FMR and EPR in Ni@C nanocomposites: Size and concentration effects

One-domain Ni@C nanoparticles encapsulated in carbon coating have been investigated depending on the size and concentration of Ni in carbon. The nanoparticles of nickel were prepared with the average diameters changing in a broad range of 4–45 nm, and the concentration of Ni in C varies in 2–12 wt%. To prepare the Ni@C nanocomposites the solid solutions of nickel phthalocyanine–metal-free phthalocyanine (NiPc) _x (H₂Pc)_1–x , 0 ≤ x ≤ 1 were synthesized and the solidphase pyrolysis of these compounds was performed. In the case of ultradispersive Ni nanoparticles (the interval of quantum dots is 1–10 nm), a considerable shift of the resonance field and broadening of resonance absorption field were revealed in the spectra of FMR at room temperature. The data were interpreted taking into account the essential contribution of the surface magnetic anisotropy, the magnetic field of which far exceeds the magnetic field of volume anisotropy.     

Raman light scattering and electron microscopy of nanocomposites with the metal core-carbon shell structure

The structural state of carbon in nanocomposites that are based on metals (Fe, Ni, Co, and Ag) encapsulated in carbon and produced by the gas-phase synthesis has been investigated using Raman spectroscopy and high-resolution transmission electron microscopy. The average diameter of particles of the initial nanocomposites after the gas-phase synthesis, including the carbon shell, is less than 15 nm and can vary as a function of the conditions and regimes used for their preparation. The shell of the initial nanocomposites, irrespectively of the metal core type, consists of carbon fragments in the form of curved layers with sizes of less than 10 nm in the lateral direction. In the initial nanocomposites, there is no periodicity in the packing of carbon layers in the radial and lateral directions. The structure of the coating is assumed to be similar to the glassy carbon structure characterized by a curvature of carbon layers in different directions, which requires that, in addition to conventional hexagonal cells, the layers should contain pentagonal and heptagonal cells. Heat treatment of the initial nanocomposites Fe@C and Ni@C in butane (700°C, 60 min) not only significantly increases the thickness of the carbon coating but also increases the degree of ordering of curved carbon fragments in the lateral and radial directions. In the composites with Fe, Ni, and Co, along with this form of carbon, semiconducting nanotubes with a diameter of 1.3–1.5 nm are also formed. The composites with silver nanoparticles exhibit the effect of time-fluctuating giant enhancement of the Raman scattering intensity.     

Atomic physics at the planned new RIKEN facility

Combining Mössbauer spectroscopy with magnetic property measurement, we have studied Fe-N and (Fe, Ni)-N powders for magnetic recording. The typical particles of the core (α-Fe)/shell (γ′-Fe₄N) structure have been successfully prepared. All the products are stabilized in a multi-organic solution. It has been found that the coercivity can be changed from 300 to 800 Oe by adjusting the shape of the particles. The special saturation magnetization of the particles can be adjusted from 120 to 180emu/g and their chemical stability is improved by substituting nickel for iron in γ′-Fe₄N. Following experiments for corrosion resistance, it is expected that (Fe, Ni)-N and the core/shell particles will be applied as recording media in the near future.     

Synthesis and magnetic properties of carbon-coated Ni/SiO2 core/shell nanocomposites

A simple method for the synthesis of carbon-coated Ni/SiO₂ core/shell nanocomposites is reported. The Ni nanoparticles were coated with silica layers via a combined procedure of sol-gel fabrication and hydrogen reduction prior to carbon coating via acetylene decomposition at an appropriate temperature. It was found that the anti-acid ability of the Ni/SiO₂ composites was greatly enhanced after carbon coating. The results of magnetization measurement show that the real part (μ′) of complex permeability of the as-obtained sample is almost independent of frequency, and the imaginary part (μ″) stays small up to a frequency of 1 GHz. The encapsulation of Ni particles with SiO₂ results in the rise of Ni nanoparticles resistivity. The outcome is the reduction in effect of eddy current at high frequency, making the real part μ′ almost constant and the imaginary part μ″ very small. Thus, this simple method may be effective for preparing composites of soft magnetic properties, especially in the high-frequency range. Supported by the Jiangsu Postdoctoral Foundation of Jiangsu Province and the Major Project of National Basic Research Program of China (Grant No. 2005CB623605)     

Hierarchical Carbon‐Encapsulated Iron Nanoparticles as a Magnetically Separable Adsorbent for Removing Thiophene in Liquid Fuel

Hierarchical carbon‐encapsulated iron nanoparticles (Fe@Cs) with typical core/shell structure are successfully synthesized from starch and iron nitrate by an easy‐to‐handle process at different carbonization temperatures (600–1000 °C). The nanoparticles are characterized by transmission electron microscopy (TEM), X‐ray diffraction (XRD), nitrogen adsorption, and Fourier transform infrared spectroscopy (FTIR). The results show that the carbonization temperature has an important effect on the morphology, the core shape, the diameters, and the porous structure as well as performance of Fe@Cs. Fe@C samples carbonized at 900 °C (Fe@C‐900) show the relatively perfect quasi‐spherical bcc‐Fe core/carbon shell porous structure and their diameters are in a narrow range of 20–50 nm. The adsorption capabilities of Fe@C samples obtained at different carbonization temperatures for removal of thiophene from model oils are evaluated and compared in a batch‐type adsorption system. It has been found that among all of the samples measured, Fe@C‐900 shows the highest adsorption capability with an increase of 54% for thiophene in comparison with that of the commercial activated carbon. The feasibility of the as‐prepared Fe@C‐900 as a magnetically separable adsorbent is also demonstrated.     

Electronic structure and resonant X-ray emission spectra of carbon shells of iron nanoparticles

The electronic structure of carbon shells of carbon encapsulated iron nanoparticles carbon encapsulated Fe@C has been studied by X-ray resonant emission and X-ray absorption spectroscopy. The recorded spectra have been compared to the density functional calculations of the electronic structure of graphene. It has been shown that an Fe@C carbon shell can be represented in the form of several graphene layers with Stone-Wales defects. The dispersion of energy bands of Fe@C has been examined using the measured C Kα resonant X-ray emission spectra.     

Synthesis of NiAu alloy and core–shell nanoparticles in water-in-oil microemulsions

NiAu alloy nanoparticles with various Ni/Au molar ratios were synthesized by the hydrazine reduction of nickel chloride and hydrogen tetrachloroaurate in the microemulsion system. They had a face-centered cubic structure and a mean diameter of 6–13 nm, decreasing with increasing Au content. As Au nanoparticles did, they showed a characteristic absorption peak at about 520 nm but the intensity decreased with increasing Ni content. Also, they were nearly superparamagnetic, although the magnetization decreased significantly with increasing Au content. Under an external magnetic field, they could be self-organized into the parallel lines. In addition, the core–shell nanoparticles, Ni₃Au₁@Au, were prepared by the Au coating on the surface of Ni₃Au₁ alloy nanoparticles. By increasing the hydrogen tetrachloroaurate concentration for Au coating, the thickness of Au shells could be raised and led to an enhanced and red-shifted surface plasmon absorption.     

Morphology,chemical composition,and electrical characteristics of hybrid (Ni-C) nanocomposite structures grown on the van der Waals GaSe(0001) surface

The morphology and chemical composition of metal (Ni), carbon, and composite (Ni-C) nanostructures grown on oxidized and unoxidized (0001) surfaces of a layered GaSe crystal by electron beam vacuum evaporation of the material from a liquid ion source in an electric field have been investigated using atomic force microscopy and X-ray photoelectron spectroscopy. It has been demonstrated that this technology makes it possible to grow nanostructures with different morphologies depending on the growth mode and substrate surface state. Dense homogeneous arrays of nickel nanoparticles (Ni@C) (with geometrical sizes of ～1–15 nm and a lateral density of higher than 10¹⁰ cm^?2) encapsulated into carbon shells, as well as carbon layers (with a thickness of the order of several nanometers), are grown on the unoxidized van der Waals GaSe(0001) surface, whereas Ni-C composite nanostructures are grown on the oxidized surface. The formation of oxide nanostructures on the van der Waals surface and their chemical composition have been examined. Vertical hybrid Au/Ni/(Ni-C)/n-Ga₂O₃(Ni@C)/p-GaSe structures grown on the GaSe(0001) surface contain Ni@C nanoparticles embedded in the wide-band-gap n-Ga₂O₃ oxide. The current-voltage characteristics of these structures at temperatures close to T = 300 K exhibit specific features of the Coulomb blockade effect.     

Synthesis of titania/carbon nanocomposites by polymeric precursor method

Here we describe a single chemical route to obtain highly dispersed nanometric Ni particles embedded in titania/carbon matrixes (amorphous and crystalline). The synthesis of these nanocomposites is based on a polymeric precursor method. The metallic Ni nanoparticles (1-15 nm) were obtained in a single process. We also present the results of photocatalytic experiments involving a series of nanocrystalline composites based on TiO₂/carbon with embedded Ni nanoparticles as nanocatalysts for rhodamine 6G degradation in aqueous solution and investigate the effects of the structure and properties of the nanocomposites on their photocatalytic applications. The effect of the different annealing treatments on the formation of TiO₂ nanophases (anatase and/or rutile), the size of Ni particles and the role of the residual carbon phase on the final solid are also described.     

Study of Diffusion Bonding of 45 Steel through the Compacted Nickel Powder Layer

The microstructure of the transition zone and powder spacer, the concentration distribution of chemical elements over the width of the diffusion-bonded joint, and microhardness of 45 steel–compacted Ni powder spacer–45 steel layered composites formed by diffusion bonding have been investigated. It has been shown that the relative spacer thickness χ < 0.06 is optimal for obtaining a high-quality joint has been formed under a compacting pressure of 500 MPa. The solid-state diffusion bonding is accompanied by sintering the nickel powder spacer and the formation of the transition zone between the spacer and steel. The transition zone consists of solid solution of nickel in the α-Fe phase and ordered solid solution of iron in nickel (FeNi₃).     

Synthesis and magnetic properties of Cu-coated Fe composite nanoparticles

The Fe/Cu nanocomposites with iron as core and copper as shell have been successfully synthesized by a two-step reduction method. A spherical nanoparticle of γ-Fe was first fabricated by the reduction of ferrous chloride, and then the Fe particle was coated by nanocrystalline Cu through the reduction of copper sulfate. The thickness of copper shell has been tuned by varying the initial concentration of copper sulfate. The morphology, crystalline structure, chemical composition and magnetic properties of the products were investigated by using transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) and vibrating sample magnetometer (VSM). It was found that the saturation magnetization (M_s) values of the Fe/Cu core–shell particles are varied owing to the different thickness of copper layer. Though the M_s value of the Fe/Cu nanocomposite is lower than that of pure iron nanoparticles, the higher M_s value (22.411 emu/g) of the Fe/Cu composites is also investigated. The result of the thermogravimetric analysis (TGA) showed the enhanced antioxidation capacity of the Fe/Cu nanocomposites. This kind of nanocomposites combined the excellent magnetism of iron and the electronic, thermal conductivity of copper, suggesting potential application as a novel electromagnetic material.     

Synthesis and coating methods of biocompatible iron oxide/gold nanoparticle and nanocomposite for biomedical applications

This detailed review presents an overview of current research on the synthesis, surface modification, and applications of Iron oxide (Fe₃O₄) nanoparticles and iron oxide/gold (Fe₃O₄/Au) nanocomposites. The different synthesis techniques of Fe₃O₄ with various basic organic and inorganic modifications are presented. The applicability and role of inorganic and organic coating on iron oxide/gold core/shell schemes were explored. The trade-off between choices for surface functionalization related to specific applications such as imaging contrast agent, drug delivery carrier and therapeutic device using iron oxide/gold core/shell was also elaborated. The versatility of combining iron oxide/ and gold as nanocomposite as the choice for biomedical application is demonstrated in MRI, CT scan, drug delivery, biosensors, and hyperthermia application.     

The role of synthetic parameters in the magnetic behavior of relative large hcp Ni nanoparticles

The controllable synthesis of relatively large nickel nanoparticles via thermal decomposition of nickel acetate tetrahydrate in oleylamine in the presence of 1-adamantane carboxylic acid (ACA) and trioctylphosphine oxide (TOPO) is reported. High crystalline hcp nanoparticles of different sizes have been prepared at 290 °C, whereas at relative lower temperatures fcc are favored. The particle size was varying between 50 and 150 nm by properly adjusting the proportion of the capping ligands. TOPO-to-ACA ratio was also found to have an influence on the magnetic properties through the potential formation of a NiO shell. Pure hcp Ni nanoparticles over 50 nm in size served as models to illuminate the magnetic behavior of this metastable hexagonal Ni phase. Contrary to the net ferromagnetic characteristics of fcc Ni nanoparticles in the same size range, hexagonal structured particles exhibit superparamagnetic behavior at room temperature and a weak ferromagnetic contribution below 15 K.     

Low temperature synthesis of iron containing carbon nanoparticles in critical carbon dioxide

We develop a low temperature, organic solvent-free method of producing iron containing carbon (Fe@C) nanoparticles. We show that Fe@C nanoparticles are self-assembled by mixing ferrocene with sub-critical (25.0 °C), near-critical (31.0 °C) and super-critical (41.0 °C) carbon dioxide and irradiating the solutions with UV laser of 266-nm wavelength. The diameter of the iron particles varies from 1 to 100 nm, whereas that of Fe@C particles ranges from 200 nm to 1 μm. Bamboo-shaped structures are also formed by iron particles and carbon layers. There is no appreciable effect of the temperature on the quantity and diameter distributions of the particles produced. The Fe@C nanoparticles show soft ferromagnetic characteristics. Iron particles are crystallised, composed of bcc and fcc lattice structures, and the carbon shells are graphitised after irradiation of electron beams.     

Synthesis and characterization of nanostructured iron compounds prepared from the decomposition of iron pentacarbonyl dispersed into carbon materials with varying porosities

This work describes the production and characterization of carbon-iron nanocomposites obtained from the decomposition of iron pentacarbonyl (Fe(CO)₅) mixed with different carbon materials: a high surface area activated carbon (AC), powdered graphite (G), milled graphite (MG), and carbon black (CB). The nanocomposites were prepared either under argon or in ambient atmosphere, with a fixed ratio of Fe(CO)₅ (4.0 mL) to carbon precursor (2.0 g). The images of scanning electron microscopy and the analysis of textural properties indicated the presence of nanostructured Fe compounds homogeneously dispersed into the different classes of pores of the carbon matrices. The elemental Fe content was always larger for samples prepared in ambient atmosphere, reaching values in the range of 20–32 wt%. On the other hand, samples prepared under argon showed reduced Fe content, with values in the range 5–10 wt% for samples prepared from precursors with low surface area (G, MG, and CB) and a much higher value (~19 wt%) for samples prepared from the precursor of high surface area (AC). Mössbauer spectroscopy and X-ray diffractometry showed that the nanoparticles were mostly composed of iron oxides in the case of the samples prepared in oxygen-rich ambient atmosphere and also for the AC-derived nanocomposite prepared under argon, which is consistent with the large oxygen content of this precursor. For the other precursors, with reduced or no oxygen content, metallic iron and iron carbides were found to be the dominant phases in samples prepared under oxygen-free atmosphere. The samples prepared in ambient atmosphere and the AC-derived sample prepared under argon exhibited superparamagnetic behavior at room temperature, as revealed by temperature-dependent magnetization curves and Mössbauer spectroscopy.     

Synthesis and investigation of the structure of nanocomposites based on nickel nanoparticles dispersed in a phthalocyanine matrix

A method based on doping of pure nickel phthalocyanine (NiPc) with a polycrystalline potassium powder at relatively low temperatures (300°C) has been proposed for the synthesis of a magnetic nanocomposite containing nickel nanoparticles stabilized in the NiPc matrix. The structural analysis of the synthesized nanoparticles and changes in the NiPc initial matrix has been performed using X-ray diffraction, X-ray absorption spectroscopy, and transmission electron microscopy. It has been found that, at the doping level used in this study, the synthesized samples of the K_xNiPc nanocomposites contain from 9 to 18% Ni in the form of metallic magnetic nanoparticles with an average size of more than 40 nm. It has been shown that the formation of nanoparticles is accompanied by a relative misorientation of persistent NiPc molecules with the unchanged structure of each of these molecules. The stabilization of nickel nanoparticles by the phthalocyanine matrix leads to the fact that the synthesized nanocomposites acquire time-conserving magnetic properties.     

Phase states and magnetic properties of iron nanoparticles in carbon nanotube channels

The structure, phase composition, and magnetic properties of carbon nanotubes filled with iron nanoparticles and obtained by thermolysis of a mixture of ferrocene and C₆₀ fullerene or ferrocene and orthoxylene at a temperature of 800°C are investigated. Electron microscopy, X-ray diffraction, and Mössbauer spectroscopy data lead to the conclusion that carbon nanotubes are multilayer systems partially filled with iron nanoparticles and/or nanorods. Metallic inclusions in nanotube channels form α-Fe, γ-Fe, and Fe₃C phases. The concentration of each phase in the samples is determined. It is shown that 10–20-nm iron clusters in nanotubes exhibit magnetic properties typical of bulk phases of iron. High elasticity of carbon nanotube walls facilitates stabilization of the high-temperature γ-Fe phase; the relative concentration of this phase in a sample can be increased by lowering the concentration of ferrocene in the initial reaction mixture.     

Study of structural and magnetic properties of superparamagnetic Fe3O4/SiO2 core–shell nanocomposites synthesized with hydrophilic citrate-modified Fe3O4 seeds via a sol–gel approach

This paper describes a simple way for the coating of magnetite nanoparticles (MNPs) with amorphous silica. First, MNPs were synthesized by controlled co-precipitation technique under N₂ gas and then their surface was modified with trisodium citrate in order to achieve particles with improved dispersibility. Afterward, magnetite-silica core/shell nanocomposites were prepared by a sol–gel approach, using magnetic fluid including electrostatically stabilized MNPs as seeds. The prepared samples were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, zeta potential analysis and vibrating sample magnetometer (VSM) in order to study their structural and magnetic properties. FT-IR and XRD results imply that resultant nanocomposites are consisted of two compounds; Fe₃O₄ and SiO₂ and TEM images confirm formation of their core/shell structure. TEM images also show increase in silica shell thickness from ∼5 to ∼24 nm with increase in amount of tetraethyl orthosilicate (TEOS) used during the coating process from 0.1 to 0.3 mL. Magnetic studies indicate that Fe₃O₄ nanoparticles remain superparamagnetic after coating with silica although their M_s values are significantly less than pristine MNPs. These core/shell nanocomposites offer a high potential for different biomedical applications due to having superparamagnetic property of magnetite and unique properties of silica.     

Effect of pH, citrate treatment and silane-coupling agent concentration on the magnetic, structural and surface properties of functionalized silica-coated iron oxide nanocomposite particles

Superparamagnetic iron oxide nanoparticles were synthesized by coprecipitation of iron chloride salts at various pH values (9, 10, 11 and12) that were adjusted using an ammonia solution. Increasing the pH from 9 to 12 led to decreases in the size of iron oxide nanoparticles from 7.9±1.4 to 5±0.6 nm and the saturation magnetization (M_s) from 82.73 to 67.14 emu/g, respectively, when analyzed with transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). X-ray diffraction patterns as well as M_s values showed that magnetite is the dominantly synthesized phase in the examined pH values. Unmodified iron oxide nanoparticles were coated with silica via the hydrolysis and condensation of tetraethyl orthosilicate (TEOS), designated P1 particles. The size distribution diagram of P1 particles showed two regions with mean sizes of 143.3±15.4 and 216.9±13.7 nm corresponding to silica and iron oxide@silica particles, respectively. Stabilization of iron oxide nanoparticles using sodium citrate prior to coating with silica (P2 particles) resulted in nanocomposites with a mean size of 275±16.1 nm and an M_s value of 2.9 emu/g. Subsequently, the surface of P2 particles was functionalized by amine groups using N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (EDS). Results obtained from the measurement of zeta potential revealed that the highest value of isoelectric point (PI) change, indicating a more efficient surface functionalization, occurs when the EDS concentration of 90 mM is used, as compared to that for particles aminated using 25 and 180 mM EDS.     

Comparative study of core–shell iron/iron oxide gold covered magnetic nanoparticles obtained in different conditions

In this study, we report the synthesis and characterization of the core–shell Fe covered with Au shells nanoparticles with mean diameters between 5 and 8 nm. The inverse micelles method was utilized to produce the samples. X-ray diffraction studies show that both core–shell systems have the expected crystalline structure. High resolution transmission electron microscopy and atomic emission spectroscopy techniques give additional information concerning the structure and composition of nanoparticles. An intermediate shell of amorphous oxidized iron was found between the magnetic Fe core and the external gold shell. The magnetic behavior of different core–shell samples shows no hysteresis loop indicating the superparamagnetic behavior of Fe@Au systems. The superparamagnetic behavior is also evidenced from FC and ZFC dependences of the magnetization versus temperature. By using the temperature dependence of the thermoremanent magnetization combined with magnetization versus applied magnetic field, the effective anisotropy constant was determined. The Fe/Au interface contribution to the effective anisotropy constant was calculated and discussed in relation with the combined shape and stress anisotropies.