Control of the crystal habit and magnetic properties of Co nanoparticles through the stirring rate

Mrad, Khouloud; Schoenstein, Frédéric; Nong, H. T. T.; Anagnostopoulou, Evangelia; Viola, Arnaud; Mouton, Ludovic; Mercone, Silvana; Ricolleau, Christian; Jouini, N.; Abderraba, Manef; Lacroix, Lise‐Marie; Viau, Guillaume; Piquemal, Jean‐Yves

doi:10.1039/c7ce00714k

Cited by 28 publications

(26 citation statements)

References 35 publications

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“…following a protocol where noble metal seeds are generated in situ from a noble metal precursor added concomitantly with the Co precursor at the beginning of the reaction, was developed by several groups to synthesize cobalt 33,34,35 and cobalt-nickel 36,37,38,39,40 nanorods or nanowires, as building blocks for permanent magnet materials 41,42 or as catalysts. 33,43,44,45,46 The one-pot in situ SMG methods present the advantage to be scalable to several grams per batch 42,47 but the drawback compared to the ex situ methods is the difficulty to study properly the nucleation steps as far as the seeds are not isolated. Several studies reported that the addition of a small amount of nucleating agent was mandatory to induce cobalt anisotropic growth and formation of nanowires, 38,43,48,49 but despite this empirical support on the role of the noble metal, no clear mechanism of the seeding process was described.…”

Section: Introductionmentioning

confidence: 99%

One-Pot Seed-Mediated Growth of Co Nanoparticles by the Polyol Process: Unraveling the Heterogeneous Nucleation

et al. 2019

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The one-step seed-mediated synthesis is widely used for the preparation of ferromagnetic metal nanoparticles (NPs) since it offers a good control of particle morphology. Nevertheless, this approach suffers from a lack of mechanistic studies because of the difficulties to follow in real time the heterogeneous nucleation and to predict structure effects with seeds that are generated in situ. Here, we propose a complete scheme of the hetero-nucleation process involved in onepot seed-mediated syntheses of cobalt nanoparticles in liquid polyols, relying on geometrical phase analysis (GPA) of high-resolution HAADF-STEM images and in situ measurements of the molecular hydrogen evolution. Cobalt particles of different shapes (rods, platelets or hourglass like particles) were grown by reducing cobalt carboxylate in liquid polyols in presence of iridium or ruthenium chloride as nucleating agent. A reaction scheme was established by monitoring the H2 evolution resulting from the decomposition of metal hydrides, formed in situ by β-elimination of metal alkoxides, and from the polyol dehydrogenation, catalytically activated by the metal particles. This is a very good probe for both the noble metal nucleation and the heterogeneous 2 nucleation of cobalt, showing a good separation of these two steps. Ir and Ru seeds with a size in the range 1-2 nm were found exactly in the center of the cobalt particles, whatever the cobalt particle shape, and high-resolution images revealed an epitaxial growth of the hcp Co on fcc Ir or hcp Ru seeds. The microstructure analysis around the seeds evidenced two different ways of relaxing the lattice mismatch between the seeds and the cobalt, with the presence of dislocations around the Ir seeds and compression zones of the cobalt lattice near the Ru seeds. The relationship between the nature of the nucleating agent, the reaction steps and the microstructure is discussed.

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Section: Introductionmentioning

confidence: 99%

One-Pot Seed-Mediated Growth of Co Nanoparticles by the Polyol Process: Unraveling the Heterogeneous Nucleation

et al. 2019

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“…As it can be seen, the magnetic characteristics improved when the aspect ratio increased. The magnetization at saturation Ms increased from 133 to 152.5 emu/g, which represents respectively 80% and 92% of the bulk expected value [8]. The variations in the coercive field H C as a function of the aspect ratio and as a function of the stacking fault density are given respectively in Figure 9a,b.…”

Section: Magnetic Propertiesmentioning

confidence: 90%

“…Indeed, Soumare et al [6] showed that, starting from cobalt laurate, the polyol process led to cobalt nano-rods with lengths up to 300 nm and diameters around 20 nm. The anisotropic growth of these nano-rods along the c axis of the hcp structure was explained on the basis of the preferential adsorption of laurate species on the different faces of the hcp habit [8,23]. Based on the estimation of the chemical potential of the ligand (laurate), it was established that, in the first step of the crystal growth, the laurate is preferentially adsorbed on the lateral facets.…”

Section: Shape and Structure Characterizationsmentioning

confidence: 99%

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Enhanced Magnetic Behavior of Cobalt Nano-Rods Elaborated by the Polyol Process Assisted with an External Magnetic Field

et al. 2020

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Cobalt nano-rods with the hexagonal close-packed (hcp) structure were prepared by reduction of the long-chain carboxylate Co (II) precursor in polyol. The application of an external magnetic field (µ0H = 1.25 T) during the nucleation and growth steps resulted in a noticeable modification of the mean aspect ratio (length/diameter) of the particles. The particle morphology was also modified as the nano-rods did not exhibit conical heads at their extremities anymore, which are observed for particles prepared without application of an external magnetic field. Besides, the stacking faults density along the c axis of the hcp structure in the cobalt nano-rods has been found to decrease with the increase in the applied magnetic field. The coercive field of randomly oriented nano-rods increased with the aspect ratio, showing the highest value (i.e., 5.8 kOe at 300 K) for the cobalt nano-rods obtained under the highest applied magnetic field. For partially oriented Co nano-rods in toluene solution, the magnetic properties were significantly enhanced with a coercive field of 7.2 kOe at 140 K, while the magnetization saturation reached 92% of the bulk. The MR/MS value was about 0.8, indicating a good orientation of the anisotropic particles relative to each other, making them suitable for the preparation of permanent magnets via a bottom-up approach.

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“…For both reference systems, the desired NPs were recovered by alternating centrifugation, washing and drying. These experimental conditions were chosen following previous works, which first established that the length of the polyol molecules and the stirring rate may be critical for the crystal growth mechanism: the shorter the polyol chains; the aggregated as polycrystal the ferrite particles produced and the gentler the stirring, the better the crystallinity of the precipitates …”

Section: Methodsmentioning

confidence: 99%

Magnetic Traits in CoO‐Core@CoFe₂O₄‐shell Like Nanoparticles

et al. 2019

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Antiferromagnetic (AF) CoO pseudo-single crystals 125 AE 20 nm in diameter were prepared by forced hydrolysis in polyol and used as seeds to grow a poly-and nanocrystalline (5 nm in diameter) ferrimagnetic (F) CoFe 2 O 4 shell. Transmission electron microscopy (TEM) showed a certain epitaxy between the face-centered cubic lattices of the AF rock-salt core and the F spinel shell. Magnetometry measurements do not evidence any exchange bias feature between the two phases in direct contact. The hysteresis loops recorded between 7 and À7 T after cooling the composite particles from 400 K (between the Néel and the Curie temperatures of the AF and F phases, respectively) to 5 K, with and without a cooling magnetic field (m 0 H FC = +7 T), are the same, meaning that the AF core does not allow any spin pinning on the CoFe 2 O 4 nanocrystals. Nevertheless, differences exist between these magnetic data and those collected on two CoFe 2 O 4 reference particles: the low temperature coercive field of the composite is between those of 5 nm sized single-crystals and their assembly as submicrometer polycrystals. This order was also observed for the blocking temperature, although the constituting CoFe 2 O 4 nanocrystals of all these particles, the composite and the two references, were the same. Advanced analysis by X-ray diffraction and TEM established that the coherent crystallographic domain length of the ferrite phase and the magnetic behavior of the three systems are interdependent. Depending on the growth conditions of the CoFe 2 O 4 phase, a relative epitaxy between the nanocrystals may exist. It is much greater in the polycrystalline CoFe 2 O 4 particles than in their CoOÀCoFe 2 O 4 core-shell counterparts, and does not occur in free single crystal CoFe 2 O 4 particles.[a] Dr.

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Control of the crystal habit and magnetic properties of Co nanoparticles through the stirring rate

Cited by 28 publications

References 35 publications

One-Pot Seed-Mediated Growth of Co Nanoparticles by the Polyol Process: Unraveling the Heterogeneous Nucleation

One-Pot Seed-Mediated Growth of Co Nanoparticles by the Polyol Process: Unraveling the Heterogeneous Nucleation

Enhanced Magnetic Behavior of Cobalt Nano-Rods Elaborated by the Polyol Process Assisted with an External Magnetic Field

Magnetic Traits in CoO‐Core@CoFe₂O₄‐shell Like Nanoparticles

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Control of the crystal habit and magnetic properties of Co nanoparticles through the stirring rate

Cited by 28 publications

References 35 publications

One-Pot Seed-Mediated Growth of Co Nanoparticles by the Polyol Process: Unraveling the Heterogeneous Nucleation

One-Pot Seed-Mediated Growth of Co Nanoparticles by the Polyol Process: Unraveling the Heterogeneous Nucleation

Enhanced Magnetic Behavior of Cobalt Nano-Rods Elaborated by the Polyol Process Assisted with an External Magnetic Field

Magnetic Traits in CoO‐Core@CoFe2O4‐shell Like Nanoparticles

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Magnetic Traits in CoO‐Core@CoFe₂O₄‐shell Like Nanoparticles