Size-tuneable synthesis of nickel nanoparticles

A facile method is described for synthesising nickel nanoparticles via the thermal decomposition of an organometallic precursor in the presence of excess n-trioctylphosphine as a capping ligand. For the first time, alkylamines with different chain lengths were employed as size-limiting agents in this synthesis. A direct correlation is demonstrated between the size of the alkylamine ligands used and the mean diameter of the nickel nanoparticles obtained. The use of bulky oleylamine as a size-limiting agent over a reaction period of 30 min led to the growth of nickel nanoparticles with a mean diameter of 2.8 ± 0.9 nm. The employment of less bulky N,N-dimethylhexadecylamine groups led to the growth of nickel nanoparticles with a mean diameter of 4.4 ± 0.9 nm. By increasing the reaction time from 30 to 240 min, while employing oleylamine as the size-limiting agent, the mean diameter of the nickel nanoparticles was increased from 2.8 ± 0.9 to 5.1 ± 0.7 nm. Decreasing the amount of capping ligand present in the reaction system allowed further growth of the nickel nanoparticles to 17.8 ± 1.3 nm. The size, structure and morphology of the nanoparticles synthesised were characterised by transmission electron microscopy and powder X-ray diffraction; while magnetic measurements indicated that the particles were superparamagnetic in nature.     

Structural analysis of nickel doped cobalt ferrite nanoparticles prepared by coprecipitation route

Magnetic nanoparticles of nickel substituted cobalt ferrite (Ni_xCo_1−xFe₂O₄:0≤x≤1) have been synthesized by co-precipitation route. Particles size as estimated by the full width half maximum (FWHM) of the strongest X-ray diffraction (XRD) peak and transmission electron microscopy (TEM) techniques was found in the range 18–28±4 nm. Energy dispersive X-ray (EDX) analysis confirms the presence of Co, Ni, Fe and oxygen as well as the desired phases in the prepared nanoparticles. The selective area electron diffraction (SAED) analysis confirms the crystalline nature of the prepared nanoparticles. Data collected from the magnetization hysteresis loops of the samples show that the prepared nanoparticles are highly magnetic at room temperature. Both coercivity and saturation magnetization of the samples were found to decrease linearly with increasing Ni-concentration in cobalt ferrite. Superparamagnetic blocking temperature as determined from the zero field cooled (ZFC) magnetization curve shows a decreasing trend with increasing Ni-concentration in cobalt ferrite nanoparticles.     

Fabrication of Ni nanodots templated by nanoporous polysulfone membrane: Structural and Magnetic properties

Two-dimensional nickel nanodots were prepared using a simple polymer based lithography process on silicon substrates. The nanoporous polysulfone membranes were fabricated using a phase inversion polymerization process. Nickel nanodots were then grown using the polysulfone membrane as a mask. The structures were written by depositing a certain thickness of nickel using electron beam evaporation. After lift off, the structural properties of the samples were studied using atomic force microscopy and grazing incidence X-ray diffractometry. The dots were found to have diameters in the range of 75–120 nm and heights of 3–5 nm. The magnetization and magnetic domain arrangement of the Ni nanodots were analyzed using vibrating sample magnetometer and magnetic force microscopy respectively. The nanodots were found to exhibit excellent soft ferromagnetic properties with a preferred easy axis of magnetization.     

Synthesis and characterization of magnetite nanopowders

We have synthesized the iron oxide nanoparticles using the newly developed mechanical ultrasonication method with the FeSO₄ · 7H₂O. We have also investigated the crystallographic structural properties, morphology, and magnetic properties of the nanopowders. According to the high resolution X-ray diffraction result, the as-synthesized iron oxide nanoparticles were magnetite (Fe₃O₄). The particle size of the magnetite nanoparticles was about 6 nm confirmed by transmission electron microscopy image. The particle shape was almost a sphere confirmed by scanning electron microscopy image. The coercivity and saturation magnetization of the as-synthesized iron oxide nanopowders were 114 Oe, and 3.7 emu/g, respectively.     

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.     

Investigation of the magnetization reversal of a magnetic dot array of Co/Pt multilayers

The magnetization reversal behavior of a dot array consisting of Co/Pt multilayers with perpendicular magnetic anisotropy was investigated. The size of the dots was varied from 200 nm down to 40 nm, while keeping the filling factor constant at about 0.16. The structural properties were determined by scanning electron microscopy, whereas the magnetic investigation was performed using SQUID and MFM techniques. It was observed that the dot size has a severe impact on the magnetization reversal mechanism where only the smallest dots with a size of 40 nm are found to be in a magnetic single-domain state. Moreover, the patterning process leads to a degradation of the multilayer, leading to a reduction of the switching field and an increase of the switching field distribution with decreasing dot size. In addition, micromagnetic simulations were performed to understand the magnetization reversal mechanism in more detail.     

Cu1.5[Cr(CN)6]⋅6.5H2O nanoparticles: synthesis, characterization, and magnetic properties

We have synthesized nanoparticles of Cu_1.5[Cr(CN)₆]⋅6.5H₂O of varying size by using poly(vinylpyrrolidone) (PVP) as a protecting polymer. The particle size variation has been achieved by varying the amount of the PVP surfactant with the reactants. The prepared nanoparticles have been investigated by using X-ray diffraction, transmission electron microscopy, and direct-current magnetization techniques. The nanoparticles crystallize in a face centred cubic structure (space group: Fm3m). The approximate particle sizes for the three samples are 18, 9, and 5 nm, respectively. Non-PVP nanoparticles (18 nm) show a magnetic ordering temperature of 65 K. A decrease in the magnetic ordering temperature was observed with decreasing particle size. These nanoparticles are magnetically very soft, showing negligibly small values of the coercivity and remanent magnetization. The maximum magnetization and spontaneous magnetization values at 5 K are found to decrease with decreasing particle size. The observed magnetization behaviour of the nanoparticles has been attributed to the increasing surface spin disorder with decreasing particle size.     

Effects of dipolar interactions on magnetic properties of granular solids

The magnetic behavior of superparamagnetic Co nanoparticles (2–4 nm in diameter) dispersed in an amorphous, insulating SiO₂ matrix was studied. Conventional fittings of magnetization curves present mean magnetic moments which diminish with decrease in temperature. In order to treat this anomalous behavior, we have applied the interacting superparamagnetic model (ISP). Mean diameters obtained from transmission electron microscopy (TEM) were compared with values obtained applying ISP model.     

EDLC characteristics with high specific capacitance of the CNT electrodes grown on nanoporous alumina templates

Well-ordered nanoporous alumina templates were fabricated by two-step anodization method by applying a constant voltage of 40 V in oxalic acid solution or of 25 V in sulfuric acid solution. The cylindrical pore diameter and pore density of the templates utilized for the carbon nanotube (CNT) growth were 86 ± 5 nm and 1.2 × 10¹⁰ cm⁻² in oxalic acid solution and 53 ± 1 nm and 3.1 × 10¹⁰ cm⁻² in sulfuric acid solution, respectively. The CNTs with uniform diameter of 50 ± 10 nm (oxalic acid) and 44 ± 2 nm (sulfuric acid) were grown on the porous alumina template as electrode materials for the electrochemical double layer capacitor (EDLC). The EDLC characteristics were examined by measuring the capacitances from cyclic voltammograms and the charge–discharge curves. The specific capacitances of the CNT electrodes are 30 ± 1 F/g (Φ = 50 ± 10 nm) and 121 ± 5 F/g (Φ = 44 ± 2 nm). The high specific capacitance of the CNT electrode was achieved by using nanoporous alumina templates with the high pore density and the small and uniform pore diameter.     

Template-free synthesis of hollow <Emphasis Type="Italic">α</Emphasis>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> microspheres

Nearly monodisperse hollow α-Fe₂O₃ microspheres composed of nanoparticles have been successfully synthesized through a facile template-free hydrothermal method. The products were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. It is shown that the hollow α-Fe₂O₃ microspheres consist of well-aligned α-Fe₂O₃ nanoparticles with a mean diameter of about 15 nm. This facile reaction route presents an efficient method for mass production of monodisperse hollow magnetic nanomaterials. The final α-Fe₂O₃ microspheres exhibit special magnetic properties with a small remnant magnetization of 0.09 emu g⁻¹ and a high coercivity of 1121.67 Oe at room temperature.     

CVD synthesis and radial deformations of large diameter single-walled CNTs

Single-walled carbon nanotubes (CNTs) were synthesized by a chemical vapor deposition (CVD) method on transmission electron microscopy (TEM) silica coated nickel grids using carbon monoxide as carbon source and iron nanoparticles as catalyst. The produced CNTs were as large as 11 nm in diameter. Investigations on the CNT deformations based on high-resolution TEM images showed that the deformation of CNTs due to their interaction with the substrate occurs at diameters larger than 2.7 nm. Small deformation of free standing tubes was found to occur at diameters above approximately 4.5 nm.     

Magnetite hollow spheres: solution synthesis,phase formation and magnetic property

Polycrystalline magnetite hollow spheres with diameter of about 200 nm and shell thickness of 30–60 nm were prepared via a facile solution route. For the reaction, ethylene glycol (EG) served as the reducing agent and soldium acetate played the role of precipitator. In addition, polyvinylpyrrolidone (PVP) served as a surface stabilizer. The morphologies and structures were characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The intermediate products at different stages were also studied to shed light on the evolution of phase formation. It revealed that the hollow structure formed via self-assembly of nanocrystallites (about 15 nm) using sodium acetate as mild precipitator. Evidences further pointed out that the Ostwald ripening process well explained the growth mechanism of the hollow structure. Magnetization measurements showed that the coercivity of magnetite hollow spheres at low temperature is about 200 Oe and the saturation magnetization is about 83 emu g⁻¹, roughly 85% that of the bulk phase, close to the value of its solid counterpart. In addition, a freezing transition was observed at 25 K.     

High-precision sample stage for photoemission microscopy of organic films

A high-precision sample stage for photoemission microscopy has been constructed to translate the sample by ±3 mm with accuracy better than 100 nm. The stage is actuated by step motors settled outside the vacuum. The accuracies of the translations were measured by observing a standard patterned sample with a photoemission electron microscope (PEEM) of 50 nm resolution. The accuracy was nearly independent of the distance of each translation step and the error was not accumulated by repeated steps. After round-trip translations up to 0.2 mm, the sample came back to the original position with accuracy of ±50 nm. The performance of the stage was demonstrated by observing growth processes of lead phthalocyanine (PbPc) films formed on graphite.     

Superparamagnetic nickel nanoparticles obtained by an organometallic approach

Nickel nanoparticles were prepared by decomposition of the organometallic precursor Ni(COD)₂ (COD=cycloocta-1,5-diene) dissolved in organic media in the presence of anthranilic acid as stabilizer. Transmission electron microscopy revealed nickel nanoparticles with a mean size of 4.2 ± 1.1 nm and selected area electron diffraction showed the formation of fcc nickel. FTIR spectroscopy confirmed the presence of modified anthranilic acid on the surface of the Ni nanoparticles suggesting that it is able to interact with the metal particles. The magnetic response of the nanoparticles was established as being of superparmagnetic character, for which a detailed quantitative analysis resulted in a mean magnetic moment of 2652 μ_B per particle together with a blocking temperature of 32 K.     

Nanocomposite Ni–TiN coatings prepared by ultrasonic electrodeposition

Nanocomposite Ni–TiN coatings were prepared by ultrasonic electrodeposition and the effects of ultrasonication on the coatings were studied. X-ray diffraction analysis was utilized to detect the crystalline and amorphous characteristics of the composite coatings. The surface morphology and metallurgical structure were observed by scanning electron microscopy, high-resolution transmission electron microscopy and scanning probe microscopy. The results showed that ultrasonication had great effects on TiN nanoparticles in composite coatings. The moderate ultrasonication conduced to homogeneous dispersion of TiN particles in the coatings. Moreover, the TiN nanoparticles that entered and homogeneously dispersed in the composite coating led to an increase in the number of nuclei for nucleation of nickel grains and inhibition of grain growth. Therefore, the introduction of ultrasonication and TiN nanoparticles resulted in the formation of smaller nickel grains. The average grain diameter of TiN particles was ∼33 nm, while Ni grains measured approximately 53 nm.     

FMR and TEM Studies of Co and Ni Nanoparticles Implanted in the SiO<Subscript>2</Subscript> Matrix

Fused silica plates have been implanted with 40 keV Co⁺ or Ni⁺ ions to high doses in the range of (0.25–1.0) × 10¹⁷ ions/cm², and magnetic properties of the implanted samples have been studied with ferromagnetic resonance (FMR) technique supplemented by transmission electron microscopy, electron diffraction and energy dispersive X-ray spectroscopy. The high-dose implantation with 3d-ions results in the formation of cobalt and nickel metal nanoparticles in the irradiated subsurface layer of the SiO₂ matrix. Co and Ni nanocrystals with hexagonal close packing and face-centered cubic structures have a spherical shape and the sizes of 4–5 nm (for cobalt) and 6–14 nm (for nickel) in diameter. Room-temperature FMR signals from ensembles of Co and Ni nanoparticles implanted in the SiO₂ matrix exhibit an out-of-plane uniaxial magnetic anisotropy that is typical for thin magnetic films. The dose and temperature dependences of FMR spectra have been analyzed using the Kittel formalism, and the effective magnetization and g-factor values have been obtained for Co- and Ni-implanted samples. Nonsymmetric FMR line shapes have been fitted by a sum of two symmetrical curves. The dependences of the magnetic parameters of each curve on the implantation dose and temperature are presented.     

Synthesis of wurtzite-phase ZnS nanocrystal and its optical properties

The wurtzite phase of ZnS nanocrystal has been prepared by annealing in 200–600 °C temperature range, its cubic phase of 2–3 nm size, prepared through soft chemical method. Results of isochronal experiments of 2 h at different temperatures indicate that visible transformation to wurtzite from cubic ZnS appears at a temperature of 400 °C, which is about three times smaller than that of bulk ZnS phase transition temperature. The phases, nanostructures, and optical absorption characteristics are obtained through X-ray diffraction, transmission electron microscopy, and UV–visible absorption spectroscopy. A stable and green photoluminescence emission peaked at 518 nm is observed from the 600 °C annealed samples, under ultraviolet light excitation.     

Facile synthesis of monodispersed α-Fe2O3 microspheres through template-free hydrothermal route

Monodisperse α-Fe₂O₃ microspheres have been selectively synthesized through a facile hydrothermal method without the assistance of any surfactant, employing FeCl₃·6H₂O and NH₄NaHPO₄ as initial materials. The products were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. α-Fe₂O₃ microspheres with average size about 250 nm were constructed by single crystalline nanoparticles with average diameter about 15 nm. The investigation on the evolution formation revealed that growth temperature was critical to control the assembly of the fresh formed nanocrystallites, and the microsphere formation was proved to be the Ostwald ripening process by tracking the structures of the products at different growth temperature. α-Fe₂O₃ microspheres showed a weak ferromagnetic behavior with a remanent magnetization of 0.208 emu g⁻¹ and a coercivity of 1,034.27 Oe at room temperature.     

Synthesis of nanocrystalline Co-Ni alloys by precursor approach and studies on their magnetic properties

Nanocrystalline Co-Ni alloys with different compositions were prepared by polyol reduction of mixed cobalt nickel hydroxides. The precursors (mixed cobalt nickel hydroxides) were prepared by co-precipitation. Powder X-ray diffraction analysis indicated the formation of fcc phase in the alloys and their crystallite size in the range 17-25 nm. Scanning electron microscopy and transmission electron microscopy studies revealed the morphology of the particles as being close to spherical, and the energy dispersive X-ray analysis showed the stoichiometry of the alloys. The magnetization as a function of field and temperature of the alloys, measured using a superconducting quantum interference device, showed superparamagnetic behavior with negligible coercivity and remanence values.     

Nano-designing of Mg doped phosphate tungsten bronzes and SiO2 composite obtained by ultrasonic spray pyrolysis method

In this study, the structure and substructure of SiO₂–Mg phosphate tungsten bronzes, MgPTB, (MgHPW₁₂O₄₀ · 29H₂O) obtained by ultrasonic spray pyrolysis method from a silica sol, and a MgPTB solution, obtained by the ion exchange method, as precursors were investigated.The mechanism of the formation of aerosol droplets is discussed. Phase composition, structure and substructure of SiO₂–MgPTB particles were investigated by X-ray powder diffraction (XRPD) analysis, transmission electron microscopy (TEM), and scanning electron microscopy (SEM).Good agreement between the theoretically predicted values for the mean diameters of particles and subparticles (1.27 μm and 75.4 nm, respectively) and the experimentally obtained ones (1.17 μm and 65–90 nm) was found.This agreement confirms the applicability of the model to get a satisfactory prediction of the most important data related to the nano-structural design of SiO₂–MgPTB powders.