Formation of Co–Au Core–Shell Nanoparticles with Thin Gold Shells and Soft Magnetic ε-Cobalt Cores Ruled by Thermodynamics and Kinetics

Abstract

Bimetallic core−shell nanoparticles (CSNPs), where a ferromagnetic core (e.g., Co) is surrounded by a noblemetal thin plasmonic shell (e.g., Au), are highly interesting for applications in biomedicine and catalysis. Chemical synthesis of such structures, however, requires multistep procedures and often suffers from impaired oxidation resistance of the core. Here, we utilized a one-step environmentally friendly laser ablation in liquid technique to fabricate colloidal Co−Au CSNPs with core−shell yields up to 78% in mass. An in-depth analysis of the CSNPs down to single-particle levels revealed the presence of a unique nested core−shell structure with a very thin gold-rich shell, a nanocrystalline ε-cobalt sublayer, and a nested gold-rich core. The generated Co−Au CSNPs feature soft magnetic properties, while all gold-rich phases (thin shells and nested cores) exhibit a face-centered cubic solid solution with substantial cobalt substitution. The experimental findings are backed by refined thermodynamic surface energy calculations, which more accurately predict the predominance of solid solution and core−shell phase structures in correlation with particle size and nominal composition. Based on the Co−Au bulk phase diagram and in conjunction with previously reported results on the Fe−Au core−shell system as well as Co− Pt controls, we deduce four general rules for core−shell formation in non-or partially miscible laser-generated bimetallic nanosystems.

Impact of Sterilization on the Colloidal Stability of Ligand-Free Gold Nanoparticles for Biomedical Applications

“…102 The immediate cooling of generated particles in LAL due to the surrounding liquid often leads to the kinetic stabilization of defect-rich and metastable phases. 46,103 In contrast, coalescence and surface atom diffusion are thermodynamically driven processes that may transform the particles to a more thermodynamically stable state with fewer defects. We could not observe any potential impacts of sterile filtration or autoclaving on the laser-generated colloidal AuNPs having diameters of 10 and 30 nm.…”

“…41 Laser ablation in liquid (LAL) is one of the simple, green approaches to obtain ligand-free colloidal NPs. 42−45 LAL has been established as a reliable method to generate monometallic and multimetallic NPs by irradiating the corresponding metallic target by a pulsed laser in the given liquid medium, 42,46 with scalability recently demonstrated to reach more than 8 g/h, 47 equivalent to several dozens of liters of colloid per hour at the concentration level addressed in this paper. In the case of ligand-free NPs, like in LAL, the stabilization occurs due to a net surface charge of the particles caused by ion adsorption.…”

Solvent-particles interactions during composite particles formation by pulsed laser melting of α-Fe2O3

“…In the last decades, laser processing has become an important route for synthesizing nanoparticles 1 . The pulsed laser ablation in liquid (LAL) method uses a focused laser beam that can provide high energy density to small areas on the target leading to rapid growth of explosive nanoparticles of pure metals 2 , 3 , metallic glasses 4 , bimetallic particles with various architectures like alloys 5 – 13 , or core–shell 14 ). Using an unfocused laser beam of moderate fluence for irradiation of nanoparticles dispersed in liquid medium results in a slightly different phenomenon.…”

Doped metal clusters as bimetallic AuCo nanocatalysts: insights into structural dynamics and correlation with catalytic activity byin situspectroscopy

“…As these nanoalloys are formed by rearrangement of metal atoms during pretreatment and reaction, and due to the defined structure of the clusters, it is possible to understand their evolution using spectroscopic techniques. 9 Another interesting bimetallic system is Co-Au, since the physicochemical properties of both metals complement each other and enhance, among others, the magnetic 15,16 , catalytic 17,18 or electrocatalytic 19,20 properties. Moreover, the synergy of the biocompatibility of Au and the magnetic properties of Co expands its applicability to biomedicine and biodiagnostics.…”

“…21,22 A challenge is the high oxidation sensitivity of cobalt, which becomes more evident as the particle size decreases. 15 Several approaches have been explored for the preparation of Co-Au nanoalloys, including chemical synthesis, 15,23 evaporation, 24 or laser ablation 16 . However, these methods results in the formation of nanoparticles with more than 100 atoms, where the plasmonic properties of gold also play a role.…”