Investigation of a new In2O3-based selective H2 gas sensor with low power consumption

The nanometer-size In₂O₃ was synthesized via a reverse microemulsion. A new catalytic combustion-type In₂O₃-based H₂ gas sensor was developed based on the technology for fabricating the direct-heating-type sensor and a surface-modifying process. A dense SiO₂ layer near the surface of the sensor was formed by chemical vapor deposition (CVD) of hexamethyldisiloxane (HMDS). The SiO₂ layer, which acted as a molecular sieve, reduced the penetration of large molecular, such as C₂H₅OH, CH₄, i-C₄H₁₀, into the sensing layer, resulting in the improvement of selectivity to H₂. The sensitive properties and the working mechanism of the sensor were presented. The In₂O₃ nanoparticles prepared by microemulsion were characterized by transmission electron microscopy and X-ray diffraction.     

In2O3 microcrystals obtained from rapid calcination in domestic microwave oven

The simple way to prepare In₂O₃ microcrystals is reported in this paper. The precursor, In(OH)₃ microstructures, were obtained using the Microwave-Assisted Hydrothermal (MAH) Method. By annealing as-prepared In(OH)₃ precursor at 500 °C for 5 min in a domestic microwave oven (MO), In₂O₃ microcrystals were prepared, inheriting the morphology of their precursor while still slightly distorted and collapsed due to the In(OH)₃ dehydration process which was studied by thermal analysis. The In(OH)₃ and In₂O₃ were characterized using powder X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and Raman spectroscopy. These techniques confirm the chemical dehydration of In(OH)₃ and the formation of In₂O₃ powders. The domestic MO promotes a rapid structural organization as compared with a CF (conventional furnace). The MAH method and the subsequent annealing in a domestic MO were shown to be a low cost route for the production of In₂O₃, with the advantages of lower temperature and smaller time.     

Factors affecting CO oxidation over nanosized Fe2O3

Nanocrystallite iron oxide powders with different crystallite sizes were prepared by co-precipitation route. The prepared powders with crystallite size 75, 100 and 150 nm together with commercial iron oxide (250 nm) were tested for the catalytic oxidation of CO to CO₂. The influence of different factors as crystallite size, catalytic temperature and weight of catalyst on the rate of catalytic reaction was investigated using advanced quadrupole mass gas analyzer system. It can be reported that the rate of conversion of CO to CO₂ increased by increasing catalytic temperature and decreasing crystallite size of the prepared powders. The experimental results show that nanocrystallite iron oxide powders with crystallite size 75 nm can be recommended as a promising catalyst for CO oxidation at 500 °C where 98% of CO is converted to CO₂. The mechanism of the catalytic oxidation reaction was investigated by comparing the CO catalytic oxidation data in the absence and presence of oxygen. The reaction which was found to be first order with respect to CO is probably proceeded by adsorption mechanism where the reactants are adsorbed on the surface of the catalyst with breaking OO bonds, then CO pick up the dissociated O atom forming CO₂.     

Preparation of Y2O3-Al2O3-SiO2 glasses by combustion synthesis melt-casting under high gravity

Y₂O₃-Al₂O₃-SiO₂ glasses were prepared by combustion synthesis melt-casting under high gravity. The properties of the glasses strongly depended on the starting compositions and preparation conditions. With a higher SiO₂ content in the starting compositions, the glass-forming ability of the melt was improved, but the density and hardness of the prepared glasses decreased. Crystallization occurred more frequently for larger samples and by using quartz crucibles instead of graphite ones. By increasing the high-gravity factors, both the density and hardness of the samples were improved. It is proposed that enhancing the high-gravity field facilitates the removal of bubbles from the melt.     

High sensitivity chlorine gas sensors using CdIn2O4 thick film prepared by co-precipitation method

Gas-sensing properties to dilute Cl₂ have been investigated for CdIn₂O₄ thick film sensors prepared by co-precipitation method. Cadmium nitrate and indium nitrate were mixed in de-ionized water. The 0.1 M NaOH was added to the mixed solution. The co-precipitate obtained was washed, filtered, dried, and calcined at 600-900 °C for 4 h. The CdIn₂O₄ sensor prepared using the powder calcined at 600 °C showed high sensitivity (S=R_g/R_a) to dilute Cl₂ at 250 °C. In particular, the CdIn₂O₄ sensor showed the sensitivity as high as 1200 even to 0.2 ppm Cl₂. The crystal structure and surface morphology were examined by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively.     

Influence of fuels on the morphology of undoped Y3Al5O12 and photoluminescence of Y3Al5O12:Eu prepared by a combustion method

Undoped and Eu-doped yttrium aluminum garnet nano-powders were prepared by a facile combustion method with citric acid/ethylene diamine tetraacetic acid (EDTA) as fuels and nitrates as oxidizers. The precursors and powders calcined at 1030 °C were investigated using thermogravimetric (TG), differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscope (SEM), and Brunauer-Emmett-Teller (BET) surface area measurements. It was found that the powders could be indexed with a garnet structure. The grains were in shape of hemispherical with sizes between 60 nm and 100 nm. With decreasing the citric acid/EDTA ratio, the crystallite size decreased steadily and the specific surface area increased. Investigations of photoluminescence (PL) revealed that as-synthesized YAG:Eu³⁺ phosphor samples exhibited an orange emission band with a main peak at 591 nm under the excitation of 394 nm. As citric acid amounts increased, the quality of crystallinity became higher and the luminescent properties were monotonously enhanced.     

Synthesis and electrochemical properties of LiV3O8 phase

Lithium vanadium oxide was synthesized by a new method in which LiOH, V₂O₅ and NH₄OH were used as the starting materials to synthesize a precursor containing Li and V, and then obtain the resulting product by calcining the precursor. The LiV₃O₈ compound prepared by this synthesis method gave a good charge-discharge and cycle performance. A specific capacity of 258 mAh/g is obtained in the range of 1.8−4.0 V in the first cycle and 247 mAh/g in the eighth cycle.     

Microwave-assisted synthesis of cobalt oxalate nanorods and their thermal conversion to Co3O4 rods

Cobalt oxalate nanorods have been successfully synthesized by a simple microwave-assisted solution approach using an ionic liquid 1-n-butyl-3-methyl imidazolium tetrafluoroborate. Upon thermal decomposition at 400 °C, cobalt oxalate nanorods could be converted to Co₃O₄ rods consisting of nanoparticles. The products were characterized using X-ray powder diffraction, transmission electron microscopy, thermogravimetric analysis and differential scanning calorimetric analysis.     

Low temperature preparation of nanocrystalline Sr0.5Ba0.5Nb2O6 powders using an aqueous organic gel route

Nano-sized Sr_0.5Ba_0.5Nb₂O₆ (SBN50) ceramic powders have been synthesized by an aqueous organic gel route. Homogeneous Sr-Ba-Nb precursor gels are prepared with Ba-EDTA, Sr-EDTA, and Nb-citrate complex as source of Sr, Ba, and Nb, respectively. Citric acid and ethylenediaminetetraacetic acid (EDTA) were used as the chelating agents. The structural variation of the SBN powder with annealing temperature was studied by TG-DTA, FT-IR and XRD. The precursor gel on calcination at 800 °C for 2 h produces a pure tungsten bronze SBN phase and the corresponding average particle size is 30-50 nm. The influences of the pH and the molar ratio of citric acid:Nb cation on the formation of homogeneous Sr-Ba-Nb precursor gels were also studied. The results show that a homogeneous Sr-Ba-Nb precursor gel with no precipitate is formed at pH 8 and the optimum molar ratio of citric acid and the metal cations is 3:1.     

Polyol method for the preparation of nanosized Gd2O3, boehmite and other oxides

A modified microwave-assisted polyol method was applied to prepare nanoparticulate ceramic powders of different oxides, i.e. Gd₂O₃, AlO(OH) (boehmite) and TiO₂. Due to the good dielectric properties of the utilised solvents, such as ethylene glycol, diethylene glycol and 1,4 butanediol, a significant decrease in reaction time was achieved under microwave heating. In the case of AlO(OH) and Gd₂O₃, <5 nm primary particle size were obtained. Boehmite was found to be intercalated with the solvent. The general applicability of the process is shown and the advantages in terms of properties and processibility are described. The powders thus prepared were investigated using X-ray diffractometry, electron microscopy and physisorption.     

Soft chemistry synthesis and crystal structure of MgxCu3−xV2O6(OH)4·2H2O

Mg_xCu_3−xV₂O₆(OH)₄·2H₂O (x ∼ 1), with similar crystal structure as volborthite Cu₃V₂O₇(OH)₂·2H₂O, was successfully prepared by a soft chemistry technique. The method consists of mixing magnesium nitrate and copper nitrate with a boiling solution of vanadium oxide (obtained by reacting V₂O₅ with few mL of 30 vol.% H₂O₂ followed by addition of distilled water). When ammonium hydroxide NH₄OH 10% was added (pH 7.8), a green yellowish precipitate was obtained. Using X-ray powder diffraction data, its crystal structure has been determined by Rietveld refinement. Compared to volborthite, the vanadium coordination changes from tetrahedral VO₄ to trigonal bipyramidal VO₅, and magnesium replaces copper, preferably, in the less distorted octahedron. At 300 °C, the phase formed is similar to the high pressure (HP) monoclinic Cu₃V₂O₈ phase. However at higher temperature, 600 °C, the phase obtained is different from known Cu₃V₂O₈ phases.     

Solution combustion synthesis of MgAl2O4 using fuel mixtures

The paper reveals a new perspective concerning the rational fuel selection and the logical elaboration of the recipes for the MgAl₂O₄ solution combustion synthesis. It was shown that Mg(NO₃)₂·6H₂O and Al(NO₃)₃·9H₂O exhibit different behavior with respect to urea, glycine and β-alanine. Urea proved to be the most adequate fuel for Al(NO₃)₃·9H₂O, while β-alanine proved to be the most appropriate fuel for Mg(NO₃)₂·6H₂O. Considering that there is a predilection of metal nitrates with respect to these fuels, in the case of MgAl₂O₄ combustion synthesis best results were achieved when fuel mixtures (urea and β-alanine, urea and glycine) were used. The use of fuel mixtures allowed the formation of pure, nanocrystalline MgAl₂O₄ directly from the combustion reaction, without any subsequent annealing step. The use of a single fuel (urea, glycine or β-alanine) led to the formation of an amorphous powder, which required further annealing in order to achieve the formation of crystalline MgAl₂O₄.     

Large-scale fabrication of H2(H2O)Nb2O6 and Nb2O5 hollow microspheres

Hollow micro-sized H₂(H₂O)Nb₂O₆ spheres constructed by nanocrystallites have been successfully synthesized via a bubble-template assisted hydrothermal process. In the reaction process, H₂O₂ acts as a bubble generator and plays a key role in the formation of the hollow structure. An in situ bubble-template mechanism has been proposed for the possible formation of the hollow structure. The spherelike assemblies of these H₂(H₂O)Nb₂O₆ nanoparticles have been transformed into their corresponding pseudohexagonal phase Nb₂O₅ through a moderate annealing dehydration process without destroying the hierarchical structure. Optical properties of the as-prepared hollow spheres were investigated. It is exciting that the absorption edge of the hollow Nb₂O₅ microspheres shifts about 18 nm to the violet compared with bulk powders in the UV/vis spectra, indicating its superior optical properties.     

Preparation and characterization of porous ultrafine Fe2O3 particles

Porous ultrafine Fe₂O₃ particles were prepared by homogeneous precipitation method. Fe³⁺ and urea were chosen as starting materials and anionic surfactant as the template. It is shown that the reaction results in the precipitation of a gelatinous hydrous iron oxide/surfactant mixture, which gives ultrafine Fe₂O₃ particles after drying and calcinations. The products were characterized by XRD, TEM, TG/DTA and BET. Conventional XRD patterns show that the products are mixture of γ-Fe₂O₃ and α-Fe₂O₃ phase after being sintered at 350 °C, and γ-Fe₂O₃ transforms entirely to α-Fe₂O₃ when sintered at 650 °C. The low-angle XRD patterns indicate that the mesostructure can only exist between 350 and 400 °C. TEM results show that the Fe₂O₃ particles have diameters of about 30 nm and lengths ranging from 100 to 120 nm; in each particle, there are several vermiculate-like mesopores with diameter of about 20-25 nm. The BET surface areas in excess of 50 m²/g are obtained after calcinations at 350 °C. The BJH desorption average pore width is around 22 nm, which is in agreement with the TEM results. The results show that anionic surfactant and sintering temperature are important to obtain this special morphology.     

Effect of synthesis conditions on the nanopowder properties of Ce0.9Zr0.1O2

In this work, the synthesis of nanocrystalline Ce_0.9Zr_0.1O₂ powders via the gel-combustion method, using different fuels, and following either stoichiometric or non-stoichiometric pH-controlled routes is investigated. The objective is to evaluate the effect of synthesis conditions on the textural and morphological properties, and the crystal structure of the synthesized materials. The solids were characterized by nitrogen physisorption, Scanning Electron Microscopy (SEM), X-ray powder diffraction (XPD), and Carbon-Hydrogen-Nitrogen Elemental Analysis (CHN).All the powders exhibited nanometric crystallite size, fluorite-type structure and negligible carbon content. Synthesis conditions strongly affect the average crystallite size, the degree of agglomeration, the specific surface area and the pore volume. Our results indicate that, by controlling the synthesis conditions it is possible to obtain solids with custom-made morphological properties.     

Synthesis and characterization of Bi2Fe4O9 powders

Bi₂Fe₄O₉ have been successfully prepared using ethylenediaminetetraacetic (EDTA) acid as a chelating agent and ethylene glycol as an esterification agent. Heating of a mixed solution of EDTA, ethylene glycol, and nitrates of iron and bismuth at 140 °C produced a transparent polymeric resin without any precipitation, which after pyrolysis at 250 °C was converted to a powder precursor for Bi₂Fe₄O₉. The precursors were heated at 400–800 °C in air to obtain Bi₂Fe₄O₉ powder and differential scanning calorimetry (DSC), thermogravimetric (TG), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) techniques were used to characterize the precursors and the derived oxide powders. XRD analysis showed that well-crystallized and single-phase Bi₂Fe₄O₉ with orthorhombic symmetry was obtained at 700 °C for 2 h and BiFeO₃ and Fe₂O₃/FeCO₃ were intermediate phases before the formation of Bi₂Fe₄O₉. Bi₂Fe₄O₉ powders show weak ferromagnetism at room temperature.     

Raman scattering characterization of bismuth based mixed oxides with Bi2O3 related structures

Orthorhombic Bi₁₇Ln₇O₃₆ (=Bi_0.708Ln_0.292O_1.5) and triclinic BiLnO₃ (=Bi_0.5Ln_0.5O_1.5) with Ln = Tm, Yb and Lu, as well as a cubic δ-type Bi_0.7Yb_0.3O_1.5, have been characterized at room temperature by Raman scattering spectrometry. The Raman spectrum deconvolution of the δ-Bi₂O₃ type cubic form allowed us to establish a correlation between the different phases of these mixed metallic oxides for the lattice modes as well as for the internal species. Using the empirical relation from Hardcastle, the BiO bond lengths have been estimated from the stretching frequency value observed at 635, 597 or 639 cm⁻¹ for cubic, orthorhombic or triclinic forms, respectively. A comparison with the X-ray results is discussed. The peritectoïdic transformation BiTmO₃ → δ + Tm₂O₃ has also been characterized by Raman spectroscopy.     

A mild solvothermal route to α-Fe2O3 nanoparticles

A mild solvothermal route has been developed to synthesize α-Fe₂O₃ nanoparticles using Fe(NO₃)₃ as a starting material. The results from XRD and TEM indicate the α-Fe₂O₃ powders possess a rhombohedrally centered hexagonal structure, and the size of particles from alcohothermal method at 160 °C is about 50-100 nm.