The supercapacitor is considered as a promising candidate for energy storage due to its high power performance, long life cycle, and low maintenance cost. [ 3 ] Pseudocapacitive materials, such as transition metal oxides, are being explored for use in supercapacitors with a large specifi c capacitance and high energy density. [ 4 ] However, pseudocapacitors often suffer from the low rate capability and poor stability, because the active materials are usually insulating or semiconducting, which hinders the fast electron transport required for high charge/discharge rates. As an ideal matrix, graphene is commonly used for growth of functional nanomaterials. [ 1a , 2c , 5 ] Recently, nanocomposites made by graphene and transition metal oxides have attracted wide attention in the fi eld of supercapacitors due to their synergetic effect, arising from the combination of the redox reaction of metal oxides with the high surface area/conductivity of graphene, to improve the electrochemical performance. [ 6 ]

Preparation of novel 3D graphene networks for supercapacitor applications

We report the fabrication of vertically aligned NiO nanowalls on nickel foils using a plasma assisted oxidation method. Electrochemical properties of as-synthesized NiO nanowalls were evaluated by galvanostatic cycling and cyclic voltammetery. The results show a capacity of ∼638 (mA h)/g (at 1.25C rate), with excellent capacity retention of up to 85 cycles, when cycled in the range, 0.005−3.0 V vs Li. The superior electrochemical performance of NiO nanowalls in comparison to the previously reported results on nanosized NiO particles can be attributed to its large surface area and shorter diffusion length for mass and charge transport. A possible reaction mechanism is discussed. We also report that electron field emission studies show that the verticllay aligned NiO nanowalls are efficient field emitters with a turn-on field of 7.4 V/µm and a maximum current density of ∼160 µA/cm2 can be achieved.

Fabrication of NiO Nanowall Electrodes for High Performance Lithium Ion Battery

Magnetic ordering in nanosized (100 and 1500 nm) nickel oxide NiO powders, prepared by the plasma synthesis method, was studied using Raman scattering spectroscopy in a wide range of temperatures from 10 to 300 K. It was observed that the intensity of two- magnon band decreases rapidly for smaller crystallites size. This effect is attributed to a decrease of antiferromagnetic spin correlations and leads to the antiferromagnetic-to- paramagnetic phase transition .

https://iopscience.iop.org/article/10.1088/1742-6596/93/1/012039/pdf

Raman scattering in nanosized nickel oxide NiO

Nanoporous Ni-Co binary oxide layers were electrochemically fabricated by deposition followed by anodization, which produced an amorphous layered structure that could act as an efficient electrocatalyst for water oxidation. The highly porous morphologies produced higher electrochemically active surface areas, while the amorphous structure supplied abundant defect sites for oxygen evolution. These Ni-rich (10-40 atom % Co) binary oxides have an increased active surface area (roughness factor up to 17), reduced charge transfer resistance, lowered overpotential (∼325 mV) that produced a 10 mA cm(-2) current density, and a decreased Tafel slope (∼39 mV decade(-1)). The present technique has a wide range of applications for the preparation of other binary or multiple-metals or metal oxides nanoporous films. Fabrication of nanoporous materials using this method could provide products useful for renewable energy production and storage applications.

Efficient Electrocatalytic Oxygen Evolution on Amorphous Nickel–Cobalt Binary Oxide Nanoporous Layers

Mott insulators are identified here with ordinary magnetic insulators. The insulating gap, local moment, and effective spin hamiltonian aspects are qualitatively explained by means of a novel set of solutions of the Hartree-Fock equations. The apparent conflict between Bloch's theorem and localized-electron phenomenology is thereby resolved in an elementary manner. This Hartree-Fock approach also sheds considerable light on the physical mechanisms responsible for the associated metal-insulator (Mott) and other related phase transitions, as observed in V2O3 and several other materials. With some generalizations and refinements, this theoretical picture is shown to also account semiquantitatively for a number of detailed properties of NiO and CoO, two of the most extensively studied Mott insulator materials. A wide variety of experimental data for NiO is surveyed in order to determine reasonable values for its effective Hubbard hamiltonian parameters, suitably generalized for the 3d electrons. The ...

Electronic structure of Mott insulators

Strong two-magnon ($2M$) Raman scattering has been observed from NiO along with first- and second-order phonon scattering The shape of the $2M$ scattering peak is well described by a simple-cubic Green's function for interacting spin waves with an intersublattice exchange ${J}_{2}$ = 148 ${\mathrm{cm}}^{\ensuremath{-}1}$ The temperature shift of the scattering peak is identical to that of the 2000-${\mathrm{cm}}^{\ensuremath{-}1}$ ir absorption peak described by Newman and Chrenko, but energy considerations require the absorption peak to be a phonon sideband of the $2M$ scattering peak, which is not seen in absorption

Infrared Absorption and Raman Scattering by Two-Magnon Processes in NiO

Raman spectra have been observed in AgGa${\mathrm{S}}_{2}$, AgGa${\mathrm{Se}}_{2}$, and CuGa${\mathrm{S}}_{2}$ using argon, krypton, and dye lasers. Of the 13 Raman-active vibrations, we have observed 12 of the modes of AgGa${\mathrm{S}}_{2}$, 9 modes of AgGa${\mathrm{Se}}_{2}$, and all the modes of CuGa${\mathrm{S}}_{2}$. The modes of AgGa${\mathrm{S}}_{2}$ which are also infrared active are in good agreement with the modes determined from a Kramers-Kronig analysis of the ir reflectivity. Agreement is also found with several of the transitions previously determined from the ir spectrum of CuGa${\mathrm{S}}_{2}$. The intensity of the ${A}_{1}$ mode of AgGa${\mathrm{S}}_{2}$ and CuGa${\mathrm{S}}_{2}$ dominates the other Raman lines when the laser excitation is well below the band gap. A resonant interference effect decreases the intensity of this mode as the band gap is approached.

Lattice vibrations of AgGa S 2 , AgGa Se 2 , and CuGa S 2

Temperature‐dependent Raman scattering by two magnons has been observed in both green and black (from excess oxygen) NiO single crystals, peaking at 1555 cm−1 at 1.4°K. The shape of the band is reasonably well described by a dominant intersublattice exchange of J2=148 cm−1, in excellent agreement with the value J2=153 cm−1, obtained from neutron scattering.1 The high‐frequency cutoff of the Raman scattering lies near 1900 cm−1 in agreement with the J1=0 two‐magnon joint density‐of‐states cutoff at 176 cm−1. The absorption line2 peaking at 1940 cm−1 has been reexamined in light of this data, and is found to exhibit a cutoff on the high‐frequency side at 2460 cm−1, and a threshold near the peak of the Raman scattering. These data support a model for the absorption band in which two magnons are created simultaneously with a photon, with the cutoff corresponding to the creation of two zone boundary magnons and an L0 phonon of 580 cm−1 at k=0. Such a phonon sideband of a two‐magnon transition probably derives ...

Infrared Absorption and Raman Scattering by Two‐Magnon Processes in NiO

The energy- and momentum-dependent cross section for 200-keV electrons scattered from thin Be foils has been measured. For energy losses within 30 V of the $K$ edge and for momentum transfers out to $q=4$ ${\mathrm{\AA{}}}^{\ensuremath{-}1}$ we compare the experimental results with a theoretical one-electron band-structure calculation which includes matrix elements. Within a few volts of the edge the results agree but then begin to deviate significantly at high energies. Many-electron excitations are proposed as a mechanism to partially account for these discrepancies.

Inelastic electron scattering near the K edge in Be

Electron-energy-loss spectra of the Sm 3d (or 4d) core level are found to be identical to within roughly-equal1 eV, irrespective of the valence state of the samarium. However, an energy shift of about 6 eV is observed for the excitation from the 3p level between the divalent and the trivalent samarium. This is explained as arising from the different final states for the d and p core excitation spectra. The strong Hund's-rule coupling for the 4f/sup 7/ final-state configuration overcomes the Coulomb interaction and leads to the nearly identical spectra for the 3d core excitations. Atomic total energy calculations using a relativistic, spin-polarized, density-functional Dirac procedure are performed and the results are in good agreement with the experimental observations.

A. E. Meixner

Papers

Infrared Absorption and Raman Scattering by Two-Magnon Processes in NiO

Lattice vibrations of AgGa S 2 , AgGa Se 2 , and CuGa S 2

Infrared Absorption and Raman Scattering by Two‐Magnon Processes in NiO

Inelastic electron scattering near the K edge in Be

Inelastic electron scattering spectra of normal and mixed-valent samarium compounds