Showing papers on "Nickel published in 2004"

Atomic-scale imaging of carbon nanofibre growth

Abstract: The synthesis of carbon nanotubes with predefined structure and functionality plays a central role in the field of nanotechnology1,2, whereas the inhibition of carbon growth is needed to prevent a breakdown of industrial catalysts for hydrogen and synthesis gas production3. The growth of carbon nanotubes and nanofibres has therefore been widely studied4,5,6,7,8,9,10. Recent advances in in situ techniques now open up the possibility of studying gas–solid interactions at the atomic level11,12. Here we present time-resolved, high-resolution in situ transmission electron microscope observations of the formation of carbon nanofibres from methane decomposition over supported nickel nanocrystals. Carbon nanofibres are observed to develop through a reaction-induced reshaping of the nickel nanocrystals. Specifically, the nucleation and growth of graphene layers are found to be assisted by a dynamic formation and restructuring of mono-atomic step edges at the nickel surface. Density-functional theory calculations indicate that the observations are consistent with a growth mechanism involving surface diffusion of carbon and nickel atoms. The finding that metallic step edges act as spatiotemporal dynamic growth sites may be important for understanding other types of catalytic reactions and nanomaterial syntheses.

Reproducible resistance switching in polycrystalline NiO films

Abstract: Negative resistance behavior and reproducible resistance switching were found in polycrystalline NiO films deposited by dc magnetron reactive sputtering methods. Oxygen to argon gas ratio during deposition was critical in deciding the detailed switching characteristics of either bi-stable memory switching or mono-stable threshold switching. Both metallic nickel defects and nickel vacancies influenced the negative resistance and the switching characteristics. We obtained a distribution of low resistance values which were dependent on the compliance current of high-to-low resistance switching. At 200°C, the low-resistance state kept its initial resistance value while the high-resistance state reached 85% of its initial resistance value after 5×105s. We suggested that the negative resistance and the switching mechanism could be described by electron conduction related to metallic nickel defect states existing in deep levels and by small-polaron hole hopping conduction.

β-Nickel Hydroxide Nanosheets and Their Thermal Decomposition to Nickel Oxide Nanosheets

Abstract: Powders of single-crystalline β-nickel hydroxide (β-Ni(OH)2) nanosheets with the hexagonal structure have been successfully synthesized by the hydrothermal method at 200 °C using nickel acetate as the nickel source and aqueous ammonia as both an alkaline and complexing reagent The yields of β-Ni(OH)2 nanosheet powders were higher than 924% This method is simple and low-cost for large-scale production of powders of single-crystalline β-Ni(OH)2 nanosheets Single-crystalline nickel oxide (NiO) nanosheets have been synthesized by thermal decomposition method at 400 °C for 2 h using single-crystalline β-Ni(OH)2 nanosheets as the precursor The sheet shape of β-Ni(OH)2 was sustained after thermal decomposition to NiO The as-prepared products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetric analysis (DSC), and thermogravimetric analysis (TG)

Synthesis of p-aminophenol from p-nitrophenol over nano-sized nickel catalysts

Abstract: p-Aminophenol was synthesized by catalytic hydrogenation of p-nitrophenol on nano-sized nickel catalysts prepared by a chemical reduction method from aqueous solutions. The catalysts were characterized by XRD, EDS, SEM, HRTEM and Mastersizer 2000. Analysis results show that as-prepared catalysts are pure f.c.c. nickel and are prone to aggregation; the average particle size of nickel catalysts is 57 nm and there are high-density defects on particle surfaces. In hydrogenation reactions of p-nitrophenol, the hydrogenation rate is zero-order dependent on nitro aromatics and increases with increasing of hydrogen pressure. Compared with commercial Raney Ni, catalytic properties (activity, selectivity, and stability) of the as-prepared nickel are superior. The reason proposed for higher catalytic activity of nano-sized nickel is a combination effect of the small particle size and high-density surface defects. The partial sintering of nano-sized nickel might lead to the deactivation of the catalytic activity of nano-sized nickel.

The synthesis, characterization and electrochemical behavior of the layered LiNi0.4Mn0.4Co0.2O2 compound

Abstract: The mixed transition metal layered compound, LiNi0.4Mn0.4Co0.2O2, with the α-NaFeO2 layer structure has been synthesized and characterized. The optimum temperature of synthesis was found to be 800–900 °C. Rietveld refinement showed that cobalt suppresses transition metal ion migration into the Li sites whereas nickel promotes the migration. XPS analysis shows that the Co and about 20% of the Ni and Mn are in the 3+ oxidation state, while 80% of the Ni and Mn are in the 2+ and 4+ oxidation states, respectively. LiNi0.4Mn0.4Co0.2O2 shows Curie–Weiss paramagnetic behavior above 150 K, and the value of the Curie constant is consistent with the above oxidation states. In lithium electrochemical cells the composition LiNi0.4Mn0.4Co0.2O2 gave the highest reversible capacity among the studied compositions. It shows excellent rate capability, giving reversible capacities ranging from 180 to 155 mA h g−1 at current densities from 0.1 to 2.0 mA cm−2.

Fabrication of Nanoporous Nickel by Electrochemical Dealloying

Abstract: The etching of the more active component from a homogeneous alloy is important in corrosion reactions and has been exploited in the synthesis of high surface area nanoporous materials. In this paper we show that nanoporous films can be obtained by selective electrochemical etching of the more noble component in a system where the more active component is passivated. We demonstrate that nanoporous nickel films can be obtained by a two-step process involving electrodeposition of a homogeneous NixCu1-x alloy followed by electrochemical etching of the copper from the alloy. The composition, lattice parameter, saturation magnetization, and Curie temperature of the electrodeposited NixCu1-x alloys can be precisely controlled by varying the deposition conditions. Nanoporous nickel can be formed by electrochemically etching the copper from the alloy. The nanoporous structures are characterized by a three-dimensional network of interconnected pores and exhibit enhanced coercivity and reduced magnetic anisotropy. T...

Efficient and selective nickel-catalyzed addition of H-P(O) and H-S bonds to alkynes.

Abstract: The cheap nickel catalysts are more reactive than the corresponding noble metal catalysts in the catalytic additions of a variety of P(O)-H bonds and an S-H bond to alkynes, affording regio- and stereoselectively both the Markovnikov and the anti-Markovnikov adducts, respectively, in high yields. A related five-coordinate hydrido nickel complex in the catalysis is successfully isolated, which can react readily with an alkyne to give the addition products.

Nickel-Catalyzed Arylcyanation of Alkynes

Abstract: A nickel catalyst coordinated by trimethylphosphine is found to effect the addition reaction of Ar−CN bonds in aromatic nitriles across alkynes to give rise to various β-arylalkenenitriles.

Synthesis and characterization of mesoporous alumina with nickel incorporated for use in the partial oxidation of methane into synthesis gas

Abstract: Mesoporous alumina catalysts that incorporate nickel (Ni-Alumina) with different Ni/Al molar ratios of 1:2, 1:5 and 1:10 were synthesized by a one-step sol–gel method using lauric acid as a template The prepared Ni-Alumina catalysts showed a relatively high surface area with a narrow pore size distribution after calcination at 700 °C; these effects were independent of the Ni/Al molar ratio Although weak interactions between nickel oxide and alumina were observed in the Ni-Alumina catalysts with increasing the amounts of nickel precursor, nickel catalysts finely dispersed on an alumina support were obtained in all cases The Ni-Alumina catalysts were found to be highly active in the partial oxidation of methane For the purposes of comparison, a nickel catalyst impregnated on a commercially available alumina support was prepared (Ni-IMP) Various characterization results showed that the 1:10 Ni-Alumina catalyst had stronger metal-support interactions and a more favorable catalyst structure for obtaining finely dispersed nickel particles, compared to the Ni-IMP catalyst The deactivation of catalysts examined in this work was not due to catalyst sintering, but mainly to the carbon deposition The Ni-Alumina catalyst having smaller nickel particles and lower levels of carbon deposition had a more stable catalytic activity than the Ni-IMP catalyst

Local Structure and Cation Ordering in O3 Lithium Nickel Manganese Oxides with Stoichiometry Li [ Ni x Mn ( 2 − x ) / 3Li ( 1 − 2x ) / 3 ] O 2 NMR Studies and First Principles Calculations

Abstract: Short-range ordering in Li[Ni x Mn ( 2 - x ) / 3 Li ( 1 - 2 x ) / 3 ]O 2 was investigated with 6 Li NMR and first principles structure computations. NMR indicates that the tendency for Ni 2 + to replace Li + in the Li + layers decreases with decreasing nickel content. Li in the Ni/Mn layers preferentially occupies sites near Mn 4 + and avoids the Ni 2 + ions, leading to nonrandom configurations. Calculations indicate that the ground state of Li(Ni 0 . 5 Mn 0 . 5 )O 2 contains zigzag rows of Ni 2 + and Mn 4 + ions. Although a disordering temperature of approximately 1000 K is calculated, ordered fragments persist above the phase transition and these materials contain significant short-range order, even when quenched from high temperature.

[Ni2O(l-Asp)(H2O)2]·4H2O: A Homochiral 1D Helical Chain Hybrid Compound with Extended Ni−O−Ni Bonding

Abstract: The hydrothermally synthesized 1D homochiral nickel aspartate oxide [Ni2O(l-Asp)(H2O)2]·4H2O is based on infinite helical chains of edge- and corner-sharing nickel octahedra. It represents the first example of a chiral one-dimensional compound with an extended helical TM−O−TM subnetwork synthesized as an optically pure product. Its structure was determined ab initio by real-space methods from laboratory powder diffraction data.

A comparison of the magnetic characteristics of nanocrystalline nickel, zinc, and manganese ferrites synthesized by reverse micelle technique

Abstract: Nanocrystalline nickel, zinc and manganese ferrites synthesized by reverse micelle synthesis technique were characterized by high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction techniques, and the magnetic behavior studied by Superconducting Quantum Interference Device (SQUID). The three ferrites exhibit blocking temperatures of 16, 20, and 35 K, respectively. The difference in the blocking temperatures was attributed to the magnetocrystalline anisotropy and L–S coupling. The saturation magnetization of the three ferrites at 300 K was 25, 8, and 6 emu/g, respectively and at 2 K was 35, 17, and 15 K, respectively. The lower saturation magnetization in the case of nickel and manganese ferrite compared to their bulk counterparts is attributed to a core-shell like particle morphology. The increase in the saturation magnetization in the case of zinc ferrite nanoparticles is attributed to the change in the cation distribution from a normal spinel to a mixed spinel structure in the nanocrystalline form.

Effect of grain size on the tribological behavior of nanocrystalline nickel

Abstract: Nanocrystalline nickel (8–28 nm) deposits were produced by direct and pulse current electrodeposition. The microhardness and microstrains in the deposits were estimated. Tribological testing indicated that the coefficient of friction (COF) for nanocrystalline nickel was almost half that of polycrystalline nickel. An important and interesting result is that extremely low COF of 0.16 can be obtained with nanocrystalline Ni coatings.

Synthesis and One-Dimensional Self-Assembly of Acicular Nickel Nanocrystallites under Magnetic Fields

Abstract: Polycrystalline nickel wires with an average length of 10 μm and diameter of about 200 nm were prepared at 70 °C by a hydrothermal process with a 0.25 T magnetic field applied. Studies show that the magnetic field induced one-dimensional assembly of acicular Ni nanocrystallites with dimensions of 200 nm in length and 10−30 nm in diameter, leading to the formation of the polycrystalline nickel wires. Magnetic measurements show saturation magnetization (Ms) of the sample prepared in a 0.25 T external magnetic field is higher than that of the sample synthesized without an external magnetic field applied. It is suggested that the one-dimensional self-assembly of acicular nickel nanocrystallites with their magnetic easy axes [111] aligned along the magnetic line of force results in the improvement of the magnetic properties.

Visible light photocatalysis in transition metal incorporated titania-silica aerogels

Abstract: Transition metal salts were added to aerogels of TiO2–SiO2 mixtures. The resulting M–TiO2–SiO2 (M = transition metal, in general present in the form of oxides) materials had very high surface areas (600–1000 m2/g) and pore volumes (1–4 cm3/g), and contained highly dispersed transition metal oxides. Several samples were active photocatalysts for acetaldehyde oxidation to carbon dioxide using visible light; in particular chromium, cobalt, and nickel. These intimate aerogel mixtures were only active when SiO2 was present (i.e. M–TiO2 aerogels were not). High oxidation states of the transition metal ions (e.g. Cr6+, Co3+) appear to be important in bringing about visible light photocatalyst activity.

Solid-state coordination chemistry of the oxomolybdate-organodiphosphonate/nickel-organoimine system: structural influences of the secondary metal coordination cation and diphosphonate tether lengths.

Abstract: The hydrothermal reactions of a molybdate source, a nickel(II) salt, tetra-2-pyridylpyrazine (tpyprz), and organodiphosphonic acids H2O3P(CH2)nPO3H2 (n = 1−5) of varying tether lengths yielded a series of organic−inorganic hybrid materials of the nickel−molybdophosphonate family. A persistent characteristic of the structural chemistry is the presence of the {Mo5O15(O3PR)2}4- cluster as a molecular building block, as noted for the one-dimensional materials [{Ni2(tpyprz)2}Mo5O15{O3P(CH2)4PO3}]·6.65H2O (6·6.65H2O) and [{Ni2(tpyprz)2}Mo5O15{O3P(CH2)5PO3}]·3.75H2O (8·3.75H2O), the two-dimensional phases [{Ni4(tpyprz)3}{Mo5O15(O3PCH2CH2PO3)}2]·23H2O (3·23H2O) and [{Ni3(tpyprz)2(H2O)2}(Mo5O15)(Mo2O4F2){O3P(CH2)3PO3}2]·8H2O (5·8H2O), and the three-dimensional structures [{Ni2(tpyprz)(H2O)3]Mo5O15{O3P(CH2)3PO3)}]·H2O (4·H2O) and [{Ni2(tpyprz)(H2O)2}Mo5O15 {O3P(CH2)4PO3}]·2.25H2O (7·2.25H2O). In the case of methylenediphosphonic acid, the inability of this ligand to tether adjacent pentanuclear clusters precludes t...