Harnessing Spontaneous Crystallization of Amorphous Nanoparticles for the Preparation of Metastable Cobalt

• Published in: Journal of physical chemistry. C 2024-07, Vol.128 (27), p.11456-11464
• Main Authors: Yeghishyan, Armenuhi, Malakpour Estalaki, Sina, Chilukuri, Rajeev, Zakaryan, Marieta K., Luo, Tengfei, Manukyan, Khachatur
• Format: Article
• Language: English
• Subjects: C: Physical Properties of Materials and Interfaces
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.4c03697

This study explores the synthesis of amorphous cobalt (a-Co) nanoparticles and investigates their spontaneous crystallization to prepare metastable γ-Co with a face-centered cubic structure. Crystallization is investigated through two distinct regimes: self-sustaining spontaneous crystallization (SSC) and volume spontaneous crystallization (VSC), utilizing either localized short heat impulses or uniform external heating, respectively. In the SSC regime, the heat generated by the crystallization elevates the temperature beyond the γ-Co ↔ ε-Co transition threshold (690 K), stabilizing the γ-Co phase and inhibiting the formation of the more stable ε-Co phase, which features a hexagonal close-packed structure. Conversely, the VSC regime not only triggers crystallization but also promotes the sintering of nanoparticles, effectively reducing porosity and enhancing the mechanical properties of the γ-Co nanomaterial. Molecular dynamics simulations, high-resolution electron microscopy imaging, and thermal analysis of the process provide detailed insights into the atomic-scale mechanism of spontaneous crystallization. These investigations reveal that clusters containing γ-Co with hexagonal close-packed stacking faults form at the initial stage of the process. As the process progresses, these clusters grow into larger crystallites and eventually transform into large grains at the expense of smaller grains and the disordered phase, leading to a significant reduction in stacking faults until the entire structure stabilizes into γ-Co.