Design And Preparation Of Two-demensional Tp-COF And Its Composite Materials For Photocatalytic Hydrogen Production

Visible-light-driven water splitting provides a one-stone-two-bird strategy to obtain hydrogen that not only can reduce the emission of carbon dioxide,but also solve the energy crisis.Since the pioneering work of Fujiashima et al in 1972,semiconductor photocatalysts have attracted widespread attention in the field of photocatalysis.In the past decades,domestic and overseas scientists have devoted themselves to the development of high-efficiency and low-cost semiconductor photocatalysts.Among them,some semiconductor photocatalysts show excellent performance of photocatalytic hydrogen production.However,there are still many obstacles,such as low visible light absorbance,low surface area,fast charge carrier recombination and photo-corrosion in the traditional semiconductor photocatalysts,limiting their wide application.Covalent Organic Frameworks（COFs）has become the research hotspot of a new generation of semiconductor photocatalysts due to its high surface area,outstanding visible-light absorption,excellent physicochemical stability and structural designability.In recent years,the research of COF photocatalysts has achieved remarkable success.A series of COF photocatalysts have been successfully developed and show excellent hydrogen evolution under visible-light irradiation.The preeminent photocatalytic hydrogen evolution of COF photocatalysts can be attributed to the following four points:ⅰ.The large conjugated structure is conducive to the absorption of visible light;ⅱ.The porous frame structure provides more active sites for mass transfer;ⅲ.Structural designability enables the adjustment of light absorption,band structure and photocatalytic performance;ⅳ.Theπ-πinteractions between the neighbor layers provides another path for the transport of charge-carriers.However,due to the high recombination rate of photogenerated electrons and holes in pure COFs and the low carrier utilization,which limits their practical application to a certain extent.Therefore,this paper took Tp-COF（Tp:1,3,5-triformylphloroglucinol,β-keto-enamine-based COF）photocatalysts as the research object,and improved the photocatalytic hydrogen production of Tp-COF photocatalyst through the following three strategies.（1）Taking Tp-DB-COF（DB:4,4’-diaminobiphenyl benzidine）as framework object,the influence of the electronic effects of functional groups on the structure,morphology,photoelectric properties and photocatalytic hydrogen production of Tp-DB-COF were investigated from the molecular level.In this work,a series of Tp-DB-X-COFs（X=H₂,（CH₃）₂,（OCH₃）₂ and（NO₂）₂）photocatalysts were successfully synthesized.The results show that the introduction of different functional groups has little effect on the structure of Tp-DB-COF,but will affect the morphology,specific surface area and light adsorption of materials.Tp-DB-（OCH₃）₂ shows the highest hydrogen evolution rate of 1.23 mmol g^-1 h^-1,followed by Tp-DB-（OCH₃）₂(0.81 mmol g^-1 h^-1)and Tp-DB(0.60 mmol g^-1 h^-1),and Tp-DB-（OCH₃）₂ with the lowest hydrogen evolution of 0.015 mmol g^-1 h^-1.Through a series of mechanism exploration experiments,it is found that the introduction of electron donating group in Tp-DB can enhance the conjugation of its structure,which is beneficial to the transport of photogenerated carriers in the covalent layer and between layers.The separation efficiency of photogenerated electrons and holes in COFs is improved,and consequently,the photocatalytic hydrogen evolution rates of materials are also significantly enhanced.（2）The noble metal-free cocatalyst was deposited on the surface of Tp-Pa-2-COF（Pa-2:2,5-dimethyl-p-phenylenediamine）by in-situ photodeposition.A series of M（M=Ni,Co and Cu）/Tp-Pa-2 composite photocatalysts were constructed and the process of in-situ photodeposition of some metal was investigated.The results show that,compared with pure Tp-Pa-2-COF,the visible-light absorption and photocatalytic hydrogen evolution rates of all M/Tp-Pa-2 composite materials are enhanced.Among them,3%Ni/Tp-Pa-2 exhibits the best photocatalytic hydrogen production performance(2.89 mmol g^-1 h^-1)in the presence of sodium ascorbate as sacrificial agent.In this system,the first 3 h is the induction process,involving the hydrolysis of Ni（OAc）₂ to form Ni（OH）₂.After the induction period,the hydrogen production tends to be stable and is still stable after 32 h.A series of mechanism exploration show that the high activity of Ni/Tp-Pa-2 composite photocatalysts can be attributed to the efficient transfer of photogenerated electrons generated by Tp-Pa-2 to the cocatalyst（Ni（OH）₂）,which improves the separation efficiency of photogenerated,and thus enhancing the activity of photocatalytic hydrogen production.This work not only improves the photocatalytic hydrogen production of Tp-COF,but also realizes the synchronous process of deposition of noble metal-free cocatalyst and hydrogen production.（3）In order to improve the separation and transfer efficiency of photogenerated carriers and construct the noble metal-free photocatalytic system,the two-dimensional conductive metal organic frameworks（MOFs,Cu₃（HHTP）₂）and Tp-COF（Tp-Pa-1,（Pa-1:p-phenylenediamine））were combined by constructing heterojunction strategy.A series of Cu₃（HHTP）₂/Tp-Pa-1 hybrid photocatalysts with different load of Cu₃（HHTP）₂ were successfully synthesized through the coordination of Cu ions with the carbonyl oxygen and enamine nitrogen groups in Tp-Pa-1.As a results,the optimized photocatalyst with Cu₃（HHTP）₂:Tp-Pa-1 ratio of 1:15 achieves the highest hydrogen evolution rate of 1.76 mmol·h^-1·g^-1 under visible-light irradiation,which is about 93 times higher than that of the pure Tp-Pa-1 and even slightly higher than that of the Tp-Pa-1 with Pt（3 wt%）as a cocatalyst.Additionally,the hybrid photocatalyst exhibits high stability and excellent cycling performance.The mechanism analysis shows that the Cu₃（HHTP）₂/Tp-Pa-1 hybrid materials can significantly promote the charge transfer and inhibit the recombination of photogenerated electron-hole pairs,thus effectively improving the hydrogen evolution rates.In this paper,the hydrogen evolution activity of Tp-COF photocatalysts was successfully improved by the regulation of electronic effects of functional groups,in-situ supported noble metal-free cocatalysts and constructing MOF/COF heterojunction.It provided a theoretical basis for the design and development of novel photocatalysts with high efficiency and low lost,and promotes the application of Tp-COF photocatalysts.