Rationally Tuning Triazine-based COFs With Electronic Effect Of Substituent For Visible-light Photocatalytic Degradation Of Tetracycline:Performance And Mechanisms

Discharge of industrial and agricultural wastewater resulted in the increased concentration of TC in aqueous environment,giving threat to human health.Compared with traditional methods（physical and biological method）,photocatalytic oxidation method displays great potential in application due to its many advantages such as high efficiency,purity,low energy consumption and easy to be operated.However,traditional photocatalysts still suffer from some limits 1)most of them are metal semiconductors and have low visible light utilization;2)the reported non-metallic materials（g-C₃N₄）have low visible light photocatalytic performance and their visible light photocatalytic performance cannot be tuned.Covalent organic frameworks（COFs）are a class of porous crystalline material.Itsπ-πlayered structure,large conjugated system and high surface area are beneficial for electron-hole transport and separation.Therefore,it exhibits great application potential in photocatalytic oxidation.Triazine-based COFs have been widely researched due to its good visible-light photocatalytic performance.However,there are still many shortcomings in triazine-based COFs such as broad energy band,low electron-hole separation efficiency and short lifetime of charge carriers.Therefore,we rationally introduce electron-donating groups（-R,R=OH,OMe）and electron-withdrawing group（-Br）into triazine-based COF-H with electronic effect of substituent,realizing adjustment of its structure and photoelectric properties,further tuning its visible-light photocatalytic performance.Based on this,we establish push-pull effect into triazine-based COFs with electronic effect of substituent,realizing enhancement of photoelectric properties of COF-Br,therefore,increasing its visible-light photocatalytic performance.The relationship between structure and performance of COFs tuned with electronic effect of substituent was elaborated.And the visible-light photocatalytic degradation of TC over triazine-based COFs was evaluated.Furthermore,the mechanism of visible-light photocatalytic degradation of TC over COFs was revealed.Researches in this study are displayed as below:（1）In this paper,we introduce electron-donating groups（-R,R=OH,OMe）and electron-withdrawing group（-Br）into triazine-based COF-H with electronic effect of substituent.After introduction of-OH,-OMe and-Br,the surface area was larger and more photocatalytic active sites were provided.The reaction rate constant（k）of visible-light photocatalytic degradation of TC over COF-R follows k_COF-Br=4.5 k_COF-OMe=20.6k_COF-H.After four cycles,the degradation efficiency of COF-OMe and COF-Br was only decreased by 7.5%and 7.7%within 30 minutes after illumination,respectively.Both experimental and theoretical results revealed that introduction of electron-donating side substituents improved the energy level,while electron-withdrawing side functionalities reduced the energy band level.Altogether,both electron-donating or-withdrawing substituents narrowed the band gap of COFs.Compared with electron-donating substituent,introduction of electron-withdrawing substituent lowered the CBM and VBM,significantly enhancing the electronic affinity and the oxidation ability of hole（h⁺）.Furthermore,introduction of electronic side substituents promoted charge carrier separation and surpassed their recombination without tautomerization.（2）In order to further improve the photoelectric properties of COF-Br,then improve its photocatalytic performance,the electron-donating R（R=OMe,OM₃e,OM₈e）was introduced into COF-Br,establishing a“push-pull”effect COF-Br@R.The degradation rate constant k of visible-light photocatalytic performance of COF-Br@R was 1.26,1.28 and 1.35 times of COF-Br and the mineralization was increased by 1.13,1.31 and 1.59 times,respectively.After four cycles,the degradation efficiency of COF-Br@R was only decreased by 7.3%,7.5%and 6.3%within 20 min after illumination.Introduction of push-pull effect increased charge carrier mobility in the smallest structure unit,electron hole separation efficiency and suppressed the electron hole recombination.Additionally,introduction of push-pull effect maintained a low VB position with simultaneous high CB position,which was beneficial for generation of h⁺with high oxidation capacity and formation of·O₂^-.Then electron utilization was improved,thereby the visible-light photocatalytic performance was enhanced.The toxicity of intermediates during degradation process was lower than TC in the COF-Br@R system and it was positive for environment.Based on the above research results,we tune the structure and photoelectric property of triazine-based COFs by introducing electronic effect of substituent via simply modification of structure.The visible-light photocatalytic degradation of TC over COFs was evaluated.Then the mechanism of substituents with electronic effect tuning the structure and performance of triazine-based COFs was revealed.Finally,the removal mechanism of tetracycline was revealed.It provides new ideas and technical theories for directional development of new carbon materials with good visible-light response.