Ni Shell Research Articles

Structure determination of chitosan-stabilized Pt and Pd based bimetallic nanoparticles by X-ray photoelectron spectroscopy and transmission electron microscopy

Chitosan (CTS)-stabilized bimetallic nanoparticles were prepared at room temperature (rt.) in aqueous solution. Palladium (Pd) and platinum (Pt) were selected as the first metals while iron (Fe) and nickel (Ni) functioned as the second metals. In order to obtain the noble metal core-transition metal shell structures, bimetallic nanoparticles were prepared in a two-step process: the preparation of mono noble metallic (Pd or Pt) nanoparticles and the deposition of transition metals (Fe or Ni) on the surface of the monometallic nanoparticles. The structures of the nanoparticles were studied using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The XPS results show that Pd and Pt exist mainly in zero valences. The presence of Fe and Ni in the bimetallic nanoparticles affects the binding energy of Pd and Pt. Moreover, the studies of O 1s spectra indicate the presence of Fe or Ni shells. The analyses of TEM micrographs give the particle size and size distributions while the high-resolution TEM (HRTEM) micrographs show the existence of noble metal core lattices. The results confirm the formation of noble metal core-transition metal shell structures.

Alumina-supported Cu@Ni and Ni@Cu core–shell nanoparticles: Synthesis, characterization, and catalytic activity in water–gas-shift reaction

Novel Cu@Ni catalysts consisting of a Cu core and a Ni shell and Ni@Cu catalysts have been synthesized by successive reduction of Cu and Ni sources dissolved in ethanol. A combination of transmission electron microscopy (TEM), UV–vis spectroscopy, X-ray diffraction (XRD), CO chemisorption, as well as thermogravimetric analysis (TGA) was used to characterize the core–shell nanoparticle catalysts. The powder XRD patterns, CO chemisorption data, and UV–vis spectra indicated the formation of core–shell structures in the bimetallic Cu–Ni nanoparticles. Cu5@Ni5 and Ni5@Cu6.5 nanoparticles with an average particle size of 4.7 and 5.4nm, respectively, were observed in TEM images. Alumina-supported Ni@Cu nanoparticle catalysts were found to possess similar activity in the water–gas-shift (WGS) reaction to supported Cu catalysts, while supported Cu@Ni catalysts showed comparable WGS activity to supported Ni catalysts and reduced extent of undesirable methanation side-reaction.

Preparation of magnetic Ni/wollastonite and zeolite P/Ni/wollastonite composite fibers

Magnetic composite fibers with wollastonite (WO) core and Ni shell were prepared by electroless plating using natural mineral WO fibers as supporters. Zeolite P layers were then loaded on the surface of Ni/WO fibers by in situ hydrothermal method. The sample structure and properties were characterized via SEM, XRD, SQUID, EDX, and N2 adsorption. The compact Ni–P coatings on the surface of WO fibers resulted in good magnetic properties of the Ni/WO fibers and zeolite P/Ni/WO fibers. After two crystallization cycles, a continuous zeolite P layer could be formed on the surface of Ni–P coating. The zeolite loading improved the stability of Ni–P coating in the acid solution, resulting in that the zeolite P/Ni/WO fibers can be used at a wide pH range of 3–7. Zeolite P/Ni/WO fiber exhibited a good sorption property for Cu2+ in the aqueous solution (12.48mg/g) and could be reused after regeneration, indicating that it could be an effective and reusable adsorbent for metal ions from a liquid system.

Controllable assembly of Ag/C/Ni magnetic nanocables and its low activation energy dehydrogenation catalysis

Double-shelled Ag/C/Ni nanocables have been synthesized through a deposition covering process of Ni nanoparticles (NPs) onto Ag/C pentagonal prism nanowires (NWs). The proposed synthesis mechanism is corroborated by scanning electron microscopy, transition electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and UV-vis absorption spectroscopy. The resulting Ag/C/Ni nanocables with an average diameter of ∼270 nm are made up of Ag NW core (∼200 nm diameter) with internal amorphous C layer (∼10 nm thickness) and outer Ni shell (∼25 nm thickness). The UV-vis absorption spectroscopy analysis indicates that the covering of the Ni shell on the Ag/C nanowire can dampen the surface plasmon resonance (SPR) of Ag wire core and lead to a red-shifted SPR absorption peak. In particular, compared with Ni NPs, the resultant double-shelled Ag/C/Ni magnetic nanocables exhibits higher catalytic activity for the dehydrogenation toward aqueous ammonia borane under ambient atmosphere, and its calculated activation energy is lower than those of many bimetallic catalysts.

One-step seeding growth of controllable Ag@Ni core–shell nanoparticles on skin collagen fiber with introduction of plant tannin and their application in high-performance microwave absorption

Controllable magnetic Ag@Ni core–shell nanoparticles (NPs) have been designed and constructed on the bayberry tannin (BT) grafted skin collagen fiber (SCF) though a simple one-step route. Due to the mild and naturally occurring reduction ability of BT, when the SCF-BT made contact with the mixture of Ag+/Ni2+ solution, BT was able to preferentially reduce Ag+ into Ag NPs, and subsequently, the formed Ag NPs served as in situ seeds for the over growth of magnetic Ni shell by the reduction of NaBH4. The systematic TEM and EDS analysis confirmed that the as-prepared Ag@Ni NPs on SCF-BT were a typical core–shell structure. Magnetic study of core–shell NPs indicated that their magnetic properties could be tuned by modulating their shell thickness with the change of Ag+/Ni2+ molar ratio. The diameter and size distribution of SCF-supported Ag@Ni core–shell NPs also can be controlled by varying the grafting degree of BT on SCF, as characterized by TEM. A novel and important application of these SCF-supported Ag@Ni core–shell NPs composites is use as high-performance microwave absorption materials in the whole X-band, C-band and some part of S-band with maximum reflection loss (RL) of −51 dB. The further analysis of electromagnetic parameters indicated that the enhancement of dielectric loss properties of SCF-supported Ag@Ni core–shell NPs is introduced by the multiple defective site polarization and interfacial polarization in bimetallic interface. In addition, owing to the combinated magnetic property on the nickel shell, the SCF-supported Ag@Ni core–shell NPs also exhibited a significant eddy current effect and anisotropic energy effect for the microwave absorption. To the best of our knowledge, this is the first time to explore tailoring the magnetic property, electromagnetic property and microwave absorption performance of functional core–shell NPs by tuning their core–shell microstructures. The present work has a significant potential for the development of novel, lightweight, low-cost, flexible and highly efficient microwave absorbing materials.

Epitaxial Growth of Au@Ni Core−Shell Nanocrystals Prepared Using a Two-Step Reduction Method

Au@Ni core−shell nanocrystals were prepared using a two-step reduction method. First, mixtures of octahedral, triangular and hexagonal platelike, decahedral, and icosahedral Au core seeds were prepared by reducing HAuCl4·4H2O in ethylene glycol (EG) using microwave (MW) heating in the presence of polyvinylpyrrolidone (PVP) as a polymer surfactant. Then, Ni shells were overgrown on Au core seeds by reducing Ni(NO3)2·6H2O in EG with NaOH and PVP using oil bath heating. Resultant crystal structures were characterized using transmission electron microscopic (TEM), high-resolution TEM, TEM−energy dispersed X-ray spectroscopic (EDS), selected area electron diffraction (SAED), and X-ray diffraction (XRD) measurements. Because a very large mismatch (13.6%) exists in lattice constants between Au (0.4079 nm) and Ni (0.3524 nm), the epitaxial growth of Ni shells over Au cores was expected to be difficult. Nevertheless, about 40 monolayers of Ni{111} shells were grown epitaxially on flat planes and sharp corners of Au{111} cores after heating reagent solution at 175 °C for 2 h. The SAED patterns showed Ni layers parallel to Au layers. This result is contrasted with our recent result for Au@Cu with a smaller lattice mismatch between Au and Cu (11.4%). In the case of Au@Cu, although the epitaxial growth of Cu{111} shells over Au core was observed, the growth rate on sharp corners was slower than that on flat {111} facets. Our results and reported data show that the lattice mismatch is not a significant factor for the crystal growth of Au@Ni on sharp corners.

Magnetic Core−Shell Nanoparticles from Nanoscale-Induced Phase Segregation

Nanoscaling of the nickel phosphides profoundly alters their domains of stability. An original mechanism of “nanoscale-induced phase segregation” was uncovered in the present study: the appearance of two crystallized phases inside each single nanoparticle (here, Ni2P and Ni fcc), where the single-phase product (Ni3P) would have been preferred at the bulk scale. This behavior was obtained by reacting at low temperature (<220 °C) substoichiometric amounts of white phosphorus (P4) on well-defined monodisperse Ni nanoparticles in solution. Phosphorus insertion inside the Ni fcc nanoparticles triggers the crystallization of a Ni2P core surrounded by a Ni shell. The crystallization process was monitored by HRTEM, EFTEM, XRD, and SQUID analyses and revealed a direct transformation of Ni fcc to a core−shell structure without any other NixPy crystallized intermediate. This core−shell Ni−P system was tuned by adjusting the amount of P4 added, providing tunable magnetic shells supported on monodisperse nanoparticles.

Facile synthesis of near-monodisperse Ag@Ni core–shell nanoparticles and their applicationfor catalytic generation of hydrogen

Magnetically recyclable Ag–Ni core–shell nanoparticles have been fabricatedvia a simple one-pot synthetic route using oleylamine both as solvent andreducing agent and triphenylphosphine as a surfactant. As characterized bytransmission electron microscopy (TEM), the as-synthesized Ag–Ni core–shellnanoparticles exhibit a very narrow size distribution with a typical size of14.9 ± 1.2 nm and a tunable shell thickness. UV–vis absorption spectroscopy study shows thatthe formation of a Ni shell on Ag core can damp the surface plasmon resonance(SPR) of the Ag core and lead to a red-shifted SPR absorption peak. Magneticmeasurement indicates that all the as-synthesized Ag–Ni core–shell nanoparticlesare superparamagnetic at room temperature, and their blocking temperaturescan be controlled by modulating the shell thickness. The as-synthesized Ag–Nicore–shell nanoparticles exhibit excellent catalytic properties for the generation ofH2 from dehydrogenation of sodium borohydride in aqueous solutions. The hydrogengeneration rate of Ag–Ni core–shell nanoparticles is found to be much higher than that ofAg and Ni nanoparticles of a similar size, and the calculated activation energyfor hydrogen generation is lower than that of many bimetallic catalysts. Thestrategy employed here can also be extended to other noble-magnetic metal systems.

Rapid synthesis of Ag@Ni core–shell nanoparticles using a microwave-polyol method

Mixtures of AgNO 3 and NiSO 4·6H 2O, NiCl 2·6H 2O, or Ni(NO 3) 2·6H 2O were reduced in ethylene glycol (EG) in the presence of NaOH and poly(vinylpyrrolidone) (PVP) under microwave (MW) heating for 10 min. Then, we succeeded in the synthesis of Ag core–Ni shell nanoparticles, denoted as Ag@Ni, in high yield. The formation of Ag@Ni particles was confirmed using energy dispersed X-ray spectroscopic (EDS) measurements and selected area electron diffraction (SAED) patterns. The growth mechanism of Ag@Ni is discussed. The UV–Vis spectra of Ag@Ni were similar to those of Ni particles.

Magnetic Cu–Ni (core–shell) nanoparticles in a one-pot reaction under microwave irradiation

We successfully prepared face-centered cubic (fcc) Cu-Ni (core-shell) nanoparticles by intramolecular reduction of formate complexes of Cu(2+) and Ni(2+) with long-chain amine ligands in a one-pot reaction within an extremely short time realized only under microwave irradiation. Observation by an HAADF-STEM technique showed that the nanostructure in one particle consisted of a Ni-rich shell and a Cu-rich core. Cu(4)Ni(6) nanoparticles with an average size of 11.7 nm were comprised of a Cu core with a diameter of ca. 6.0 nm, a Ni shell ca. 1.6 nm thick and a 0.9 nm thick interlayer of mixed Cu-Ni alloy between the Cu core and the Ni shell. Both the oxidation characteristics and the magnetic properties were dramatically affected by the molar ratios of Cu : Ni in the Cu-Ni nanoparticles. The magnetization of Cu(3)Ni(7) and Cu(4)Ni(6) comprised of a diamagnetic Cu-rich core, ferromagnetic Ni-rich shell and antiferromagnetic NiO-rich layer on the particle surface showed an exchange bias (209 and 143 Oe, respectively).

Dipole directed ring assembly of Ni-coated Au-nanorods

We demonstrate the synthesis of novel core-shell nanoring superstructures consisting of non-dipolar Au nanorods coated with magnetic Ni shells. We show that the magnetic dipole induced self-assembly can be tuned by selective reduction of Ni on gold nanorods. Superstructures range from nanorings containing physically separated particles to solid rigid nanorings.

Controlled synthesis of MnFe2O4–Ni core–shell nanoparticles

Magnetic nanoparticles have been of significant interest for various applications such as data information storage [1, 2], biomedical drug delivery [3], magnetic resonance imaging [4], and energy harvesting [5] due to their unique characteristics as a function of size. To improve the functionality of magnetic sensors various studies are being conducted on magnetic material–metal composites [6–11]. In this study, we focused on the synthesis of ceramic–metal core–shell particles via an aqueous method as schematically illustrated in Fig. 1a [12]. MnFe2O4 core particles were synthesized by a solvothermal method, and then Ni shells were coated onto the core particles by ionic interaction between Ni ions and negatively charged MnFe2O4 particles. The motivation behind the synthesis of nanoparticles is that they could potentially lead to the realization of new type of magnetic field sensors that utilizes thermal detection to quantify the changes in external magnetic field. An oscillating magnetic field in the surrounding will induce eddy current in the conductive shells that could result in the joule heating of the core. Furthermore, the oscillating magnetic field will induce strain in the core due to magnetostriction which will create deformation in the matrix. The result of these two effects will be to create a finite temperature difference with the surrounding. Ceramic– metal core–shell particles are also promising for designing composites that exhibit electromagnetostrictive (EMS) effect where current flowing on conductive shells could induce a magnetic bias in insulating magnetostrictive grains, resulting in an induced strain in the grains through electromagnetoelastic coupling. If a ferroelectric layer is present on top of the metal shell in the configuration ceramic/metal/ceramic, then the strain from magnetostrictive grains will result in generation of electric charge on ferroelectric layer. Sensing can be conducted by monitoring the output electric charge as a function of applied magnetic field. The immediate advantage of such an approach is that the entire EMS effect would be electrically controllable. However, the first and foremost challenge in achieving these effects is to control the thickness of the nickel shell in range of 1–5 nm which are coated on magnetostrictive cores with diameters of few hundred nanometers.

Engineering Magnetic Properties of Ni Nanoparticles by Non-Magnetic Cores

This work focuses on the magnetism-control study of nickel (Ni) nanoparticles (NPs) with non-magnetic cores of noble metals (M = Ag, Au). Monodispersed M NPs used as templates were synthesized by reducing their salt with a weak reductant—oleylamine (OA). The OA acted as surfactant and non-coordinating reagent in the preparation of Ni NPs with a narrow size distribution. Then Ni atoms were controlled to grow upon the M NPs by controlling the decomposition of nickel acetylacetonate in OA. The as-prepared M and core/shell M/Ni NPs exhibit spherical shape, as characterized by transmission electron microscopy. The systematic study indicated that the Ni shell exhibits face-centered cubic crystal structure and is relatively stable in air. The hydrophobic OA molecules on the surface could be exchanged by hydrophilic imidazole to make the samples water-soluble. Magnetic study of core/shell NPs indicated that their blocking temperatures could be tuned by modulating either core size or shell thickness. The novel mag...

Evolution of Ni:B2O3 core-shell structure and magnetic properties on devitrification of amorphous NiB particles in air

The Ni:B2O3 nanoparticles have been synthesized by a novel two-step chemical reduction method, under ambient conditions and subsequent annealing in air. The evolution of this structure has been followed through a sequence of annealing treatments. Structural and magnetic investigations on as-prepared and annealed samples revealed that as-prepared samples are weakly magnetic and develop into spherical ferromagnetic entities with superior magnetic properties. The saturation magnetization (Ms∼42 emu/g) and coercivity (Hc∼482 Oe) at room temperature clearly suggest that the air annealed (500 °C) samples are protected from the oxidation due to formation of B2O3 protective layer. The magnetization results are consistent with the observed microstructural changes and satisfactorily explained on the basis of a core-shell model, where we consider each particle as a magnetically heterogeneous system consisting of a ferromagnetic core of Ni and nonmagnetic shell of B2O3. These results suggest that this kind of nanocomposite systems might have significant potential as a recording media.

Molecular dynamics simulation of the energetic reaction between Ni and Al nanoparticles

Molecular dynamics simulations are used to simulate the energetic reaction of Ni and Al particles at the nanometer scale. The effect of particle size on reaction time and temperature for separate nanoparticles has been considered as a model system for a powder metallurgy system. Coated nanoparticles in the form of Ni-coated Al nanoparticles and Al-coated Ni nanoparticles are also analyzed as a model for nanoparticles embedded within a matrix. The differences in melting temperature and phase change behavior, e.g., the volumetric expansion of Al between Al and Ni, are expected to produce differing results for the coated nanoparticle systems. For instance, the volumetric expansion of Al upon melting is expected to produce large tensile stresses and possibly rupture in the Ni shell for Ni-coated Al. Simulation results show that the sintering time for separate and coated nanoparticles is nearly linearly dependent on the number of atoms or volume of the sintering nanoparticles. We have also found that nanoparticle size and surface energy are important factors in determining the adiabatic reaction temperature for both systems at nanoparticle sizes of less than 10 nm in diameter.

Assembly and magnetic properties of nickel nanoparticles on silicon nanowires

The directed assembly of magnetic Ni nanoparticles at the tips of silicon nanowires is reported. Using electrodeposition Ni shells of thickness from 10 to 200 nm were selectively deposited on Au catalytic seeds at the ends of nanowires. Magnetic characterization confirms a low coercivity (≈115 Oe) ferromagnetic behavior at 300 K. This approach to multifunctional magnetic-semiconducting nanostructure assembly could be extended to electrodeposition of other materials on the nanowire ends, opening up additional approaches to device integration. Such magnetically functionalized nanowires offer an approach to developing highly localized magnetic probes for high resolution magnetic resonance force microscopy.

Numerical simulation of the thermal response of Al core/Ni shell nanometer-sizedparticles

Molecular dynamics simulations have been used to investigate the thermal response to agradual temperature rise of Al core/Ni shell unsupported particles with a radius of about4 nm and different shell thickness. The different melting points permit a clean separation ofthe Al and Ni solid-to-liquid transition processes, the Al core melting at temperatures atwhich the Ni shell still has a well defined solid character. The latent volume releasedby Al at melting gives rise to a considerable tensile strain in the Ni shell. Themechanical response of the latter depends on its thickness. A sudden failure and aconsequent fragmentation of the core/shell system is observed at small shell thicknessvalues.

Quasi‐Epitaxial Growth of Ni Nanoshells on Au Nanorods

Gold nanorods have been extensively studied during the past decade because of their interesting optical properties, arising from localized surface–plasmon resonances (SPRs). Because of the shape anisotropy of nanorods, two well-defined SPRs can occur, namely, those parallel and transversal to their long axis, which offers a high degree of optical control when illuminated with polarized light. Additionally, nanorod assemblies can be constructed with various orientations, offering potential applications in waveguiding and construction of electronic devices. The technique of manipulating gold nanorods for exploitation of such properties is, however, not largely developed and, therefore, there is a need for novel ways to guide nanorod assembly. For example, gold nanorod alignment has been achieved by stretching in a polymer matrix, or applying an external electric field, which allowed detailed optical characterizations to be made. On the other hand, Au rod strings were made by bio-recognition-driven assembly, selective binding of chemical functionalities, and by assembly on modified carbon nanotubes. We present in this communication a first approach toward magnetic functionalization of gold nanorods, which is intended as a tool for magnetic manipulation and directed assembly. Additionally, the same technique can be exploited for the synthesis of magnetic nanoshells, which are rare but can be expected to display a large magnetic anisotropy, with potential uses in data storage. The surface modification of gold nanorods has often been accomplished through coating with various materials, such as Ag, Ag2S/Ag2Se, [17] SiO2, [18,19] Pd, or Pt. However, the synthesis of composite nanomaterials containing gold nanoparticles and a magnetic material (transition metals in particular) is complicated, especially in the case of gold nanorods, since they easily reshape at moderately high temperatures that are typically used for synthesis in organic solvents. Thus, for the coating of Au rods with magnetic metals, a low-temperature process is required that can simultaneously involve a slow reduction, so as to avoid nucleation in solution. For these reasons, we have chosen nickel as the coating material, because the synthesis of air-stable Ni colloids by reduction with hydrazine, at low temperature and in the presence of the surfactant cetyl-trimethylammonium bromide (CTAB, also used for Au nanorod synthesis) has been reported previously. As this process requires the use of a suitable catalyst, and gold was found to be inefficient, we used gold nanorods with platinum nanocrystals selectively grown at their tips, which were recently developed by us. The transmission electron microscopy (TEM) images in Figure 1 illustrate the various steps during the synthesis, including the starting Au rods, the growth of Pt tips, and finally the uniform coating with Ni shells of various thickness. Based on our previous work, we used gold nanorods coated with platinum at a Au/Pt molar ratio of 20, which leads to the growth of small Pt crystals on both tips of the Au rods, as shown in Figure 1B. The use of a minimum amount of Pt was intended to preserve as much as possible the optical response of the original Au rods and promote preferential Ni reduction at the tips. However, reduction of NiCl2 with hydrazine in the presence of Au/Pt rods stabilized with CTAB was found to lead to homogeneous growth of Ni shells around the rods, as indicated by the roughness and contrast observed in Figure 1C–E. It is remarkable that when using pure Au rods as seeds no Ni reduction was achieved, while just by including the Pt tips, the reduction took place invariably over the whole rod. Both energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) (see Supporting Information) have confirmed the presence of both nickel and gold in the final, composite nanorods (the Pt signal was too low in EDS because of its small relative amount, but Pt could be identified from XPS). The process allowed a fine tuning of the thickness of the Ni shell, through varying the molar ratio between Ni and Au, as shown in Figure 1C–E for molar ratios of 1, 5, and 10. By increasing the Ni/Au ratio, both the length and width increased similarly, resulting in a gradual decrease of the aspect ratio from 5.1 down to 2.6 (see Supporting Information, Fig. S1). Before proceeding with a more detailed analysis of the prepared nanoparticles, we carried out a qualitative demonstration of their magnetic character, by means of a hand-held permanent magnet. First of all, during the washing step of the synthesis (see Experimental) we used the magnet for separation of the particles from the solvent, observing complete separation (Fig. S2) over periods of few hours (longer for thinner C O M M U N IC A TI O N

Assembly of Ni/γ-Fe 2O 3 shell/core nanowires

In the present work, an assembly of Ni/γ-Fe 2O 3 shell/core nanowire arrays was successfully performed by employing the electrodeposition technique combined with the chemical precipitation method using the silanized porous anodic alumina template(PAAT). The microstructure of the shell/core nanowires was characterized by means of a transmission electron microscopy and an X-ray diffractometer. The diameter of the shell/core nanowires is about 50 nm. The thickness of Ni shell can be controlled, e.g., from 4–6 nm to 8–10 nm by varying electrodeposition parameters, and the Ni shell is identified in amorphous state, it coats a polycrystalline γ-Fe 2O 3 core.

Thermal explosion in Ni-Al system: influence of reaction medium microstructure

The thermal explosion (TE) phenomenon was investigated in the Al-Ni system (Al-rich compositions) with various reactant medium microstructures. The parameters of TE (e.g., ignition temperature, reaction times, etc.) obtained for Al particles clad by Ni were compared with those for Al+Ni powder mixtures having different particle sizes. The heating rate was varied in the range 100 to 900 K/min, and its influence on TE characteristics was also studied. The results show that ignition process is related to either phase (melting of reactant or first product formed) or microstructural (breaking of the Ni shell in case of clad particles) transformations and depends on the reactant medium microstructure. The latter influences the process of initial product formation, which in turn affects the interaction rate during the post-ignition stages. The obtained results were discussed and compared with those reported in the literature. In general, it is suggested that TE in heterogeneous gasless systems differs from explosions in conventional homogeneous media. In the latter, the operating conditions define ignition temperature, while in the former, it is related to the system phase diagram and thus is a characteristic property of the system.