Carbon-encapsulated Cobalt Nanoparticles Research Articles

Bagasse derived N-doped graphitic carbon encapsulated cobalt nanoparticles as an efficient bifunctional catalyst for water splitting reactions

The utilization of non-precious metal-based electrodes to unlock cost-effective and scalable processes for the production of clean energy is highly desirable. Herein, we reported a straightforward hydrothermal carbonization method for the synthesize of nitrogen-doped graphitic carbons integrated with cobalt (Co@NGC-600, Co@NGC-700, and Co@NGC-800) from sugarcane bagasse and their electrocatalytic applications in oxygen and hydrogen evolution reactions. The graphitic nature of the Co integrated N-doped carbon materials was confirmed by Raman and XPS analysis. FESEM and HRTEM analysis confirms the dispersion of Co on N-doped carbon nanosheets. The Co@NGC-800 catalyst shows low overpotential and Tafel slope, requiring only 304 mV (84.1 mV dec−1) for oxygen electrode and 234 mV (118 mV dec−1) for HER at a current density of 10 mA cm−2. Moreover, Co@NGC-800 materials showed better activity than Co@NGC-700, Co Co@NGC-600, and Co@NC. The active Co@NGC-800 bifunctional electrocatalysts pair contributes to the fabrication of a water electrolyser with a 10 mA/cm2 current density at 1.68 V. The Co@NGC-800 // Co@NGC-800 electrocatalyst exhibits good stability over a period of 24 h with negligible potential aberration during the process of water splitting. The electrocatalytic performance of the prepared catalyst in a 1.0 M KOH solution exhibits admirable bifunctional activity and remarkable stability. In fact, it outperforms cobalt-based and carbon-supported electrocatalysts, including highly valued commercial catalysts like IrO2 and Pt@C. The solar cell configuration, utilizing the energetic bifunctional Co@NGC-800 electrocatalyst, can continuously generate hydrogen and oxygen. The overall water-splitting process of Co@NGC-800 catalyst delivered at a cell voltage of 1.68 V, enabling sustained and efficient oxygen and hydrogen evolution at a large scale.

Cobalt catalyst encapsulated by N-doped carbon for selective hydrogenation of aldehyde to alcohol

Furfuryl alcohol is an important industrial chemical and an intermediate for the synthesis of other high-value products. Here, an explosion-assisted strategy was developed to synthesize nitrogen-doped carbon-encapsulated cobalt nanoparticles catalyst Co-NC. The selective hydrogenation of furfural to furfuryl alcohol can be achieved over Co-NC in aqueous solution successfully. The presence of cobalt nitrate enhanced the explosion process which promoted its decomposition in turn. The high-temperature reduction process accelerated the derivation of carbon nitride into N-containing carbon encapsulating on cobalt nanoparticles. The N-containing coating improves the hydrophilia and catalytic activity of catalyst, thus achieving the selective hydrogenation of aldehydes in aqueous solution.

Graphitic carbon-encapsulated cobalt nanoparticles embedded 1D porous hollow carbon nanofibers as advanced multifunctional electrocatalysts for overall water splitting and Zn-air batteries

Physical mixing of monofunctional noble metal catalysts, such as Pt/C or Ru/IrO2, increases the commercial cost and stability risk of electrodes. Therefore, it is desirable to develop a multifunctional electrocatalyst for zinc-air batteries and integrated electrolytic devices. To develop an effective way to fabricate high-performance multifunctional electrocatalysts by modifying advanced nanostructures, a coaxial electrospinning approach with in-situ synthesis and subsequent carbonization was used to construct a highly integrated three-function catalyst composed of graphitic carbon-encapsulated cobalt nanoparticles embedded into one-dimensional (1D) porous hollow carbon nanofibers (CoNC-HCNFs). Under the synergistic effect of the active material and the advanced nanostructure, the as-prepared CoNC-HCNFs demonstrated an operating overpotential of 186 mV (10 mA cm−2) for the hydrogen evolution reaction (HER), a half-wave potential of 0.83 V (vs. RHE at 10 mA cm−2) for the oxygen reduction reaction (ORR), and a potential of 1.58 V (10 mA cm−2) for the oxygen evolution reaction (OER). With their exceptional multifunctional activities, two CoNC–HCNF-based aqueous zinc-air batteries (ZABs) in series could drive an alkaline water electrolyzer for splitting water. Furthermore, due to the superior mechanical flexibility and rechargeability of the solid-state ZAB, it has great application prospects in powering portable and wearable electronics. This research is expected to offer inspiration for the development of other excellent MOF-based hollow carbon nanofibers and to enable them to be adopted more widely in electrochemical energy conversion and energy storage.

Encapsulated cobalt nanoparticles as a recoverable catalyst for the hydrolysis of sodium borohydride

The significant challenges encountered when replacing precious-metal-based catalysts in the hydrolysis of sodium borohydride (NaBH4) are related to the low durability and efficiency of nonprecious metals. In this study, we synthesized nitrogen-doped mesoporous graphitic carbon encapsulated cobalt nanoparticles (Co@NMGC) with a core–shell structure by the carbonization of ethylenediaminetetraacetic acid (EDTA)-derived carbon. These structures were then used as catalysts for the hydrolisis of NaBH4. Among different calcination temperatures, 500 ​°C resulted in the best catalytic activity, with a hydrogen production rate of 3575 ​mL ​min−1 g−1 at 25 ​± ​0.1 ​°C and low activation energy of 35.2 ​kJ ​mol−1 for NaBH4 hydrolysis. The maximum hydrogen production rate using the cobalt-based complex catalyst was about three-fold that achieved using the pure cobalt catalyst. We found that the complex catalyst showed high durability, retaining 82.5% of the initial catalytic activity after 20 hydrolysis cycles. The catalyst was also magnetic, making it 100% recyclable. Density functional theory (DFT) calculations showed that the graphitic carbon shell promoted electron penetration from the Co nanoparticles into the graphitic carbon surface, which prevented oxidation of the Co nanoparticles, while the Co core provided better conductivity. This work provides a novel method for the synthesis of shelled-structure catalysts, which have potential applications in the hydrolysis of sodium borohydride.

3D Frameworks with Variable Magnetic and Electrical Features from Sintered Cobalt-Modified Carbon Nanotubes.

3D frameworks of carbon nanotubes (CNTs) uniformly decorated by cobalt oxide or carbon-encapsulated cobalt nanoparticles were obtained by spark plasma sintering for the first time. The influence of the sintering temperature ( TS) and Co content on the morphology, structure, and electrical and magnetic properties of the obtained materials was investigated by Raman spectroscopy, electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and in situ magnetometry. It was shown that application of the SPS technique allowed simultaneous compaction of the material, formation of CNT framework, and Co oxide reduction. The appearance of the carbon shell around 4-10 nm Co particles was observed at TS > 600 °C. At higher TS, the Co particle size increased (up to 300 nm at 1400 °C), whereas the carbon shell ordered and thickened. The formation of large-size few-layers graphene sheets was observed at TS = 1400 °C. Electrical conductivity of the composites was found to be higher than that of sintered pristine CNTs and varied in the range of 500-12 500 Sm/m. Magnetic experiments demonstrated soft magnetization of the samples and the coercivity of 200-300 Oe. Thus, the obtained CNT-based material is simultaneously compact, formable, electroconductive, and ferromagnetic. Its properties can be tuned by variation of the sintering parameters. Synthesized cobalt-modified carbon 3D structures are promising for the application in magnetic separation, catalysis, fuel cells, and electromagnetic shielding.

Electromagnetic and microwave absorbing properties of carbon-encapsulated cobalt nanoparticles

Electromagnetic and microwave absorption of carbon-encapsulated cobalt nanoparticles, which were synthesized by the CVD method using NaCl as the catalytic support, have been investigated. The real and imaginary parts of permittivity and permeability of carbon-encapsulated cobalt nanoparticle composites rise with the increasing of nanoparticle concentration. The carbon-encapsulated cobalt nanoparticles/paraffin composite achieves a reflection loss below −10 dB in the frequency range of 7.4–11.8 GHz, and the minimum value reaches −18.6 dB at 8.8 GHz. The microwave-absorbing peaks of composites containing carbon-encapsulated cobalt nanoparticles shifts to low frequency region with the increasing of nanoparticle concentration.

Carbon-encapsulated cobalt nanoparticles: synthesis, properties, and magnetic particle hyperthermia efficiency

A facile and low-cost method for structuring carbon-encapsulated cobalt nanoparticles (Co@C) is presented. Three samples were solvothermally prepared in one step at 220 °C and one in two steps at 200 °C. Three different polyols such as propylene glycol, triethylene glycol, and tetraethylene glycol were used as carbon sources, solvents, and reducing agents. The samples were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and Raman spectroscopy. Concerning the crystal structure of the particles, a mixture of hcp/fcc Co phases was obtained in three of the samples, independently of the polyol used. The coexistence of cubic and hexagonal phases was revealed both from XRD and high-resolution TEM (HRTEM). The formation of the cubic fcc structure, despite the relatively low reaction temperature, is attributed to the role of the interface between carbon coating and metallic core. The presence of carbon coating was demonstrated by Raman spectrometry, exhibiting the characteristic D and G graphitic bands, and by HRTEM observations. All samples showed ferromagnetic behavior with saturation magnetization up to 158 emu/g and coercivity up to 206 Oe. From the magnetic particle hyperthermia measurements recorded at a frequency of 765 kHz, a maximum SLP value of 241 W/g was obtained.

Fabrication and Properties of Carbon-Encapsulated Cobalt Nanoparticles over NaCl by CVD.

Carbon-encapsulated cobalt (Co@C) nanoparticles, with a tunable structure, were synthesized by chemical vapor deposition using Co nanoparticles as the catalyst and supported on a water-soluble substrate (sodium chloride), which was easily removed by washing and centrifugation. The influences of growth temperature and time on the structure and magnetic properties of the Co@C nanoparticles were systematically investigated. For different growth temperatures, the magnetic Co nanoparticles were encapsulated by different types of carbon layers, including amorphous carbon layers, graphitic layers, and carbon nanofibers. This inferred a close relationship between the structure of the carbon-encapsulated metal nanoparticles and the growth temperature. At a fixed growth temperature of 400 °C, prolonged growth time caused an increase in thickness of the carbon layers. The magnetic characterization indicated that the magnetic properties of the obtained Co@C nanoparticles depend not only on the graphitization but also on the thickness of the encapsulated carbon layer, which were easily controlled by the growth temperatures and times. Optimization of the synthesis process allowed achieving relatively high coercivity of the synthesized Co@C nanoparticles and enhancement of its ferromagnetic properties, which make this system promising as a magnetic material, particularly for high-density magnetic recording applications.

Graphitisation and microstructure transformation of carbon-encapsulated cobalt nanoparticles synthesised by chemical vapour deposition using alumina powder as catalytic support

Carbon-encapsulated cobalt nanoparticles (Co@C) are prepared by the chemical vapour deposition of methane at 773–1073 K. This paper investigates the graphitisation and microstructure transformation of Co@C governed by synthesis temperature. The results show that carbon shells consisting of amorphous carbon or small graphite fragments begin forming on Co nanoparticles from 773 to 873 K. The products transform into onion-like carbon-encapsulated cobalt nanoparticles at 973 K. At 1073 K, a higher graphitisation of Co@C is achieved, with the appearance of carbon nanotubes. With a rising synthesis temperature, the carbon shells undergo the transformations from amorphous carbon to graphite fragments and finally to onion-like carbon. The microstructural transformations are demonstrated by scanning electron microscope, transmission electron microscope and Raman characterisations. Therefore, controlling the synthesis temperature is a feasible and easy approach one can take to adjust the structure and graphitisation of Co@C in the chemical vapour deposition process.

In situ oxidation of carbon-encapsulated cobalt nanocapsules creates highly active cobalt oxide catalysts for hydrocarbon combustion.

Combustion catalysts have been extensively explored to reduce the emission of hydrocarbons that are capable of triggering photochemical smog and greenhouse effect. Palladium as the most active material is widely applied in exhaust catalytic converter and combustion units, but its high capital cost stimulates the tremendous research on non-noble metal candidates. Here we fabricate highly defective cobalt oxide nanocrystals via a controllable oxidation of carbon-encapsulated cobalt nanoparticles. Strain gradients induced in the nanoconfined carbon shell result in the formation of a large number of active sites featuring a considerable catalytic activity for the combustion of a variety of hydrocarbons (methane, propane and substituted benzenes). For methane combustion, the catalyst displays a unique activity being comparable or even superior to the palladium ones.

Electromagnetic characteristics and microwave absorption properties of carbon-encapsulated cobalt nanoparticles in 2–18-GHz frequency range

Carbon-encapsulated cobalt nanoparticles (Co(C)) with a diameter of 10–50nm were synthesized by an arc discharge method. The inner crystal cobalt core was completely encapsulated by an outer carbon shell, which consisted of both amorphous carbon and 15–20 layers of graphite-like carbon. The electromagnetic characteristics of Co(C) embedded in paraffin at 10, 40, 50, 70, and 80wt.% (2, 10, 15, 29, and 41vol.%, respectively) at 2–18GHz were investigated. The distinctive core–shell microstructure of Co(C) and the carbon shells were mainly responsible for the high complex permittivity. The real part of complex permeability (μ) decreased and the imaginary part of μ remained small with increasing frequency, which revealed that good insulation between the metallic cobalt cores was achieved by carbon encapsulation. The major electromagnetic absorption mechanism was dielectric loss. The minimum calculated reflection loss (RL) was −52dB at 7.54GHz with 50wt.% Co(C) for a 3-mm Co(C)/paraffin composite. The measured RL of a Co(C)/epoxy coating with 40wt.% loading at 3mm showed good agreement with that of calculated RL. The electromagnetic property of Co(C) can be tailored through both control Co(C) concentrations and coating thickness.

Development of Magnetic Levitation 3-D Cultures of Breast and Colon Cancer Cells Using Carbon Encapsulated Cobalt Magnetic Nanoparticles

Innovative technologies are required for creation of in vitro models that mimic in vivo systems and can be used in drug discovery and development, disease models for cancer, stem cell research, and tissue engineering. Therefore, we established nanomagnetically levitated three-dimensional (3-D) cultures of breast and colon cancer cells using carbon encapsulated cobalt nanoparticles.

Carbonization of solvent and capping agent based enhancement in the stabilization of cobalt nanoparticles and their magnetic study

We describe a hybrid synthetic protocol, the solvated metal atom dispersion (SMAD) method, for the synthesis and stabilization of monodisperse amorphous cobalt nanoparticles. By employing an optimized ratio of a weakly coordinating solvent and a capping agent monodisperse colloidal cobalt nanoparticles (2 ± 0.5 nm) have been prepared by the SMAD method. However, the as-prepared samples were found to be oxidatively unstable which was elucidated by their magnetic studies. Oxidative stability in our case was achieved via a pyrolysis process that led to the decomposition of the organic solvent and the capping agent resulting in the formation of carbon encapsulated cobalt nanoparticles which was confirmed by Raman spectroscopy. Controlled annealing at different temperatures led to the phase transformation of metallic cobalt from the hcp to fcc phase. The magnetic behaviour varies with the phase and the particle size; especially, the coercivity of nanoparticles exhibited strong dependence on the phase transformation of cobalt. The high saturation magnetization close to that of the bulk value was achieved in the case of the annealed samples. In addition to detailed structural and morphological characterization, the results of thermal and magnetic studies are also presented.

Structure and Magnetic Properties of Carbon Encapsulated Co Nanoparticles Prepared by the Pyrolysis of Mixture of Sucrose and Cobalt Nitrate

Carbon-encapsulated Cobalt nanoparticles were synthesized with sucrose as carbon source and cobalt nitrate as metal source in nitrogen atmosphere. The weight of purified sample could arrive around 12-20 grams at one time. The as-prepared CECNPs sample was characterized by transmission electron microscopy (TEM), X-ray diffraction technique (XRD), vibrating sample magnetometer (VSM) and Raman spectroscopy. The results showed CECNPs was typical core / shell structure on nanometer scale, the core was cobalt single crystal, the shell was graphitic carbon, and the particle size was about 30nm. The magnetic measurement revealed that CECNPs was ferromagnetic material with the quite small remanent magnetizations Mr of 3.7emu/g and coercive forces Hc of 275Oe at room temperature.

Preparation and magnetic behavior of carbon-encapsulated cobalt and nickel nanoparticles from starch

Carbon-encapsulated cobalt and nickel nanoparticles with core/shell structure have been successfully synthesized with maize-derived starch as carbon source and metal nitrate as metal precursors in flowing hydrogen. The as-prepared M@Cs materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction technique (XRD) and vibrating sample magnetometer (VSM). The effects of the metal precursors on the structure and the size of the M@Cs materials were investigated, and the magnetic properties of the M@Cs materials were measured. The results show that the structure and the size of the M@Cs materials are different in terms of the different metal precursors. The Co@Cs materials are made of the fcc-Co core and the graphitic carbon shell, of which the core diameter is in a range of 20–35 nm, while the Ni@Cs materials are composed of fcc-Ni core and the amorphous carbon shell, of which the core diameter ranges from 30 to 50 nm. The hysteresis loops of the as-made M@Cs materials show that some of the nanoparticles are in a superparamagnetic state at room temperature. A mechanism is proposed to explain the growth process of the M@Cs materials. It is believed that the starch with the helical structure is responsible for the formation of the M@Cs materials featuring the core/shell structure.

Magnetic anisotropy in carbon encapsulated Co/CoO “lines” with large exchange bias

Carbon encapsulated cobalt nanoparticles were prepared by Co(II) reduction and post-annealing in inert argon gas. Cobalt nanoparticles form parallel “lines” with approximately equal space of 35 μm on the single-crystal silica substrate in the presence of a magnetic field of 3 kGs. Magnetic measurement results indicate that this system exhibits a strong magnetic anisotropy. Along the line direction, it was ferromagnetic with coercive force of 85 Oe, while perpendicular to the substrate plane, only a narrow hysteresis loop was observed. The samples were oxidized at 573 K for various times under air atmosphere. Coercivity was enhanced due to the exchange coupling between ferromagnetic Co and antiferromagnetic CoO. Compared with the random dispersed Co/CoO nanoparticles that was prepared under the same annealing and oxidation condition, a larger exchange bias was obtained in the magnetic aligned specimen. The mechanism of coercivity enhancement and larger exchange bias was discussed.

Large-scale preparation of carbon-encapsulated cobalt nanoparticles by the catalytic method

With strategical selection of the water-soluble sodium chloride (NaCl) as the supporting material, we realized large-scale production of carbon-encapsulated cobalt nanoparticles with a productivity of almost 100%. The products can be fully separated from the supporting materials by simple washing process. When NaCl, NaF and Al 2O 3 were used as supporting materials, the morphologies of the carbon products changed from carbon-encapsulated magnetic nanoparticles (CEMNs) to an intermediate state (quasi-nanocages) between CEMNs and carbon nanotubes (CNTs), and then to CNTs, respectively. NaCl shows strong inhibiting effects on the growth of CNTs, favoring the formation of CEMNs.

Preparation, Texture, and Magnetic Properties of Carbon Nanotubes/Nanoparticles Doped with Cobalt

Formation of cobalt-encapsulating carbon nanotubes/nanoparticles from the decomposition products of Co(CO)3NO was investigated by carrying out the reaction in a specially arranged closed cell at different temperatures. Multiwalled carbon nanotubes along with carbon-encapsulated cobalt nanoparticles were generated via a catalytic carbon monoxide disproportionation reaction over the in situ formed cobalt nanoparticles on an MgO support. The carbonaceous materials were separated from the product by a simple acid-treatment method. Structure and composition of the products were characterized by X-ray diffraction (XRD), transmission electron microscopy, and thermogravimetric analysis. It was observed that nanotube formation is favored at 1000 °C, whereas lower temperatures produced mainly nanoparticles with or without the encapsulated cobalt. The average outer diameter of the nanotubes was 26 nm with an inner diameter of 15 nm. As revealed from XRD, the encapsulated cobalt particles were in their high-temperatu...

Controlled Synthesis of Carbon-Encapsulated Co Nanoparticles by CVD

Carbon-encapsulated cobalt nanoparticles are prepared over a range of temperaturesand partial pressures by CVD from cyclohexane on prepared substrates consisting of ground powders of SiO2 impregnated with Co(NO3)2 and (NH4)2Mo2O7. The encapsulated cobalt nanoparticles are freed from the incorporated SiO2 and molybdenum by treating the deposition product with aqueous hydrofluoric acid. Extracted material is dispersed in ethanol and deposited on a carbon-coated grid prior to investigation by high-resolution TEM.

Controlled Synthesis of Carbon-Encapsulated Co Nanoparticles by CVD

Carbon-encapsulated cobalt nanoparticles are prepared over a range of temperaturesand partial pressures by CVD from cyclohexane on prepared substrates consisting of ground powders of SiO2 impregnated with Co(NO3)2 and (NH4)2Mo2O7. The encapsulated cobalt nanoparticles are freed from the incorporated SiO2 and molybdenum by treating the deposition product with aqueous hydrofluoric acid. Extracted material is dispersed in ethanol and deposited on a carbon-coated grid prior to investigation by high-resolution TEM.