Nickel Oxide Surface Research Articles

Revealing the Charge Storage Mechanism of Nickel Oxide Electrochromic Supercapacitors.

Nickel oxide (NiO) is considered one of the most promising positive anode materials for electrochromic supercapacitors. Nevertheless, a detailed mechanism of the electrochromic and energy storage process has yet to be unraveled. In this research, the charge storage mechanism of a NiO electrochromic electrode was investigated by combining the in-depth experimental and theoretical analyses. Experimentally, a kinetic analysis of the Li-ion behavior based on the cyclic voltammetry curves reveals the major contribution of surface capacitance versus total capacity, providing fast reaction kinetics and a highly reversible electrochromic performance. Theoretically, our model uncovers that Li ions prefer to adsorb at fcc sites on the NiO(1 1 1) surface, then diffuse horizontally over the plane, and finally migrate in the bulk. More significantly, the calculated theoretical surface capacity (106 mA h g-1) accounts for about 77.4% of the total experimental capacity (137 mA h g-1), indicating that the surface storage process dominates the whole charge storage, which is in accordance with the experimental results. This work provides a fundamental understanding of transition-metal oxides for application in electrochromic supercapacitors and can also promote the exploration of novel electrode materials for high-performance electrochromic supercapacitors.

Efficiency enhancement of perovskite solar cells based on Al2O3-passivated nano-nickel oxide film

Interface passivation plays a significant role in reducing surface defects in semiconductor materials. In this paper, we present a kind of Al2O3-passivated nickel oxide (NiO) film as hole transport layer for the perovskite solar cells (PSCs). The optimal Al2O3 insulating film between the NiO and perovskite active layers has a capability to decrease the defect states density at the surface of NiO and suppress the carriers recombination at NiO/perovskite interface. The passivation process could apparently improve the efficiencies of corresponding PSCs from 5.64 to 7.12%. Steady-state photoluminescence and transient time-resolved photoluminescence (TRPL) results demonstrate the restraint of charge recombination at the NiO/perovskite interface with passivation. The Hall effect test further shows that the electrical properties of the NiO film can be improved by adjusting the precursor concentration of the Al2O3, which has a certain significance for the application of interface engineering in PSCs. Furthermore, this method has the advantages of simple operation, low cost, and strong reproducibility, which is also good for the working stability of PSCs.

Ultrahigh-Loading of Ir Single Atoms on NiO Matrix to Dramatically Enhance Oxygen Evolution Reaction.

Engineering single-atom electrocatalysts with high-loading amount holds great promise in energy conversion and storage application. Herein, we report a facile and economical approach to achieve an unprecedented high loading of single Ir atoms, up to ∼18wt%, on the nickel oxide (NiO) matrix as the electrocatalyst for oxygen evolution reaction (OER). It exhibits an overpotential of 215 mV at 10 mA cm-2 and a remarkable OER current density in alkaline electrolyte, surpassing NiO and IrO2 by 57 times and 46 times at 1.49 V vs RHE, respectively. Systematic characterizations, including X-ray absorption spectroscopy and aberration-corrected Z-contrast imaging, demonstrate that the Ir atoms are atomically dispersed at the outermost surface of NiO and are stabilized by covalent Ir-O bonding, which induces the isolated Ir atoms to form a favorable ∼4+ oxidation state. Density functional theory calculations reveal that the substituted single Ir atom not only serves as the active site for OER but also activates the surface reactivity of NiO, which thus leads to the dramatically improved OER performance. This synthesis method of developing high-loading single-atom catalysts can be extended to other single-atom catalysts and paves the way for industrial applications of single-atom catalysts.

Low-Temperature (

A scan-mode low-temperature (<40 °C) atmospheric-pressure helium (He) dielectric-barrier discharge jet (DBDjet) is applied to treat nickel oxide (NiO) thin films for p–i–n perovskite solar cells (PSCs). Reactive plasma species help reduce the trap density, improve the transmittance and wettability, and deepen the valence band maximum (VBM) level. A NiO surface with the lower trap density surface of NiO allows better interfacial contact with the MAPbI3 layer and increases the carrier extraction capability. MAPbI3 can better crystallize on a more hydrophilic NiO surface, thereby suppressing charge recombination from the grain boundary and the interface. Further, the deeper VBM allows better band alignment and reduces the probability of nonradiative recombination. NiO treatment using He DBDjet with a scan rate of 0.3 cm/s can improve PSC efficiency from 13.63 to 14.88%.

Stability of hexafluoroacetylacetone molecules on metallic and oxidized nickel surfaces in atomic-layer-etching processes

Adsorption of enol hexafluoroacetylacetone (hfacH) on nickel oxide (NiO) fcc (100) and metallic Ni fcc (100) surfaces and the stability of the adsorbate was examined using first-principles quantum mechanical simulations. It was shown that an hfacH molecule can be unstable and dissociate on an Ni metal surface. On an NiO surface; however, an hfacH molecule can be deprotonated and form a hexafluoroacetylacetonate anion (hfac−) bonded stably with positively charged Ni atoms of the surface. The results are consistent with observations of the interaction of hfacH with NiO and Ni surfaces in earlier experiments. The results also explain the mechanisms of the adsorption steps in the thermal atomic layer etching of Ni based on the cyclic processes of surface oxidation and formation of volatile organo-nickel complexes.

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.

ToF-SIMS Investigation of the Initial Stages of MeCpPt(CH3)3 Adsorption and Decomposition on Nickel Oxide Surfaces: Exploring the Role and Location of the Ligands

The chemistry of trimethyl(methylcyclopentadienyl)platinum(IV) (MeCpPt(CH3)3), a platinum organometallic compound often used for the chemical deposition of Pt films, on nickel oxide surfaces was in...

Band offset determination of p-NiO/n-TiO2 heterojunctions for applications in high-performance UV photodetectors

Nickel oxide (NiO)-decorated titanium dioxide (TiO2) heterojunction photodetectors were prepared by two-step anodization. Surface scattering of NiO particles was successfully controlled by varying second-step anodizing voltage, with substantially less clustering of NiO particles on the TiO2 nanotubes (NTs) observed as the voltage increased. Fabricated photodetectors exhibited higher sensitivity to UV light as NiO surface dispersion increased. Electronic bandgap of TiO2 and that of NiO was determined as ~ 3.35 eV and ~ 3.80 eV, respectively. Introduction of NiO particles on well-ordered TiO2 NTs narrowed the bandgap of TiO2, and the difference between work functions of TiO2 and NiO produced sufficient built-in electric field to separate the electron–hole pairs. This led to an enhanced performance of NiO/TiO2 heterojunction photodetectors, which showed high values of responsivity (86 A/W), external quantum efficiency (292%), and detectivity (2.2 × 1010 Jones) under 365 nm UV light illumination. The valence and conduction band offsets at the interface of the NiO/TiO2 heterojunction were determined as ~ 1.54 eV and ~ 1.99 eV, respectively.

A density functional theory analysis on syngas adsorption on NiO (100) surface

Oxygen carrier (OC) design can effectively improve the performance of chemical looping combustion (CLC) and realize fossil fuel combustion at low energy-cost CO2 capture. This study describes the adsorption principles of syngas (i.e. CO and H2) on a clean nickel oxide (100) surface under single and multiple nearest neighbor effects. The results show that the adsorption stability of CO and H2 is mostly weakened by the first neighbor compared to the rest. With the same species as nearest neighbors (uniform adsorption), syngas adsorption stability is reduced when the number of neighbors increases. Similarly, when compared to uniform adsorption, the adsorption stability of CO and H2 is slightly stronger with neighboring sites occupied with different species (hybrid adsorption). In addition, a lower degree of symmetry tends to strengthen CO and H2 adsorption. Results from this analysis show that the adsorption stability of CO and H2 with neighbors are highly related to steric, hybrid and symmetry effects. An electronic property analysis was performed to further support the key role of hybrid neighboring effects in the adsorption process of syngas on the nickel oxide (100) surface.

INVESTIGATION OF ADSORPTION ABILITY OF DIAMINOMALEODINITRILE ON NICKEL SURFACE BY ELLIPSOMETRIC METHOD

The difficulty of the discharge of nickel ions due to the adsorption of the addition of diaminomeleodinitrile on the electrode surface contributes to the formation of a fine crystalline precipitate with high reflectivity. Therefore, in this work, we studied the adsorption capacity of diaminomeleodinitrile on the surface of nickel from borate buffer solution in the cathodic and anodic regions. The values of its free adsorption energy were determined, which were at a potential of -0.65 V - 39.7 kJ / mol, and at 0.2 V - 66.2 kJ / mol. It was determined that diaminomeleodinitrile has adsorbed on the surface of nickel from borate buffer solution at pH = 7.4 mainly due to the forces of chemical interaction. It has been established that on the oxidized nickel surface, the adsorption of diaminomeleodinitrile begins in the region of lower concentrations compared to the “clean” surface. On average, a monolayer has formed within 60-75 min, while changes in the phase angle of the reflected light are small, due to the small size of the adsorbed molecule. It was determined that on the oxidized surface these changes are more than on the “clean” one. By method of reflective ellipsometry, which allows to measure and analyze the differences in the polarization parameters of a plane polarized light flux incident at an angle on two homogeneous media with different optical properties, the thickness of the resulting monolayer was determined. On an oxidized surface, it was ~ 0.5 nm, and on a “clean” surface, ~ 0.2 nm. When comparing these thicknesses with the size of the diaminomeleodinitrile molecule, it can be assumed that at a potential of 0.2 V, the diaminomeleodinitrile molecules are drawn out at an angle to the nickel surface, and at a potential of -0.65 V they are adsorbed flat on the nickel surface.

Surface area- and mass-based comparison of fine and ultrafine nickel oxide lung toxicity and augmentation of allergic response in an ovalbumin asthma model

Background: The correlation of physico-chemical properties with mechanisms of toxicity has been proposed as an approach to predict the toxic potential of the vast number of emerging nanomaterials. Although relationships have been established between properties and the acute pulmonary inflammation induced by nanomaterials, properties’ effects on other responses, such as exacerbation of respiratory allergy, have been less frequently explored.Methods: In this study, the role of nickel oxide (NiO) physico-chemical properties in the modulation of ovalbumin (OVA) allergy was examined in a murine model. Results: 181 nm fine (NiO-F) and 42 nm ultrafine (NiO-UF) particles were characterized and incorporated into a time course study where measured markers of pulmonary injury and inflammation were associated with NiO particle surface area. In the OVA model, exposure to NiO, irrespective of any metric was associated with elevated circulating total IgE levels. Serum and lung cytokine levels were similar with respect to NiO surface area. The lower surface area was associated with an enhanced Th2 profile, whereas the higher surface area was associated with a Th1-dominant profile. Surface area-normalized groups also exhibited similar alterations in OVA-specific IgE levels and lung neutrophil number. However, lung eosinophil number and allergen challenge-induced alterations in lung function related more to particle size, wherein NiO-F was associated with an increased enhanced pause response and NiO-UF was associated with increased lung eosinophil burden.Conclusions: Collectively, these findings suggest that although NiO surface area correlates best with acute pulmonary injury and inflammation following respiratory exposure, other physico-chemical properties may contribute to the modulation of immune responses in the lung.

Activating peroxydisulfate by morphology-dependent NiO catalysts: Structural origin of different catalytic properties

Understanding how the morphology of a nanocatalyst tailors its catalytic performance is a crucial issue for the rational design and fabrication of active heterogeneous catalysts at the nanometer level. Herein, we demonstrate novel findings of peroxydisulfate (S2O82−; PDS) activation catalyzed by nickel oxide (NiO) nanocatalysts of different morphologies, in which oxygen vacancies (VO) were the defective sites that facilitated the chemical bonding with PDS molecules and promoted the reactivity of VO-connected nickel ions for PDS activation, thus elucidating the structural origin of its catalytic activities. The morphologically tunable NiO with various VO concentrations exhibited different catalytic performance for the removal of phenol (a model organic pollutant). Based on an electron paramagnetic resonance (EPR) study, radical competition reactions, and quenching tests, the main reactive species was revealed to be the non-radical PDS-NiO complex, which can effectively attack C6H5O− to yield intermediates attached to NiO surface. This work not only improves the fundamental understanding of active sites in morphologically tunable transition metal oxides, but also provides valuable guidelines for the rational design and synthesis of high-performance, morphology-dependent nanocatalysts.

NiO2 and Fe2O3 Surface Modified Glassy Carbon Electrodes for the Electrocatalytic Oxidation of Bioanalytes

Electrochemical detection of analytes is desirable due to high sensitivity, vast portability, low cost, and rapid response. Clinically relevant amino acids such as arginine, alanine, serine, and valine may be used as biomarkers for the diagnosis of various diseases, such as obesity and major depressive disorder.1 Typically, changes in metabolite concentrations can be detected by liquid chromatography-mass spectrometry2 or nuclear magnetic resonance spectroscopy.3 These methods tend to be long, tedious, and complicated with low throughput. Using electrochemical sensors could provide a more simplistic approach. One of the challenges of detecting amino acids is that they are not typically electrochemically active. However, in the presence of metal oxide nanoparticles, such as ZnO, MnO2, NiO, and CuO, immobilized on the working electrode surface, electrocatalytic reactions of biomolecules occur.4,5 For example, a glassy carbon electrode modified with NiO nanoparticles was able to electrochemically activate serine, alanine, and glycine under basic aqueous conditions.4 Electrocatalytic activity was observed in both cyclic voltammetry and chronoamperometry experiments. Another example includes a report that metal electrocatalysts, particularly Ni compounds, are better at activating biomolecules containing alcohol functional groups at increased pH.5 Ideally, sensors that can detect biomolecules at physiological pH are preferred to minimize sample preparation in evaluating biofluids. Discussed herein are the surface modifications of a glassy carbon electrode with iron oxide and nickel(IV) oxide surfaces through electrodeposition and passivation. The sensors were tested through detection of arginine, alanine, serine, and valine at physiological pH by cyclic voltammetry in a flow cell. We will report on the found sensitivity towards arginine, alanine, serine, and valine. In addition, the stability of the surfaces was also investigated. The sensitivity and stability of both sensor modification, towards amino acid detection, will be compared. The authors would like to acknowledge supported by CIBBR (P20GM113131), an Institutional Development Award (IDeA) from the National Institute of General Medical Sciences of the National Institutes of Health.

Developing a Robust Recombination Contact to Realize Monolithic Perovskite Tandems With Industrially Common p-Type Silicon Solar Cells

Although two-terminal perovskite-silicon tandem solar cells have rapidly increased in efficiency, they have only been demonstrated with n-type silicon, which currently constitutes less than 5% of the global photovoltaics market. In this paper, we realize the first two-terminal perovskite tandem with p-type silicon by developing a recombination contact that enables voltage addition without damaging either subcell. We find that silicon interband recombination contacts are limited by a SiO x charge-extraction barrier, which forms during oxidative top-cell fabrication. A sputtered 30-nm indium tin oxide layer is found to protect the silicon cell surface from oxidation, while forming a recombination contact with the p-type nickel oxide hole transport layer for the perovskite top cell. Using this recombination contact we achieve voltage addition between the perovskite top cell and aluminum back-surface field p-type silicon bottom cell. We also find that minimizing moisture on the nickel oxide surface is important for achieving a stable open-circuit voltage under illumination. The recombination contact developed herein could play an important role in near-future developments.

Enhanced Nano-Catalyst Infiltration of Anode-Supported SOFCs through Surface Modification of Electrodes By Catechol Surfactants

The wet impregnation/infiltration method has been a well-utilized route to incorporate nanoparticles inside the electrode architecture by co-firing the metal salt solutions that are wicked to the internal pore structure. Impregnation/infiltration of catalytically active nanoparticles brings an extension of the phase boundaries in the active area which reduces the polarization resistance. There are three main infiltration parameters that impact on reducing the electrode polarization resistance and over potential: a) nano-catalyst concentration, b) nano-catalyst location after infiltration, and c) nano-catalyst decoration. It should be noted that those factors are all interrelated and can fail or be achieved altogether. The nanocatalyst concentration is usually controlled by performing repetitive infiltration steps. In these experiments, co-firing is needed after each infiltration step, to open up free network path inside the electrode, then the process is repeated until the desired amount is achieved. One major potential risk of repetitive infiltration is the clogging of the porosity. This labor-intensive protocol also suffers from inhomogeneous deposition and gas starvation issues during the cell operation due to the inefficient decoration of the infiltrates at the active area where the deepest region of the electrode is located. Otherwise, the metal nitrate solution dries at the top or middle region of the electrode and the nano-catalysts sits far away from the active layer, hence the impact in polarization resistance reduction can be seen less. Due to these issues, there is a need to control the deposition rate and achieve a homogenous nano-catalyst distribution and reduce the number of infiltration steps. One solution is adding wetting agents to the precursor mixture to enhance the penetrability. Pressure or vacuum assisted infiltration can also help to wick the solution through the electrode. In this study, catechol-based molecules which are a specific type of bio-monomers found in the nature of multi-functional end-groups, such as dopamine, poly-norepinephrine (pNE), and derivatives are used as electrode surface modifiers. Apart from the neurotransmitter functions, these end-groups are naturally capable of strong adhesive properties when bio-monomers are linked and polymerized. The polymerized surfactant can bind with the metal cation by chelation. Within the past decades, catechol based molecules are used as bio-derived surfactants in various coating studies. Mostly, studies have been focused on planar, dense and single-phase substrates. However, SOFC electrodes of percolated porous architecture vary tens to hundreds of microns in thickness. The porosity level depends on the SOFC type and manufacturing method, but can vary between 10-50%. In addition, the electrodes consist of multi-constituents. For example, conventional anode electrode comprises nickel oxide (NiO) and yttria stabilized zirconia (YSZ) mixture and the conventional cathode is composed of lanthanum strontium manganite (La1−xSrxMnO3 , LSM) and yttria stabilized zirconia (YSZ). The motivation of this work is to monitor the long-term stability of the polymerized norepinephrine assisted CeO2, ceria nano-catalyst infiltrated commercial button SOFCs by single step infiltraton. Investigating the optimum nano-catalyst concentration for that particular off-the-shelf SOFC architecture is also a focus area for this work. The initial part of the study investigated the deposition and polymerization kinetics of the catechol-surfactant film on NiO, YSZ and LSM surfaces. Deposition parameters such as immersion time, solid loading and polymerization technique were all studied. The study aim was to pinpoint the acceptable polymerization degree and adhered film/particle layer thickness range to enhance homogeneous coating of porous SOFC electrode walls by preventing pore clogging. The experiments showed that the pNE coating layer can be tuned from 25 nm to 150 nm. In the second part of the study, these coated substrates were immersed into the cerium nitrate solution to form CeO2, ceria, nanoparticles to observe the ceria layer coarsening as a function of the pNE layer thickness and cerium precursor immersion time. The study assisted in understanding the optimal ceria decoration after the annealing process at the SOFC operating temperature (750oC). Experiments showed that tuning the immersion time and solid loading induced nano-catalyst decoration from discrete particles to film formation onto the planar substrates. The optimal protocols were applied to impregnate anode-supported SOFC button cells. The electrochemical performance and the stability of the infiltrated anode-supported SOFCs were measured over 300 hours and the performance was correlated to the decoration of the nano-catalyst impregnated electrodes. The developed impregnation protocol resulted in >20% increase in power density at 750° when the same protocol was applied to a commercial anode-supported SOFC. Acknowledgements: The authors would like to thank James Poston at NETL-Morgantown for his assistance in SEM/EDS characterization. The WVU Shared Research Facilities are also acknowledged for their assistance in materials characterization.

Oxidative dissolution of NiO in aqueous electrolyte: An impedance study

The present contribution reports on the analysis of the electrochemical properties of screen-printed nickel oxide (NiO) in aqueous electrolyte when NiO is in the configuration of thin film (thickness, l ≤ 7 μm). This type of NiO samples presents mesoscopic morphology. The latter characteristic combined to the intrinsic electroactivity of the oxide leads to the observation of proportionality between NiO thickness and current density. In particular, NiO undergoes two distinct processes of oxidation with reversible features in aqueous media. These redox processes occur in the solid state and, as such, involve mass/charge transfer between the NiO electrode and the electrolyte. On the other hand, the electrochemical oxidation of NiO is accompanied by the progressive chemical dissolution of the oxide when the latter is in the oxidized state. This is a consequence of the elevated content of stored charge on NiO surface when it interfaces a high polarity solvent like water. In the present work we have considered electrochemical impedance spectroscopy (EIS) for the investigation of the charge transfer/transport properties of NiO during the simultaneous occurrence of chemical and electrochemical processes. The study proposes models of electrical circuits for analyzing the interfacial phenomena involved in the aqueous oxidation of NiO.

Specific capacitance, energy and power density coherence in electrochemically synthesized polyaniline-nickel oxide hybrid electrode

Abstract In the present manuscript, polyaniline (PAni), nickel oxide (NiO), and a fine nanosheet-type network of PAni-NiO hybrid film electrodes are prepared using electrodeposition technique and envisaged as electrode materials in electrochemical capacitor application. Furthermore, these film electrodes are characterized for their structural and morphological studies with the help of X-ray diffraction and scanning electron microscopy measurements, respectively. Atomic force microscopy revelation affords the roughnesses of electrode surfaces. Contact angle elucidation explodes the hydrophilic nature of PAni, NiO and PAni-NiO hybrid electrode surfaces. Electrochemical measurements confirm that the PAni-NiO hybrid electrode shows synergic effect as its specific capacitance measured at a scan rate of 5 mV/s in 1 M Na 2 SO 4 electrolyte is noticeably higher (936.36 F/g) than the PAni and NiO electrodes (601 and 263.5 F/g). All these electrodes exhibit remarkable capacity retention performance after 2000 cycles. The nanosheet network of PAni-NiO hybrid electrode offer a great reaction surface area for rapid ion and electron transfer with good stability as well, which all are advantageous for enhancing the electrochemical capacitance performance.

In Situ DRIFTS and NAP-XPS Exploration of the Complexity of CO2 Hydrogenation over Size-Controlled Pt Nanoparticles Supported on Mesoporous NiO

Pt nanoparticles (4.8 nm) were anchored onto the surface of mesoporous nickel oxide (NiO) supports. Pt/NiO samples were compared with pristine NiO and Pt/SBA-15 silica catalysts in CO2 hydrogenation to form carbon monoxide, methane, and ethane at 473–673 K. Pt/NiO (1%) samples were ∼20 times and ∼1.5 times more active at 493 K compared with Pt/SBA-15 and NiO catalysts, respectively. However, the Pt-free NiO support has an activity of 120% compared to that of Pt/NiO catalysts at 673 K. In the case of 1% Pt/SBA-15 catalyst, the selectivity toward methane was 13%, whereas it was 90 and 98% for NiO and 1% Pt/NiO at 673 K, respectively. Exploration of the results of the reactions was performed by near-ambient pressure X-ray photoelectron spectroscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy. In the case of pure NiO, we found that the surface of the support was mainly covered by elemental Ni under reaction conditions, where the Ni/NiOx system is responsible for the high activity o...

N-Modified NiO Surface for Superior Alkaline Hydrogen Evolution.

Boosting the sluggish kinetics of the hydrogen evolution reaction in alkaline environments is key for the large-scale application of water-alkali and chlor-alkali electrolysis. In this study, nitrogen atoms are used to precisely modulate electrochemical active sites on the surface of nickel oxide with low-coordinated oxygen atoms, to achieve enhanced kinetics in alkaline hydrogen evolution. Theoretical and experimental results demonstrate that surface charge redistribution after modulation facilitates both the initial water dissociation step and the subsequent recombination of Had from low-coordinated oxygen sites and desorption of OHad- from nickel sites, thus accelerating the overall hydrogen evolution process. The N-modulated nickel oxide enriched in low-coordinated oxygen atoms exhibits significantly enhanced activity with a small overpotential of -100 mV at the current density of -10 mA cm-2 and a robust stability over 90 h for hydrogen evolution in 1.0 m KOH.

Al2O3-Interlayer-Enhanced Performance of All-Inorganic Silicon-Quantum-Dot Near-Infrared Light-Emitting Diodes

Efficient all-inorganic silicon-quantum-dot (Si-QD) near-infrared light-emitting diodes (LEDs) have been fabricated by using nickel oxide (NiO) and zinc oxide (ZnO) as the transport layers of holes and electrons, respectively. It is found that the LED performance may be significantly improved by the atomic layer deposition of an Al2O3 interlayer between Si QDs and NiO. The improvement is due to the fact that the Al2O3 interlayer can not only suppress the exciton quenching induced by the traps at the NiO surface and the accumulated holes at the NiO/Si-QD interface, but also reduce the leakage of carriers. The optimum thickness of the Al2O3 interlayer is found to be ~5.7 nm, which leads to the increase of the optical power density by a factor of ~7 (from ~2 to $14~\mu \text{W}$ /cm2) and that of the external quantum efficiency by a factor of ~10 (from ~0.01% to 0.1%) for the all-inorganic Si-QD near-infrared LED on glass. In addition, it is shown that the Al2O3 interlayer may also improve the performance of flexible all-inorganic Si-QD near-infrared LEDs on poly(ethylene terephthalate).