Ni-doped MgO Research Articles

Synthesis and dielectric studies of Ni doped MgO nanostructure

Pure and Ni-doped MgO nanostructures were synthesized using reverse micelle approach, which produced a size distribution with narrow range effectively. By using X-ray diffraction, the face cantered cubic structure of each sample was confirmed. Energy dispersive analysis of X-ray confirms composition of all the samples. By using the high-resolution transmission mode of electron microscopy, the morphology and size of the nanostructure were validated. Utilizing reflectance spectra, the Kubelka–Munk equation has been used to calculate the optical bandgap which is ref shifted with doping. Vibrational mode analysis was done using Raman spectroscopy. Broadband dielectric spectroscopy was used to measure the dielectric parameters revealing improved dielectric performance for Ni doped MgO nanostructure. Because of the generation of oxygen vacancies by dopant addition, lower frequency dielectric loss and a higher dielectric constant were achieved. The impedance behavior in various frequency regimes demonstrated how effectively conductive grains and insulating grain borders contributed. Looking at the literature survey, this is very first attempt to study the temperature and frequency dependent dielectric study for Ni doped MgO nanostructure.

Effective UV–visible-infrared light-driven photothermocatalytic dry reforming of methane on Ni/Ni-MgO caused by a novel synergetic effect and photoactivation

Light-driven photothermocatalytic dry reforming of methane (DRM) is a promising technology which can efficiently convert two greenhouse gases of CO2 and CH4 into fuels and realize effective solar energy storage simultaneously. It is greatly challenging to achieve high fuel production rate with a relatively low light intensity and restrain the coking side-reactions that cause catalyst deactivation. Herein, a novel nanocomposite of Ni nanoparticles (NPs) loaded on Ni-doped MgO (Ni/Ni-MgO) was prepared. Very high production rate (r) of H2 and CO (82.78 mmol min−1 g−1 and 98.05 mmol min−1 g−1) as well as a light-to-fuel efficiency (η, 28.8%) are obtained with concentrated UV–vis IR irradiation at a relatively low light intensity (78.8 kw m−2). Ni/Ni-MgO displays excellent durability due to its very low coking rate of 2.46 × 10−3 gc h−1 g−1catalyst, reduced by 11.6 times as compared to that of a reference sample of Ni NPs loaded on MgO (Ni/MgO). This is attributed to a synergetic effect between Ni NPs and Ni-MgO, in which the active oxygen of Ni-doped MgO participates in the oxidation of carbon species as one of the rate-determining steps of DRM, thus facilitating DRM on Ni NPs and greatly improving coking resistance. A photoactivation is found to greatly reduce activation energy and thus enhances catalytic activity due to the substantial facilitation of carbon oxidation by light irradiation.

A program system for self-consistent embedded potentials for ionic crystals

Embedded-cluster models of crystalline solids are important to allow accurate wave function methods to be applicable to solids. The ab initio model potential method for embedding ionic solids has historically been shown to be a viable tool. While useful, the method has been limited by the need to generate such potentials for each crystal structure and the lack of a freely available program for generating ab initio model potentials. Herein, this is remedied by showcasing a new, AFL licensed, program, SCEPIC, which can be used in combination with Molcas or OpenMolcas codes to derive ab initio model potentials for ionic crystals.The applicability of ab initio model potentials derived via SCEPIC is evaluated for three simple ionic solids: MgO, CaO and CaF2. The following questions are addressed: (i) the capability of the method to reproduce the density matrix from periodic density functional theory calculations, (ii) the feasibility of performing geometry optimisations, (iii) the possibility to model band gaps of insulators and (iv) the ligand-field splitting of Ni-doped MgO. Going beyond the classical restriction of parametrising ab initio model potentials only at the Hartree–Fock level-of-theory, this work additionally address the sensitivity of results to the underlying Hamiltonian used to derive the potentials. The results demonstrate that good agreement with periodic density functional theory calculations can be achieved, geometry optimisations are feasible and accurate band gaps and ligand-field splittings can be computed.

Nonlinear optical, IR and orbital properties of Ni doped MgO nanoclusters: A DFT investigation

Abstract In the present study, density functional theory calculations have been performed to explore the changes in the electronic properties of Mg12O12 nanocluster through exohedral doping with nickel. The changes in zero-point Energies (ΔZPE), thermal Energies (ΔE), thermal Enthalpies (ΔH), and thermal Free Energies (ΔG) of all positions are also calculated and compared with interaction energies. The HOMO-LUMO gaps of Ni-doped MgO nanocages are reduced significantly mainly due to the increase in the energies of HOMO. The Ni atom profoundly increases the polarizability (α) and first hyperpolarizabilities (βo) of MgO nanocluster however, a position dependent effect is observed. The value of α (and βo) changed from 219 (0.3) au for pristine MgO to 261 (1061), 263 (1122), and 262 (1240) au for positions P1, P2, and P3, respectively. These interesting results will be beneficial for the potential use of the MgO nanocluster in novel electronic and high-performance NLO materials.

Electronic and magnetic properties of transition metal doped MgO sheet: A density-functional study

The stable geometries, electronic and magnetic properties of MgO sheets with Mg atom substituted by 3d transition metals (TM) (Mn, Fe, Co, and Ni) were investigated by First-principles calculations. The atomic structures of Co- and Ni-doped MgO sheets undergo larger distortions than those of Mn- and Fe-doped MgO sheets. Mn, Fe and Co atoms are found to be more suitable to dope into MgO sheet compared with Ni atom. The band gaps of MgO sheets are tunable and the magnetic properties of MgO sheets can be modified, which mainly originate from the d-orbitals of TM atoms. The present results indicate that the TM-doped MgO sheets maybe have potential applications in spintronics and magnetic data storage.

Theoretical Study of Hydrogen Permeation through Mixed NiO–MgO Films Supported on Mo(100): Role of the Oxide–Metal Interface

This work presents a periodic density functional study of the adsorption and permeation of atomic H on Ni-doped MgO oxide thin films supported on a Mo(100) surface. We find that the binding of atomic H is affected by the presence of a metallic support. The chemisorption energies increase considerably when the oxide film is supported. The H permeation through the NiO-MgO oxide was also studied. H migration through the unsupported NiO-MgO oxide is thermodynamically inhibited, while the presence of the metallic Mo makes permeation thermodynamically favored. We attribute the promoting effect to the different character of adsorbed H at the unsupported Ni-doped MgO oxide and at the oxide-Mo interface. In the former case, H forms hydroxyl groups, whereas in the latter case it appears as hydride due to the formation of strong metal-hydrogen bonds. These results illustrate the important role that the oxide-metal interface could play in the mechanism for pure and mixed oxides reduction.

Density functional study of CO and NO adsorption on Ni-doped MgO(100)

The adsorption of small molecules such as NO or CO on surfaces of magnetic oxides containing transition metals is difficult to model by current density functional approximations. Two such oxides are NiO(100) and Ni-doped MgO(100). Here we compare the results of a theoretical model of the Ni-doped MgO(100) surface with experimental results on NiO(100), which introduces some uncertainty into a quantitative theory-experiment comparison. In the present work, we tested seven meta-GGA and hybrid metafunctionals, in particular, three developed by the Minnesota group (M05, M06-L, and M06), and TPSS, TPSSh, TPSSKCIS, and B1B95; six GGA functionals, including BP86, PBE, and four other functionals that are modifications of PBE (PBEsol, SOGGA, revPBE, and RPBE); five hybrid GGA functionals (B3LYP, PBE0, B97-2, B97-3, and MPWLYP1M); and one unconventional functional of the generalized gradient type with scaled correlation called MOHLYP. The Minnesota meta-GGA functionals were found in the past to be very good choices when transition metal atoms were present; the other functionals chosen are a selection from the most currently used and most promising sets of functionals for bulk solids and surfaces and for transition metals. The difficulty is due to the charge transfer between open shells in the case of NO and to the weak character of the interaction in the case of CO. It is shown that the M06 hybrid meta functional applied to NO or CO on a model of the Ni-doped MgO(100) surface is able to provide a good description of both adsorbate geometries and binding energies. The M06 vibrational frequency shifts are more accurate than for other functionals, but there is still room for improvement.

Magnetic properties of Ni-doped MgO diluted magnetic insulators

Magnesium oxide doped with Ni has been studied in two different forms: one in which the Ni ions are incorporated into the substitutional sites and the other in which Ni is present both in substitutional sites and in the form of metallic Ni precipitates embedded in the MgO matrix. Magnetic properties of these materials have been studied and correlated with the microstructural properties. There is a significant difference in the magnetic properties between the two forms. From these studies the authors envisage that a diluted magnetic insulator will be paramagnetic in the absence of intrinsic defects such as vacancies and interstitials. MgO is a good system to perform the present studies as it can be synthesized as a high quality crystal devoid of defects to a high degree. Moreover, the magnetic properties of the Ni precipitates can be used to compare the results when a diluted magnetic semiconductor material is not fully devoid of nanoclusters/precipitates and secondary phases.

Catalytic dissociation of N 2O on pure and Ni-doped MgO surfaces

The catalytic activity of pure and Ni-doped MgO surfaces in N 2O decomposition has been investigated theoretically with DFT B3LYP cluster model calculations. The barrier to abstraction of O from N 2O to form a surface peroxo group, O 2 2 - , is found to be rate limiting on both pure and Ni-doped MgO. In the presence of Ni impurities, however, the barrier is reduced from 1.37 eV to 1.19 eV, thus accounting for the experimentally observed increase in catalytic activity as function of the Ni content. Dissociation reaction was found to take place also by direct interaction with surface Ni ion with a competing activation barrier of 1.22 eV. However, this latter process is less exothermic (1.45 eV vs 2.19 eV). The O abstraction by a surface oxygen is followed by O diffusion and recombination with final desorption of an O 2 molecule from the surface. This process occurs preferentially on pure MgO while requires higher barriers in the presence of Ni. This can explain the observed decrease of catalyst activity when the Ni concentration becomes higher than 50%.

Bonding of NH3, CO, and NO to NiO and Ni-doped MgO: a problem for density functional theory

Recent experimental results (Hoeft et al 2001 Phys. Rev. Lett. 87 086101) have questionedthe capability of current theoretical methods for describing the bonding ofNH3, CO, and NO with the NiO(100) surface. We show that these systems doindeed represent a challenge to theory. For different reasons, density functionaltheory (DFT) fails in describing the bonding of these molecules to the NiO surface.The gradient-corrected functionals which work better for the properties ofNH3/NiO and CO/NiO (energies, geometries, vibrations) provide wrong answers for NO/NiO andvice versa. This is not due to the well-known difficulty as regards DFT describing theinsulating character of NiO. In fact, exactly the same problem is found for isolatedNi2+ impurities in MgO. A correct description of the bonding of both closed-shell(NH3 and CO) and open-shell (NO) molecules toNixMg1−xO is obtained only after inclusion of dynamical correlation and dispersion forces viawavefunction-based methods. However, even with correlated calculations someuncertainties exist regarding the predicted value of the energy of adsorption of NO onNiO. While CASPT2 calculations reach reasonable agreement with experiment,the results of approximate coupled-cluster calculations (the multi-configurationcoupled-electron-pair approach) substantially underestimate the adsorption energy.

Bonding of NO on NixMg1−xO powders: an EPR and computational study

Abstract In this paper, we review the reactivity of the paramagnetic NO molecule with Ni-doped MgO based on electron paramagnetic resonance (EPR) spectra and density functional theory (DFT) cluster model calculations. The EPR spectrum of the NiO/MgO sample recorded at 77 K under NO pressure is characterized by two distinct signals, one assigned to monomeric NO physisorbed on low-coordinated Mg2+ ions and the other to NO adsorbed on terrace Ni2+ ions. The latter interaction is found to be at least three times larger than the former. The nature of NO bonding to Ni2+ ions is analyzed in detail. The EPR spectrum proves the presence of one unpaired electron localized on the Ni 3d shell. The DFT calculations indicate that the unpaired electron sits on the dx2–y2 orbital, while the Ni2+ dz2 electron is coupled with the NO π xz ∗ unpaired electron, producing a NONi2+ bond with dominant covalent character.

The treatment of the spin coupling in the bonding of NO to the Ni-doped MgO (100) surface

The results are reported of density functional theory (DFT) and explicitly correlated wave-function (CASSCF and CASPT2) calculations on the bonding of NO with the Ni-doped MgO(100) surface. The surface is represented by means of a cluster of ions embedded in point charges. A comparison is made between unrestricted (spin polarized) and spin restricted approaches. While the geometry of the surface complex is described in quite an accurate way by a spin unrestricted DFT approach, e.g., using the B3LYP functional, the spin distribution does not correspond to that of the real physical situation. In fact, the spin polarized DFT treatment shows three unpaired electrons, two with spin up and one with spin down, while EPR experiments show clearly the existence of a single spin localized on an Ni 3d shell. A spin restricted B3LYP treatment, on the other hand, gives a correct spin distribution and geometry but fails in reproducing the adsorption energy. Other exchange-correlation functionals behave in a similar or even worse way. The CASPT2 results, by contrast, are in substantial agreement with the experiment, showing the importance of treating on the same footing the spin and electron correlation as well as the multi-configuration character of the wavefunction.

Bonding of NO to NiO(100) and NixMg1−xO(100) surfaces: A challenge for theory

The NO/NiO(100) system represents an excellent test case for the theory of surface chemical bond since accurate information about geometry, adsorption strength, and spin properties is available from experiments performed on NiO and Ni-doped MgO powders, single crystals, and thin films. We used cluster models to describe the NO/NiO interaction in combination with density functional theory (DFT) and wave function-based methods. We have identified four major aspects of the interaction: (1) the bonding cannot be described by a single determinant; (2) a spin-polarized DF-B3LYP approach gives reasonable adsorption properties at the price of a physically incorrect spin distribution; (3) a key ingredient of the interaction is the Coulomb repulsion within the Ni 3d shell; since this term is described very differently depending on the exchange-correlation functional it can result in overbound generalized gradient approach or Becke, Lee, Yang, and Parr or in strongly unbound (HFLYP) systems depending on the DFT approach; (4) the proper inclusion of the dynamical correlation is essential to treat the on-site Coulomb repulsion within the Ni 3d shell and to provide an accurate bond strength. In fact, the explicitly correlated complete-active-space second-order perturbation theory method gives results in overall agreement with the experiment. This shows the importance of treating on the same footing spin and electron correlation as well as the multiconfiguration character of the wave function.

Spectral Analysis of Transition Metal-Doped MgO "Matched Emitters" for Thermophotovoltaic Energy Conversion

A new, thermally excited Co/Ni-doped MgO ceramic emitter for TPV energy conversion is described in this work, and termed the “matched emitter” because its emissive power spectrum is very efficiently matched with the portion of the electromagnetic spectrum that can be converted directly into electrical energy by infrared responding GaSb photovoltaic cells. Ligand Field Theory calculations are used to estimate the crystal field splitting energies at high temperatures for Co and Ni-doped MgO matched emitters. Experimental measurements of the high temperature (1300–1400°C) emissive power spectrums for Co and Ni-doped MgO emitters are compared with predictions obtained from ligand field calculations for what is believed to be the first time. It was found that crystal field splitting energies of 10 Dq = 9070 cm−1 represented the “best fit” for the Co-doped MgO high temperature emissive power spectrum, and 10 Dq = 7950 cm−1 for the Ni-doped MgO spectrum. These values are only slightly lower than values reported for corresponding single crystal, transition metal doped laser materials that were measured at or well below room temperature.

Spectrally selective, matched emitters for thermophotovoltaic energy conversion processed by tape casting

The thermophotovoltaic (TPV) generator converts radiant energy from a high temperature emitter element into electric power using infrared responding photovoltaic cells. Spectral control is a primary issue in TPV applications. Conventional TPV generators have relied on filters to achieve selectivity and spectral control with near-blackbody ceramic emitters. Several practical problems have limited the success of this approach, particularly the present lack of a satisfactory wide-band infrared filter. A new, spectrally selective emitter is described in this work, and will be called the “bandgap matched” emitter because its emissive power spectrum is very efficiently matched with the infrared response of the GaSb photovoltaic cell. The superior spectral efficiency has been achieved with a novel combination of spectrally active, transition metal dopants within an infrared-transparent magnesium oxide ceramic matrix. High mechanical integrity, thermal shock resistance, excellent heat transfer characteristics, and near-ideal spectral efficiency have been achieved by fabricating composite Co- or Ni-doped MgO from thin ceramic ribbons made by the tape casting process.

Adsorption and Decomposition of H2S on MgO(100), NiMgO(100), and ZnO(0001) Surfaces: A First-Principles Density Functional Study

The adsorption and dissociation of H2S on MgO(100), Ni-doped MgO(100), and ZnO(0001) was studied using first-principles density-functional calculations (DFT-GGA) and periodic supercells. The bonding of H2S and its S-containing dissociated species (HS and S) is substantially stronger on ZnO(0001) than on MgO(100), making dissociation easier on zinc oxide. This behavior can be explained by the smaller ionicity in ZnO, which leads to a larger electron density around the Zn atoms and a larger reactivity toward S-containing molecules. Replacing some of the metal centers of MgO(100) with Ni atoms enhances the binding of S-containing species through new electronic states associated with the Ni 3d levels and located above the occupied {O 2p + Mg 3s} bands. In addition, structural defects, like steps, expose metal centers with lower coordination and larger reactivity than pentacoordinated Mg atoms in MgO(100). A simple model based on perturbation theory and band−orbital mixing is able to explain the differences in...

Magnetic susceptibilities of Co- and Ni-doped MgO

The temperature dependence (1.7-300 K) of the DC magnetic susceptibilities chi for the systems MpMg1-pO (M=Co,Ni) with p<pc (where pc approximately=0.14 is the percolation threshold) are reported. Two sets of samples were studied for both dopants: two lightly doped samples with p<0.01 and two more heavily doped samples with p=0.06 to 0.12. For the lightly doped samples, results are analysed in terms of contributions from single magnetic ions, exchange-coupled pairs and any contributions from the spin-orbit coupling. For Co2+ in MgO, a contribution from spin-orbit coupling leads to a temperature-dependent magnetic moment ( mu T=3.84 mu B at 5 K to 5.15 mu B at 300 K) and an apparent Curie-Weiss variation, whereas for Ni2+ in MgO, the paramagnetic chi P follows the Curie-law variation. From this analysis, concentrations of the dopants are determined. Further, the temperature independence of chi PB/ chi PA for two doping levels A and B at low concentrations is used to show that any contributions from exchange-coupled pairs are negligible. The more heavily doped samples show non-linear variations of chi P-1 against T, presumably due to contributions from magnetic clusters of different sizes.

Radiation-enhanced impurity aggregation in MgO.

Electron irradiation at high temperatures of pure, Fe-, and Ni-doped MgO suppresses the ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Ni}}^{2+}$ optical absorption bands, and leads to the production of precipitates containing Fe and Ni. The results suggest metallic precipitates. This process is due to radiation-induced reduction and precipitation, in which displacement damage and radiation-enhanced diffusion play an important role. In the case of Fe, the results indicate ${\mathrm{Fe}}^{2+}$ as the diffusing entity. Comparison between electron and \ensuremath{\gamma}-ray irradiations show that ${\mathrm{Fe}}^{2+}$ ions which are paired with other impurities do not readily diffuse under electron irradiation.

Effet of Nikel Alloying on the Thermal Diffusivity/ondutivity of MgO Single rystals

The thermal diffusivity/conductivity of Ni-doped MgO single crystals was measured as a function of temperature by the laser-flash method. Nickel in the form of the substitutional solid solution, Ni2+, has a much greater effect on the thermal diffusivity / conductivity than precipitated Ni colloids. These results are discussed in terms of phonon and photon scattering effects.

Nickel Colloids in Reduced Nickel-Doped Magnesium Oxide

High-temperature treatment in a reducing atmosphere of Ni-doped MgO crystals resulted in the formation of colloids with an average size of about 50 nm. The colloids are coherent nickel precipitates having a fcc structure ($a=0.352$ nm), and occasionally a bcc structure ($a=0.288$ nm). The ratio of magnetic to nuclear scattering amplitudes for the (111) precipitate reflection was about 17% below the value for nickel. Optical absorption measurements indicate that a broad optical band centered at \ensuremath{\sim} 2.2 eV is associated with the colloidal precipitates.