Wang Mei, Leff Asher C, Li Yue, Woehl Taylor J
Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, Maryland 20742, United States.
Sensors and Electron Devices Directorate, Combat Capabilities Development Command, United States Army Research Laboratory, Adelphi, Maryland 20783, United States.
ACS Nano. 2021 Feb 23;15(2):2578-2588. doi: 10.1021/acsnano.0c07131. Epub 2021 Jan 26.
Colloidal synthesis of alloyed multimetallic nanocrystals with precise composition control remains a challenge and a critical missing link in theory-driven rational design of functional nanomaterials. Liquid-phase transmission electron microscopy (LP-TEM) enables direct visualization of nanocrystal formation mechanisms that can inform discovery of design rules for nanocrystal synthesis, but it remains unclear whether the salient flask synthesis chemistry is preserved under electron beam irradiation during LP-TEM. Here, we demonstrate controlled LP-TEM synthesis of alloyed AuCu nanocrystals while maintaining the molecular structure of electron beam sensitive metal thiolate precursor complexes. flask synthesis experiments formed alloyed nanocrystals containing on average 70 atomic% Au using heteronuclear metal thiolate complexes as a precursor, while gold-rich alloys with nearly no copper formed in their absence. Systematic dose rate-controlled LP-TEM synthesis experiments established a range of electron beam synthesis conditions that formed alloyed AuCu nanocrystals that had statistically indistinguishable alloy composition, aggregation state, and particle size distribution shape compared to flask synthesis, indicating the flask synthesis chemistry was preserved under these conditions. Reaction kinetic simulations of radical-ligand reactions revealed that polymer capping ligands acted as effective hydroxyl radical scavengers during LP-TEM synthesis and prevented oxidation of metal thiolate complexes at low dose rates. Our results revealed a key role of the capping ligands aside from their well-known functions, which was to prevent copper oxidation and facilitate formation of prenucleation cluster intermediates via formation of metal thiolate complexes. This work demonstrates that complex ion precursor chemistry can be maintained during LP-TEM imaging, enabling probing nonclassical nanocrystal formation mechanisms with LP-TEM under reaction conditions representative of flask synthesis.
精确控制合金成分的多金属纳米晶体的胶体合成仍然是一个挑战，也是功能纳米材料理论驱动的合理设计中关键的缺失环节。液相透射电子显微镜（LP-TEM）能够直接观察纳米晶体的形成机制，这有助于发现纳米晶体合成的设计规则，但尚不清楚在LP-TEM的电子束照射下，烧瓶合成中的显著化学过程是否得以保留。在这里，我们展示了在保持对电子束敏感的金属硫醇盐前体配合物分子结构的同时，通过LP-TEM可控合成合金化的AuCu纳米晶体。烧瓶合成实验使用异核金属硫醇盐配合物作为前体，形成了平均含有70原子%金的合金纳米晶体，而在没有这种配合物的情况下，几乎不形成含铜的富金合金。系统的剂量率控制LP-TEM合成实验确定了一系列电子束合成条件，在此条件下形成的合金化AuCu纳米晶体，其合金成分、聚集状态和粒径分布形状与烧瓶合成相比在统计学上无显著差异，这表明在这些条件下烧瓶合成化学过程得以保留。自由基-配体反应的反应动力学模拟表明，聚合物封端配体在LP-TEM合成过程中起到了有效的羟基自由基清除剂的作用，并在低剂量率下防止了金属硫醇盐配合物的氧化。我们的结果揭示了封端配体除了其众所周知的功能外的关键作用，即防止铜氧化，并通过形成金属硫醇盐配合物促进预成核簇中间体的形成。这项工作表明，在LP-TEM成像过程中可以维持复杂离子前体化学过程，从而能够在代表烧瓶合成的反应条件下用LP-TEM探测非经典的纳米晶体形成机制。
