Molybdenum Bronze Research Articles

Preparation and characterization of layered molybdenum trioxide pillared with chromia

The well-ordered polyhydroxyacetato-Cr 3+ species intercalated molybdenum trioxide by the reaction of an aqueous solution of Cr(OAc) 3 with a dispersion of layered sodium molybdenum bronze ([Na(H 2O) 5] 0.25MoO 3) was prepared. X-ray fluoroscopy (XRF), TGA and near/UV–VIS spectra showed that the intercalates may mainly be trimers and tetramers. The first chromia-pillared molybdenum trioxide with an interlayer distance of 10.9 Å was then obtained after calcined at 350 °C in a flow of mixed nitrogen and oxygen (4% O 2, v/v). The obtained porous material has a surface area of 82.4 m 2 g −1, with ca. 60% of pores having r p<20 Å. XPS result suggested that the chromia pillars existed as Cr 2O 3 in the interlayer of the material.

Synthesis, Characterization, and Electronic Structure of a New Molybdenum Bronze SnMo4O6.

Synthesis, Characterization, and Electronic Structure of a New Molybdenum Bronze SnMo₄O₆.

Crystal growth and electrical properties of thallium molybdenum purple bronze TlMo 6O 17 by molten salt electrolysis

Large size and high quality single crystals of quasi-two-dimensional thallium molybdenum purple bronze TlMo 6O 17 have been grown by electrolytic reduction of molten salt of Tl 2CO 3MoO 3. The crystal structure is trigonal with space group P 3 ̄ m1 determined by X-ray diffraction and four-circle single crystal diffraction. The lattice parameters of the unit cell are: a= b=5.5282 Å, and c=13.6991 Å. The temperature dependence of resistivity and magnetic susceptibility confirmed that a metal-to-metal transition occurs near 110 K.

Optical and electrochromic properties of heated and annealed MoO3 thin films

Heating and annealing effects on the optical properties of slightly blue substoichiometric molybdenum trioxide thin films are reported partially. During heating and annealing, different levels of coloration seem to be generated by simultaneous reduction and proton injection into thin films, which cause the formation of hydrogen molybdenum bronzes of different concentrations. The optical band gap, Eo, of MoO3 thin film when annealed in between 313 and 473 K is decreased to 2.82 eV, and this reduction in Eo has been explained with a view to phonon and polaron species. Moreover, temperature-dependent change in the optical band gap has also been interpreted in terms of band gap slope, B, and the real part of the refractive index, n. Urbach slope, σ, also goes down to 0.67 eV−1 due to annealing treatment. Using the oscillator model, a phonon energy, hνo, of about 0.08 eV was found for the reported MoO3 thin film which is very close to the characteristic phonon energy of MoO3. Over and above, the intensity for the blue absorption band, over the photon energy range 0.4–3.0 eV, also reaches a maximum as MoO3 thin film is annealed at 473 K for 1 h and then decreases by 32% after annealing at the same temperature for 2 h. This phenomenon is due to polaronic band intensity which rises and falls during annealing. Half-width of the blue band, as found by fitting polaron model, is changed only by 3% on annealing the MoO3 thin film at 473 K as compared to its value at room temperature.

ChemInform Abstract: Synthesis, Characterization, and Electronic Structure of a New Molybdenum Bronze SnMo4O6.

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Direct Synthesis of Hydrogen Coinserted Hydrated Sodium and Potassium Molybdenum Bronzes: Their Characterization and Selective Preparation of Purple, Blue, and Red Molybdenum Bronzes

We succeeded in synthesizing the hydrogen coinserted hydrated potassium molybdenum bronzes (HKBz) with the 8- and 6-coordination structures by a direct new reduction method. By this method, the 8-coordination type was obtained. According to NMR results, the behavior of protons in the 8-coordination type was different from those in the 6-coordination type. We obtained the single phase of purple sodium molybdenum bronze when hydrogen coinserted hydrated sodium bronze (HNaBz) controlled the content of hydrogen, and sodium was heated in nitrogen. Also, we obtained the blue and red potassium bronzes by heating HKBz and controlling the contents of hydrogen and potassium.

Electronic structures of two-dimensional metallic oxides and bronzes

The electronic structures of some molybdenum and tungsten oxides or bronzes exhibiting Peierls transitions are investigated at room temperature. The detection of a weak conduction band, well separated from a large valence band, evidences the metallic character of each oxide. The distributions of the valences of the different transition metals are analyzed by XPS. In each oxide, the presence of atleast two contributive components to the main core levels reveals a mixed valence state of the transition metal. But the proportions of the different components do not reflect the distribution of the cationic valences, as expected from the crystallographic structures. To understand this disagreement, we suggest that two alternative ways, including or rejecting a screening effect generated by the conduction electrons contribute to the photoemission processes and alter the real distribution of the cationic charges.

A New Synthetic Route for Mixed-Valence Compounds: Leaching Treatments of Hydrogen Molybdenum Bronze

In order to explore the possibilities of a new synthetic route, based on selective extraction of the same atomic species with different valences, for mixed-valence compounds, leaching treatments of HxMoO3 with various x values with various kinds of solutions were investigated. Both oxidation-type (pseudooxidation) and reduction-type (pseudoreduction) products could be obtained by these simple treatments without oxidizing or reducing agents. Their formation mechanisms were elucidated from various investigations of solutions as well as solids. Moreover, novel hydrated hydrogen–alkali metal co-insertion compounds of layered molybdenum oxide with larger hydrogen contents were obtained by the treatments.

Formation of bronzes during temperature-programmed reduction of MoO3 with hydrogen—an in situ XRD and XAFS study

The temperature-programmed reduction of molybdenum trioxide (MoO3) from 300 to 773 K in 50 vol.% hydrogen in He (105 Pa) at different heating rates (0.1, 0.2, and 5 K/min) was investigated by in situ X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS). At heating rates of ∼0.1 and ∼0.2 K/min, the formation of the molybdenum bronze H0.34MoO3 was observed by in situ XRD in the early stage of the reduction of MoO3. At a heating rate of 5 K/min, the formation of a more disordered bronze (HxMoO3 with x∼0.07) prior to the detection of the product phase MoO2 was observed by in situ XAFS. In both studies, the consumption of the bronze was found prior to the complete reduction of MoO3. A simplified mechanism for the temperature-programmed reduction of MoO3 in hydrogen is proposed that includes: (i) incorporation of H2 in the MoO3 bulk and formation of a more or less ordered bronze, (ii) consumption of the bronze and formation of nucleation site for MoO2, (iii) nucleation of MoO2 and nuclei growth.

Magnetoresistance properties of quasi-two-dimensional charge-density wave compounds AMo6O17 (A=Na, K, and Tl)

Temperature dependence of resistivity in quasi-two-dimensional purple bronzes AMo6O17 (A=Na, K, and Tl) has been comparatively studied between 2 and 300 K at different magnetic fields. Huge positive magnetoresistance (MR) behavior below the Peierls transition temperature TP∼110 K was found in both K/Tl purple bronzes for H∥c, but only a weak one in the Na purple bronze. At lower temperature TM∼16 K, a second anomaly of the resistivity, which moved to higher temperatures under a magnetic field, was also observed in both K/Tl compounds, with no corresponding anomaly in the Na purple bronze. For H⊥c, the MR effect becomes much weaker in all three compounds. These results suggest that the second anomaly is possibly related to the second Peierls instability, and the huge MR effect results from the large modification of the third FS along the [100] direction induced by the magnetic field, in which the third FS is imperfectly nested in K/Tl purple bronzes and absent in the Na purple bronze.

Proton NMR study of the lowest-hydrogen-content molybdenum bronzeH0.26MoO3

Temperature and frequency dependences of proton spin-lattice relaxation time have been measured in the lowest-hydrogen-content molybdenum bronze ${\mathrm{H}}_{0.26}{\mathrm{MoO}}_{3}$ by pulsed NMR method. Temperature dependence of proton NMR spectra have been also observed. It is revealed that protonic motion becomes active above about 165 K and the fluctuation of dipolar interaction of protons arising from the protonic motion governs the spin-lattice relaxation in the temperature range between 213 and 333 K. The activation energy is determined to be $1.30\ifmmode\times\else\texttimes\fi{}{10}^{4}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$ and the correlation time ${\ensuremath{\tau}}_{c}$ is $1.29\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\mathrm{s}$ at room temperature. The protonic motion governing the spin lattice relaxation is discussed and it is suggested that the protonic motion is lattice diffusion. The diffusion constant D is estimated to be $3.27\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}{\mathrm{m}}^{2}/\mathrm{s}.$

Syntheses of hydrated molybdenum bronzes by reduction of MoO3 with NaBH4.

Hydrated molybdenum bronzes have been prepared by reduction reaction of MoO3 with NaBH4 in ethanol and DMSO. The reduction reactions in both solvents occur smoothly; thus, the layered structure of MoO3 is maintained in the product. Divalent cation Ca2+ has been intercalated between the MoO3 layers, which leads to highly reduced molybdenum bronze (Mo5.26+). Solvated molybdenum bronze catalyzes the reduction reaction of DMSO by NaBH4, producing CH3SCH3. The structure model of hydrated sodium molybdenum bronze has also been reinvestigated by using the Rietveld analysis. The hydrated molybdenum bronze crystallizes in an orthorhombic structure, in which the structure of Mo octahedron layers is closely related to that in MoO3. However, the structure refinement reveals that the Mo octahedron in the MoO3 layers is axially distorted, which is different from that in MoO3 but similar to an isoelectron compound H0.33MoO3.

Effect of reduction temperature on the transformation of MoO 3 to MoO x with a large surface area

The surface area of MoO 3 was enlarged by H 2 reduction. When the samples reduced at different temperatures were compared at a certain extent of reduction, MoO 3 reduced at 573 and 623 K exhibited much larger surface areas than that at 673 K. MoO 3 reduced at temperature below 623 K had a surface area of 170–180 m 2 g −1 and pores with the diameter of 0.6–3.0 nm when the reduction degree was about 50%. Since the surface area and the porous structure of MoO 3 reduced at 623 K changed little by treatment at 673 K in an N 2 atmosphere, the small surface area of MoO 3 reduced at 673 K cannot be elucidated by thermal stability. XRD studies showed that reduction of MoO 3 at temperature below 623 K proceeded via the formation of hydrogen molybdenum bronze, H x MoO 3, while the H x MoO 3 phase was not observed at 673 K. We suggest from these results that the enlargement of surface area and the formation of micropores can occur only when reduction of MoO 3 proceeds via the H x MoO 3 phase.

Infrared and EPR studies of the red potassium molybdenum bronze K0.33MoO3

Infrared reflectance and electron-paramagnetic-resonance (EPR) of the red bronze K0.33MoO3are presented. The infrared measurements at 300 K indicated that red bronze K0.33MoO3is a semiconductor with very strong anisotropy in the range. The presence of three sharp lines at bout 900 cm−1for P⊥bis in fact due to the intramolecular oscillations of oxygen inside one octahedron. The phonon oscillators near 500 cm−1for P⊥band P∥bassociate with the modification of the effective charge. The EPR signals can be attributed to electrons localized in the isolated Mo5+paramagnetic center. The difference of relative intensities of the EPR lines in 300 K and 100 K is due to the electronic excitation. The inhomogeneous broadening of the resonance lines is due to a g-factor distribution arising from crystalline field and spin-orbit coupling.

Calorimetric Study of Hydrated Potassium Molybdenum Bronze

Abstract Hydrated and dehydrated potassium bronzes, K0.25H0.07MoO3·0.54H2O (HyK-H2) with the six-coordination structure and K0.25H0.07MoO3 (DehyK) with the eight-coordination one, were synthesized by a new direct method. The standard enthalpy of formation was determined by solution calorimetry to be -4950 ± 1 kJ mol-1 for HyK-H2 and -4817 ± 1 kJ mol-1 for DehyK, respectively. According to the thermodynamic data, HyK-H2 is stable.

Cu/Mo Nanostructured Alloy Thin Films

The synthesis of nanostructured materials was pioneered by the inert gas condensation and compaction technique. We investigate magnetron sputtering as an alternative processing route in comparison with thermal evaporation. We have studied the elaboration process and alloy formation by annealing as a function of molybdenum content. The sputtered material was investigated by means of X-ray diffraction, scanning electron microscopy, electron dispersion spectroscopy, atomic force microscopy and Vickers hardness tester. The results indicate that Cu/Mo alloys are semi-hard nanostructures with colors that change from black to yellow-golden when the Mo content increases. Hard nanostructured composite alloys were obtained at 500 °C only at high Mo content. The yellow-golden alloys can be considered as molybdenum bronzes and can be used for integrated circuits (IC), electrical contacts, metallic resistors and high voltage sockets.

Cu/Mo Nanostructured Alloy Thin Films

The synthesis of nanostructured materials was pioneered by the inert gas condensation and compaction technique. We investigate magnetron sputtering as an alternative processing route in comparison with thermal evaporation. We have studied the elaboration process and alloy formation by annealing as a function of molybdenum content. The sputtered material was investigated by means of X-ray diffraction, scanning electron microscopy, electron dispersion spectroscopy, atomic force microscopy and Vickers hardness tester. The results indicate that Cu/Mo alloys are semi-hard nanostructures with colors that change from black to yellow-golden when the Mo content increases. Hard nanostructured composite alloys were obtained at 500 °C only at high Mo content. The yellow-golden alloys can be considered as molybdenum bronzes and can be used for integrated circuits (IC), electrical contacts, metallic resistors and high voltage sockets.

CHARGE DENSITY WAVE INSTABILITY IN TlMo6O17

The charge density wave instability in the quasi-two-dimensional conductor thallium purple molybdenum bronze TlMo 6 O 17 was carefully examined by studying the temperature dependence of resistivity, thermoelectric power (TEP) behavior and magnetic susceptibility. A metal-to-metal transition was confirmed near 110 K in TlMo 6 O 17 due to the partial opening of a gap at the Fermi surface and the driving of charge density wave.

Quantum Mechanical Investigations on the Insertion Compounds of Early Transition Metal Oxides

We present the results of electronic structure ab initio Hartree-Fock calculations performed on the perovskite, hexagonal, pyrochlore and layered polymorphic structures of MoO3 and WO3. We also examine their sodium and potassium insertion compounds, i.e. the molybdenum and tungsten bronzes; the materials examined are at the basis of applications in electrochromic devices and rechargeable batteries. Results of the calculations allow to gain insight into the processes occurring at the atomic level during the insertion process: we examine the relative stability of the host MoO3 and WO3 frameworks under addition of electrons to the conduction band, and extraframework ions in the empty interstices of the host lattice, which occur upon electrochemical insertion. The extraframework ions show a templating behaviour for the structure of the host transition metal oxide framework, reminiscent of zeolite synthesis. We exploit the knowledge gained in the latter field of research to examine the feasibility of the synthesis of novel microporous transition metal oxide lattices with composition MoO3 and WO3.

Preparation of Porous Chromia-Pillared Molybdenum Trioxide

Abstract A chromia-pillared layered molybdenum trioxide has been prepared by combining a Cr(Oac)3 aqueous solution with a suspension of the molybdenum bronze for the first time; the resulting material after calcination at 350 °C is porous with a surface area of 82 m2 g−1.