Electrochemical characterization of core-shell carbon-encapsulated magnetic nanoparticles

In this paper we studied the electrochemical behaviour of core-shell carbon-encapsulated magnetic nanoparticles (CEMNPs). CEMNPs have core diameters between 15 and 35 nm and are comprised of Fe, Fe₃C and NdC₂ nanoparticles encapsulated in crystalline carbon cages. Direct current cyclic voltammetry (CV) studies showed that carbon-encapsulated magnetic nanoparticles are stable in electrolyte environments. The graphitic coating perfectly isolates the encapsulated particles from the electrolyte in a wide range of potentials. CEMNP-based electrodes have low resistance (0.43-1.44 Ω cm²) and posses a specific capacity of 10-40 F g^− 1, which depends on the surface area and the crystallinity. It was shown, that CEMNPs are interesting multi-functional materials with a high potential to be used in various electrochemical devices.     

Preparation and characterization of carbon-encapsulated iron nanoparticles and their catalytic activity in the hydrogenation of levulinic acid

This study describes the synthesis of carbon-encapsulated iron nanoparticles using an ultrasonic method and also investigates their catalytic activity. These nanoparticles have been prepared using ultrasonic irradiation followed by annealing at various temperatures. As the annealing temperature of as-prepared α-Fe₂O₃ nanoparticles increased, the sample transformed into γ-Fe₂O₃, Fe₃O₄, and Fe nanoparticles via the reduction process without requiring any additional reducing agents such as H₂ gas, thus, creating a carbon shell surrounding the nanoparticles. By controlling the experimental conditions, Fe nanoparticles of various sizes can be formed with diameters in the range 100–800 nm; these nanoparticles are tightly encapsulated by 20-nm-thick carbon shells. Because of their high saturation magnetization 212 emu g^?1, the carbon-encapsulated Fe nanoparticles can be used for magnetic resonance imaging with a dramatically enhanced efficiency compared to commercially available T ₂ contrast agents. Moreover, the carbon-encapsulated Fe nanoparticles showed its superior catalytic activity and reusability for the hydrogenation of biomass-derived levulinic acid to GVL (99.6 %) in liquid phase.     

Synthesis process of gold nanoparticles in solution plasma

We describe the dynamics of the synthesis of gold nanoparticles by a glow discharge in aqueous solutions. A pulsed power supply was used to generate discharges in the aqueous solutions. The initial [AuCl₄]⁻ ion concentration and the voltage applied between the electrodes were varied. The [AuCl₄]⁻ ion was reduced by the H radicals generated in the discharge. The reduction rates were calculated from the changes in the [AuCl₄]⁻ ion concentration during the discharge time. Dendrite-shaped nanoparticles of about 150 nm size were formed in discharge during 1 min. The pH of the solution decreased gradually with the increase of the discharge time. The decrease in pH led to the dissolution of gold nanoparticles. The reduction and the dissolution rates increased proportionately with the applied voltage. The size of the gold nanoparticles decreased at 20 nm after running the discharge during 45 min. Moreover gold nanoparticles with exotic shapes, such as triangle, pentagon, and hexagon were also observed. The particles were confirmed to be as polycrystalline gold nanoparticles by electron diffraction patterns. In summary, when the reduction rate lowered as a result of dissolution, anisotropic nanoparticles were formed and continued to grow in size in the solution.     

Controlled multistep purification of single-walled carbon nanotubes

A controlled and scalable multistep purification method has been developed to remove iron impurity and nonnanotube carbon materials from raw single-walled carbon nanotubes (SWNTs) produced in the HiPco (high-pressure CO) process. In this study, iron nanoparticles, coated by carbon, are exposed and oxidized by multiple step oxidation at increasing temperatures. To avoid catalytic oxidation by iron oxide of carbon nanotubes, the exposed and oxidized iron oxide is deactivated by reaction with C(2)H(2)F(4) or SF(6). The iron fluorides are removed by a Soxhlet extraction with a 6 M HCl solution. The purity and quality of each sample were determined by thermogravimetric analysis (TGA), Raman spectrometry, ultraviolet-visible-near-IR (UV-vis-near-IR) spectrometry, fluorescence spectrometry, and transmission electron microscope (TEM) spectroscopy. The purity and yield of SWNTs are improved due to reduced catalytic activity of the iron oxide. Greater iron oxide removal also resulted from oxidation at higher temperatures.     

High efficiency removal of dissolved As(III) using iron nanoparticle-embedded macroporous polymer composites

Novel nanocomposite materials where iron nanoparticles are embedded into the walls of a macroporous polymer were produced and their efficiency for the removal of As(III) from aqueous media was studied. Nanocomposite gels containing α-Fe₂O₃ and Fe₃O₄ nanoparticles were prepared by cryopolymerisation resulting in a monolithic structure with large interconnected pores up to 100 μm in diameter and possessing a high permeability (ca. 3 × 10⁻³ m s⁻¹). The nanocomposite devices showed excellent capability for the removal of trace concentrations of As(III) from solution, with a total capacity of up to 3 mg As/g of nanoparticles. The leaching of iron was minimal and the device could operate in a pH range 3-9 without diminishing removal efficiency. The effect of competing ions such as SO₄²⁻ and PO₄³⁻ was negligible. The macroporous composites can be easily configured into a variety of shapes and structures and the polymer matrix can be selected from a variety of monomers, offering high potential as flexible metal cation remediation devices.     

Development of redox deposition of birnessite-type MnO2 on activated carbon as high-performance electrode for hybrid supercapacitors

Birnessite-type MnO₂/activated carbon nanocomposites have been synthesized by directly reducing KMnO₄ with activated carbon in an aqueous solution. It is found that the morphologies of MnO₂ grown on activated carbon can be tailored by varying the reaction ratio of activated carbon and KMnO₄. An asymmetric supercapacitor with high energy density was fabricated by using MnO₂/activated carbon (MnO₂/AC) nanocomposite as positive electrode and activated carbon as negative electrode in 1 M Na₂SO₄ aqueous electrolyte. The asymmetric supercapacitor can be cycled reversibly in the cell voltage of 0–2 V, and delivers a specific capacitance of 50.6 F g⁻¹ and a maximum energy density of 28.1 Wh kg⁻¹ (based on the total mass of active electrode materials of 9.4 mg), which is much higher than that of MnO₂/AC symmetric supercapacitor (9.7 Wh kg⁻¹).     

High capacity and good cycling stability of multi-walled carbon nanotube/SnO2 core-shell structures as anode materials of lithium-ion batteries

The multi-walled carbon nanotube/SnO₂ core-shell structures were fabricated by a wet chemical route. The electrochemical performance of the core-shell structures as anode materials of lithium-ion batteries was investigated. The initial discharge capacity and reversible capacity are up to 1472.7 and 1020.5 mAh g⁻¹, respectively. Moreover, the reversible capacity still remains above 720 mAh g⁻¹ over 35 cycles, and the capacity fading is only 0.8% per cycle. Such high capacities and good cyclability are attributed to SnO₂ network structures, excellent mechanical property and good electrical conductivity of the multi-walled carbon nanotubes.     

Influence of Fe nanoparticles diameters on the structure and electron emission studies of carbon nanotubes and multilayer graphene

In this paper we report the effect of Fe film thickness on the growth, structure and electron emission characteristics of carbon nanotubes (CNTs) and multilayer graphene deposited on Si substrate. It is observed that the number of graphitic shells in carbon nanostructures (CNs) varies with the thickness of the catalyst depending on the average size of nanoparticles. Further, the Fe nanoparticles do not catalyze beyond a particular size of nanoclusters leading to the formation of multilayer graphene structure, instead of carbon nanotubes (CNTs). It is observed that the crystallinity of CNs enhances upon increasing the catalyst thickness. Multilayer graphene structures show improved crystallinity in comparison to CNTs as graphitic to defect mode intensity ratio (I_D/I_G) decreases from 1.2 to 0.8. However, I_2D/I_G value for multilayer graphene is found to be 1.1 confirming the presence of at least 10 layers of graphene in these samples. CNTs with smaller diameter show better electron emission properties with enhancement factor (γ_C = 2.8 × 10³) in comparison to multilayer graphene structure (γ_C = 1.5 × 10³). The better emission characteristics in CNTs are explained due to combination of electrons from edges as well as centers in comparison to the multilayer graphene.     

Facile size and shape control of templated Au nanoparticles under microwave irradiation

In this work we prepared icosahedral gold particles and gold nanoplates using potassium tetrachloroaurate as precursor and poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers as both reductant and capping agent under microwave irradiation. The products were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The size and shape of the resultant nanoparticles could be tuned by changing the chloride ion dosage and reaction temperature. With lower dosage of chloride ion, a lower proportion of irregular shaped nanoparticles and smaller gold decahedra and icosahedra were observed. Increasing the molecular ratio of [AuCl₄]/[Cl⁻] and reaction temperature could increase the proportion of gold nanoplates in the final product. Typically when the reaction proceeded at 120 °C with [AuCl₄]/[Cl⁻] = 10, > 90% of the product was nanoplatelets.     

Nanodimensional carbon materials formed in a gas discharge plasma

We have experimentally studied the formation of nanodimensional carbon materials from a methane-hydrogen gas mixture activated by a dc discharge. The range of discharge voltages and currents ensuring stable deposition of carbon films was determined. Data on the carbon-containing components of the activated gas phase were obtained by in situ optical emission spectroscopy of the gas discharge plasma. It is shown that the formation of nanodiamond and nanographite particles, as well as carbon nanotubes, in the deposited films is correlated with the presence of C₂ carbon dimers in the gas phase. A mechanism of the noncatalytic formation of carbon nanotubes from platelike graphite nanoparticles is proposed.     

Growth of carbon nanotubes on diatomite

We report the formation of vertical carbon nanotubes utilizing diatomite as a substrate. This new material combines the advantages of carbon nanotubes and diatomite in one material. The SEM investigations showed that the average diameter of the carbon nanotubes was 60 nm and the growth was through the tip growth mechanism. Raman spectroscopy was also used for the carbon nanotubes characterization and showed two intensive peaks around 1350 cm⁻¹ and 1580 cm⁻¹ and several peaks at low frequency range from 100 cm⁻¹ to 500 cm⁻¹ which are assigned to the radial breathing mode (RBM) and used as a characteristic of single wall carbon nanotubes. The photoluminescence measurements at the room temperature showed two very narrow intensive overlapping peaks near the ultraviolet range at energy of about 3 eV. And there are two peaks with lower intensity in the infrared region at 830 nm and at 940 nm (or 1.49 eV, 1.3 eV respectively).     

Fabrication of magnetic activated carbon by carbothermal functionalization of agriculture waste via microwave-assisted technique for cationic dye adsorption

The agriculture shell wastes were carbothermally converted to magnetic activated carbon by a microwave-assisted decoration of iron oxide nanoparticles onto the shell surface. The influence of ternary catalytic mixtures, including zinc, iron II and III chlorides on the cationic dye adsorption efficiency was addressed by the composite impregnations onto the almond or walnut shell powders, explored to the carbonization. The efficiency was maximized by determination the proportions of used salts. The best results were obtained with loading FeCl₃ onto the walnut shell in which the proportion of salt was 50%. Although the load of magnetic particles onto the adsorbent normally lead to decrease in efficiency, the prepared powder exhibited the appropriate performance above 99%. It should be point out that the dye adsorption efficiency of magnetic activated carbons fabricated by carbothermal functionalization was 7–10% higher than those produced in the nitrogen atmosphere. The adsorbent displayed the nano-porous structure with average pore diameter about 2 nm, providing a surface area around 1000 m²·g⁻¹ for the removal of dye in a dynamic system. The maximum adsorption capacity was determined to be 130 mg·g⁻¹ in the neutral condition.     

Preparation of porous SnO2 helical nanotubes and SnO2 sheets

We report a surfactant-free chemical solution route for synthesizing one-dimensional porous SnO₂ helical nanotubes templated by helical carbon nanotubes and two-dimensional SnO₂ sheets templated by graphite sheets. Transmission electron microscopy, X-ray diffraction, cyclic voltammetry, and galvanostatic discharge–charge analysis are used to characterize the SnO₂ samples. The unique nanostructure and morphology make them promising anode materials for lithium-ion batteries. Both the SnO₂ with the tubular structure and the sheet structure shows small initial irreversible capacity loss of 3.2% and 2.2%, respectively. The SnO₂ helical nanotubes show a specific discharge capacity of above 800 mAh g⁻¹ after 10 charge and discharge cycles, exceeding the theoretical capacity of 781 mAh g⁻¹ for SnO₂. The nanotubes remain a specific discharge capacity of 439 mAh g⁻¹ after 30 cycles, which is better than that of SnO₂ sheets (323 mAh g⁻¹).     

Removal of acid orange 7 by guava seed carbon: A four parameter optimization study

The preparation of carbon from waste materials is a recent and economic alternative for the removal of dyes. In this study four samples of carbon were obtained by thermal treatment at 1000 °C using as precursor the guava seed with different particle sizes. The Taguchi method was applied as an experimental design to establish the optimum conditions for the removal of acid orange 7 in batch experiments. The chosen experimental factors and their ranges were: pH (2–12), temperature (15–35 °C), specific surface area (50–600 m² g⁻¹) and adsorbent dosage (16–50 mg ml⁻¹). The orthogonal array L₉ and the larger the better response category were selected to determine the optimum removal conditions: pH 2, temperature 15 °C, S_esp 600 m² g⁻¹ and dosage 30 mg ml⁻¹. Under these conditions a total removal of acid orange 7 was achieved. Moreover, the most significant factors were the carbon specific surface area and the pH. The influence of the different factors on the adsorption of acid orange 7 from solution is explained in terms of electrostatic interactions by considering the dye species and the character of the surface.     

A facile approach to Fe3O4@Au nanoparticles with magnetic recyclable catalytic properties

A novel approach has been developed to synthesize gold-coated iron oxide nanoparticles. Fe₃O₄ nanoparticles were initially prepared by co-precipitation method and subsequently coated with gold layer under oleylamine reduction of AuCl₄⁻ at room temperature. The core-shell nanoparticles showed magnetic recoverable catalytic activity for the reduction of 4-nitrophenol with NaBH₄.     

Potentiodynamical co-deposited manganese oxide/carbon composite for high capacitance electrochemical capacitors

Manganese oxide/carbon composite materials were prepared by introducing the carbon powders into the potentiodynamical anodic co-deposited manganese oxide in 0.5 mol L^− 1 MnSO₄ and 0.5 mol L^− 1 H₂SO₄ mixed solution at 40 °C. The surface morphology and structure of the composite material were examined by scanning electron microscope and X-ray diffraction. Cyclic voltammetry tests and electrochemical impedance measurements were applied to investigate the performance of the composite electrodes with different ratios of manganese oxide and carbon. These composite materials with rough surface, which consisted of approximately amorphous manganese oxide, were confirmed to possess the ideal capacitive property. The highest specific capacitance of manganese oxide/carbon composite electrode was up to 410 F g^− 1 in 1.0 mol L^− 1 Na₂SO₄ electrolyte at the scan rate 10 mV s^− 1. The synthesized composite materials exhibited ideal capacitive behavior indicating a promising electrode material for electrochemical supercapacitors.     

Co2SnO4/activated carbon composite electrode for supercapacitor

A new kind of Co₂SnO₄-based electrode materials for supercapacitor was synthesized by co-precipitation method. The microstructure and surface morphology of Co₂SnO₄ were characterized by X-ray diffraction and scanning electron microscopy, respectively. Cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy were employed for the determination of specific capacitance and the equivalent series resistance of Co₂SnO₄/activated carbon composite electrode in KCl solution. It was shown that the composite electrode with 25 wt% Co₂SnO₄ had excellent specific capacitance up to 285.3 F g⁻¹ at the current density of 5 mA cm⁻². In addition, the composite electrode exhibited excellent long-term stability and, after 1000 cycles, 70.6% of initial capacitance was retained. Regarding the low cost, easy preparation, steady performance and environment friendliness, Co₂SnO₄/activated carbon composite electrode could have potentially promising application for supercapacitor.     

Bulk synthesis of carbon nanocapsules and nanotubes containing magnetic nanoparticles via low energy laser pyrolysis of ferrocene

A one-step synthesis route to carbon nanocapsules and nanotubes containing Fe and Fe₃C nanoparticles is reported. Low power laser assisted pyrolysis of ferrocene yielded carbon nanocapsules (30-100 nm in diameter) and multi-wall carbon nanotubes (30-80 nm in diameter). The developed route is fast and enables one to synthesize the products at a rate of 84 mg/min. The iron content in the product (10-42 wt.%) can be varied by modulating the buffer gas pressure during the synthesis process.     

Magnetic properties and methylene blue adsorptive performance of CoFe2O4/activated carbon nanocomposites

Owing to the unique microporous structure and high specific surface area, activated carbon (AC) could act as a good carrier for functional materials. In this paper, CoFe₂O₄/AC nanocomposites were prepared by a facile hydrothermal method for the adsorption of dyes in wastewater. The results indicated that CoFe₂O₄ nanoparticles presented the spinel structure and existed in the pores of AC. The saturation magnetization (Ms) increased with the CoFe₂O₄ content, while the surface area and pore volume decreased. For the larger magnetic moment, very few CoFe₂O₄ were needed to maintain the higher surface area of CoFe₂O₄/AC nanocomposites. The sample-5 (CoFe₂O₄:C = 1:200) possessed the surface area of 1096.85 m² g⁻¹ (close to 1243.35 m² g⁻¹ of AC) and Ms of 5.11 emu g⁻¹, which were sufficient for magnetic separation in wastewater treatment. 99% methylene blue could be adsorbed in 50 min, and then the CoFe₂O₄/AC nanocomposites could be separated from the solution easily by an outer magnet.