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).     

Sol–gel synthesis and structural characterization of niobium-silicon mixed-oxide nanocomposites

(Nb₂O₅) _x ·(SiO₂)_1−x gels of four different compositions with x = 0.025 (2.5Nb), 0.050 (5Nb), 0,10 (10Nb) and 0.20 (20Nb) were synthesized at room temperature from niobium penta-chloride and tetra-ethoxysilane and their structural evolution with the temperature was examined by X-ray diffraction, thermogravimetry/differential thermal analysis, Raman and IR spectroscopy (Fourier transform). The synthesis procedure tuned in this work allowed to obtain for each studied composition transparent chemical gels in which the niobium dispersion resulted to be strongly dependent on the Nb₂O₅ loading: it was on the atomic scale for the 2.5Nb and 5Nb gel samples whereas the gel structure of the 10Nb and 20Nb appears formed by phase separated niobia-silica nanodomains. All dried gels keep their amorphous nature up to 873 K, while at higher temperatures crystallization of T- and H-Nb₂O₅ polymorphs were observed according to the Nb₂O₅ loading: at low loading T-Nb₂O₅ was the main crystallising phase, whereas at higher one the H-Nb₂O₅ prevails. Particularly, T-Nb₂O₅ was the sole crystallising phase in the whole explored temperature range for the 2.5Nb, keeping its nanosize up to 1273 K for all samples except for the 20Nb.     

Novel sulfur-doped niobium pentoxide nanoparticles: fabrication, characterization, visible light sensitization and redox charge transfer study

Novel sulfur-modified niobium(V) oxide nanoparticles (SNON) that firstly exhibited good visible light sensitization were fabricated by a modified sol–gel technique using a very stable sol containing niobium(V) chloride, oxalic acid, isopropanol as chelating agent and thiourea as sulfur source. The resulting S-doped Nb₂O₅ nanomaterials were characterized by cyclic voltammetry (CV), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDAX), scanning electron microscope (SEM), ultra-violet diffuse reflectance (UV-DRS) and thermogravimetry thermal Analysis (TG-DTA). As against the response of unmodified niobium(V) oxide nanoparticles (UNON), the doped samples show different electrochemical response indicating an induced charge transfer across the niobium pentoxide/solution interface, thus forming two anodic peaks and a cathodic peak. This important observation was confirmed by UV-DRS in terms of band bending due to sulfur doping. Upon sulfur-modification, the absorption edge extends into the visible light region. The SEM observation shows that the SNPN existed in the mode of polycrystalline structure and the average grain size 63 nm. The EDAX analysis of undoped Nb₂O₅ and sulfur doped Nb₂O₅ shows the Nb₂O₅ (98%) and S (2%) content of nanopowder. These SNON nanoparticles are expected to be suitable candidates as visible light niobium(V) oxide nanoparticles sensitization.     

Separation of niobium and tantalum with phenylarsonic acid

Phenylarsonic acid permits satisfactory separation of niobium and tantalum and estimation of tantalum from an oxalate solution containing sulphuric acid up to pH 5.8. For complete precipitation of niobium the pH should exceed 4.8. In mixtures, tantalum is precipitated below pH 3.0 and niobium is then precipitated above pH 5.0. When the oxalate concentration is high, recovery of niobium with cupferron is recommended. When the ratio of Nb₂O₅, to Ta₂O₅ exceeds 2:1, reprecipitation of tantalum is necessary. The effect of interfering ions is studied.     

Characterization of niobium(v) oxide received from different sources

The characterization of three commercial powders of niobium(V) oxide received from two producers was made. The thermal behavior of Nb₂O₅ up to melting point and its microstructure were studied using X-ray powder diffraction, thermoanalytical methods (DSC/TG), infrared spectroscopy (IR) and scanning microscopy. Analysis of the obtained results revealed that the starting structure of niobium(V) oxide and its thermal behavior depend on the origin of niobia. Depending on the origin of the powder and of its thermal treatment, three polymorphs of Nb₂O₅ can be observed. Sintering of powders above 1200 °C results in the formation of single phase, H-Nb₂O₅.     

Synthesis of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>〈B〉 solid precursors and LiNbO<Subscript>3</Subscript>〈B〉 batches and their phase compositions

A process has been developed for preparing boron-doped niobium pentoxides Nb₂O₅〈B〉 to be used as precursors in the sysnthesis of nithium biobate batches LiNbO₃〈B〉 having tailored dopant concentrations. Solutions of various origins were used to isolate Nb₂O₅〈B〉. A method has been advanced to account for boron loss as volatile compounds upon the heat treatment of niobium hydroxide in order to determine the boron amount to be added to niobium hydroxide in the form of H₃BO₃. The boron concentration in LiNbO₃〈B〉 during lithium niobate synthesis is shown to be independent of the origin of the Nb₂O₅〈B〉 precursor with the same as-batch boron concentration. The phase compositions of Nb₂O₅〈B〉 and LiNbO₃〈B〉 have been characterized by X-ray powder diffraction and IR spectroscopy and boron concentrations have been determined for the synthesis of single-phase lithium niobate batches for use in the production of optically uniform single crystals and pore-free piezoelectric ceramics.     

On potassium hexaniobates released in the course of preparation of concentrated niobium(V) alkaline solution

Solutions containing 500 g L^?1 Nb₂O₅ were obtained by sintering Nb₂O₅ with potash followed by leaching with water. The potassium niobate released from niobium(V) alkaline solutions was examined with an application of methods of physical and chemical analysis. Hydrates K₈Nb₆O₁₉·4H₂O and K_7.5[H_0.5Nb₆O₁₉]·14H₂O were first obtained.     

Characterization of Thermally Stable Brønsted Acid Sites on Alumina‐Supported Niobium Oxide after Calcination at High Temperatures

Thermally stable Brønsted acid sites were generated on alumina‐supported niobium oxide (Nb₂O₅/Al₂O₃) by calcination at high temperatures, such as 1123 K. The results of structural characterization by using Fourier‐transform infrared (FTIR) spectroscopy, TEM, scanning transmission electron microscopy (STEM), and energy‐dispersive X‐ray (EDX) analysis indicated that the Nb₂O₅ monolayer domains were highly dispersed over alumina at low Nb₂O₅ loadings, such as 5 wt %, and no Brønsted acid sites were presents. The coverage of Nb₂O₅ monolayer domains over Al₂O₃ increased with increasing Nb₂O₅ loading and almost‐full coverage was obtained at a loading of 16 wt %. A sharp increase in the number of hydroxy groups, which acted as Brønsted acid sites, was observed at this loading level. The relationship between the acidic properties and the structure of the material suggested that the bridging hydroxy groups (Nb? O(H)? Nb), which were formed at the boundaries between the domains of the Nb₂O₅ monolayer, acted as thermally stable Brønsted acid sites.     

Lowervalent hexaniobate cluster in aqueous HCl: an equilibrium redox study

Summary It has been established⁽¹⁾ that hydrated niobium(V) oxide is, in fact, hexameric isopolyniobic acid, H₈Nb₆O₁₉·xH₂O, containing a protonated oxoniobate(V) cluster. It has also been shown^(2,3) that the stoichiometric and nonstoichiometric oxides as well as niobates, soluble or insoluble, formed under various conditions, are really derivatives of H₈Nb₆O₁₉. The amphoteric H₈Nb₆O₁₉ is soluble in conc. H₂SO₄ maintaining its hexanuclear structure⁽⁴⁾ and exists in the form of a SO₃ adduct of H₈Nb₆O₁₉. In the latest communication⁽⁵⁾ the hexameric cluster has been shown to exist even in the subnormal niobium oxidation states. The aqueous H₂SO₄ solution of niobium(V) when reduced with zinc forms various dark red-brown crystalline salts of the anion [Nb₆O₇(O·SO₃)₁₂]^16–. This cluster anion has niobium in an average nonintegral oxidation state of +3.67 and the Nb₆O₁₉ unit is coordinated to a maximum of twelve SO₃ groups. The present communication describes the potentiometric investigation of the reduced oxoniobate cluster in aqueous HCl. There are reports that strongly acidic niobium(V) solutions are reduced either electrolytically or by metals^(6–9). These workers proposed that niobium(III) was formed in solution although no detailed investigation on the species was made.     

Decomposition of CCl4 by TT-phase niobium oxide

TT-phase of niobium oxide was found to decompose CCl₄ into CO₂ most selectively (without formation of COCl₂) at 453 K among oxides, amorphous Nb₂O₅, TT-phase Nb₂O₅, SiO₂–Al₂O₃, Al₂O₃, V₂O₅, TiO₂ and ZrO₂.     

Incorporation of niobium into bridged silsesquioxane based silica networks

In this work different synthesis routes were evaluated with the aim of optimizing the incorporation of niobium within a hybrid silica matrix on an atomic scale. The fast kinetics of the hydrolysis/polycondensation of the organic Nb precursor Nb(OEt)₅ entails a segregation of the resulting material into Nb₂O₅ and a silica based network. To overcome this effect we (a) performed a prehydrolysis of 1,2-bis-triethoxy-ethane (BTESE) prior to adding niobium penta-ethoxide, or (b) attempted to reduce the availability of Nb via a complexation of Nb by either acetylacetone or 2-methoxyethanol. The network organization was evaluated from results of Fourier transform infrared as well as ¹³C, ²⁹Si and ¹⁷O MAS NMR spectroscopy. Whereas the prehydrolysis of BTESE and the addition of 2-methoxyethanol induced only moderate mixing of Nb and Si, leading to a network in which islands of Nb₂O₅ are linked to the hosting silica based matrix via Nb–O–Si bonds, the use of acetylacetonate lead to a mixing of Nb and Si on the atomic scale, forming a mixed Nb–O–Si network without any extended clusters of segregated Nb₂O₅. The Si–C–C–Si bridge from the silsesquioxane is found to survive the condensation process and is even present in the resulting materials after annealing at 200 °C.     

Isolation of a Dodecanuclear Polyoxo Cluster {Nb12O21} Showing a Rare Case of Five-fold Coordinated Niobium(V) Centers with a Square Pyramidal Geometry

The reactivity of the complexing anthracene-9-carboxylate ligand has been investigated with a niobium(IV) tetrachloride precursor (NbCl₄ ⋅ 2THF) in isopropanol solvent. This resulted in the crystallization of a molecular assembly containing two distinct {Nb₁₂O₂₁} cores surrounded by multiple isopropanolate and anthracenoate ligands. The compound is formulated [Nb₁₂₍(μ₃-O)₃(μ-O)₁₈(C₁₅H₉O₂)₈(OⁱPr)₁₀] ⋅ [Nb₁₂(μ₃-O)₂(μ-O)₁₉(C₁₅H₉O₂)₈(OⁱPr)₁₀] illustrating the two different dodecameric oxo-clusters, for which the niobium(IV) precursor was oxidized in the niobium(V) state during the reactional process. The two distinct {Nb₁₂O₂₁} units mainly differs by the environment of the niobium centers, which exhibits unexpected five-fold coordination (square pyramid) for some of them, together with the classical six-fold coordination (octahedron) as usually found for niobium(V). In the crystallization process, the. IR spectroscopy was used to analyze the esterification reaction occurring between the anthracene acid an isopropanolate ligands responsible of the production of water used in the oxo-condensation of the niobium centers. ⁹³Nb Solid state NMR was tentatively used to assess the occurrence of the different niobium environments.     

The investigation of NbO2 and Nb2O5 electronic structure by XPS,UPS and first principles methods

In this paper, the electronic structures of NbO₂ and Nb₂O₅ are theoretically and experimentally analyzed. The oxides in the samples are mainly consisted of NbO₂ and NbO, whereas the outmost layer of the samples is NbO₂. After exposure to air, the outermost layer on all niobium samples is Nb₂O₅. The photoelectrons from the first 2–4 Å contribute to the spectra, so the valence band structure of NbO₂ and Nb₂O₅ can be confirmed from ultraviolet photoelectron spectroscopy (UPS). By comparing the UPS with density of state results, the electronic structure of NbO₂ and Nb₂O₅ can be distinguished from each other, and then the electronic structure was deconvoluted into several electronic states. The agreement between experimental result and theory is, in the best case, satisfactory. Copyright © 2013 John Wiley & Sons, Ltd.     

Studies on the effect of milling on reduction of niobium pentaoxide with aluminium

The effect of milling on the aluminothermic reduction of niobium pentaoxide was investigated. Charges of Nb₂O₅ and Al (containing 5 % excess Al) were milled for different time (2, 5 and 10 h). XRD profile of milled samples indicated no phase formations during milling; only peak broadening were seen. Milled and unmilled charges were heated in a thermal analyser up to 1,400 °C. Products of milled charges showed formation of Nb, NbO and Al₂O₃; whereas unmilled hand mixed charges showed formations of Nb₃Al along with Nb and Al₂O₃. The tendency of milled charges towards Nb formations without the presence of aluminides was explained from the increase in surface area of charges caused by particle reduction.     

Niobium(V) oxide doped with Mg<Superscript>2+</Superscript> and Gd<Superscript>3+</Superscript> cations: Synthesis and structural studies

Methods for direct doping of niobium pentoxide with photovoltaically inactive Mg²⁺ and Gd³⁺ cations were developed for subsequent use in the synthesis of a stock for growing single crystals of lithium niobate with improved optical characteristics. The Raman spectra of doped pentoxides Nb₂O₅: Mg and Nb₂O₅: Gd revealed their island structures.     

Electrochemical deposition of electrochromic niobium oxide films from an acidic solution of niobium peroxo complexes

Deposition of electrochromic niobium(V) oxide films from an acidic solution of niobium peroxo complexes on a transparent conducting cathode in the form of an SnO₂ film on glass was studied. With an increase in the negative potential of the deposition of niobium(V) oxide films from a solution of niobium peroxo complexes at pH 2.5, the structure and composition of the films changed. A study of the electrochromic properties of Nb₂O₅ films revealed broadening of the bands in the electrochromic coloration spectrum with an increase in the negative potential of the deposition.     

Beiträge zur Untersuchung anorganischer nichtstöchiometrischer Verbindungen. XIV. Oxydationsprodukte von orthorhombischem Nb12O29 Elektronenoptische Untersuchung

Contributions to the Investigation of Inorganic Non-stoichiometric Compounds. XIV. Oxidation Products of Orthorhombic Nb₁₂O₂₉, Electron Optical Investigation An electron optical investigation shows that the orthorhombic starting material Nb₁₂O₂₉(BII) is well ordered. The oxidation products Nb₂O₅(Ox1BII) and Nb₂O₅(Ox2BII) are different from each other in structures as well as in their reactions. Nb₂O₅(Ox1BII) is unstable in the electron beam and differs from BII by characteristic point-defects. The radiation load can lead to the reduction to BII or to a transition into a defect structure with R-type-tunnels. The not well ordered structure of Nb₂O₅(Ox2BII) is stable in the electron beam. Characteristic is the sequence of [2×5] and [3×4] blocks, the latter in two different orientations. The observed composition O/Nb = 2.500 can be described by the present structural modell assuming vacant niobium tetrahedral sites. The large structural differences between the oxidation products of the orthorhombic and the monoclinic Nb₁₂O₂₉ are remarkable.     

Phase relations in the system V/Nb/O. V. Investigation of mixed crystals V1-xNbxO2

Mixed crystals V_1-xNb_xO₂ exist over the whole area of the quasibinary line VO₂-NbO₂. The existence of Nb_{5+ beside V3+ and V4+ on the V-rich side and V3+ beside Nb5+ and Nb4+ on the Nb-rich side of the mixed crystals is demonstrated by XANES-measurements. The compound VNbO}4_(V0.5_Nb0.5_O2_{) is described as a double oxide with vanadium only as V3+ and niobium only as Nb5+. At this point the electric resistivity of the solid solution shows a maximum.}     

Nitridation of Niobium Pentoxide Films in Ammonia by Rapid Thermal Processing

The feasibility of niobium oxynitride formation through nitridation of niobium pentoxide films in ammonia by rapid thermal processing (RTP) was investigated. Niobium films 200 and 500 nm thick were deposited by sputtering on Si(100) wafers covered by a 100 nm thick thermally grown SiO₂ layer. These as‐deposited films exhibited distinct texture effects. They were processed in three steps using an RTP system. The as‐deposited niobium films were first nitridated in an ammonia atmosphere at 1000 °C for 1 min and then oxidised in molecular oxygen at temperatures ranging from 400 to 600 °C. Those samples in which a single Nb₂O₅ phase was determined after oxidation were additionally nitridated in ammonia at 1000 °C for 1 min. Investigations show that surface roughness of the samples after oxidation of niobium films first nitridated in ammonia is lower than after direct oxidation of as‐deposited films in oxygen, although the niobium pentoxide phase formed after annealing was the same in both cases. We explain this result as being due to the large expansion of the niobium lattice during the direct oxidation of the niobium film in molecular oxygen and also to the high oxidation rate of the as‐deposited niobium film in oxygen. By incorporation of oxygen in the crystal lattice of niobium and rapid formation of niobium pentoxide, substantial intrinsic stress was built up in the film, frequently resulting in delamination of the film from the substrate. Nitrogen hinders the diffusion of oxygen in nitridated films, which leads to a decrease of the oxidation rate and thus slower formation of Nb₂O₅. Nitridation of the completely oxidised niobium films in ammonia leads to the formation of niobium oxynitride and niobium nitride phases.     

Synthesis of MgNb<Subscript>2</Subscript>O<Subscript>6</Subscript> nanocrystalline powders by an improved citrate sol–gel technique

MgNb₂O₆ nanocrystalline powders have been synthesized at a low temperature by improved citrate sol–gel method in this paper. The high quality solution of Nb⁵⁺ was prepared using Nb₂O₅ as the starting material. The crystal structure and microstructure of MgNb₂O₆ powders were characterized by XRD and SEM techniques, and the effects of preparation craft including pH value and the proportion of citric acid to the niobium ions on the crystal structure and microstructure of powders were also investigated. XRD and TG/DTA results show that the single phase of MgNb₂O₆ for synthesized powders can be obtained by calcining the precursor at 700 °C. SEM results indicate that the average particle size of MgNb₂O₆ exhibits a significantly dependence on the pH values and the proportion of citric acid to the niobium ions, where it was found that particle size of a 20 nm can be obtained for the MgNb₂O₆ powders by sol–gel process.