The Preparation Of Highly Efficient Graphitic Carbon Nitride Based Photocatalysts For The Removal Of Aqueous Pollutants And Mechanism Study

With the rapid development of global industry,energy crisis and environmental problem have become more and more serious.Therefore,to effectively alleviate the contradictory between energy crisis and environmental problems,some suitable solutions is urgently needed.In recent years,photocatalysis technology have attracted great attentions owing to its advantages,such as low energy consumption,mild reaction conditions,convenient operations and high reaction efficiency,which also owns the great potential for environmental pollution removal.The key-point for photocatalytic technology development is to exploit efficient photocatalysts.In this decade,graphite carbon nitride（g-C₃N₄）,a kind of polymeric metal-free semiconductor materials with a narrow band gap of 2.7 eV,which can be excited by visible light,have become the hotspot in the field of photocatalysis.g-C₃N₄ owns some advantages such as excellent physical-chemical stability,environmental friendly,cost effectiveness,and so on.In this study,a series of work have been conducted about the preparation of novel g-C₃N₄ based photocatalysts and its application for the treatment of environmental pollutant in aqueous solution.Some strateg ies for the design of g-C₃N₄ based nanocomposites have been used,such as element doping,construction of heterojunction,application of Z-scheme mechanism,surfaces plasma resonance effect,up-conversion of quantum dots and plasmonic metal oxide.And then these g-C₃N₄ based nanocomposites have been used for the photocatalytic removal of dyes,antibiotics and heavy metals to investigate the photocatalytic performance.Based on the experimental results,proper reaction mechanisms have been proposed.The detail research contents and experiment results of this dissertation have be summarized as follows:（1）Highly efficient Z-scheme Ag₂CrO₄/g-C₃N₄-N nanocomposites have been fabricated via a facial precipitation method.In the nanocomposite,g-C₃N₄ nanosheets can act as the substrate,which is good for the attachment of Ag₂CrO₄ nanoparticles on its surface.The experiment results demonstrated that the prepared Ag₂CrO₄/g-C₃N₄-N nanocomposite owns excellent visible light response ability and high activity for photocatalytic degradation of methyl orange（MO）and rhodamine B（RhB）under visible light,solving the problem of insufficient light response ability for g-C₃N₄.The optimal composites with the mass ratio of Ag₂CrO₄ to g-C₃N₄-N as 50%（CNA-50）showed the highest photocatalytic activity for MO degradation.Under the optimal condition,MO molecule can be completely removed in the presence of CNA-50within 21 min under invisible light irradiation,and the reaction constant reached0.0964 min^-1,which was 5.9 and 10.8 times than those of pure Ag₂CrO₄ and pure g-C₃N₄-N,respectively.TheformationofAg₂CrO₄/g-C₃N₄-NZ-scheme heterojunction and enhanced visible response ability contributed to the improved photodegradation efficiency.In the reaction process,the photogenerated electrons in the conduction band(E_CB)of Ag₂CrO₄ tend to transfer and recombine with the photogenerated holes in the valence band(E_VB)of g-C₃N₄,which can inhibit the recombination efficiency and maintain high redox ability.Furthermore,owing to the efficient transfer of the photogenerated electrons from Ag₂CrO₄ to g-C₃N₄-N,the photocorrosion of Ag₂CrO₄ nanoparticles is strongly suppressed,which makes the prepared Ag₂CrO₄/g-C₃N₄-N nanocomposites own excellent stability.（2）A novel metal-free phosphorus doped porous ultrathin carbon nitride nanosheets（PCN-S）were successfully prepared via the element doping and thermal exfoliation method.Various characterization results show that PCN-S owns lots of in-plane pores on its surface,large specific surface area and broad light response to the whole visible light region.The BET results show that the specific surface area of PCN-S is 102.5 m²g^-1,which is larger than pristine g-C₃N₄.The P atom can replace the N atom in g-C₃N₄,which makes the prepared PCN-S have broad light absorption ability in the whole visible light region.Then the prepared PCN-S are added in the mixed Cr（VI）/2,4-DCP reaction solution for simultaneous photocatalytic reduction of Cr（VI）and oxidation of 2,4-DCP.The result show that the initial concentrations of the Cr（VI）and 2,4-DCP will influence the photocatalytic degradation efficiency in a certain concentration range.When the mixture solution is composed by 20 mg/L of Cr（VI）and 80 mg/L of 2,4-DCP,the synergistic effect of the two pollutants reached the maximum,and all the Cr（VI）and 2,4-DCP can be removed within 120 min under visible light irradiation.Control experiment indicates that acid reaction condition and the dissolved oxygen can improve the photocatalytic activity of this reaction system,and PCN-S also owns good reutilization and stability.（3）To further promote the photogenerated electrons and holes transfer rate,BiVO₄ nanoparticles were adhered on the surface of PCNS to form PCNS/BVO composites via a one-pot impregnated precipitation method.Experiment results show that when 400 mg of PCNS is added,the prepared PCNS/BVO-400 photocatalyst presents the best performance,showing a tetracycline（TC）removal efficiency of96.95%and reaction constant of 0.0492 min^-1 within 60 min,higher than that of pure PCNS and pristine BiVO₄.Owing to the band gap matchment,the prepared PCNS/BVO composites conform to the Z-scheme reaction mechanism in the reaction process,which can promote the transfer and separation of the photogenerated electron and holes.Meanwhile,HPLC-MS/MS technology was used to identify the intermediate products,and results show that the N-demethylation process and hydroxylation process were the main degradation processes for TC in this reaction system.Mineralization experiment presents that this reaction system owns high mineralization ability and can completely decompose contaminate into CO ₂ and H₂O.The real water treatment experiment shows the great potential of the prepared PCNS/BVO composite for realistic wastewater treatment.（4）A novel ternary dual Z-scheme Ag@PCNS/BiVO₄ photocatalysts have been successfully fabricated by the photo-deposition of Ag into PCNS/BiVO₄ composites.Ag can be treated as the“electron bridge”for the transfer of photogenerated charges.The prepared Ag@PCNS/BiVO₄ composite achieve improved photodegradation rate for the ciprofloxacin（CIP）removal,presenting 92.6%removal efficiency and 0.0203min^-1 of reaction constant under visible light irradiation within 120 min,and also presenting enhanced photocatalytic activity under near infrared light irradiation.Besides,it presents good mineralization ability with 28.5%TOC removal efficiency of CIP.The remarkable photodegradation performance of prepared Ag@PCNS/BiVO ₄for CIP removal can be contributed to the following two reasons:1)The synergetic effect among the Ag,PCNS and BiVO₄ in ternary Ag@PCNS/BiVO₄ composite can enhance the visible light utilization ability;2)The strong local surface plasma resonance effect（LSPR）caused by Ag nanoparticles lead to the establishment of the local electric field,which contributed to the dual Z-scheme charge transfer pathway in the reaction process.This dual Z-scheme charge transfer mechanism can bring both fast transportation of the photogenerated electron-hole pairs and strong redox ability,which results in the remarkable photocatalytic performance of the prepared Ag@PCNS/BiVO₄.（5）A novel Ag/N-GQDs/g-C₃N₄ nanocomposite with full-spectrum light response ability has been successfully prepared via a simple process.The contents of both NGQDs and Ag NPs show great influence on the photocatalytic activity of the hybrid nanocomposites,and the optimum Ag/NGQDs/g-C₃N₄（with the content of 0.5%N-GQDs and 2.0%Ag NPs）presents a 92.8,90.11,and 31.3%remove efficienct for TC molecule under light irradiation with the wavelengths larger than 365,420 and760 nm,respectively,indicating that the prepared Ag/N-GQDs/g-C₃N₄ presents boosted photocatalytic performance not only in visible light but also in NIR light.The combination of the LSPR effect from Ag NPs with the up-converted effect of N-GQDs leads to the enhanced light transfer and absorption ability of the Ag/N-GQDs/g-C₃N₄nanocomposites,which results in the generation of quantities of photogenerated charges.The enhanced photocatalytic activity and removal efficiency are mainly attributed to synergistic effect among Ag,N-GQDs and g-C₃N₄,which not only enhances the light absorption and transformation process but also accelerate s photoexcited charge transportation and migration effect.（6）Novel Z-scheme W₁₈O₄₉/g-C₃N₄ nanograss composites have been successfully synthesized via the hierarchical assembly of nonstoichiometric tungsten oxide(W₁₈O₄₉)onto the surface of g-C₃N₄ nanosheets.The prepared W₁₈O₄₉/g-C₃N₄nanograss composites present high photocatalytic activity under both full-spectrum and NIR light irradiation,and show highly removal efficiency for bothcolorful methylene blue（MB）and colorless CIP.The boosted photocatalytic activity of the prepared W₁₈O₄₉/g-C₃N₄ nanograss composites originates from the unique dual-channel charge-carrier transfer path:Z-scheme-improved charge-carriers separation and plasmon-induced“hot electrons”injection process.The unique nanograss structure can bring more light reflection and refraction process,which improve the utilization of light energy.Control experiments demonstrated that the oxygen vacancies would greatly influence the photocatalytic activity of W₁₈O₄₉/g-C₃N₄ nanograss composites greatly,and H₂O₂ can consume the oxygen vacancy while NaBH₄ can recovery them.Radical experiment indicated that·OH and·O₂^-could both generated under full-spectrum or NIR light irradiation,which further demonstrate the co-existence of Z-scheme mechanism and“hot electrons”injunction process.