Study On Modification Of Molecular Structure Of G-C₃N₄ For Efficient Photocatalytic Hydrogen Evolution

With the rapid development of modernization level,energy crisis have become an important factor to affect people’s life and social development.Photocatalytic technology can convert sunlight to other as energy.Recent years,it has attracted extensive interests of researchers.However,traditional photocatalysts such as TiO₂,due to the wide band gap,can only be excited under ultraviolet light,which will affects the utilization of sunlight greatly.Graphite carbon nitride（g-C₃N₄）,as a metal free organic polymer semiconductor with the responds of visible light,high chemical stability and thermal stability,has become a research focus in the field of photocatalysis in recent years.However,the material has a narrow response range to visible light（the maximum absorption edge is about 460 nm）and a high recombination rate of photo-generated carrier,which will restricts the development of the catalyst.Taking into account the molecular structure and organic properties of graphite carbon nitride,a series of modified g-C₃N₄ with wide visible response range and low photo-generated carrier recombination rate were prepared by adjusting the molecular structure and grafting other organic monomers.The main results of this paper are listed as follows.（1）The modified g-C₃N₄ were obtained by thermal processing bulk g-C₃N₄ in HCl solution at different temperature.The ¹H solid NMR,zeta potential,BET,Raman and SEM were adopted to study the effects of the thermal treatment and HCl treatment to the structure of layered graphite carbon nitride.After thermal treatment in HCl solution,the formed H-bond network system in g-C₃N₄ was proposed,which will promote the proton exchange between g-C₃N₄ and H₂O,and improve the photocatalytic performance for hydrogen evolution.In addition,the effect of H-bond between H⁺and H₂O was studied by DFT calculation.Finally,after a simple calcination,the materials were restored to its original state,and the formed H-bond network system was broken.The results of photocatalytic performance for hydrogen evolution from the restored materials further certified the effect of the formed H-bond network system in g-C₃N₄.（2）g-C₃N₄ was treated in acetic acid at high temperature to improve its photocatalytic performance.Elemental analysis,¹³C NMR,EPR,TG-DTA and other characterization methods were used to analyze the composition and structural changes of the modified samples.The pretreatment led to the generation of nitrogen deficient g-C₃N₄,and the possible scheme process was proposed.In addition,the nitrogen deficient g-C₃N₄ promotes the photocatalytic performance for the degradation of RhB and evolution of hydrogen.（3）Taking into account the high similarity of structure between formamide and urea,formamide was selected as an organic monomer to copolymerize with urea,and then nitrogen deficient g-C₃N₄ was obtained.Meanwhile,XRD,FTIR,DRS,EPR and PL were adopted to investigate the morphology,structure and optical property of the prepared materials.The amount of nitrogen defects can be controlled by the variable time in the thermal polymerization process.The materials exhibited a wider range of visible light responds,reduced the recombination rate of photo-generated carriers.The formation of nitrogen deficient g-C₃N₄ was proved.The generated nitrogen defects and their contents play important roles on photocatalytic hydrogen evolution.In addition,the HOMO and LUMO of the nitrogen deficient g-C₃N₄ were investigated by DFT calculation.（4）As a kind of bidentate ligand,acetylacetone are very useful in the synthesis of heterocyclic compounds.In the study,acetylacetone was used as an organic monomer to polymerize with urea.¹³C NMR,EPR,DRS and XPS were adopted to analyze the structure,morphology and optical property of the products.The electron donor in CN network was introduced by the thermal polymerization process.The modified g-C₃N₄ exhibits excellent performance for photocatalytic hydrogen evolution under visible light irradiation.At 450 nm,apparent quantum efficiency as high as 18.8%was achieved.In addition,the fusion of acetylacetone in g-C₃N₄ was also found applicable to other precursors（dicyandiamide,thiourea）by the synthetic route.Optical absorption and photocatalytic hydrogen evolution under visible light was improved similarly,highlighting the universality of the current strategy.Density functional theory（DFT）calculations certified the proposed structure of incorporating acetylacetone in g-C₃N₄ and explained the enhanced ability to harvest photons due to the introduction of electron donor in CN network.