Three-dimensional porous carbon nanofiber networks decorated with cobalt-based nanoparticles: A robust electrocatalyst for efficient water oxidation

Electrochemical water splitting used for generating hydrogen has attracted increasingly attention due to energy and environmental issues. It is a major challenge to design an efficient, robust and inexpensive electrocatalyst to achieve preferable catalytic performance. Herein, a novel three-dimensional (3D) electrocatalyst was prepared by decorating nanostructured biological material-derived carbon nanofibers with in situ generated cobalt-based nanospheres (denoted as CNF@Co) through a facile approach. The interconnected porous 3D networks of the resulting CNF@Co catalyst provide abundant channels and interfaces, which remarkably favor both mass transfer and oxygen evolution. The as-prepared CNF@Co shows excellent electrocatalytic activity towards the oxygen evolution reactions with an onset potential of about 0.445 V vs. Ag/AgCl. It only needs a low overpotential of 314 mV to achieve a current density of 10 mA/cm² in 1.0 M KOH. Furthermore, the CNF@Co catalyst exhibits excellent stability towards water oxidation, even outperforming commercial IrO<inf>2</inf> and RuO<inf>2</inf> catalysts.

Field dependent transition to the non-linear regime in magnetic hyperthermia experiments: Comparison between maghemite, copper, zinc, nickel and cobalt ferrite nanoparticles of similar sizes

Further advances in magnetic hyperthermia might be limited by biological constraints, such as using sufficiently low frequencies and low field amplitudes to inhibit harmful eddy currents inside the patient's body. These incite the need to optimize the heating efficiency of the nanoparticles, referred to as the specific absorption rate (SAR). Among the several properties currently under research, one of particular importance is the transition from the linear to the non-linear regime that takes place as the field amplitude is increased, an aspect where the magnetic anisotropy is expected to play a fundamental role. In this paper we investigate the heating properties of cobalt ferrite and maghemite nanoparticles under the influence of a 500 kHz sinusoidal magnetic field with varying amplitude, up to 134 Oe. The particles were characterized by TEM, XRD, FMR and VSM, from which most relevant morphological, structural and magnetic properties were inferred. Both materials have similar size distributions and saturation magnetization, but strikingly different magnetic anisotropies. From magnetic hyperthermia experiments we found that, while at low fields maghemite is the best nanomaterial for hyperthermia applications, above a critical field, close to the transition from the linear to the non-linear regime, cobalt ferrite becomes more efficient. The results were also analyzed with respect to the energy conversion efficiency and compared with dynamic hysteresis simulations. Additional analysis with nickel, zinc and copper-ferrite nanoparticles of similar sizes confirmed the importance of the magnetic anisotropy and the damping factor. Further, the analysis of the characterization parameters suggested core-shell nanostructures, probably due to a surface passivation process during the nanoparticle synthesis. Finally, we discussed the effect of particle-particle interactions and its consequences, in particular regarding discrepancies between estimated parameters and expected theoretical predictions. Copyright 2012 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi. org/10.1063/1.4739533]

Magnetic hyperthermia investigation of cobalt ferrite nanoparticles: Comparison between experiment, linear response theory, and dynamic hysteresis simulations

Considerable effort has been made in recent years to optimize materials properties for magnetic hyperthermia applications. However, due to the complexity of the problem, several aspects pertaining to the combined influence of the different parameters involved still remain unclear. In this paper, we discuss in detail the role of the magnetic anisotropy on the specific absorption rate of cobalt-ferrite nanoparticles with diameters ranging from 3 to 14 nm. The structural characterization was carried out using x-ray diffraction and Rietveld analysis and all relevant magnetic parameters were extracted from vibrating sample magnetometry. Hyperthermia investigations were performed at 500 kHz with a sinusoidal magnetic field amplitude of up to 68 Oe. The specific absorption rate was investigated as a function of the coercive field, saturation magnetization, particle size, and magnetic anisotropy. The experimental results were also compared with theoretical predictions from the linear response theory and dynamic hysteresis simulations, where exceptional agreement was found in both cases. Our results show that the specific absorption rate has a narrow and pronounced maxima for intermediate anisotropy values. This not only highlights the importance of this parameter but also shows that in order to obtain optimum efficiency in hyperthermia applications, it is necessary to carefully tailor the materials properties during the synthesis process. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729271]

Layer-by-Layer assembled cobalt ferrite nanoparticles for chemical sensing

Ultra-thin (thicknesses of 50-90 nm) nanocomposite films of cobalt ferrite nanoparticles (np-CoFe2O4, 18 nm in diameter) and polyelectrolytes (doped polyaniline-PANI, poly-3,4-ethylenedioxy thiophene: polystyrene sulfonic acid-PEDOT:PSS, and sulfonated lignin-SL) are assembled layer-by-layer onto interdigitated microelectrodes aiming at to create novel nanostructured sensoactive materials for liquid media chemical sensors. The nanocomposites display a distinctive globular morphology with nanoparticles densely-packed while surrounded by polyelectrolytes. Due to the presence of np-CoFe2O4 the nanocomposites display low electrical conductivity according to impedance data. On the other hand, this apparent shortcoming turns such nanocomposites much more sensitive to the presence of ions in solution than films made exclusively of conducting polyelectrolytes. For example, the electrical resistance of np-CoFe2O4/PEDOT:PSS and PANI/SL/np-CoFe2O4/SL architectures has a 10-fold decrease when they are immersed in 20 mmol. L-1 NaCl solution. Impedance spectra fitted with the response of an equivalent circuit model suggest that the interface created between nanoparticles and polyelectrolytes plays a major role on the nanocomposites electrical/dielectrical behavior. Since charge transport is sensitive to nanoparticle-polyelectrolyte interfaces as well as to the physicochemical conditions of the environment, the np-CoFe2O4-based nanocomposites can be used as sensing elements in chemical sensors operated under ac regime and room temperature.

Dielectric properties of cobalt ferrite nanoparticles in ultrathin nanocomposite films

Multilayered nanocomposite films (thickness 50-90 nm) of cobalt ferrite nanoparticles (np-CoFe2O4, 18 nm) were deposited on top of interdigitated microelectrodes by the layer-by-layer technique in order to study their dielectric properties. For that purpose, two different types of nanocomposite films were prepared by assembling np-CoFe2O4 either with poly(3,4-ethylenedioxy thiophene):poly(styrene sulfonic acid) or with polyaniline and sulfonated lignin. Despite the different film architectures, the morphology of both was dominated by densely-packed layers of nanoparticles surrounded by polyelectrolytes. The dominant effect of np-CoFe2O4 was also observed after impedance spectroscopy measurements, which revealed that dielectric behavior of the nanocomposites was largely influenced by the charge transport across nanoparticle-polyelectrolyte interfaces. For example, nanocomposites containing np-CoFe2O4 exhibited a single low-frequency relaxation process, with time constants exceeding 15 ms. At 1 kHz, the dielectric constant and the dissipation factor (tan ᵟ) of these nanocomposites were 15 and 0.15, respectively. These values are substantially inferior to those reported for pressed pellets made exclusively of similar nanoparticles. Impedance data were further fitted with equivalent circuit models from which individual contributions of particle's bulk and interfaces to the charge transport within the nanocomposites could be evaluated. The present study evidences that such nanocomposites display a dielectric behavior dissimilar from that exhibited by their individual counterparts much likely due to enlarged nanoparticle- polyelectrolyte interfaces.

Simple synthesis and spectroscopic studies on cobalt added ZnO nanocrystals

Cobalt doped zinc oxide nanoparticles were prepared through simple wet chemical method. X-ray diffraction studies confirm the prepared particles are in wurtzite structure. Scanning Electron Microscopy studies show the shape and morphology of the particles. To identify the presence of cobalt in ZnO, Energy Dispersive X-ray analysis was done. Optical absorption measurements show the presence of exciton peak at 375 nm. Photoluminescence studies were done with the excitation wavelength of 330 nm, which shows the emission because of exciton recombination and oxygen vacancy.

High Value of Proton Relaxivity Achieved by Graphene Oxide-Cobalt Ferrite Nanoparticle Composite: A Potential Contrast Agent in Magnetic Resonance Imaging

This work investigates the potential of graphene oxide-cobalt ferrite nanoparticle (GO-CoFe2O4) composite as image contrast enhancing material in Magnetic Resonance Imaging (MRI). In the preset work, GO-CoFe2O4 composites were produced by a two-step synthesis process. In the first step, graphene oxide (GO) was synthesized, and in the second step CoFe2O4 nanoparticles were synthesized in a reaction mixture containing GO to yield graphene GO-CoFe2O4 composite. Proton relaxivity value obtained from the composite was 361 mM(-1)s(-1). This value of proton relaxivity is higher than a majority of reported relaxivity values obtained using several ferrite based contrast agents.

Microwave absorbers designed from PVDF/SAN blends containing multiwall carbon nanotubes anchored cobalt ferrite via a pyrene derivative

Lightweight and flexible electromagnetic shielding materials were designed by selectively localizing multiwall carbon nanotubes (MWNTs) anchored magnetic nanoparticles in melt mixed co-continuous blends of polyvinylidene fluoride (PVDF) and poly(styrene-co-acrylonitrile) (SAN). In order to facilitate better dispersion, the MWNTs were modified using pyrenebutyric acid (PBA) via pi-pi stacking. While one of the two-targeted properties, i.e., high electrical conductivity, was achieved by PBA modified MWNTs, high magnetic loss was accomplished by introducing nickel (NF) or cobalt ferrites (CF). Moreover, the attenuation by absorption can be tuned either by using NF (58% absorption) or CF (64% absorption) in combination with PBA-MWNTs. More interestingly, when CF was anchored on to MWNTs via the pyrene derivative, the minimum reflection loss attained was -55 dB in the Ku band (12-18 GHz) frequency and with a large bandwidth. In addition, the EM waves were blocked mostly by absorption (70%). This study opens new avenues in designing flexible and lightweight microwave absorbers.

Increased carbon nanotube area density after catalyst generation from cobalt disilicide using a cyclic reactive ion etching approach

We used a cyclic reactive ion etching (RIE) process to increase the Co catalyst density on a cobalt disilicide (CoSi2) substrate for carbon nanotube (CNT) growth. Each cycle of catalyst formation consists of a room temperature RIE step and an annealing step at 450 °C. The RIE step transfers the top-surface of CoSi2 into cobalt fluoride; while the annealing reduces the fluoride into metallic Co nanoparticles. We have optimized this cyclic RIE process and determined that the catalyst density can be doubled in three cycles, resulting in a final CNT shell density of 6.6 × 10 11 walls·cm-2. This work demonstrates a very effective approach to increase the CNT density grown directly on silicides. © 2014 AIP Publishing LLC.

High-frequency ferromagnetic resonance on ultrafine cobalt particles

We report on high-frequency (300-700 GHz) ferromagnetic resonance (HF-FMR) measurements on cobalt superparamagnetic particles with strong uniaxial effective anisotropy. We derive the dynamical susceptibility of the system on the basis of an independent-grain model by using a rectangular approach. Numerical simulations give typical line shapes depending on the anisotropy, the gyromagnetic ratio, and the damping constant. HF-FMR experiments have been performed on two systems of ultrafine cobalt particles of different sizes with a mean number of atoms per particles of 150 +/- 20 and 310 +/- 20. In both systems, the magnetic anisotropy is found to be enhanced compared to the bulk value, and increases as the particle size decreases, in accordance with previous determinations from magnetization measurements. Although no size effect has been observed on the gyromagnetic ratio, the transverse relaxation time is two orders of magnitude smaller than the bulk value indicating strong damping effects, possibly originating from surface spin disorders.

Magnetic characterization of Co1-xNixFe2O4 (0 <= x <= 1) nanoparticles prepared by co-precipitation route

Magnetic nanoparticles of Ni-doped cobalt ferrite [Co1-xNixFe2O4(0 <= x <= 1)] synthesized by coprecipitation route have been studied as a function of doping concentration (x) and particle size. The size of the particles as determined by X-ray diffractometer (XRD) and transmission electron microscope (TEM) analyses was found in the range 12-48 nm. The coercivity (H-C) and saturation magnetization (M-S) showed a decreasing behavior with increasing Ni concentration. M-S of all the samples annealed at 600 degrees C lies in the range 65.8-13.7 emu/gm. Field-cooled (FC) studies of the samples showed horizontal shift (exchange bias) and vertical shift in the magnetization loop. Strong decrease in exchange bias (H-b) and vertical shift (delta M) was found for low Ni concentrations while negligible decrease was found at higher concentrations. The presence of exchange bias in the low Ni-concentration region has been explained with reference to the interface spins interaction between a surface region (with structural and spin disorder) and a ferrimagnetic core region. M(T) graphs of the samples showed a decreasing trend of blocking temperature (T-b) with increasing Ni concentration. The decrease of T-b with increasing Ni concentration has been attributed to the lower anisotropy energy of Ni+2 ions as compared to Co+2 that increases the probability of the jump across the anisotropy barrier which in turn decreases the blocking temperature of the system.

Effect of Crystallographic Texture on Magnetic Characteristics of Cobalt Nanowires

Cobalt nanowires with controlled diameters have been synthesized using electrochemical deposition in etched ion-track polycarbonate membranes. Structural characterization of these nanowires with diameter 70, 90, 120 nm and length 30 mu m was performed by scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray diffraction techniques. The as-prepared wires show uniform diameter along the whole length and X-ray diffraction analysis reveals that [002] texture of these wires become more pronounced as diameter is reduced. Magnetic characterization of the nanowires shows a clear difference of squareness and coercivity between parallel and perpendicular orientations of the wires with respect to the applied field direction. In case of parallel applied field, the coercivity has been found to be decreasing with increasing diameter of the wires while in perpendicular case; the coercivity observes lower values for larger diameter. The results are explained by taking into account the magnetocrystalline and shape anisotropies with respect to the applied field and domain transformation mechanism when single domain limit is surpassed.

Studies on the magnetism of cobalt ferrite nanocrystals synthesized by hydrothermal method

Fe-Co/CoFe2O4 nanocomposite and CoFe2O4 nanopowders were prepared by the hydrothermal method. The structure of magnetic powders were characterized by X-ray diffraction diffractometer (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), thermal gravity analysis (TGA) and differential thermal analysis (DTA) analysis, X-ray photoelectron spectrometry (XPS), and Fourier transform infrared spectra (FTIR) techniques, while magnetic properties were determined by using a vibrating sample magnetometer (VSM) at room temperature.

Uniaxial anisotropy and novel magnetic behaviors of CoFe2O4 nanoparticles prepared in a magnetic field

CoFe2O4 nanoparticles prepared by chemical coprecipitation method in a magnetic field exhibit novel magnetic properties. The average particle diameter was about 2 nm and larger depending on the post annealing temperature. Magnetization measurements indicate that smaller nanoparticles are superparamagnetic above their respective blocking temperatures. In the blocked state, these nanoparticles exhibit interesting behaviors in the magnetic hysteresis measurements. Constricted, or wasp waisted with extremely narrow waist, hysteresis curves have been observed in the magnetization versus field sweeps. For larger nanoparticles, the room temperature hysteresis is typical of a ferromagnet with an open loop, but the loop closes at lower temperature. The novel magnetic behavior is attributed to the directional order of Co ions and vacancies in CoFe2O4 established during the coprecipitation of the nanoparticles under an applied field.

Preparation of cobalt hexacyanoferrate nanowires using carbon nanotubes as templates

A simple and convenient method for preparation of cobalt hexacyanoferrate (CoHCF) nanowires by electrodeposition was reported. Multiwall carbon nanotubes (MWNTs) were used as templates to fabricate CoHCF nanowires. MWNTs could affect the size of CoHCF nanoparticles and made them grow on the sidewalls of carbon nanotubes during the process of electrodeposition. Thus CoHCF nanowires could be obtained by this method. Field-emission scanning electron microscopy, UV-vis spectroscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy were used to characterize these nanowires. These results showed the CoHCF nanowires could be easily and successfully obtained and it gave a novel approach to prepare inorganic nanowires.