银纳米结构的稳定性以及其等离子共振对光(电)催化增强的影响

Photocatalysis, a process harvesting energy directly from sunlight, have the potential to solve the energy crisis and environmental issue facing to human beings. After several decades of research, the photocatalysis is still trade-off due to many limitation of the photocatalysts and their constructions. For example, the minimum thickness required for sufficient light absorption of incident light is always much larger than the charge diffusion length of majority semiconductors, indicating a very low efficiency of charge transportation. To solve this, the thickness of the film or nanomaterials has to be shortened. However, the light absorption will be insufficient. Surface plasmon resonance (SPR) of metal has been considered as an efficient method to solve this problem due to enhanced optical field around the metal and the charge exchange between metal and semiconductor. Ag nanostructures with tunable sizes and morphologies have a broad absorption from near ultraviolet to near infrared region and have been attracted much attention for SPR enhanced photocatalysis. However, application of Ag for photocatalysis is limited due to the low chemical and electrochemical stability of the Ag nanostructures. In this proposal, we start from the efforts to stabilize Ag nanostructures in air or at the photocatalytic media, avoid the direct contact between Ag nanostructures and environmentals and then develop several photocatalytic systems with high efficiency and stability. One common approach to improve the stability of Ag nanostrcutres is to introduce a protective layer with pin-hole and crack free insulate shell such as SiO2, Al2O3 and HfO2. Due to the localized effect of SPR of the Ag nanostructures, the thickness of the insulator should be short enough for enhanced light absorption of semiconductor and charge interaction between metal and semiconductor. Graphene, a monolayer of carbon atoms packed into a honeycomb crystal plane, exhibits great chemical and photo stability and is impenetrable to gas molecules as small as helium atom. Graphene has been studied as a promising candidate for effective anti-oxidation layer for many metals. Graphene is also considered as a promising candidate as the protection of Ag nanostructures. In this proposal, the graphene as the protective layer will be introduced for anti-corrosion for Ag nanostructures. Meanwhile, the synergistic effect of graphene as the protection layer and the media for efficent charge separation and transportation is expected for this novel photocatalytic or electro-photocatalytic process.

光催化被认为能解决人类目前所面临的能源危机及环境污染问题。然而当前的光催化材料一个重要的问题就是所需要完全吸收光的厚度大大超过半导体电荷的自由程，导致只有少量光激发的电荷用于催化反应。金属等离子体共振效应能够在降低材料厚度同时具有很好吸光度而且提高电荷利用效率。Ag纳米结构具有可控尺寸和形貌，其共振峰分部从近紫外到近红外，从而在等离子体增强的光催化中被广泛研究。但是Ag纳米结构由于化学稳定性不好，其应用受到限制。本项目拟从保护Ag纳米结构出发，在其与半导体之间引入中间层来避免在催化反应中Ag纳米结构和外部环境的接触而导致的氧化，从而实现催化体系的高活性和稳定性的目的。研究将在Ag纳米结构表面覆盖一层厚度可控的绝缘层（SiO2，Al2O3，HfO2等）或者引入石墨烯作为中间层来构建新型光催化剂或光电电极，实现高效稳定的催化体系。

有效利用太阳能的一个方法是将太阳能转化成化学能并加以存储利用。光或者光电分解水分为四电子过程的析氧反应和二电子的析氢反应。二者过程都存在过电势和稳定性的问题。特别是相比较于二电子的析氢反应，四电子的析氧反应动力学上更加的困难。尽管Pt、Ir和Rh材料是高效的析氢析氧催化剂，然而其高昂成本和资源稀缺限制它们的广泛应用。近年来廉价过渡态金属(Ni, Fe, Co，Mn)的化合物表现出优良的析氢析氧电催化性能。催化性能可以通过表面缺陷位点的调控、界面活性位点的调控、结构的优化与构建、化学掺杂、不同成分复合协同效应等手段来实现提高催化活性与稳定性的目的。这里，研究注重于理解超薄过渡态金属氧化物析氧反应的机理并设计多尺度的多级结构在保持大比表面积同时促进电荷的传输，从而达到高反应活性与稳定性的OER催化剂。同时，基于金属磷化物表面调控与掺杂的手段研究金属磷化物的析氢反应机制并在此基础上提高催化剂性能。并着力于构建稳定高效的析氢析氧催化剂，为其规模使用打下一定的基础。