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）是一个新兴的科学领域，在多相催化特别感兴趣。它们的特点是元素复杂，至少有五种元素的组成几乎相等，并且仍然具有令人惊讶的简单的固溶体晶体结构。液体激光烧蚀（LAL）是一种很有前途的合成HEA NP的方法，因为它可以扩展到g/h范围，并且在不需要有机表面配体或支撑材料的情况下提供胶体颗粒。然而，LAL对固溶体HEA NP的形成机理尚不清楚。此外，该技术合成的完全混合HEA NP的组成范围（原子比）尚未得到系统的研究。在之前的实验（本项目的第一阶段）中，我们已经探索了双金属系统的相关科学问题，重点是FeAu。我们发现理想混合固溶体颗粒在分离结构上形成的普遍程度是由目标的组成、颗粒大小和激光脉冲持续时间驱动的。此外，还发现了一种独特而复杂的分离Fe@AuFe核壳结构，这可能是由于所含元素的表面能和熔点高度不匹配造成的。在这个项目中，我们的目的是阐明这些发现是否可转移到HEA NP。通过LAL合成I) CoCrFeMnNi II) AgAuCuPdPt中的NP，并调整激光脉冲时间等工艺参数以获得元素分布均匀且氧化最小的固溶体HEA结构。在接下来的步骤中，我们将研究HEA NP中特定选择的过量元素（如合金II中的Ag和Pt）是否以及在多大程度上会促使颗粒发生元素偏析，是否会形成核壳结构，特别是这些转变如何依赖于NP大小。这些检查需要利用和开发高度先进的基于STEM/EDX和saed的方法，这些方法允许在单个NP中以原子分辨率区分多个元素和晶体结构。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仔细检查HEA NP表面和主体之间的结构和组成不匹配，特别是表面组成驱动催化的潜在适用性。最后，通过原位TEM加热实验对激光制备的HEA NP的组成和相结构的变化进行了研究。在此，我们将探讨HEA NP的潜在亚稳态，并在此基础上阐明向热力学平衡转变的机制。这些研究将辅以分子动力学和蒙特卡罗模拟的计算模型。

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.