人工光催化分解水产氢器件的设计构筑及其电子转移机制研究

Visible light-driven water splitting to produce hydrogen is now considered as an attractive alternative energy source leading to solar energy conversion and storage. This project aims to design and fabricate hybrid and unimolecular photochemical molecular devices (PMDs) to improve quantum efficiency of photocatalytic hydrogen-evolving. The hybrid PMDs are fabricated by encapsulating of photosensitive molecules and noble metal nanoparticles into the cavities of metal-organic molecular cages (MOC) with multiple catalytic units and multiple photosensitizing units, respectively. The unimolecular PMDs are prepared by self-assembling of multiple catalytic units and photosensitizing units into a single MOC. In hybrid PMDs, the intermolecular electron transfer between MOCs and photosensitizer/nanoparticles could be promoted by the host-guest chemistry, i.e. space-confinement effect and host-guest interaction. In unimolecular PMD, the intramolecular electron and energy transfer from photosensitizing units to catalytic units could be facilitated via multiple and independent channels based on highly ordered and symmetry structure of MOC with proper organization of the photo- and catalytic -units. The time-resolved spectra, including ultra-fast transient absorption and photoluminescence, are employed to research the mechanism of photoinduced electron and energy transfer by detecting electron-relaxation dynamics in ground and excited states. Ultimately, recognizing and understanding the photocatalytic mechanism of PMDs in homogeneous phase. This project aims at the efficient utilization of solar energy and the key issue involves the electron and energy transfer in photocatalysis. The implement of this project will enrich the strategies of designing and fabricating traditional-molecular photocatalysts and open up the new methods of adjusting and controlling electron and energy transfer. It possesses important theory significance and practical value for the efficient utilization of solar energy.

本项目研究以太阳能的高效利用为目的，设计基于多个光敏中心/多个催化中心的复合型和单分子型金属-有机笼产氢器件：利用分子笼主体空间限域效应，将具有吸光功能的光敏分子或具有产氢功能的贵金属纳米粒子组装入分子笼空腔内，构筑复合型产氢器件；采用自组装策略，将多个光敏中心和多个催化中心在分子笼内有序对称地排列，构筑出具有多个相互等价电子转移通道的单分子器件。本项目的核心科学问题涉及到光催化过程中的电子转移和能量传递：分别通过复合型产氢器件的主客体相互作用以及单分子产氢器件的多通道电子转移来促进光敏中心向催化中心的电子定向转移；进一步利用超快光谱技术，从不同时间尺度研究金属-有机分子笼产氢器件在光催化过程中的电子转移，揭示电子转移与质子还原反应的耦合机制。本项目研究，将丰富传统分子基光催化剂的设计构筑策略，开辟分子水平调控电子转移和能量传递的新思路，对于太阳能的高效利用，具有重要的理论意义和实际价值。

本项目围绕 [Pd6(RuL3)8]28+(MOC-16)金属有机分子笼产氢器件和原子结构精确的Au25(SG)18纳米簇，通过课题组前期发展的配位自组装策略（Adv. Mater, 2018, 30, 1704576），成功实现[Pd6(RuL3)8]28+分子笼和Au25(SG)18纳米簇在二维C3N4载体表面和三维金属有机框架载体内部的多相化组装，构筑出复合型产氢催化剂。(一) [Pd6(RuL3)8]28+分子笼集多个光敏中心和多个催化中心于一体，组装入金属有机框架及其衍生载体内部后，提高了分子笼组成和结构稳定性，同时在光催化分解水产氢研究中，获得了较均相分子笼催化剂更高的产氢性能；稳态和瞬态光谱研究表明，金属有机框架载体在保留[Pd6(RuL3)8]28+分子笼固有的皮秒时间尺度内多通道等价定向高效电子转移特性的同时，主客体限域效应抑制了载流子的无辐射复合，将分子笼电子激发三重态的寿命从纳秒范畴延长至微秒量级；基于不同载体亲疏水性和三维孔道结构的对比，结合负载后[Pd6(RuL3)8]28+产氢性能的差异，揭示了光催化分解水产氢过程中水分子直接参与了氢气的生成，水分子O-H键的断裂是反应的决速步骤，而牺牲试剂因载体的传质限制，分别起到单电子和两电子还原剂的作用。(二) 研究了Au25(SG)18纳米簇与金属有机框架中金属-氧/氮簇相互作用而诱导产生的不同Au电荷态对催化性能的影响；通过与具有氧化还原性的金属氧化物载体的对照，揭示了金属有机框架中的金属-氧簇作为模型化的氧化物载体，在保持Au25(SG)18纳米簇结构完整性的同时，保持Au25(SG)18优异的催化性能。具有吸光与催化双功能的Au25(SG)18纳米簇在金属有机框架载体内部固载后，获得了稳定的去除硫醇配体保护的Au25纳米簇，研究了金纳米粒子与金纳米簇在光催化分解水产氢中独特的尺寸效应。（三）本项目研究，开辟了调控电子转移和能量传递的新思路，为设计、构筑以及优化人工光催化产氢器件提供科学依据和理论指导，对于太阳能的开发和高效利用，具有重要的理论意义和实际价值。