Structural Characterization of Nickel Oxide Nanowires by X-ray Absorption Near-Edge Structure Spectroscopy

Misoriented high-entropy iridium ruthenium oxide for acidic water splitting

Designing an efficient catalyst for acidic oxygen evolution reaction (OER) is of critical importance in manipulating proton exchange membrane water electrolyzer (PEMWE) for hydrogen production. Here, we report a fast, nonequilibrium strategy to synthesize quinary high-entropy ruthenium iridium-based oxide (M-RuIrFeCoNiO2) with abundant grain boundaries (GB), which exhibits a low overpotential of 189 millivolts at 10 milliamperes per square centimeter for OER in 0.5 M H2SO4. Microstructural analyses, density functional calculations, and isotope-labeled differential electrochemical mass spectroscopy measurements collectively reveal that the integration of foreign metal elements and GB is responsible for the enhancement of activity and stability of RuO2 toward OER. A PEMWE using M-RuIrFeCoNiO2 catalyst can steadily operate at a large current density of 1 ampere per square centimeter for over 500 hours. This work demonstrates a pathway to design high-performance OER electrocatalysts by integrating the advantages of various components and GB, which breaks the limits of thermodynamic solubility for different metal elements.

In Situ H2 Reduction of Al2O3-Supported Ni- and Mo-Based Catalysts

Nickel (Ni)-promoted Molybdenum (Mo)-based catalysts are used for hydrotreatment processes in the chemical industry where the catalysts are exposed to high-pressure H2 at elevated temperature. In this environment, the catalyst transforms into the active phase, which involves the reduction of the oxide. Here, we report on the first in situ study on the reduction of alumina supported Ni- and Mo-based catalysts in 1 mbar H2 using ambient-pressure X-ray photoelectron spectroscopy (APXPS). The study confirms that mixing Ni and Mo lowers the reduction temperature of both Ni- and Mo-oxide as compared to the monometallic catalysts and shows that the MoO3 reduction starts at a lower temperature than the reduction of NiO in NiMo/Al2O3 catalysts. Additionally, the reduction of Ni and Mo foil was directly compared to the reduction of the Al2O3-supported catalysts and it was observed that the reduction of the supported catalysts is more gradual than the reduction of the foils, indicating a strong interaction between the Ni/Mo and the alumina support.

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.

Nickel oxide nanowires: vapor liquid solid synthesis and integration into a gas sensing device

In the field of advanced sensor technology, metal oxide nanostructures are promising materials due to their high charge carrier mobility, easy fabrication and excellent stability. Among all the metal oxide semiconductors, nickel oxide (NiO) is a p-type semiconductor with a wide band gap and excellent optical, electrical and magnetic properties, which has not been much investigated. Herein, we report the growth of NiO nanowires by using the vapor liquid solid (VLS) technique for gas sensing applications. Platinum, palladium and gold have been used as a catalyst for the growth of NiO nanowires. The surface morphology of the nanowires was investigated through scanning electron microscopy to find out which catalyst and growth conditions are best for the growth of nanowires. GI-XRD and Raman spectroscopies were used to confirm the crystalline structure of the material. Different batches of sensors have been prepared, and their sensing performances towards different gas species such as carbon monoxide, ethanol, acetone and hydrogen have been explored. NiO nanowire sensors show interesting and promising performances towards hydrogen.

Synthesis of wide band gap nanocrystalline NiO powder via a sonochemical method

A sonochemistry-based synthesis method was used to produce nanocrystalline nickel oxide powder with ≈ 20 nm average crystallite diameter from Ni(OH)(2) precursor. Ultrasound waves were applied to the primary solution to intensify the Ni(OH)(2) precipitation. Dried precipitates were calcined at 320°C to form nanocrystalline NiO particles. The morphology of the produced powder was characterized by transmission electron microscopy. Using sonochemical waves resulted in lowering of the size of the nickel oxide crystallites. FT-IR spectroscopy and X-ray diffraction revealed high purity well-crystallized structure of the synthesized powder. Photoluminescence spectroscopy confirmed production of a wide band-gap structure.

Effect of reaction conditions on size and morphology of ultrasonically prepared Ni(OH)(2) powders

Modern electrochemical devices require the morphological control of the active material. In this paper the synthesis of nickel hydroxide, as common active compound of such devices, is presented. The influence of ultrasound in the synthesis of nickel hydroxide from aqueous ammonia complexes is studied showing that ultrasound allows the fabrication of flower-like particles with sizes ranging in between 0.7 and 1.0μm in contrast with the 6-8μm particles obtained in the absence of ultrasound. The influence of gas flow, temperature of the process and surfactants in the ultrasonically prepared powders is discussed in term of shape, size and agglomeration of the particles. Adjusting the experimental condition, spherical or platelet-like particles are obtained with sizes ranging from 1.3μm to 200nm.

Oxygen K-edge XANES investigation of NicMg(1-c)O solid solutions

A series of Ni(c)Mg(1-c)O solid solutions are characterized by means of synchrotron radiation X-ray diffraction and X-ray absorption near-edge-structure spectroscopy at oxygen K-edge (532 eV). A dramatic drop of the pre-edge peak intensity is observed in the Ni(c)Mg(1-c)O system upon dilution. It can be attributed to a decrease of 3d(8)(Ni(2+))-2p(O(2-)) mixing upon dilution with magnesium ions due to a decrease of the number of 3d vacancies as nickel ion is replaced by magnesium ion. Similarly, the decrease of the number of 4s and 4p vacancies also leads to a decrease of 4s4p(Ni(2+))-2p(O(2-)) hybridization, and hence a drop of intensities of features B and C. The features E and F are more sensitive to the increase of the degree of disorder upon dilution than feature D, revealing that the latter is mainly dependent by the medium-range order.

A simple solution route to single-crystalline Sb2O3 nanowires with rectangular cross sections

We report a simple solution route to large-scale synthesis of uniform, single-crystalline, and well-faceted orthorhombic antimony trioxide (Sb(2)O(3)) nanowires with rectangular cross sections by direct air oxidation of bulk metal antimony (Sb) in a mixed solution made of ethylenediamine (EDA) and deionized water (DIW). The as-synthesized products were analyzed by range of methods, such as XRD, SEM, EDX, TEM, SAED, HRTEM, FTIR, Raman, UV-vis absorption, and photoluminescence (PL) spectra. The as-synthesized Sb(2)O(3) nanowires with rectangular cross sections are usually hundreds of micrometers in length, typically 80-100 nm in width, and 60-80 nm in thickness. The novel room temperature photoluminescence properties of Sb(2)O(3) nanowires with rectangular cross sections displayed a significant UV luminescence with a strong emission band at 374 nm, which was reported for the first time, indicating the as-synthesized products with an optical band gap E(g) = 3.3 eV. It is expected that as-synthesized Sb(2)O(3) nanowires would be a new member of functional materials and used in the manufacture of advanced nanodevices.

Sol-Gel Template Synthesis and Photoluminescence of n- and p-Type Semiconductor Oxide Nanowires

A sol-gel template technique has been put forward to synthesize single-crystalline semiconductor oxide nanowires, such as n-type SnO2 and p-type NiO. Scanning electron microscopy and transmission electron microscopy observations show that the oxide nanowires are single-crystal with average diameters in the range of 100-300 nm and lengths of over 10 microm. Photoluminescence (PL) spectra show a PL emission peak at 401 nm for n-type semiconductor SnO2, and a PL emission at 407 nm for p-type semiconductor NiO nanowires, respectively. Correspondingly, the observed violet-light emission at room temperature is attributed to near-band-edge emission for SnO2 nanowires and the 3d(7)4s-->3d8 transition of Ni2+ for NiO nanowires.