Single Crystalline NiO Research Articles

Regulating Lattice Oxygen on the Surfaces of Porous Single-Crystalline NiO for Stabilized and Enhanced CO Oxidation

The long-range ordered lattice structure and interconnected porous microstructure of porous single crystals (PSCs) provide structural regularity and connectivity in remote electron movement to stabilize oxygen vacancies and activate lattice oxygen linked to surface active sites. In this work, we prepare NiO powder, single-crystal (SC) NiO, and PSC NiO. NiO contains a significant amount of oxygen vacancies. We find that the structure of porous NiO can create more oxygen vacancies. We load Pt onto these NiO crystals by atomic layer deposition (ALD) to activate lattice oxygen on definite NiO surfaces. The results show that Pt-loaded NiO effectively exhibits CO oxidation performance, in which Pt-loaded PSC NiO completely oxidizes CO at 65 °C. With 1% CO fully adsorbed, the density of activate lattice oxygen becomes an essential factor affecting performance. PSC NiO with deposited Pt clusters exhibited stable CO oxidation catalysis when run in air at ~65 °C for 300 h.

Long-distance spin current transmission in single-crystalline NiO thin films

Spin current transmission in antiferromagnets is one of the most interesting focuses in antiferromagnetic spintronics. Its study may provide access to spin superfluid physics, which gives rise to metastable spin supercurrents. In this work, we investigate the spin current transmission in a single-crystalline NiO (111) and NiO (001) of the NiO/Fe20Ni80 and Pt/NiO/Fe20Ni80 epitaxial films by the spin-pumping enhanced damping of FeNi. The spin transmission length is estimated to be = 50 ± 4 nm and for NiO (111) and NiO (001), respectively. The results are succinctly discussed in the framework of the spin superfluidity mechanism.

Emergent Spin Glass Behavior Created by Self-Assembled Antiferromagnetic NiO Columns in Ferrimagnetic NiFe2O4.

Spin glass (SG) is a magnetic state with spin structure incommensurate with lattice and charge. Fundamental understanding of its behavior has a profound impact on many technological problems. Here, we present a novel case of interface-induced spin glass behavior via self-assembly of single-crystalline NiO microcolumns in a single-crystalline NiFe2O4 matrix. Scanning transmission electron microscopy indicates that the hexagonal-shaped NiO columns are along their [211] direction and oriented along the [111] direction of the NiFe2O4 matrix. Magnetic force microscopy reveals magnetic anisotropy between NiO columns (antiferromagnetic transition temperature TN ∼ 523 K) and NiFe2O4 matrix (ferrimagnetic transition temperature TFI ∼ 860 K). This leads to spin disorder/frustration at atomically sharp NiFe2O4/NiO interfaces responsible for spin glass behavior below TSG ∼ 28 K. Our results demonstrate that self-assembly of magnetically distinct microstructures into another crystalline and magnetically ordered matrix is an effective way to create novel spin states at interfaces.

Characterization of the soft X-ray spectrometer PEAXIS at BESSY II.

The performance of the recently commissioned spectrometer PEAXIS for resonant inelastic soft X-ray scattering (RIXS) and X-ray photoelectron spectroscopy and its hosting beamline U41-PEAXIS at the BESSY II synchrotron are characterized. The beamline provides linearly polarized light from 180 eV to 1600 eV allowing for RIXS measurements in the range 200-1200 eV. The monochromator optics can be operated in different configurations to provide either high flux with up to 1012 photons s-1 within the focal spot at the sample or high energy resolution with a full width at half maximum of <40 meV at an incident photon energy of ∼400 eV. The measured total energy resolution of the RIXS spectrometer is in very good agreement with theoretically predicted values obtained by ray-tracing simulations. PEAXIS features a 5 m-long RIXS spectrometer arm that can be continuously rotated about the sample position by 106° within the horizontal photon scattering plane, thus enabling the study of momentum-transfer-dependent excitations. Selected scientific examples are presented to demonstrate the instrument capabilities, including measurements of excitations in single-crystalline NiO and in liquid acetone employing a fluid cell sample manipulator. Planned upgrades of the beamline and the RIXS spectrometer to further increase the energy resolution to ∼100 meV at 1000 eV incident photon energy are discussed.

Unconventional mesoporous single crystalline NiO by synergistically controlled evaporation and hydrolysis

Mesoporous single-crystal NiO was synthesized by controlling metal ion hydrolysis and solvent evaporation rates, exhibiting excellent electrochemical activities.

Tailoring Nucleation at Two Interfaces Enables Single Crystalline NiO Nanowires via Vapor-Liquid-Solid Route.

Here we show a rational strategy to fabricate single crystalline NiO nanowires via a vapor-liquid-solid (VLS) route, which essentially allows us to tailor the diameter and the spatial position. Our strategy is based on the suppression of the nucleation at vapor-solid (VS) interface, which promotes nucleation only at the liquid-solid (LS) interface. Manipulating both the supplied material fluxes (oxygen and metal) and the growth temperature enables enhancement of the nucleation only at the LS interface. Furthermore, this strategy allows us to reduce the growth temperature of single crystalline NiO nanowires down to 550 °C, which is the lowest growth temperature so far reported.

Effect of Grain-Boundaries in NiO Nanosheet Layers Room-Temperature Sensing Mechanism under NO2

The discovery of gas sensing properties of single-crystalline nanostructures comparable or even better than their polycrystalline counterparts has triggered the attention of the gas sensor research community. To eleborate the sensing mechanisms of single-crystalline and polycrystalline nanostructures, the single-crystalline (SC) NiO hexagonal nanosheet and the nanoparticle self-assembled polycrystalline (PC) NiO nanosheet were prepared for room-temperature NO2 sensing studies. The gas sensing studies revealed that the SC NiO exhibited a remarkably higher response to NO2 at room temperaure than the PC NiO. The correlation between the structural features of NiO and the room temperature NO2 sensing performances was discussed in detail via the grain boundary scattering theory. It was found that the scattering potential played a vital role in the sensing process. Because the absence of grain boundary in the SC NiO reduced the NO2 chemisorption-induced carrier scattering at interfaces, the SC NiO performed the ...

(Invited) Structure and Stability of Pt-Y Alloy Particles for Oxygen Reduction Studied by Electron Microscopy

Nanostructured catalysts play an increasingly large role in contemporary society and understanding their behavior in their working state has been primary area of focus for many research groups over the past decades. With dwindling supplies of the precious metals often used in these catalysts, their effective use becomes increasingly important. Recent developments in electron microscopy as well as computational modelling now allow us to gain new insight into the structure and stability of these catalysts in an environment resembling their working conditions. Such insights will enable scientists to effectively synthesize stable nanostructures with an increased performance and lifetime. For heterogeneous catalysts in the form of supported metal nanoparticles, surfaces play a crucial role. This is valid for both electrocatalysts as well as traditional heterogeneous catalysts. Reactions take place on the surfaces exposed to the reactants while the interface between the nanoparticles and the support maintains the high dispersion by preventing them from sintering. Studying such systems using state-of-the-art electron microscopy techniques can provide new knowledge of their working state and what processes govern their performance and stability. Aberration corrected transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) combined with energy dispersive X-ray spectroscopy (EDS) can help unravel the structure of alloy materials. Specifically, PtxY and PtxGd alloy nanoparticles were studied. These catalysts are used for the oxygen reduction reaction (ORR) and show an activity which is more than 5 times higher compared to the best commercial Pt-based catalyst. Furthermore, these catalysts both retain more than 60% of their activity after accelerated aging tests. The electron microscopy investigations reveal the elemental distribution within the nanoparticles showing a core-shell type structure after ORR treatment. Figure 1 shows high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images of platinum-yttrium nanoparticles in the form of core-shell structures. A core-shell structure is formed after the ORR chemical treatment, with an alloyed core embedded in a 1nm Pt-rich shell, due to the segregation of the Y from the first few atomic layers of the particle and its dissolution into the electrolyte. The Pt-rich shell prevents further dissolution of the rare earth metal protecting the alloyed core. Using Density Functional Theory (DFT), Pt-Hg and Pd-Hg were identified as promising candidates for the electrochemical production of hydrogen peroxide H2O2. The active surface was predicted to consist of reactive Pt or Pd atoms surrounded by more inert Hg atoms. Electrochemical measurements on the two catalysts have shown a performance exceeding the current state-of-the-art. Using Electron microscopy the structure and composition of the nanoparticle alloys has been determined and it confirms the theoretical predictions. In Pt-Hg nanoparticles, EDS studies has indicated the presence of both elements on single particles, although high-resolution imaging has shown particles where pure Pt was the only matching structure. In the case of Pd-Hg, a core-shell structure was found, with a pure Pd core and a Pd-Hg shell. Through atomic resolution STEM, the structure of the alloy in the shell of different particles was revealed, showing the formation of an ordered alloy structure. Studying deactivation mechanisms is an important discipline in catalysis. Environmental transmission electron microscopy (ETEM) provides a unique platform where catalysts can be studied in situ in a reactive environment. Using ETEM, the structure and morphology of catalysts and how the structures behave during deactivation can be investigated in an environment resembling that in a catalytic reactor in real time. Here the sintering of nanoparticles were studied. Model catalytic systems consisting of supported nickel, palladium, gold and platinum nanoparticles were studied in situ in the microscope and time-lapsed image sequences recorded. Analysis of these image sequences reveal that both particle migration and Ostwald ripening can take place. These studies reveal the underlying mechanisms governing the sintering process. The technique was also used to study the activation and deactivation processes of Ni-YSZ cermets used in solid oxide fuel cells where we have shown the reduction of single crystalline NiO to metallic Ni followed by the reoxidation to polycrystalline NiO. Electron microscopy provides a versatile platform for investigations of catalysts. Using various EM techniques, new insight into the active phase and deactivation processes of catalysts can be obtained. Figure 1

Effect of Ni2+ and O2− vacancies on the electrical and optical properties of nanostructured nickel oxide synthesized through a facile chemical route

Nanostructured nickel oxide samples were synthesized through a novel chemical route using nickel chloride and ethanol amine. The DC electrical conductivity of the as prepared sample is found to be enhanced by 3–6 orders of magnitude in comparison to single crystalline NiO. The conductivity increases with increase in the annealing temperature. The dominant conduction mechanism is found to be that due to large polarons associated with holes in the O2p band. Study of DC conductivity in vacuum and air indicate that the oxygen vacancies are filled up on annealing thereby causing an increase in the concentration of uncompensated Ni2+ concentration. The relative concentration of Ni2+ and O2− vacancies influence the optical absorption also. It is observed that the presence of O2− vacancies inhibit the low energy (2–4eV) intraionic 3d8–3d8⁎ transitions while the increase in the percentage of uncompensated Ni2+ vacancies enhances these transitions. The PL emission in the UV range (2.4–3.4eV) is unaffected by the O2− vacancy concentration while 490nm emission due to O2− vacancies decreases with increase in annealing temperature.

Ultrafast Synthesis of Yolk-Shell and Cubic NiO Nanopowders and Application in Lithium Ion Batteries

A continuous one-pot method was employed to synthesize yolk-shell and single-crystalline cubic NiO powders in a few seconds. Submicrometer-sized NiO yolk-shell particles were prepared by spray pyrolysis at 900 °C. Single-crystalline cubic NiO nanopowders were prepared by one-pot flame spray pyrolysis from NiO vapors. Particle surface areas of the yolk-shell and single-crystalline cubic NiO powders as obtained using the Brunauer-Emmett-Teller method were 8 and 5 m(2) g(-1), respectively. The mean crystallite sizes of the yolk-shell-structured and cubic NiO powders were 50 and 80 nm, respectively. The yolk-shell and single-crystalline cubic NiO powders delivered discharge capacities of 951 and 416 mA h g(-1), respectively, after 150 cycles, and the corresponding capacity retentions measured after the first cycle were 106 and 66%, respectively. The yolk-shell-structured NiO powders showed rate performance better than that of the single-crystalline cubic NiO nanopowders. Even at a high current density of 1 A g(-1), the discharge capacity of the yolk-shell-structured NiO powders was as high as 824 mA h g(-1) after 50 cycles, in which the current densities were increased stepwise.

Facile synthesis of single-crystalline NiO nanosheet arrays on Ni foam for high-performance supercapacitors

Single-crystalline NiO nanosheet arrays on Ni foam as a binder-free electrode for high-performance supercapacitors, fabricated by a facile and cost-effective hydrothermal approach, exhibit a high specific capacitance, good rate capability, and excellent cycling stability.

Preparation and supercapacitor application of the single crystal nickel hydroxide and oxide nanosheets

The single crystalline Ni(OH)2 nanosheets were synthesized by a simple chemical precipitation method using nickel chloride as precursors and ammonia as precipitating agent. The Ni(OH)2 nanosheets were successfully converted to NiO nanosheets via calcination under appropriate conditions. Analytical methods such as X-ray diffraction (XRD) spectra, scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and Fourier transformed infrared (FTIR) spectra were employed to characterize the morphology and microstructure of the final products. The experimental results revealed that Ni(OH)2 nanosheets were shape-preserved transformed to NiO nanosheets at 250°C for 24h. Ni(OH)2 and NiO nanosheets were directly functionalized as supercapacitor electrodes for potential energy storage applications, whose charge–discharge properties, electrochemical impedance spectra, cyclic voltammetry, and cycle performance were examined. The experimental results show that the single-crystalline NiO nanosheets are a promising candidate for the supercapacitor electrode. They exhibit a high specific capacitance, prompt charge/discharge rate.

High-quality single crystalline NiO with twin phases grown on sapphire substrate by metalorganic vapor phase epitaxy

High-quality single crystalline twin phase NiO grown on sapphire substrates by metalorganic vapor phase epitaxy is reported. X-ray rocking curve analysis of NiO films grown at different temperatures indicates a minimum full width at half maximum of the cubic (111) diffraction peak of 0.107° for NiO film grown at as low as 550 °C. Detailed microstructural analysis by Φ scan X-ray diffraction and transmission electron microscopy reveal that the NiO film consists of large single crystalline domains with two different crystallographic orientations which are rotated relative to each other along the [111] axis by 60°. These single crystal domains are divided by the twin phase boundaries.

Synthesis and photocatalytic activity of monodisperse single crystalline NiO octahedrons by the selective adsorption of Cl− ions

Monodisperse NiO octahedrons have been synthesized by heating NiCl2 solutions on the Si substrates at 400–500°C for 1h. The NiO octahedrons are single-crystalline with the body-centered cubic structure and have controllable sizes in the range of 166–257nm. Control over the size of NiO octahedrons was achieved by adjusting the heating temperature. The formation of NiO octahedrons results from the selective adsorption of Cl– ions on unsaturated Ni(II) sites of NiO (111) surface. The NiO octahedrons with exposed {111} facets show higher intrinsic photocatalytic activity than Degussa P25 TiO2, and the photocatalytic activity was enhanced with a decrease on the size of NiO octahedrons.

An amperometric glucose biosensor based on the immobilization of glucose oxidase on the platinum electrode modified with NiO doped ZnO nanorods

A novel glucose biosensor was fabricated by immobilizing glucose oxidase (GOD) on a platinum electrode modified with NiO doped ZnO nanorods (NRs), which were adopted to construct an amperometric glucose biosensor for the first time. ZnO single-crystalline nanorod and NiO doped ZnO nanorods are prepared by hydrothermal method and characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The electrochemical behavior of the platinum electrode modified with NiO doped ZnO nanorod was investigated in phosphate buffer solution (PBS). The effect of molar ratio of Ni/Zn in the doped ZnO nanorod, pH value, applied potential, temperature was investigated in detail. Under the optimal experimental condition, the constructed glucose biosensor showed a good linearly relationship between the concentration of glucose and the electrochemical current, a high sensitivity of 61.78μA/mMcm2, and a fast response time of <5s. All results demonstrate that the NiO doped ZnO NRs can provide a new platform for glucose biosensor design and other biological applications.

Improved electrochemical properties of single crystalline NiO nanoflakes for lithium storage and oxygen electroreduction

A facile strategy has been developed to realize the controllable synthesis of single crystalline NiO nanoflakes. According to the SEM, TEM and BET analysis, it was found that the m-NiO nanoflakes have a hexagonal structure with an average pore diameter of 4.4 nm, and exhibit a high surface area of 113.4 m2 g−1 and a pore volume around 0.142 cm3 g−1. The m-NiO nanoflakes exhibit a significantly improved electrochemical performance as an anode of lithium-ion batteries (LIBs) compared with bulk NiO based on cyclic voltammograms and galvanostatic measurement. Additionally, the m-NiO nanoflakes showed remarkable advanced activity for catalyzing oxygen reduction reaction (ORR ) relative to the bulk NiO electrode and the bare electrode. Further, rotating disk electrodes demonstrated that the m-NiO nanoflakes, as the support of the Pt nanoparticles, shows significantly improved activity for ORRs.

Aligned NiO nanoflake arrays grown on copper as high capacity lithium-ion battery anodes

Transition metal oxides are promising candidates for lithium-ion battery electrodes, while their performances are generally limited by their poor electrical conductivity and cycling stability. In this paper, we report the growth of aligned, single-crystalline NiO nanoflake arrays directly on copper substrates by a modified hydrothermal synthesis and post-annealing. The close contact of NiO nanoflakes on a current collector (e.g. Cu) allows for efficient charge transport, and waives the need for adding ancillary conducting materials or binders. In addition, the mesopores inside the NiO nanoflakes and the spacing between the adjacent aligned nanoflakes provide efficient ion transport pathways as well as sufficient flexibility for electrode volume expansion. As proof-of-concept, anodes made of NiO nanoflakes directly grown on Cu showed a high capacity and excellent cycling stability. The capacity was retained at 720 mA h g−1 over 20 cycles at a current density of 100 mA g−1, almost equal to the theoretical value of NiO and much higher than the NiO products formed in the same growth solution. Even at a high discharge–charge rate of 1 A g−1 (1.5 C), the NiO nanoflakes grown on Cu were capable of retaining a capacity of 500 mA h g−1 over 40 cycles. Our report suggests that NiO nanoflakes may serve as a promising anode material for a high-power lithium-ion battery.

Synthesis and field emission properties of nickel oxide nanoplatelets prepared by simple novel technique

Single crystalline NiO nanoplatelets were successfully synthesized by new facile method at 200 °C. The morphology and microstructure were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD measurement indicated that the prepared sample had typical cubic structures. The SEM investigation confirmed that the product was of the form of nanoplatelets. These nanoplatelets have average width length 250 nm and thickness of 20 nm. The field emission measurements demonstrate that the NiO nanoplatelets show a promising field emission property. The improved field emission is attributed to the local field enhancement factor at the nanoplatelets. The results confirm the importance of the morphology of nanomaterial in field emission.

Single-crystalline Ni(OH)2 and NiO nanoplatelet arrays as supercapacitor electrodes

Vertically aligned Ni(OH)2 and NiO single-crystalline nanoplatelet arrays were directly grown on the fluorine-doped tin oxide (FTO) substrate by a simple hydrothermal method. The effects of the hydrothermal parameters on the morphology and crystal structure of the nanoarray film were investigated. Controlling the ammonia and persulfate concentrations was the key to controlling the morphology of the nanoarray film. The experimental results showed that the single-crystalline NiO nanoplatelet array was a promising candidate for the supercapacitor electrode. It exhibited a high specific capacitance, prompt charge/discharge rate, and good stability of cycling performance. It is believed that the vertically oriented aligned single-crystalline NiO nanoplatelet array is beneficial to the charge transfer in the electrode and to the ion transport in the solution during redox reaction.

A Template and Catalyst-Free Metal-Etching-Oxidation Method to Synthesize Aligned Oxide Nanowire Arrays: NiO as an Example

Although NiO is one of the canonical functional binary oxides, there has been no report so far on the effective fabrication of aligned single crystalline NiO nanowire arrays. Here we report a novel vapor-based metal-etching-oxidation method to synthesize high-quality NiO nanowire arrays with good vertical alignment and morphology control. In this method, Ni foils are used as both the substrates and the nickel source; NiCl(2) powder serves as the additional Ni source and provides Cl(2) to initiate mild etching. No template is deliberately employed; instead a nanograined NiO scale formed on the NiO foil guides the vapor infiltration and assists the self-assembled growth of NiO nanowires via a novel process comprising simultaneous Cl(2) etching and gentle oxidation. Furthermore, using CoO nanowires and Co-doped NiO as examples, we show that this general method can be employed to produce nanowires of other oxides as well as the doped counterparts.