Synthesis of Ultra-Fine Columbite Powder MgNb2O6 by the Polymerized Complex Method

Pure MgNb₂O₆ powders with high specific surface area and high crystallinity have been successfully synthesized by the Polymerized Complex (PC) Method. A solution of water, citric acid, ethylene glycol and niobium and magnesium ions, was polymerized at 130°C by the water elimination and was calcined at 400°C for 2 h. The precursor formed was calcined at temperatures from 700°C to 1000°C in air to obtain the columbite phase. Thermal analysis, Raman spectroscopy, x-ray diffraction, adsorption/desorption hysteresis and scanning electron microscopy were used to investigate the polymer decomposition and the columbite phase evolution. No evidence for phase separation of crystalline MgCO₃ and Nb₂O₅ in Raman spectra and x-ray diffraction pattern as distinct intermediates were found, and the MgNb₂O₆ probably is formed through a single-step decomposition of oxycarbonate intermediate, as other double oxide synthesized by the PC Method, as indicated by the thermal decomposition of the precursor.     

UV-Visible and IR Spectroelectrochemical Properties of V2O5 Crystalline Films Charged/Discharged in Extended Potential Range

Electrochemical properties of amorphous and crystalline V₂O₅ films, dip-coated from V-oxoisopropoxide sols and thermally treated at various temperatures (100, 150, 200 and 300°C), have been studied in extended potential range, i.e. from 1.4 to –1.6 V vs. Ag/AgCl in 1M LiClO₄/propylen carbonate (PC) electrolyte. The formation of various lithiated (-, -, - and -Li _x V₂O₅) phases was correlated with the values of insertion coefficient x obtained from cyclic voltammograms (CV) of crystalline V₂O₅ films (300°C). Reversible charging was observed when films were cycled up to –1.0 V vs. Ag/AgCl, while the extension of the potential to –1.3 V vs. Ag/AgCl change the CV of films irreversibly. Charging of crystalline V₂O₅ films was followed by the help of in-situ UV-visible spectroscopy, that revealed the intensity variations of the polaron absorption above 600 nm and the presence of the absorbing V³⁺ species between 550 and 650 nm. Ex-situ IR spectra of the crystalline films charged/discharged at –1.6V/1.4V vs. Ag/AgCl confirmed the amorphisation of the films' structure.     

Influence of the crystallinity on the Li+ intercalation process in Nb2O5 films

Nb₂O₅ thin films were prepared by the Pechini method. The effect of the film crystallinity on the electrochemical and electrochromic properties was investigated. A relationship between the crystalline structure and the Li⁺ intercalation/extraction process, stability and kinetics was observed. A significant decrease in the electrochemical response was observed as a function of the number of cycles for films treated at 400 and 450 °C. However, as the calcination temperature increases this effect disappears. XRD studies shown that at 400 °C, the material is amorphous, evolving to orthorhombic phase. The transmittance variation as well as the coloration efficiency increases as the temperature is increased. In the initial cycles the intercalation charge is higher for the amorphous oxide than for the orthorhombic phase. However, the variation in the optical density is small. On the other hand, the charge of the orthorhombic phase oxide does not change. These results suggest that there are two different processes associated with Li⁺ intercalation, but only one of them leads to the coloration process.     

Methane Combustion Over Fe, Co and Ni Modified Manganese Oxide Catalysts

Fe-Mn, Co-Mn and Ni-Mn composite oxide catalysts based on high specific surface area MnO₂ precursor were prepared and applied to catalytic combustion of CH₄. Results were compared with that of unmodified MnOx and 1wt.% Pd/-Al₂O₃. Below 450°C, manganese oxide catalysts show higher activity than Pd/-Al₂O₃, while the modified manganese oxide catalysts exhibit higher activity than the unmodified one below 420°C. All catalysts were characterized by means of N₂-BET, XRD, TG-DTA and H₂-TPR. Due to the interaction between Fe, Co or Ni oxides and manganese oxide, the activity of the oxygen species of the modified catalysts is improved, which leads to the increase of their CH₄ combustion activity.     

The effect of composition variables on precursor degradation and their consequence on Nb2O5 film properties prepared by the Pecchini Method

The calcination of Nb₂O₅ prepared by the Pecchini method was investigated using thermal analysis. The polymer precursor was prepared with different citric acid/ethylene glycol (CA/EG) and citric acid/niobium precursor (CA/Nb) molar ratios. The results suggest a strong influence of the CA/EG molar ratio, mainly on the polymerization degree. The CA/Nb ratio also modifies the degradation process of the precursor solution. The effect of the precursor compositional variation was also observed in the oxide structure, as well as in its electrochemical and electrochromic behavior. Films prepared using high CA/EG and a Nb salt precursor amount have the highest values of charge density and coloration efficiency. Moreover XRD data for this sample show a crystalline structure while the samples prepared with low CA/EG ratio are amorphous.     

Preparation of La0.75Sr0.25MnO3 Nano-Phase Powders and Films from Alkoxide Precursors

The first purely alkoxide-based sol-gel route to nano-phase powders and thin films of perovskite La_0.75Sr_0.25MnO₃ is described. The phase and microstructure evolution on heat treatment of free gel films to form the target nano-phase oxide were investigated by TGA, IR spectroscopy, powder XRD, SEM and TEM-EDS. The xerogel consisted of a hydrated oxo-carbonate, without remaining alkoxo groups or solvent. Heating at 5°C·min^–1 decomposed the carbonate groups and yielded the pure perovskite La_0.75Sr_0.25MnO₃ at 760°C. The cell dimensions were virtually unchanged from the first observation of perovskite at 680°C, to 1000°C, 4 h. The monoclinic cell of La_0.75Sr_0.25MnO₃ obtained at 1000°C, 4 h, had the dimensions a = 5.475(1), b = 5.504(2), c = 7.771(1) Å, = 90.50(2), fitting the literature data quite well. Crack-free, homogenous, 150 nm thick La_0.75Sr_0.25MnO₃ films were prepared by spin-coating Si/SiO₂/TiO₂/Pt and polycrystalline -Al₂O₃ substrates with a 0.6 M alkoxide solution, followed by heating at 5°C·min^–1 to 800°C, 30 min.     

Epitaxial Growth of Sr x Ba1 − x Nb2O6 Thin Films Prepared from Sol-Gel Process

In this study, we prepared Sr _x Ba_{1 – x} Nb₂O₆ (x = 0.3, 0.5 and 0.7) thin film on 0.75 wt% La doped SrTiO₃ (100) and (110) single crystal substrates. A homogeneous coating solution was prepared with Sr and Ba acetates and Nb(OEt)₅ as raw materials, and acetic acid and diethlene glycol monomethyl ether as solvents. The substrates were coated with the solution by spin coating method. As-coated thin films were heated from 973 to 1273 K in air. The grains of the thin film on La doped SrTiO₃ (100) were pillar shaped and arranged in right angle to each other. On the other hand, the grains of these thin films on La doped SrTiO₃ were pillar shape and arranged in one direction. The crystallographic relationship of the thin film between Sr _x Ba_{1 – x} Nb₂O₆ and substrate that the 130 and 310 direction of the thin film on the substrate were oriented with c-axis in parallel to the substrate surface. On the other hand, (hk0) phase diffractions of Sr _x Ba_{1 – x} Nb₂O₆ thin film on the substrate (110) were investigated in the XRD theta-2theta measurement. It is expected that the Sr _x Ba_{1 – x} Nb₂O₆ (x = 0.3, 0.5 and 0.7) were highly oriented or epitaxial growing on La doped SrTiO₃ (110) single crystal substrate.     

A new approach to the preparation of microcrystalline ZnNb2O6 photocatalysts via a water-soluble niobium-citrate-peroxo compound

A water-soluble niobium-citrate-peroxo compound was synthesized by using Nb₂O₅ as a precursor. This niobium compound solution was successfully applied to the preparation of microcrystalline ZnNb₂O₆ photocatalysts via a water-based sol-gel method. The results indicated that pure ZnNb₂O₆ could be obtained in a temperature range from 750 to 950 °C. The absorption edge of ZnNb₂O₆ located at about 305 nm, corresponding to a band gap of ca. 4.06 eV. The photocatalytic activities of the as-prepared samples for the methyl orange degradation were evaluated under UV light (λ = 254 nm). It was found that the sample obtained at 850 °C showed the highest photocatalytic activity due to its suitable surface area and crystallinity.     

Preparation and Characterization of SiO2-MxOy(M = V, Sn, Sb) Thin Films from Silicic Acid and Metal Chlorides

The preparation of SiO₂-M _x O _y (M = V, Sn, Sb) binary oxide thin films by sol-gel method was investigated. The reaction of silicic acid with metal chloride (M = Sn and Sb) or oxychloride (M = V) formed homogeneous solutions. The dip-coating of slide glass and silicon wafer followed by heat treatment gave oxide films having Si—O—M bond. The changes of FT-IR spectra as a function of heat treatment temperature and molar composition confirmed the Si—O—M bonds. The sheet resistance of films increased with an increase on heat treatment temperature and decrease in the content of metal oxide M _x O _y . X-ray diffraction peaks were observed for the SiO₂-V₂O₅ films with high V₂O₅ contents and heat-treated above 250°C, while the others were amorphous. Oxide films heat treated at 500°C had a thickness between 340–470 nm.     

Preparation and Optical Characterization of Sol-Gel Deposited Pb(Zr0.45Ti0.55)O3 Films

Homogeneous crack-free lead zirconate titanate (Pb(Zr_0.45Ti_0.55)O₃: PZT 45/55) films were prepared by a chemically modified sol-gel process using lead acetate trihydrate, zirconium n-propoxide, and titanium isopropoxide precursors. The coating solutions were modified by the addition of diethanolamine. Single and multilayer films were deposited with a 2000 rpm spin rate on fused silica and MgO(100) substrates. Multiple spin coating with an intermediate heat treatment in air at 400°C for 3 min between coatings was performed to obtain films up to 2 m in thickness. The formation of the tetragonal perovskite structure was found to depend on the intermediate firing temperature, final annealing temperature, and annealing time. A 650°C rapid thermal annealing treatment in oxygen was required to crystallize the PZT film into the perovskite structure. The films were characterized using optical spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and thermo-gravimetry and differential scanning calorimetry (TG-DSC). The optical constants of the PZT films were evaluated from spectral transmittance and reflectance measurements. Optical constants are presented over the visible and near infrared region.     

Structure and Optical Properties of 0.1BiFeO3-0.9SrBi2Nb2O9 Thin Films Using a Modified Sol-Gel Technique

Fe-doped SrBi₂Nb₂O₉ precursor solution was synthesized using bismuth nitrate Bi(NO₃)₃·5H₂O, strontium nitrate Sr(NO₃)₂, iron nitrate Fe(NO₃)₃·9H₂O, and niobium ethoxide Nb(OC₂H₅)₅ as starting materials, ethylene glycol monomethyl ether (C₃H₈O₂) as the solvent. 0.1BiFeO₃-0.9SrBi₂Nb₂O₉ thin films were prepared on fused quartz substrates using sol-gel processing. The surface morphology and crystal structure and optical properties of the thin films were investigated. The thin film annealing at 400°C were found to be amorphous, and the thin films crystallize to a perovskite structure after a post-deposition annealing at 600°C for 1 h in air. The grain of thin film was evenly distributed. The thin films exhibit the designed optical transmission, while the optical transition is indirect in nature. Their optical band gap is about 2.5 eV.     

Photoelectrochemical Properties of Sol-Gel Nb2O5 Films

Structural, optical, electro and photoelectrochemical properties of amorphous and crystalline sol-gel Nb₂O₅ coatings have been determined. The coatings are n-type semiconductor with indirect allowed transition and present an overall low quantum efficiency ( < 4%) for UV light to electric conversion. The photoconducting behavior of the coatings is discussed within the framework of the Gärtner and Södergren models. Improvement can be foreseen if Nb₂O₅ coatings can be made of 10–20 nm size nanoparticles.     

V2.38Nb10.7O32.7: A V2O5-Nb2O5 mixed oxide tunnel structure related to the tetragonal tungsten bronzes

A crystal structural model for the orthorhombic compound V_2.38Nb_10.7O_32.7, which is known as “V₂Nb₉O_27.5”, was developed by means of selected area electron diffraction (SAED), Rietveld refinement and high resolution electron microscopy (HREM). The metastable compound is obtained by thermal decomposition of freeze-dried precursors as chain-like agglomerated nanoparticles or by reaction of V₂O₅ with fresh-precipitated Nb₂O₅ as more compact micro-scaled crystals. With the latter, it was possible to identify its structure for the first time (space group Cmmm). The tetragonal tungsten bronze (TTB)-type structure shows high potential for ionic intercalation, since easily reducible [V⁵⁺₂O²⁻] units are implemented in the tunnels of a rigid niobium oxide framework.     

Preparation and performances of nanosized Ta2O5 powder photocatalyst

Nanosized-Ta₂O₅ powder photocatalyst was successfully synthesized by using sol-gel method via TaCl₅ butanol solution as a precursor. Ta₂O₅ species can be formed under 500 °C via the decomposition of the precursor. The crystalline phase of Ta₂O₅ powder photocatalyst can be obtained after being calcined above 600 °C for 4 h. The crystal size and particle size of Ta₂O₅ powder photocatalyst was about 50 nm. A good photocatalytic performance for the degradation of gaseous formaldehyde was obtained for the nanosized-Ta₂O₅ powder. The Ta₂O₅ powder formed at 700 °C for 4 h and at 650 °C for 12 h showed the best performance. The calcination temperature and time play an important role in the crystallization and photocatalytical performance of nanosized-Ta₂O₅ powder.     

Sol-gel synthesis of mesoporous CaCu3Ti4O12 thin films and their gas sensing response

A new sol-gel synthesis procedure of stable calcium copper titanate (CaCu₃Ti₄O₁₂—CCTO) precursor sols for the fabrication of porous films was developed. The composition of the sol was selected in order to avoid the precipitation of undesired phases; ethanol was used as solvent, acetic acid as modifier and poly(ethyleneglycol) as a linker agent. Films deposited by spin-coating onto oxidized silicon substrates were annealed at 700 °C. The main phase present in the samples, as detected by X-ray diffraction and Raman spectroscopy, was CaCu₃Ti₄O₁₂. Scanning electron microscopy analysis showed that mesoporous structures, with thicknesses between 200 and 400 nm, were developed as a result of the processing conditions. The films were tested regarding their sensibility towards oxygen and nitrogen at atmospheric pressure using working temperatures from 200 to 290 °C. The samples exhibited n-type conductivity, high sensitivity and short response times. These characteristics indicate that CCTO mesoporous structures obtained by sol-gel are suitable for application in gas sensing.     

Preparation and Spectroscopic Characterization of Blue CoAl2O4 Coatings

Optically selective thin films of CoAl₂O₄ with a spinel structure were produced for an automotive lamps application by the sol-gel process using aluminum sec-butoxide, ethylacetoacetate chelating agent and cobalt nitrate hexahydrate. The use of two metal-oxide precursors is advantageous over the single bimetallic alkoxide precursor (aluminum cobalt isopropoxide), because it allows us to vary the Co/Al ratio in the precursor solution. We found that the Co/Al ratio should not exceed 0.3 if we are to achieve films with the characteristic blue colour at 700°C. The structural characteristics of the oxide powders were determined from infrared (IR) spectra and X-ray diffraction (XRD) analysis, while the optical properties of the films were investigated with UV-VIS spectroscopy.     

Refinement of the cation distribution in Nb-Mo double oxide

The structure of Nb₂Mo₃O₁₄ double oxide is refined from powder data using synchrotron radiation and the anomalous scattering effect; space group P $ \bar 4 The structure of Nb₂Mo₃O₁₄ double oxide is refined from powder data using synchrotron radiation and the anomalous scattering effect; space group P 2₁ m is found for the material. It is demonstrated that in the tetragonal unit cell with parameters a = 23.173 ?, c = 4.0027 ? Nb⁵⁺ and Mo⁶⁺ ions are stochastically distributed in MO₆ octahedra and MO₇ pentagonal bipyramids of the polygonal network structure of the Mo₅O₁₄ type. Original Russian Text Copyright ? 2008 by T. Yu. Kardash, L. M. Plyasova, V. M. Bondareva, and A. N. Shmakov __________ Translated from Zhurnal Strukturnoi Khimii, Vol. 49, No. 4, pp. 729–735, May–June, 2008.     

Fabrication and characterization of three-dimensionally ordered macroporous niobium oxide

Three-dimensionally ordered macroporous (3-DOM) niobium oxide was fabricated by aqueous organic gel method through the interstitial spaces between polystyrene spheres assembled on glass substrates. Freshly precipitated hydrous niobium oxide (Nb₂O₅·nH₂O), which was prepared starting from Nb₂O₅, was used in combination with citric acid in an aqueous solution and then was transferred as a niobium source to synthesize 3-DOM Nb₂O₅. The morphologies of porous Nb₂O₅ were characterized by scanning electron microscope (SEM). The thermal decomposition and phase composition of 3-DOM Nb₂O₅ were investigated by Fourier transform infrared spectroscopy (FT-IR), Thermogravimetric–differential thermal analysis (TG–DTA), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS).     

Phase formation, crystal chemistry, and properties in the system Bi2O3-Fe2O3-Nb2O5

Subsolidus phase relations have been determined for the Bi₂O₃-Fe₂O₃-Nb₂O₅ system in air (900-1075 °C). Three new ternary phases were observed—Bi₃Fe_0.5Nb_1.5O₉ with an Aurivillius-type structure, and two phases with approximate stoichiometries Bi₁₇Fe₂Nb₃₁O₁₀₆ and Bi₁₇Fe₃Nb₃₀O₁₀₅ that appear to be structurally related to Bi₈Nb₁₈O₅₇. The fourth ternary phase found in this system is pyrochlore (A₂B₂O₆O′), which forms an extensive solid solution region at Bi-deficient stoichiometries (relative to Bi₂FeNbO₇) suggesting that ≈4-15% of the A-sites are occupied by Fe³⁺. X-ray powder diffraction data confirmed that all Bi-Fe-Nb-O pyrochlores form with positional displacements, as found for analogous pyrochlores with Zn, Mn, or Co instead of Fe. A structural refinement of the pyrochlore 0.4400:0.2700:0.2900 Bi₂O₃:Fe₂O₃:Nb₂O₅ using neutron powder diffraction data is reported with the A cations displaced (0.43 Å) to 96g sites and O′ displaced (0.29 Å) to 32e sites (Bi_1.721Fe_0.190(Fe_0.866Nb_1.134)O₇, Fd3¯m (#227), ). This displacive model is somewhat different from that reported for Bi_1.5Zn_0.92Nb_1.5O_6.92, which exhibits twice the concentration of small B-type cations on the A-sites as the Fe system. Bi-Fe-Nb-O pyrochlores exhibited overall paramagnetic behavior with large negative Curie-Weiss temperature intercepts, slight superparamagnetic effects, and depressed observed moments compared to high-spin, spin-only values. The single-phase pyrochlore with composition Bi_1.657Fe_1.092Nb_1.150O₇ exhibited low-temperature dielectric relaxation similar to that observed for Bi_1.5Zn_0.92Nb_1.5O_6.92; at 1 MHz and 200 K the relative permittivity was 125, and above 350 K conductive effects were observed.