated catalytic experiments,and its structure and morphology remained unchanged after photo catalytic reaction.To further improve the separation efficiency of photogenerated electrons and holes of g-C₃N₄ photocatalyst,the combination of thinner layer g-C₃N₄ nanosheets and CeO₂with good conductivity were prepared to form CeO₂/g-C₃N₄ heterojunction photocatalyst,and it was used for photocatalytic removal of SMX in water.The results of structural and morphological tests showed that CeO₂ and g-C₃N₄ were closely bonded to form heterostructure,which increased the separation rate of photoelectron and hole.CeO₂/g-C₃N₄ heterojunction composite photocatalyst prepared with CeO₂ mass ratio of 5%(5CeO₂/g-C₃N₄)had the weakest PL emission peak,and its transient response photocurrent density was 0.20mA/m²,which was 42.90% higher than that of pure g-C₃N₄,and 81.8%higher than that of pure CeO₂.The catalytic experiment results showed that 5CeO₂/g-C₃N₄ obtained the highest SMX photocatalytic performance.When the initial concentration of SMX was 500?g/L,the dosage of photocatalyst was 50mg/L,the removal efficiency was up to 99.2% after 1h of illumination,which was8.25% and 91.83% higher than that of g-C₃N₄ and CeO₂,respectively.The property of 5CeO₂/g-C₃N₄ composite photocatalyst was stable,and the removal rate of SMX was still above 96% by it after 5 times of reuse.In addition,in order to improve the visible light absorption ability of the photocatalyst and to promote the separation of photoelectrons and holes,the CNTs/g-C₃N₄ composite photocatalysts were prepared by combining g-C₃N₄ and carbon nanotubes(CNTs)with high adsorptivity and electrical conductivity.The DRS results showed that the absorption range for visible light of CNTs/g-C₃N₄-1 photocatalyst increased when the mass ratio of CNTs was 1%,and the absorption edge of composite photocatalyst increased from 460nm to 470nm,the enhancement of the absorption of visible light was beneficial to the photocatalytic reaction.The transient photocurrent desity results indicated that the photocurrent density of CNTs/g-C₃N₄-1 photocatalyst was 0.23mA/cm²,which was 64.29%higher than that of pure phase g-C₃N₄ photocatalyst.The removal efficiencies of Rh B and SMX by CNTs/g-C₃N₄ were 98.1% and 99.4% after 1h of visible light irradiation,which were 16.23%and 8.47% improved compared to the pure g-C₃N₄.The results demonstrated that the photocatalytic activity of g-C₃N₄ based composite photocatalyst combined with CNTs was enhanced.The photocatalytic ability of systhesized composite photocatalyst CNTs/CeO₂/g-C₃N₄ was improved further,the removal rate of SMX was nearly 100% and the removal rate of Rh B was 99.2% under 1h of visible light irradiation.The enhancement of the photocatalysis performance of the composite photocatalyst was mainly attributed to the enlarged visible light absorption range and the enhanced photoelectric separation.The capturing experiments of active species and the LC-MS measurement for intermediate products of SMX by photocatalysis were carried out.The results showed that h⁺,·OH and·O₂^- were the active factors of the SMX and Rh B photocatalytic removal by g-C₃N₄ based photocatalysts,and·O₂^- was the most important active free radical.The photocatalysis process of SMX by g-C₃N₄-based photocatalysts can be summarized as bond breaking,substitution reaction,hydroxylation and oxidation reaction.The results of repeated phptpcatalysis experiments and the structural and morphological characterization of the photocatalysts before and after photocatalysis showed that these g-C₃N₄-based photocatalysts had solid structure and catalytic stability,which was favorable for their popularization and application in photocatalysis technology.To sum up,the g-C₃N₄ based photocatalysts with higher photocatalytic degradation performance were prepared in this study,the problem of the rapid recombination of photoinduced electrons and holes of g-C₃N₄ was solved,leading to the enhanced ability to remove the organic pollutants in water.This study provided a new idea for the removal of pollutants in water.