多枝状贵金属纳米晶的可控合成、性质及其生长机理的研究

Noble metal nanocrystals have exhibited remarkable physical and chemical properties, such as electronic, optical and catalytical properties, which are closely related to their size, shape, surface structure and so on. Experimental and theoretical calculations have proven that nonspherical Au and Ag nanocrystals show outstanding properties on both surface plasmon resonance (SPR) and surface enhanced Raman scattering (SERS), due to the anisotropic distribution of the electromagnetic field. Particularly, Au nanocrystals containing multiple sharp branches have SPR bands that are tunable into the near-infrared region, and show great SERS enhancements resulting from the "lightning-rod effect" of sharp branches. Therefore, multi-branched Au nanocrystals have exhibited promising biological applications, such as biosensors, diagnostics and photothermal therapy. In this proposal, we'll control the growth kinetics of branched Au nanocrystals to obtain several types of multi-branched Au nanostructures with large aspect ratio and sharp branches, which show superiority on SPR and SERS properties. Furthermore, the branched bimetal nanocrystals of a core-shell structure will be controlled synthesized, where multi-branched Au nanocrystals act as "template" or "crystal seeds" and other novel metal (such as Ag, Pt and Pd) can grow on the surface of Au to form a shell. Meanwhile, the relationship between multi-branched nanostructure and the corresponding properties will be systematically investigated. In addition, the high resolution transmission electronic microscopy (HRTEM) and in-situ X-ray absorption fine structures (XAFS) spectroscopy will be exploited as powerful tools to follow the structural change of surface atoms on the branch tips and elucidate the growth mechanism of branched nanocrystals, respectively. Understanding the growth mechanism will be helpful to direct the designed synthesis of noble metal nanocrystals with other unique shapes.

贵金属纳米晶独特的电学、光学和催化等性质通常与其尺寸、形状和表面结构等因素密切相关。实验和理论计算都证明非球形Au、Ag纳米晶由于电磁场的各向异性分布而在表面等离子共振（SPR）和表面增强拉曼散射（SERS）性质上优势显著。其中，具有多个尖锐分枝的Au纳米晶的SPR带可调至近红外区域，尖锐分枝的"避雷针效应"也使其SERS大幅增强。这些特性使多枝Au纳米晶在生物医学领域（如光热治疗）有广泛的应用前景。本项目通过控制多枝Au纳米晶的生长动力学，最终得到几种尺寸均匀、SPR和SERS性质优异的多枝Au纳米结构，并以此为模板生长具有其他贵金属壳层的多枝双金属纳米晶。通过精确调控分枝长径比、尖锐度等，系统研究多枝结构与其性质间的关系。此外，利用高分辨透射电镜和原位X射线吸收精细结构谱，研究分枝表面原子结构变化和多枝纳米晶的生长机理，从而指导特殊形状贵金属纳米晶的设计合成。

基于贵金属的多枝状纳米晶体由于其独特的表面结构以及多样的结构组成，在催化、表面等离子共振等性能上有明显优势，因而受到广泛关注。尤其是含Pt的纳米材料，由于其在催化、电催化、能源等领域的重要应用，更是受到研究者的长期青睐。. 经过3年多的研究，本项目研究团队以Pt基纳米晶为主要研究对象，以X射线吸收精细结构（XAFS）为主要表征手段，在多枝状Pt纳米晶、Pt-M（M= Fe, Co, Ni）双金属纳米晶和多级结构Pt纳米晶的制备上取得了突破性的进展，并发现Pt-Ni双金属纳米晶在一定比例下（如4：1）对氨硼烷水解的催化能力几乎与纯Pt纳米晶相同，并通过XAFS表征和分析证明Ni在双金属纳米晶中的氧化态可能是影响其催化性能的重要因素。本项目中还自行设计搭建了原位XFAS表征测试池，对多枝状Pt纳米晶的成核及生长机理进行了原位研究，结果表明：快速充分还原条件下，H2PtCl6直接被还原为零价Pt原子，然后Pt原子进一步团聚成核、生长为Pt纳米晶；而在连续不充分的还原条件下，H2PtCl6先还原为低氧化态的团簇聚合物PtnClx，然后被进一步还原为零价Pt原子簇或晶核，最后生长为Pt纳米晶。. 在合成方法上，还尝试了通过恒电流法沉积制备Pt纳米晶，并通过对电流强度、沉积时间和前驱液浓度的调控，得到了几种具有多级结构的Pt纳米颗粒。但这些沉积产物的均匀性、分级可控性、产量仍需得到进一步的发展。本项目在贵金属的可控合成及生长机理的深入理解等方面都为该领域的其他研究者们提供了方法的借鉴和参考。