摘要
Carbon nitride(C_(3)N_(4))is promising for photocatalytic hydrogen production,but photogenerated electrons and holes in C_(3)N_(4)usually tend to exist as excitons due to intrinsic Coulomb interactions making its photocatalytic activity unsatisfactory.Herein,a well‐designed intramolecular C_(3)N_(4)‐based donor‐acceptor(D‐A)photocatalytic system was constructed to promote exciton dissociation.Due to its good chemical compatibility with melamine and appropriate sublimation property,2‐amino‐4,6‐dichloropyrimidine unit was chosen as the monomer to react with melamine to construct intramolecular D‐A system(CNCl_(x)).The hydrogen evolution rate of CNCl_(0.15)is 15.3 times higher than that of bulk C_(3)N_(4)under visible light irradiation,with apparent quantum efficiency of 13.6%at 420 nm.The enhanced activity is attributed to introduced electron‐withdrawing−Cl group as terminal group in the resulted CNCl_(x) samples,which can build internal electric field to promote the exciton dissociation into free electron and hole.In addition,lower work function value of CNCl_(x) samples indicates that internal electric field can help free electrons and holes transfer to the surface of CNCl_(x) samples for photocatalytic reaction.
聚合物氮化碳(C_(3)N_(4))因具有可见光响应特性、良好的化学稳定性、无毒性等优点而成为一类极具吸引力的光催化剂.遗憾的是,由于本征库仑相互作用,C_(3)N_(4)中的光生电子和空穴通常以激子的形式存在,导致迁移到表面的光生电子和空穴数量减少,从而降低了光催化活性,因此人们做了大量的研究工作来促进激子解离成自由电子和空穴.D-A体系可以诱导内部电场的产生,从而促进激子解离成自由电子和空穴,因此,构建供体-受体(D-A)体系是一种有效地促进激子解离的方法.然后在内电场作用下,自由电子和空穴也能够更加容易地转移到共聚物表面,从而发生相应的光催化还原和氧化反应.本文选择了2-氨基-4,6-二氯嘧啶(C_(4)H_(3)Cl_(2)N_(3))作为单体,与三聚氰胺共聚形成分子内共聚物(CNCl_(x))来构建D-A体系.由于分子结构相似,C_(4)H_(3)Cl_(2)N_(3)与C_(3)N_(3)(NH_(2))_(3)分子具有良好的化学相容性.在共聚过程中,C_(4)H_(3)Cl_(2)N_(3)在219~222℃升华,三聚氰胺在300℃升华,在温度继续升高到550ºC的过程中,气相混合物充分混合并发生共聚反应.在共聚过程中,如果C_(4)H_(3)Cl_(2)N_(3)分子与C_(3)N_(3)(NH_(2))_(3)反应,那么三聚氰胺沿着这个方向的聚合将终止,因此吸电子-Cl基团将全部位于共聚分子的末端.相较于体相C_(3)N_(4),CNCl_(x)样品活性均有所提高,且随着-Cl基团数量的增加,CNCl_(x)样品活性先提高后降低,其中CNCl_(0.15)样品活性最高.CNCl_(0.15)在可见光下的析氢速率是体相C_(3)N_(4)的15.3倍,在420 nm处的表观量子效率为13.6%.对RhB,MO和苯酚的降解速率分别为体相C_(3)N_(4)的5.82,7.93和9.53倍.构建分子内D-A体系以后,C_(3)N_(4)活性提高主要是因为随着末端-Cl基团的增加,材料的吸光能力和激子解离效率提高.而且-Cl基团也可以充当电子的俘获位点,浓度进一步升高会降低电荷转移的效率使活性降低.EIS的奈奎斯特图和i-t曲线结果表明,CNCl_(0.15)的电弧半径最小,光电流最大,说明其具有最低的电阻和最高的载流子传输效率.紫外光电子能谱测试结果表明,CNCl_(x)功函数值较小,电子更容易在内部电场的作用下移动到表面,而过量的-Cl基团增加了CNCl_(0.2)的功函数值,导致CNCl_(0.2)样品的光催化活性降低.
基金
国家自然科学基金(21676193,21978200,51661145026).
