Cobalt Microspheres Research Articles

Effect of reaction pH value on the microtopography of cobalt microspheres and microwave absorbing properties of Co@MWCNTs

Chemical plating as a simple and effective method has been widely used in the production of metal materials, which can essential to reduce the density and improve the application of metal materials. In this paper, the cobalt microspheres were prepared by chemical plating while the polystyrene-microspheres were as a template. It was found that the pH value of reaction had a significant impact on the microtopography of Co-microsphere. XRD results of the Co-microsphere illustrated that, the crystal shape of Co-microspheres were affect by the pH value. Otherwise, the SEM results indicated that the flower-like cobalt microspheres were prepared while the pH value was 12. In addition, the microtopography of Co-microsphere had a significant effect on microwave absorption property of Co@MWCNTs. The reflection loss results demonstrated that, the Co-12@MWCNTs had a good microwave absorption performance, while the maximum reflection loss was −38.94 dB and the effective absorbing bandwidth was 4.28 GHz at a thickness of 2 mm. Therefore, it is necessary to study the influence of pH value on the microtopography of Co-microspheres and microwave absorbing mechanism of Co@MWCNTs.

Ослабление диполь-дипольного взаимодействия в ансамблях микросфер кобальта с немагнитным ядром

Magnetic microspheres with nonmagnetic acrylic glass core were fabricated using electroless deposition. There are favorable conditions for magnetic flux closure within each particle. As a result, the dipole-dipole interaction in the powder of such microspheres is reduced drastically. This is the key difference between powders of magnetic microspheres and powders of full magnetic spherical particles witch behavior is significantly affected by the dipole-dipole interaction. The magnetic hysteresis in the cobalt microspheres with nonmagnetic core is significantly large then in the full cobalt particles produced using the same technique.

Dipole-dipole interaction weakening in ensembles of cobalt microspheres with a nonmagnetic core

Magnetic microspheres with nonmagnetic acrylic glass core were fabricated using electroless deposition. There are favorable conditions for magnetic flux closure within each particle. As a result, the dipole-dipole interaction in the powder of such microspheres is reduced drastically. This is the key difference between powders of magnetic microspheres and powders of full magnetic spherical particles witch behavior is significantly affected by the dipole-dipole interaction. The magnetic hysteresis in the cobalt microspheres with nonmagnetic core is significantly large then in the full cobalt particles produced using the same technique. Keywords: Magnetic microspheres, magnetic dipole-dipole interaction.

SOLVOTHERMAL SYNTHESIS AND CHARACTERIZATION OF POROUS Co MICROSPHERES

Porous cobalt (Co) microspheres were successfully synthesized via a facile surfactant-free solvothermal route. These microspheres have an average diameter of about 2 µm, assembled by many primary Co nanosheets. A possible formation mechanism of porous Co microspheres is discussed. The magnetic hysteresis loop shows a ferromagnetic feature of porous Co microspheres and the saturation magnetization (Ms) and coercivity (Hc) are 115.3 J/T·kg and 6.883 kA/m, respectively.

Preparation of topographically remarkable cobalt microspheres for colorimetric detection of dopamine and elucidation of its sensing mechanism

In this work, a new colorimetric probe is prepared and demonstrated for the detection of dopamine (DA), an important neurotransmitter. The proposed colorimetric probe is prepared using bovine serum albumin (BSA) modified cobalt hydroxide microspheres (CoMSPs@BSA) and subsequently explored for the selective colorimetric detection of DA. The sensing studies are monitored using UV–visible absorbance spectroscopy. Interestingly upon addition of DA, an intense blue color is formed suggesting the potential use of this probe for naked eye detection of DA. The mechanism behind the sensing is investigated using time dependent UV–visible absorbance spectroscopic study and cyclic voltammetry. These studies clearly indicate the ligand exchange reaction is responsible for the resultant blue color formation. Further using this colorimetric probe a concentration range from 5.3 to 211 µM DA could easily be detected. Limit of detection of this DA sensor is found to be 5 × 10−6 M. Similarly a linear relation is observed from the calibration graph plotted between the concentration and absorption intensity corresponding to ligand exchange of BSA by DA. Moreover, this sensing probe (CoMSPs@BSA) is found to be highly selective among a wide range of other commonly interfering biomolecules. Topographically remarkable cobalt microspheres are prepared using a simple method and explored for the colorimetric detection of dopamine. The sensing mechanism is elucidated using UV–visible spectroscopic and electrochemical studies.

Defect engineering of cobalt microspheres by S doping and electrochemical oxidation as efficient bifunctional and durable electrocatalysts for water splitting at high current densities

Herein, we perform a facile one-step deep eutectic solvent electrodeposition approach to fabricate a series of S-doping highly porous Co-S films by in situ grown on Ni foam (Co-S/NF), which exhibits a superior catalytic activity and robust nature for both oxygen and hydrogen evolution reaction (OER and HER) in alkaline media. The doping of S is evidenced to induce a structural and size transition on the Co deposited layer, which leads to a significant increase on the electrochemical surface area and more accessible active sites for enhancing the catalytic activity. The formation of oxygen deficiencies-rich Co-O/OH species with a phase transformation from Co-S species is noticed during OER. Benefiting from the merits of in situ growth, the Co-S films obtained at an optimal S doping level (denoted as Co-S-50/NF) can robustly drive high current densities of 100 and 500 mA cm−2 with low overpotentials of 322 and 368 mV for OER, and 124 and 155 mV for HER. The water electrolyzer assembled with Co-S-50/NF requires only 1.79 and 2.08 V to reach 100 and 500 mA cm−2 in 1.0 M KOH, and 1.72 V to achieve 500 mA cm−2 in 30 wt% KOH at 353 K for over 150 h.

Characteristics of Co3O4 and cobalt nanostructured microspheres: Morphology, structure, reduction process, and magnetic properties

Nanostructured magnetic cobalt microspheres were synthesized using a two-stage method. First, nanostructured cobalt oxide Co3O4 microspheres were produced by ultrasonic spray pyrolysis at different temperatures from 800 to 1100C. Second, the nanostructured powders obtained were reduced to metal cobalt in an hydrogen atmosphere at different temperatures from 220 to 350C. In this study the evolution of the morphology from Co3O4 nanostructured microspheres to nanostructured Co, resulting from the Co3O4 reduction process, and the structure of metal cobalt, were investigated. It was found that the metal cobalt agglomerates have the same shape and mean diameter (∼ 1,5μm) as those of the cobalt oxide, Co3O4. The amount of Co microspheres with smaller diameters decreased with increasing reduction temperature. Also, the β-Co content increased with increasing reduction temperature.

Size-controlled synthesis, growth mechanism and magnetic properties of cobalt microspheres

Monodispersed cobalt microspheres with remarkably uniform morphology and tunable size have been successfully synthesized via a facile and effective solvothermal route. It is found that sodium hydroxide has a great influence on the size and shape of the resulting cobalt powders. Through adjusting the molar ratio of NaOH to Co2+, the size of cobalt microspheres can be precisely manipulated. In addition, the magnetic properties of the as-synthesized samples demonstrate that cobalt microspheres exhibit ferromagnetic property related to their sizes. More importantly, this study not only provides a facile and effective strategy to prepare the cobalt microspheres with tunable sizes, but also gives a reference for the synthesis of other magnetic functional materials.

Elaboration, characterization and magnetic properties of cobalt nanoparticles synthesized by ultrasonic spray pyrolysis followed by hydrogen reduction

Nanostructured cobalt microspheres were synthesized by a two-stage method. First, cobalt oxide Co3O4 nanostructured microspheres were synthesized by spray pyrolysis at 700, 800, 900, 1000, and 1100, °C. Second, the nanostructured powders thus obtained were reduced to metal cobalt in a hydrogen atmosphere at 300°C. For a sample obtained at 1000°C, the influence of the reduction temperatures (220, 240, 270, 300, and 350°C) on the metal cobalt powders properties was investigated. All the samples obtained were characterized by means of X-ray diffraction (XRD), coherent length determination and scanning electron microscopy (SEM). In addition, an average size of the particles, a specific surface area, and an effective diameter were determined. Finally, the magnetic properties of the metal cobalt, such as coercive force, saturation, and residual magnetization were measured by vibration sample magnetometer (VSM). The dependence on synthesis conditions of nanostructured metal cobalt microspheres properties was highlighted.

Effects of microstructure and filling ratio on electromagnetic properties of Co microspheres

Cobalt microspheres with diameters of 1.5–3.5µm were synthesized by a liquid phase reduction method. The effects of hydrogen annealing on microstructure evolution and electromagnetic properties of Co microspheres were investigated. The influence of filling ratio on the electromagnetic properties of specimens containing Co microspheres as fillers was also examined. The results indicated that the annealing leads to increase in Co microspheres' permittivity as the improved conductivity that developed during annealing contributes to enhanced dielectric relaxation. High filling ratio is found to be favorable for achieving high electromagnetic properties and thus higher electromagnetic absorbing performances, which is of technical significant for application in low frequency band. Coatings containing 30, 45 and 50vol% Co particles as fillers present excellent EMA performance, even very thin thickness is applied. High electromagnetic wave absorbing efficiency of −10dB was observed at thickness of 1.5mm in C band and the electromagnetic wave absorption bandwidth reaches up to 6.3GHz (6.7–13GHz) when the filling volume is 45vol%.

Three-dimensional cobalt microspheres composed of nanosheets assembled via facile hydrothermal method

We report a template-free approach to prepare three-dimensional cobalt microspheres composed of nanosheets by a facile hydrothermal method. The synthesized 3D cobalt microspheres with diameter of approximately 1–2µm have a nanosheet of ca. 18nm. The samples were characterized by X-ray diffraction, SEM, TEM, and VSM techniques. It was found that the initial concentration of Co(NO3)2·4H2O and surfactant played an important role in the formation of 3D cobalt microspheres. The Ms value of 3D cobalt microspheres which obtained under the cationic surfactant is about 122.55emu/g. Due to the specific structure and magnetic properties, these 3D cobalt microspheres are expected to have potential applications as candidates for catalysis, sensor and energy storage.

Low temperature preparation of a graphene-cobalt microsphere hybrid by borohydride-initiated reduction for enriching proteins and peptides.

A graphene-cobalt microsphere hybrid was prepared by the chemical reduction of a mixture containing graphene oxide and cobalt chloride. It was found that a little amount of potassium borohydride or sodium borohydride could initiate the hydrazine-reduction of the mixture at a low temperature of 80 °C. The structure of the material was investigated by scanning electron microscopy, transmission electron microscopy, energy dispersive spectroscopy, vibrating sample magnetometery, Fourier transform infrared spectroscopy, Raman spectroscopy, and Brunauer-Emmett-Teller (BET) techniques. The average size of the cobalt microspheres on graphene sheets was measured to be ∼590 nm. Magnetic investigations indicated that the graphene-cobalt microsphere hybrid exhibited ferromagnetic behavior at room temperature. In addition, the magnetic hybrid was successfully employed in the enrichment and identification of low-abundance proteins and peptides in combination with mass spectrometry.

Preparation Biomagnetic Sorbent for Cleaning Crude Oil Spills

The objective of this study is preparing a novel environmental friendly light weight magnetic sorbent consisting of bacteria and cobalt by chemical process to further cleaning crude oil spills in sea. The observations by ESEM indicated that a thin granular layer covered on the surface of model microorganism of Staphylococcus epidermildis which provided host for absorbing cobalt ions and depositing cobalt particulates. The results showed that composite present flower-like microspheres with large-scale yields. The high temperature treatment of amorphous bacteria composited cobalt microspheres showed that it well crystallized corresponding to Co3O4 at 800 °C. The microorganism composited magnetic sorbent opens an important alternation absorbent for timely oil spill cleanup.

Facile synthesis of hollow porous cobalt spheres and their enhanced electromagnetic properties

We report a template-free approach to prepare hollow porous cobalt spheres by combining a facile solvent-thermal route with a chemical reduction process. The synthesized cobalt microspheres with diameter of approximately 2–4 μm have a shell thickness of about 150 nm. The hollow Co microsphere is built from particle-like ligaments with diameter of ca. 80–150 nm and there are a lot of pores with size of several tens of nm on the shell. The hollow porous cobalt spheres exhibited saturation magnetization (Ms) of 163.0 emu g−1 and large coercivity (Hcj) of 307.9 Oe. The epoxy resin composites with 30 wt% and 60 wt% hollow porous cobalt sample showed efficient EM wave absorption characteristics (RL < −20 dB) in the ranges of 11.3–18.0 GHz and 4.0–12.0 GHz over absorber thicknesses of 1.4–2.0 mm and 1.4–4.0 mm, respectively. Our study demonstrates the potential application of hollow porous ferromagnetic materials as a more efficient electromagnetic wave absorber.

SELF-ASSEMBLED COBALT CHAINS INDUCED BY MAGNETIC FIELD THROUGH HYDROTHERMALLY REDUCING COBALT IONS

Co uniform hierarchical microspheres were grown on a large scale at 200° by a simple hydrothermal method with the help of the surfactant polyvinylpyrrolidone (PVP). During the synthesis, the external magnetic field as a controlling factor assembled cobalt microspheres to form chain-like structures vertically on substrate. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the product. The results indicate that three-dimensional (3D) Co microspheres are constructed by the cross-sectional nanoplatelets shape with a mean thickness of ~ 250 nm. The mechanism involved for the formation of the chain-like structures is discussed. The results of the vibrating sample magnetometer (VSM) show that the magnetic Co chains possess the saturation magnetization of 125.2 emu/g. Our work may shed some light on the design of other well-defined complex nanostructures, and as-grown architectures may have potential applications.

Selective synthesis and characterization of metallic cobalt, cobalt/platinum, and platinum microspheres

Metallic cobalt microspheres have been successfully synthesized via a facile hydrothermal method in the presence of poly (N-vinyl-2-pyrrolidone) (PVP), and the diameters of these spheres are in the range of 1–2 μm. Bimetallic Co/Pt microspheres and hollow noble metal Pt microspheres have also been selectively synthesized by adjusting the amount of reactants of replacement reaction in which corresponding noble metal compound and metallic cobalt microspheres are used as starting materials. The study on catalytic activities of the as-prepared metallic microspheres on hydrolysis of ammonia-borane for hydrogen generation reveals that Co/Pt (molar ratio 5:1) bimetallic microspheres are far more active than the hollow noble metal Pt microspheres.

Reduction of Complex Precursor Cobalt Bis(4-pyridine carboxylate) Tetrahydrate under an External Magnetic Field

The reduction of complex precursor cobalt bis(4-pyridine carboxylate) tetrahydrate under a magnetic field was investigated. Particular cobalt potato-like hierarchical structures (10-50 μm) composed of nanoflakes (10-20 nm in thickness) were obtained without a magnetic field applied due to spherical clusters consisting of cobalt complexes formed before reduction. When a 0.35 T external magnetic field was applied, one-dimensional microchains comprising potato-like cobalt microspheres were formed via the same route, which has a length of hundreds of microns, while the assembly of nanoflakes induced by the magnetic field was not observed. The experiment results indicate that the interactions among the complex molecules (or nanoflakes) are stronger than the magnetic interaction between the magnetic dipolar and the applied magnetic field. As a result, the spherical clusters of cobalt complex and the potato-like spheres are stable under the applied magnetic field. The applied field determines the assembly of potato-like spheres to form one-dimensional microchains only.

A facile aqueous phase synthesis of cobalt microspheres at room temperature

Cobalt microspheres constructed by the assembly of nanoplatelets have been synthesized by a wet chemical reductive procedure at room temperature with the help of glycerin and citric acid and without additional surfactants. The size of the microspheres is about 2–5 μm and that of the nanoplatelets assembled the microspheres is tens of nanometers in thickness. In this synthetic system, cobalt acetate was employed as Co source, sodium hydroxide was used to manipulate the pH value of the reaction system, and hydrazine hydrate was used as a reducing agent. A series of experiments were performed with different amounts of glycerin, from 0.5 mL to 4 mL, the results reveal that the formation of cobalt microspheres is assisted by glycerin. The shape, structure, and magnetic properties of the final products were investigated by XRD, SEM and VSM. This kind of Co nanostructures shows a ferromagnetic behavior at room temperature with enhanced coercivity, and has potential uses in magnetic recording devices and other related nanodevices. A possible mechanism for the formation of microspheres is proposed.

Shape-controlled synthesis and self-assembly of hexagonal cobalt ultrathin nanoflakes

Hexagonal cobalt microspheres composed of ultrathin nanoflakes were obtained via a hydrothermal reduction route by using the metal complex cobalt bis (4-pyridine carboxylate) tetrahydrate as a precursor of cobalt. The diameter of these spheres is about 2 μm. The thickness of the cobalt nanoflakes is about 10 nm along the easy magnetization axis [0 0 1], which is approximate to the critical size (12 nm) for single-domain behavior in Co. The hysteresis loop of the sample shows an interesting soft magnetic behavior with low coercivity (24.5 Oe). It is noticed that the coercivity is much lower than that of the hcp cobalt nanoflakes reported previously. The peculiar magnetic properties may result from both the anisotropy and the self-assembly manner of the nanoflakes.

Surfactant-assisted hydrothermal synthesis of chains self-assembled by cobalt microspheres

Magnetic cobalt chains, self-assembled by microspheres of hexagonal-phase cobalt, have been synthesized at 100 °C via a hydrothermal reduction route in the presence of cobalt chloride, the surfactant sodium dodecylsulfate (SDS) (or cetyltrimethylammonium bromide CTAB) and the complex reagent sodium tartrate. As-synthesized, the chains are 100–300 μm in length and the cobalt microspheres, which consist of nanosheets with an average thickness of about 60 nm, are 5–10 μm in diameter. The magnetic hysteresis loops at 5 K and 300 K of the chains of microspheres show ferromagnetic characteristics. The morphologies of the microspheres can be controlled by adjusting the concentrations of the surfactant and the complex reagent and also the reaction temperature.