Fabrication Of Graphitic Carbon Nitride Based Photocatalysts For Degradation Of Organic Pollutants And Ammonia Production

With the rapid development of industry and the acceleration of urbanization,environmental pollution and energy shortage are the main problems that need to be urgently solved.Semiconductor photocatalysis technology has shown the great potential in the fields of pollutant removal,energy conversion and organic synthesis,which is attributed to the intriguing properties of less energy consumption,mild reaction conditions,sustainable development and high reaction efficiency.The efficient semiconductor photocatalysts are the key point of photocatalysis technology.Graphitic carbon nitride(g-C₃N₄),as a kind of layered organic metal-free polymer materials,is mainly composed of carbon(C)and nitrogen(N)elements,which is integrated with the properties of simple preparation and non-toxicity.Owing to the moderate band gap,physicochemical stability and strong regulation ability,it is intensively applied in photocatalysis fields.However,the practical application of g-C₃N₄ is limited by high charge carrier recombination,less active sites,and relatively weak optical harvest ability.In this study,ultrathin g-C₃N₄ nanosheet was prepared by thermal exfoliation strategy,and a series of g-C₃N₄-based hybrid composites were prepared through doping elements,coupling with semiconductors and introducing defects.The composition,morphologies,charge carrier separation and optical properties of the prepared materials were measured by various characterizations.Furthermore,the photocatalytic activities of as-synthesized materials were investigated by degradation of organic pollutants and nitrogen fixation.Based on the experimental results,the possible photocatalytic mechanism for the enhanced photocatalytic performance was proposed.The detailed experimental results and research contents are summarized as follows:(1)The ultrathin g-C₃N₄ nanosheet photocatalyst was fabricated through thermal exfoliation method,and 2D/2D Z-scheme heterojunction composite of Bi₅O₇I/g-C₃N₄was successfully designed through in-situ precipitation strategy.Bi₅O₇I/g-C₃N₄heterojunction photocatalyst shows excellent photocatalytic performance for degradation of ciprofloxacin(CIP).Among all the prepared photocatalysts,Bi₅O₇I/g-C₃N₄ hybrid composite shows the highest photocatalytic performance.Under visible light irradiation,the degradation efficiency of CIP can reach to 84.7%.The improvement of the photocatalytic performance of Bi₅O₇I/g-C₃N₄ composite is attributed to the increased yield of reactive oxygen species(ROS),which is beneficial from direct Z-scheme heterojunction.The difference of the working functions between Bi₅O₇I and g-C₃N₄ catalysts causes charge alignment,promoting the formation of built-in electric field(BIEF).When the hybrid composite is excited by visible light,BIEF compel the charge transfer from type-II into Z-scheme model,enhancing charge carrier separation.Therefore,the catalyst can possess excellent redox ability,promoting ROS production,and further improving the photocatalytic performance.In addition,the unique 2D/2D contact between Bi₅O₇I and g-C₃N₄catalyst shortens the transfer distance of charge carriers,further enhancing charge carrier separation efficiency.(2)The ultrathin g-C₃N₄nanosheet was prepared by thermal exfoliation method assisted with HNO₃,and Z-scheme composite of Ag@Ag₃PO₄/g-C₃N₄/r GO was fabricated through photoreduction through in-situ precipitation strategy assisted with photoreduction.Among all the prepared catalysts,Ag@Ag₃PO₄/g-C₃N₄/r GO composite shows the highest photocatalytic performance for degradation of paracetamol(APAP).Under visible light irradiation,about 84.2%of APAP is degraded over Ag@Ag₃PO₄/g-C₃N₄/r GO composite.The degradation process is consistent with pseudo first-order kinetics model,and the reaction rate constant is4.5716 min^-1.The enhancement of the photocatalytic performance is mainly due to the highly efficient charge carrier separation,the enhancement of the optical harvest ability and the high adsorption ability between organic pollutants and solid catalyst.Based on the trapping experiments,it is clearly found that·O₂^-and h⁺are the main functioned active species in the photocatalytic reaction systems.Furthermore,Ag NPs act as charge transfer mediator in the prepared hybrid composite.The local surface plasma resonance effect(LSPR)extends the optical harvest ability,possibly result in high amount of photoinduced charge carriers.r GO is favorable for capturing photoinduced electrons,promoting the highly efficient charge carrier separation.Furthermore,r GO can enhance the adsorption ability between organic pollutants and solid catalyst,which is beneficial to increasing the contact between active species and pollutants.(3)The ternary photocatalyst of Ag I/Bi OI/g-C₃N₄ was prepared by in-situ precipitation strategy.Under visible light irradiation,the ternary composite can be self-assembled into Ag@Ag I/Bi OI/g-C₃N₄ dual Z-scheme composites.The dual Z-scheme heterojunction promotes the spatial distribution of charge carriers and maintains the excellent redox ability,which is beneficial to the formation of active species.Therefore,the photocatalytic performance of the prepared composite is enhanced.Compared with single and binary catalysts,Ag I/Bi OI/g-C₃N₄ composite shows the highest photocatalytic performance for degradation of tetracycline(TC).The degradation rate of TC can reach to 90.2%under 50 min visible light irradiation.The degradation process is in accordance with pseudo first-order kinetic model,and the corresponding reaction rate constant is 0.0389 min^-1.Based on trapping experiments and ESR measurements,it is clearly found that·O₂^-and h⁺are the main active species in the reaction system.However,·OH has weak effect on the photocatalytic reaction activity.The recycle experiments show that the prepared composite possesses excellent stability,which may have promising potential in environmental remediation.(4)Boron(B)doped defective ultrathin g-C₃N₄ nanosheet(BNUCNx)was prepared by the calcination method assisted with Na BH₄.As an anionic defect site,nitrogen vacancy(Nvc)can effectively enhance the chemisorption of N₂,providing abundant active sites for the cleavage of N?N.Furthermore,Nvc can capture photoinduced electrons,promoting the charge carrier separation and reducing the interface transfer resistance of charge carriers.Based on DFT calculation results,the introduction of B element can further improve the chemisorption of N₂.In addition,cyano(-C?N)can shorten the band gap energy,which is favorable for improving the utilization efficiency of solar energy.The experimental results show that the prepared catalyst possesses excellent photocatalytic performance for nitrogen fixation,and NH₃ conversion efficiency is 435.28?mol g^-1 h^-1.DFT calculation results show that nitrogen defect and B dopant can effectively improve the chemisorption of N₂.The increased N-N bond length of adsorbed N₂ can weaken the stability of N?N,which is favorable for the polarization and cleavage of N₂.(5)The hybrid composite of BiO QDs/UCNx composite was successfully prepared through electrostatic interaction.When the mass ratio of Bi O QDs is 6.7%,the hybrid photocatalyst(Bi UCNx-5)shows the highest photocatalytic performance for nitrogen fixation with NH₃ conversion efficiency of 573.25?mol g^-1 h^-1.The enhancement of the photocatalytic performance is mainly attributed to the dual active sites of N₂ and the spatial charge carrier separation.When UCNx closely contacts with Bi O QDs,the electrons tend to transfer from UCNx to Bi O QDs until their Fermi levels achieve into equilibrium state.The charge difference between UCNx and Bi O QDs leads to the formation of built-in electronic field(BIEF),and the band structure of UCNx bends upward.When Bi UCNx-5 is excited by visible light,photoinduced electrons on UCNx tends to flow into Bi O QDs driven by BIEF.Attributed to the presence of Schottky barrier,the electrons accumulated on Bi O QDs are not easy to return back to the UCNx.In addition,nitrogen defects are favorable for chemisorbing N₂ and capturing photoinduced electrons,further enhancing NH₃ conversion.More importantly,the spatial charge carrier separation tends to promote high amount of energetic electrons in the charge-carrier-involved photocatalytic reaction,avoiding electrons flow into defect states in the solid catalyst.