Core-Shell Nanoparticles Driven by Surface Energy Differences in the Co-Ag, W-Fe, and Mo-Co Systems

Koten, Mark A.; Mukherjee, Pinaki; Shield, Jeffrey E.

doi:10.1002/ppsc.201500019

Cited by 26 publications

(23 citation statements)

References 35 publications

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“…However, the spontaneous formation of this structure clearly simplified the fabrication process. This minimum size is slightly below the 8 nm reported by Koten et al [37] for similar bimetallic systems. Given that the power used for the fabrication of both NP types was the same, a similar thermal energy should then be expected.…”

Section: Morphological and Structural Characterizationcontrasting

confidence: 53%

Spontaneous Formation of Core@shell Co@Cr Nanoparticles by Gas Phase Synthesis

et al. 2020

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This work presents the gas phase synthesis of CoCr nanoparticles using a magnetron-based gas aggregation source. The effect of the particle size and Co/Cr ratio on the properties of the nanoparticles is investigated. In particular, we report the synthesis of nanoparticles from two alloy targets, Co90Cr10 and Co80Cr20. In the first case, we observe a size threshold for the spontaneous formation of a segregated core@shell structure, related to the surface to volume ratio. When this ratio is above one, a shell cannot be properly formed, whereas when this ratio decreases below unity the proportion of Cr atoms is high enough to allow the formation of a shell. In the latter case, the segregation of the Cr atoms towards the surface gives rise to the formation of a shell surrounding the Co core. When the proportion of Cr is increased in the target (Co80Cr20), a thicker shell is spontaneously formed for a similar nanoparticle size. The magnetic response was evaluated, and the influence of the structure and composition of the nanoparticles is discussed. An enhancement of the global magnetic anisotropy caused by exchange bias and dipolar interactions, which enables the thermal stability of the studied small particles up to relatively large temperatures, is reported.

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Section: Morphological and Structural Characterizationcontrasting

confidence: 53%

Spontaneous Formation of Core@shell Co@Cr Nanoparticles by Gas Phase Synthesis

et al. 2020

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“…There are other experimental results that are seemingly inconsistent with this study. 66 , 67 However, we note that the comparisons to the experimental results are not always easy as the surface segregation can be induced by factors other than heat treatment, such as the presence of adsorbates or substrate/support. 68 …”

Section: Resultsmentioning

confidence: 99%

General Trends in Core–Shell Preferences for Bimetallic Nanoparticles

et al. 2021

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Surface segregation phenomena dictate core–shell preference of bimetallic nanoparticles and thus play a crucial role in the nanoparticle synthesis and applications. Although it is generally agreed that surface segregation depends on the constituent materials’ physical properties, a comprehensive picture of the phenomena on the nanoscale is not yet complete. Here we use a combination of molecular dynamics (MD) and Monte Carlo (MC) simulations on 45 bimetallic combinations to determine the general trend on the core–shell preference and the effects of size and composition. From the extensive studies over sizes and compositions, we find that the surface segregation and degree of the core–shell tendency of the bimetallic combinations depend on the sufficiency or scarcity of the surface-preferring material. Principal component analysis (PCA) and linear discriminant analysis (LDA) on the molecular dynamics simulations results reveal that cohesive energy and Wigner–Seitz radius are the two primary factors that have an “additive” effect on the segregation level and core–shell preference in the bimetallic nanoparticles studied. When the element with the higher cohesive energy also has the larger Wigner–Seitz radius, its core preference decreases, and thus this combination forms less segregated structures than what one would expect from the cohesive energy difference alone. Highly segregated structures (highly segregated core–shell or Janus-like) are expected to form when both the relative cohesive energy difference is greater than ∼20%, and the relative Wigner–Seitz radius difference is greater than ∼4%. Practical guides for predicting core–shell preference and degree of segregation level are presented.

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“…Utilizing the surface segregation mechanism, one can produce core-shell nanostructures by simply evaporating both core and shell materials simultaneously in the gas phase, rendering the process 'continuous' without the need of a separate coating process. [38][39][40][41][42] Note that in situ heat treatment in gas phase synthesis methods has been reported to be efficient for phase transformation. 43 In our setup, heat-induced surface segregation occurs when the agglomerates of bimetallic nanoparticles, synthesized by spark ablation, pass through a tube furnace during which the agglomerates become spherical core-shell structures.…”

Section: Introductionmentioning

confidence: 99%