Investigation of the oxide shell forming on ɛ-Co nanocrystals

Braidy, Nadi; Behal, S. K.; Adronov, Alex; Botton, Gianluigi A.

doi:10.1016/j.micron.2007.10.017

Cited by 6 publications

(5 citation statements)

References 23 publications

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“…Despite the highest saturation magnetization exhibited by such metals (e.g., Fe, Co, and Ni) in their pure forms, extreme sensitivity toward oxidation also limits their utilization . When exposed to air, the formation of small oxide shells on ferromagnetic NPs may significantly alter their magnetic properties and also potentially hinder their catalytic performance for selected reactions. − In the case of Co, a CoO overlayer is often observed with a diameter of 3–5 nm. , However, in our case, the magnetic properties of the metallic ε-Co phase are retained, which excludes the formation of such oxide layers since these cores are protected by noble-metallic thin Au-rich shells. It should be noted that the magnetization values obtained for the Co–Au structures are still comparable to the values often reported for biomedical applications based on nanoentities. , Unlike the hard-magnetic materials, the magnetic behavior of these Co–Au structures can be easily influenced by small external fields owing to their soft magnetic characteristics.…”

Section: Resultsmentioning

confidence: 71%

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

et al. 2021

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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.

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Section: Resultsmentioning

confidence: 71%

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

et al. 2021

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“…For most conventional TEMs, SAED is effective to select areas down to 500 nm. For the analysis of multiple crystalline phases, the Debye ring pattern's radial intensity profile is comparable to the powder x‐ray diffraction pattern . Recording the SAED over a region that is overhanging a hole avoids interference from the amorphous film.…”

Section: Methodsmentioning

confidence: 99%

Experimental methods in chemical engineering: Transmission electron microscopy—TEM

et al. 2020

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Thanks to an accelerating voltage in the range of 30 to 300 kV, an electron beam can pass through a thin specimen and form an image with sub-Ångström spatial resolution. When impinging on a thin crystalline specimen, the fast electrons scatter and diffract. The transmitted electron pattern depends on the local thickness, density, crystal structure, and chemical nature of the sample. The transmission electron microscope (TEM) shapes the incoming electron beam using magnetic lenses onto the specimen and, using a different set of magnetic lenses, focuses the projected electron pattern to a camera. The final image magnification and contrast are controlled using the parameters from the electron gun, apertures positioned along the optical path, and magnetic lenses. With this combination of lens and aperture, TEM offers two possible modes of operation: (a) imaging, including high-resolution electron microscopy to reveal the size, shape, crystallinity, and morphology of materials; and (b) diffraction, to determine the crystalline nature of a region of interest of a thin film, particle, or collection of particles. Chemical engineers have taken advantage of both of these modes to analyze their samples and inform their research. A bibliometric study conducted using the WoS database places TEM as one of the preferred microscopy tools to study advanced materials such as thin films, nanomaterials, and composites used in particular for the development of applications related to energy storage and conversion (catalysis, photocatalysis, electrochemistry, and batteries) and environment (adsorption, waste-water treatment, and filtration). K E Y W O R D Selectron diffraction, interfaces, metrology, nanomaterials, transmission electron microscopy

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“…Consider a sub-region of half width 'L' such that L< R. The integrated EELS signal, M(E), for this sub-region contains information from the analysis volume whose projection in the yz-plane is represented by ABCD in figure 1. Assuming that the analysis volume has a constant dimension 'λ' along the x-axis (this assumption will be discussed in more detail further on), equation (1) reduces to:…”

Section: Mathematical Backgroundmentioning

confidence: 99%

“…the 'core'). Such core-shell structures are frequently found in nanotechnology in the form of nanoparticles or nanowires [1][2][3][4][5][6]. To investigate the structure, and composition of such materials with high spatial reolution, techniques such as energy dispersive X-ray (EDX) spectroscopy and electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) are routinely used.…”

Section: Introductionmentioning

confidence: 99%

Characterising the surface and interior chemistry of core–shell nanoparticles using scanning transmission electron microscopy

Mendis

Craven

2011

Ultramicroscopy

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Investigation of the oxide shell forming on ɛ-Co nanocrystals

Cited by 6 publications

References 23 publications

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

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

Experimental methods in chemical engineering: Transmission electron microscopy—TEM

Characterising the surface and interior chemistry of core–shell nanoparticles using scanning transmission electron microscopy

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