量子点表面配体相关的电荷转移的研究

Photocatalysis reactions have shown its unique capability in solving energy problem, environment pollution and developing new synthetic chemistry in organic chemistry. Quantum dots, due to their excellent and flexible tenability in structural, optical and electrical properties, have been regarded as ideal candidates as photosensitizer/catalyst. However, the advantages of quantum dots in photocatalysis reactions are far from been fully realized, mainly because of the unintentionally controlled structure of quantum dots for photocatalysis reactions. Surface ligands would increase the distance between quantum dots and acceptors, leading to slow charge transfer rate. Because of the smaller dielectric constant of ligand layer than that of inorganic components, the influence of ligand layer on charge transfer rate would be much larger than that of inorganic components. Additionally, the motion of acceptor molecule would be suppressed due to enhanced spatial hindrance within the ligand layer. This would lead to useless charge transfer circle. Considering the significant effect of ligand layer on charge transfer, we propose that to obtain quantum dots for efficient photocatalysis reactions, surface ligand should be preliminarily optimized. Aiming at simultaneously obtaining long-lived charge separation and efficient mass transported, this study would explore the effect of ligands on the diffusion process of acceptor molecule within ligand layer, and their contributions to integral charge separation circle. With these knowledge, further optimizing inorganic component would be possible.

光催化在解决能源危机、缓解环境污染以及发展新的有机合成方法等方面有不可替代的作用。量子点因其优异、可调的结构和相应的光、电性质，是光敏化剂/催化剂的理想材料。然而现有的量子点的光催化效率都不高，一个重要的原因是没有为光催化量身定制合适的量子点。量子点表面致密的配体会增加量子点与有机分子间的距离，减少电荷转移的速率。由于更小的介电常数，相对无机部分，配体层的对电荷转移速率的影响更大。此外，配体层还会限制有机分子的运动，导致无效的电荷转移循环。因此要得到高性能的量子点光敏化剂/催化剂，首先要优化量子点表面配体。本项目将着重研究有机分子在量子点配体层内扩散行为对两者间完整的电荷分离过程的影响。希望通过本研究就能够明确配体的影响机制，优化同时得到长时间的电荷分离态和高的物质传输效率，为进一步优化量子点无机部分作为光敏化剂/催化剂提供基础。

尽管量子点作为光催化剂已经成功应用于能源、环境、精细合成等领域，其低下的光催化效率限制了工业化应用。配体通过与量子点表面原子配位，不仅可以改变量子点的表面态，减慢激子复合速率，还为量子点提供溶液分散性，增加量子点与反应物的接触，但配体的存在会阻碍反应物靠近，减慢电荷转移和物质传输。本项目研究了配体动力学在量子点与受体分子发生电荷转移过程中的影响。当配体足够时，强配体如羧酸盐在室温下会在表面形成致密的配体层，导致只有少量受体分子可以进入到配体层的空隙中，引起准静态电荷转移。弱配体如脂肪胺由于和量子点表面较弱的结合能力，可以引起吸附电荷转移和碰撞电荷转移。吸附、准静态和碰撞电荷转移的转移速率依次减慢，但准静态电荷转移的物质传输速率最慢，然后依次是吸附和碰撞电荷转移。减少配体虽然有利吸附电荷转移和碰撞电荷转移，但是产生的表面悬键却会增加非辐射复合的速率。可控组装量子点不仅可以减少表面悬键，还能提高光生电荷的分离效率。用CdSe/CdS核壳量子点作为光催化剂加氢还原2,3,5-三甲基对苯醌，用脂肪胺取代原本的羧酸镉配体，可以提升5倍以上的量子点的光催化效率，而多孔聚集体则可增加50倍以上的光催化活性。以苯甲醇为质子源，520nm绿光为激发光，2,3,5-三甲基对苯醌到2,3,5-三甲基对苯酚的TOF可达每个量子点18000 h-1以上，量子产率为8%。对于醛和酮的加氢还原，通过改变质子的供应速率，我们可以选择性调控产物为一元醇或频哪醇，且TOF可达每个量子点4000 h-1以上。本项目的研究成果揭示了配体的动力学、数量对反应活性的影响机理，为设计高光催化量子产率的量子点结构提供了实验依据。