Combustion synthesis of magnesium ferrite as liquid petroleum gas (LPG) sensor: Effect of sintering temperature

We report synthesis of Spinel type magnesium ferrite (MgFe₂O₄) material by a simple, inexpensive combustion method with glycine as a fuel and their application as a gas sensor for reducing gases (LPG, Acetone, Ethanol, Ammonia). The dependence of reducing gas sensing properties on the structural and surface morphological properties has been studied as an effect of sintering temperatures. The structural and surface morphological properties were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The MgFe₂O₄ were highly oriented along (311) with the spinel type crystal structure. The SEM observation reveals that porous morphology decreases due to the grain growth as sintering temperature increases. The mechanism of reducing gas sensing by the MgFe₂O₄ pellets is explained on the basis of adsorbed oxygen on the sensor surface. The selectivity and maximum response of 71% to 2000 ppm of LPG was observed at 698 K with the (MgFe₂O₄) material sintered at 1173 K.     

The nature of antimony-enriched surface layer of Fe-Sb mixed oxides

Antimony segregation is a common feature in Fe-Sb mixed oxides, which have been widely applied as catalysts in selective oxidation and ammoxidation reactions. This paper attempts to shed a light on the cause of such a common feature and on the nature of the antimony-enriched surface layer over FeSbO₄ by means of XPS surface analysis. Single-phase FeSbO₄ samples prepared by different methods were studied, and the antimony in their surface layer is a mixture of both Sb⁵⁺ and Sb³⁺ rather than single Sb⁵⁺. Their surface composition is close to FeSb₂O₆, which could be described as (FeSbO₄)(Sb₂O₄)_δ, δ = 0.5, and it is not “Fe(II)Sb(V)₂O₆” as suggested in literature. Fe-Sb mixed oxides with Sb/Fe > 1 (mol/mol) are mixtures of FeSbO₄ and Sb₂O₄, and the surface of FeSbO₄ grains would be a layer of (FeSbO₄)(Sb₂O₄)_δ, δ ≥ 0.5. Fe-Sb mixed oxides with Sb/Fe < 1 are mixtures of FeSbO₄ and Fe₂O₃, and the surface of FeSbO₄ grains would be a layer of (FeSbO₄)(Sb₂O₄)_δ, δ ≤ 0.5, but the remaining Fe₂O₃ would be encapsulated by a layer of FeSbO₄.     

Preparation of spherical and uniform-sized ferrite nanoparticles with diameters between 50 and 150 nm for biomedical applications

Spherical uniform-sized iron ferrite nanoparticles were synthesized by adding a disaccharide and seed ferrite crystals into an aqueous reaction solution. The average size range 50-150 nm was controlled by choosing one out of five disaccharides and by changing the amount of the seed crystals. The particles had a saturation magnetization and a crystalline structure which are similar to those of intermediate Fe₃O₄-γ-Fe₂O₃. When coated with citrate, the particles with nearly 100 nm diameter were stably suspended in water for 2 days. These novel particles will be utilized as magnetic carriers in biomedical applications.     

The effect of surface morphology on the response of Fe2O3-loaded vanadium oxide nanotubes gas sensor

The effect of surface morphology on the response of an ethanol sensor based on vanadium nanotubes surface loaded with Fe₂O₃ nanoparticles (Fe₂O₃/VONTs) was investigated in this work. The particle size of Fe₂O₃ loaded on VONTs was varied by using novel citric acid-assisted hydrothermal method. In the synthesis progress, citric acid was used as a surfactant and chelate agent, which ensured the growth of a uniform Fe₂O₃ loading on the nanotubes surface. The ethanol sensing properties was then measured for these Fe₂O₃/VONTs at 230-300 °C. The results showed that the sensor response increased with the particles size and the loading amount of Fe₂O₃. It appears that the load of Fe₂O₃ on the VONTs surface increases the concentration of oxygen vacancies and decreases the concentration of free electrons. The effects of morphology on the sensor resistance were interpreted in terms of the Debye length and the difference in the number of active sites.     

Size-controlled synthesis of superparamagnetic iron oxide nanoparticles and their surface coating by gold for biomedical applications

The size mono-dispersity, saturation magnetization, and surface chemistry of magnetic nanoparticles (NPs) are recognized as critical factors for efficient biomedical applications. Here, we performed modified water-in-oil inverse nano-emulsion procedure for preparation of stable colloidal superparamagnetic iron oxide NPs (SPIONs) with high saturation magnetization. To achieve mono-dispersed SPIONs, optimization process was probed on several important factors including molar ratio of iron salts [Fe³⁺ and Fe²⁺], the concentration of ammonium hydroxide as reducing agent, and molar ratio of water to surfactant. The biocompatibility of the obtained NPs, at various concentrations, was evaluated via MTT (3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide) assay and the results showed that the NPs were non-toxic at concentrations <0.1 mg/mL. Surface functionalization was performed by conformal coating of the NPs with a thin shell of gold (∼4 nm) through chemical reduction of attached gold salts at the surface of the SPIONs. The Fe₃O₄ core/Au shell particles demonstrate strong plasmon resonance absorption and can be separated from solution using an external magnetic field. Experimental data from both physical and chemical determinations of the changes in particle size, surface plasmon resonance optical band, phase components, core–shell surface composition, and magnetic properties have confirmed the formation of the mono-dispersed core–shell nanostructure.     

Relationships between surface compositions and properties of surfaces of mixed fumed oxides

Fumed oxides SiO₂/Al₂O₃ (SA), SiO₂/TiO₂ (ST) and Al₂O₃/SiO₂/TiO₂ (AST) at different content of alumina and titania were investigated by one-pass temperature-programmed desorption (OPTPD) time-of-flight mass-spectrometry (TOFMS), Auger electron spectroscopy (AES), NMR, FTIR, thermally stimulated depolarization current (TSDC), microcalorimetry, adsorption of nitrogen, water, (dimethylamino)azobenzene (DMAAB) and metal ions (Pb(II) and Ni(II)). It was shown that all the studied adsorption/desorption and energetic properties of mixed fumed oxides depend strongly on the surface content of alumina (shown as a surface content of aluminum, ) in SA and AST and titania (shown as a surface content of titanium, ) in ST and AST. Many of these properties demonstrate clear correlations with the and values over the total range of alumina and titania content in the materials.     

Synthesis and magnetic properties of ordered barium ferrite nanowire arrays in AAO template

BaFe₁₂O₁₉ nanowire arrays having single magnetic domain size (≤460 nm) in anodic aluminum oxide (AAO) templates were prepared by sol-gel and self-propagating high-temperature synthesis techniques. The diameter of the nanowire arrays is approximately 70 nm and the length is about 2-4 μm. The specimens were characterized using X-ray diffraction, vibrating sample magnetometer, field emission scan electron microscope, atomic force microscopy and microwave vector network analyzer. The magnetic properties of BaFe₁₂O₁₉ nanowire arrays embedded in AAO templates were measured by VSM with a field up to 1274 KA/m at room temperature. The results indicate that the nanowire arrays exhibit large saturation magnetization and high coercivity in the range of 6000 Oe and an obvious magnetic anisotropy with the easy magnetizing axis along the length of the nanowire arrays, probably due to the shape anisotropy and magneto-crystalline anisotropy. Finally the microwave absorption properties of the nanowires were discussed.     

Synthesis and characterization of vanadium nanoparticles on activated carbon and their catalytic activity in thiophene hydrodesulphurization

Vanadium nanoparticles (∼7 nm) stabilized on activated carbon were synthesized by the reduction of VCl₃·3THF with K[BEt₃H]. This material was characterized by inductive coupled plasma-atomic emission spectroscopy (ICP-AES), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) analyses. The catalytic performance of the carbon-supported vanadium was studied using thiophene hydrodesulfurization (HDS) as model reaction at 300 °C and P = 1 atm. The catalytic activity of the vanadium carbide phase on the activated carbon carrier was more significant than that of the reference catalysts, alumina supported NiMoS. The method proposed for the synthesis of such a catalyst led to an excellent performance of the HDS process.     

In-situ bridging of SnO2 nanowires between the electrodes and their NO2 gas sensing characteristics

A simple and efficient way of making highly sensitive SnO₂ nanowire-based gas sensors without an individual lithography process was studied. The SnO₂ nanowires network was floated upon the Si substrate by separating the Au catalyst layer from the substrate. As the electric current is transported along the networks of the nanowires, not along the surface layer on the substrate, the gas sensitivities could be maximized in this networked and floated structures. The sensitivity was 5-30 when the NO₂ concentration was 1-10 ppm. The response time was ca. 20-60 s.     

Effect of Li2O and CoO-doping of CuO/Fe2O3 system on its surface and catalytic properties

Physicochemical, surface and catalytic properties of pure and doped CuO/Fe₂O₃ system were investigated using X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX), nitrogen adsorption at −196 °C and CO-oxidation by O₂ at 80-220 °C using a static method. The dopants were Li₂O (2.5 mol%) and CoO (2.5 and 5 mol%). The results revealed that the increase in precalcination temperature from 400 to 600 °C and Li₂O-doping of CuO/Fe₂O₃ system enhanced CuFe₂O₄ formation. However, heating both pure and doped solids at 600 °C did not lead to complete conversion of reacting oxides into CuFe₂O₄. The promotion effect of Li₂O dopant was attributed to dissolution of some of dopant ions in the lattices of CuO and Fe₂O₃ with subsequent increase in the mobility of reacting cations. CoO-doping led also to the formation of mixed ferrite Co_xCu_1−xFe₂O₄. The doping process of the system investigated decreased to a large extent the crystallite size of unreacted portion of Fe₂O₃ in mixed solids calcined at 600 °C. This process led to a significant increase in the S_BET of the treated solids. Doping CuO/Fe₂O₃ system with either Li₂O or CoO, followed by calcination at 400 and 600 °C decreased its catalytic activity in CO-oxidation by O₂. However, the activation energy of the catalyzed reaction was not much affected by doping.     

Hydrothermal synthesis of C-doped Zn3(OH)2V2O7 nanorods and their photocatalytic properties under visible light illumination

A visible light-driven photocatalyst, C-doped Zn₃(OH)₂V₂O₇, prepared by a hydrothermal method was studied. The as-prepared catalyst was characterized by SEM, XRD, DRS, and XPS, and exhibited efficient photocatalytic activity in the degradation of methylene blue (MB) under visible-light irradiation. Besides decoloring, the decomposition of MB was also observed, further demonstrating the performance of the photocatalyst. The carbon existing on the surface of Zn₃(OH)₂V₂O₇ nanorods was free and in carbide form. Dye degradation followed first-order kinetics, and was explained on the basis of the Langmuir-Hinshelwood mechanism.     

A study of nano-sized surface coating on LiMn2O4 materials

The present study used the Pechini process with a heat treatment to synthesize LiMn₂O₄ powder (LMO). After surface coating, a nano-sized oxidative film of Li-Ni-Mn formed on the surface of the LMO-Ni powders. The concentration of Mn³⁺ for the LMO-Ni powder was lower and the average electrovalence of Mn exceeded the theoretical value, resulting in the initial capacity of the LMO-Ni powder being lower than the LMO powder. However, the LMO-Ni powder with the Li/Ni film not only restrained Mn ions from dissolving into the electrolyte, but also improved the charge-discharge cycling capacity.     

Heat generation ability in AC magnetic field and their computer simulation for Ti tube filled with ferrite powder

The heat generation ability of needle-type materials was studied for the application of thermal coagulation therapy in an AC magnetic field. Although the Ti tube without the MgFe₂O₄ powder or Ti rod showed poor heat generation abilities in an AC magnetic field, the temperature was significantly increased by the presence of ferrite powder in the Ti tube. We confirmed using a computer simulation that the eddy loss of the Ti tube was increased by the enhanced magnetic flux density due to the ferrite powder in the Ti tube. The heat generation of the ferrite filled Ti tube was increased by utilization of the quenched MgFe₂O₄ powder from elevated temperature. The relative magnetic permeability of the quenched ferrite was enhanced with the decrease in the inverse ratio of the cubic spinel structure. The heat generation ability was increased with the increase in the relative magnetic permeability of the Ti tube with ferrite powder. The calculated joule loss based on the experimental results showed an agreement with those using the computer simulation.     

Formation of CdO films from chemically deposited Cd(OH)2 films as a precursor

Formation of cadmium hydroxide at room temperature onto glass substrates from an aqueous alkaline cadmium nitrate solution using a simple soft chemical method and its conversion to cadmium oxide (CdO) by thermal annealing treatment has been studied in this paper. The as-deposited film was given thermal annealing treatment in oxygen atmosphere at 450 °C for 2 h for conversion into cadmium oxide. The structural, surface morphological and optical studies were performed for as-deposited and the annealed films. The structural analyses revealed that as-deposited films consists of mixture of Cd(OH)₂ and CdO, while annealed films exhibited crystalline CdO. From surface morphological studies, conversion of clusters to grains after annealing was observed. The band gap energy was changed from 3.21 to 2.58 eV after annealing treatment. The determination of elementals on surface composition of the core-shell nanoparticles of annealed films was carried out using X-ray photoelectron spectroscopy (XPS).     

Preparation and magnetic properties of BaFe12O19/Ni0.8Zn0.2Fe2O4 nanocomposite ferrite

Nanocomposite of hard (BaFe₁₂O₁₉)/soft ferrite (Ni_0.8Zn_0.2Fe₂O₄) have been prepared by the sol–gel process. The nanocomposite ferrite are formed when the calcining temperature is above 800 °C. It is found that the magnetic properties strongly depend on the presintering treatment and calcining temperature. The “bee waist” type hysteresis loops for samples disappear when the presintering temperature is 400 °C and the calcination temperature reaches 1100 °C owing to the exchange-coupling interaction. The remanence of BaFe₁₂O₁₉/Ni_0.8Zn_0.2Fe₂O₄ nanocomposite ferrite with the mass ratio of 5:1 is higher than a single phase ferrite. The specific saturation magnetization, remanence magnetization and coercivity are 63 emu/g, 36 emu/g and 2750 G, respectively. The exchange-coupling interaction in the BaFe₁₂O₁₉/Ni_0.8Zn_0.2Fe₂O₄ nanocomposite ferrite is discussed.     

A Study on the Rheological and Magnetic Properties of Polymer-Bonded Magnets Using Polycarbonate as the Bonding Material

The effect of three metal oxides on the magnetic properties of polymer bonded magnets (PBMs) was studied. The three PBMs, using polycarbonate (PC) as binder and 5 wt% of Fe₃O₄, Fe₂O₃, or CuO nanoparticles, were prepared by melt extrusion in a twin screw extruder followed by compression molding. Transmission electron microscopic (TEM) images showed a better dispersion for the PC/Fe₃O₄ nanocomposite compared with that of the other nanocomposites. The dynamic intersection frequency (ω_c), which is related to the crossing of the G′ and G^″ curves, showed that there was more homogeneity in the PC/Fe₃O₄ and PC/Fe₂O₃ nanocomposites. The curves of saturation magnetization for the three nanocomposites showed that there was a relationship between the magnetic properties and the homogeneity of the nanoparticles studied by rheometry. Because the magnetic strength of PC/Fe₃O₄ was greater than that of the other nanocomposites, it was concluded that not only the intrinsic magnetic property of the filler was an important factor to increase the magnetic property, but also the homogeneity of the filler within the matrix had an important role.     

Mössbauer characterization of calcium–ferrite oxides prepared by calcining Fe2O3 and CaO

Calcium ferrite oxides were prepared by calcining a mixture powder of iron- and calcium oxide. The ⁵⁷Fe-Mössbauer spectra of the calcium ferrites oxides were measured, revealing that the products should be Ca₂Fe₂O₅ and CaFe₂O₄, the ratio of which was dependent of the Fe/Ca atomic ratio of the mixture powder.     

The control of the diameter of the nanorods prepared by dc reactive magnetron sputtering and the applications for DSSC

The TiO₂ nanorod arrays, with about 1.8 μm lengths, have been deposited on ITO substrates by dc reactive magnetron sputtering at different target-substrate distances. The average diameter of these nanorods can be modified from about 45 to 85 nm by adjusting the target-substrate distance from 90 to 50 mm. These nanorods are highly ordered and perpendicular to the substrate. Both XRD and Raman measurements show that the nanorods prepared at different target-substrate distances have only an anatase TiO₂ phase. The nanorods prepared at the target-substrate distance less than 80 mm have a preferred orientation along the (2 2 0) direction. However, this preferred orientation disappears as the target-substrate distance is more than 80 mm. These TiO₂ nanorods have been used as the electrodes for dye-sensitized solar cells (DSSCs). The highest conversion efficiency, about 4.78%, has been achieved for TiO₂ nanorods prepared at 80 mm target-substrate distance.     

Template-free synthesis of ZnV2O4 hollow spheres and their application for organic dye removal

Hollow ZnV₂O₄ spheres with the shell aggregated by small nanoparticles were successfully synthesized through a facile one-pot template-free solvothermal method. The as-prepared product was characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and Brunauer-Emmett-Teller N₂ adsorption-desorption analyses. The formation of ZnV₂O₄ hollow spheres was based on flowerlike intermediate products supported reduction-dissolution-aggregation process at the expense of consumption of all the flowerlike products. The obtained ZnV₂O₄ hollow spheres showed a good adsorption capacity of methylene blue (MB) organic dye, which might be attributed to their special structural feature with large surface area. The adsorption kinetics and isotherm of MB on ZnV₂O₄ hollow spheres were also studied.     

Synthesis of urchin-like Co3O4 hierarchical micro/nanostructures and their photocatalytic activity

Urchin-like Co₃O₄ hierarchical micro/nanostructures have been successfully synthesized by calcining urchin-like precursor CoCO₃, which are prepared by a facile hydrothermal route. The particle size of the urchin-like Co₃O₄ could be easily controlled by altering the calcination temperature. The morphology and structure of the as-prepared urchin-like products were characterized by XRD, FESEM and TEM. Photocatalytic measurement demonstrates that these urchin-like Co₃O₄ micro/nanostructures show good photocatalytic effect and their degradation efficiency is strongly dependent on their particle size. Furthermore, a plausible reaction mechanism is also proposed to illustrate the photocatalytic processes of Co₃O₄.