摘要
Designing high-efficiency photocatalysts by the construction of organic/inorganic heterojunctions is considered to be an effective approach for improving photocatalytic hydrogen evolution reaction(HER)activity.This work designed and built unique S-scheme heterojunctions by in-situ growing inorganic WO_(3) nanoparticles with excellent oxidation ability on fused-sulfone-modified covalent organic frameworks(FS-COF)with strong reduction ability.It is found that FS-COF and WO_(3) have a well-matched staggered band alignment.The best-designed FS-COF/WO_(3)-20%exhibits a maximum photocatalytic HER rate of 24.7 mmol g^(-1) h^(-1) under visible light irradiation,which is 1.4 times greater than the pure FS-COF.Moreover,photogenerated electron-hole pairs can be separated and utilized more efficiently thanks to the FS-COF/WO_(3) heterojunction's ability to create a favorable internal electric field resulting from the difference in work functions between FS-COF and WO_(3),which speeds up the transfer dynamics of photoinduced electrons from WO_(3) to FS-COF through an additional interfacial electron-transfer channel obeying the directional S-scheme migration mechanism.Furthermore,the S-scheme migration mechanism of photoinduced charge carriers instead of the type-II mechanism was confirmed by the signal intensity of•O_(2)−species from spin-trapping electron paramagnetic resonance spectra over the single component and the formed heterojunction.It ensures the photoexcited electrons maintain on the lowest unoccupied molecular orbital of FS-COF with a strong reduction ability to participate in photocatalytic HER,resulting in a significantly boosted H_(2) evolution rate.Based on organic/inorganic coupling,this work offers a strategy for creating particular S-scheme heterojunction photocatalysts.
光催化技术能将太阳能转化为清洁的化学能,被认为是解决环境污染和能源短缺问题的可行方法.近几十年来,开发高效的析氢光催化剂受到了广泛的关注.然而,无机金属氧化物光催化剂由于光学吸收利用效率有限、光生电子-空穴对分离效率低,且缺乏足够的驱动力满足过电位需求,限制了其在光催化析氢反应中的实际应用.因此,有必要开发具有可见光响应的新型有机化合物析氢光催化剂.共价有机框架(COFs)是一类有名的可见光驱动型析氢光催化剂,通过共价键连接有机结构单元而形成的结晶多孔聚合物.然而,大多数COFs表面都会发生严重的电荷载流子复合.为了增强COFs光催化剂中电荷载流子的分离,可以将COFs与具有适当能带结构的无机金属氧化物半导体结合起来,形成有机/无机杂化的COFs基异质结光催化剂,从而提高光催化析氢反应活性.本文采用溶剂热法在具有强还原能力的砜修饰共价有机框架(FS-COF)上原位生长具有优异氧化能力的无机WO_(3)纳米颗粒,设计并构建了独特的有机/无机杂化S型异质结.研究发现,FS-COF与WO_(3)具有很好的交错能带排列.最佳设计的FS-COF/WO_(3)-20%(即WO_(3)占FS-COF的质量百分数为20%)在可见光下的最大光催化析氢速率为24.7 mmol g^(-1)h^(-1),是纯FS-COF的1.4倍.并且FS-COF/WO_(3)-20%样品的光催化析氢速率超过了已报道的很多COFs基异质结.此外,由于FS-COF/WO_(3)的功函数不同,FS-COF/WO_(3)异质结能够产生有利的内建电场,因此可以更有效地实现光生电子-空穴对分离,从而通过一个额外的界面电子转移通道,加速光生电子从WO_(3)向FS-COF的转移动力学过程,该电子转移通道遵从定向S型迁移机制.此外,自旋捕获电子顺磁共振谱表明,在单组分和形成的异质结上的·O_(2)^(-)信号强度的对比验证了光诱导载流子遵从S型迁移机制而非Ⅱ型迁移机制.这保证了光激发电子保持在具有强还原能力的FS-COF的最低未占分子轨道上以参与光催化析氢反应,从而显著提高了H_(2)的析出速率.总之,本文通过原位溶剂热法将还原性FS-COF与氧化性WO_(3)纳米颗粒杂化,构建了具有定向内建电场的有机/无机杂化S型异质结.该异质结具有高效的界面电子传输通道,大大提升了光催化析氢速率.该策略不仅为有机/无机杂化光催化剂的构建提供了新思路,更为设计兼具强氧化还原能力与高效电荷分离特性的S型异质结体系奠定了一定理论基础,对开发高效太阳能-氢能转换材料具有重要参考.
基金
国家自然科学基金(21403184)
江苏省高等学校基础科学(自然科学)研究重大项目(22KJA430008).
