Modification Design Of G-C₃N₄ For Photocatalytic Water Spliting Hydrogen Evolution

In many fields of utilizing solar energy,photo-chemical energy conversion based on photocatalytic reduction technology is considered to be one of the effective ways to make full use of solar energy.The realization of efficient photocatalytic water splitting to produce hydrogen with high calorific value is a very challenging topic in the field of materials and energy.As the core of photocatalytic water splitting for hydrogen evolution,the design and development of efficient,stable and low-cost photocatalysts has become a research hotspot.As a typical two-dimensional polymer semiconductor,graphitic carbon nitride（g-C₃N₄）has suitable energy band structure,good stability and unique electronic structure,and is a potential photocatalytic semiconductor material.However,the thermodynamic and kinetic defects of g-C₃N₄ in the field of photocatalysis,such as insufficient photoresponse ability,poor charge separation and transfer performance,and slow surface reaction rate,lead to unsatisfactory efficiency of photocatalytic water splitting for hydrogen evolution.Therefore,exploring ways to optimize the thermodynamics and kinetics in the photocatalytic process from the aspects of electronic structure,charge carrier transport efficiency,and surface reaction kinetics,and are crucial for improving the performance of g-C₃N₄-based photocatalytic water splitting for hydrogen evolution.In this dissertation,the porous g-C₃N₄ nanosheets modified with organic functional groups such as cyano and amino groups were firstly synthes ized with the assistance of inorganic ammonium salts.O n this basis,the influence mechanism of the construction of microstructure and the modification of different functional groups on the light absorption,charge separation and transfer performance,and surface reaction kinetics of g-C₃N₄ was studied.Useing The functional group-modified porous g-C₃N₄nanosheets as matrix materials,to light absorption properties,charge separation and transfer properties were further optimized by constructing different t ypes of heterojunctions.The structure and properties of the samples were characterized,and the charge separation and transfer processes and the mechanism of photocatalytic water splitting for hydrogen evolution were explored.The main research work and results are as follows:（1）The g-C₃N₄ （HCN-5）with a“bibendum-like”structure was synthesized by introducing ammonium acetate during the synthesis process,which consists of microtubes formed by regular encircling nanotubes.During the synthesis process,the hydrolysis and pyrolysis of ammonium acetate releases acetic acid,acetamide and ammonia gas to form micro-tubes in a"volcanic eruption-like"mechanism.Acetamide self-assembles and inducing plane bending through hydrogen bonding forms nanotubes.First-principles calculations show that the difference in quantum confinement effect in the axial and radial directions of the nanotubes leads to the directional transfer of photogenerated electrons along the axial direction,which promotes the separation of photogenerated charges.The results show that the photocatalytic water splitting for hydrogen evolution efficiency of HCN-5 under 350-780 nm light irradiation reaches169.6μmol·h^-1,about 10 times that of the bulk g-C₃N₄(17.28μmol·h^-1).（2）The functional group-modified porous g-C₃N₄ nanosheets（NCN-x（x=0.5,1,2,3,5））were one step synthesized with NH₄Cl-assistance.The study found that the amount of NH₄C l has a great influence on the microstructure of NCN-x and the amounts of functional groups introduced.When the mass ratio of NH₄C l to melamine is 5:1,the total pore volume of the prepared NCN-5 can reach 0.457 cm³·g^-1,the thickness is less than 50 nm,and a large amount of-C≡N,-OH and-NH₂ are present in NCN-5.Due to the existence of-C≡N,-OH and-NH₂,the electronic transition type in NCN-x adds n-π*and n-σ*transitions on the basis of singleπ-π*transition,and the band gap gradually decreases from 2.70 e V in bulk g-C₃N₄ to 2.20 e V.In the plane,the electron donating group-πplane-electron withdrawing group forms the electron transfer path of D-π-A.Due to the hydrogen bonding effect and the reduction of the interlayer spacing,the transport resistance of photogenerated carriers between adjacent planes is reduced.The optimization of these two aspects effectively improves the charge separation and transfer efficiency.The photocatalytic water splitting for hydrogen evolution efficiency of NCN-5 under the light irradiation of 350-780 nm reached 266.8μmol·h^-1,which was about 15 times that of bulk g-C₃N₄（17.28μmol·h^-1）.（3）The composite aerogel（CNGO）of NCN-5/graphene oxide（GO）was constructed by hydrothermal method,and it was decorated with g-C₃N₄ quantum dots（CNQDS）to obtain a CNGO/CNQDS three-dimensional composite photocatalytic materials.Its microstructure and performance were characterized and tested.The results show that NCN-5 and GO from a heterostructure through hydrogen bonding and van der Waals forces.Due to the good electrical conductivity and“hot electron”effect of GO,the photogenerated electrons generated by NCN-5 and GO can rapidly transfer and enrich in the active sites on the surface of NCN-5.The construction of aerogels effectively prevents agglomeration of 2D heterostructures.CNQDs can not only effectively improve the utilization of light at 650-850 nm,but also facilitate local charge transfer,and can themselves serve as catalytically active sites.The photocatalytic hydrogen production efficiency of CNGO/CNQDs at 350-780 nm and700 nm were 330.7μmol·h^-1 and 1.57μmol·h^-1,respectively.（4）The NCN-5/WC Schottky heterojunction（WCCN）was constructed by using NCN-5 as the matrix material and compounding with WC with metal-like properties,and its microstructure and properties were studied.The results show that the space potential difference at the heterointerface facilitates the rapid extraction of photogenerated electrons from NCN-5 by WC through the Schottky junction and promotes the separation of photogenerated electrons and holes.The high DOS of WC d orbital electrons and the built-in electric field caused by the potential difference between different crystal planes make the photogenerated electrons still have hydrogen evolution activity.This makes the hydrogen production efficiency of WCCN under350-780 and 800-2500 nm il umination 2 and 5 times that of NCN-5,respectively.（5）Oxygen vacancy-rich non-stoichiometric tungsten oxide(WO_3-x)was prepared by oxidizing WC nanoparticles,and it was combined with NCN-5 to construct a heterostructure（WON）.The microstructure and properties of the samples were characterized and analyzed.The results show that oxygen vacancies lead to strong metal-like localized surface plasmon resonance（LSPR）in WO_3-x.There is a C-O bond-like effect between the C atoms in NCN-5 and the O atoms in WO_3-x at the heterojunction interface.The WON integrates two charge transfer mechanisms,the semiconductive S-scheme charge transfer and LSPR“hot electron”injection process similar to traditional plasmonic metal/semiconductor system,which not only improves photogenerated carrier separation,but also exhibits UV-Vis-NIR-driven H₂ evolution activity.The photocatalytic hydrogen production rates of WON under the illumination of 350-780 nm and 800-2500 nm reached 542.9μmol·h^-1 and 1.2μmol·h^-1,respectively.The AQ Y values reached 25.64%and 23.12%under 365 and 380 nm monochromatic light,respectively.