Energy funneling and charge separation in CdS modified with dual cocatalysts for enhanced H_(2) generation

双助催化剂负载CdS的能量捕集和电荷分离增强产H_(2)

Anchoring molecular cocatalysts on semiconductors has been recognized as a general strategy to boost the charge separation efficiency required for efficient photocatalysis.However,the effect of molecular cocatalysts on energy funneling(i.e.,directional energy transfer)inside semiconductor photocatalysts has not been demonstrated yet.Here we prepared CdS nanorods with both thin and thick rods and anchored the conjugated molecules 2‐mercaptobenzimidazole(MBI)and cobalt molecular catalysts(MCoA)sequentially onto the surface of nanorods.Transient absorption measurements revealed that MBI molecules facilitated energy funneling from thin to thick rods by the electronic coupling between thin and thick nanorods,which is essentially a light‐harvesting antenna approach to enhance the charge generation efficiency in the reaction center(here the thick rods).Moreover,MBI and MCoA molecules selectively extracted photogenerated holes and electrons of CdS nanorods rapidly,leading to efficient charge separation.Consequently,CdS/MBI/MCoA displayed 15 times enhanced photocatalytic H_(2) evolution(1.65 mL)than pure CdS(0.11 mL)over 3 h of illumination.The amount of H_(2) evolution reached 60 mL over 48 h of illumination with a high turnover number of 26294 and an apparent quantum efficiency of 71%at 420 nm.This study demonstrates a novel design principle for next‐generation photocatalysts.

利用半导体光催化分解水产氢是将太阳能转换为化学能的有效策略之一.然而,现有催化剂体系的太阳能-氢能转化率较低,制约了人工光合作用的长远发展.因此,急需开发一种新的捕光策略通过储存和释放化学能来提高能源利用效率.由于无机半导体具有低的激子结合能,光生电子-空穴对在室温下可以迅速解离.因此,只要相邻的半导体畴是电子耦合的,并提供驱动力,能量迁移就可以通过独立的电子和空穴传输过程有效进行.然而,在半导体光催化剂中利用能量捕集(即定向能量迁移)的系统很少被涉及.尽管分子助催化剂常被负载在半导体光催化剂上来提高电荷分离和催化效率,但分子助催化剂对半导体光催化剂内部能量捕集的影响尚未被阐明.本文制备了粗细不同的CdS纳米棒,并将共轭分子2-巯基苯并咪唑(MBI)和钴分子催化剂(MCoA)依次锚定在纳米棒表面.CdS和CdS/MBI的透射电子显微镜照片表明,CdS纳米棒的直径不是完全均一的,主要集中在20–50 nm.由HRTEM照片观察到修饰MBI分子后CdS表面出现一层无定形膜,进一步修饰MCoA分子后CdS表面的膜厚变化不明显.XPS谱结果表明,修饰了MBI的CdS,其Cd 3d和S 2p均呈现向高结合能迁移的趋势,证实了CdS和MBI间存在一定的相互作用.此外,CdS/MBI/MCoA的Co 2p峰相比于钴天冬氨酸呈现向低结合能迁移的趋势,揭示了其电子密度的增加.CoA,CdS/CoA和CdS/MBI/MCoA的XANES和EXAFS谱可以证实CoA分子与MBI分子存在相互作用,而不是直接键合在CdS纳米棒的表面.在瞬态吸收谱中,修饰了MBI的CdS在490 nm处的漂白峰变弱,而位于500 nm处的漂白峰变强.同时,B2相比于B1的形成动力学曲线有一个轻微的延迟.由此证实,MBI分子在CdS上的修饰加速了电荷由细棒向粗棒的迁移,这增强了反应位点的电荷产生效率.此外,修饰了MBI的CdS其动力学曲线呈现一个加速的衰减,与加入(NH_(4))_(2)SO_(3)的衰减趋势一致,从而证实了MBI的空穴捕获作用.与单纯的CdS相比,CdS/MBI/MCoA催化剂动力学曲线呈现明显的衰减,且与加入电子捕获剂BQ的整体效果一致,说明MCoA分子的电子捕获作用.瞬态吸收谱揭示了MBI分子借助粗细纳米棒之间的电子耦合加速了从细棒到粗棒的能量捕集,这本质上是一种提高反应中心(此处为粗纳米棒)电荷产生效率的光捕集天线方法.MBI和MCoA分子选择性地快速捕获CdS纳米棒的光生成空穴和电子,从而实现了高效的电荷分离.活性测试结果表明,CdS/MBI/MCoA光照3 h后的光催化产H_(2)活性(1.65 mL)比纯CdS(0.11 mL)高15倍.光照48 h后,H_(2)的生成量达到60 mL,并且转化数为26294.在420 nm处的表观量子效率达到71%.综上,本研究为未来光催化剂的发展提供了一种新颖的设计思路.