Preparation Of Pyridyl COFs Nanosheets And Application Of Visible-light-driven Overall Water Splitting To Produce Hydrogen

Photocatalytic overall water splitting to produce hydrogen is considered to be a green and effective way to solve the current energy crisis and environmental pollution problems.In terms of thermodynamics,the overall water splitting process is an uphill reaction that cannot be spontaneously carried out.Therefore,a reasonable construction of an effective photocatalyst and the release of H₂ and O₂ by the visible light-driven overall water splitting reaction is an effective solution to break the bottleneck of the current photocatalytic overall water splitting.As a new class of crystalline porous materials,covalent organic frameworks(COFs)have the advantages of strong visible light absorption,high stability,adjustable structure and energy band,and have shown excellent photocatalytic performance.At present,in the presence of a sacrificial agent,the visible light-catalyzed performance of COFs for hydrogen or oxygen production has been very outstanding,but the overall water splitting of COFs and the production of hydrogen and oxygen have not yet been achieved.Although the energy level structure of many COFs meets the thermodynamic requirements of overall water splitting due to the lack of suitable hydrogen production and hydrogen production active sites,it is difficult to overall water splitting kinetics.In response to the above scientific problems,this paper constructed COFs with different N sites,studied the role of N sites in total water solution,and realized the overall water splitting performance of COFs for the first time.Constructed COFs nanosheets with similar structures but with different N sites,and researched the hydrogen production,oxygen generation and overall water splitting properties of COFs nanosheets with different N sites.Using[2,2'-bipyridine]-5,5'-diamine,2,5-aminopyridine,benzidine and 1,3,5-trialdehyde phloroglucinol reacts to construct TpBpy-COFs,Tp Pa-Py-COF and TpBD-COF.Pt nanoparticles are loaded as promoters by in-situ method,self-assembly method or photoreduction method,and the Pt-X%/COF-NS photocatalyst with nanosheet morphology is obtained by ultrasonic thin layer.The successful synthesis of crystalline COFs nanosheets and the loading of Pt nanoparticles were determined by characterizing the structure and morphology of a series of materials.The energy band structure characterization is used to determine that the series of materials meet the thermodynamic requirements of photocatalytic overall water splitting.The photocatalytic overall water splitting performance shows that only the TpBpy-COF material exhibits the properties of overall water splitting,and the TpBpy-COF nanosheets with 5%PtNPs in situ restricted loading show the best activity,the evolution rates of H₂ and O₂ reached 130?mol/g/h and 64?mol/g/h,the AQY at 450 nm is 2.8%,and the STH value is 0.23%under AM 1.5G simulated sunlight.In the presence of the sacrificial agent,Pt-5%/TpBpy-NS has the best hydrogen and oxygen evolution activity.In-depth research of the overall water splitting mechanism of bipyridyl COFs,and confirmed the synergistic effect between the pyridine N atom and the same ring C atom in the overall water splitting reaction.The photo/electrochemical characterization of a series of materials proves that the morphology of the nanosheets enhances the use of light and the migration of photo-generated charges on the two-dimensional conjugate plane,proves that confinement of PtNPs in the COFs channel can maximize the promotion of light separation and utilization of generated charges.Combination performance testing and photoelectric characterization results prove that the presence of N atoms in the pyridine structure can effectively construct polarity in the frameworks,thereby facilitating the separation of photo-generated charges.The results of density functional theory on the photocatalytic reaction active sites and catalytic mechanism show that in the TpBpy-COF framework,the N atom of bipyridine is the active site for hydrogen production and the C atom in the same ring is the active site for oxygen production.The absorption of hydrogen protons by the pyridine N atom in the overall water splitting reaction will increase the oxygen generation activity of the C site,indicating that hydrogen evolution and oxygen evolution sites can work together,thereby reducing the energy barrier of the overall water splitting reaction.