Preparation Of Composites Based On Covalent Organic Framework For Photocatalytic Hydrogen Production

The massive use of fossil fuels,on the one hand,leads to its rapid depletion,and on the other hand,causes serious environmental pollution.Therefore,the development of clean energy to replace fossil energy is the focus of research in energy,environment,chemical industry and other fields.Hydrogen energy is considered as an ideal clean energy because of its high combustion value and clean product.Over the past few decades,converting and storing solar energy in the form of H₂ energy by splitting water with light has developed into a key technology.In order to effectively drive the proton reduction half reaction,the development of high activity and low cost photocatalyst is a major challenge.Covalent organic framework（COFs）is a new kind of porous organic polymer.It is composed of light elements,with low density,chemical tunability and nano-sized pore structure.The excellent crystallinity of COFs will facilitate the internal charge transfer and inhibit the recombination of carriers.Most importantly,the high crystallinity of COFs can minimize the charge capture at defects.Especially,COFs is composed of covalent bonds and can exist stably in strong acidity or basicity,oxidation and reduction environments.As a consequence,COFs is considered as a potential photocatalytic material.Compared with some traditional single-component inorganic semiconductors,two-dimensional COFs（2D COFs）have good photochemical activity,extendedπconjugated system and larger specific surface area.Nevertheless,due to the exciton effect caused by the Coulomb interaction between photoelectron-hole pairs,the high recombination rate makes it difficult to further enhance the H₂ generation activity of 2D COFs.At present,a lot of work has shown that the composite photocatalyst composed of two or more kinds of semiconductor materials can accelerate the separation and migration of photocarriers and boost the photocatalytic activity.Based on this,in this paper,a typical ketoenamine based 2D COFs material（TpPa-1-COF）is selected as the matrix.A series of photocatalytic materials with better H₂ evolution performance were obtained by combining TpPa-1-COF with tin disulfide（SnS₂）,nickel phytate（PA-Ni）and perylene-3,4,9,10-tetracarboxylic diimide（PDINH）semiconductor composites.XRD,FT-IR,XPS,SEM,TEM,DRS and photoelectric chemical analysis were used to investigate the morphology,structure,energy band and charge separation of the prepared composite photocatalyst,revealing the photocatalytic H₂ evolution mechanism of the developed composite photocatalyst.The research contents of this paper are as follows:（1）The TpPa-1-COF photocatalyst was synthesized by mechanical grinding method.Then,a novel 2D-2D SnS₂/TpPa-1-COF nanocomposite was prepared by hydrothermal route.The structure and morphology of the prepared products were characterized by XRD,FT-IR,XPS,SEM and TEM.The results of photocatalytic H₂ generation under simulated solar illumination（λ≥420 nm）illustrate that 2D-2D SnS₂/TpPa-1-COF heterojunctions not only speed up the separation of photogenerated charge carriers but also facilitate the H₂production kinetics and expand the range of visible light response to orange light（600 nm）.Especially,the maximum photocatalytic H₂production rate of the 2D-2D SnS₂/TpPa-1-COF heterojunction without the addition of cocatalyst Pt reaches 37.11μmol h^-1,which is 21.7-fold and 2-fold higher than those of individual TpPa-1-COF and 3 wt%Pt/TpPa-1-COF,respectively.The enhanced catalytic activity can be attributed to the formation of heterojunctions with well-matched band positions and the higher charge transfer efficiency of 2D-2D layered structures.In addition,the photochemical test further verified that SnS₂/TpPa-1-COF heterojunction can effectively promote the migration and separation of interfacial photocarriers.（2）PA-Ni was used as a cocatalyst to enhance the photocatalytic performance of TpPa-1-COF materials.The PA-Ni/COF composites with different PA-Ni additions were successfully synthesized by morphology and structure analysis.A series of PA-Ni/COF composite materials show excellent photocatalytic activity.The material containing 5 wt%PA-Ni（5%PA-Ni/COF）has the best H₂ production rate of 20.64μmol h^-1,12.1 times of that of pure TpPa-1-COF.The H₂generation rate of 5 wt%Pt as cocatalyst（5%Pt/COF）is twice as high as 11.41μmol h^-1.At the same time,PA-Ni/COF composites still maintained high H₂ evolution activity after repeated H₂production tests.Through photochemical and electrochemical tests,it is confirmed that PA-Ni/COF composite material has improved the light absorption capacity and photocarrier separation capacity,thus promoting the improvement of its photocatalytic H₂ generation performance.（3）The organic semiconductor PDINH with TpPa-1-COF band matching was selected.It can dissolve in concentrated sulfuric acid and then self-assemble into supramolecular and crystallize out in water.Therefore,PDINH/COF pure organic Z-type heterojunction was constructed for the first time by in-situ growth of self-assembled PDINH supramolecules on the surface of TpPa-1-COF.The results of photocatalytic H₂ evolution test show that the best ratio of PDINH/COF composite（2:20）has a H₂ generation rate of 17.23μmol h^-1,which is 10 times that of pure TpPa-1-COF.The construction of heterojunction can effectively enhance the performance of photocatalytic H₂ evolution.The band matching between PDINH and TpPa-1-COF was demonstrated by band analysis and photoelectrochemical test,and the electron transport mode was proved to be direct Z-type.