Preparation And Hydrogen Production Of Upconversion Enhanced NaGdF₄@Tp-COFs Photocatalyst

A large number of uncontrolled exploitation and use of fossil fuels will produce a large amount of greenhouse gases,which has caused a series of environmental problems.Hydrogen is considered as an alternative to fossil energy due to its advantages of renewable,environmentally friendly and high energy density.Photocatalytic decomposition of aquatic hydrogen has become a hot research direction in this field because of its environmental protection and no secondary energy consumption.Covalent organic framework（COFs）materials have the advantages of high stability,large specific surface area,adjustable structure and energy band,and have become an excellent catalyst in the field of photocatalytic hydrogen production.However,COFs also have the disadvantages of photo-generated carrier recombination and poor sunlight response.To solve the above scientific problems,this paper selects the classical upconversion nanoparticle luminescence materials,and constructs the Schiff base TpPa-COF as the model to construct the composite material,and realizes the efficient photocatalytic hydrogen evolution activity.The main research contents are as follows:1.Firstly,NaGdF₄-1 with anti-Stokes luminescence properties was coated in the TpPa-2-COF layer structure,and NaGdF₄-1@TpPa-2-COF composite was constructed.NaGdF₄-1 was used to enhance the response ability of the composite in the near infrared light region,and improve the photocatalytic hydrogen evolution activity.Hexaseptic NaGdF₄-1was prepared by solvothermal method,NaGdF₄-1 was activated by liquid phase method to obtain NaGdF₄-1,which was then put into the synthesis system of TpPa-2-COF to obtain NaGdF₄-1@TpPa-2-COF series composites.Its structure and morphology were analyzed by a series of physical characterization.The luminescence range of NaGdF₄-1 and the band structure of NaGdF₄-1@TpPa-2-COF were determined by UV-vis-NIR diffuse reflectance,upconversion luminescence spectrum and band potential analysis.It was proved that the composite meets the thermodynamic requirements of photocatalytic hydrogen evolution.The photocatalytic hydrogen production activity test showed that the hydrogen production rate of NaGdF₄-1（15%）@TpPa-2-COF composite reached 39.32mmol·h-1·g-1,which was 12.1 times of that of TpPa-2-COF alone（3.25 mmol·h-1·g-1）.2.In order to broaden the range of sunlight response and further enhance the charge transfer ability,TpPa-1-COF with higher conduction potential than NaGdF₄-1was selected,and NaGdF₄-1 was coated in TpPa-1-COF,and then NaGdF₄-1@TpPa-1-COF composite was prepared.Similar to the previous preparation methods,NaGdF₄-1@TpPa-1-COF series composites were prepared in situ by solvothermal method,and their structures and morphologies were analyzed through a series of physical characterization.The luminescence range of NaGdF₄-1 was determined by UV-vis-NIR diffuse reflectance and upconversion luminescence spectra,and it can be absorbed and utilized by TpPa-1-COF.The energy band structure of the composite material was calculated,and it was proved that the composite material can construct heterojunction and meet the thermodynamic requirements of photocatalytic hydrogen evolution.The test of photocatalytic hydrogen production activity at 420<λ<800 nm showed that the hydrogen production rate of NaGdF₄-1（15%）@TPa-1-COF composite reached 26.17 mmol·h-1·g-1.Was 11.53 times that of TpPa-1-COF alone（2.27mmol·h-1·g-1）;The test of photocatalytic hydrogen production activity atλ>420 nm showed that the hydrogen production rate of NaGdF₄-1（15%）@TPa-1-COF composite reached 57.13 mmol·h-1·g-1,which was 25.29 times that of TpPa-1-COF alone（2.27mmol·h-1·g-1）.Through a series of photoelectric tests and TEM characterization,the heterojunction types and electron transfer sites of the composites were further determined,and the photocatalytic mechanism was obtained.