Preparation and Properties of Monodisperse Magnetic Cobalt Colloids Grafted with Polyisobutene (original) (raw)
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Synthesis and Characterization of Large Colloidal Cobalt Particles
Langmuir, 2006
Large colloidal environmentally stable silica-coated cobalt particles were synthesized by combining the sodium borohydride reduction in aqueous solution and the Stöber method. Low size polydisperse cobalt spheres with an average size of 95 nm were synthesized by using a borohydride reduction method and were subsequently coated with a thin layer of silica by means of hydrolysis and condensation of tetraethylorothosilicate (TEOS) in an aqueous/ ethanolic solution. The large uniform cobalt spheres consist of smaller metallic Co clusters, explaining the superparamagnetic behavior of the spheres. The particles were investigated by transmission electron microscopy (TEM) and vibrating sample magnetometry (VSM).
Surface effects on the magnetic properties of ultrafine cobalt particles
Physical Review B, 1998
Monodispersed nanoparticles of cobalt have been prepared by an original method using the decomposition under hydrogen of an organometallic precursor in the presence of a stabilizing polymer. Two colloids ͑Coll-I and Coll-II͒ have been obtained by changing the organometallic concentration in the polymer. Observation by high-resolution transmission electronic microscopy ͑HRTEM͒ showed Co particles well isolated and regularly dispersed in the polymer with a very narrow size distribution centered around 1.5 nm ͑Coll-I͒ and 2 nm ͑Coll-II͒ diameter. These particles are superparamagnetic above the blocking temperature 9 K ͑Coll-I͒ and 13.5 K ͑Coll-II͒. The particle size deduced from the analyses of the magnetic susceptibilities and magnetization curves are consistent with those measured by HRTEM. Magnetization at 5 K seems to saturate in fields up to 5 T leading to an enhanced mean magnetic moment per atom for both samples, where ͗ Co ͘ϭ1.94Ϯ0.05 B for the smallest particles. High-field magnetization measurements, up to 35 T, increases nearly linearly with the applied field. This is equivalent to an increase of the mean magnetic moment with ͗ Co ͘ϭ2.1Ϯ0.1 B at 35 T for the smallest particles. The effective magnetic anisotropies are found to be larger than that of the bulk materials and decrease with increasing particle size. This set of data allows us to conclude that the enhanced magnetization, its increase with applied magnetic field, and the enhanced effective magnetic anisotropy are associated with the large influence of the surface atoms and are more significant with decreasing size.
Magnetic properties and microstructure of cobalt nanoparticles in a polymer film
Solid State Communications, 2003
Superparamagnetic properties of self-aggregated cobalt nanoparticles in the perfluorinated sulfo-cation membrane (MF-4SK) prepared by ion-exchange method were investigated by transmission electron microscopy (TEM) and superconducting quantum interference device (SQUID) magnetometry at various temperatures. Our experimental results show that cobalt nanoparticles in MF-4SK exhibit superparamagnetic properties above the blocking temperature ðT B Þ; which varies from , 80 to , 300 K depending on the cobalt concentration at 100 Oe applied field. The average particle radius of 3.8 nm inferred from Langevin function fit for the concentration of 7.8 £ 10 19 cobalt atoms per 1 g of polymer film is in good agreement with TEM observation. This experimental evidence suggests that cobalt nanoparticles in the polymer film obey a single-domain theory. The results are discussed in the light of current theory for the superparamagnetic behavior of magnetic nanoparticles.
Magnetic properties of colloidal cobalt nanoclusters
Co nanoclusters were synthesized by an inverse-micelle chemical route. The magnetic and microstructural properties of the nanoparticles have been analyzed as a function of the surfactant (AOT and DEHP) and the drying method. Microstructural analysis has been performed by TEM and XANES; magnetic properties have been studied by hysteresis loops and zero-field cooling – field cooling (ZFC-FC) curves. TEM images show 2 to 4 nm sized particles spherical in shape. XANES measurements point out a significant presence of Co3O4with metallic Co and some Co2+ bound to the surfactant. The presence of antiferromagnetic Co3O4 explains the magnetic transition observed at low T in both ZFC-FC measurements and hysteresis loops. Finally, the presence of magnetic interactions explains the bigger effective cluster size obtained from hysteresis loops fits (6-10 nm) compared to the sizes observed by TEM (2-4 nm).
Large-scale synthesis of defined cobalt nanoparticles and magnetic metal–polymer composites
Nanoscale, 2009
We present a method where 3-cobalt nanoparticles with an average diameter of 4.5 nm can be synthesized in a controlled process and in significantly larger quantities than previously reported in the literature, based on the thermal decomposition of dicobaltoctacarbonyl in the presence of oleic acid and trioctylphosphine oxide (TOPO). Moreover, since the resulting particles are coated with an oleate layer, as shown by infrared (IR) spectroscopy, the colloids can be re-dispersed in organic solvents. These dispersions are suitable for the preparation of nanocomposites by a simple procedure, i.e. mixing of the cobalt dispersion with a polymer solution followed by casting and solvent evaporation. Magnetization measurements confirm the expected superparamagnetic behavior for both the cobalt nanoparticles and the metal-polymer nanocomposites.
Microstructure and magnetic properties of colloidal cobalt nano-clusters
Journal of Magnetism and Magnetic Materials, 2010
The magnetic response of nanometer sized Co nanoparticles (NP) prepared using reverse micelle solutions are presented. The use of complementary structural and morphological probes (like transmission electron microscopy, high resolution electron microscopy, X-ray absorption spectroscopy) allowed to relate the magnetic properties to the size, morphology, composition and atomic structure of the nanoparticles. All data agree on the presence of a core-shell structure of NPs made of a metallic Co core surrounded by a thin Co-oxide layer. The core-shell microstructure of NPs affects its magnetic response mainly raising the anisotropy constant.
ChemInform, 2012
Recently, magnetic nanocomposites (MNC) have aroused significant scientific and technological interests because their properties strongly rely on the interplay between those of the constituent components. Here, using three types of cobalt-based MNCs, we demonstrate how their physical behaviour, including thermal, electrical and magnetic, can be strongly affected by such interplays. First, using Au core -Co shell nanoparticles (NPs), we demonstrate that their thermal stabilities are critically dependent on various boundaries and they structurally transform from the core-shells to the peanut structures via several intermediate states by a series of energy minimizations including the grain boundaries, Co/Au interface and strain. Second, the microstructures of the MNC are co-determined by the properties of the individual components, which in turn will strongly affect their overall properties. We illustrate this by a careful study of the electron transport in cobalt/poly (3-hexylthiophene, 2, 5-diyl) (P3HT) hybrid thin films, and show that they satisfy a fluctuation-induced tunnelling model that is strongly depended on their microstructures; moreover, a magnetoresistance in these thin films was also observed. Finally, the magnetic properties and phase stability of MNCs can also be strongly altered as a result of this interplay. Three phase transformations are observed in cobalt ferrofluids for T ∼ 10-300 K, namely second order magnetic phase transformations (blocked-unblocked transition) at the blocking temperature of the magnetic NP, first order magnetic and structural phase transformations at the solvent melting temperature, T M , and second order premelting transformation at T PM < T < T M . These transformations show specific magnetic signatures in field-cool and zero-field-cool magnetization measurements and are qualitatively in agreement with predictions using M-spectrum theory.
Journal of Magnetism and Magnetic Materials, 2017
In this work, for first time the ferrofluids based on the cobalt ferrite (CoFe 2 O 4) nanoparticles were prepared by the co-precipitation method at different reaction times (0.5-6.5 hours). Crystal structure, morphology and magnetic properties of the cobalt ferrite nanoparticles and the ferrofluids based on the nanoparticles were studied by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and vibrating sample magnetometer (VSM). The XRD patterns of CoFe 2 O 4 nanoparticles synthesized at different reaction times indicated that all samples are single phase in accordance with inverse cubic spinel structure with space group Fd-3m, and no impurity phase was observed. By increasing the reaction time to 3.5 h, the lattice parameter and the average crystallites size increased and then afterwards decreased by increasing the reaction time. The microscopic studies indicated the formation of nanosized particles with nearly spherical in shape, whereas the average particle size for all samples is found to be less than 50 nm. The results of VSM also showed that the saturation magnetization and coercivity field of the cobalt ferrite nanoparticles and the ferrofluids were influenced by reaction time, whereas the ferrofluids have lower values of magnetic parameters than that of nanoparticles.
Synthesis and characterization of magnetic cobalt ferrite nanomaterials
2010
Magnetic fluids which constitute a suspension of magnetic nanoparticles in a carrier liquid are termed ferrofluids. Ferrofluids are smart fluids exhibiting both magnetic and fluid like characteristics. In the present study, we report on ferrofluidswhich are suspensions of magnetite (Fe 3 O 4) nanoparticles in carrier fluids of varying viscosity. The magnetite nanoparticles have been synthesized by a chemical co-precipitation method.X-Raydiffraction of the as prepared sample revealsa single phase of magnetite.SEM characterisation reveals regular morphology and a narrow size distribution for the synthesized nanoparticles. These are dispersed in pure water and mixtures of water and glycerol using suitable surfactants and ultrasonicated. Our studies reveal that the stability of the ferrofluids have a strong dependence on the viscosity of the carrier fluid as well as the pH, with high viscosity and low pH yielding more stable ferrofluids. While low concentration of the magnetic component renders it impossible to see signatures of its presence in Raman spectroscopy, SQUID magnetometryreveals the presence of the magnetic component suggesting a distribution in size of the nanoparticles with a broad blocking temperature range. Further, a magnetic anomaly is seen at the freezing point of the carrier fluid in the ZFC-FC measurements for the ferrofluid made with 50% v/v aqueous glycerol solution.
Size and shape control for water-soluble magnetic cobalt nanoparticles using polymer ligands
Journal of Materials Chemistry, 2008
We report a synthesis of monodisperse water-soluble magnetic Co nanoparticles using a facile reduction method in aqueous media in the presence of alkyl thioether end-functionalized poly(methacrylic acid) (PMAA-DDT) ligands. The size and shape of the nanoparticles are both tunable by varying synthesis conditions. The size of the spherical nanoparticles can be tuned between 2-7.5 nm by changing the concentration of the polymer. Our synthesis approach also provides a route for producing much larger spherical nanoparticles of 80 nm as well as anisotropic nanorods of 15 Â 36 nm. The spherical nanoparticles are superparamagnetic at room temperature. The nanoparticles can be stable in water for up to eight weeks when 0.12 mM PMAA-DTT with molecular weight of 13500 g mol À1 is used as ligand.