Sandia Octahedral Molecular Sieves Research Articles

Considerable humidity response of a well-aligned SOMS micro-wire flexible sensor by moisture-induced releasing of trapped electrons.

Sandia Octahedral Molecular Sieves micro-wires (SOMS MWs) that exhibit ultra-high response to moisture and a short response time can be produced easily in an environmentally friendly mass production process. They are excellent candidate materials for use in humidity sensors. SOMS MWs were synthesized using niobium pentoxide as a precursor in concentrated sodium hydroxide solution. To fabricate humidity-sensing devices, electrophoresis was utilized to align the SOMS MWs on a polyethylene terephthalate (PET) substrate. The degree of alignment of SOMS MWs can be tuned by controlling the electric field during electrophoresis. Both well-aligned SOMS MWs (S-1.00) and randomly distributed SOMS MWs (S-0.00) exhibit maximum sensitivities to humidity (RRH/RDRY) of almost 104 and 107 respectively, and both exhibit short response times (34 and 38 s) and recovery times (7 and 10 s); these MWs outperform metal oxide ceramic-based materials in sensing humidity. Furthermore, the humidity response of S-1.00 exceeds that of S-0.00 by three orders of magnitude, and this result cannot be explained with reference to the Grotthuss mechanism. Therefore, the moisture-induced carriers from trapped electrons contribute significantly to the humidity response of SOMS MWs. In addition, with outstanding humidity sensing-performance under extreme bending conditions and superior durability after being bent hundreds of times, the well-aligned SOMS MW sensor is a favorable candidate for emerging multifunctional electronic-skin.

Preparation of One-Dimension Orthorhombic NaNbO<sub>3</sub> Long Rods by Combining Hydrothermal Method with Post-Heat Treatment

One-dimensional orthorhombic NaNbO3 long rods were successfully prepared by combining a traditional hydrothermal method with the post-heat treatment. In the hydrothermal reaction process, sandia octahedral molecular sieves (SOMS) were synthesized at 160°C using Nb2O5 and NaOH as the initial reactants. Then, the effects of different post-heat treatment conditions on the morphology and crystallinity of product were investigated. Results showed that post-heat temperature could promote the crystal of the product to form orthorhombic NaNbO3, and the axis of the long rod ran along the [001] direction of NaNbO3 orthorhombic unit cell. Our study may provide a new approach for the preparation and investigation of one-dimensional alkali niobate photocatalytic materials and template grain for textured alkali niobium.

Preparation and characterization of orthorhombic NaNbO3 Long Bar

In this study, we successfully prepared perovskite-type NaNbO3 long bars with orthorhombic phase. At first, one-dimensional sandia octahedral molecular sieves (SOMS) were synthesized by the hydrothermal treatment at 160°C using Nb2O5 and NaOH as the initial reactants. The SOMS exhibited ribbon-like Na2Nb2O6·6/5H2O fiber structures with high length-to-width ratio. Then, the prepared ribbon-like Na2Nb2O6·6/5H2O fibers changed into the perovskite structure by the post-heat treatment at 450°C. The surface morphology and phase structure of the products were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) respectively; the fine structures of the products were investigated by means of Raman spectrum and high-resolution transmission electron microscopy (HRTEM). Furthermore, the mechanism of formation of NaNbO3 long bars was studied based on the experimental data. Our study may provide a new approach for the preparation and investigation of one-dimensional alkali niobate photocatalytic materials and template grain for textured alkali niobium.

Franconite, NaNb2O5(OH)·3H2O: structure determination and the role of H bonding, with comments on the crystal chemistry of franconite-related minerals

AbstractThe crystal structure of franconite, NaNb2O5(OH)·3H2O, has been characterized by single-crystal X-ray diffraction using material from Mont Saint-Hilaire, Québec, Canada. Results givea= 10.119(2),b= 6.436(1),c= 12.682(2) Å and β = 99.91(3)° and confirm the correct space group asP21/c. The crystal structure, refined toR= 4.63% andwR2=11.95%, contains oneNasite, two distorted octahedralNbsites and nineOsites. It consists of clusters of four edge-sharing Nb(O,OH)6octahedra, linked through shared corners to adjacent clusters, forming layers of Nb(O,OH)6octahedra. These alternate along [100] with layers composed of NaO(H2O)4polyhedra, the two being linked together by well defined H bonding. The predominance of H bonding, essential to the mineral, results in a perfect {100} cleavage. Chemical analyses (n= 7) of four crystals give the empirical formula (Na0.73Ca0.13☐0.14)∑=1.00(Nb1.96Ti0.02Si0.02Al0.01)∑=2.01O5(OH)·3H2O (based on nine oxygens) or ideally NaNb2O5(OH)·3H2O. Franconite is crystallo-chemically related to SOMS [Sandia Octahedral Molecular Sieves; Na2Nb2−xMxO6−x(OH)x·H2O with M = Ti, Zr, Hf], a group of synthetic compounds with strong ion-exchange capabilities. Both hochelagaite (CaNb4O11·nH2O) and ternovite (MgNb4O11·nH2O) have X-ray powder diffraction patterns and cation ratios similar to those of franconite indicating that these minerals probably have similar structures.

In Situ Observation of Dehydration-Induced Phase Transformation from Na2Nb2O6–H2O to NaNbO3

We have monitored the phase transformation from a Sandia octahedral molecular sieve Na2Nb2O6–H2O to a piezoelectric NaNbO3 nanowire through in situ X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements at high temperatures. After dehydration at 288 °C, the Na2Nb2O6–H2O becomes significantly destabilized and transforms into NaNbO3 with the increase of time. The phase transformation time is exponentially proportional to the inverse of temperature, for example, ∼105 s at 300 °C and ∼101 s at 500 °C, and follows an Arrhenius equation with the activation energy of 2.0 eV. Real time TEM investigation directly reveals that the phase transformation occurs through a thermally excited atomic rearrangement due to the small difference of Gibbs free energy between two phases. This work may provide a clue of kinetic control for the development of high piezoelectric lead-free alkaline niobates and a deep insight for the crystallization of oxide nanostructures during a hydrothermal process.

Surface sprouting growth of Na2Nb2O6·H2O nanowires and fabrication of NaNbO3 nanostructures with controlled morphologies

The one-dimensional (1D) Sandia octahedral molecular sieves (SOMS) Na2Nb2O6·H2O nanowires were prepared by the reaction of Nb powder with NaOH via a one-step hydrothermal methods. The products were characterized by SEM, TEM, XRD, IR and EDX. A plausible sprouting growth mechanism is proposed for the formation of Na2Nb2O6·H2O nanowires based on the systematic investigation. Na2Nb2O6·H2O nanoribbons-based complex flowerlike structure were first grew on the metallic niobium surface, the growth process of SOMS nanowire is similar to the plant seed sprouting and growing, Na2Nb2O6·H2O nanowires were finally formed at the depletion of metallic niobium powder. In the end, we showed that Na2Nb2O6·H2O nanowires is hydrothermally synthesized in the 15M NaOH solution at 423K, which can be easily converted into NaNbO3 nanowires by calcination, while the NaNbO3 cubes were obtained through the same hydrothermal process at a higher temperature of 453K.

Temperature-induced solid-phase oriented rearrangement route to the fabrication of NaNbO3nanowires

We proposed here a temperature-induced solid-phase oriented rearrangement route to the fabrication of NaNbO(3) nanowires by using sandia octahedral molecular sieves (SOMS) Na(2)Nb(2)O(6) x H(2)O as a precursor. The SOMS precursor was prepared by using metal Nb powder as a raw material through the hydrothermal approach.

Tunable Conductivity of Collapsed Sandia Octahedral Molecular Sieves

The structure−property relationship between atomic cation substitution and bulk-scale conductivity in perovskites has been studied systematically. A series of Na−Nb perovskites has been synthesized via two methods, (1) ion exchange or (2) synthetic metal doping of microporous Sandia Octahedral Molecular Sieve (SOMS, Na2Nb2-xMxO6-x(OH)x·H2O, where 0 ≤ x ≤ 0.4) phases, followed by calcination to the perovskite phase. We show that the oxygen conductivity can be altered by substitution of the A- and B- site cations in the perovskite structure Na1-xAxNb1-yMyO3-d, where 0 ≤ x ≤ 0.1). Furthermore, we are able to show improved ion conductivity over that of yttria-stabilized zirconia with the Na0.9Mg0.1Nb0.8Ti0.2O3-y phase with our highest conductivity observed (0.176 S/cm) at 1173 K. These fundamental studies allow us to develop perovskites with optimal electric properties.

Role of Water in the Ion Selectivity of Niobate-Based Octahedral Molecular Sieves

The role of occluded water in ion exchange selectivity is examined in a class of molecular sieves named Sandia Octahedral Molecular Sieves (SOMS: Na2Nb2-xMxO6-x(OH)x·H2O (i.e., M = Ti; 0 < x < 0.4)). SOMS exhibit a high selectivity for divalent cations only when the framework Nb(5+) are substituted by M(4+) atoms. Vibrational dynamics of the water molecules with varying charge balancing cations and M atoms are studied by inelastic neutron scattering (INS) measurements and 1H MAS NMR and correlated to density functional theory and molecular dynamics data. The experimental INS spectra were compared with those of ice Ih to characterize the changes induced by confinement on the occluded H2O and the resulting hydrogen-bonding network. Data indicates that, with increasing M(4+) content and divalent ion exchange, the trend of occluded water molecules is to change from extended rigid ice-like networks to restored bulk-like arrangements with increased solvation effects on the channel charge-balancing cations.

A New and Industrially Favorable Method for Synthesizing Sandia Octahedral Molecular Sieve (SOMS‐1) Na2Nb1.6Ti0.4O5.6 (OH)0.4×H2O: Synthesis, Structure, and Scale‐up.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Prototype Sandia Octahedral Molecular Sieve (SOMS) Na2Nb2O6×H2O: Synthesis, Structure and Thermodynamic Stability.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Prototype Sandia Octahedral Molecular Sieve (SOMS) Na2Nb2O6·H2O: Synthesis, Structure and Thermodynamic Stability

A new microporous phase Na2Nb2O6·H2O, which transforms to NaNbO3 perovskite on heating, has been synthesized by the hydrothermal method. Rietveld analysis of powder synchrotron X-ray diffraction data reveals that the structure comprises a framework of [NbO6] and [NaO6] octahedra with other Na+ being located in the channels (space group C2/c; a = 17.0511(9) A; b = 5.0293(2) A; c = 16.4921(9) A; β =113.942(2)°). This phase belongs to the recently synthesized Sandia octahedral molecular sieves (SOMS) family, Na2Nb2-xMxO6-x(OH)x·H2O (M = Ti, Zr) and is the archetype for the substituted structures. Using drop-solution calorimetry into molten 3Na2O·4MoO3 at 974 K, the enthalpies of formation of Na2Nb2O6·H2O from the constituent oxides and from the elements have been determined to be −295.4 ± 4.8 and −2895.5 ± 6.4 kJ/mol, respectively. From the drop-solution calorimetric data for Na2Nb2O6·H2O and its dehydrated perovskite phase, the enthalpy of the dehydration reaction, Na2Nb2O6·H2O → 2NaNbO3 + H2O, has been der...

Raman Spectroscopy of Na2Nb2O6•H2O and Na2Nb2-xMxO6-x(OH)x•H2O (M = Ti, Hf) Ion Exchangers

The Raman spectra of Sandia Octahedral Molecular Sieves Na 2 Nb 2 - x M x O 6 - x (OH) x .H 2 O (M = Ti, Hf; x ≤ 0.2) (SOMS-Ti and SOMS-Hf) and their parent compound Na 2 Nb 2 O 6 .H 2 O (SOMS) were measured between 24 °C and 600 °C. The spectra of all three materials as well their variations with temperature are almost identical, which indicates that the M 4 + for Nb 5 + disordered substitution has little effect on the structure of the niobate framework. The annealing above 250 °C results in dehydration reflected in the Raman spectra by disappearance of water-related bands at ∼1700 cm - 1 and 3100-3400 cm - 1 and irreversible changes of some Raman line parameters. As a whole, however, the spectral changes in the frequency range, corresponding to internal oxygen vibrations, are rather modest in contrast to the drastic changes of the X-ray diffraction patterns. This shows that although the SOMS structure rearranges in the dehydrated phase, the basic constituting elements, NbO 6 octahedra and Na-centered polyhedra, remain practically unchanged. At further annealing above ∼500 °C the Raman spectrum transforms irreversibly into a spectral structure consisting of three broad bands centered (at 24 °C) at 232, 600, and 876 cm - 1 , while the X-ray pattern indicates transformation to a perovskite-like structure We argue that the Raman spectrum in this case reflects the density of vibrational states of a strongly disordered oxygen sublattice.

Sandia octahedral molecular sieves (SOMS): structural and property effects of charge-balancing the M(IV)-substituted (M = Ti, Zr) Niobate framework.

Sandia octahedral molecular sieves (SOMS) is an isostructural, variable composition class of ion exchangers with the general formula Na(2)Nb(2-x)M(IV)(x)O (6-x)(OH)(x).H(2)O (M(IV) = Ti, Zr; x = 0.04-0.40) where up to 20% of the framework Nb(V) can be substituted with Ti(IV) or Zr(IV). This class of molecular sieves is easily converted to perovskite through low-temperature heat treatment (500-600 degrees C). This report provides a detailed account of how the charge imbalance of this Nb(V)-M(IV) substitution is compensated. X-ray powder diffraction with Rietveld refinement, infrared spectroscopy, thermogravimetric analysis, (23)Na MAS NMR, and (1)H MAS NMR were used to determine how the framework anionic charge is cation-balanced over a range of framework compositions. All spectroscopic evidence indicated a proton addition for each M(IV) substitution. Evidences for variable proton content included (1) increasing OH observed by (1)H MAS NMR with increasing M(IV) substitution, (2) increased infrared band broadening indicating increased H-bonding with increasing M(IV) substitution, (3) increased TGA weight loss (due to increased OH content) with increasing M(IV) substitution, (4) no variance in population on the sodium sites (indicated by Rietveld refinement) with variable composition, and (5) no change in the (23)Na MAS NMR spectra with variable composition. Also observed by infrared spectroscopy and (23)Na MAS NMR was increased disorder on the Nb(V)/M(IV) framework sites with increasing M(IV) substitution, evidenced by broadening of these spectral features. These spectroscopic studies, along with ion exchange experiments, also revealed the effect of the Nb(V)/M(IV) framework substitution on materials properties. Namely, the temperature of conversion to NaNb(1-x)M(IV)(x)O(3) (M = Ti, Zr) perovskite increased with increasing Ti in the framework and decreased with increasing Zr in the framework. This suggested that Ti stabilizes the SOMS framework and Zr destabilizes the SOMS framework. Finally, comparing ion exchange properties of a SOMS material with minimal (2%) Ti to a SOMS material with maximum (20%) Ti revealed the divalent cation selectivity of these materials which was reported previously is a function of the M(IV) substitution in the framework. A thorough investigation of this class of SOMS materials has revealed the importance of understanding the influence of heterovalent substitutions in microporous frameworks on material properties.