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

• Applicants: Professor Dr.-Ing. Stephan Barcikowski; Professor Dr. Lorenz Kienle
• Subject Area: Solid State and Surface Chemistry, Material Synthesis ; Synthesis and Properties of Functional Materials
• Term: since 2015
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 277627168

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.

• DFG Programme: Research Grants