Magneto Optical Properties of FeB<Subscript>x</Subscript>Fe<Subscript>2−x</Subscript>O<Subscript>4</Subscript> Nanoparticles

In this study, FeB_xFe_2?xO₄ nanoparticles (NPs) were synthesized by the polyol method. The M–H hysteresis curves exhibit superparamagnetic characteristics that are both coercivity and remanent magnetization values are negligible. The particle size dependent Langevin function was applied to calculate the magnetic particle dimensions around 9 nm. The measured magnetic moments of NPs are in range of (1.52–2.2) µ_B and almost half or less with respect to 4 µ_B of bulk Fe ferrite. Magnetic anisotropy was specified as uniaxial and calculated effective anisotropy constants (K _eff ) are between 43.3 × 10⁴ and 19.4 × 10⁴ emu/g. The UV–Vis diffuse reflectance spectroscopy and Kubelka–Munk theory were used to determine the optical properties. The estimated optical band gap values (2.15–2.48 eV) of FeB_xFe_2?xO₄ NPs are bigger with respect to reported values (1.88–2.12 eV) for Fe₃O₄ NPs in the literature. The bigger E _g values are mainly attributed to B concentration and partly to quantum confinement effect.     

Biodiesel Preparation from Soybean Oil by Using a Heterogeneous Ca<Subscript>x</Subscript>Mg<Subscript>2−x</Subscript>O<Subscript>2</Subscript> Catalyst

Abstract  

An effective heterogeneous catalyst, Ca_xMg_2−xO₂, was prepared and tested for soybean oil transesterification with methanol. The catalysts were characterized by using X-ray diffraction , Fourier transform infrared spectra, thermo gravimetric and differential thermal analysis , and Hammett indicator method. The catalyst with Ca/Mg ratio of 1.0 and calcined at 800 °C exhibited high catalytic activities. Under the suitable transesterification conditions (methanol/oil ratio 12:1, catalyst loading 6 wt%, reaction time 5 h, at reflux of methanol), the oil conversion of 91.3% could be achieved. The catalyst can be easily recovered and reused without significant deactivation.     

Speciation and Electronic Structure of La<Subscript>1−x</Subscript>Sr<Subscript>x</Subscript>CoO<Subscript>3−δ</Subscript> During Oxygen Electrolysis

Cobalt-containing perovskite oxides are promising electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolyzers. However, a lack of fundamental understanding of oxide surfaces impedes rational catalyst design for improved activity and stability. We couple electrochemical studies of epitaxial La_1?xSr_xCoO_3?δ films with in situ and operando ambient pressure X-ray photoelectron spectroscopy to investigate the surface stoichiometry, adsorbates, and electronic structure. In situ investigations spanning electrode compositions in a humid environment indicate that hydroxyl and carbonate affinity increase with Sr content, leading to an increase in binding energy of metal core levels and the valence band edge from the formation of a surface dipole. The maximum in hydroxylation at 40% Sr is commensurate with the highest OER activity, where activity scales with greater hole carrier concentration and mobility. Operando measurements of the 20% Sr-doped oxide in alkaline electrolyte indicate that the surface stoichiometry remains constant during OER, supporting the idea that the oxide electrocatalyst is stable and behaves as a metal, with the voltage drop confined to the electrolyte. Furthermore, hydroxyl and carbonate species are present on the electrode surface even under oxidizing conditions, and may impact the availability of active sites or the binding strength of adsorbed intermediates via adsorbate–adsorbate interactions. For covalent oxides with facile charge transfer kinetics, the accumulation of hydroxyl species with oxidative potentials suggests the rate of reaction could be limited by proton transfer kinetics. This operando insight will help guide modeling of self-consistent oxide electrocatalysts, and highlights the potential importance of carbonates in oxygen electrocatalysis.     

Exploring the Redox Behavior of La<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>Mn<Subscript>1−x</Subscript>Al<Subscript>x</Subscript>O<Subscript>3</Subscript> Perovskites for CO<Subscript>2</Subscript>-Splitting in Thermochemical Cycles

Mixed oxides with perovskite structure have been proposed as promising alternative for the solar fuel production via thermochemical redox cycles. For this work, the system La_0.6Sr_0.4Mn_1?xAl_xO₃ (x?=?0 to 0.8) was selected according to its high thermal stability and rapid oxidation kinetics, and the influence of the Al/Mn ratio on the redox properties was investigated. The characterization of the five oxides samples with different Al content confirmed the high redox capacity and the favorable behavior of these materials in consecutive cycles, as analyzed thermogravimetrically. The results show that following reduction at 1300?°C in inert atmosphere up to 0.32 mmol g^?1 of O₂ are released, while a 10-cycle reaction test confirms the feasibility of long term operation with these perovskites. It was observed that the reduction extent was enhanced with increasing the Al-content, but the oxidation degree is maximum for compositions near x?=?0.5, corresponding to an O₂ release of 0.318 mmol g^?1 (δ?=?0.132). After selecting the compositions with more promising redox properties, additional reactions were performed in a lab-scale fixed bed reactor with injection of CO₂ in the oxidation step at 900?°C in order to generate CO. In these tests, the most interesting results were obtained for the perovskite La_0.6Sr_0.4Mn_0.6Al_0.4O_3, with reduction extent of 0.266 mmol?g^?1, but the production of CO is in comparison significantly lower (0.114 mmol?g^?1). Further studies are required to determine the best operation conditions for thermochemical cycles using those materials.     

Synthesis of magnetically separable core–shell structured Ni<Subscript>x</Subscript>Fe<Subscript>1−x</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript>@TiO<Subscript>2</Subscript> nanoparticles photocatalysts for the degradation of organic dyes

In this paper, the core–shell structured NiFe₂O₄@TiO₂ nanoparticles and nanochains as photocatalysts were successfully prepared through hydrothermal and hydrolysis method. The as-prepared core–shell structure was composed of a magnetic NiFe₂O₄ core and photocatalytic titanium oxide coating shell. SEM and TEM images characterized the morphology of NiFe₂O₄@TiO₂ nanoparticles. Moreover, the results of XRD patterns proved that the TiO₂ coating shell consisted of anatase. The VSM measurements showed that the saturation magnetization values of NiFe₂O₄ and NiFe₂O₄@TiO₂ nanoparticles was 65 and 53 emu/g, respectively. The photocatalyst of NiFe₂O₄@TiO₂ nanoparticles exhibited the outstanding recyclable performance for RhB. And, the photo_degradation ration of maintained 69 % after the photocatalyst experienced ten photocatalysis experiments, which is better than that of Fe₃O₄@TiO₂ photocatalysts.     

Structural,Optical and Mössbauer Study of Ba<Subscript>1 − x</Subscript>Cu<Subscript>x</Subscript>Fe<Subscript>12</Subscript>O<Subscript>19</Subscript> (0.5 ≤ x) Nano Hexaferrites

In this study, a novel Ba_1?xCu_xFe₁₂O₁₉ (0.5?≤?x) nano-hexaferrites were prepared by a simple and cost-effective sol–gel auto-combustion method using barium nitrates, iron nitrate, copper (II) acetate monohydrate and citric acid, and its structural, optical properties and hyperfine interactions were reported. Structural properties were analyzed through XRD (X-ray diffraction), Scanning electron microscopy (SEM), and TEM (Transmission electron microscopy), while percent diffuse reflectance spectroscopy (DRS) and Mössbauer spectrometer were used for analyzing the optical and magnetic properties of the resultant products. The observed Mössbauer studies proved the ferromagnetic nature of nanoparticles (NPs) samples. The crystallite size (XRD) varies in a range of (23.30–35.12) nm. The direct optical energy band gap (E _g ) of all samples were calculated by Tauc plots where the E _g values are found in a small range of 1.97–2.15 eV. The experimental evidences signify the promising use of newly prepared nano-hexaferrites in the development of materials in various industrial devices and far better than the conventional available hexaferrites materials.     

Preparation of Nanostructured Si<Subscript>x</Subscript>N<Subscript>1−x</Subscript> Thin Films by DC Reactive Magnetron Sputtering for Tribology Applications

In this work, nanostructured silicon nitride (Si_xN_1?x) thin films were prepared by reactive magnetron sputtering using an Ar/N₂ gas mixture of 1:1. The structures and fractional compositions of the prepared samples were determined by x-ray diffraction (XRD) and electron-dispersion x-ray diffraction (EDS) patterns as functions of inter-electrode distance. They showed that the prepared films were polycrystalline and the partial amount of silicon (x) is in the range 0.825–0.865 as the inter-electrode distance was in the range 2.5–7.5 cm. The particle sizes of the prepared nanostructures were determined by field-effect scanning electron microscopy (FE-SEM) to be about 38 nm. The measured Vickers microhardness of the prepared films showed relatively high values (570-750 kg.f/mm²) and decreased with decreasing film thickness, which is inversely proportional to the inter-electrode distance. These results encourage using these nanostructures for coating of wearable tools in industrial tribology applications.     

Direct-hydrothermal synthesis of LiFe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>PO<Subscript>4</Subscript> cathode materials

Carbon free LiFe_1−x Mn _x PO₄ (x = 0, 0.05, 0.1, 0.2, 0.4) cathode materials were prepared by a direct-hydrothermal process at 170 °C for 10 h. The structural and electrochemical properties of the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), charge–discharge experiments, cyclic voltammetry (CV) and alternating current (AC) impedance spectroscopy. The electrochemical performance of LiFePO₄ prepared in this manner showed to be positively affected by Mn²⁺-substitution. Among the Mn²⁺-substitution samples, the LiFe_0.9Mn_0.1PO₄ exhibited an initial discharge capacity of 141.4 mA h g⁻¹ at 0.1 C, and the capacity fading is only 2.7% after 50 cycles.     

Spatial localization of holes in La<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CuO<Subscript>4</Subscript>

The parameters of the tensor of the electric field gradient (EFG) in cation sites of the La_{2 ? x} Sr _x CuO₄ lattice have been determined by the method of emission Mössbauer spectroscopy on ⁵⁷Co(^57m Fe), ⁶⁷Cu(⁶⁷Zn), ⁶⁷Ga(⁶⁷Zn), and ¹⁵⁵Eu(¹⁵⁵Gd) isotopes. There is no quantitative agreement between the calculated (the pointcharge model) and experimental values of the main component of the tensor EFG V _zz , which is explained by the absence of the reliable data on the Sternheimer coefficients for Fe³⁺, Zn²⁺, and Gd³⁺ ions. Based on the comparison of the calculated and experimental dependences of V _zz on x it was shown that the holes appearing during the substitution of La³⁺ for Sr²⁺ are localized preferably on the oxygen atoms that are in the same plane as the copper atoms, which is in agreement with the model discussed in the literature and assumes that the mechanism responsible for the high-temperature superconductivity of solid solutions La_{2 ? x} Sr _x CuO₄ is the interaction between the conductivity electrons and two-atomic two-electron centers with negative correlation energy.     

Synthesis and proton incorporation in BaCe<Subscript>0.9−<Emphasis Type="Italic">x</Emphasis></Subscript>Zr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Y<Subscript>0.1</Subscript>O<Subscript>3−δ</Subscript>

BaCe_0.9−x Zr _x Y_0.1O_3−δ powders were synthesized by a solid-state method at 1,400 °C. Two compositions were studied (x = 0.3 and x = 0.7). Pellets were prepared and conventionally sintered in air at 1,700 °C. Then, the samples were heated at 600 °C for 3 h in different reducing atmospheres: dry hydrogen, wet hydrogen and wet deuterium. After each treatment, the proton diffusion depth profile was obtained using Secondary Ion Mass Spectroscopy (SIMS). Protons were not incorporated in the material when the gas was not wet, and the isotope effect suggests that protons present in the pellet come from water and not from hydrogen.     

Thermal expansion and phase transitions in K<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cs<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>BSi<Subscript>2</Subscript>O<Subscript>6</Subscript> borosilicate solid solutions

Solid solutions K_{1 − x} Cs _x BSi₂O₆ (x (atomic fraction) = 0.00, 0.20, 0.30, 0.35, 0.40, 0.80, 0.90,1.00) have been prepared by solid-phase synthesis and crystallization of glasses in the KBSi₂O₆-CsBSi₂O₆ borosilicate series. The thermal behavior of the solid solutions has been investigated using the annealing and quenching techniques, dilatometry, and high-temperature X-ray powder diffraction. It has been shown that solid solutions with x = 0.00–0.35 and 0.40–1.00 correspond to space groups I $ \bar 4 $ \bar 4 3d and I a $ \bar 3 $ \bar 3 d, respectively. The cubic-cubic phase transition I $ \bar 4 $ \bar 4 3d ai Ia $ \bar 3 $ \bar 3 d occurs in the composition range x = 0.35−0.40 at room temperature. In the series of solid solutions with x = 0−0.30, the reversible cubic-cubic polymorphic transition I $ \bar 4 $ \bar 4 3d ai ia $ \bar 3 $ \bar 3 d with an increase in the temperature has been revealed, the temperature of the polymorphic transition has been determined, and the thermal expansion in both polymorphic modifications has been studied using high-temperature X-ray diffraction and dilatometry. The solid solutions belonging to the space group Ia $ \bar 3 $ \bar 3 d are characterized by a lower thermal expansion than the solid solution belonging to the space group I $ \bar 4 $ \bar 4 3d. According to the calculations, the equivalents α/γ for the space groups I $ \bar 4 $ \bar 4 3d and Ia $ \bar 3 $ \bar 3 d are 0.03 and 0.02 (in cesium atomic fractions per degree Celsius), respectively.     

Solvothermal Synthesis and Catalytic Properties of Nanocrystalline ZnFe<Subscript>2−x</Subscript>Al<Subscript>x</Subscript>O<Subscript>4</Subscript> (x = 0, 1, 2) Spinels in Aniline Methylation

Abstract  

ZnFe_2−xAl_xO₄ (x = 0, 1, 2) spinels were obtained by microwave-assisted solvothermal method using 1,4-butanediol as reaction medium. The results of XRD and HRTEM studies have indicated higher nanocrystallinity of aluminium containing spinels, and N₂ adsorption–desorption measurements have revealed their enhanced textural properties, in particular much higher specific surface areas. NH₃-TPD method and cyclohexanol test have shown the variation of surface acid–base properties with the changes in the spinel composition. All studied spinels were active in aniline methylation and proved to be selective for N-methylation leading to N-methylaniline and N,N-dimethylaniline. The main advantage of aluminium containing spinels is that N-alkylation with methanol is possible at significantly lower temperature (200–260 °C) when pure zinc ferrite is almost inactive.     

Polyol Mediated Solvothermal Synthesis and Characterization of CuIn<Subscript>(1−x)</Subscript>Ga<Subscript>x</Subscript>S<Subscript>2</Subscript> Nanocrystals

Gallium-substituted copper indium sulfide (CuIn_(1?x)Ga_xS₂) nanoparticles have been synthesized by a convenient solvothermal method without usage of surfactants or toxic reductants such as hydrazine. Thiourea, sodium hydroxide, CuCl₂·2H₂O, InCl₃ and GaCl₃ were used as starting materials and ethylene glycol as solvent and reducing agent. The reactions were performed at 200 °C for 16 h. Effect of sodium hydroxide on the reaction products is analyzed. The powders are mainly characterized by X-ray diffraction, Raman spectroscopy, transmission electron microscopy, BET surface area measurements and UV–Vis absorption spectroscopy. The results show that the gallium is successfully incorporated into the chalcopyrite crystal structure. The homogeneous powders obtained are constituted of nanoparticles with sizes in the range 20–30 nm and exhibit a specific surface area close to 65 m²/g. Further, the possible mechanism for the formation of CuIn_(1?x)Ga_xS₂ nanocrystals is explained. The optical band gap energies of the nanoparticles were in the range 1.48–1.75 eV.     

Synthesis of La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3 + δ</Subscript> manganites by pyrohydrolysis of nitrates

Single-phase finely dispersed perovskite-like manganites La_{1 ? x} Sr _x MnO_{3 + δ} (0 ≤ x ≤ 0.33) with an average particle size of approximately 3μm were synthesized by the pyrohydrolytic method from a stoichiometric mixture of the corresponding metal nitrates at a temperature of 500°C in a water vapor atmosphere. The parameter δ was changed as a result of the subsequent heat treatment. It was established that the manganite La_0.67Sr_0.33MnO_{3 + δ} synthesized by the pyrohydrolytic method is characterized by a more pronounced change in the magnetoresistance as compared to the manganite that had the same composition but was synthesized according to the conventional ceramic technique.     

Optical absorption and disorder in the As<Subscript>40</Subscript>S<Subscript>60 − <Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> vitreous alloys

The absorption edge spectra of vitreous alloys in the As-S-Se system are measured in the temperature range 77–300 K. The parameters of the fundamental absorption edge and the electron-phonon interaction in As₄₀S_{60 ? x} Se _x (x = 0–40) glasses are determined, and the influence of the compositional disorder on these parameters is investigated. The contributions of the static structural and temperature disorders to the smearing of the fundamental absorption edge of the alloys under study are evaluated.     

Magnetic and structural inhomogeneities of the La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3 + δ</Subscript> manganites

The specific features revealed in the behavior of the parameters of the crystal structure and the lattice dynamics of the La_{1 ? x} Ca _x MnO_{3 + δ} manganites upon the transition to a magnetically ordered state are investigated using X-ray powder diffraction and inelastic neutron scattering. The results obtained are explained in terms of an inhomogeneous state of the samples. This state manifests itself in the form of local regions that are enriched or depleted in charge carriers, with the former regions characterized by the highest temperature of ferromagnetic ordering in the system under investigation. A comparison of the evolution of the crystal structure with variations in the temperature both in the presence and in the absence of a magnetic field has demonstrated that the inhomogeneities are eliminated by an external magnetic field and that the regions enriched in charge carriers become predominant in the system.     

Thermal expansion and phase transitions in K<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Rb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>BSi<Subscript>2</Subscript>O<Subscript>6</Subscript> leucite borosilicate solid solutions

Continuous solid solutions and the reversible phase transition from the I-43d cubic phase to the Ia-3d cubic phase are revealed in the borosilicate series K_{1 ? x} Rb _x BSi₂O₆. Samples in the KBSi₂O₆-RbBSi₂O₆ system are prepared by solid-phase synthesis and crystallization of glasses and investigated using the annealing and quenching technique, high-temperature X-ray diffraction, and dilatometry. The above polymorphic phase transition is observed in all solid solutions at temperatures in the range from 330 to 430°C depending on the composition: an increase in the rubidium content in the solid solution leads to a gradual decrease in the phase transition temperature. The linear thermal expansion coefficients α are determined for solid solutions of different crystalline modifications and glasses. The linear thermal expansion coefficients α for the I-43d low-temperature phase are equal to (20–23) × 10^?6 K^?1 according to the X-ray diffraction data and (21–24) × 10^?6 K^?1 according to the dilatometric data. The values of α for the Ia-3d high-temperature phase lie in the range (4–9) × 10^?6 K^?1 according to the X-ray diffraction data and in the range (6–9) × 10^?6 K^?1 according to the dilatometric data. The linear thermal expansion coefficients for both modifications decrease with an increase in the rubidium content in the solid solutions. The linear thermal expansion coefficients for glasses α = (10–11) × 10^?6 K^?1 are close to those for the high-temperature modification and virtually independent of the sample composition. The I-43d (cubic) ai I4₁/a (tetragonal) o Ia-3d (cubic) polymorphic phase transitions in the KBSi₂O₆ compound are revealed by differential scanning calorimetry (DSC) and dilatometry. Their reversibility is confirmed by the DSC data.     

Sol-Gel Synthesis,Thermal Behavior of Nanopowders and Chemical Stability of La<Subscript>1 – <Emphasis Type="Italic">x</Emphasis></Subscript>Ho<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>PO<Subscript>4</Subscript> Ceramic Matrices

Nanosized powders of orthophosphates in the LaPO₄–HoPO₄–H₂O system have been synthesized to determine the mutual solubility of LaPO₄ · nH₂O and HoPO₄ · nH₂O initial components and to obtain ceramic matrices by sintering them. Formation of hexagonal, monoclinic or tetragonal solid solutions was revealed, and their limits and thermal stability were determined. A series of limited hexagonal LaPO₄ · nH₂O-based solid solutions was observed within the 0 ≤ x ≤ 0.6 concentration range up to 600°C. Further they transformed to monoclinic LaPO₄-based form within the 0 ≤ x ≤ 0.3 concentration range. Solubility of LaPO₄ · nH₂O and LaPO₄ in tetragonal HoPO₄(· nH₂O) is lower (≤10 mol %). Specific surface area of La_1–xHo_xPO₄ · nH₂O powders was in the range of 90.5–165.0 m²/g depending on x. Leaching rate of La³⁺ and Ho³⁺ from La_1–xHo_xPO₄ matrices in nitric acid solution (pH 1–2) was determined to be 10^–5–10^–2 g/(cm² day) for both ions.     

Study of Oxygen Reactivity in La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CoO<Subscript>3−δ</Subscript> Perovskites for Total Oxidation of Toluene

Abstract  

The total oxidation of toluene is studied over catalytic systems based on perovskite with general formula AA′CoO_3-δ (A = La, A′ = Sr). The systematic and progressive substitution of La³⁺ by Sr²⁺ cations in the series (La_1−x Sr _x CoO_3−δ system) of the perovskites have been studied to determine their influence in the final properties of these mixed oxides and their corresponding reactivity performance for the total oxidation of toluene as a model volatile organic compound with detrimental effects for health and environment. The structure and morphology of the samples before and after reaction have been characterized by XRD, BET and FE-SEM techniques. Additional experiments of temperature programmed desorption of O₂ in vacuum and reduction in H₂ were also performed to identify the main surface oxygen species and the reducibility of the different perovskites. It is remarkable that the La_1−x Sr _x CoO_3−δ series presents better catalytic performance for the oxidation of toluene, with lower values for the T₅₀ (temperature of 50 % toluene conversion) than the previously studied LaNi_1−y Co _y O₃ series.