Colloidal High Entropy Alloy (HEA) Nanoparticles by kinetically controlled Laser Ablation Synthesis in Liquids- Formation mechanism and their integration into Biphasic Core-Shell Morphologies

高熵合金(HEA)纳米粒子(NP)是多相催化领域的一个新兴科学领域。它们的特点是元素复杂，至少有五种元素的组成接近相同，并且仍然具有令人惊讶的简单的固溶体晶体结构。液体中激光烧蚀(LAL)是一种很有前途的合成HEA纳米粒子的方法，因为它可以扩展到g/h范围，并且不需要有机表面配体或载体材料就可以胶体形式提供粒子。然而，到目前为止，对LAL固溶体HEA NP的形成机理还不够清楚。此外，用这种技术合成的完全混合的HEA NP的组成范围(原子比)还没有得到系统的检验。在以前的实验中(本项目的第一阶段)，我们已经探索了双金属系统的相关科学问题，重点是FeAu。我们发现，在分离结构上形成理想混合固溶体颗粒的盛行程度是由靶的组成、颗粒大小和激光脉冲持续时间决定的。此外，还确认了一种独特而复杂的Fe@AuFe核壳结构，这种结构可能是由于所含元素的表面能和熔点高度不匹配而出现的。在这个项目中，我们的目的是阐明这些发现是否可以转移到HEA NP。其中Np来自I)CoCrFeMnNi II)AgAuCuPdPt将由LaL合成，并将采用激光脉冲宽度等工艺参数来获得元素分布均匀且氧化程度最低的固溶HEA结构。在接下来的步骤中，我们将调查HEA NP中特定选择的元素的过量是否以及在多大程度上会导致粒子进入元素偏析，是否会形成核壳结构，特别是这些转变如何依赖于NP的大小。这些检查需要使用和开发非常先进的基于STEM/EDX和SAED的方法，这些方法允许在原子分辨率下区分单个NP中的多种元素和晶体结构。在另一种方法中，我们将使用STEM/EDX和EELS以及循环伏安法和XPS来密切检查HEA NP的表面和主体之间的结构和组成不匹配，特别是与表面组成相关的表面组成驱动着潜在的催化应用。最后，通过原位电子显微镜加热实验，考察了激光制备的HEA纳米粒子的成分和相结构的变化。在这里，我们将探索HEA NP的潜在亚稳性，并在此基础上阐明其向热力学平衡的转变机制。这些研究将得到使用分子动力学和蒙特卡罗模拟的计算建模的补充。

High entropy alloys (HEA) nanoparticles (NP) are an emerging scientific field of particular interest in heterogeneous catalysis. They are characterized by elemental complexity with at least five elements at near equal compositions and still possess a surprisingly simple solid solution crystal structure. Laser ablation in liquids (LAL) is a promising method for the synthesis of HEA NP as it is scalable to the g/h range and provides the particles as colloids without the need for organic surface ligands or support materials. However, the formation mechanism of solid solution HEA NP by LAL is up to date insufficiently understood. Furthermore, the compositional range (atomic ratios) for fully mixed HEA NP synthesized by this technique has not been systematically examined.In previous experiments (1st phase of this project) we already explored related scientific questions for bimetallic systems with an emphasis on FeAu. We found that the prevalence of the formation of ideally mixed solid solution particles over segregated structures is critically driven by the composition of the target, the particle size, and the laser pulse duration. This was complemented by the identification of a unique and complex segregated Fe@AuFe core-shell structure, which probably emerges due to a high mismatch in surface energy and melting point in the contained elements. In this project, we aim to elucidate whether these findings are transferable to HEA NP. Thereto NP from I) CoCrFeMnNi II) AgAuCuPdPt will be synthesized by LAL and process parameters like laser pulse duration will be adapted to yield solid solution HEA structures with homogeneous elemental distribution and minimized oxidation. In a consecutive step, we will investigate whether and to what extent the excess of specifically chosen elements in the HEA NP, e.g. Ag and Pt in alloy II), would drive the particles into elemental segregation, whether a core-shell structure would form, and particularly how these transitions depend on NP size. These examinations necessitate the utilization and development of highly advanced STEM/EDX- and SAED-based methods, which allow differentiation of multiple elements and crystal structures within a single NP at atomic resolution. In another approach, we will closely examine structural and compositional mismatches between the surface and the bulk of HEA NP using STEM/EDX and EELS as well as cyclic voltammetry and XPS, particularly relevant as surface composition drives potential applicability in catalysis. Finally, an examination of changes in composition and phase structure in laser-fabricated HEA NP will be conducted by in situ TEM heating experiments. Here we will explore the potential metastability of the HEA NP and based on this elucidate the transformation mechanism towards thermodynamic equilibrium. These studies will be complemented by computational modeling using Molecular Dynamics and Monte Carlo simulations.