Morphological control of Ni/NiO core/shell nanoparticles and production of hollow NiO nanostructures

Chemical synthesis coupled with a microwave irradiation process allowed for the control of size (6–40 nm), shape, and shell thickness of Ni/NiO core/shell nanoparticles. In this unique synthetic route, the size of Ni nanoparticles (NiNPs) was strongly influenced by the nickel salt-to-stabilizer ratio and the amount of the stabilizer. Interestingly, it was observed that the shape of the nanoparticles was altered by varying the reaction time, where longer reaction times resulted in annealing effects and rupture of the stabilizer micelle leading to distinct shapes of Ni/NiO core/shell nanostructures. Product cooling rate was another important parameter identified in this study that not only affected the shape, but also the crystal structure of the core/shell nanoparticles. In addition, a simple and cost-effective method of microwave irradiation of NiNPs led to the formation of distinctly shaped hollow NiO nanoparticles. These high surface area core/shell nanoparticles with well-controlled morphologies are important and can lead to significant advancement in the design of improved fuel cells, electrochromic display devices, and catalysis systems.     

The effects of temperature upon NiO formation and oxygen removal on Ni(110)

Oxygen chemisorption and NiO nucleation and growth on Ni(110) have been studied with low energy electron diffraction and Auger electron spectroscopy. Changes in the Auger peak energies and shapes were shown to occur only upon NiO formation. The effects of step-changes in temperature upon NiO nucleation and growth were studied and it was shown that temperature steps or annealing during the chemisorption regime did not significantly affect either chemisorption or NiO formation. During NiO growth, temperature steps to a higher temperature caused reduced growth rates, while steps to lower temperature caused increased growth rates. The reaction rate constant from the island growth model was calculated and shown to agree with literature data. The values obtained from temperature step measurements agreed within a factor of two with those obtained for reactions without temperature steps. Therefore, no systematic temperature effect upon the NiO nuclei density was observed for Ni(110). The activation energy for growth of NiO was found to be 5.5 kcal/mole. Dissolution of oxygen into bulk nickel was also studied and it was shown that bulk diffusion of oxygen in nickel was not rate controlling. Rather, surface phase transitions were observed which allowed incorporation of oxygen over the temperature range of 150°C to greater than 800°C, depending on the quantity of oxygen already incorporated.     

Temperature-dependent formation and shrinkage of hollow shells in hemispherical Ag/Pd nanoparticles

It is shown that both the growth and shrinkage of hollow shells in Ag/Pd hemispherical core-shell nanostructures take place at the same temperature. The crossover time, t _cr, between these regimes is shifted to lower values with increasing temperature. This result confirms that the growth and shrinkage regimes are controlled by the faster and slower diffusion coefficients (D _Ag and D _Pd), respectively. The pore radius, confirming recent theoretical predictions, linearly depends on the initial particle radius, and the slope of this straight line increases with the average composition of the faster component.     

Four-fold hollow site for S in a Ni(100) raft on NiO(100)

The reaction of H₂S with NiO(100) has been studied by polarization-dependent surface EXAFS. The results evidence reduction of the selvedge to form a Ni raft having S in four-fold sites with a S–Ni bond length of 2.21±0.02 Å. The Ni–Ni in-plane distance is 2.77±0.09 Å, representing a 6±4% contraction compared to that in NiO(100).     

Formation and strain distribution of Ni/NiO core/shell magnetic nanoparticles fabricated by pulsed laser deposition

Nucleation and growth lead to substantial strain in nanoparticles embedded in a host matrix.The distribution of strain field plays an important role in the physical properties of nanoparticles.Magnetic Ni/NiO core/shell nanoparticles embedded in the amorphous Al2O3 matrix were fabricated by pulsed laser deposition.The results from a high-resolution transmission electron microscope also revealed that the core/shell nanoparticles consist of a single crystal Ni core with a faced-centered cubic structure(Space ...     

Magnetic behavior and physicochemical properties of Ni and NiO nano-particles

A series of nano-crystalline Ni/NiO particles was synthesized by a combustion route depending upon the glycine-nitrate process. The as prepared samples were characterized by X-ray diffraction (XRD), scanning electron micrograph (SEM), transmission electron micrograph (TEM), nitrogen adsorption isothems at 77 K and vibrating sample magnetometer (VSM) techniques.The XRD results revealed that the Ni powder crystallizes was formed with the cubic phase when the molar ratio of glycine to nitrate is 1.5. Above or below that molar ratio, NiO phase coexists as an impurity along with the Ni phase. The SEM and TEM measurements of the as synthesized powders showed that the particles are irregular in shapes and have porous morphology. Increasing the ratio between glycine and Ni-nitrate resulted in slightly agglomeration and grain growth of nano-particles with subsequent decrease in the value of surface area depending upon high combustion heat. The magnetization value of Ni measured at room temperature is very close to the value observed for commercial Ni powder.     

Electron spin resonance study of NiO antiferromagnetic nanoparticles

The electron spin resonance (ESR) spectra of antiferromagnetic nanoparticle NiO specimens have been investigated as a function of temperature at x-band (microwave) frequencies. Below the nominal Néel temperature, the x-band resonances arising from the bulk antiferromagnets, including NiO particles with diameters greater than 100 Å, all vanish due to the emergence of large molecular exchange fields. The ESR resonance signals of 60 Å antiferromagnetic nanoparticles, however, persist to the lowest temperatures. These nanoparticle resonance lines shift to lower fields rapidly as the temperature is decreased, while the lineshapes broaden and distort.     

High temperature instability of hollow nanoparticles

The concept of the Gibbs energy balance is used to derive the evolution equation of both surfaces of a hollow nanosphere. The process is driven by reducing the surface energy and the elastic energy stored in the bulk. A semianalytical solution is provided for the time period during which a hollow nanosphere shrinks to a compact nanospherical particle. Comparison with recently reported results is performed.     

Spontaneous formation of large-area monolayers of well-ordered nanoparticles via a wet-coating process

Monolayers of well-ordered close-packed silica nanoparticles (NPs) with a diameter of 25 nm were spontaneously formed on silicon substrates over a large area and at a high rate via a wet-coating process using a capillary coater. The effects of zeta potential, dispersion solvent, and substrate friction on the NP self-assembly induced by the solvent evaporation were investigated. Experimental results showed that the solvent and the substrate had a larger impact on the order of formed monolayers than the zeta potential did, which are discussed from the viewpoint of the Derjaguin-Landau-Verwey-Overbeek (DLVO) and non-DLVO theories. Based on the above studies, we proposed a solution to fabrication of well-ordered NP monolayers without boundary defects at the surface coverage of 1 via the wet-coating process.     

Preparation of NiO and CoO nanoparticles using M2+-oleate (M = Ni,Co) as precursor

The preparation of NiO and CoO nanoparticles was reported. The dot-like NiO and flower-like CoO nanoparticles were obtained using M²⁺-oleate (M = Ni, Co) as precursor via thermal decomposition method. Transmission electron microscopic (TEM) images monitored the growth of NiO and CoO nanoparticles. When the reaction complex including M²⁺-oleate (M = Ni, Co) precursor, oleic acid and 1-octadecene was heated to the refluxing temperature (320 °C), the formed NiO and CoO nanoparticles were needle-like and very small, indicating low growth speed. However, when the reaction complex was kept refluxing for 30 min, dot-like NiO and flower-like CoO nanoparticles were observed, suggesting the accelerated growth at this refluxing stage. The difference of the morphology of the resultant NiO and CoO nanoparticles resulted from the difference of their growth mode. Selected-area electron diffraction (SAED) patterns showed the face-centered cubic structures of NiO and CoO nanoparticles. The magnetic property of the nanoparticles was studied using vibrating sample magnetometer (VSM).     

Oxidation behaviour of SiC coatings

Amorphous silicon carbide (SiC) films were deposited on silicon substrates by radio-frequency magnetron sputtering. The films were oxidized in air in the temperature range 400–900 °C and for times from 1 to 16 h. Neutron reflectivity measurements provided information on the thickness, density and roughness of the SiC and on the formed SiO₂ layers. Fourier transform infrared spectroscopy was used to determine the bond structure of the formed SiO₂ and changes in the bonding of SiC after exposure at the oxidation temperature. The surface morphology of the oxidized films was characterized by atomic force microscopy measurements. The oxidation kinetics is initially fast and as the SiO₂ layer is formed it slows down. The SiC consumption varies linearly with time at all oxidation temperatures. Exposure of the SiC at the oxidation temperature affects its density and to some degree its bond structure, while the formed SiO₂ has density and bond structure as that formed by oxidation of Si under the same conditions. PACS  66.30.Ny; 68.47.Gh; 68.55.J-     

Synthesis of hollow nanoparticles by plasma polymerization

A capacitively coupled low pressure plasma is used to produce particles by plasma polymerization of various organic precursors. The particles produced by this method are spherical, submicron in diameter, and contain a hollow center. Sixteen organic molecules were tested for the ability to produce hollow particles. Of these, 12 were found to reproducibly result in the formation of hollow particles while the other 4 either produced non-hollow particles or no particles at all under the conditions investigated here. Cyclohexane derivatives and the presence of an aromatic ring correlate strongly with a tendency to form hollow centers. The particle sizes vary from 70 nm to nearly 1 m and the core diameter is on the average 18 of the particle diameter. The corresponding shell thickness ranges from 24 nm for styrene to about 360 nm for cyclohexane. To explain the formation of hollow centers a mechanism is proposed based on the hypothesis that the solid phase is formed via crosslinking and solidification of viscous drop. This mechanism explains the observed linear relationship between core size and particle size.     

Spontaneous formation of polymer nanoparticles by good-solvent evaporation as a nonequilibrium process

In recent years, polymer nanoparticles have been investigated with great interest due to their potential applications in the fields of electronics, photonics, and biotechnology. Here, we report the spontaneous formation of polymer nanoparticles from a clear solution containing a nonvolatile poor solvent by slow evaporation of a volatile good solvent. During evaporation of the good solvent, the solution gradually turns turbid. After evaporation, polymer nanoparticles of homogeneous shape and size are dispersed in the poor solvent. Homogeneous nucleation and successive growth of polymer particles takes place during the dynamic nonequilibrium process of solvent evaporation. The size of the particles, ranging from tens of nanometers to micrometer scale, depends on both polymer concentration and the solvent mixing ratio. Because of the physical generality of the particle formation mechanism, this procedure is applicable to a wide variety of polymers with suitable combinations of solvents. Here, we also show unique features, surface structures and surface properties of polymer nanoparticles prepared by this method.     

Exchange bias and training effect in Ni/Ag-doped NiO bilayers

A series of polycrystalline Ag-doped Ni_1−xAg_xO/Ni bilayers with x up to 0.2 were prepared by magnetron sputtering. X-ray diffraction, atomic force microscopy and transmission electron microscopy analyses reveal that Ag doping significantly reduces the mean NiO grain size and leads to the appearance of Ag nanoparticles on the surface of the Ag-doped NiO films. As x increases, the exchange bias field and coercivity at room temperature decrease as a consequence of the reduced thermal stability of smaller NiO grains and the screening effect resulting from the interfacial Ag nanoparticles. At lower temperatures, a slight enhancement of the exchange bias field is observed in the Ag-doped sample, indicating that the Ag doping increases the uncompensated NiO spin density. In addition, our studies find that the training effect of the Ag-doped sample can be well described by a spin configurational relaxation model, regardless of the presence of Ag nanopartiles at the interface.     

Oxidation of Ni3Al–Ni3Nb alloys

Conclusions The Ni₃Al–Ni₃Nb alloys are oxidized as a result of the diffusion of oxygen ions toward the interface between the alloy and the oxidation product. This diffusion produces a relatively thick inner layer of complex composition; in addition, diffusion of nickel ions toward the interface between the oxidation product and the gas results in the formation of a thin outer layer of NiO. At any temperature, NiO in the inner oxide is reduced to Ni by niobium atoms. During the initial stages of the oxidation, the reduction occurs at the oxide-alloy inter face; during the later stages, it occurs at the interface between the oxide and the suboxide layer. Protective double oxides of NiO · Nb₂O₅ (t = 700–725 °) and NiO · Al₂O₃ (t = 800–850 °) form in the oxidation product. An -Nb₂O₅ conversion occurs at 825–900 ° and considerably reduces the oxidizability of the alloys. The -Nb₂O₅ lattice probably contains fewer oxygen vacancies than the -Nb₂O₅ lattice and thus has better protective properties.Translated from Izvestiya VUZ. Fizika, No. 12, pp. 75–83, December, 1969.     

单畴Ni球观测及NicoreNiOshell的铁磁/反铁磁耦合研究

 使用微波辅助聚合方法制备了单分散单畴Ni纳米球，由MFM发现，尺度分布在100～180 nm的Ni球的一个相关特征是条型磁畴结构。用XRD、TEM、XPS以及EDAX测量了由Ni球进一步制备的Ni_coreNiO_shell高度球型纳米结构。用VSM 和SQUID进一步讨论了其铁磁/反铁磁界面耦合效应，估算了交换耦合场与粒子尺寸的关系。     

Influence of ambient gas on formation process of Si nanoparticles by laser ablation

We described the influence of a type of gas and its pressure upon the size distribution of Si nanoparticles fabricated by laser ablation in an ambient gas and the plume dynamics during the synthesis. The plume dynamics was investigated by laser-induced fluorescence and ultraviolet Rayleigh scattering. Based on the results, the importance of the gas flow within the ablation plume in the formation of the nanoparticles is understood.