Core-Shell nanoparticle formation in FeAu and CoAu during laser ablation in liquid synthesis is driven by particle size, melting temperature and surface energy Christoph Rehbock, Jacob Johny and Stephan Barcikowski Technische Chemie I and Center for Nanointegration Duisburg-Essen (CENIDE), Germany Laser ablation in liquids is a versatile technique for the synthesis of colloidal nanoparticles, which particularly excels in the generation of alloy nanoparticles, while overall nanoparticle composition is primarily ruled by the used target. However, which phase structure emerges in these particles is dependent on the constituent´s solid solubility in the bulk state, oxidation tendencies, elemental composition, and particle size are to date still insufficiently explained. In our work, we focused on the FeAu and CoAu alloy systems. Thereto we laser-ablated corresponding alloy targets with varying compositions in acetone and analyzed their phase structure with advanced (S)TEM/EDX methods. In FeAu we could identify the emergence of a Fe@Au core-shell morphology, primarily formed from iron-rich targets with iron molar fractions > 0.5 and particles with diameters > 20 nm a finding in good accordance with a simple thermodynamic model. [1,2] Analysis of the particles´ ultrastructure revealed that the core was composed of pure BCC iron, while the shell was a gold-rich FeAu alloy. [3] In situ heating experiments in the TEM further indicated transfer into a mixed solid solution for gold-rich particles as well as total elemental segregation and formation of a truncated-bipyramidal structure for iron-rich particles as predicted by theory [4]. These findings confirm the metastable nature of the formed core-shell nanoparticles. We further highlighted the generation of similar Co@Au core-shell morphologies, though with a higher tendency towards more complex nested-core-shell morphologies and ε-Co-phases, while controls with PtCo showed no core-shell structures.[5] These findings seem to indicate that the emergence of a core-shell structure during laser ablation synthesis is ruled by I) pronounced miscibility gaps in the bulk phase diagram, II) a sufficiently large nanoparticle diameter, III) deviating melting temperatures and IV) a mismatch in surface energy. Finally, we demonstrate the transferability of these findings to the AuFeCo ternary alloy system. References 1. Tymoczko, A.; Kamp, M.; Prymak, O.; Rehbock, C.; Jakobi, J.; Schürmann, U.; Kienle, L.; Barcikowski, S. Nanoscale 2018, 10 (35), 16434. 2. Tymoczko, A.; Kamp, M.; Rehbock, C.; Kienle, L.; Cattaruzza, E.; Barcikowski, S.; Amendola, V. Nanoscale Horizons 2019, 4 (6), 1326. 3. Kamp, M.; Tymoczko, A.; Popescu, R.; Schürmann, U.; Nadarajah, R.; Gökce, B.; Rehbock, C.; Gerthsen, D.; Barcikowski, S.; Kienle, L. Nanoscale Advances 2020, 2, 3912-3920. 4. Kamp, M.; Tymoczko, A.; Schürmann, U.; Jakobi, J.; Rehbock, C.; Rätzke, K.; Barcikowski, S.; Kienle, L. Crystal Growth & Design 2018, 18 (9), 5434. 5. Johny, J.; Kamp, M.; Prymak, O; Tymoczko, A.; Wiedwald, U.; Rehbock, C.; Schürmann, U.; Popescu, R.; Gerthsen, D.; Kienle, L.; Shaji, S.; and Barcikowski, S. J. Phys. Chem. C 2021, 125, 9534–9549.
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