Preparation Of Imines Covalent Organic Framework Based Composite Photocatalysts And Its Water Splitting Performance

With the massive use of fossil fuels,the energy crisis is becoming a huge challenge for global sustainable development.It has become an urgent problem to seek new clean energy to replace traditional fuel.Converting solar energy into hydrogen energy through photocatalysis is considered to be one of the technologies with great potential to solve the above problems.As a kind of organic semiconductor materials,Covalent Organic Frameworks（COFs）are porous crystalline polymers formed by organic structures linked by covalent bonds.Due to its periodic structural units,ordered and adjustable pore structure,high specific surface area,good chemical and thermal stability,wide range of visible light response,andπ-πconjugated structure,COFs has a broad application prospect in catalysis,energy storage,adsorption and other fields.Especially in the aspect of photocatalysis,COFs shows incomparable advantages by virtue of its own characteristics.In this paper,it is studied in the following two parts:（1）The step-scheme（S-scheme）heterojunction has an advantage in the field of photocatalysis because it can promote the separation of photogenerated electrons and hole pairs while retaining the strongest redox ability.In this paper,the S-scheme heterojunction was synthesized by the composition of covalent imine porous organic framework（COF-LZU1）and the prepared hollow cube CdS.By adjusting the proportion of each component,it was found that the composite with 1.5%COF content had the highest hydrogen production performance,with a hydrogen production rate of 8670μmol·g^-1·h^-1,about 2.1 times that of pure CdS,and the apparent quantum efficiency（AQE）was about 8.9%under 420 nm monochromatic light.Through systematic characterization analysis,it is found that there is a built-in electric field pointing from COF to CdS at the interface between CdS and COF.Under the action of the built-in electric field,the photogenerated electrons from the CdS conduction band recombine with the holes in the COF valence band,while the photogenerated electrons with strong reduction ability in the COF conduction band and the photogenerated holes with strong oxidation ability in the CdS valence band are retained,which is beneficial to the photocatalytic hydrogen evolution performance.This work provides a theoretical basis for the design and preparation of organic/inorganic S-scheme heterojunction photocatalysts.（2）Rapid charge recombination and low surface reaction kinetics are two main factors restricting the photocatalytic performance of covalent organic frameworks.In order to promote the charge separation of TpPa-1-COF,WO₃/TpPa-1-COF composites were prepared by in-situ growth method.Compared with WO₃ and TpPa-1-COF,the performance of composite photocatalyst was significantly improved.Especially when the WO₃ mass ratio in the composite is 3%,3%WO₃/TpPa-1-COF has the best photocatalytic hydrogen evolution rate,reaching 19.89 mmol·g^-1·h^-1,about 4.8 times that of pure TpPa-1-COF.Its AQE at 420 nm is 12.4%.X-ray photoelectron spectroscopy（XPS）showed that there was a built-in electric field at the interface between WO₃ and TpPa-1-COF,which drove the photogenerated carriers to migrate according to the S-scheme mechanism.Therefore,WO₃/TpPa-1-COF composites have high charge separation efficiency and strong redox ability,which is also confirmed by the photoelectric chemistry results.This work provides theoretical knowledge for constructing S-scheme heterojunction reasonably for photocatalytic cracking of aquatic hydrogen.