Non-stoichiometric NiO Research Articles

Electrochromic performances of nonstoichiometric NiO thin films

Electrochromic (EC) performances of Ni3+ containing NiO thin films, called modified NiO thin films, prepared either by pulsed laser deposition or by chemical route are reported. When cycled in lithium based electrolyte, the comparison of the EC behavior of nonstoichiometric NiO thin films points out a larger optical contrast for the films synthesized by chemical route with the absence of an activation period on early electrochemical cycling due in particular to a larger porosity. Herein we demonstrate faster kinetics for modified NiO thin films cycled in lithium ion free electrolyte. Finally, X-ray absorption spectroscopy is used for a preliminary understanding of the mechanism involved in this original EC behavior linked to the film characteristics including their disorder character, the presence of Ni3+ and their porous morphology.

Precursors of active Ni species in Ni/Al2O3 catalysts for oxidative dehydrogenation of ethane

Ni/Al2O3 catalysts for oxidative dehydrogenation (ODH) of ethane were prepared by impregnation of Al2O3 with nickel acetate or nickel nitrate, and by mechanical mixing of NiO and Al2O3. The Ni-based catalysts were characterized by N2 adsorption-desorption, X-ray diffraction, diffuse reflectance UV-visible diffuse reflectance spectroscopy, and temperature-programmed reduction of hydrogen. The results showed that formation of crystalline NiO particles with a size of < 8 nm and/or non-stoichiometric NiO species in the Ni/Al2O3 catalysts led to more active species in ODH of ethane under the investigated reaction conditions. In contrast, tetrahedral Ni species present in the catalysts led to higher selectivity for ethene. Formation of large crystalline NiO particles (22–32 nm) over Ni/Al2O3 catalysts decreased the selectivity for ethene.

Coexistence of intrinsic and extrinsic origins of room temperature ferromagnetism in as implanted and thermally annealed ZnO films probed by x-ray absorption spectroscopy

The present work reports the structural and ferromagnetic properties of 200 KeV Ni2+ ion implanted ZnO films before and after annealing at 650 °C in air and correlates these properties with their electronic structures. X-ray absorption spectroscopy (XAS) at Zn and Ni K-edges reveal that Zn metal cluster is absent in ZnO matrix, but there is an indication of clustering of Ni metal as a secondary phase. The XAS analysis of O K-edge shows (i) non-stoichiometric NiO segregation in as-implanted film and its absence/reduction in air annealed film, (ii) partial substitution of Ni2+ ions at tetrahedral Zn sites, and (iii) presence of lattice defects such as oxygen vacancies in both films. The observed ferromagnetic behavior of as-implanted and air annealed films has both intrinsic as well as extrinsic origins. The intrinsic and extrinsic origins are discussed in terms of bound magnetic polaron model and presence of antiferromagnetic NiO phase/ferromagnetic Ni metal clusters, respectively.

Magnetization reversal in exchange-biasedNi∕NiOlayered structures

The dependence of the asymmetric magnetization reversal, exchange field, and coercive field on the NiO microstructure and thickness is studied in polycrystalline $\mathrm{Ni}∕\mathrm{Ni}\mathrm{O}$. Probing structural properties in real and reciprocal space, we show that the modification of the NiO growth conditions results in nonstoichiometric NiO. Exchange and coercive fields are strongly enhanced in nonstoichiometric $\mathrm{Ni}∕\mathrm{Ni}\mathrm{O}$ bilayers. Probing exchange properties in trilayers, we show that the coercive and exchange field enhancements are of different origins but depend on the NiO domain state. Modifying the internal part of the antiferromagnet allows a control of the exchange properties. Studying the Ni magnetization reversal, we reveal the contribution of nonuniform reversal modes and coherent rotation as a function of the angle between the applied field and the Ni easy axis. We observe two distinct critical angles revealing the significant role of nonuniform magnetization reversal in the asymmetry and its angular dependence.

Ni–Nb–O mixed oxides as highly active and selective catalysts for ethene production via ethane oxidative dehydrogenation. Part I: Characterization and catalytic performance

This work demonstrates the high potential of a new class of catalytic materials based on nickel for the oxidative dehydrogenation of ethane to ethylene. The developed bulk Ni–Nb–O mixed oxides exhibit high activity in ethane ODH and very high selectivity (∼90% ethene selectivity) at low reaction temperature, resulting in an overall ethene yield of 46% at 400 °C. Varying the Nb/Ni atomic ratio led to an optimum catalytic performance for catalysts with Nb/Ni ratio in the range 0.11–0.18. Detailed characterization of the materials with several techniques (XRD, SEM, TPR, TPD-NH 3, TPD-O 2, Raman, XPS, electrical conductivity) showed that the key component for the excellent catalytic behavior is the Ni–Nb solid solution formed upon the introduction of niobium in NiO, evidenced by the contraction of the NiO lattice constant, since even small amounts of Nb effectively converted NiO from a total oxidation catalyst (80% selectivity to CO 2) to a very efficient ethane ODH material. An upper maximum dissolution of Nb 5+ cations in the NiO lattice was attained for Nb/Ni ratios ⩽0.18, with higher Nb contents leading to inhomogeneity and segregation of the NiO and Nb 2O 5 phases. A correlation between the specific surface activity of the catalysts and the surface exposed nickel content led to the conclusion that nickel sites constitute the active centers for the alkane activation, with niobium affecting mainly the selectivity to the olefin. The incorporation of Nb in the NiO lattice by either substitution of nickel atoms and/or filling of the cationic vacancies in the defective nonstoichiometric NiO surface led to a reduction of the materials nonstoichiometry, as indicated by TPD-O 2 and electrical conductivity measurements, and, consequently, of the electrophilic oxygen species (O −), which are abundant on NiO and are responsible for the total oxidation of ethane to carbon dioxide.

Effect of heat treatment on the properties of non-stoichiometric p-type nickel oxide films deposited by reactive sputtering

Non-stoichiometric p-type NiO thin films were deposited by radio frequency reactive magnetron sputtering from a Ni target with equal Ar and O 2 flow rates. The non-stoichiometry of as-deposited NiO presented an O-rich NiO phase analyzed by Rutherford backscattering spectrometry. The properties of sputtered NiO films, such as crystallinity, transmittance, bonding configuration, and hole carrier concentration after annealing at various temperatures in nitrogen were examined using X-ray diffractometry, ultraviolet-visible spectroscopy, X-ray photoelectron spectrometry and Hall measurement, respectively. Furthermore, the weight change and phase transition of sputtered NiO heated in nitrogen ambient were measured by thermogravimetric analysis and differential scanning calorimetry. Our results reveal that the heat treatment on NiO films leads to the out-diffusion of excess oxygen atoms, which may be attributed to their interstitial nature in the sputtered NiO structure.

Anodic oxidation of a Ni–Cr alloy in a molten electrolyte—effect of temperature and melt composition

A study of the anodic oxidation of a Ni–10 wt.% Cr alloy in molten NaOH and NaOH–Li 2CO 3 is presented, emphasising the role of temperature and carbonate addition in the mechanism of growth, the composition and the structure of the surface oxide films. The potential ranges of passivation, passivity and transpassivity have been determined by voltammetry. Impedance measurements were performed in the passive potential region to investigate the conductivity mechanism of the oxide films as depending on the temperature and melt composition. The surface composition of the oxides has been estimated by X-ray photoelectron spectroscopy (XPS) and their in-depth composition by Auger electron spectroscopy (AES). As a result, the main passivation product on the Ni surface was found to be a non-stoichiometric and/or partly hydrated NiO. A bilayer structure of the surface film is hypothesised, the inner barrier layer being tentatively identified with a Cr-substituted NiO and the outer deposited layer as a mixture of non-stoichiometric NiO and a Ni–Na or Ni–Li oxides. The transport of nickel cation vacancies through the barrier sublayer was assumed to be the rate-limiting step of the overall process in the passive region. The increase in thickness of the surface film with temperature is interpreted as due to the increase of the rate of cation vacancy transport through the barrier layer, leading to an increased secondary-film precipitation rate.

Effect of Nonstoichiometry of Nickel Oxides on Their Supercapacitor Behavior

How electrochemical capacitance is affected by the non-stoichiometric properties of nickel oxide (NiO x ) films is reported. Nonstoichiometric NiO x thin-film electrodes were prepared for use in a supercapacitor by reactive radio-frequency (rf) sputtering of a nickel metal target. The oxygen partial pressure in the sputtering gas was systematically varied to fabricate various nonstoichiometric NiO x films. As the oxygen partial pressure increased in the sputtering gas, hole concentration increased from 4.7 X 10 1 6 to 1.0 X 10 1 8 cm - 3 while the specific capacitance of the NiO x films decreased from 148 to 80 F/g. A mechanism to explain the relationship between the specific capacitance and nonstoichiometry is proposed. The specific capacitance values of these NiO x films are determined by the number of Ni 2 + states that can be oxidized to Ni 3 + as a result of the double injection of OH - ions and holes.

XPS depth profile analysis of non‐stoichiometric NiO films

AbstractAntiferromagnetic NiO films used for pinning layers in spin valve systems were prepared by reactive sputtering from an NiO target with variation of the oxygen/argon mixture in the sputter gas. Using XPS depth profiling we investigated the NiOx overstoichiometry (x &gt; 1) in the films that was found by other methods and the chemical changes in the films during annealing. A direct detection of Ni3+ failed because of strong preferential sputtering with the formation of metallic Ni during sputtering. By means of factor analysis a qualitative description of the overstoichiometry is possible. Indirectly it could be shown that metallic Ni is already formed in the oxide layers during annealing. It can be concluded that the stability of the NiOx films decreases with increasing deviation from stoichiometry (x &gt; 1). Copyright © 2004 John Wiley &amp; Sons, Ltd.

Influence of surface acceptor–donor couples on conductivity and other electrochemical properties of nonstoichiometric NiO at 200 °C

Nonstoichiometric nickel oxide, a p-type semiconductor, was tested at 200 °C as an electrode material for proton conductor-based sensors. When measured in air, its potential was found to be dependent on variable concentrations of oxidisable gaseous species, and conversely, at fixed concentrations of the latter compounds the potential was dependent on the pressure of the oxygen. The potential variations, due to changes in surface hole concentrations, induced us to measure the conductivity of the oxide under the same conditions, i.e. in the presence of a donor–acceptor couple. The results obtained were in agreement with the previous observations. A kinetic model for steady state surface hole concentration is proposed to explain the above electrochemical properties.

Characterization of Ni–Mg–Al mixed oxides and their catalytic activity in oxidative dehydrogenation of n-butane and propene

Hydrotalcite-type materials with different Ni:Mg:Al ratios have been prepared and characterized by different methods (powder X-ray diffraction, differential thermal analysis and thermogravimetric analysis, FT-IR and VIS–UV/diffuse reflectance spectroscopies and specific surface area and pore size distribution assessment). Samples calcined at 600°C, where the layered structure has been destroyed, are formed by non-stoichiometric NiO–MgO oxides, and their catalytic activity in n-butane and propene oxidative dehydrogenation has been studied; production of methane and ethylene, in addition to CO 2 and H 2O, is observed. The sample without Ni is not active in these reactions, but, among those containing Ni, that with the lowest content exhibits the largest activity.

Surface and Catalytic Properties of NiO and Fe2O3 Solids

The surface characteristics, viz., the specific surface area SBET, the total pore volume Vp and the mean pore radius r̄, of NiO and Fe2O3 were determined from N2 adsorption isotherms conducted at −196°C for the different adsorbents preheated in air at temperatures in the range 300–800°C. The catalytic activities exhibited in CO oxidation by O2 on the various solids were investigated at temperatures varying between 150°C and 400°C. The effect of heating the NiO and Fe2O3 solids in CO and O2 atmospheres at 175–275°C on their catalytic activities was also studied. The results showed that increasing the calcination temperature in the range 300–800°C resulted in a progressive decrease in the SBET value of NiO and Fe2O3. The computed values of the apparent activation energy for the sintering of the oxides were 71 and 92 kJ/mol, respectively. The sintering of NiO and Fe2O3 took place mainly via a particle adhesion mechanism. The catalytic activity of NiO decreased progressively on increasing its calcination temperature from 300°C to 800°C, due to a decrease in its SBET value and the progressive removal of excess O2 which was present as non-stoichiometric NiO. This treatment also decreased the catalytic activity of Fe2O3. The decrease was, however, more pronounced when the temperature increased from 300°C to 400°C which was a result of the crystallization of the ferric oxide into the α-Fe2O3 phase. An increase in the calcination temperature for both oxides from 300°C to 800°C did not modify the mechanism of oxidation of CO by O2 over the various solids but rather changed the concentration of catalytically active sites. Heating NiO and Fe2O3 in CO and O2 atmospheres at 175–275°C modified their catalytic activities, with Fe2O3 being influenced to a greater extent than NiO.

Ni(III) doped NiO as the electrode material for electrochemical devices employing protonic conductors

Abstract The essential prerequisite for a material to be used as electrode for protonic conductors is to possess electron or hole conductivity and to be able to insert reversibly protons coming from the electrolyte. The most common examples of such compounds are some interstitial hydrides or protonic insertion compounds like PdH N or H A WO 3 , respectively. Furthermore they must possess other important requirements: chemical stability, high reversibility, compatibility with the electrolytes and so on. The aim of this work was to see whether the non-stoichiometric NiO 1 + x , which is a semiconductor, can be used to advantage for this purpose. A mixture of this compound with a typical protonic conductor, in which the Ni(III) content was increased anodically, has proved to behave as an electrode with a high stability and relatively high reversibility.

Characterization of nickel species on several γ-alumina supported nickel samples

Studies of the chemical preparation, surface areas, pore distributions, powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectra (XPS) of several γ-alumina supported nickel samples active for the catalytic hydrogenation of hexanedinitrile have been carried out. NiO crystallites showed to be larger than the pore mouths of the support. Pore distributions lie in the mesopore region with diameters between 20 and 100 Å. Powder XRD detects the oxidized and reduced-nickel phases present and does not detect the nickel aluminate phase, due to the lack of crystallinity of the latter. SEM micrographs detect the presence of octahedral NiO crystallites and amorphous shelling nickel aluminate. XPS results show the presence of surface Ni 2+ (in the form of stoichiometric and non-stoichiometric NiO, and stoichiometric and non-stoichiometric nickel aluminates) and surface reduced nickel. A deconvolution of the experimental curves was carried out in order to obtain a better assignment of the surface nickel species present. Nickel aluminate is detected at calcination temperatures > 623 K and covers all the surface of the support with layers between zero and several atoms thick, depending on calcination temperature and nickel concentration. Catalytically active reduced nickel (for nitrile hydrogenations) either as naked crystallites or as encapsulated nickel inside voided non-stoichiometric aluminate shells lie on top of the underlying catalytically inactive nickel aluminate when precursor calcination temperatures are higher than 623 K and reduction temperatures of 673 K. NiO transformation into nickel aluminate collapses the cubic NiO surface morphology of the small NiO crystallites giving rise to voided shells of nickel aluminate which may hide encapsulated reducible NiO.

Effect of composition on the physicochemical properties of nickel aluminium hydrotalcites

Ni-Al hydrotalcite compounds with the structural formula [Ni1−xAlx(OH)2]x+ [(CO3)x/2 ·mH2O]x− were synthesized in the composition range Al/Ni+Al (x)=0.20–0.50 by sequential precipitation. The samples were characterized by X-ray diffraction, infrared absorption, thermogravimetry-differential scanning calorimetry-evolved gas analysis, transmission electron microscopy and BET surface-area measurements. The crystallinity and the lattice parameters of the samples are significantly affected with respect to composition. Hydrothermal treatments performed on the aged samples enhanced the crystallinity. The nitrate ion is also present along with carbonate ion as the charge-balancing anion in the interlayer space. The thermal stability of these materials increased with decrease in Ni/Al atomic ratio. Thermal calcination of these compounds yielded non-stoichiometric NiO whose crystallinity and thermal stability depends on the composition as well as on calcination temperature.

NiO Reducibilities: Structural and Catalytic Properties of Their Pure and Potassium-Doped Reduced Forms

Abstract The study of the activation energies of reduction of stoichiometric and nonstoichiometric potassium-free and potassium-doped NiO was carried out by means of temperature-programmed reduction experiments, using two compared theoretical methods. Nonstoichiometric NiO shows the lower initial activation energy of reduction, whereas stoichiometric NiO shows a higher initial activation energy for starting the autocatalytic nucleation. Studies of the chemical preparation, BET surface areas, X-ray diffraction, X-ray photoelectron spectra, scanning electron microscopy, and measurements of catalytic activities of the nickel samples for the catalytic hydrogenation of 1,6-hexanedinitrile, in a continuous process at 1 atm pressure and 443 K, in the absence of ammonia, were also carried out. Surface areas dcrease when NiO reduction temperature increase. XRD, XPS, TPR, and SEM measurements detect NiO incipiently reduced between 463 and 498 K, a NiO reducibility inhibitor character for potassium, and 99.9% reduction above 573 K for all NiO forms. 100% selectivities with respect to 6-aminohexanenitrile may be obtained at 60% conversions for catalysts with potassium contents of 10.5 × l0 −4 g K 2 O/g Ni on reduced nonstoichiometric NiO. A mechanism for the continuous process is proposed.

Surface characterization and catalytic properties of several graphite supported potassium-free and potassium-doped nickel catalysts

Abstract Studies of the chemical preparation, activation energies of reduction by temperature-programmed reduction. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and catalytic activities of several graphite supported potassium-free and -doped nickel catalysts were carried out for the catalytic hydrogenation of 1,6-hexanedinitrile in a continuous process at 1 atm pressure, 443K, and in the absence of ammonia. Activation energies of reduction for the graphite supported non-stoichiometric NiO were higher than those from the unsupported non-stoichiometric NiO. Increasing potassium contents increase the activation energies of reduction. XRD measurements showed the presence of graphite, NiO and Ni phases, and also the loss of graphite due to gasification at temperatures higher than ca. 723 K. XPS showed the surface crystalline phases present and indicated the inhibiting effects of graphite and potassium upon the reducibility of NiO, the metallic nickel sintering and the graphite gasification at high temperatures. The catalytic conversions increase at higher NiO reduction degrees, potassium drives selectivity towards 6-aminohexanenitrile, and as graphite prevents metallic nickel from sintering, conversions remain high at temperatures of 723 K. 100% Selectivities with respect to 6-aminohexanenitrile were obtained at conversions higher than 50% for catalysts with potassium contents of 1.4·10−3 g K2O/g catalyst and 0.21 g Ni/g graphite. A mechanism is proposed.

Morphology of coprecipitated nickel/alumina catalysts with low alumina content

A series of low-alumina Ni/Al 2O 3 catalysts, designed for carbon monoxide hydrogenation, was examined by temperature-programmed reduction, the BET method, oxygen chemisorption, and X-ray diffraction. Pre-reduced catalysts consisted of nickel crystallites and amorphous non-stoichiometric NiO·Al 2O 3 residues. According to the proposed model of the catalysts the residues appeared in two forms: as clusters situated between/on the nickel crystallites and as inclusions encapsulated within large nickel crystallites. The size of the nickel crystallites, the size and composition of the clusters, and the amount of inclusions were estimated. A decrease of alumina content in the catalysts resulted in an increase in the size of the nickel crystallites, and did not affect the size, composition, and abundance of the clusters.

Surface characterization and hydrogenation properties of several nickel/α-alumina catalysts

Studies of the chemical preparation, X-ray photoelectron spectra (XPS), activation energies of reduction, temperature-programmed reduction (TPR), X-ray diffraction (XRD) and catalytic activities of several nickel/α-alumina catalysts have been carried out for the catalytic hydrogenation of hexanedinitrile, in a continuous process at 1 atm pressure, 443 K, and in the absence of ammonia. XPS results show complete reduction of non-stoichiometric NiO on α-alumina at temperatures higher than 623 K and higher surface nickel dispersion with increasing nickel content and decreasing reduction temperatures. Activation energies of reduction for the α-alumina-supported non-stoichiometric NiO were higher than those of the unsupported non-stoichiometric NiO. TPR results show that the initial and final temperatures of reduction of the α-alumina-supported non-stoichiometric NiO are higher with unsupported NiO, confirming the inhibiting effect of α-alumina on NiO reduction. XRD measurements show the presence of α-alumina, NiO and Ni phases, and also the increase in crystallite size with increasing reduction temperature. Catalytic conversions increase with the nickel content and selectivities toward 6-aminohexanenitrile increase at lower nickel contents, high space velocities, and higher metallic sintering, probably owing to the presence of a higher content of specific crystal sites responsible for the production of 6-aminohexanenitrile. A mechanism is proposed.

Structural and catalytic properties of several potassium-doped nickel/α-alumina solids

Studies of the chemical preparation, activation energies of reduction, temperature-programmed reduction (TPR), X-ray diffraction (XRD), X-ray photoelectron (XP) spectra and catalytic activities of several potassium-doped nickel/α-alumina catalysts have been carried out for the catalytic hydrogenation of hexanedinitrile, in a continuous process at 1 atm pressure, 443 K, and in the absence of ammonia. Activation energies of reduction for the potassium-doped non-stoichiometric NiO/α-alumina precursors were higher than those obtained for the potassium-free NiO/α-alumina and for the unsupported non-stoichiometric NiO. TPR results show that there is an inhibiting effect of α-alumina and potassium on NiO reduction. XRD measurements reveal the presence of α-alumina, NiO and Ni phases, the latter being present in larger quantities at higher reduction temperatures and lower potassium contents. XRD spectra also show fairly constant nickel particle sizes for all catalysts tested, XPS results confirm the inhibiting effect of potassium upon the reducibility of NiO. The XPS intensity ratios of surface Ni : NiO match the BET area ratios Ni : NiO of the samples. Conversions of the catalysed reactions decrease with increasing potassium content of the catalysts, probably because of poisoning of the latter with a Thorpe cycling formation of an enamine at high basic K2O contents. Selectivities towards 6-aminohexanenitrile (which may reach 100% at 85% conversion, thereby of potential interest for the manufacture of nylon-6,6) increase with potassium content of the catalysts. The less selective catalysts also yield 1,6-diaminohexane and azacycloheptane. A lack of oligomerization reactions is observed, and a mechanism is proposed.