Controllable Exfoliation And Photocatalytic Hydrogen Evolution Of Covalent Triazine Frameworks

As we all know,solar light-driven photocatalytic hydrogen evolution based on water splitting provides a promising route to meet the ever-increasing energy demands while simultaneously mitigating environmental issues.The development of efficient and environmentally friendly photocatalysts has always been the key to this technology.Recently,the emerging organic semiconductors—covalent organic framework compounds（COFs）have attracted wide attention from researchers in this field because of its good physical and chemical stability,high exciton-and charge-mobilities and adjustable structure and properties as well as other unique advantages,which is considered to be a promising organic photocatalyst.At present,the utilizing of some highly promising COFs in the field of photocatalysis has made pioneering progress.However,these methods involve the use of rare precursors to control the structural properties of the catalyst,and the experimental conditions are usually harsh.Based on the status of COFs-based materials in the field of photocatalysis,we developed a simple and effective chemical stripping method to regulate the photocatalytic activity of covalent triazine frameworks（CTFs）.In this paper,CTFs nanosheets and defects-rich CTFs nanosheets were successfully prepared,and then were applied in the field of photocatalytic water splitting.The main research contents and results of this article are as follows:（1）We reported the exfoliation of bulk-like CTFs（obtained by a superacid catalyzed method）with the assistance of using sulfuric acid as effective intercalant to prepare few-layered CTFs nanosheets（FL-CTF-H₂SO₄）,and which was applied in photocatalytic water splitting to produce hydrogen.The experimental results showed that the exfoliation of bulk-like CTFs into few-layered CTFs nanosheets could shorten the migration distance of photogenerated carriers inside CTFs,effectively suppress the electron-hole recombination,and thus enhance the performance for hydrogen evolution from water splitting of few-layered CTFs nanosheets.（2）Based on the above results,adjust the energy band structure of the few-layered CTFs nanosheets by introducing defect energy levels to further optimize the photocatalytic activity is of few-layered CTFs nanosheets.With the assistance of using sulfuric acid and ammonium persulfate as effective intercalant and mild oxidant,respectively,we exfoliated the bulk-like CTF into few-layered CTFs nanosheets with defects,which was applied in photocatalytic water splitting to produce hydrogen.Both the experiments and density functional theory（DFT）calculations revealed that controlling the relative content of defect states in few-layered CTFs nanosheets by the reaction time could adjust the energy band position and band structure of few-layered CTFs nanosheets,thus further optimizing its performance of photocatalytic water splitting to produce hydrogen.When the reaction time was 2h,few-layered CTFs nanosheets with the best photocatalytic activity（FL-CTF-2）could be obtained with an apparent quantum yield of 11.14%and a hydrogen evolution rate of 1527μmol h^–1 g^–1.The excellent photocatalytic activity of FL-CTF-2 can be attributed to the introduction of defect states,which enhanced its ability to absorb visible light,and effectively inhibited the recombination of internal photogenerated electron and hole pairs.