Preparation And Photocatalytic Properties Of Covalent Organic Frameworks Photocatalysts

Energy shortage and environmental pollution caused by the increasing consumption of fossil energy have become the two crucial issues in human society.Solar power is abundant and renewable in nature and converting solar energy into hydrocarbon fuels through artificial photosynthesis is one of the most ideal ways to solve these problems in the future.The semiconductors have obvious advantages in preparing solar fuels via photocatalytic water splitting and carbon dioxide reduction reactions.However,the development of semiconductors in practical applications is limited by the narrow spectral response to solar light,severe photogenerated charge recombination and limited catalytic sites.Covalent organic frameworks（COFs）are porous and crystalline materials with tunable structure and function,large specific surface area,ordered porosity and conjugated structure.COFs can be adjusted by using diverse organic ligands and metallic cites to enhance the light absorption range and optimize the microscopic morphology.The large specific surface area and ordered porosity provide more active cites for photocatalytic reaction,which are beneficial to the contact between the substrate molecules and the catalytic sites.The highly conjugated structure is beneficial to improving the efficiency of charge migration.Compared with the traditional inorganic semiconductors,COFs have the properties of both inorganic semiconductors and organic compounds.It is conducive to the study of the internal charges and energy transferring pathways and photocatalytic mechanism.Based on the strategy of electron-rich donor（D）and electron-deficient acceptor（A）,a series of porphyrin-based COFs and triazine-based COFs photocatalysts were designed and synthesized.The D-A structures in COFs promote the separation of photogenerated charges and improve the light absorption of the materials.The photocatalytic CO₂ reduction of porphyrin-based COFs and the photocatalytic water splitting of triazine-based COFs were conducted to investigate the structure-activity relationship,which provide effective strategies for the design of COFs photocatalysts.The main research contents are as follows:（1）Porphyrins have strong and wide absorption in visible light region.Based on the unique electronic structure and optical properties of porphyrins,the donor-acceptor two-dimensional covalent organic frameworks PD-COF was constructed via the Schiff-base reaction with 5,10,15,20-tetra-（4-aminophenyl）porphyrin（TAPP）as electron donor and 2,5-Dibutyl-3,6-bis（4-formylphenyl）-pyrrolo[3,4-c]pyrrole-1,4-dione（DPP-CHO）as electron acceptor.At the same time,the NiPD-COF was constructed with5,10,15,20-tetra-（4-aminophenyl）porphyrinato Ni（II）（NiTAPP）to compare the morphology and photocatalytic efficiency.The results of photocatalytic CO₂ reduction experiment prove that NiPD-COF shows higher CO production efficiency of 40μmol g^-1 h^-1 than PD-COF of 20.9μmol g^-1 h^-1 within an operation time of 25 hours,in the absence of any additional photosensitizers or noble metallic co-catalyst.From the results of morphology characterization,NiPD-COF material is of multilayer nanosheet structure,which is beneficial to shorten the transmission pathway of photo-generated charges from the inside of the material to the surface.The photoelectric performance test results confirm that the NiPD-COF has more efficient charge separation and transport.Density functional theory（DFT）calculations are used to study the migration pathway of photogenerated electrons.The internal reductive quenching cycle of NiTAPP fragments for NiPD-COF leads to its higher photocatalytic efficiency than that of PD-COF.（2）The transition metal complexes are widely used as photocatalyst in photocatalytic CO₂ reduction,due to their unique photoelectric performance and redox properties.To increase the quantity of photocatalytic active sites,the porphyrin based covalent organic frameworks containing bipyridine group were constructed with5,10,15,20-tetra-（4-aminophenyl）porphyrinato Zn（Zn TAPP）and 5,5’-Diformyl-2,2’-bipyrine as starting materials.The ZnPBp-COF can be easily decorated with transition metal complexes with the bipyridine blocks acting as the reacting cites.In this chapter,we use three complexes cis-Ru（bpy）₂Cl₂,[C₅（CH₃）₅RhCl₂]₂ and Ir₂（ppy）₄（μ-Cl）₂ as starting materials forming three new COFs.ICP-OES measurement is used to determine the metal contents.ZnPBp-COF-Ru shows the highest photocatalytic efficiency of CO₂reduction at a rate of 96μmol g^-1 h^-1for CO and 66μmol g^-1 h^-1for CH₄.The characterization results of morphology show that the crystal sizes of ZnPBp-COF-Ru,ZnPBp-COF-Rhand ZnPBp-COF-Ir are smaller than the precursor,which shorten the pathway of charge transferring to the surface of COFs materials.The Ru（bpy）₂²⁺,[RhC₅（CH₃）₅]³⁺and Ir（ppy）₂³⁺in frameworks acts as photocatalytic active sites,which can also enhance the photocatalytic CO₂ reduction efficiency.（3）The triazine units in the frameworks with the features of high conjugation and electron withdraw effect can enhance the efficiency of charge migration.Covalent triazine frameworks（CTFs）with conjugated structure and high nitrogen content are confirmed to possess enormous potentials in photocatalytic water splitting.In this chapter,the new covalent triazine frameworks（PhBp-CTF）with triazine as linker was constructed under a mild condition.The PhBp-CTF shows excellent H₂ production efficiency with a rare of 3175μmol g^-1 h^-1.The PhBp-CTF-Ir was synthesized through the complexation reaction with Ir₂（ppy）₄（μ-Cl）₂.The photocatalytic experiment reflects that PhBp-CTF-Ir shows a higher rate of H₂ generation at 4805μmol g^-1 h^-1.The higher photocatalytic efficiency attribute to the higher charge mobility in the PhBp-CTF-Ir and Ir（ppy）₂³⁺ligand as proton reduction catalyst.