| The influence of population growth and industry development has caused global demand for energy to soar.With limited fossil fuel reserves and growing environmental concerns,converting solar energy into clean transportable fuels such as hydrogen is key to achieving sustainable development.As a clean,efficient,safe and sustainable new energy,hydrogen energy is regarded as the most promising clean energy in the 21st century and the strategic energy development direction of mankind.However,building a sufficiently efficient and stable system for converting solar to hydrogen energy remains a major challenge.Therefore,it is necessary for us to develop efficient catalysts to meet our hydrogen energy needs.Covalent organic frameworks（COFs）are a new kind of porous organic materials,which are composed of organic building units connected by strong covalent bonds.It is a kind of good crystalline porous structure material,which is used in many fields（gas storage,adsorption,photoelectricity and catalysis,etc.）.COFs have their unique chemical and physical characteristics,such as chemical tunability,nano-size pore structure,high specific surface area,conjugation,good crystallinity,etc.These advantages mean that COFs have the potential to become the pioneer of photocatalytic research.Although COFs have shown great development potential as photocatalysts,the research on COFs photocatalytic decomposition of aquatic hydrogen is still in the primary stage and faces many problems and challenges.In this paper,seven kinds of COFs were purposefully designed and synthesized.By starting from the structure-performance relationship,a new method to improve the efficiency of COFs photocatalytic decomposition of water was explored,and a new idea of COFs design and synthesis was developed.The specific research contents are as follows:（1）In this paper,three structure-related COFs with different proportions of hydrazone bond and ketone amine bond were constructed by fixing the main frame structure and changing the way of bonding groups,thus realizing a tunable COFs platform.The effects of ketone amine and/or ketone bond on COFs were compared.Different proportions of ketamine and hydrazone bond can change the structure order,visible light absorption range,band gap width,electron hole migration rate,etc.Under visible light irradiation,the stable rate of 12109.6μmol g-1 h-1 of pure ketenamine bonded COFs was much better than that of ketenamine bonded hydrazone bonded and pure hydrazone bonded counterparts(473.4 and 92μmol g-1 h-1).Importantly,through the combination of photoelectric chemical experiment and theoretical calculation reveals the ketene amine as the join key COFs has good photocatalytic activity,this is because the ketene COFs amine groups,has the stronger ability of electron delocalization,more efficient electron-hole separation and transfer ability,and in the light of inducing reduction reaction has a better forbidden band width.This conclusion provides an idea and method for the structural design of photocatalytic COFs.（2）The effective separation and transfer of photogenerated charges is a key objective to improve the photocatalytic activity of COFs.Although donor-acceptor systems are an attractive way to induce electron-hole separation,conjugated systems tend to lead to electron-hole recombination.In the natural photocatalytic systemⅡ,the photocatalytic efficiency is greatly improved due to the existence of two acceptors,so we want to introduce this strategy into COFs synthesis to improve the performance of photocatalytic hydrogen production.Here,four COFs were synthesized through the Schiff-base reaction to investigate whether the presence（simultaneous or not）of triazine and ketone as acceptors and the anilino as electron donor in COFs improve electron-hole separation efficiency.Evidence indicates that,when triazine and ketone are simultaneously present in the COF,electron delocalization is more extensive,which can lead to efficient charge separation and electron transfer to Pt-based co-catalysts for catalysis.The COF comprising two acceptors displayed the highest photocatalytic hydrogen production rate(6286.1μmol g-1 h-1-41.2 times and 3.4 times as large as those of the COFs containing only triazine or ketone,respectively)and the highest apparent quantum yield（0.64%at 420 nm）.Moreover,the influence of the donor-acceptor distance on the electron-hole separation was investigated by changing the length of a bridging biphenyl ring.Evidence indicated that the transport distance of a single phenyl group is more favorable for the catalytic reaction.This work affords an insight and support for the design of efficient COF photocatalysts. |
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