Undoped NiO Research Articles

Size and Surface Effects of Ce-Doped NiO and Co3O4 Nanostructures on Ferromagnetism Behavior Prepared by the Microwave Route

We investigated the structure and magnetic properties of Ce-doped NiO and Co3O4 samples. The multiemission centers created in the nanostructures observed from PL spectra were obviously due to the formation of interstitials (Nii and Coi) and oxygen vacancies (VO) by the incorporation of Ce3+ ions into the lattice site which affects the band gap structure of the material. Room-temperature magnetic study was demonstrated for Ce-doped NiO and Co3O4 nanostructures using a vibrating sample magnetometer. The origin of magnetism in these samples was related to the finite size and importantly oxygen vacancies created by the Ce ion dopant, which was confirmed from the decrease of magnetization value at room temperature compared to undoped NiO and Co3O4 samples.

Ru-Al codoping to mediate resistive switching of NiO:SnO2 nanocomposite films

The Ru-Al codoped NiO:SnO2 nanocomposite films are revealed to exhibit bipolar resistive switching. The switching mechanism is well explained by the formation/rupture of filamentary paths due to the field-induced migration of oxygen vacancies and oxygen ions. Compared with that of the undoped NiO:SnO2 film, the ON/OFF ratio of Ru-Al codoped samples is largely improved. This is ascribed to the increased content of oxygen vacancies and trapped states between the equilibrium Fermi level and conduction band induced by the interstitial defects of Ru and Al.

Structural, optical, and electrical properties of NiO-In composite films deposited by radio frequency cosputtering

In-doped NiO films with indium concentrations ranging from 0 to 30.3 at. % were deposited on glass substrates to investigate corresponding structural, optical, and electrical property variations. The x-ray diffraction patterns show that all films display only NiO peaks. When In atoms were added to NiO films, the NiO peaks shifted to lower angles, indicating that the lattice parameters of the films increased due to the larger In ions substituting for the smaller Ni ions. An electrical resistivity (ρ) too high to be measured occurred when the indium concentration in the NiO film was less than 15.6 at. %. The ρ value dropped significantly to 0.06 Ω·cm as the indium concentration increased to 26.9 at. %. Upon further raising the In to 30.3 at. %, the ρ value decreased further to 0.01 Ω·cm. All the In-doped NiO films showed n-type conduction. The transmittance of undoped NiO film is as high as 96%. On raising the indium concentration to 15.6, 19.9, 26.9, and 30.3 at. %, the transmittances decreased further to 68%, 62%, 57%, and 47%, respectively. Introducing higher In concentrations improved the films’ thermal stability of electrical resistivity.

Cu-doped NiO for aqueous asymmetric electrochemical capacitors

In this work, the influence of Cu doping on the electrochemical properties of NiO was investigated regarding its performance as electrodes in aqueous supercapacitors. NiO and Ni1−xCuxO (x=0.5–3%) cathode materials were synthesized via a citrate-gel process. Ni1−xCuxO samples have the same crystal structures as those of the un-doped NiO, indicating that copper atoms enter into the lattice of nickel mono-oxide. Compared to NiO, Cu-doping materials significantly improve the specific capacitance and rate capability. Especially, among all of the investigated samples, Ni0.99Cu0.01O presents the best electrochemical performance as high as 559 and 389Fg−1, which are 90.14% and 86.12% higher than those of NiO at 0.3 and 10A/g, respectively. This work demonstrated that Cu-doping is an effective way to improve the pseudo-capacitance of nickel monoxide.

Room-temperature ferromagnetism in nanocrystalline Fe-doped NiO powders synthesized by a simple direct thermal decomposition method

Room temperature ferromagnetism was observed in ∼26–34 nm of nanocrystalline Fe-doped NiO powders synthesized by a simple direct thermal decomposition method. The undoped and Fe-doped NiO samples were thoroughly characterized using x-ray diffraction (XRD), energy-dispersive x-ray spectroscopy (EDXs), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The magnetic properties of the prepared samples were determined using vibrating sample magnetometry (VSM). The pure NiO sample exhibits antiferromagnetic behavior, whereas all the Fe-doped samples are ferromagnetic having the specific magnetizations of ∼0.033–0.102 emu/g at 10 kOe. Our results indicate that the ferromagnetic property is intrinsic to the Fe-doped NiO system and is not a result of any secondary magnetic phase or cluster formation.

Structural, optical and electrical properties of undoped and Li-doped NiO thin films prepared by sol–gel spin coating method

Un-doped and lithium doped nickel oxide thin films have been prepared on glass substrates by the sol–gel spin coating technique using nickel acetate and lithium chloride as source materials. The thickness of the films was increased by increasing the number of layers successively deposited on the same substrate. The effect of the number of layers on the structural, optical and electrical properties of NiO thin films was studied. The results of investigations of the samples show that four is the most suitable number of layers of undoped NiO films with high optical transparency. Then, by using these optimized deposition parameters, lithium doped nickel oxide films are prepared. The surface morphology and crystalline structure of the thin films were investigated by atomic force microscopy and X-ray diffraction (XRD), respectively. A modification in the morphology of the films was observed while increasing the Li amount in the solution. XRD patterns show that the films are polycrystalline. The preferred orientations were evaluated from XRD data. The average transmittance of the NiO films in the visible region increases with the increase of Li concentration. The electrical measurements show that the resistance of the films decreases as the lithium content increases.

High conductivity nickel oxide thin films by a facile sol–gel method

Li-doped NiO thin films (Li:NiO) were deposited on glass substrates using a facile sol–gel method. The effects of Li doping concentration on the structural, electrical and optical properties of Li:NiO films were examined. The structural properties of Li:NiO films were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy. XRD revealed the Li:NiO films to have a polycrystalline bunsenite structure. The surface morphology of the Li:NiO films exhibited nanocrystalline grains with a randomly oriented morphology. The resistivity decreased substantially to 1.33kΩcm at 0.11mol%, which is one order of magnitude lower than that of the undoped NiO film. The mean transmittance decreased from 70% to 60% with increasing Li doping concentration from 0.02mol% to 0.19mol%.

Electrical conductivity of Ni–YSZ composites: Variants and redox cycling

Short-term changes in the electrical conductivity of different Ni–YSZ composites (cermets) were measured by an in-situ 4-point DC technique. The isothermal reduction was carried out in dry, humidified or wet hydrogen at temperatures from 600 to 850°C. The cermets reduced at 600°C showed a stable conductivity of about 1100S/cm, which increased to an enhanced ~2000S/cm upon re-oxidation and subsequent re-reduction cycling at the same temperature. At 850°C, a rapid initial conductivity loss was observed; upon re-reduction after the re-oxidation both the conductivity and its loss rate were largely the same as in the initial reduction. The presence of steam had an accelerating effect on the conductivity loss at 850°C. In addition to cermets with a typical microstructure, different modified microstructures and compositions were tested. In the modified cermets, Al, Mg, and Ce were used as Ni dopants alongside with undoped Ni. Scanning electron microscopy of cermets reduced in different conditions showed increasing particle size and loss of metal-to-metal percolation in the samples reduced at higher temperatures, and a very fine microstructure in the high conductivity sample re-oxidised at 600°C. Short-term conductivity changes due to microstructural changes in both the standard and modified cermets with different Ni doping were compared by re-oxidation at 600°C and subsequent thermal excursions up to 1000°C by normalising the conductivity to a constant temperature. Modified cermets show reduced conductivity loss on both isothermal heating and high temperature ramping. Most stable cermets were the ones with undoped NiO and modified microstructure, as well as modified microstructures using Ti and Mg, or Ce secondary oxide coatings. Master sintering curve approach was successfully implemented to analyse the conductivity loss data. The MSC analysis yielded apparent activation energies for Ni sintering within the composite of 375kJ/mol when heated in dry H2 and 440kJ/mol when under wet H2.

Improved electrochromic performances of NiO based thin films by lithium addition: From single layers to devices

Aiming at enhancing the electrochromic properties of anodically colored NiO thin films, lithium doped NiO thin films were grown on FTO/glass substrates, by the pulsed laser deposition (PLD) method. Optimized conditions, namely a room temperature substrate under 10Pa oxygen pressure were used. Comparison with undoped NiO thin films indicates that lithium doping deteriorates NiO cubic phase (111) preferred orientation and also induces lattice disorder. The investigation of the electrochromic properties of Li–Ni–O thin films in aqueous liquid electrolyte, 1M KOH, on the one hand and in hydrophobic lithium conductive ionic liquid, 0.3M LiTFSI in BMITFSI, on the other hand, demonstrates an improvement in the electrochromic performances with lithium doping. Finally, electrochromic devices built on the association of WO3 and Li–Ni–O thin films and using the above quoted ionic liquid blended with PMMA as electrolyte are reported. Good electrochromic performances and neutral color are shown.

Reproducible resistance switching of defect-engineered NiO x with metallic Nb impurity

The effect of Nb doping on the resistance switching characteristics of NiO x films was investigated. Pt/Nb-doped NiO x /Pt metal–insulator–metal stacks were fabricated using NiO x films with various Nb contents sputtered by reactive dc magnetron sputtering. The resistance switching behaviors of the metal–insulator–metal stacks were then examined in conjunction with a study on the physical properties such as the chemical bonding of NiO x films. Nb doping of NiO x at a T dep of 400 °C and an O 2 partial pressure of 5% resulted in an improved endurance of SET/RESET processes with a narrower distribution of V SET, and a larger memory window compared to un-doped NiO x films. NiO x with 5.47% Nb deposited at an O 2 partial pressure of 15% showed bistable resistance switching behavior while undoped NiO x material, deposited under the same condition did not. A study of the chemical bonding states by X-ray photoelectron spectroscopy showed that the Nb-doping of NiO x films produced an increase in the density of Ni 0 and a reduction in the density of Ni 3+, compared to corresponding values for undoped NiO x films deposited under the same condition. The resistive switching behavior of NiO x was enhanced by defect engineering with metal impurity with different oxidation valence.

Study of Magnetic and Electrical Properties of Nanocrystalline Mn Doped NiO

Diluted Magnetic Semiconductors (DMS) are intensively explored in recent years for its applications in spintronics, which is expected to revolutionize the present day information technology. Nanocrystalline Mn doped NiO samples were prepared using chemical co-precipitation method with an aim to realize room temperature ferromagnetism. Phase formation of the samples was studied using X-ray diffraction-Rietveld analysis. Scanning electron microscopy and Energy dispersive X-ray analysis results reveal the nanocrystalline nature of the samples, agglomeration of the particles, considerable particle size distribution and the near stoichiometry. Thermomagnetic curves confirm the single-phase formation of the samples up to 1% doping of Mn. Vibrating Sample Magnetometer measurements indicate the absence of ferromagnetism at room temperature. This may be due to the low concentration of Mn2+ ions having weak indirect coupling with Ni2+ ions. The lack of free carriers is also expected to be the reason for the absence of ferromagnetism, which is in agreement with the results of resistivity measurements using impedance spectroscopy. Arrhenius plot shows the presence of two thermally activated regions and the activation energy for the nanocrystalline Mn doped sample was found to be greater than that of undoped NiO. This is attributed to the doping effect of Mn. However, the dielectric constant of the samples was found to be of the same order of magnitude very much comparable with that of undoped NiO.

Effect of W impurity on resistance switching characteristics of NiO x films

In this work, we investigated the effect of W doping on the resistance switching behavior of NiO x films. The W doping is expected to produce cation-rich NiO x film because W has high valence state compared to Ni in NiO x . To measure resistance switching behavior, Pt/NiO x /Pt and Pt/NiO x /TiN MIM stacks were fabricated by reactive dc magnetron sputtering with various W doping contents in NiO x . Also, physical properties such as atomic density and chemical bonding states of NiO x were characterized in addition to the resistance switching characteristics. Increasing W doping into NiO x produced the higher resistance value of high resistance states (HRS) compared to the undoped NiO x films with large memory window. Even though Pt/NiO x with W/Pt stacks showed a unipolar resistance switching behavior, Pt/NiO x with W/TiN stacks showed bipolar resistance switching. Composition and chemical bonding states in the deposited films were investigated by x-ray photoelectron spectroscopy (XPS). W doping into NiO x films produced higher density of metallic Ni (Ni 0) than that of NiO x films deposited at the same conditions. This work demonstrated that the resistive switching behavior of NiO x can be enhanced by doping NiO x with W of different oxidation valence.

Investigation of magnetic behaviour of Ni–Fe–O prepared by the solid state method

Polycrystalline Fe doped NiO was prepared by the solid state method with the aim of exploring its suitability as a functional material for spintronics applications. Phase pure samples (up to 2% doping of Fe) as evident from x-ray diffraction were characterized to investigate the magnetic, electric and dielectric properties. Magnetic measurements using the vibrating sample magnetometer clearly reveal ferromagnetic behaviour at room temperature. The activation energies of the doped samples, determined from impedance spectroscopy, were found to be relatively higher than that of undoped NiO. It is also observed that Fe doping has no significant effect on the dielectric properties of the samples. Thermogravimetric analysis, carried out to find the Curie temperature (TC), leads to a remarkable insight into the observed ferromagnetism by identifying the presence of secondary phase NiFe2O4.

Temperature effect on the electrical properties of undoped NiO thin films

Undoped NiO thin films have been prepared onto glass substrate by e-beam evaporation of the element Ni in vacuum at ∼2 × 10 −4 Pa. The as-deposited Ni films were then oxidized in air by heating about 2 h at a temperature of 470 K and then the oxidized Ni films are turned into NiO thin films. From the deposition time and film thickness after annealing in air, an effective deposition rate of NiO thin films was about 6.67 nms −1. X-ray diffraction (XRD) study shows the NiO films are amorphous in nature. SEM studies of the surface morphology of NiO films exhibit a smooth and homogeneous growth on the entire surface. The elemental composition of NiO films is estimated by Energy Dispersive Analysis of X-rays (EDAX) method. The effects of temperature on the electrical properties of NiO thin films were studied in details. The heating and cooling cycles of the samples are reversible in the investigated temperature range after successive heat-treatment in air. Thickness dependence of conductivity is well in conformity with the Fuchs–Sondheimer theory. Temperature dependence of electrical conductivity shows a semiconducting behavior with activation energy. The thickness dependence of activation energy as well as thermopower studies was done within 293–473 K temperature range, respectively. Thermopower study indicates the NiO films a p-type semiconductor. Optical study in the wavelength range 0.3 < λ<1.2 μm range exhibits a high transmittance in the visible as well as in the near infra-red. Calculation from the optical data, the NiO sample exhibits a band gap at 3.11 eV, which does agree well with earlier reported values. These studies may be of importance for the application of this material in energy efficient surface coating devices.

Resistance switching characteristics in Li-doped NiO

We investigated the effects of lithium (Li)-doping on bi-stable resistance switching in polycrystalline NiO film in the temperature range of 10 K&amp;lt;T&amp;lt;300 K. Compliance-dependent resistive switching transport revealed some distinctive and interesting features not observed in undoped NiO films previously studied. An analysis of the temperature dependence of the resistive switching transport showed that Li-doping could modify the thermal properties of the off-state leading to a stable on/off switching operation. It is clearly shown that doping Li in NiO can improve NiO’s retention properties and stability of on/off switching voltages.

Fabrication and characterization of In–Ga–Zn–O/NiO structures

Undoped and Li-doped NiO films have been prepared by radio frequency sputtering and electron-beam evaporation techniques. These sputtered films prepared at a lower pressure of 2.6 × 10 − 1 Pa show no diffraction peaks and their surfaces are smooth. These films are p-type semiconductors and their resistivity is dependent on an O 2-partial pressure. A minimum resistivity of 0.39 Ω cm is attained for a Li-doped NiO film prepared at RF power of 20 W in 50% O 2 atmosphere. In contrast, Li-doped NiO films prepared by electron-beam evaporation exhibit no diffraction peaks, but their surface is not smooth. The fabrication of pn-junction is carried out between NiO films and amorphous n-type transparent oxide films, e.g., In–Zn–O and In–Ga–Zn–O. No rectification characteristic is seen at an interface of In–Zn–O and Li-doped NiO films, while the rectification characteristic is achieved in In–Zn–O/In–Ga–Zn–O/NiO/Au device. The threshold voltage of 2.3 V is estimated from the current vs. voltage characteristic.

Properties of nickel oxide thin films deposited by RF reactive magnetron sputtering

The NiO films were deposited by RF reactive magnetron sputtering from a Ni target in a mixture of oxygen and argon gases onto heated Si and Corning 7059 glass substrates. The influences of process parameters including RF power, O 2/Ar ratio, and substrate temperature on the film properties such as O/Ni atomic ratio, crystallographic structure, preferred orientation, transmittance and resistivity were then investigated. The results show that deposition rate is an approximately linear function of RF power. Only the (200) diffraction peak is found in the XRD patterns of the deposited NiO films. The values of O/Ni atomic ratio in the thin films are between 1.87 and 1.80. It decreases as the substrate temperature is increased. The transmittances of undoped NiO films show a strong dependence on the O 2/Ar ratio. With increasing O 2/Ar ratio, the transmittance decays. The film resistivity increases with increased substrate temperature. The conduction in nickel oxide is due to the presence of Ni +3 ions, which are formed due to nickel vacancies in the films.

Electron and hole doping in NiO

We have investigated hole doped (by lithium) and electron-doped (by nickel metal) NiO with photoemission (PES), inverse photoemission (IPES) and low and high energy electron energy loss spectroscopy (EELS). Both types of doping create empty states approximately in the middle of the charge transfer gap of undoped NiO.

Die entscheidende Mitwirkung der Metall- und Sauerstoffionen von Nickeloxid beim Ablauf der Oxidation von o-Xylol

The Decisive Cooperation of Metal and Oxygen Ions of Nickel Oxide During the Oxidation of o-Xylene As can be concluded from the experimental results at 450°C in the reaction mixtures consisting of N2O2-o-Xylene, both nickel oxide pure and doped with Li2O or In2O3 is unsuited as catalyst for the oxidation of o-xylene to phthalic anhydride. In contrast to NiO which ionosorbs both oxygen and o-xylene, NiOLi2O, a strong ionosorbent for o-xylene, prevents the ionosorption of oxygen because of the large concentration of holes. Since gaseous oxygen does not react with ionosorbed o-xylene but a reduction of nickel oxide to metallic nickel has been observed in spite of the fact of enough oxygen in the gas phase it can be assumed that o-xylene is forced to remove oxygen ions from the NiO lattice under generation of oxygen-ion vacanxies and nickel atoms. The predominant portions of the reaction products are H2O and CO2. With undoped nickel oxide and NiOIn2O3 which were not reduced under the same experimental conditions, the reaction products had roughly the same composition. The reduction of NiOLi2O however will be prevented in a gas mixture with a high oxygen pressure which oxidizes the formed nickel atoms on the surface of NiOLi2O to nickel oxide making possible the entrance of oxygen from the gas phase and, therefore, the oxidation of o-xylene. A turbulent motion of a 2-component catalyst powder from NiO-1mole% Li2O covered with ionosorbed o-xylene and ZnO-1mole% In2O3 covered with ionosorbed oxygen in the same gas mixture resulted in the same reaction products as in the presence of sole NiOLi2O under simultaneous reduction of nickel oxide. From that we can conclude that the oxidation of o-xylene by oxygen ions of NiO occurs more easily than the reaction with ionosorbed oxygen on ZnOIn2O3 which obviously seems to be bounded too strongly. This result is also confirmed by the prevention of the oxidation of gaseous o-xylene in the presence of only ZnOIn2O3. Finally, the operation of the carrier catalyst V2O5/TiO2 which is employed for the oxidation of o-xylene to phthalic anhydride will be prevented to a large extent in simultaneous presence of nickel oxide either pure or doped with Li2O and In2O3 in roughly the same amount. This result can be mentioned as a proof for the interaction of a 2-component catalyst the mechanism of which is at present not satisfactorily understood.

Sintering of LixMi1?xO solid solutions at 1200�C

The sintering process of Li x Mi1−x O solid solutions, with differentx values, was examined. Sintering was performed in air at 1200°C. An intense shrinkage in lithiated nickel oxide, with respect to the undoped NiO, was observed. This behaviour is attributed to the existence of a gradient in the chemical potential of the lithium ion between the surface and the interior of the particles, due to lithium oxide evaporation, which acts as the driving force of the mass transport by volume diffusion.