全有机COF/PUP S型光催化剂分解水制氢的性能研究

All-organic covalent organic frameworks/perylene diimide urea polymer S-scheme photocatalyst for boosted H_(2) generation

能源危机和环境污染已经成为了人类当前迫切需要解决的问题,太阳能是丰富而取之不尽的资源,合理高效地利用太阳能,是解决上述问题的理想途径之一.共价有机框架(COFs)光催化剂以地球上丰富的元素为基础,因其通过分子工程可实现光学和电子性质调节而受到广泛关注.由于具有高结晶度和有序多孔结构,COFs被设计和用于多种光催化应用.然而,载流子分离性能较差严重地制约了COFs的光催化性能.本文通过在低成本的有机氧化端苝二酰亚胺脲聚合物(PUP)上原位生长二维三嗪基亚胺连接的COF(TATF-COF)纳米片来构建一个全有机S型异质结光催化剂(TATF-COF/PUP),并用于可见光光催化分解水产氢.首先,以咪唑为溶剂通过溶剂热方法制备了PUP纳米带,在室温下以2,4,6-三(4-氨基苯基)-1,3,5-三嗪(TA)和三(4-甲酰苯基)胺(TF)为单体,在邻二氯苯/正丁醇(4:1 v/v,4.0 mL)溶剂中以三氟甲磺酸钪为催化剂合成了TATF-COF/PUP异质结光催化剂,并以抗坏血酸为牺牲剂测试了光催化剂的产氢活性.在可见光照射下,TATF-COF/PUP异质结光催化剂的最高可见光光催化分解水产氢速率为94.5 mmol g^(-1) h^(-1),是纯TATF-COF的3.5倍;并且经过连续8次循环反应(每次3 h)TATF-COF/PUP体系催化活性仅减少12.7%,表现出较好的稳定性.同时,在420 nm波长的光照下,表观量子效率达到了19.7%,该结果表明在TATF-COF/PUP光催化剂体系中,S型异质结形成较强的界面内建电场能够有效促进TATF-COF与PUP间光生电子-空穴对的分离.利用X射线衍射光谱、X射线光电子能谱、红外和固体碳谱等对TATF-COF/PUP体系的组成和结构进行分析,结果证实成功合成了TATF-COF和PUP,且两种材料很好地复合.通过紫外-可见漫反射光谱研究了样品材料的光吸收能力;通过瞬态和稳态表面光电压、稳态及瞬态荧光光谱等研究了材料的电荷载流子复合和转移行为,发现相比于纯TATF-COF,TATF-COF/PUP的光生电子空穴对具有更高的分离效率.通过理论计算、紫外光电子能谱和电子自旋共振等确定了材料内部电场的方向和在光照条件下光生载流子的迁移,进而明确了S型异质结的成功构建,且通过TATF-COF和PUP之间的耦合作用,极大地提升了TATF-COF光催化产氢的活性.本文通过构建S型内建电场,合理设计了紧密的全有机异质结界面,可以提高TATF-COF的光催化性能,从而加速光生电子在异质结中的分离和利用.基于S型异质结的耦合可以作为一种通用策略,扩展到各种COFs基半导体光催化剂的改性中,解决COFs基半导体载流子分离性能较差的问题,为COFs基高效光催化产氢材料的设计和开发提供参考.

Conjugated covalent organic frameworks(COFs)hold great promise in photocatalytic hydrogen evolution owing to their high crystallinity,large surface area,and distinct structure.However,COFs exhibit poor charge separation.Therefore,investigating highly effective COF-based photocatalysts is crucial.For the first time,conjugated COF/perylene diimide urea polymer(PUP)all-organic heterostructure with S-scheme interfacial charge-transfer channels was successfully developed and manufactured via in situ coupling of the two-dimensional triazine-based imine-linked COF(denoted as TATF-COF)with PUP.The optimal photocatalytic hydrogen-evolution rate of 94.5 mmol h^(-1) g^(-1) for TATF-COF/PUP is 3.5 times that of pure TATF-COF and is comparable to or even higher than that of the previously reported COF-based photocatalysts,resulting in an apparent quantum efficiency of up to 19.7%at 420 nm.The improved directional S-scheme charge transfer driven by the tuned built-in electric field and enhanced oxidation and reduction reaction rates of the photogenerated carriers contribute synergistically to the boosted photocatalytic H_(2) evolution.Experiments and theoretical studies reveal plausible H_(2) evolution and spatial S-scheme charge-separation mechanisms under visible-light irradiation.This study provides advanced methods for constructing all-organic S-scheme high-efficiency photocatalysts by the modulation of band structures.