Influence of hydrothermally modified γ-Al2O3 on the properties of NiMo/γ-Al2O3 catalyst

The influence of hydrothermal modification of γ-Al₂O₃ on the properties of NiMo/γ-Al₂O₃ catalyst was investigated in this paper. The experimental results showed that the use of the modified γ-Al₂O₃ in the preparation of the NiMo/γ-Al₂O₃ catalyst led to the increase of the dispersion of the surface Mo and Ni oxides, favored the formation of the poly-molybdates and promoted the reduction of the active Mo oxides owing to the increase of the surface acidity of the modified γ-Al₂O₃. Therefore, the NiMo/γ-Al₂O₃ catalyst supported on the modified γ-Al₂O₃ exhibited a higher hydrodenitrogenation (HDN) activity than that supported on the untreated γ-Al₂O₃ in the temperature range of 300-340 °C.     

Preparation and high-temperature properties of Au nano-particles doped α-Al2O3 composite coating on TiAl-based alloy

Au nano-particles doped α-Al₂O₃ composite coatings were successfully prepared on TiAl-based alloy by electrodeposition, using the Al₂O₃ sols with minor addition of HAuCl₄ solution. The even distribution of Au nano-particles (<2.0 wt.%) in the α-Al₂O₃ matrix has been observed. Isothermal oxidation tests of the samples coated with the as-prepared novel coatings at 900 °C in static air for 200 h shown that the oxygen inward diffusion can be effectively suppressed to a low level. The results of high-temperature cyclic oxidation test at 900 °C in air revealed that the oxidation and spallation resistance of TiAl-based alloy were improved significantly under thermal cycling. In the as-prepared coatings, cracks were shielded by means of crack bridging and the fracture resistance of the formed scales can be improved by toughening effects of the composite structure. Surface scratching tests after the cyclic oxidation exhibited that the adhesion of the formed composite scale on TiAl-based alloy was remarkably improved by the Au nano-particles doped α-Al₂O₃ composite coating.     

A comparison of electronic structure and optical properties between N-doped β-Ga2O3 and N-Zn co-doped β-Ga2O3

The electronic structure and optical properties of N-doped β-Ga₂O₃ and N-Zn co-doped β-Ga₂O₃ are investigated by the first-principles calculation. In the N-Zn co-doped β-Ga₂O₃ system, the lattice parameters of a, b, c, V decrease and the formation energy of N-Zn co-doped β-Ga₂O₃ is smaller in comparison with N-doped β-Ga₂O₃. There are two shallower acceptor impurity levels in N-Zn co-doped β-Ga₂O₃. Comparing with N-doped β-Ga₂O₃, the major absorption peak is red-shifted and the impurity absorption edge is blue-shifted for N-Zn co-doped β-Ga₂O₃. The results show that the N-Zn co-doped β-Ga₂O₃ is found to be a better method to push p-type conductivity in β-Ga₂O₃.     

Anomalous magnetic properties of an iron film system deposited on fracture surfaces of α-Al2O3 ceramics

An iron film percolation system is fabricated by vapour-phase deposition on fracture surfaces of α-Al2O3 ceramics. The zero-field-cooled （ZFC） and field-cooled （FC） magnetization measurement reveals that the magnetic phase of the film samples evolve from a high-temperature ferromagnetic state to a low-temperature spin-glass-like state, which is also demonstrated by the temperature-dependent ac susceptibility of the iron films. The temperature dependence of the exchange bias field He of the iron film exhibits a minimum peak around the temperature T=5 K, which is independent of the magnitude of the cooling field Hcf. However, for T 〉 10K, （1） He is always negative when Hcf=2kOe and （2） for Hcf= 20 kOe （1Oe≈80 A/m）, He changes from negative to positive values as T increases. Our experimental results show that the anomalous hysteresis properties mainly result from the oxide surfaces of the films with spin-glass-like phase.     

Electronic structure of α-Al2O3 slabs: A local environment study

In this work we performed an ab initio/Density Functional Theory (DFT) study of structural and electronic properties of the (0 0 1) α-Al₂O₃ surface. For this study we used two methods with different basis set: the Full-Potential Augmented Plane Wave plus local orbital (FP-APW+lo) and a linear combination of numerical localized atomic orbital basis sets, employing the WIEN2k code and the SIESTA code, respectively. In order to calculate the structural and electronic properties of the reconstructed surface, we calculated the final equilibrium atomic position with the SIESTA code and then the electric-field gradient (EFG) at Al sites was calculated with the FP-APW+lo code using the optimized positions. Using this procedure we found equilibrium structures with comparative lower energy than those obtained using only the FP-APW+lo method. The EFG tensor and the local structure for Al were studied as a function of the depth from the surface for the relaxed structures. We found that distances down to 6 Å from the surface are sufficient to converge the EFG and the Al–O distances to bulk values. The predicted bulk EFG at the Al site is in good agreement with available experimental values. These results can be used for local probes purposes, e.g., in the case of doping, with important sensitivity for probes located close to the top of the surface, in particular for distances smaller than 6 Å.     

Microwave synthesis, characterization, and electrochemical properties of α-Fe2O3 nanoparticles

Ultrafine α-Fe₂O₃ nanoparticles with an extremely narrow distribution were synthesized by microwave heating. Transmission electron microscopy (TEM) images showed that most primary particles have ellipsoid shapes, and the average diameter of the primary particles was less than 10 nm. The electron diffraction pattern and fringes in some particles in TEM images showed that these nanoparticles were single crystals. The BET surface area of the freeze-dried product was 217 m²/g. The initial discharge capacity of the α-Fe₂O₃ nanoparticles exceeded 1007 mA/g (cut-off voltage: 0.5 V). This large capacity corresponds to that calculated by assuming the reduction of Fe³⁺ to Fe⁰. The α-Fe₂O₃ nanoparticles also work as a rechargeable electrode material. The charge-discharge test between 4 V and 1.5 V gave a good rechargeable capacity of about 150 mAh/g.     

First-principles calculations on the electronic and vibrational properties of β-V2O5

Using a first-principles approach based on density functional theory,this paper studies the electronic and dynamical properties of β-V2O5.A smaller band gap and much wider split-off bands have been observed in comparison with αV2O5.The Ramanand infrared-active modes at the Γ point of the Brillouin zone are evaluated with LO/TO splitting,where the symbol denotes the longitudinal and transverse optical model.The nonresonant Raman spectrum of a βV2O5 powder sample is also computed,providing benchmark theoretical results for the assignment of the experimental spectrum.The computed spectrum agrees with the available experimental data very well.This calculation helps to gain a better understanding of the transition from αto β-V2O5.     

Effects of N-doping concentration on the electronic structure and optical properties of N-doped β-Ga2O3

The electronic structures and the optical properties of N-doped β-Ga2O3 with different N-doping concentrations are studied using the first-principles method.We find that the N substituting O（1） atom is the most stable structure for the smallest formation energy.After N-doping,the charge density distribution significantly changes,and the acceptor impurity level is introduced above the valence band and intersects with the Fermi level.The impurity absorption edges appear to shift toward longer wavelengths with an increase in N-doping concentration.The complex refractive index shows metallic characteristics in the N-doped β-Ga2O3.     

Synthesis and properties of Au-Fe3O4 and Ag-Fe3O4 heterodimeric nanoparticles

Monodisperse Au-Fe 3 O 4 heterodimeric nanoparticles (NPs) were prepared by injecting precursors into a hot reaction solution.The size of Au and Fe 3 O 4 particles can be controlled by changing the injection temperature.UV-Vis spectra show that the surface plasma resonance band of Au-Fe 3 O 4 heterodimeric NPs was evidently red-shifted compared with the resonance band of Au NPs of similar size.The as-prepared heterodimeric Au-Fe 3 O 4 NPs exhibited superparamagnetic properties at room temperature.The Ag-Fe 3 O 4 heterodimeric NPs were also prepared by this synthetic method simply using AgNO 3 as precursor instead of HAuCl 4.It is indicated that the reported method can be readily extended to the synthesis of other noble metal conjugated heterodimeric NPs.     

Investigation of structural,physical and optical properties of CeO2–Bi2O3–B2O3 glasses

xCeO₂–30Bi₂O₃–(70−x) B₂O₃ glasses are synthesized by using the melt quench technique. A number of studies such as XRD, density, molar volume, optical band gap, refractive index and FTIR spectroscopy are employed to characterize the glasses. The band gap decreases from 2.15 to 1.61 eV, refractive index increases from 2.67 to 2.93 and density increases from 4.151 to 4.633 g/cm³. The decrease in band gap with CeO₂ doping approaches the semiconductor behavior. FTIR spectroscopy reveals that incorporation of CeO₂ into glass network helps to convert the structural units of [BO₃] into [BO₄] and results in Bi–O bond vibration of [BiO₆].     

Electronic and thermodynamic properties of α-Pu2O3

Based on density functional theory+U$\text{density functional theory}+U$ calculations and the quasi-annealing simulation method, we obtain the ground electronic state for α-Pu₂O₃ and present its phonon dispersion curves as well as various thermodynamic properties, which have seldom been theoretically studied because of the huge unit cell. We find that the Pu–O chemical bonding is weaker in α-Pu₂O₃ than in fluorite PuO₂, and subsequently a frequency gap appears between oxygen and plutonium vibration density of states. Based on the calculated Helmholtz free energies at different temperatures, we further study the reaction energies for Pu oxidation, PuO₂ reduction, and transformation between PuO₂ and α-Pu₂O₃. Our reaction energy results are in agreements with available experiment. And it is revealed that high temperature and insufficient oxygen environment are in favor of the formation of α-Pu₂O₃.     

Improving protein resistance of α-Al2O3 membranes by modification with POEGMA brushes

A kind of protein-resistant ceramic membrane is prepared by grafting poly(oligo (ethylene glycol) methyl ether methacrylate) (POEGMA) brushes onto the surfaces and pore walls of α-Al₂O₃ membrane (AM) by surface-initiated atom-transfer radical polymerization (SI-ATRP). Contact-angle, Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and field-emission scanning electron microscopy (FESEM) were measured to confirm that the surfaces and pore walls of the ceramic porous membranes have been modified by the brushes with this method successfully. The protein interaction behavior with the POEGMA modified membranes (AM-POEGMA) was studied by the model protein of bovine serum albumin (BSA). A protein-resistant mechanism of AM-POEGMA was proposed to describe an interesting phenomenon discovered in the filtration experiment, in which the initial flux filtrating BSA solution is higher than the pure water flux. The fouling of AM-POEGMA was easier to remove than AM for the action of POEGMA brushes, indicated that the ceramic porous membranes modified with POEGMA brushes exhibit excellent protein resistance.     

Shape- and field-dependent Morin transitions in structured α-Fe2O3

We report shape- and field-dependent magnetic properties of ellipsoid-, spindle-, flattened- and rhombohedra-shaped α-Fe₂O₃ samples prepared by solvothermal technique. We observed that a magnetic spin-flip mechanism, mostly known as Morin transition (T_M), depends on the shape of α-Fe₂O₃ as well as on the applied magnetic field. In each of these structures the obtained value of T_M was less than its bulk value of 263 K. We observed that T_M shifted from highest 251.4 K for ellipsoid to lowest 220.8 K for rhombohedra structure, with intermediate values of T_M for the other two structures. However, for rhombohedra structure T_M shifted from 220.8 to 177.5 K under the external magnetic field of 100 Oe-30 kOe, respectively. The observed lowering of T_M in the structured sample was analyzed in terms of elementary size, shape of the nanocrystallites, lattice parameters and occupancy of Fe⁺³ ions as well. These parameters were determined from the Rietveld refinement process using MAUD software.     

Growth of α-Al2O3:C crystal with highly sensitive optically stimulated luminescence

In this work, an α-Al₂O₃:C crystal was directly grown by the temperature gradient technique (TGT) using Al₂O₃ and graphite powders as the raw materials. The optical, optically stimulated luminescence (OSL) properties and dosimetric characteristics of as-grown crystal were investigated. As-grown α-Al₂O₃:C crystal shows strong absorption band at 205, 230 and 256 nm. Three-dimensional thermoluminescence (TL) emission spectrum of the crystal shows a single emission peak at ∼415 nm. The OSL decay curve can be fitted to two exponentials, the faster component and the slower component. The OSL response of the crystal shows a linear-sublinear-saturation characteristic. As-grown α-Al₂O₃:C crystal shows excellent linearity in the dose range from 5×10⁻⁶ to 50 Gy. For doses higher than the saturation dose (100 Gy), the OSL sensitivity decreases as the dose increases.     

Solvothermal synthesis and characterization of α-Fe2O3 nanodiscs and Mn3O4 nanoparticles with 1,10-phenanthroline

α-Fe₂O₃ nanodiscs and Mn₃O₄ nanoparticles have been prepared by the 1,10-phenanthroline as complexing agent in the presence of sodium hydroxide under hydrothermal conditions. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR) spectra. The average diameter of α-Fe₂O₃ nanodiscs is of 2 μm. In the case of Mn₃O₄ sample, the Mn₃O₄ crystallites are nanoparticles with an average size of 34 nm. A formation mechanism for the α-Fe₂O₃ and Mn₃O₄ nanomaterials was proposed.     

Magnetic properties of nanosized γ-Fe2O3 and α-(Fe2/3Cr1/3)2O3, prepared by thermal decomposition of heterometallic single-molecular precursor

Thermal decomposition of the trinuclear complex [Fe₂CrO(CH₃COO)₆(H₂O)₃]NO₃ at 300, 400 and 500 °C gave γ-Fe₂O₃ nanoparticles along with amorphous chromium oxide, while decomposition of the same starting compound at 600 and 700 °C led to the formation of α-(Fe_2/3Cr_1/3)₂O₃ nanoparticles. Size of γ-Fe₂O₃ nanoparticles, determined by X-ray diffraction, was in the range from 9 to 11 nm and increased with formation temperature growth. Average size of α-(Fe_2/3Cr_1/3)₂O₃ nanoparticles was about 40 nm and almost did not depend on the temperature of its formation. γ-Fe₂O₃ nanoparticles possessed superparamagnetic behavior with blocking temperature 180-250 K, saturation magnetization 29-35 emu/g at 5 K, 44-49 emu/g at 300 K and coercivity 400-600 Oe at 5 K. α-(Fe_2/3Cr_1/3)₂O₃ nanoparticles were characterized by low magnetization values (2.7 emu/g at 70 kOe). Such magnetic properties can be caused by non-compensated spins and defects present on the surface of these nanoparticles. The increase of α-(Fe_2/3Cr_1/3)₂O₃ formation temperature led to decrease of magnetization (being compared for the same fields), which may be caused by decrease of the quantity of defects or non-compensated spins (due to decrease of particles' surface).     

Synthesis of α-Fe2O3 nanobelts and nanoflakes by thermal oxidation and study to their magnetic properties

α-Fe₂O₃ nanobelts and nanoflakes have been successfully synthesized by oxidation of iron-coated ITO glass in air. The X-ray diffraction, Raman spectrum and scanning electron microscopy are carried out to characterize the nanobelts and nanoflakes. The formation mechanism has been presented. Significantly, the magnetic investigations show that the magnetic properties are strongly shape-dependent. The magnetization measurements of belt-like and flake-like α-Fe₂O₃ in perpendicular exhibit ferromagnetic feature with the coercivity (H_c) and saturation magnetization (M_s) of 334.5 Oe and 1.35 emu/g, 239.5 Oe and 0.12 emu/g, respectively. For the parallel, belt-like and flake-like α-Fe₂O₃ also exhibit ferromagnetic feature with the H_c and M_s of 205.5 Oe and 1.44 emu/g, 159.6 Oe and 0.15 emu/g, respectively.     

Structure and photoluminescence properties of epitaxial SnO2 films grown on α-Al2O3 (0 1 2) by MOCVD

SnO₂ thin films have been successfully deposited on α-Al₂O₃ (0 1 2) substrates by metalorganic chemical vapor deposition (MOCVD) in the temperature range 500-700 °C. The films were epitaxially grown in the tetragonal SnO₂ phase and were (1 0 1) oriented. In-plane orientation relationship [0 1 0]SnO₂||[1 0 0]Al₂O₃ and [1 0 1?]SnO₂||[1? 2? 1]Al₂O₃ was determined between the film and substrate. Photoluminescence (PL) spectra measured at room temperature revealed that the film grown at 700 °C showed an intense ultra-violet (UV) PL peak at 333 nm, which was a band-edge emission peak in SnO₂ films. At a temperature of 13 K, a new broad PL band centered at about 480 nm was observed. The corresponding PL mechanisms are discussed in detail.     

Pillaring and photocatalytic properties of mesoporous α-Fe2O3/titanate nanocomposites via an exfoliation and restacking route

Mesoporous α-Fe₂O₃-pillared titanate nanocomposites have been successfully synthesized through an exfoliation−restacking route. Powder X-ray diffraction and N₂ adsorption-desorption isotherms revealed that the α-Fe₂O₃ pillared titanate has an interlayer distance of 3.27 nm, a specific surface area of 66 m²/g and an average pore size of 7.6 nm, suggesting the formation of a mesoporous pillared structure. UV-vis diffuse reflectance spectra show a red shift, indicative of a narrow band gap energy of ∼2.1 eV compared to the host layered titanate, which is essential in creating a visible light photocatalytic activity. The results of degradation of rhodamine B reveal that the present pillared mesoporous composites exhibit better photocatalytic activities than those of the pristine materials under visible irradiation, based on the band gap excitement and the dye-sensitized path, originated from their high surface area, mesoporosity and the electronic coupling between the host and the guest components.