| 1. |
- Deppert, Knut, et al.
(författare)
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One-step Gas-phase Synthesis of Core-shell Nanoparticles via Surface Segregation.
- 2019
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Konferensbidrag (refereegranskat)abstract
- A great amount of research effort has been devoted to theproduction of core-shell nanoparticles for applications in variousfields including biomedical imaging, catalysis, and plasmonics.Such attention to core-shell nanoparticles arise from the fact thatthey can exhibit enhanced physical and/or chemical properties.Furthermore, core-shell particles with distinctly new propertiescompared to those of the constituent materials can be designedby tuning, for example, their size, shell thickness, and structure [1,2].Although chemical synthesis techniques are currently the mostpopular methods for fabricating core-shell nanoparticles,interface and surface contaminations are often an unavoidableissue in the solution-based approaches. Aerosol based methodsare cleaner alternatives and have been used to produce core-shellnanoparticles [3-6]. Here we present aerosol core-shellnanoparticles generated via spark discharge generation (SDG) [7].Cu-Ag core shell nanoparticles were fabricated via surfacesegregation using SDG accompanied by sintering directly in thegas phase. The surface segregation employed in this methodrefers to the phenomenon of the enrichment of one componentof a mixture in the surface region and is attributed to theinterplay between the atomic radii, cohesive energy, and surfaceenergy of the core and shell materials [8].Depending on the sintering temperature, the SDG-generatednanoparticles form Janus-like or core-shell structures. Themorphology, crystallinity, and composition of the SDG-generatedbimetallic nanoparticles were investigated by scanning electronmicroscopy, high-resolution transmission electron microscopy,and energy-dispersive X-ray spectroscopy. Molecular dynamicssimulations were carried out to investigate the structuralevolution of Cu-Ag nanoparticles during heating and coolingprocesses corresponding to the sintering. This appealingly simpleone-step gas-phase synthesis method presented here can beemployed for other bimetallic systems.
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| 2. |
- Deppert, Knut, et al.
(författare)
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Predicted trends of core-shell preferences for bimetallic nanoparticles by molecular dynamics
- 2020
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Konferensbidrag (refereegranskat)abstract
- We present predicted trends of core-shell preferences obtained by molecular dynamics (MD) simulations for 28 bimetallic nanoparticle systems (approximately 4 nm in diameter) composed of 8 metals; Ag, Cu, Au, Pd, Fe, Co, Ni, and Pt. The two single-element FCC crystals were heated up to the melting temperature to form a bimetallic system and subsequently cooled to a room temperature. The core-shell preferences were quantified by identifying surface atoms in the MD results using a method based on the alpha-shapes method. Three different types of structures were observed in the solidified bimetallic systems; a) mixed b) core-shell c) Janus-like. Our MD simulations were carried out for significantly larger nanoparticles than the previous DFT calculations. It also provided more detailed information on the core-shell preferences and clearly distinguished Janus-like from core-shell structures. The deciding factors in core-shell preferences are investigated from the general trends found in this study.
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| 3. |
- Eom, Namsoon, et al.
(författare)
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A Continuous One-Step Aerosol Method for Producing Core@Shell Cu@Ag Nanoparticles
- 2019
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Konferensbidrag (refereegranskat)abstract
- The synthesis of core@shell nanoparticles has predominantly been carried out by chemical methods or physical deposition of a shell material post-growth onto core particles. By combining immiscible materials that differ in surface energy, we demonstrate the spontaneous atomic rearrangement to monodisperse Cu@Ag core@shell particles directly in the aerosol phase, starting from pure Cu and Ag electrodes with the spark discharge generation (SDG) method. The morphology and crystallinity were investigated by SEM and HRTEM, and the composition was confirmed by STEM EDX, which indicated that the Ag shell acts as an oxidation barrier for the Cu core. The current material system may find applications in antibacterial coatings. The method presented can be extended to other bimetallic systems with applications in catalysis, plasmonics and as seed-particles for nanowire growth. Owing to the simple, zero-waste and continuous production method, SDG is an ideal platform for such nanoparticle generation and investigation.
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| 4. |
- Eom, Namsoon, et al.
(författare)
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Core-shell Cu-Ag Nanoparticles Produced by Spark Discharge Generation
- 2019
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Konferensbidrag (refereegranskat)abstract
- The synthesis of core@shell nanoparticles has predominantly been carried out by chemical methods or physical deposition of a shell material post-growth onto core particles. By combining immiscible materials that differ in surface energy, we demonstrate the spontaneous atomic rearrangement to monodisperse Cu@Ag core@shell particles directly in the aerosol phase, starting from pure Cu and Ag electrodes with the spark discharge generation (SDG) method. The morphology and crystallinity were investigated by SEM and HRTEM, and the composition was confirmed by STEM EDX, which indicated that the Ag shell acts as an oxidation barrier for the Cu core. The current material system may find applications in antibacterial coatings. The method presented can be extended to other bimetallic systems with applications in catalysis, plasmonics and as seed-particles for nanowire growth. Owing to the simple, zero-waste and continuous production method, SDG is an ideal platform for such nanoparticle generation and investigation.
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| 5. |
- Eom, Namsoon, et al.
(författare)
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General Trends in Core-Shell Preferences for Bimetallic Nanoparticles
- 2021
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 15:5, s. 8883-8895
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Tidskriftsartikel (refereegranskat)abstract
- 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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| 6. |
- Eom, Namsoon, et al.
(författare)
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General Trends in Core-shell Preferences for Bimetallic Nanoparticles
- 2021
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Konferensbidrag (refereegranskat)abstract
- Core-shell nanoparticles have gathered much attention of the scientific community owing to their potential applications in various fields including biomedical imaging and catalysis. Predicting core-shell preference1-3 is, however, still largely based on a few experimental observations and limited theorical studies, and hence development of new core-shell nanoparticles is normally built on a trial-and-error approach. Here we present general trends of core-shell preferences for 45 bimetallic nanoparticle systems studied by molecular dynamics (MD) and Monte Carlo (MC) simulations. Simulations were performed using LAMMPS code and the embedded-atom method (EAM) potentials were employed for simulating the interactions between atoms in the bimetallic nanoparticle systems composed of 10 metals; Ag, Cu, Au, Pd, Fe, Co, Ni, Pt, Al, and Mo. In order to quantify the core-shell preference, the MD/MC results were analysed to identify surface atoms using the alpha-shapes method. The core and shell compositions of the preferred equilibrium structures of bimetallic combinations were then used to categorize each combination into one of four different types depending on the level of core- shell tendency: mixed, core-shell, highly segregated core-shell, Janus-like. The categorized MD/MC results were also analysed using principal component analysis (PCA) and linear discriminant analysis (LDA) to determine the primary factors that dictate core-shell tendency. Eight possible factors were considered, and cohesive energy and atomic radius are found to be the two primary factors that have an ‘additive’ effect on the segregation level and core-shell preference in the bimetallic nanoparticles studied. In the majority of the investigated combinations, the element with higher cohesive energy has smaller atomic radius and tend to occupy the core. 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 atomic radius difference is greater than ~ 4 %. However, when the element with higher cohesive energy has larger atomic radius, the core-shell tendency decreases. The general trend observed in the current study can be used as a guide in nanoparticle synthesis methods in which heat-induced surface segregation phenomena play an essential role, and in predicting the equilibrium structures of bimetallic nanoparticles.
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| 7. |
- Eom, Namsoon, et al.
(författare)
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Sintering Mechanism of Core@Shell Metal@Metal Oxide Nanoparticles
- 2021
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 125:29, s. 16220-16227
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Tidskriftsartikel (refereegranskat)abstract
- Metal oxide shell layers are promising candidates to improve the performance of metal nanoparticles (NPs) in various applications. However, despite a significant amount of experimental work on metal@metal oxide (M@MO) NPs, computational modeling is scarce, particularly on the sintering mechanism, which plays a crucial role in both the synthesis and performance of NPs. Here, we present atomic diffusion and sintering dynamics of M@MO NPs investigated using molecular dynamics based on the ReaxFF potentials. The coalescence process of the metal NPs with amorphous oxide shell is mainly facilitated by the relatively mobile surface atoms and grain-boundary-like diffusion, and thus, it is similar to reported mechanisms for crystalline nanoparticles. Intriguingly, atomic trajectory tracing reveals that surface diffusion is highly localized, contrary to the common understanding of freely moving high-mobility surface atoms. These atomic descriptions provide valuable insights for designing functional NPs with oxide layers and establishing more accurate accounts of the sintering mechanism.
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| 8. |
- Eom, Namsoon, et al.
(författare)
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Sintering Mechanism of Core@Shell Metal@Metal-Oxide Nanoparticles
- 2021
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Konferensbidrag (refereegranskat)abstract
- Adding metal-oxide shell layers is a promising route for improving the performance of metal nanoparticles (NPs) in various applications. However, despite the rapidly growing interest and a significant amount of experimental work on metal@metal-oxide NPs, computational modeling is relatively scarce, particularly on the sintering mechanism, which plays a crucial role in both the synthesis and performance of NPs. It is well known that the primary sintering mechanisms are surface diffusion and grain boundary diffusion in crystalline materials (Zachariah, 1999) and viscous flow in amorphous clusters (Eggersdorfer, 2011). Therefore, the sintering mechanism of metal@metal-oxide NPs gives rise to fundamental scientific questions as they generally exhibit crystalline cores with amorphous shells. Here, we present atomic diffusion and sintering dynamics of metal@metal-oxide NPs investigated using molecular dynamics based on the ReaxFF potentials.We have investigated three metal@metal-oxide core@shell NPs as model systems: Ni@NiO, Cu@CuO, and Fe@Fe2O3, all of which are actively studied for various catalytic applications. Sintering MD simulations of two core@shell clusters are analysed using an atom- tracking approach together with crystallinity and mean square displacement (MSD) analysis. The main sintering mechanisms are found to be surface and grain-boundary- like diffusion, similar to that of crystalline NPs (Figure 1). Intriguingly, atomic trajectory tracing (Figure 2) reveals that surface diffusion is highly localized and that it is mainly the surface atoms near the contact region that actively participate in the sintering. In other words, contrary to the common understanding of freely moving high mobility surface atoms (Jose-Yacaman, 2005), atoms located away from the contact region remain distant during the early stage of the sintering process.We expect the sintering mechanism observed in metal@metal-oxide core@shell NPs here to be particularly relevant for small metal nanoclusters as they usually have a thin surface oxide layer. It can also open up promising new directions in designing aerosol NPs via sintering.
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| 9. |
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| 10. |
- Snellman, Markus, et al.
(författare)
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A thermal evaporator for aerosol core-shell nanoparticle synthesis
- 2024
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Ingår i: Journal of Aerosol Science. - 0021-8502. ; 175
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Tidskriftsartikel (refereegranskat)abstract
- Segregated bimetallic nanoparticles like core-shell nanoparticles are of interest in various fields including biomedicine, catalysis, and optoelectronics. Aerosol technology is an optimal platform to control nanoparticle size, structure, and composition, which are some of the most important parameters tuning the material performance for the intended applications. Here, we develop a novel evaporator design to coat core particles on-line with a shell directly in the gas phase. The evaporator employs a local heater that decouples heating the evaporating material from the aerosol particles to limit core-shell alloying. We characterize the system by evaporating Zn onto core particles of Au, Sn, and Bi and demonstrate the core-shell particle formation with controllable shell thickness in each material system. We discuss simple models to explain the observed growth process inside the evaporator and the resulting shell formation.
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