Sodium Tungsten Bronzes Research Articles

Hollow Sodium Tungsten Bronze (Na0.15WO3) Nanospheres: Preparation, Characterization, and Their Adsorption Properties

We report herein a facile method for the preparation of sodium tungsten bronzes hollow nanospheres using hydrogen gas bubbles as reactant for chemical reduction of tungstate to tungsten and as template for the formation of hollow nanospheres at the same time. The chemical composition and the crystalline state of the as-prepared hollow Na0.15WO3nanospheres were characterized complementarily, and the hollow structure formation mechanism was proposed. The hollow Na0.15WO3nanospheres showed large Brunauer–Emment–Teller specific area (33.8 m2 g−1), strong resistance to acids, and excellent ability to remove organic molecules such as dye and proteins from aqueous solutions. These illustrate that the hollow nanospheres of Na0.15WO3should be a useful adsorbent.

Theory of the color change ofNaxWO3as a function of Na-charge doping

We report theoretical investigations of the coloration of ${\text{WO}}_{3}$ upon charge insertion using sodium tungsten bronze $({\text{Na}}_{x}{\text{WO}}_{3})$ as a model system. Our results explain well the systematic color change of ${\text{Na}}_{x}{\text{WO}}_{3}$ from dark blue to violet, red-orange, and finally to golden yellow as sodium concentration $x$ increases from 0.3 to unity. Proper accounts for both the interband and the intraband contributions to the optical response are found to be very important for a detailed understanding of the coloration mechanism in this system.

Three-dimensional band structure of highly metallicNa0.8WO3by angle-resolved photoemission spectroscopy

Three-dimensional electronic structure of highly metallic sodium tungsten bronze, ${\text{Na}}_{0.8}{\text{WO}}_{3}$, is investigated by high-resolution angle-resolved photoemission spectroscopy. The experimentally determined valence-band structure along the momentum directions both parallel and perpendicular to the surface has been compared with the results of ab initio band-structure calculation. The angle-resolved photoemission spectroscopy spectra for different photon energies reveal that possibly the oxygen vacancies in the system are responsible for the evolution of density of states at the top of $\ensuremath{\Gamma}$ point in experimental valence band. The band dispersion around $\ensuremath{\Gamma}(X)$ point leading to an electronlike Fermi surface is well predicted by the band calculation. As we move from bulk-sensitive to more-surface-sensitive photon energy, we found emergence of Fermi surfaces at $X(M)$ and $M(R)$ points similar to the one at $\ensuremath{\Gamma}(X)$ point, suggesting the reconstruction of surface due to rotation/deformation of ${\text{WO}}_{6}$ octahedra.

Electronic structure of sodium tungsten bronzesNaxWO3by high-resolution angle-resolved photoemission spectroscopy

The electronic structure of sodium tungsten bronzes, $Na_xWO_3$, for full range of x is investigated by high resolution angle-resolved photoemission spectroscopy (HR-ARPES). The experimentally determined valence band structure has been compared with the results of ab initio band-structure calculation. The HR-ARPES spectra taken in both the insulating and metallic phase of $Na_xWO_3$ reveal the origin of metal-insulator transition (MIT) in the sodium tungsten bronze system. In the insulating $Na_xWO_3$, the near-EF states are localized due to the strong disorder caused by the random distribution of $Na^+$ ions in $WO_3$ lattice. While the presence of an impurity band (level) induced by Na doping is often invoked to explain the insulating state found at low concentrations, there is no signature of impurity band (level) found from our results. Due to disorder and Anderson localization effect, there is a long-range Coulomb interaction of conduction electrons; as a result, the system is insulating. In the metallic regime, the states near $E_F$ are populated and the Fermi level shifts upward rigidly with increasing electron doping (x). The volume of electronlike Fermi surface (FS) at the $\Gamma (X)$ point gradually increases with increasing Na concentration due to $W 5dt_2_g$ band filling. A rigid shift of $E_F$ is found to give a qualitatively good description of the FS evolution.

Synthesis of tungsten bronze powder and determination of its composition

Sodium tungsten bronze powders were synthesized by thermal reduction of a gas/melt system at high temperature. Samples having a cubic structure with different compositions were prepared. The initial melt included Na2WO4 ,W O 3 and 10-40% mol. NaCl while the reducing gas was hydrogen at 750 � C. An original mechanism of controlling the powders size and distribution was suggested and discussed. A quantita- tive novel and simple method to determine the bronze composition based on TGA data was developed. An increase in the NaCl content led to a decrease of the crystals size and improved the powder uniformity. Fine powders, in the 2-5 lm size range, were synthesized from melt with 40% mol of NaCl. The stoichiometry parameter x of the obtained bronzes ranged from 0.8 to 0.92. An excellent agreement between x values deter- mined by the classical XRD route and the proposed TGA method was demonstrated.

Metal–insulator transition in sodium tungsten bronzes, [formula omitted], studied by angle-resolved photoemission spectroscopy

We report high-resolution angle-resolved photoemission spectroscopy on sodium tungsten bronzes, Na x WO 3 , which exhibit a metal–insulator transition as a function of x. We found that the near- E F states are localized in Na x WO 3 ( x ⩽ 0.25 ) due to the strong disorder caused by the random distribution of Na + ions in the WO 3 lattice, which makes the system insulating. In the metallic regime we found that the rigid shift of band structure can explain the metallic Na x WO 3 band structure with respect to Na doping.

Angle-Resolved Photoemission Spectroscopy of the InsulatingNaxWO3: Anderson Localization, Polaron Formation, and Remnant Fermi Surface

The electronic structure of the insulating sodium tungsten bronze, Na(0.025)WO(3), is investigated by high-resolution angle-resolved photoemission spectroscopy. We find that near-E(F) states are localized due to the strong disorder arising from random distribution of Na+ ions in the WO(3) lattice, which makes the system insulating. The temperature dependence of photoemission spectra provides direct evidence for polaron formation. The remnant Fermi surface of the insulator is found to be the replica of the real Fermi surface in the metallic system.

A green route for microwave synthesis of sodium tungsten bronzes Na xWO 3 (0< x

A green route has been developed for microwave synthesis of sodium tungsten bronzes Na x WO 3 (0< x<1) from Na 2WO 4, WO 3 and tungsten powder. The hybrid microwave synthesis was carried out in argon atmosphere using CuO powder as the heating medium. Tungsten powder is used as the reducing agent instead of the alkali metal iodides previously used for the microwave synthesis of oxide bronzes. The prepared samples were characterized by powder X-ray diffraction, energy-dispersive X-ray analysis and scanning electron microscopy, and their phase constitutions, crystal structures and morphologies are in consistence with that in the literature. This synthesis method is simple, green and atom economic, and promising for preparation of other oxide bronzes and related compounds.

Point Contact Spectroscopy and Temperature Dependence of Resistivity of Metallic Sodium Tungsten Bronzes - Role of Optical Phonons

In this paper we report the results of electrical resistivity (1.5≤ T ≤ 300K) and point contact spectroscopy (PCS) measurements on single crystals of metallic sodium tungsten bronze with varying sodium content. We have shown that the electron-phonon coupling function as measured through PCS can explain quantitatively the large temperature dependence of resistivity ρ seen in these materials over the entire temperature range. The electron-phonon coupling function shows predominately large peaks for phonon frequency range of 30 meV ≤ ω ≤ 100 meV which match well with the calculated optical phonons for WO6 octahedron. The integrated electron-phonon coupling constant λ from this data is ≈ 0.25-0.45, depending on the Na content.

Density-functional study of charge doping inWO3

The addition of electron donors to the vacant A site of defect-perovskite structure tungsten trioxide causes a series of structural and chemical phase transitions; for instance, in the well-known case of the sodium tungsten bronzes (NaxWO3). Here we calculate the effect of the addition of electronic charge to WO3 without the complication of also including sodium ions. Our density functional theory method enables isolation of electronic effects from the additional size, chemical, and disorder effects present in experimental samples. Our calculated low-temperature phase diagram between x=0 and x=1 moves from the initial low-temperature monoclinic phase through a second (centered) monoclinic phase, an orthorhombic phase, a tetragonal antiferroelectric, and an aristotypic cubic phase, in broad agreement with the experimentally-observed transformations in NaxWO3. Our work confirms that the observed structural transformations are driven primarily by electronic factors. We find that the dominant electronic effect is the covalent interaction between the tungsten 5d and oxygen 2p orbitals.

Mechanochemical synthesis of sodium tungsten bronze nanocrystalline powders

Nanocrystalline powders of sodium tungsten bronze Na x WO 3 ( x∼0.88) have been prepared by mechanochemical process using starting materials of Na pieces and WO 3 powders in a planetary ball mill. The synthesis reactions proceed with increasing milling time and are almost completed after 44 h. Phase-pure nanoparticles of Na x WO 3 with average size of 17 nm were directly obtained after a simple washing process to remove the by-product of Na 2WO 4. The resistivity was measured in the temperature range from 77 to 300 K. The sample displays semiconducting behavior and can be characterized by three-dimensional variable-range hopping. The mechanochemical process seems to be an attractive route to fabricate tungsten bronzes because of several advantages such as easy preparation, less cost, operating at low temperature and suitability for a large-scale production.

High temperature magnetic anomalie in Na xWO 3 single crystals at low sodium content

Sodium tungsten bronze single crystals with low sodium content ( x≃0.14) were grown by the electrochemical reduction method. The magnetic and transport properties of the obtained Na x WO 3 samples were investigated. The results of SQUID magnetic measurements show presence of a magnetic anomalie at 150 K. It indicates the possible existence of the high-temperature superconducting phase in sodium tungsten bronzes.

Crystal structure of a new monophosphate tungsten(V) bronze with hexagonal tunnels Na 4(WO 3) 8(PO 2) 4

A new monophosphate tungsten(V) bronze Na 4(WO 3) 8(PO 2) 4 has been isolated in melted Na–W–P–O system by sealed tube reaction. Its structure has been determined by single crystal X-ray diffraction in the centrosymmetric space group P2 1/ m with parameters a=6.6065(2), b=5.2711(2), c=8.8898(2) Å, β=99.5898(2)°, Z=2, R=0.055, R W=0.065. The obtained Na 4(WO 3) 8(PO 2) 4 is related to the series of monophosphate tungsten bronzes with hexagonal tunnels A x (WO 3) 2 m (PO 2) 4 with A=Na, K. The framework {(WO 3) 8(PO 2) 4 4−} 3∞ consists of ReO 3 type staircase-like layers built up of WO 6 octahedra interlinked by PO 4 tetrahedra and contains hexagonal tunnels running along [010] where the sodium atoms are located. The edge-sharing distorted pentagonal pyramids NaO 6 form infinite zigzag chains along the channels. A comparison of the Na 4(WO 3) 8(PO 2) 4 structure with that of sodium and potassium monophosphate tungsten bronzes is carried out.

Chemical synthesis and electronic conduction properties of sodium and potassium tungsten bronzes

NaxWO3 and KxWO3 gels have been prepared by reduction of WO3 in aqueous solution with NaBH4 or KBH4 induced by re-addition of hydrochloric acid. These tungsten bronze gels, as good precursors, were annealed at 600 or 850°C to yield crystalline product. Their compositions are Na0.56WO3 and K0.58WO3, respectively, as determined by energy dispersive analysis. X-ray diffraction patterns indicate that NaxWO3 annealed at 600°C is a metastable tetragonal phase, which transforms to cubic perovskite phase when annealed at 850°C; while KxWO3 annealed at either 600 or 850°C has a tetragonal structure. Resistivity measurement indicates that Na0.56WO3 shows a change from metallic to anomalous semiconducting behavior accompanied by phase transformation from tetragonal to cubic phase. This is in contrast to metallic behavior in cubic NaxWO3 (x>0.25) generally reported before. Both of the samples of K0.58WO3 annealed at 600 and 850°C are semiconducting.

The surface structure of the metallic sodium tungsten bronze Na 0.667WO 3(001)

Scanning tunnelling microscopy has been used to identify a number of surface reconstructions on the (001) surface of the cubic metallic sodium tungsten bronze, Na 0.667WO 3. Which is dominant has been found to depend critically on sample preparation. As well as a ( 2 × 2 ) R45° reconstruction that bears a striking similarity to that of the parent material, tungsten trioxide, regions of (2×1) periodicity are observed that can only be explained in terms of an Na y O surface layer. In the current work, we relate the effect of sample preparation on the surface electronic structure of Na 0.667WO 3(001) with that on the atomic structure by comparing photoemission spectra with STM images. Particular interest is focused on band gap defect states in photoemission spectra which, in contrast to similar states in spectra from WO 3, do not appear to correlate with the appearance of localised defects or highly reduced terraces in STM images. The existence of peroxide-like oxygen dimers at the (2×2) reconstructed surface, on the other hand, is characterised by the appearance of identifiable states in the valence band spectrum.

X-Ray and Electron Diffraction Study of the Tetragonal Sodium Tungsten Bronze, Na0.10WO3, with Distorted Perovskite Structure

The crystal structure of the tetragonal sodium tungsten bronze, Na0.10WO3, was investigated by single-crystal X-ray and electron diffraction methods. The average structure has space groupP4/nmmand cell constantsaav=5.2492(5) Å,cav=3.8953(4) Å, andZ=2. The superstructure has space group very probablyP4 and cell constantsas=7.423(3) Å,cs=7.791(1) Å, andZ=8. Full-matrix least-squares refinements resulted in a conventionalRvalue of 0.041 [86 observed unique reflections,I>2σ(I)] for the average structure and anRvalue of 0.086 [537 observed unique reflections,I>2σ(I)] for the superstructure. The general features of the average structure, which is of distorted perovskite type, are the same as those reported earlier (A. Magnéli, 1951.Acta Chem Scand. 5, 670). The two axial bonds of the WO6octahedron are not equal (1.95 Å ) as observed in the previous work but have significantly different lengths [1.63(6) and 2.27(6) Å]. The superstructure, reported for the first time, results from the average structure by the combined effects of tilting of the WO6octahedra about c leading to a doubling of this axis (cs=2cav) and of shape deformation of these octahedra in theabplane leading to a supercell in this plane withas=2aav.

Superconductivity of sodium tungsten bronze with cubic structure

Sodium tungsten bronze NaxWO3 with cubic structure is reported to exhibit bulk superconductivity at sodium content 0.16≤x≤0.4. The superconducting transition temperatureTc and the temperature dependence of the critical fieldHc2 have been measured for the samples with different x. It was concluded that the enhancement ofTc at low x-values occurs due to softening of the phonon spectrum. The possible reason for positive curvature of theHc2 vsT curve is discussed.

Real space observation of peroxide-like dimers on a sodium tungsten bronze surface by scanning tunnelling microscopy

Oxygen dimers on the (2 × 1) reconstructed surface of Na0.65,WO3(001) have been observed by scanning tunnelling microscopy, with an O–O separation (2.2 Å) that is significantly less than twice the ionic radius of the oxide ion (2.8 Å).

NASICON electrode for detecting sodium ions

Solid electrolytes are used for electrochemical devices like batteries, fuel cells and sensors. NASICON, a ceramic Na + conductor, has been used as a sensitive membrane for an Na +-sensitive electrode for several years. In this contribution we report on the stability of NASICON in aqueous solutions and on the characteristic of an Na +-sensitive electrode with NASICON as membrane. Such a membrane can be prepared by conventional ceramic techniques but also by the screen printing technique. An all thick film electrode with NASICON as sensitive membrane and sodium tungsten bronze as internal reference electrode is less sensitive to Na + than an electrode with a membrane in pellet form.

ChemInform Abstract: Preparation of Sodium Tungsten Bronze (cubic) by the Reduction of Tungstates with Hydrogen.

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