Formation of MgO whiskers on the surface of bulk MgB<Subscript>2</Subscript> superconductors during in situ sintering

Magnesium oxide (MgO) whiskers (with diameters of about 60–80 nm) formed on the surface of bulk polycrystalline MgB₂ superconductor at a relative low temperature (720 °C) during in situ sintering process. The reaction between Mg and B powders begins at a temperature below melting point of Mg and maintains till about 750 °C. The residual Mg powders evaporate and react with trace oxygen to form MgO vapor as the temperature exceeds the melting temperature of Mg and a low supersaturation is required for the growth of MgO whiskers. The preformed MgB₂ and MgO crystals act as substrates and the melted Mg powders on the surface of them serve as catalysts during the growth process of MgO whiskers. The growth process of MgO whiskers is dominated by a self-catalytic vapor–liquid–solid (VLS) mechanism.     

Enhanced adsorption performance of hierarchical S-doped Ni(OH)<Subscript>2</Subscript> hollow nanocomposites for the removal of Congo red

In this work, nonmetallic S was doped into hierarchical Ni(OH)₂ hollow microspheres by ethanol solvothermal method using thiourea as sulfur source. Although the morphology of precursor Ni(OH)₂ is maintained, the surface states and pore properties had greatly changed after S doping. Using the as-prepared S-doped Ni(OH)₂ as adsorbents for the removal of Congo red (CR), the S-doped Ni(OH)₂ exhibited much better adsorption capacity compared with undoped Ni(OH)₂. The adsorption behavior of both Ni(OH)₂ and S-doped Ni(OH)₂ followed the pseudo-second-order kinetic model and intraparticle diffusion model. The equilibrium data of Ni(OH)₂ could be better fitted by Langmuir model, while Freundlich model could be better used to describe the S-doped Ni(OH)₂ with a much larger adsorption capacity toward CR. The tuned microstructure and changed surface states of adsorbent after S doping may be responsible for the enhanced adsorption performance. Therefore, the doping of S species into hierarchical Ni(OH)₂ paves a new way to tune the microstructure and surface states of Ni-based materials.     

Recycling of glass in synthesis of MCM-48 mesoporous silica as catalyst support for Ni<Subscript>2</Subscript>O<Subscript>3</Subscript> photocatalyst for Congo red dye removal

Glass was successfully recycled in the synthesis of mesoporous silica MCM-48 which was used as catalyst support for nickel oxide photocatalyst. The resulted products were evaluated using X-ray diffraction, scanning electron microscope and UV–Vis spectrophotometer. The precipitated nickel oxide is of Ni₂O₃ form and loading of it onto MCM-48 resulted in a reduction in the band gap energy from about 3.66 eV to about 2.4 eV. The role of MCM-48 as catalyst support for Ni₂O₃ in enhancing the adsorption capacity and photocatalytic properties of nickel oxide was evaluated through series of equilibrium studies and photocatalytic degradation of Congo red dye under visible light. Using of glass-based MCM-48 as catalyst support for Ni₂O₃ showed enhancing the adsorption capacity by 31.3 and 14.8% higher than the adsorption capacity of Ni₂O₃ and MCM-48, respectively. Also, the photocatalytic degradation percentage increased by about 67.3% relative to the Ni₂O₃ degradation percentage. The nature of MCM-48/Ni₂O₃ adsorption mechanism is chemisorption and occurs in multilayer form throughout the heterogeneous surface of the composite. The using of MCM-48 as support for Ni₂O₃ photocatalyst enhanced the adsorption capacity through increasing the total surface area. The loading process resulted in fixing of the Ni₂O₃ particles throughout the porous structure which producing more exposed active photocatalyst sites and active adsorption sites for the incident photons as well as preventing the nickel oxide particles from agglomeration. Based on the obtained results, supporting of Ni₂O₃ particles onto MCM-48 is promising active centers for the degradation of Congo red dye molecules.     

TiO<Subscript>2</Subscript> type influences fibronectin adsorption

Human fibronectin (FN) plays a key role in the biointegration of implants as the success depends on adsorption of proteins like FN [1]. Indeed FN can be an intermediary between the biomaterial surface and cells. The adsorption of human fibronectin (FN) on commercially pure titanium with a titanium oxide layer formed in a H₂O₂ solution (TiO₂ cp) and TiO₂ sputtered on Si (TiO₂ sp) was studied. Adsorption isotherms and the work of adhesion were assessed by wettability studies, X-ray photoelectron spectroscopy (XPS), and by radiolabelling of FN with ¹²⁵I, ¹²⁵I-FN. Exchangeability of bound FN by free FN, was also evaluated by the radiolabelling technique. Contact angle determinations have shown that FN displays higher affinity for the TiO₂ cp surface than for the TiO₂ sp. As expected from the surface free energy values, the work of adhesion of FN is higher for the TiO₂ cp substrate, the more hydrophilic one, and lower for the TiO₂ sp substrate, the more hydrophobic one. The adsorption isotherms were evaluated by two different techniques: radiolabelling of FN (¹²⁵I-FN) and XPS. TiO₂ cp adsorbs more FN than the TiO₂ sp surfaces as shown by the radiolabelling data. FN molecules are also more strongly attached to the former surface as indicated by the work of adhesion and by the exchangeability studies. Results using ¹²⁵I-FN also suggests that FN adsorbs as a multilayer for FN concentrations in solution higher than 100 μg/mL.     

Atomistic Modeling of Corrosion Resistance: A First Principles Study of O2 Reduction on the Al(111) Surface Covered with a Thin Hydroxylated Alumina Film

In the early stage of corrosion of Al or Al alloys (i.e., during the initiation of localized corrosion), an oxide film is generally present on the surface. This work investigates the possibility for a cathodic reaction to occur on these oxide films. We discuss realistic models of supported oxide films on Al(111) in order to disentangle the factors determining the reactivity towards O₂. Three components of the complex film formed on Al(111) can be identified: an ultrathin under‐stoichiometric Al_xO_y interface layer, an intermediate Al₂O₃ phase with γ‐alumina structure, and an hydroxylated AlOOH surface termination with boehmite structure. The electron transfer to O₂ molecules depends on the workfunction, Φe, of the metal/oxide interface and on the thickness of the inner Al₂O₃ phase. The electron transfer takes place both from the metal‐oxide interface and the oxide surface to the adsorbed O₂ molecule. Very important is the role of the hydroxyl groups at the surface: they eliminate the Al surface states and stabilize the surface; they allow the reduced O₂ ⁻ species to capture protons and transform into hydrogen peroxide in a non‐activated process. H₂O₂ is further reduced to two water molecules, in a series of two‐electron mechanisms. These reactions take place only when the internal alumina phase is ultrathin (here 0.2 nm). As soon as an Al₂O₃ inner layer develops (film thickness of about 1 nm), the film becomes unreactive and passivates the Al(111) surface. The results help to shed light on the complex reactions responsible for metal corrosion.     

Effects of post-deposition annealing temperature and ambient on RF magnetron sputtered Sm<Subscript>2</Subscript>O<Subscript>3</Subscript> gate on n-type silicon substrate

Samarium oxide (Sm₂O₃) thin films with thicknesses in the range of 15–30 nm are deposited on n-type silicon (100) substrate via radio frequency magnetron sputtering. Effects of post-deposition annealing ambient [argon and forming gas (FG) (90% N₂ + 10% H₂)] and temperatures (500, 600, 700, and 800 °C) on the structural and electrical properties of deposited films are investigated and reported. X-ray diffraction revealed that all of the annealed samples possessed polycrystalline structure with C-type cubic phase. Atomic force microscope results indicated root-mean-square surface roughness of the oxide film being annealed in argon ambient are lower than that of FG annealed samples, but they are comparable at the annealing temperature of 700 °C (Argon—0.378 nm, FG—0.395 nm). High frequency capacitance–voltage measurements are carried out to determine effective oxide charge, dielectric constant and semiconductor-oxide interface trap density of the annealed oxide films. Sm₂O₃ thin films annealed in FG have smaller amount of effective oxide charge and semiconductor-oxide interface trap density than those oxide films annealed in argon. Current–voltage measurements are conducted to obtain barrier heights of the annealed oxide films during Fowler–Nordheim tunneling.     

Theoretical investigation of the C<Subscript>60</Subscript>/copper phthalocyanine organic photovoltaic heterojunction

Molecular heterojunctions, such as the one based on copper phthalocyanine (CuPc) and carbon fullerene (C₆₀) molecules, are commonly employed in organic photovoltaic cells as electron donor-acceptor pairs. We have investigated the different atomic structures and electronic and optical properties of the C₆₀/CuPc heterojunction through first-principles calculations based on density functional theory (DFT) and time-dependent DFT. In general, configurations with the CuPc molecule “lying down” on C₆₀ are energetically more favorable than configurations with the CuPc molecule “standing up”. The lying-down configurations also facilitate charge transfer between the two molecules, due to the stronger interaction and the larger overlap between electronic wavefunctions at the interface. The energetically preferred structure consists of CuPc placed so that the Cu atom is above a bridge site of C₆₀, with one N-Cu-N bond of CuPc being parallel to a C-C bond of C₆₀. We also considered the structure of a periodic CuPc monolayer deposited on the (001) surface of a face-centered cubic (fcc) crystal of C₆₀ molecules with the lying-down orientation and on the (111) surface with the standing-up configuration. We find that the first arrangement can lead to larger open circuit voltage due to an enhanced electronic interaction between CuPc and C₆₀ molecules.      

Study of air oxidation of amorphous Zr<Subscript>65</Subscript>Cu<Subscript>17.5</Subscript>Ni<Subscript>10</Subscript>Al<Subscript>7.5</Subscript> by X-ray photoelectron spectroscopy (XPS)

The oxidation of the bulk amorphous alloy Zr₆₅Cu_17.5Ni₁₀Al_7.5 in air in its amorphous and the supercooled liquid states was studied in the temperature range 573–663 K using X-ray photoelectron spectroscopy (XPS). The oxide film mainly consisted of the oxides of Zr (as ZrO₂) and Al (as Al₂O₃). No Cu or Ni was found in the oxide film formed on the amorphous state of the alloy while significant Cu (as CuO) was present in the oxide film formed on the alloy in its supercooled liquid state. The role of the various alloying elements during oxidation at high temperatures in air is discussed in the paper. The XPS data from oxide film support the previously suggested mechanism for oxidation of this alloy, i.e. the rate controlling process during oxidation of the alloy at low temperatures (in the amorphous state) is the back-diffusion of Ni and Cu, while the oxidation at high temperatures (in the supercooled liquid state) is dominated by the inward diffusion of oxygen.     

Computational investigations of Cu-embedded MoS<Subscript>2</Subscript> sheet for CO oxidation catalysis

Elevated amount of CO levels in the atmosphere poses serious health and environmental hazards. Oxidation of CO using suitable catalysts is one of the methods to control it. By means of DFT calculations, single Cu atom doped in S vacancy of MoS₂ nanosheet is studied for CO oxidation catalysis. Cu atom is strongly confined at the S-defective site of the MoS₂ sheet, possessing high energy barrier for the diffusion to its neighboring sites. Adsorption energy, charge transfer and orbital hybridization of CO and O₂ molecules adsorbed Cu-doped MoS₂ sheet reveal that O₂ is relatively more strongly adsorbed than CO. High adsorption energy of O₂ (??2.115 eV) and large charge transfer between O₂ and Cu–MoS₂ sheet (0.493e), compared to CO, make O₂ adsorption more favorable, which extenuates CO poisoning and hence helps in the efficient CO oxidation process. The complete oxidation of CO takes place in two steps: \( {\text{CO}} + {\text{O}}_{2} \to {\text{OOCO}} \) with activation energy of 0.201 eV, succeeded by \( {\text{OOCO}} + {\text{CO}} \to 2{\text{CO}}_{2} \) without any energy barrier. Our results show that the basal plane of MoS₂ sheet gets activated by embedding it with Cu metal, which can catalyze CO oxidation reaction effectively and without poisoning issues. The high activity, stability and low cost features can possibly encourage fabricating MoS₂-based catalysts for CO oxidation reaction.     

Interfacial reaction between Al<Subscript>72</Subscript>Ni<Subscript>12</Subscript>Co<Subscript>16</Subscript> decagonal quasicrystalline particles and liquid aluminium

The interfacial reaction between Al₇₂Ni₁₂Co₁₆ quasicrystalline particles and pure Al melt at 670 °C was investigated. For all the studied samples, only one interfacial reaction product was detected at the interface by scanning electron microscopy. The product was identified to be the Al₉(Co, Ni)₂ crystalline phase, which show an rod-like morphology. The growth rate of the Al₉(Co, Ni)₂ phase layer is very fast. Based on the microstructure analysis results, it is proposed that the layer growth is initially towards the liquid phase, but changes direction towards the quasicrystalline phase by the solid state reaction of diffusional Al with the quasicrystalline phase at the Al₇₂Ni₁₂Co₁₆/Al₉(Co, Ni)₂ interface. A grain boundary grooving effect is deduced to have been involved during the reaction process.     

Synthesis and thermoanalytical study of SiO<Subscript>2</Subscript>–TiO<Subscript>2</Subscript> composites modified with macrocyclic endoreceptors

We have developed processes for the fabrication of SiO₂–TiO₂ composites containing crown ethers (CEs) with composite: CE weight ratios from 1: 0.06 to 1: 1. As oxide sources, we used titania and silica sols. The composites were characterized by differential thermal analysis, X-ray diffraction, and adsorption gravimetry. The results demonstrate that most of the water and the solvent are bound into a complex with the CE, which decomposes at temperatures from 170 to 230°C. The temperature range of CE removal depends on the SiO₂: TiO₂ and oxide: CE ratios in the composite. Our results demonstrate effectiveness of strontium cation imprint formation in an adsorbent in the sol–gel processing step, which ensures an increase in the amount of strontium cation adsorption by 20%. We have identified conditions for quantitative lanthanum, strontium, and barium adsorption on the synthesized composites.     

Tunable SiO<Subscript>2</Subscript>/Si-based nanostructures

We model the formation of a nanoscale potential well with quantum wires on the semiconductor surface near the SiO₂/Si interface owing to a special charge distribution in the oxide. We consider an SiO₂/Si structure in the form of a cylindrical substrate covered with a coaxial oxide layer. The charge distribution in the oxide is taken to have the form of charged circular rings of finite thickness, coaxial with the cylindrical substrate. The parameters of the quantum wires are analyzed in relation to the charge distribution and density. Reducing the separation between two charged rings decreases the width of the quantum wires produced on the semiconductor surface and increases their depth.     

Microstructural control of Ni-YSZ cermet anode for planer thin-film solid oxide fuel cells

Ni-Y₂O₃-stabilized ZrO₂ (Ni-YSZ) cermet anode was fabricated for solid oxide fuel cells (SOFCs) by conventional ceramic processing using NiO-YSZ composite particles. Microstructures of the anode were carefully characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The Ni-YSZ cermet anode was consisting of fine YSZ connections, as the conducting pass of oxygen ions, on the surface of Ni network, as that of electrons, with continuous pore structure and as that of gaseous species. No amorphous phases were present at the interface between Ni and YSZ, and there was an orientation relationship between Ni and YSZ grains, (111)_Ni//(111)_YSZ. The cermet anode showed a high electrical performance at 800 °C. These results indicated that the electrochemical activity of the Ni-YSZ cermet anode was enhanced with the present microstructure.     

Fabrication of a plane-parallel interface in Ni/PbZr<Subscript>0.2</Subscript>Ti<Subscript>0.8</Subscript>O<Subscript>3</Subscript> heterostructures

A process has been proposed for producing a plane-parallel ferromagnetic/ferroelectric interface, which ensures a reproducible magnetoelectric performance of Ni/PbZr_0.2Ti_0.8O₃ (PZT) heterostructures. Its principle is to smooth the initial surface profile of the PZT ceramic substrate to a submicron level by sequential deposition/sputtering of three layers 0.2, 0.1, and 0.05 μm in thickness through ion beam sputtering of a target having the same composition as the substrate, followed by the growth of a nickel film on the smoothed surface. This allows one to preclude film bulging and spalling and ensures high quality of the interface.     

Surface reactivity and electrophoretic deposition of ZrO<Subscript>2</Subscript>–MgO mechanical mixture

Electrophoretic deposition (EPD) is a precision technique useful for obtaining high quality ceramic bodies with controlled dimensions and smooth coatings. The electrophoretic deposition rate is highly dependent on the surface chemistry of the powders, especially when dealing with multi-component systems. The objective of this work is to study the surface reactivity of both ZrO₂ and MgO in ethanol suspension to provide experimental benchmarks to control EPD of a ZrO₂–3 wt% MgO mechanical mixture (Z3M) in ethanol. Infrared spectroscopy (FTIR) showed that ZrO₂ surface spontaneously reacts with ethanol, generating negative electrophoretic mobility of the particles (−0.07 × 10⁻⁸ V⁻¹ s⁻¹) measured by Electroacoustic Sonic Amplitude (ESA). MgO surface also spontaneously reacted with ethanol, but a positive electrophoretic mobility was observed in this case (0.26 × 10⁻⁸ V⁻¹ s⁻¹). Scanning Electron Microscopy of Z3M dried from ethanol suspension showed that MgO particles were located around the ZrO₂ particles, forming composite agglomerates, probably due to the electrostatic attraction between MgO and ZrO₂ particles. Homogeneous deposits could be obtained from EPD of Z3M ethanol suspensions. Mercury intrusion porosimetry showed that the ZrO₂–MgO green deposited bodies using different voltages had similar pores diameters distributions, indicating that the ZrO₂–MgO agglomerates are not affected by the increasing deposition rates.     

Monodisperse nickel nanoparticles supported on SiO<Subscript>2</Subscript> as an effective catalyst for the hydrolysis of ammonia-borane

Monodisperse Ni nanoparticles (NPs) have been synthesized by the reduction of nickel(II) acetylacetonate with the borane-tributylamine complex in a mixture of oleylamine and oleic acid. These Ni NPs are an active catalyst for the hydrolysis of the ammonia-borane (AB, H₃N·BH₃) complex under ambient conditions and their activities are dependent on the chemical nature of the oxide support that they were deposited on. Among various oxides (SiO₂, Al₂O₃, and CeO₂) tested, SiO₂ was found to enhance Ni NP catalytic activity due to the etching of the 3.2 nm Ni NPs giving Ni(II) ions and the subsequent reduction of Ni(II) that led to the formation of 1.6 nm Ni NPs on the SiO₂ surface. The kinetics of the hydrolysis of AB catalyzed by Ni/SiO₂ was shown to be dependent on catalyst and substrate concentration as well as temperature. The Ni/SiO₂ catalyst has a turnover frequency (TOF) of 13.2 mol H₂·(mol Ni)⁻¹ · min⁻¹—the best ever reported for the hydrolysis of AB using a nickel catalyst, an activation energy of 34 kJ/mol ± 2 kJ/mol and a total turnover number of 15,400 in the hydrolysis of AB. It is a promising candidate to replace noble metals for catalyzing AB hydrolysis and for hydrogen generation under ambient conditions.     

Ex Situ Spark Plasma Sintering of Short Powder-in-Tube MgB<Subscript>2</Subscript> Tapes with Open and Closed Ends

Short powder-in-tube tapes of MgB₂ in the Fe sheath were fabricated by ex situ route from a commercial powder containing some free Mg and MgO impurity phases. The final heat treatment was performed by spark plasma sintering (SPS). Tapes were with open (OT) or closed (CT) endings. Closed endings were made by folding and pressing. The MgB₂ core of the OT sample has shown a higher low-field critical current density, a higher maximum pinning force, a slightly higher disorder, smaller average MgB₂ crystallite size, a weak contact between Fe and MgB₂ core, and more macro-flux jumps. The upper and irreversibility fields were similar for OT and CT samples. In the center of the MgB₂ cores, the detected impurity phase is MgO, while at the interface with Fe, MgB₄ also occurs. Impurity phases found at interface, MgO and MgB₄, are present in the center of the bulk SPSed samples. Reactions and pinning-force-related parameters are discussed with respect to Mg behavior influenced by condition of endings. It is inferred that the presence of free Mg in the raw MgB₂ powder has an important contribution to observed differences, and its removal or control is recommended.     

Surface reactivity of reduced LaFeO3 as studied by TPD and IR spectroscopies of CO,CO2 and H2

CO, CO₂ and H₂ reactive adsorption on LaFeO₃ at 298 K has been studied as a function of the reduction temperature of the perovskite oxide by means of temperature programmed desorption (TPD) and infrared (IR) spectroscopies. TPD spectra of CO after CO adsorption contained peaks at 365 to 440 K assigned to linearly adsorbed CO and at 495 to 540 K assigned to bridged CO. TPD spectra of CO₂ after CO adsorption presented broad peaks centred at 570 and 715 K assigned to monodentate and bidentate carbonates, respectively. TPD spectra of CO₂ obtained after CO₂ adsorption contained peaks at 375 to 425 K and at 570 to 675 K. These were associated to infrared bands of monodentate and bidentate carbonates, respectively. In the CO-H₂ and H₂-CO successive adsorption on the reduced surface of LaFeO₃ the TPD peak of H₂ at 345 to 360 K is strongly inhibited and a new desorption peak appeared at 585 to 590 K. This is assumed to be due to CO adsorption on metallic Fe⁰ sites (CO-H₂ coadsorption) or to a displacement of adsorbed hydrogen from Fe⁰ to a new adsorption site (H₂-CO coadsorption). CO was found to interact more strongly than hydrogen with the adsorbent surface.Deceased, formerly of Instituto de Catálisis y Petroleoquímica, C.S.I.C., Serrano 119, 28006 Madrid, Spain.     

Three-scale analysis of BaTiO<Subscript>3</Subscript> piezoelectric thin films fabrication process and its experimental validation

A three-scale analysis of crystal growth process is newly proposed based on the first-principles calculation and on the finite element analysis in order to generate a new biocompatible piezoelectric thin film. Crystal growth process of lead-free BaTiO₃ thin films was designed and experimentally generated on SrTiO₃(100), (110), (111), and MgO(100) substrates using the radio-frequency magnetron sputtering method. Crystal structures of BaTiO₃ were measured by X-ray diffraction (XRD) θ/2θ scan. We used Pt for the electrode and measured piezoelectric strain constants d ₃₃ using the ferroelectric measurement system. As a result, analytical crystal orientation fractions on SrTiO₃(110) and (111) substrates had good quantitative agreement with experimental ones, and ones on SrTiO₃(100) and MgO(100) substrates corresponded with these experimental crystal structures. Furthermore, analytically determined piezoelectric strain constants d ₃₃ qualitatively showed a good agreement with experimental ones. Especially, for SrTiO₃(100) and MgO(100) substrates, the differences of d ₃₃ depending on orientation fractions were analyzed by the three-scale simulation accurately. Consequently, it is confirmed that the three-scale analysis is a useful simulation tool to design new biocompatible piezoelectric thin films.     

Ni-doped ZnCo<Subscript>2</Subscript>O<Subscript>4</Subscript> atomic layers to boost the selectivity in solar-driven reduction of CO<Subscript>2</Subscript>

Regulating the selectivity of CO₂ photoreduction is particularly challenging. Herein, we propose ideal models of atomic layers with/without element doping to investigate the effect of doping engineering to tune the selectivity of CO₂ photoreduction. Prototypical ZnCo₂O₄ atomic layers with/without Ni-doping were first synthesized. Density functional theory calculations reveal that introducing Ni atoms creates several new energy levels and increases the density-of-states at the conduction band minimum. Synchrotron radiation photoemission spectroscopy demonstrates that the band structures are suitable for CO₂ photoreduction, while the surface photovoltage spectra demonstrate that Ni doping increases the carrier separation efficiency. In situ diffuse reflectance Fourier transform infrared spectra disclose that the CO₂^·? radical is the main intermediate, while temperature-programed desorption curves reveal that the ZnCo₂O₄ atomic layers with/without Ni doping favor the respective CO and CH₄ desorption. The Ni-doped ZnCo₂O₄ atomic layers exhibit a 3.5-time higher CO selectivity than the ZnCo₂O₄ atomic layers. This work establishes a clear correlation between elemental doping and selectivity regulation for CO₂ photoreduction, opening new possibilities for tailoring solar-driven photocatalytic behaviors.

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