Development of Ferromagnetic Superspins in Bare Cu Nanoparticles by Electronic Charge Redistribution

We report on the results of investigating the ferromagnetic properties of bare Cu nanoparticles. Three sets of bare Cu nanoparticle assemblies with mean particle diameters of 6.6, 8.1, and 11.1 nm were fabricated, employing the gas condensation method. Curie-Weiss paramagnetic responses to a weak driving magnetic field were detected, showing the appearance of particle superspins that overcomes the diamagnetic responses from the inner core. The isothermal magnetization displays a Langevin field profile together with magnetic hysteresis appearing even at 300 K, demonstrating the existence of ferromagnetic superspins in the Cu nanoparticles. Shifting of a noticeable amount of electronic charge from being distributed near the lattice sites in bulk form toward their neighboring ions in nanoparticles was found. The extended 3d and 4s band mixture are the main sources for the development of localized 3d holes for the development of ferromagnetic particle superspins in Cu nanoparticles.     

Size dependence of the magnetic properties of Ni nanoparticles prepared by thermal decomposition method

By means of thermal decomposition, we prepared single-phase spherical Ni nanoparticles (23 to 114 nm in diameter) that are face-centered cubic in structure. The magnetic properties of the Ni nanoparticles were experimentally as well as theoretically investigated as a function of particle size. By means of thermogravimetric/differential thermal analysis, the Curie temperature T_C of the 23-, 45-, 80-, and 114-nm Ni particles was found to be 335°C, 346°C, 351°C, and 354°C, respectively. Based on the size-and-shape dependence model of cohesive energy, a theoretical model is proposed to explain the size dependence of T_C. The measurement of magnetic hysteresis loop reveals that the saturation magnetization M_S and remanent magnetization increase and the coercivity decreases monotonously with increasing particle size, indicating a distinct size effect. By adopting a simplified theoretical model, we obtained M_S values that are in good agreement with the experimental ones. Furthermore, with increase of surface-to-volume ratio of Ni nanoparticles due to decrease of particle size, there is increase of the percentage of magnetically inactive layer.     

Magnetic properties of carbon nano-particles produced by a pulsed arc submerged in ethanol

Carbon powder was produced by a pulsed arc ignited between two carbon electrodes submerged in ethanol, and was comprised of both micro- and nano-particles. The measured magnetic properties of the mixed “raw” powder at 20 and 300 K were: saturation magnetization M_s ∼ 0.90-0.93 emu/g, residual magnetization M_r = 0.022 and 0.018 emu/g, and coercive force H_c = 11 and 8 Oe, respectively. The data lead to conclusion that the powder consisted of ferromagnetic particles with a critical temperature much higher than 300 K. Magnetic particles in solution were separated by means of bio-ferrography. It was found that the magnetically separated particles included chains of ∼30-50 nm diameter spheres, and nanotubes and nanorods with lengths of 50-250 nm and diameters of 20-30 nm. In contrast, the residual particles which passed through the bio-ferrograph consisted of 1 μm and larger micro-particles, and nano-particles without any definite shape.     

Synthesis and characterization of yttrium iron garnet nanoparticles doped with cobalt

In this work, we have synthesized and characterized yttrium iron garnet nanoparticles doped with cobalt. The X-ray diffraction data showed a single phase, belonging to the cubic structure of Y₃Fe₅O₁₂. Rietveld refinement revealed variation of the angles and interionic distances (Fe³⁺(a)-O^2-Y³⁺(c) and Fe³⁺(d)-O^2--Y³⁺(c) when Fe³⁺ ions are replaced by Co³⁺ ions in the tetrahedral (d) and octahedral (a) sites of YIG. In addition, the lattice parameter a, decreases from 12.3846?Å to 12.3830?Å with the increasing of cobalt concentration. The analysis by Infrared and Raman spectroscopies has shown a slight stretching at lower wave numbers as the dopant concentration increased. The magnetic measurements confirm the substitution of Fe³⁺ by Co³⁺ in the a-sites and d-sites with the reduction of the saturation magnetization from 26.63?emu/g to 24.92?emu/g, for 0.000?≤?y?≤?0.030. Changes in the coercive field varying the dopant concentration were related to the particle size and pinning centers existence.     

Preparation and properties of poly(ethylene glycol)-block-poly(acrylic acid)-coated cobalt nanocrystals

Reported in this paper are the preparation and properties of ?-Co nanocrystals coated by poly(ethylene glycol)-block-poly(acrylic acid) (PEG-b-PAA). These particles were prepared via the thermal decomposition of Co₂(CO)₈ at 185 °C in 1,2-dichlorobenzene, in the presence of the surfactant PEG-b-PAA and the co-surfactant trioctylphosphine oxide. At a given initial Co₂(CO)₈ concentration, the size of the particles increased with increasing Co₂(CO)₈-to-PEG-b-PAA molar ratio, and could be tuned between ∼5 and ∼20 nm. The size distribution of the particles narrowed as the Co₂(CO)₈ concentrations increased. The resultant particles were dispersible in a wide range of solvents, including chloroform, N,N-dimethylforamide, and water, which solubilized PEG. Magnetic measurements revealed that the particles possessed saturation magnetization close to that of bulk Co, suggesting high purity of the particles.     

Particle size effects of gold on the kinetics of the oxygen reduction at chemically prepared Au/C catalysts

Gold nanoparticles with narrow and controlled size distributions have been synthesized chemically and deposited onto a carbon support. Using the resulting gold on carbon (Au/C) catalysts, Au particle size effects on the kinetics of the oxygen reduction reaction (ORR) were analyzed in acidic media (0.5 M H₂SO₄). From rotating ring-disk electrode (RRDE) voltammetric studies, it was found that, for bulk gold, the number of electrons, n, involved in the ORR was nearly constant at potentials above −0.2 V. On the contrary, for the catalysts with diameters less than 10-15 nm, the value of n increased as the potential became more negative, and the highest value of n was obtained when the size of Au particles was less than 3 nm. Those results showed that further reduction of H₂O₂ or direct 4-electron reduction of O₂ proceeded at relatively low overpotential on extremely small gold clusters.     

Core–shell superparamagnetic iron oxide nanoparticle (SPION) clusters: TEM micrograph analysis,particle design and shape analysis

For the first time, particle shape analysis of silica coated iron oxide (maghemite/magnetite) nanoparticle clusters (core–shell nanostructures) is discussed using computational methods. We analyzed three samples of core–shell nanostructures synthesized with different thickness of the silica shell. A new computational method is presented and successfully applied to the segmentation of the core–shell nanoparticles, as one of the main problems in image analysis of the TEM micrographs. We have introduced the “circularity coefficient”, marked with k_circ and defined as the ratio of circularity measure C₂(S) of nanoparticles core and circularity measure core–shell nanoparticles in order to answer the question how the shell affects the overall shape of the final core–shell structure, with respect to circularity. More precisely, the “circularity coefficient” determines whether the circularity of the core–shell nanoparticle is higher, lower or equal to the circularity of the core. We have also determined the shell's share in the overall area of the core–shell nanoparticle. The core–shell nanoparticle clusters here investigated exhibit superparamagnetic properties at room temperature, thus emphasizing their potential for use in practical applications such as in biomedical and particle separation. We show that the saturation magnetization strength can be easily adjusted by controlling the thickness of the silica shell.     

Biomagnetic of Apatite-Coated Cobalt Ferrite: A Core–Shell Particle for Protein Adsorption and pH-Controlled Release

Magnetic nanoparticle composite with a cobalt ferrite (CoFe₂O₄, (CF)) core and an apatite (Ap) coating was synthesized using a biomineralization process in which a modified simulated body fluid (1.5SBF) solution is the source of the calcium phosphate for the apatite formation. The core–shell structure formed after the citric acid–stabilized cobalt ferrite (CFCA) particles were incubated in the 1.5 SBF solution for 1 week. The mean particle size of CFCA-Ap is about 750 nm. A saturation magnetization of 15.56 emug^-1 and a coercivity of 1808.5 Oe were observed for the CFCA-Ap obtained. Bovine serum albumin (BSA) was used as the model protein to study the adsorption and release of the proteins by the CFCA-Ap particles. The protein adsorption by the CFCA-Ap particles followed a more typical Freundlich than Langmuir adsorption isotherm. The BSA release as a function of time became less rapid as the CFCA-Ap particles were immersed in higher pH solution, thus indicating that the BSA release is dependent on the local pH.     

Surfactant-assisted combustion synthesis of agglomerated-free,size- and shape-controlled magnetic iron oxide nanoparticles for biomedical applications

An advanced version of the solution combustion synthesis (SCS) method was developed to prepare iron oxide (IO) nanoparticles, through controlling the agglomeration, size and shape of nanoparticles by assisting a cationic surfactant, “cetyltrimethylammonium bromide (CTAB)” and ethanol. Various IO nanoparticles were prepared in the presence of different CTAB:ethanol molar compositions of 0:0, 0.27:0, 0.55:0, 0.27:17.1, 0.55:17.1 and 0.82:17.1. The morphological evolution from agglomerated-shape particles to the well-dispersed as well as the size- and shape-controlled particles depended directly on the CTAB:ethanol molar composition. A shift from ferromagnetic behavior to superparamagnetic was observed by the application of CTAB along with ethanol, where the lowest blocking temperature (T_b, 60 K), highest saturation magnetization (M_s, 83.5 emu g⁻¹), zero coercivity and remanance magnetization were revealed for the particles prepared by CTAB:ethanol molar compositions of 0.55:17.1. These particles showed an acceptable specific absorption rate (SAR) value (320 W g⁻¹) as well as no obvious hemolytic and cytotoxic effects. This work provides new insights into advancing the SCS method and thus controlling the morphology, size and shape of metal oxide nanoparticles.     

Optical properties of gold/multilayer-graphene/carbon nanotube hybrid materials

Discrete dipole approximation (DDA) method was utilized to simulate optical properties (extinction spectra and normalized electric field distribution) of multilayer graphene shell encapsulated gold nanoparticles (GNPs) and their heterostructures with carbon nanotube (CNT). The results were also compared with those for gold (Au) nanoparticles and CNT–Au nanoparticle heterostructures. Encapsulation of Au nanoparticle with a multilayer graphene shell in GNPs did not suppress optical properties and surface plasmons of the former. Tunable optical characteristics of GNPs were calculated that showed resonance peak wavelengths, corresponding to encapsulated Au nanoparticle, between ∼527 and ∼663 nm as a function of size and multilayer graphene shell thickness. Enhanced optical/plasmonic behavior and intense ‘hot spots’ were estimated for CNT coated with tightly-packed GNPs as compared to CNT coated with bare Au nanoparticles.     

Effect of apple cider vinegar agent on the microstructure,phase evolution,and magnetic properties of CoFe2O4 magnetic nanoparticles

In the present study, spinel structure CoFe₂O₄ nanoparticles were successfully synthesized by the sol-gel auto-combustion technique. The effect of apple cider vinegar (ACV) addition as an organic biocompatible agent on the size, morphology, and magnetic properties of CoFe₂O₄ nanoparticles was investigated in detail. The phase evolution, particle size, and lattice parameter changes of the synthesized phase have been estimated by using Rietveld structure refinement analysis of X-ray powder diffraction data. Also, Fourier transform infrared spectra (FT-IR) of the samples verified the presence of two expected bands correspond to tetrahedral and octahedral metal-oxygen complexes within the spinel structure. Furthermore, microstructural observations revealed that ultrafine particles have a semi-spherical morphology. It was shown that the particles size decreased from ~45 to ~17 nm with an increase in the amount of ACV. Magnetic properties were carried out by vibrating sample magnetometer (VSM) at room temperature. Both the saturation magnetization (M_s) and coercivity (H_c) were found to be significantly dependent on the crystallite size and the amount of ACV.     

Elaboration of superparamagnetic cobalt–ferrite nanocomposites from films of chitosan chelates

Cobalt–ferrite nanocomposites were synthesized from polymeric films of chelates of Co(II) and Fe(III) ions within a chitosan matrix by a solid‐state coprecipitation reaction with weight content ratios of chitosan to cobalt–ferrite of 50/50 and 25/75 w/w. Morphological and crystalline studies of the composites were performed by high‐resolution transmission electron microscopy, X‐ray diffraction, and selected area electron diffraction with a nanobeam diffraction probe. The results show nanoparticles around 4 nm with a spinel structure, consistent with the cobalt–ferrite phase. The magnetic behavior was evaluated with curves of the applied‐field‐dependent magnetization [M(H)] and the temperature‐dependent magnetization [M(T)]. Both the M(H) and M(T) curves showed typical superparamagnetic behavior, depicting an absence of hysteretic characteristics and the characteristic peak at blocking temperature in the zero‐field‐cooled curve. There was also evidence of strong interparticle and intraparticle interactions, which suggested magnetic frustration in the particle magnetic moment alignment with the applied field. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Structural,magnetic and magnetocaloric properties of CoFe2−xMoxO4 (0.0≤x≤0.3) ferrites

We have investigated structural, magnetic and magnetocaloric properties of CoFe_2-xMo_xO₄ (0.0≤x≤0.3) ferrites. Polycrystalline samples were prepared by the sol gel method and characterized by the powder X-ray diffraction and scanning electron microscopy. X-ray diffraction patterns show that all samples have a cubic spinel structure and the lattice parameter, a, decreases monotonically with increase in Mo concentration. Scanning electron micrographs indicate that most of the particles are in the range of 400–850 nm size. Magnetic measurements, performed by using a cryogen free vibrating sample magnetometer, show that these samples are soft ferromagnets in the measured temperature range. The saturation magnetization, M_s, values of Mo-doped samples are larger than the parent compound with a maximum value of ~106 emu/g for x=0.2 sample. The magnetic entropy change (−ΔS) increases with increase in applied magnetic field and shows a peak in the vicinity of blocking temperature. A maximum value of 0.56 J kg⁻¹ K⁻¹ at 5 T field has been observed for x=0.2 sample.     

An efficient magnetically modified microbial cell biocomposite for carbazole biodegradation

Magnetic modification of microbial cells enables to prepare smart biocomposites in bioremediation. In this study, we constructed an efficient biocomposite by assembling Fe₃O₄ nanoparticles onto the surface of Sphingomonas sp. XLDN2-5 cells. The average particle size of Fe₃O₄ nanoparticles was about 20 nm with 45.5 emu g^-1 saturation magnetization. The morphology of Sphingomonas sp. XLDN2-5 cells before and after Fe₃O₄ nanoparticle loading was verified by scanning electron microscopy and transmission electronic microscopy. Compared with free cells, the microbial cell/Fe₃O₄ biocomposite had the same biodegradation activity but exhibited remarkable reusability. The degradation activity of the microbial cell/Fe₃O₄ biocomposite increased gradually during recycling processes. Additionally, the microbial cell/Fe₃O₄ biocomposite could be easily separated and recycled by an external magnetic field due to the super-paramagnetic properties of Fe₃O₄ nanoparticle coating. These results indicated that magnetically modified microbial cells provide a promising technique for improving biocatalysts used in the biodegradation of hazardous compounds.     

Characterization and magnetic properties of carbon-coated cobalt nanocapsules synthesized by the chemical vapor-condensation process

Carbon-coated cobalt nanocapsules were synthesized by the chemical vapor-condensation process with cobalt carbonyl (Co₂(CO)₈) used as precursor and carbon monoxide (CO) as carrier gas. The characterization and magnetic properties of carbon-coated cobalt nanocapsules were investigated systematically. The transmission electron microscope (TEM) images showed that the as-prepared nanoparticles consist of a metal core and an amorphous carbon shell. X-ray diffraction and TEM selected area diffraction revealed the presence of f.c.c. Co phase, h.c.p. Co phase, and minority Co₂C, Co₃C phases. The saturation magnetization at room temperature of the nanocapsules is 146.9 Am² kg⁻¹, which is 90% of the bulk ferromagnetic element counterpart. The coercive force at room temperature of the nanocapsules is 0.12 T, while the ratio of remnant to saturation magnetization M_r/M_s is about 0.4. The saturation magnetization and the coercive force increase with increasing the decomposition temperature, mainly due to the increase of the size of the magnetic particles. The decomposition of the cobalt carbonyl (Co₂(CO)₈) and CO gas can decrease efficiently the oxygen content in nanocapsules. The metallic Co nanoparticles completely coated by carbon can resist the dilute acid erosion as well as the oxidation. The thermal stability of the Co nanocapsules is also studied.     

Preparation and Properties of ε-Fe3N-Based Magnetic Fluid

In this work, ε-Fe₃N nanoparticles and ε-Fe₃N-based magnetic fluid were synthesized by chemical reaction of iron carbonyl and ammonia gas. The size of ε-Fe₃N nanoparticles was tested by TEM and XRD. Stable ε-Fe₃N-based magnetic fluid was prepared by controlling the proper ratio of carrier liquid and surfactant. The saturation magnetization of stable ε-Fe₃N-based magnetic fluid was calculated according to the volume fraction of the particles in the fluid. The result shows that both the calculated and measured magnetizations increase by increasing the particle concentration. With the increasing concentration of the ε-Fe₃N particles, the measured value of the magnetic fluid magnetization gradually departs from the calculated magnetization, which was caused by agglomeration affects due to large volume fraction and large particle size.     

Size effect on magnetic properties of Zn0.95−xMgxNi0.05O nanoparticles by Monte Carlo simulation

Diluted magnetic semiconductors (DMSs) have been providing a wide research area with various conflicting results of magnetic properties which are generally originated from structural characteristics due to fabrication process. We focused on the size dependent magnetic behavior of Zn_0.95?xMg_xNi_0.05O nanoparticles as a promising novae material introducing room temperature ferromagnetism (FM) at low doping concentrations of Mg⁺² and Ni⁺² ions. Markov Chain Monte Carlo method based on Metropolis algorithm is used to simulate the system, constructed on experimental parameters such as particle size (D), lattice constants (a and c), uniaxial anisotropy constant (K), applied field (H) herewith doping concentrations of Ni (5%) and Mg (1%). However, we described the system with the Heisenberg Hamiltonian to represent the exotic nature of the DMS type materials since determining the J_ij constants by tracing the type of the exchange relation between different types of atoms as reported in former studies. In the light of hysteresis measurements, nanoparticles generated between 5?<?D <?15?nm showed strong FM among others. An exciting result is that D =?5?nm, 10?nm and 15?nm curves are so close to be overlapped. Saturation magnetization (Ms) and coercive field (H_c) had peaks of; D =?15?nm remnant magnetization (Mr) increased with increasing D up to 50?nm. Furthermore, D <?5?nm and D>?15?nm sized particles started to lose FM behavior. In addition, magnetic features of Zn_0.95?xMg_xNi_0.05O nanoparticles can be controlled via picking a fabrication method to tune the particle size.     

Size-Controlled Synthesis of Colloidal Gold Nanoparticles at Room Temperature Under the Influence of Glow Discharge

Highly dispersed colloidal gold (Au) nanoparticles were synthesized at room temperature using glow discharge plasma within only 5 min. The prepared Au colloids were characterized with UV–visible absorption spectra (UV–vis), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) equipped with an energy dispersion X-ray spectrometer (EDX). UV–vis, XPS and EDX results confirmed that Au³⁺ ions in HAuCl₄ solution could be effectively reduced into the metallic state at room temperature with the glow discharge plasma. TEM images showed that Au nanoparticles were highly dispersed. The size of colloidal Au nanoparticles could be easily tuned in the nanometer range by adjusting the initial concentration of HAuCl₄ solution. Moreover, the as-synthesized Au colloids (d _av = 3.64 nm) exhibited good catalytic activity for glucose oxidation. The nucleation and growth of colloidal Au particles under the influence of the plasma was closely related with the high-energy electrons generated by glow discharge plasma.     

Halloysite nanotubes coated with magnetic nanoparticles

Co nanoparticles were assembled on the surface of halloysite nanotubes (HNTs) to prepare one-dimensional magnetic Co-HNTs via electroless deposition. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDXS) and vibrating sample magnetometer (VSM). The cobalt nanoparticles of 3–7 nm in size were uniformly deposited on the surface of the nanotubes. The remanent magnetization (M_r), saturation magnetization (M_s) and coercivity (H_c) values of the Co-HNTs were 13.9 emu/g, 27.05 emu/g and 1659 O_e, respectively, larger than that of the pure Co nanoparticles (580.72 O_e). A mechanism of the deposition of the magnetic nanoparticles on the surface of the halloysite nanotubes is suggested. Co-HNTs showed an interesting potential in the field of magnetic devices.     

Effect of Co substitution and magnetic field on the morphologies and magnetic properties of CeO2 nanoparticles

Pure and Co-doped CeO₂ nanoparticles were synthesized successfully by the solvothermal method. The effect of Co substitution and external magnetic field on the morphologies and magnetic properties of nanoparticles was investigated. Results showed that synthesized Co-doped CeO₂ had the face-centered cubic structure and no other impurities existed in the samples with the increase of Co concentration from 5 to 75?wt%. The increasing Co concentration made the morphologies of Co-doped CeO₂ nanoparticles vary from the hollow sphere, solid sphere to rod-like shape. The applied external magnetic field of 5T decreased the nanoparticle size effectively including the diameter of hollow sphere with low Co concentration and rod-like particles with high Co concentration. Moreover, the wall thickness of hollow sphere particles was also decreased from 35?nm to 18?nm for pure CeO₂. The Co-doped CeO₂ nanoparticles showed the weak ferromagnetic behavior. With the increase of Co concentration, the saturation magnetization (M_s) value increased first and then decreased. The Co-doped CeO₂ with 30?wt% showed the highest value of 3.65?×?10^?2 emu/g (M_s). The M_s value of Co-doped CeO₂ prepared in 5T showed an increasing trend with the Co concentration. The highest value (M_s) reached 4.21?×?10^?2 emu/g for doped CeO₂ with 75?wt% Co.