Research Of Novel G-C₃N₄ Based Photocatalysts And Mechanism Of Increased Solar Fuel Performance

Photocatalytic technology is expected to convert abundant solar energy into green and clean energy（solar fuel）that is easy to store and transport,through water splitting to produce H₂ and CO₂ reduction to produce hydrocarbons.Designing efficient and robust photocatalysts and system is the key to promoting photocatalysis development.Among numerous semiconductor photocatalysts,graphite carbon nitride（g-C₃N₄）is considered as a promising semiconductor photocatlyst owning to many advantages,such as abundant sources of raw materials,simple preparation process,stable physical and chemical properties,and suitable energy band structure.Nevertheless,the large band gap and polymeric characterisitics lead to some shortcomings of g-C₃N₄,such as small specific surface area,severe photo-generated carrier recombination,and narrow visible light absorption range,which resulting in poor photocatalytic efficiency.This article aims to improve the photocatalytic H₂ production and CO₂ reduction performance of g-C₃N₄.A novel hirerachical g-C₃N₄ strucutre with promoted charge separation and transportation has been developed.In addition,a functional groups grafted g-C₃N₄ strucutre has also been developed,which shows enhanced visible light absorption and charge transfer efficiency.An efficient photocatalytic system based on functional groups grafting g-C₃N₄coupling with local electric field polarization has been further constructed.Meanwhile,the activity enhancement mechanisms of as-sythesized photocatalysts and system were revealed.1.To aviod long preparation cycle and unenvironmental protection of template method in the construction of nanostructures of g-C₃N₄,a template-free method for hydrothermal post-treatment of g-C₃N₄ with ammonium nitrate（NH₄NO₃）is developed.The prepared H-CN shows a novel hierarchical structure with vertical lamellar surface.The experimental results suggest that the formation of these vertically-oriented nanosheets are affected by the addition of NH₄NO₃ in hydrothermal system,which can provide the acidic environment and NO₃^-to bond with heptazine ring on the surface of g-C₃N₄.Due to higher specific surface area,better crystallinity,and promoted carrier separation efficiency,its hydrogen production rate under visible light is increased by 4.1 times as compared with unmodified g-C₃N₄.2.The supramolecular self-assembly method is applied to surmount the limited kinetic mass transfer of traditional bulk polymerization.A new supramolecular precursor is prepared by introducing 1-amino-2-propanol to regulate the hydrothermal process of dicyandiamide.After calcining at high temperature,the hierarchical tubular g-C₃N₄(HCN_0.7)modified with ethyl groups was obtain.The experimental and computational results show that the modified structure has many advantages.The introduced ethyl functional group can not only expand the visible light response range,but also introduce an intermediate energy level in the energy band,which increased the light absorption in the region of 450-560 nm.The generation of ethyl functional group in structure can redistribute the electron cloud,which lead to the promoted separation and migration efficiency of photo-generated electron-hole pairs.Numerous mesopores and an enlarged specific surface area can provide more active sites,which is benefit for the diffusion and adsorption process of substances.Therefore,the photocatalytic H₂ production activity of HCN_0.7 under visible light reaches 13.84 mmol/h/g（AQE at 420 nm is 18.25%）,which is 30.1 times higher than that of unmodified g-C₃N₄.3.Based on previous work,diisopropanolamine was further applied to control the structure of supramolecular precursor.A hierarchical porous carbon nitride structure with grafted ethoxy functional groups was synthesized,and polar MgO（111）was loaded on its surface to promote the local electric field polarization.The as-sythesized sample exhibits the production rate of photocatalytic CO₂ reduction to CO is as high as 122.15μmol/h/g,which is55.6 times higher than that of BCN.The experimental and theoretical calculation results show that the grafting ethoxy groups help to enhance light absorption ability,increase photo-generated charge transportation and utilization,promote CO₂ adsorption capacity,and improve reduction reaction capacity.The coupling with MgO（111）that introduce local electric field can further enhance the separation efficiency of photogenerated carriers and delocalized electron concentration.Therefore,the as-prepared sample exhibits excellent activity.This work provides new method for improving the electronic structure and surface properties of g-C₃N₄ to construct efficient photocatalytic system.