NiO Core Research Articles

A step forward towards optimized iron–nickel bimetal oxide core–shell nanoparticles and their magnetic properties

The core–shell nanoparticles of Fe3O4 and NiO are synthesized under different reaction conditions and are characterized by UV–vis and fluorescence spectroscopy, DLS, SEM, EDX, XRD, VSM, and FTIR spectroscopy. These techniques evaluated optimal synthetic conditions for desired size, morphology, stability, and crystallinity of these core–shell nanoparticles. Increasing the concentration of salt for shell material (Fe3O4 or NiO) in core–shell NPs resulted in better coating and increase in hydrodynamic size from 227 nm to 531 nm for (Fe3O4)NiO, and 244 nm to 544 nm for (NiO)Fe3O4, whereas lower concentration and higher temperature (80 °C) for (Fe3O4)NiO and lower temperature (25 °C) for (NiO)Fe3O4, higher pH 13 and 11 for (Fe3O4)NiO and (NiO)Fe3O4 respectively, and fast (quick) addition of reactants produces homogenous small sized nanoparticles. Stability is generally attained at higher concentration, higher temperature (80 °C), higher pH (13 and 11 for (Fe3O4)NiO and (NiO)Fe3O4 respectively), and slow addition (10 min) of reactants. Fluorescence enhancement is observed for (Fe3O4)NiO core–shell NPs at high concentration, temperature, pH and slow addition of reactants, representing proper coating of NiO. For (NiO)Fe3O4 NPs, fluorescence quenching is observed at higher concentration, lower temperature, higher pH, and slow addition of reactants representing proper coating of Fe3O4. XRD depicted pure cubic crystalline nature of these core–shell nanoparticles without any impurity. The identified optimized conditions are useful for obtaining nanoparticles with desired properties for various applications including sensing and catalysis etc. Magnetic properties of pure and core–shell nanoparticles show ferrimagnetic behavior for Fe3O4 NPs and weak ferromagnetic or superparamagnetic behavior NiO NPs.

Two–step pulsed laser ablation for preparation NiO@ZnO core-shell nanostructure and evaluation of their antibacterial activity

The excessive use of antibiotics led to the appearance of many strains of resistant bacteria, so it became necessary to use new antibacterial techniques. The aim of this work is the synthesis of novel core–shell NPs (NiO@ZnO) for antibacterial applications. A novel NiO@ZnO core–shell nanomaterial with a nanosize and uniform shape has been synthesised using the two-step pulsed laser ablation in liquid (PLAL) method. The first step is the laser ablation of the nickel target and the production of NiO NPs (core) colloidal, followed by the second step, the ablation of ZnO (shell) inside NiO NPs colloidal. The transmission electron microscopy results approve the formation of NiO@ZnO core–shell NPs with an average particle size of 54.4 nm for NiO particles and 60.7 nm for the NiO@ZnO core–shell. The antibacterial activity was examined against the pathogenic bacterial strains E. coli and S. aureus. We found that in the case of NiO, the inhibition rates were 62.4 and 59.2% for E. coli and S. aureus, respectively. While with NiO@ZnO NPs, this result was improved to 74.8% for E. coli and 71.2% for S. aureus. So, it was found that using the core–shell NPs increased the antibacterial activity of NPs, and the use of NiO NPs and core approved their effect as antibacterial agents due to their special properties. This behaviour is primarily because of the accumulation of the NiO@ZnO NPs on the surface of the bacteria, which leads to cytotoxic bacteria and a relatively increased ZnO, causing cell death. Furthermore, the use of a NiO@ZnO core–shell material will inhibit the bacteria from nourishing themselves on the culture medium. Therefore, core shelling metal oxides with another metal oxide or other material improves their antibacterial activity compared with using them alone.

The Reduction Step and Mechanism Analysis of the NiO-8YSZ Anode Affected by the Hydrogen Concentration

The reduction of the NiO-YSZ anode has a direct bearing on the performance of the solid oxide fuel cell, and the hydrogen concentration is a significant condition during the reduction process. Hydrogen at different concentrations were used to reduce the NiO-YSZ anode and the initial performances of the solid oxide fuel cells were tested to reveal the influence of the hydrogen concentration on the reduction of the NiO-YSZ anode. The experimental results suggested that four stages of the open circuit voltage exist during the reduction of the NiO-YSZ anode, namely rapid increase period, temporary stasis period, slow increase period and final stable period successively. The rates of the rapid increase period and the slow growth period showed significant positive correlations with the hydrogen concentration of the reduction while there was a negative correlation between the duration of the temporary stasis period and the hydrogen concentration of the reduction. The values of the temporary stasis period were 0.83V approximately, which were irrelevant to the hydrogen concentration of the reduction. During the final stable period, the variation of the open circuit voltage is less than 1mV/h, which were close to the theoretical open circuit voltage. After the reduction of the NiO-YSZ anode, the ohmic resistance of the solid oxide fuel cell was about 0.31Ω /cm2 and did not reveal a clear correlation with the hydrogen concentration of the reduction. The polarization resistance showed significant negative correlation with the hydrogen concentration of the reduction on open circuit. But under the on-load condition, the same phenomenon only occurred in low hydrogen concentration of the reduction and then the polarization resistance remained largely the same with the increasing hydrogen concentration of the reduction. There was a direct correlation between the initial performances of the solid oxide fuel cell and the polarization resistance under the on-load condition that the initial performances can be improved with the increasing hydrogen concentration during the reduction process of the NiO-YSZ anode when the hydrogen concentration was low and then the initial performances remained constant basically with the continuing increasing hydrogen concentration. The abovementioned phenomena demonstrate that the resistance on open circuit represent nothing to the performances of the solid oxide fuel cell under the on-load condition. Previous researches about the NiO reduction by hydrogen, especially the shrinking core model can be used to explain the abovementioned phenomena. The reduction of the NiO is a volume reduction process. The reaction interface of the Ni/NiO moves inwards during the reduction, forming an unreacted NiO core surrounded by a rim of metallic Ni with much porous structures finally. The H2O produced in the reduction is absorbed onto the surface of the unreacted NiO core, improving the apparent activation energy and limiting diffusion of the Ni atoms to appropriate nucleation centers. Thus, the reduction process is retarded, forming the temporary stasis period. The absorbed H2O removes out through the porous structures of the rim of the metallic Ni. Meanwhile the H2 penetrates deeper and continues to reduce the unreacted NiO core, which is the mechanism of the slow increase period. According to the shrinking core model, higher hydrogen concentration can make the unreacted NiO core smaller, lessening the H2O absorbed onto the surface. Thus, the time needed for the absorbed H2O removal is shorten, leading the duration of the temporary stasis period showed significant negative correlation with the hydrogen concentration. The previous researches attribute the activation of the solid oxide fuel cell to the coarsening of the Ni particle or the further reduction of the unreacted NiO core. In this experiment, the performance of the solid oxide fuel cell with the NiO-YSZ anode reduced by 5% hydrogen concentration was lowest, and thus its potential of the activation was highest. In the first 20 hours of the operation, its performance kept improving and achieved that of the solid oxide fuel cells with the NiO-YSZ anodes reduced by higher hydrogen concentration. But the performance of the solid oxide fuel cells with the NiO-YSZ anodes reduced by higher hydrogen concentration did not significantly improve in the first 20 hours of the operation. The comparison showed that the cause of the activation of the solid oxide fuel cell is quite possible the further reduction of the unreacted NiO core.

The Reduction Step and Mechanism Analysis of the NiO-8YSZ Anode Affected by the Hydrogen Concentration

Hydrogen of different concentrations humidified by the water vapour were used to reduce the NiO-8YSZ anodes of the solid oxide fuel cells (SOFCs) at 700℃ and 720℃. The experimental results suggested that four periods of the open circuit voltage, namely rapid increase period, temporary stasis period, slow increase period and final stable period exist successively during the reduction, the characteristics of which were closely related to the hydrogen concentration of the reduction. After the reduction, the polarization resistance on open circuit and on load showed different correlation with the hydrogen concentration of the reduction. The variation trend of the initial performances of the SOFCs corresponded highly to the polarization resistance on load. The shrinking core model of the reduction of the NiO suggested that the further reduction of the unreacted NiO core would most likely cause the activation of the SOFC.

Size-dependent magnetic properties of Mn-Co-NiO based heterostructured nanoparticles

In this work, we investigate the synthesis, along with the structural and magnetic properties, of novel Mn-Co-NiO-based heterostructured nanocrystals (HNCs). The objective is to develop novel, well-structurally ordered inverted antiferromagnetic (AFM) NiO–ferrimagnetic (FiM) spinel phase overgrowth HNCs. Inverted HNCs are particularly promising for magnetic device applications because their magnetic properties are more easily controlled by having well-ordered AFM cores, which can result in magnetic structures having large coercivities, tunable blocking temperatures, and other enhanced magnetic effects. The synthesis of the HNCs is accomplished using a two-step process: In the first step, NiO nanoparticles are synthesized using a thermal decomposition method. Subsequently, Mn-Co overgrowth phases are grown on the NiO nanoparticles via hydrothermal nanophase epitaxy, using a fixed pH level (∼5.3) of the aqueous medium. This pH level was selected based on previous work in our laboratory showing that NiO/Mn3O4 HNCs of constant size have optimal coercivity and exchange bias when synthesized at a pH of 5.0. The crystalline structure and gross morphology of the Mn-Co-NiO-based HNCs have been analyzed using X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques, respectively. Analysis using these techniques shows that the HNCs are composed of a NiO core and a CoMn2O4 overgrowth phase. Rietveld refinement of XRD data shows that the NiO core has the rocksalt (Fm3̄m) cubic crystal structure and the CoMn2O4 overgrowth has the spinel (I41/amd) crystal structure. Moreover, an increased relative amount of the CoMn2O4 overgrowth phase is deposited with decreasing NiO core particle size during the synthesis of the HNCs. The results from PPMS magnetization and high-resolution transmission electron microscopy (HRTEM) characterization of the Mn-Co-NiO-based HNCs are discussed herein.

Tuning Exchange Coupling in NiO-Based Bimagnetic Heterostructured Nanocrystals.

A series of bimagnetic heterostructured nanocrystals having an antiferromagnetic NiO core and a ferrimagnetic MnxNi1-xO and/or FiM Mn3O4 island nanophase overgrowth has been synthesized under varying aqueous solution pH conditions. The two-step self-assembly process employs a thermal decomposition method to synthesize NiO nanoparticles, followed by growth of the MnxNi1-xO and/or Mn3O4 nanophase over the NiO core using hydrothermal synthesis at pH values ranging from 2.4-7.0. The environmentally benign hydrothermal process involves pH control of the protonation vs hydroxylation reactions occurring at the nanoparticle surface. TEM analysis and Rietveld refinement of XRD data show that three distinct types of heterostructured nanocrystals occur: NiO/MnxNi1-xO core-shell-like heterostructures at the pH of 2.4, mixed NiO/MnxNi1-xO and/or/Mn3O4 core-overgrowth structures for 2.4 < pH < 4.5, and predominantly NiO/Mn3O4 core-island structures for pH > 4.5. The magnetic coercivity and exchange bias of the heterostructured nanocrystals vary systematically with the pH of the aqueous solution used to synthesize the samples. The temperature-dependent magnetization and hysteresis loop data are consistent with the nature of overlayer coverage of the NiO core. Our DFT based calculations show that the MnxNi1-xO phase has ferrimagnetic properties with a stable spin orientation along the ⟨111⟩ orientation. Furthermore, the calculations show that the magnetic anisotropy constant (K1) of the Mn3O4 phase is considerably larger than that of the MnxNi1-xO phase, which is confirmed by our experimental results. The coercivity and exchange bias field are the largest for the NiO/Mn3O4 core-island nanocrystals, synthesized at a pH value of 5.0, with robust values of nearly 6 kOe and 3 kOe, respectively. This work demonstrates the tunability of hydrothermal deposition, and concomitant magnetic coercivity and exchange bias properties, of MnxNi1-xO and/or Mn3O4 nanophase overgrowth over a NiO core with pH, that makes these heterostructured nanocrystals potentially useful for magnetic device, biomedical, and other applications.

Nitride‐Oxide‐Metal Heterostructure with Self‐Assembled Core–Shell Nanopillar Arrays: Effect of Ordering on Magneto‐Optical Properties

Magneto-optical (MO) coupling incorporates photon-induced change of magnetic polarization that can be adopted in ultrafast switching, optical isolators, mode convertors, and optical data storage components for advanced optical integrated circuits. However, integrating plasmonic, magnetic, and dielectric properties in one single material system poses challenges since one natural material can hardly possess all these functionalities. Here, co-deposition of a three-phase heterostructure composed of a durable conductive nitride matrix with embedded core-shell vertically aligned nanopillars, is demonstrated. The unique coupling between ferromagnetic NiO core and atomically sharp plasmonic Au shell enables strong MO activity out-of-plane at room temperature. Further, a template growth process is applied, which significantly enhances the ordering of the nanopillar array. The ordered nanostructure offers two schemes of spin polarization which result in stronger antisymmetry of Kerr rotation. The presented complex hybrid metamaterial platform with strong magnetic and optical anisotropies is promising for tunable and modulated all-optical-based nanodevices.

N‐ZnO/p‐NiO Core/Shell‐Structured Nanorods for Piezoelectric Nanogenerators

Energy‐harvesting technologies, which can generate electrical energy from various energy sources, such as solar, thermal, and mechanical movement that are commonly found in the local environment, supply permanent and environmentally friendly energy. In addition, related research has been in the spotlight because it provides power by harvesting and converting a naturally occurring energy source without a charging process through external power. Piezoelectric nanogenerators (PENGs) based on piezoelectric materials have attracted much attention due to their high energy‐conversion efficiency, capability for miniaturization, and light weight. Zinc oxide (ZnO), which has a wurtzite crystal structure, is a representative piezoelectric material. The defects that are inevitably present in ZnO nanorods (NRs), however, generate excessive free electrons, which reduce the piezoelectric potential and thus reduce the output characteristics. Herein, ZnO–NiO core–shell structure–based PENGs are designed to enhance their piezoelectric output performance by reducing excess electrons in the ZnO NRs using a p‐type semiconducting NiO layer. The thickness and structure of the NiO coated on the ZnO NRs are observed and analyzed by adjusting the molar concentration of solution for the NiO layer coating, and the effects of these on the piezoelectric output are discussed.

Toward Optimized Radial Modulation of the Space-Charge Region in One-Dimensional SnO2-NiO Core-Shell Nanowires for Hydrogen Sensing.

The gas-sensing properties and mechanism and the role of the shell thickness of structurally well-defined SnO2/NiO heterostructures are studied. One-dimensional (1D) SnO2/NiO core-shell nanowires (CSNWs) were produced by a two-step process; single-crystalline SnO2-core nanowires (NWs) were synthesized by vapor-liquid-solid (VLS) deposition and then decorated with a polycrystalline NiO-shell layer by atomic layer deposition (ALD). The thickness of the NiO-shell layer was precisely controlled between 2 and 8.2 nm. The electrical conductance of the sensors was decreased many orders of magnitude with the NiO coating, suggesting that the conductivity of the sensors is dominated by Schottky barrier junctions across the n(core)-p(shell) interfaces. The gas-sensing response of pristine SnO2 NWs and SnO2/NiO CSNWs sensors with various thicknesses of the NiO-shell layers was investigated toward hydrogen at various temperatures. The response of the SnO2/NiO-X (X is the number of ALD cycles) CSNWs significantly depends on the thickness of the NiO-shell layer. The SnO2/NiO-100 sensor showed the best performance (NiO-shell thickness ca. 4.1 nm), where the radial modulation of the space-charge region is maximized. The sensing response of the SnO2/NiO-100 sensor was 114 for 500 ppm of hydrogen at 500 °C, which was about four times higher than the response of pristine SnO2 NWs. The sensing mechanism is mainly based on the formation of a p-n junction at the p-NiO-shell and the n-SnO2-core interface and the modulation of the hole-accumulation region in the NiO-shell layer. The remarkable performance of the SnO2/NiO CSNWs sensors toward hydrogen is attributed to the high surface to volume ratio of the 1D SnO2 core-NWs, the conformal NiO shell layer, and the optimized shell layer thickness radially modulating the space-charge regions.

Debye-length controlled gas sensing performances in NiO@ZnO p-n junctional core–shell nanotubes

To enhance gas sensing performance, constructing p-n heterostructures are considered as a promising route for designing high-performance gas sensing materials due to their synergistic and p-n junction effects. In particular, the shell layers’ thickness often plays a vital role in the modulation of carrier transport during the gas sensing processes. In this work, we have designed a type of NiO@ZnO core–shell nanotube (CSNT) with a thickness-tuneable shell by combining electrospinning with atomic layer deposition (ALD) techniques. The results showed that the NiO@ZnO composite nanofibers possessed a uniform tubular structure and comprised of a 230 nm polycrystalline NiO core and a wrinkled porous ZnO shell with a tuneable thickness (0–50 nm) via ALD cycles. Also, gas sensing tests showed that the NiO@ZnO CSNTs with shell thickness close to the Debye length showed the highest gas sensitivity, e.g. the response to 100 ppm ethanol was 15.8, which is ~6.5 times that of the pure NiO. Moreover, the assembled sensors also showed excellent stability (almost keeping 100% after 1 month tests), increased response speed and improved gas selectivity. Furthermore, based on our series of tests and analysis, a possible gas sensing enhancement mechanism (Debye-length controlled gas sensing mechanism) has been proposed for the gas sensing behaviours of our designed sensors based on NiO@ZnO CSNTs.

Exchange-biased hybrid γ-Fe2O3/NiO core-shell nanostructures: three-step synthesis, microstructure, and magnetic properties.

A two-step solvothermal method combining a calcination process was conducted to synthesize γ-Fe2O3/NiO core-shell nanostructures with controlled microstructure. The formation mechanism of this binary system has been discussed, and the influence of microstructures on magnetic properties has been analyzed in detail. Microstructural characterizations reveal that the NiO shells consisted of many irregular nanosheets with disordered orientations and monocrystalline structures, packed on the surface of the γ-Fe2O3 microspheres. Both the grain size and NiO content of nanostructures increase with the increasing calcination temperature from 300 °C to 400 °C, accompanied by an enhancement of the compactness of NiO shells. Magnetic studies indicate that their magnetic properties are determined by four factors: the size effect, NiO phase content, interface microstructure, i.e. contact mode, area, roughness and compactness, and FM-AFM (where FM and AFM denote the ferromagnetic γ-Fe2O3 and the antiferromagnetic NiO components, respectively) coupling effect. At 5 K, the γ-Fe2O3/NiO core-shell nanostructures display certain exchange bias (HE = 60 Oe) and enhanced coercivity (HC = 213 Oe).

Synthesis and characterization of sulfonated mesoporous NiO-ICG core-shell solid sphere catalyst with superior capability for methyl ester production.

In the present research, a mesoporous NiO core–shell solid sphere was hydrothermally synthesized, using polyethylene glycol (PEG; 4000) as a surfactant and incomplete carbonized glucose (ICG) as a template. Then, thermal decomposition of ammonium sulphate was employed to convert the as-synthesized material to sulfonated mesoporous NiO–ICG catalyst, in order to accelerate conversion of waste cooking palm oil (WCPO) into ester. The structural, textural, morphological, and thermal characteristics of the synthesized sulfonated mesoporous NiO–ICG catalyst were evaluated using X-ray diffraction (XRD), Raman spectroscopy, temperature programed desorption (TPD), Brunauer–Emmet–Teller (BET), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). Furthermore, the effect of different reaction parameters against reaction time were investigated. Under the optimal transesterification conditions; catalyst loading of 1 wt%, methanol to WCPO ratio of 9 : 1, operation temperature of 100 °C and mixing intensity of 450 rpm, an optimum ester yield of 95.6% was achieved. Additionally, a recyclability study proved that the spent catalyst was highly potential to be reused for nine successive transesterification reactions without further treatment. Finally, the physicochemical characteristics of the produced WCPO methyl ester were evaluated which were highly in accordance with both European (EN; 14214) and American Standards for Testing Materials (ASTM; D6751) specifications.

Multi−interface spin exchange regulated biased magnetoelectric coupling in Cluster−assembled multiferroic heterostructural films

Cluster−assembled Ni−NiO core−shell structures were periodically immersed into multiferroic Bi5Ti3FeO15 (BTFO) film matrices to form pseudo 1–3 BTFO/Ni−NiO/BTFO heterostructural films with multiferroic/antiferromagnetic/ferromagnetic (MF/AFM/FM) multi−interfaces. A biased magnetoelectric coupling effect was obtained at room temperature by a magnetic annealing process. The biased direction depends on the orientation of the annealing magnetic field, with magnetoelectric coefficient curves moving to left after positive annealing magnetic field, while moving to right with negative annealing magnetic field. This biased magnetoelectric effect originates from the multi−interface spin exchange coupling. The immersion of cluster−assembled Ni−NiO core−shells in the multiferroic matrices enhance the interfacial coupling effect due to the large surface-to-volume ratio. Thus, the interfacial spin exchange induced biased magnetoelectric effect is enhanced and reversible. The work provides an avenue on the weak magnetic field detection, spintronic devices and multistate switching memories.

Stability of complex catalyst with NiO@TiO2 core-shell structure for hydrogen production

Core-shell NiO@TiO2 and CaO complex catalyst was prepared to improve stability used in the reactive sorption-enhanced reforming (ReSER) process for hydrogen production. NiO@TiO2 as catalyst component was prepared by synthesizing Ni(OH)2 as core via urea hydrolysis, by coating TiO2 shell via adsorption phase reaction technique, and was mixed with nano CaO as CO2 adsorbent to form a complex catalyst. The morphology of NiO@TiO2 and the complex catalyst were characterized by means of TEM, EDS, XRD and N2 physisorption measurements. Hydrogen production performance was investigated in a fixed-bed stainless steel reactor. Results showed that the complex catalyst with core-shell NiO@TiO2 structure has 36 cyclic runs of the ReSER process for hydrogen production, while only nine cycle runs obtained without TiO2 coated as shell. It was revealed that core-shell NiO@TiO2 structure could increase the surface area of complex catalyst and promote the separation of NiO core particles. It was also found that the optimum TiO2 coating time could obtain relative smaller NiO grain. And smaller initial NiO grain in core-shell structure would improve the stability of the complex catalyst.

Polyaniline/Nickel Oxide–a Core/Shell Structured Nanocomposite as Electrode Material in Supercapacitor Applications

In this paper, the supercapacitor electrode properties of a core/shell multifunctional nano composite, with unique physiochemical properties, is presented. The synthesis method of polyaniline /NiO (PAESNI) with polyaniline (PANI) shell on NiO core, their morphological studies including Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) to know their surface chemistry and most important property required for explaining super capacitor properties like Cyclic Voltammetry (CV) and Charging Discharging studies (GCD) are explained. TEM and SEM images reveal that the polyaniline shells as-prepared are narrowly dispersed on NiO nano particles and possess uniform morphologies. CV shows that the PAESNI exhibits multiple redox behaviour during potentiodynamic cycling in acidic media at different potentials. A simple and cost-effective preparation technique with hierarchical structure and good capacitive behaviour (362 Fg-1 at 1 Ag-1 current density and 372 Fg-1 at 20 mVs-1 scan rate), Energy density (50.2 WhKg-1 at 1 Ag-1 current density), Power density (2 kWKg-1) and 99% coulombic efficiency at 4 Ag-1 current density encourages its commercial use as a high performance supercapacitor electrode.

Expanding possibilities for solid-phase crystallization by exsolving tunable Pd–NiO core–shell nanostructures

The major drawbacks of solid-phase crystallization as a means of synthesizing catalysts are the lack of heterogeneous structural diversity and the comparatively low metal dispersion it affords.

Investigation of novel inverted NiO@NixCo1-xO core-shell nanoparticles

Inverse core-shell nanoparticles, comprised of an antiferromagnetic (AFM) core covered by a ferromagnetic (FM) or ferrimagnetic (FiM) shell, are of current interest due to their different potential application and due to the tunability of their magnetic properties. The antiferromagnetic nature of NiO and high Néel temperature (523 K) makes this material well suited for inverse core-shell nanoparticle applications. Our primary objective in this project has been to synthesize and characterize inverted core-shell nanoparticles (CSNs) comprised of a NiO (AFM) core and a shell consisting of a NixCo1-xO (FiM) compound. The synthesis of the CSNs was made using a two-step process. The NiO nanoparticles were synthesized using a chemical reaction method. Subsequently, the NiO nanoparticles were used to grow the NiO@NixCo1-xO CSNs using our hydrothermal nano-phase epitaxy method. XRD structural characterization shows that the NiO@NixCo1-xO CSNs have the rock salt cubic crystal structure. SEM-EDS data indicates the presence of Co in the CSNs. Magnetic measurements show that the CSNs exhibit AFM/FiM characteristics with a small coercivity field of 30 Oe at 5 K. The field cooled vs zero field cooled hysteresis loop measurements show a magnetization axis shift which is attributed to the exchange bias effect between the AFM NiO core and an FiM NixCo1-xO shell of the CSNs. Our ab initio based calculations of the NixCo1-xO rock salt structure confirm a weak FiM character and a charge transfer insulator property of the compound.

Synthesis and Characterization of Novel Inverted NiO@NixMn1-xO Core-Shell Nanoparticles

ABSTRACTMagnetic core-shell nanoparticles have the potential for numerous applications, such as in magnetic recording media, magnetic resonance imaging, drug delivery or hyperthermia, and spin valves. Inverse core-shell nanoparticles, comprised of an antiferromagnetic (AFM) core covered by a ferromagnetic (FM) or ferrimagnetic (FiM) shell, are of current interest due to the tunability of their magnetic properties. NiO is typically antiferromagnetic in nature and has a Néel temperature of 523 K. Our primary objective in this project is to synthesize and characterize inverted core-shell nanoparticles (CSNs) comprised of a NiO (AFM) core and a shell consisting of a NixMn1-xO (FM/FiM) compound. The synthesis of the CSNs was made using a two-step process. The NiO nanoparticles were synthesized using a chemical reaction method. Subsequently, the NiO nanoparticles were used to grow the NiO@NixMn1-xO CSNs using our hydrothermal nano-phase epitaxy method. XRD structural characterization shows that the NiO@NixMn1-xO CSNs have the rock salt cubic crystal structure throughout. SEM-EDS data indicates the presence of Mn in the CSNs. SQUID magnetic measurements show that the CSNs exhibit AFM/FM or AFM/FiM characteristics with a coercivity field of 425 Oe at 5 K. The field cooled vs zero field cooled hysteresis loop measurements show a significant exchange bias effect between the AFM NiO core and FM/FiM NixMn1-xO shell of the CSNs. The results of additional TEM and magnetic characterization are discussed.

Fabrication of NiO@Co3O4 core/shell nanofibres for high-performance supercapacitors

NiO@Co3O4 core/shell nanofibres (NiO@Co3O4 NFs) as a promising supercapacitors electrode material were synthesized by a facile co-electrospinning method and heat treatment. The NiO core is about 125nm in diameter and the Co3O4 shell has a thickness of ~80nm. The electrochemical properties of NiO@Co3O4 NFs in 6M KOH aqueous electrolyte are investigated. Compared with NiO (222Fg−1, 1Ag−1) and Co3O4 (284Fg−1, 1Ag−1) nanofibres, NiO@Co3O4 NFs electrode exhibits higher specific capacitance (437Fg−1, 1Ag−1), as well as good cycling stability (82.9% retention after 1000 cycles at 5Ag−1). The enhanced electrochemical performances can be attributed to the uniform core/shell fibre nanostructures. These results suggest that such NiO@Co3O4 NFs would be promising electrodes for supercapacitors.

Exchange bias in finite sized NiO nanoparticles with Ni clusters

Structural and magnetic properties of finite sized NiO nanoparticles are investigated with synchrotron X-ray diffraction (XRD), transmission electron microscopy, magnetometer and ferromagnetic resonance (FMR) spectroscopy. A minor Ni phase is detected with synchrotron XRD, attributed to the oxygen defects in the NiO core. A considerable exchange bias of ~100Oe is observed at 50K and it drops abruptly and vanishes above 150K, in association with the reduction of frozen spins. FMR data indicate a strong interaction between ferromagnetic (FM) and antiferromagnetic (AFM) phases below 150K, consistent with the picture of isolated FM clusters in AFM matrix.