Investigation On Preparation And Properties Of Covalent Organic Frameworks Photocatalysts

Energy crises and environmental pollution require people to find clean,pollution-free renewable energy as soon as possible.Hydrogen is known as the future oil because of its energy efficiency,green pollution and other characteristics.Light-driven water splitting into hydrogen is considered to be the most ideal way to obtain hydrogen energy,while efficient and stable photocatalyst is the key to achieve this technology.In recent years,covalent organic frameworks?COFs?have become a very promising semiconductor photocatalyst material due to its good light absorption and suitable band gap.However,the shortcoming of high photogenerated electrons and holes recombination rate limits its application.Therefore,we firstly introduce different functional groups on the TpPa-COF-X backbone and explore its effect on photocatalytic activity.Subsequently,due to better photocatalytic activity,TpPa-COF is selected to combine with NH₂-UiO-66in order to form a covalently bonded heterojunction material.The main contents are as follows:?1?A series of TpPa-COF-X?X=-H,-?CH₃?₂,and-NO₂?were carried out by the1,3,5-triformylphloroglucinolandcorrespondingdiamine?Paraphenylenediamine for TpPa-COF;2,5-dimethyl-p-phenylenediamine for TpPa-COF-?CH₃?₂;2-nitro-1,4-phenelynediamine for TpPa-COF-NO₂?under solvothermal conditions.We systematically investigated the differences in morphology,light-absorption intensity and band gap of these 2D COFs.The electrochemicalimpedancespectra,photocurrent-timeprofilesand photoluminescence spectrum show that the separation ability of photogenerated charges was also followed the order of TpPa-COF-?CH₃?₂>TpPa-COF>TpPa-COF-NO₂.The results of photocatalytic H₂ evolution measurements indicate that the TpPa-COF-?CH₃?₂ shows the maximum photocatalytic activity of 8.33 mmol g^-1 h^-1 is approximately 6 times and 38 times higher than the TpPa-COF and TpPa-COF-NO₂,respectively.The best photocatalytic H₂ production performance of TpPa-COF-?CH₃?₂ in these systems should be mainly attributed to the weak electron-donating ability to the conjugated system,which result in more efficient charge transferring in the inner of the material.?2?By covalently anchoring NH₂-UiO-66 onto the surface of TpPa-COF,a new type of MOF/COF hybrid materials with high surface area,porous framework and high crystallinity was synthesized.Fourier transform infrared spectra and solid-state ¹³C CP/MAS NMR spectroscopy show that NH₂-UiO-66 and TpPa-COF formed hybrid material by covalent bond;UV-vis spectroscopy and flat band potential analysis show that NH₂-UiO-66 and TpPa-COF could form type II heterojunction.The electrochemical impedance spectra,photocurrent-time profiles,photoluminescence spectrum and surface photovoltage spectroscopy suggest that NH₂-UiO-66/TpPa-COF hybrid materials can effectively promote the separation and transport of photogenerated electrons and holes.We also investigated the photocatalytic properties of NH₂-UiO-66/TpPa-COF materials with different ratios.The hydrogen production rate of NH₂-UiO-66/TpPa-COF?4:6?was as high as23.41 mmol g^-1 h^-1.It is about 20 times higher than that of the parent TpPa-COF and the best performance photocatalyst for H₂ evolution among various MOF-and COF-based photocatalysts.