NiO Nanocrystals Research Articles

Lithium doped nickel oxide nanocrystals with a tuned electronic structure for oxygen evolution reaction.

The electronic structure of catalysts influences the electrocatalytic behavior. Herein, the electronic structure of NiO nanocrystals is modified by doping with a small amount of Li+. The Li+ doped NiO nanocrystals show much better OER activity than pristine NiO and LiNiO2, which is a result of tuned Fermi levels, stronger hybridization between Ni 3d and O 2p, and narrowed energy band gap. This work provides a facile strategy to regulate NiO electronic energy bands to promote OER performance.

NiO nanocrystals encapsulated into a nitrogen-doped porous carbon matrix as highly stable Li-ion battery anodes

Though the carbon matrix is extensively investigated to enhance the electrochemical performances of metal oxides for lithium-ion batteries (LIBs), it is still difficult to expose the whole active sites in the electrode material to participate in electrochemical reactions. Herein, we report a facile MOF-derived strategy for the controlled synthesis of a hybrid structure with NiO nanocrystals uniformly distributed into nitrogen-doped porous carbon matrix (NiO@N–C). In this strategy, the simultaneously generated nitrogen-doped porous carbon matrix can confine the further growth of the resulting NiO nanocrystals during the carbonization process, in which the ultrafine particle sizes of NiO can shorten ion diffusion length and provide abundant exposed active sites for lithium-storage, while the carbon matrix can function as a buffer layer to alleviate the volume expansion. Benefiting from the advantages of the structural features and porous carbon matrix, NiO@N–C delivers a large reversible capacity of 1373 mAh g−1 at 100 mA g−1 after 200 cycles and a remarkable cycling stability up to 1000 cycles at 1 A g−1 with a capacity of 877 mAh g−1, suggesting its potential as an anode material for lithium-ion batteries.

Characterization and catalytic activity of soft-templated NiO-CeO2 mixed oxides for CO and CO2 co-methanation

Nanosized NiO, CeO2 and NiO-CeO2 mixed oxides with different Ni/Ce molar ratios were prepared by the soft template method. All the samples were characterized by different techniques as to their chemical composition, structure, morphology and texture. On the catalysts submitted to the same reduction pretreatment adopted for the activity tests the surface basic properties and specific metal surface area were also determined. NiO and CeO2 nanocrystals of about 4 nm in size were obtained, regardless of the Ni/Ce molar ratio. The Raman and X-ray photoelectron spectroscopy results proved the formation of defective sites at the NiO-CeO2 interface, where Ni species are in strong interaction with the support. The microcalorimetric and Fourier transform infrared analyses of the reduced samples highlighted that, unlike metallic nickel, CeO2 is able to effectively adsorb CO2, forming carbonates and hydrogen carbonates. After reduction in H2 at 400 °C for 1 h, the catalytic performance was studied in the CO and CO2 co-methanation reaction. Catalytic tests were performed at atmospheric pressure and 300 °C, using CO/CO2/H2 molar compositions of 1/1/7 or 1/1/5, and space velocities equal to 72000 or 450000 cm3·h−1·gcat−1. Whereas CO was almost completely hydrogenated in any investigated experimental conditions, CO2 conversion was strongly affected by both the CO/CO2/H2 ratio and the space velocity. The faster and definitely preferred CO hydrogenation was explained in the light of the different mechanisms of CO and CO2 methanation. On a selected sample, the influence of the reaction temperature and of a higher number of space velocity values, as well as the stability, were also studied. Provided that the Ni content is optimized, the NiCe system investigated was very promising, being highly active for the COx co-methanation reaction in a wide range of operating conditions and stable (up to 50 h) also when submitted to thermal stress.

Synthesis of novel Cubane [Ni4(O∩O)4(OCH3)4(OOH)4] cluster: XRD/HSA-interactions, spectral, DNA-binding, docking and subsequent thermolysis to NiO nanocrystals

Under liquid-state vibration ultrasonic mode, the Ni4(O∩O)4(OCH3)4(OOH)4 complex [O∩O-bidentate ligand = 4,4,4-trifluoro-1-phenylbutane-1,3-dione] was synthesized from metal precursor and ligand in 10% H2O2/MeOH media, as a novel [Ni4(μ3-O)4] cubane cluster. The XRD-structure of the desired cluster was unraveled that each Ni(II) metal center possesses a distorted octahedral geometry. Monitored thermal treatment to the tetranuclear metal cluster in an open atmosphere afforded nanocrystalline rod-shaped nickel oxide. Moreover, CHN-EA, FT-IR, conductivity measurement, UV–Vis., EDX, SEM, TG/DTA, and PXRD have confirmed the structure of the new cluster and NiO nanocrystals. The XRD-lattice interactions were explored to evaluate their role via computational Hirshfeld surface analysis. The molecular docking of the cluster with SS-DNA was estimated and demonstrated.

Effect of the Oxidation Degree of a Nickel Foil Surface on Its Catalytic Activity in the Reaction of Ethylene Oxidation

The effect of the oxidation degree of a nickel foil surface on the rate of catalytic oxidation of ethylene was studied by a pulse method at 600 and 700°C. It was shown that a reduced metallic surface demonstrated a high activity in partial ethylene oxidation, whereas a partially oxidized surface with an oxidation degree of ~24 formal O2 monolayers, in the total oxidation of C2H4. A SEM investigation has revealed that the oxidized surface was partially coated with nickel oxide nanocrystals. A further increase in the surface oxidation degree led to a continuous coverage of the Ni surface with oxide crystals and a dramatic decrease of catalytic activity. In addition, a low maximum of total ethylene oxidation was observed at 700°C in the range of surface oxidation degree of 95–135 O2 monolayers.

Improved electroluminescence performance of quantum dot light-emitting diodes: A promising hole injection layer of Fe-doped NiO nanocrystals

Abstract Fe-doped NiO nanocrystals (NCs) were prepared by a simple solvothermal route and applied as a hole injection layer (HIL) in quantum dots light-emitting diodes QLEDs. The transmittance of the film is more than 82%. The resulting devices show pure QD electroluminescent emissions with a maximum EL brightness of 27624 cd m−2 after doping 5% mol Fe in NiO, due to the enhanced carrier concentration and conductivity. The external quantum efficiency (EQE), current efficiency (CA) and lifetime of the QLED device showed the maximum values of 3.84%, 5.93 cd A−1 and 11490 h, respectively.

Defect induced ferromagnetism in NiO nanocrystals: Insight from experimental and DFT+U study

We investigate the defects induced magnetic properties of NiO nanocrystals prepared by mechanical alloying ball milling process. The average crystallite size reduces to ∼12nm after 40 hours of milling but the face centered cubic structure is stable both in pure and milled NiO powders. The unmilled pure NiO is anti-ferromagnetic in nature, whereas the as-milled nanoparticles are ferromagnetic at room temperature with saturation magnetization reaching a maximum of 0.85 emu/g. X-ray photoelectrons spectroscopy (XPS) shows two distinct peaks at 854.2 and 856.1 eV specifically related to the typical Ni-O bonds from Ni2+ and Ni3+ respectively. This is clearly supports the presence of non-stoichiometry in the milled samples due to decrease in Ni2+-O2−-Ni2+ spin correlation length caused by defects. These defects and crystallinity are characterized by Raman spectra which shows that the two-magnon band disappears, one-phonon (1P) longitudinal optical (LO) band broadens, two-phonon (2P) LO band exhibits red-shift and disappears eventually with decreasing of particle size. Magnetic hysteresis (M-H) loops reveals the ferro-magnetic nature of NiO nanocrystals. Considering these defects, we have also investigated the electronic and magnetic properties systematically within the framework of density functional theory. Our results suggest that the vacancies created by only oxygen as well as both nickel and oxygen are largely responsible for ferro-magnetism in nanoscale NiO powders.

Study of the electroluminescence performance of NiO-based quantum dot light-emitting diodes: The effect of annealing atmosphere

NiO is fabricated by a simple solvothermal reaction using tert-butyl alcohol and nickel acetylacetonate as precursors at 200 °C for 20 h. NiO nanocrystals were dispersed in ethanol followed by a spinning coating technique on indium tin oxide (ITO) substrates to form NiO coating as a hole injection layer in quantum dot light-emitting diodes (QLEDs). NiO coating is annealed in ultraviolet ozone, air and nitrogen atmosphere, respectively. NiO coating is analyzed by ways of atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS) and Hall effect measurements. Electroluminescence performance of NiO-based QLEDs is enhanced after ultraviolet ozone treatment due to the increased conductivity of NiO coating.

Enhanced catechol biosensing on metal oxide nanocrystal sensitized graphite nanoelectrodes through preferential molecular adsorption

Amperometric biosensors offer a viable platform for phenol trace level detection in water with potential for miniaturization and automation, towards a cost-effective analytical monitor. In this work, we rationally designed and computationally analysed two laccase biosensors based on NiO(100) and α-Fe2O3(110) nanocrystals (NCs) sensitized carbon paste electrodes (CPEs) for selective electrochemical detection of catechol. Investigation of nanoelectrode surface for catechol sensing is advantageous to design efficient detection platforms for analytes in water. Incorporation of metal-oxide NCs in more common CPEs enabled reduction of their overpotential and improved the sensitivity of detection. Excellent dispersion of 11 nm-sized NiO NCs in CPE was obtained compared to cluster type α-Fe2O3 NCs with effective Randles-Sevcik surface area being 0.089 and 0.051 cm2 respectively, as against 0.027 cm2 for bare CPE. We performed adsorption energy calculations of different aminoacids on prevalent oxide NC surfaces that showed preferred molecular arrangements and stronger binding for NiO(100) plane amidst competitive adsorption by water, enabling shorter electron transfer distance for reversible catechol oxidation on the electrode surface, and thereby its faster detection. Our electrocatalytic studies also pointed to additional catechol oxidation facilitation through co-operative redox couples of Cu(I)/Cu(II) and Ni(II)/Ni(III) on laccase-CPE-NiO(100) electrode. The biosensor response reached steady state within 6 s, one of the shortest reported in literature with low detection limit of 0.95 μM and sensitivity of 1.415 A M−1 cm−2. From the different performance studies and the real sample analyses, NiO NC modified CPE could be proposed as a competent material for catechol biosensing.

Role of magnetism on transition metal oxide surfaces in vacuum and solvent

The transition metal oxides MnO, FeO, NiO, and CoO are essential materials systems for catalysis applications and energy technologies. These materials exhibit a magnetic phase transition from paramagnetism to antiferromagnetism upon cooling. In this work, we show that an accurate treatment of the surfaces requires a description of the disordered local magnetic moments of the paramagnetic phase. We determine how the magnetic phase transition and the presence of solvent affect the surface energies using density-functional theory and a solvation model. To accurately account for the correlations in the $d$-electron system, and to match the observed magnetic order and band gaps of the room-temperature phases, we include the Hubbard-$U$ correction. In the case of MnO, FeO, and CoO, which are paramagnetic semiconductors or insulators at room temperature, we demonstrate the importance of the local magnetic moments and model the materials using special quasirandom structures. To determine the equilibrium shape of MnO, FeO, NiO, and CoO nanocrystals, we calculate the surface energies of their low-energy (100), (110), and (111) facets and perform the Wulff construction. For the (111) facet, we consider various reconstructions that remove the polar nature of the unreconstructed surface. The processing conditions of these oxide nanoparticles, in most cases, involve a solvent. To analyze the influence of the solvent environment on the surface energies of different facets and thereby the crystal shape, we calculate the surface energies of these oxides in water using the continuum solvation model VASPsol. We find that the surface energies decrease due to the dielectric screening. However, as the ratio of surface energies remains sufficiently similar, we predict that the equilibrium crystal shape is only weakly affected by the presence of the solvent.

Self‐Assembled NiO Nanocrystal Arrays as Memristive Elements

AbstractMemristive switching, a nonlinear current–voltage (I–V) characteristic, has seen a tremendous surge in interest as an approach to achieve implementation of synaptic functions. The memristive switching behavior of self‐assembled NiO nanocrystals is investigated via scanning probe microscopy, based on first‐order reversal curve current–voltage spectroscopy. Synaptic switching is clearly observed as a direct consequence of filament growth (i.e., gradually increased conductance) in the nanocrystals. A spatial dependency of the conduction in the nanocrystals suggests that there is a localization of the switching filament. The current understanding of this localization ignores features related to local lateral variation current, which can generate an excessive local heat and temperature such as electrical dissipation. The observation of low electrical dissipation at the edge of the nanocrystals shows that less energy is wasted as heat such that the bias applied can be utilized more efficiently to assist the nucleation of the filament and thus reduces the power consumption. Electrical power dissipation is also found to scale with nanocrystal height and has spatial dependence within the nanocrystals. The combination of synaptic switching and high density of the nanocrystals demonstrate that it is feasible to exploit them to create a basic architecture for neuromorphic memory devices.

3 D NiO Nanowall Hole-Transporting Layer for the Passivation of Interfacial Contact in Inverted Perovskite Solar Cells.

Nickel oxide (NiO) materials with excellent stability and favorable energy bands are desirable candidates for hole-selective contact (HSC) of inverted perovskite solar cell (PSC). However, studies that focus on addressing interfacial issues, which are induced by the poor NiO/perovskite contact or other defects, are scarce. In this study, a facile one-step hydrothermal strategy is demonstrated for the development of a 3 D NiO nanowall (NW) film as a promising HSC. The new NiO NWs HSC exhibits a robust and homogenous mesoporous network structure, which improved the NiO/perovskite interface contact, passivated the interfacial defect and improved the quality of the perovskite film. The optimized interface features enabled a power conversion efficiency (PCE) approaching 18 %. A diethanolamine (DEA) interlayer was introduced to further passivate the intrinsic defect of the NiO surface, resulting in better charge transfer with suppressed recombination loss. As a result, the champion PCE of the NiO NWs/DEA-based device was increased to 19.16 % with a high open-circuit voltage (≈1.11 V) and fill factor (>80 %), which is prominent in methylammonium lead iodide-based inverted PSCs. Furthermore, the device exhibited better stability and lower hysteresis behavior than a conventional solution-based NiO nanocrystal device.

Direct Solvothermal Synthesis of Phase-Pure Colloidal NiO Nanocrystals

Nickel(II) oxide (NiO) nanocrystals have potential applications in catalysis and photocatalysis and as precursors for nanostructured thin films of NiO used as photocathodes in p-type dye-sensitized...

Synthesis and characterization of Na-doped NiO nanocrystals

In this study, we present the effect of doping of sodium (Na) in nickel oxide nanocrystals synthesized using the co-precipitation method. The samples are characterized using X-ray diffraction (XRD) and analyzed further with the help of Raman and Fourier transform infrared (FTIR) spectroscopic data. The XRD peaks, calculated crystallite size, dislocation density, strain and FTIR and Raman spectroscopic data is compared for the samples with the different doping concentrations. The crystal growth is explained on the basis of the recorded data. The experimental data is in good agreement mutually and, with the earlier reports.

Flexible NiO nanocrystal-based resistive memory device fabricated by low-temperature solution-process

In this study, a nickel oxide (NiO) nanocrystal (NC) based flexible resistive memory device is demonstrated at temperature as low as 180 °C by ligand exchange process. The fabricated device for flexible application with structure Ni/NiO/Ni on PI substrate exhibits excellent switching characteristics with low set/reset voltages and stable resistance values in both ON and OFF states for over 100 switching cycles of memory operation. Also, this flexible memory device shows stable resistive switching properties under compressive stress with bending radius to 10 mm and consecutive bending cycles. The ReRAM fabricated by a low-temperature solution-process shows potential for next generation flexible electronics.

Holey graphene confined hollow nickel oxide nanocrystals for lithium ion storage

Utilization of the Kirkendall effect, a composite of holey graphene confined hollow nickel oxide nanocrystals is designed and successfully fabricated using an in situ etching approach. The composite exhibits remarkable lithium storage performance including high reversible specific capacity, great cycling durability, and excellent rate capability. This work offers a promising innovative architecture to designing high-performance electrode materials for advanced lithium ion batteries.

Inverted planar perovskite solar cells featuring ligand-protecting colloidal NiO nanocrystals hole transport layer

Nickel oxide (NiO) is a promising material to replace organic hole transport materials and solve the stability problem of perovskite solar cells (PSCs). Various methodologies have been applied to synthesize high quality NiO, especially for colloidal nanocrystals (NCs) route, which well behaves in the nanocrystals synthesis field. To better introduce this route to the perovskite solar cell field and produce well-performance hole transport layers (HTLs) of NiO NCs, we got inspired by ligand-protecting strategy. The stability of NiO NCs in reaction system is improved significantly, and the nucleation and growth of NiO NCs can be controlled effectively, which makes for the high crystallinity. Furthermore, the extraordinary dispersion of NiO NCs in chlorobenzene solvent can be observed, which makes the preparation of HTL films by simple spin-coating method possible. The relevant measurements and analysis manifest that the HTLs of NiO NCs with excellent transparency, good film-forming performance, admirable hole transport capability and few interface traps can be obtained by optimizing the concentration of NiO NCs, finally contributing to the photovoltaic performance of corresponding inverted planar PSCs. Our work broadens the application of colloidal chemical synthetic route in PSCs field and highlights the superiority of this protocol further.

Enlarging Surface/Bulk Ratios of NiO Nanoparticles toward High Utilization and Rate Capability for Supercapacitors

AbstractReasonable design and delicate control of microstructures are critical to achieve high energy density of active materials for pseudocapacitors that seriously depend on usable reaction interface. This work shows the effect of ultrasmall particle size on enhancing utilization and rate performance of active materials. Three types of NiO nanocrystals with different sizes of 3.36, 6.24, and 7.18 nm in average diameter are uniformly distributed on mesoporous carbon nanosheets derived from corn straw piths. The nanosheets with 3.36 nm NiO particles present an extremely high NiO utilization of 93.4% (2404 F g−1 at 0.5 A g−1), which is 2–2.5‐fold higher than materials with large sizes (6.24 and 7.18 nm). This enhancement is ascribed to more complete conversion and higher ionic/electronic conductivity from a preferable surface/bulk ratio of NiO. By coupling with commercial activated carbon, the asymmetric supercapacitors present high energy and power densities (28.53 Wh kg−1 at 375 W kg−1), with 78.3% capacitance retention after 10 000 cycles at 10 A g−1.

A self-powered photoelectrochemical aptamer probe for oxytetracycline based on the use of a NiO nanocrystal/g-C3N4 heterojunction.

A self-powered photoelectrochemical (PEC) aptamer probe is presented for the determination of oxytetracycline (OTC). The assay is based on the use of g-C3N4 and NiO nanocrystals (NCs) which form a heterojunction. The latter was prepared by two-step hydrothermal pyrolysis by using the ionic liquid 1-hydroxyethyl-3-methylimidazole chloride which functions as a morphological template to form NiO NCs. The heterojunction exhibits much better electronic conductivity, wider absorption range, higher electron-hole-separation productivity, and stronger photocurrent compared to plain g-C3N4. The heterojunction was adopted to construct a self-powered PEC aptamer probe for OTC detection. An OTC-binding aptamer was immobilized on the heterojunction and the probe was constructed. The aptamer on the probe binding with OTC can form steric hindrance for transmitting of electrons and cause the PEC signal change depending on the OTC concentration. The photocurrent decreases with increasing OTC concentration in the 0.01 to 100nM concentration range and its detection limit is 4 pM (at S/N = 3). Graphical abstract Schematic representation of a self-powered photochemical aptamer probe. The probe performs enhanced ability for oxytetracycline (OTC) determination due to the formation of NiO nanocrystals/g-C3N4 (NiO NCs/g-C3N4) heterojunction and the specification recognition of the aptamer.

Self-assembled NiO microspheres for efficient inverted mesoscopic perovskite solar cells

Perovskite solar cells (PSCs) with both normal (n-i-p) and inverted (p-i-n) mesoscopic structures usually exhibit higher efficiency than their planar counterparts because the mesoporous charge transport layers can supply heterogeneous nucleation sites for growing high quality perovskite crystals and enlarged charge separation area for better charge extraction. However, comparing with the achieved extremely high or even the certified world record efficiency of mesoscopic PSCs, the significant improvement of inverted mesoscopic PSCs has yet been made, mainly owing to the lack of suitable p-type semiconductors for preparing mesoporous hole transport layers (HTLs). Here, an emulsion-based bottom-up self-assembly strategy is used to prepare NiO microspheres from well-dispersed NiO nanocrystals. The self-assembled NiO microspheres are further used to fabricate mesoporous NiO HTLs of the inverted mesoscopic PSCs. The as-prepared mesoporous NiO HTL with self-assembled NiO microspheres can provide more suitable graded energy alignment, better charge carrier dynamics and reduced dark recombination in the device comparing with the inverted planar PSC with NiO nanocrystal HTL, contributing to obviously enhanced photovoltaic performance and nearly eliminated photocurrent-voltage hysteresis. Due to the general strategy of emulsion-based bottom-up self-assembly for microspheres synthesis, it will overcome the shortage of p-type materials for preparing efficient inverted mesoscopic PSCs.