Modification Of G-C₃N₄-based Photocatalysts For Hydrogen Evolution Reaction

Human society faces three major problems,including environmental pollution,resource shortage and ecological destruction.The scope of urbanization is constantly expanding,and the types of industrialization are also increasing.The development of the two has brought many conveniences to society and people's lives.However,the above progress and development have also brought some obstacles to people's lives.For example,in addition to too much industrial waste gas being discharged into the air,many industrial solid wastes are also discarded at will.The development of industry is inseparable from the support and demand of energy.Due to the limited amount of energy resources in China,in order to alleviate energy pressure,researchers are working hard to develop some sustainable use of green energy.Hydrogen energy has always been a clean energy carrier.Compared with other energy supply methods,the process of obtaining energy through hydrogen energy and utilizing it is not only environmentally friendly,but also consistent with the theme of sustainable development in today's society.Therefore,clean energy,which combines the advantages of low cost,environmental protection and renewable energy,is considered to be a suitable substitute for fossil fuels.Hydrogen energy is a widely distributed,abundant,green and efficient energy carrier that can be obtained by converting solar energy into chemical energy.It is a sustainable energy carrier with great development potential.Experiments in which H₂ and O₂ were produced on TiO₂ single crystal electrodes under near-ultraviolet light made it possible to produce hydrogen and oxygen by conversion between photo-chemical energy.This major discovery has prompted researchers to become interested in semiconductor catalysis and is trying to find some new materials that can be used to catalyze hydrogen evolution.At present,the focus of photocatalytic research is the development and preparation of low photo-generated carrier recombination rate,high physicochemical stability and high quantum yield catalyst.Graphite phase carbonitride?g-C₃N₄?,as a novel metal-free polymer organic semiconductor with a layered structure,has become a key research semiconductor because of its excellent photochemical stability and unique electronic structure.However,the inherent disadvantages of low visible light response,low specific surface area,and high photogenerated electron-hole recombination rate hinder the further development of g-C₃N₄.In view of the above deficiencies,various properties?bandgap,morphology,specific surface area?of g-C₃N₄ was improved by modification methods such as inorganic acid acidification,nucleobase copolymerization and biological template-assisted transition element doping.During the experiment,we analyzed the materials from apparent phase?SEM,XRD,TEM?,optical properties?FT-IR,UV-vis?,electronic valence?XPS?,and porous structure?BET,TG?.The main work is as follows:?1?Self-assembly synthesis of g-C₃N₄ with abundant nitrogen defects for photocatalytic hydrogen evolutionCatalysts with different morphologies were synthesized by hydrothermal treatment under different acidic conditions with melamine as the precursor.During the process,melamine is hydrolyzed,and the precursor and hydrolyzate are self-assembled by hydrogen bonding to form a supramolecular intermediate.After high temperature calcination,g-C₃N₄ with abundant nitrogen deficiency is obtained.The prepared sample has reduced electron-hole recombination rate and an increased fluorescence lifetime due to the expanded?-?conjugated structure of the polymer.Optical absorption characterization indicated that samples formed by surface self-assembly showed significantly extended light absorption in the visible region compared to the original g-C₃N₄.CN-10 samples exhibit excellent catalytic performance in photocatalytic hydrogen evolution reactions due to the dual advantages of their internal connections and external tubular structures.The formation of hydrogen bonds promotes the energy transfer between molecules and the delocalization of excited states.The introduction of defects introduces new energy levels under the conduction band of the catalyst,and improves the quantum yield in the catalytic process.The self-assembled tubular structure has both channel effect and shielding effect,which provides a fast transmission channel for photogenerated electrons and a barrier to suppress photo-electron-hole pair recombination.Finally,based on a series of characterization results,the NH₃-template-assisted crimping mechanism under hydrogen bonding was proposed.?2?Nucleobase copolymerization to prepare g-C3N4 with large specific surface area for photocatalytic hydrogen evolutionA sponge-like porous carbonitride with an increased specific surface area was prepared by a green synthesis method using melamine and five nucleobases as precursors.Benefiting from the good biocompatibility of nucleobases,CN-U₃₀ has a specific surface area of up to 127 m²/g,which is 12.7 times that of bulk g-C₃N₄?10m²/g?.The increased specific surface area corresponds to a rich reactive site,so CN-U₃₀ exhibits excellent hydrogen evolution performance.?3?Preparation of Mo-doped g-C₃N₄ via biological template for photocatalytic hydrogen evolutionMo-doped polymer carbonitride was synthesized using uracil as a biological template and melamine as a precursor.During the process,uracil forms a chelate intermediate with Mo⁺⁶.The electronic configuration of Mo⁺⁶ is[Kr]4d⁰5s⁰,and its empty orbitals can act as electron acceptors to accept electrons from pyrimidine ring donors.Synergistically,an electron acceptor-donor built-in electric field is formed,its successful construction not only promotes the rapid transfer of photogenerated electrons,but also provides a large number of active electrons for the reduction reaction.Combined with the results of XRD,FT-IR,EDS,solid-state NMR and XPS analysis,we proposed a hypothetical mechanism for the possible chelation-hydrogen bond coordination of bio-templated Mo-doped g-C₃N₄,in which doped Mo not only provided some new catalytically active centers,but also extended the 2D conjugated system of polymeric carbonitrides into the 3D space.