Molybdenum Bronze Research Articles

A reduction–nitridation process of molybdenum films in expanding microwave plasma: Crystal structure of molybdenum nitrides

This work reports the structure of molybdenum nitride phases crystallizing in molybdenum films coated on Si (100) substrates, and exposed to (Ar–N2–H2) gas mixtures with various gas contents in an expanding microwave plasma reactor. Compared with the numerous data found in literature, the formation of βMo2N and γMo2N phases strongly oriented in the [204] and [220] crystalline directions, respectively is unexpected. The βMo2N phase is synthesized in molybdenum films exposed to pure N2 plasma for 1h whereas the molybdenum films exposed to (Ar–33%N2–1%H2) plasma for only 30min consists almost entirely of γMo2N phase. The Mo2N crystallites about 30nm wide display a columnar morphology in the growth direction after processing in (Ar–33%N2–1%H2) plasma. In contrast with most processes where energetic ions improve the nitrogen transfer into the surface layers of the metal film, the expanding plasma process promotes the chemical reactions on the surface of the material. The remaining oxides are reduced by hydrogen species such as NHx radicals produced in (Ar–N2–H2) plasma. The reactions are exothermic and could involve the formation of intermediates such as hydrogen molybdenum bronze HxMoO3 that would promote the crystallization of the high temperature γMo2N phase to the detriment of the low temperature βMo2N phase during the reduction–nitridation process.

VO2 nanoflake arrays for supercapacitor and Li-ion battery electrodes: performance enhancement by hydrogen molybdenum bronze as an efficient shell material

VO2 nanoarrays are grown on graphene foam and the surface is coated with HMB for performance enhancement as LIB and supercapacitor electrodes.

Pore-arrayed hydrogen molybdenum bronze: Preparation and performance as support of platinum nanoparticles for methanol oxidation

A novel electrocatalyst for methanol oxidation is fabricated by decorating platinum nanoparticles on pore-arrayed hydrogen molybdenum bronze (HxMoO3). In this fabrication, pore-arrayed HxMoO3 is prepared with polystyrene monolayer as a template, and platinum nanoparticles are decorated on the resulting pore-arrayed HxMoO3 by using a current pulse technique. The fabricated electrocatalyst is investigated with a combination of physical characterizations from X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy, and electrochemical measurements including cyclic voltammetry, chronopotentiometry, chronoamperometry, and cell discharge test. It is found that the platinum decorated on pore-arrayed HxMoO3 (Pt/p-HxMoO3) is more uniform, has smaller particle size and exhibits improved electrocatalytic activity and stability for methanol oxidation, compared to that on non-pore-arrayed HxMoO3 (Pt/HxMoO3). The peak current for methanol oxidation in cyclic voltammetry is improved from 4.89 mA cm−2 for Pt/HxMoO3 to 6.41 mA cm−2 for Pt/p-HxMoO3, and the time for abrupt potential change in chronopotentiometry is enhanced from 498 min for Pt/HxMoO3 to 576 min for Pt/p-HxMoO3. The improved performance is attributed to the larger specific surface area of the pore-arrayed HxMoO3, which favors the formation of smaller Pt nanoparticles.

Tailoring acid–metal functions in molybdenum oxides: Catalytic and XPS-UPS, ISS characterization study

Formation of acid, metal and metal–acid (bifunctional) functions in titania-supported molybdena following controlled partial reduction of MoO3 using hydrogen at different temperatures are monitored by a combination of surface XPS-UPS, ISS techniques. Addition of controlled amount of sodium or potassium alkali metals to the bifunctional MoO2−x(OH)y/TiO2 (MoTi) enabled to neutralize the Brønsted acid Mo-OH functions. Sodium or potassium molybdenum bronze metallic function is formed following the addition of the alkali metal to the Mo salt following its calcination at 773K. The catalytic functions of these MoTi and Na, KMoTi systems were evaluated for the dehydration/hydrogenation, oxidation reactions of isopropanol to acetone as well as the ring shortening of cyclohexane to methylcyclopentane MCP and its oxidative dehydrogenation to benzene.

Effective model for the electronic properties of quasi-one-dimensional purple bronze Li0.9Mo6O17based onab initiocalculations

We investigate the electronic structure of the strongly anisotropic, quasi-low-dimensional purple bronze Li${}_{0.9}$Mo${}_{6}$O${}_{17}$. Building on all-electron ab initio band-structure calculations, we obtain an effective model in terms of four maximally localized Wannier orbitals, which turn out to be far from atomiclike. We find two half-filled orbitals arranged in chains running along one crystallographic direction and two full orbitals in perpendicular directions, respectively. The possibility to reduce this model to only two orbitals forming two chains per unit cell with interchain coupling is discussed. Transport properties of these models show high anisotropy, reproducing trends of the experimentally determined values for the dc conductivity. We also consider basic effects of electron-electron interactions using the (extended) variational cluster approach and dynamical mean field theory. We find good agreement with experimental photoemission data upon adding moderate onsite interaction of the order of the bandwidth to the ab initio derived tight-binding Hamiltonian. The obtained models provide a profound basis for further investigations on low-energy Luttinger-liquid properties or to study electronic correlations within computational many-body theory.

NPt0 (Hx−2nMoO3) as a promising catalyst for the oxidation of methanol. Synthesis and electrocatalytic properties

A new method for the synthesis of the catalyst systems Pt–Mo was suggested. nPt0(Hx−2nMoO3)/GC electrodes were prepared by a redox reaction between the hydrogen-containing molybdenum bronzes and potassium tetrachloroplatinate (II) in acid solutions at open circuit potential. The electrodes were characterized by CVA, SEM, X-ray microanalysis, XRD, XPS and ICP-AES. Pt0conglomerates formation with nonuniform distribution over the molybdenum bronzes surface has been revealed. nPt0(Hx−2nMoO3)/GC electrodes showed high catalytic activity (not inferior to Pt–Ru-catalyst) in the oxidation of carbon monoxide and methanol as compared with Pt/GC-electrodes. Catalytic effect is apparently achieved by effective oxidation of strongly chemisorbed species (COads, HCOads), which occurs at boundaries platinum – molybdenum oxide. Therefore nPt0(Hx−2nMoO3) can be considered as one of perspective catalysts for DMFC.

Solution‐Processed Hydrogen Molybdenum Bronzes as Highly Conductive Anode Interlayers in Efficient Organic Photovoltaics

Highly efficient and stable organic photovoltaic (OPV) cells are demonstrated by incorporating solution‐processed hydrogen molybdenum bronzes as anode interlayers. The bronzes are synthesized using a sol‐gel method with the critical step being the partial oxide reduction/hydrogenation using an alcohol‐based solvent. Their composition, stoichiometry, and electronic properties strongly correlate with the annealing process to which the films are subjected after spin coating. Hydrogen molybdenum bronzes with moderate degree of reduction are found to be highly advantageous when used as anode interlayers in OPVs, as they maintain a high work function similar to the fully stoichiometric metal oxide, whereas they exhibit a high density of occupied gap states, which are beneficial for charge transport. Enhanced short‐circuit current, open‐circuit voltage and, fill factor, relative to reference devices incorporating either PEDOT‐PSS or a solution processed stoichiometric molybdenum oxide, are obtained for a variety of bulk heterojunction mixtures based on different polymeric donors and fullerene acceptors. In particular, high power conversion efficiencies are obtained in devices that employed the s‐HxMoO2.75 as the hole extraction layer.

Preparation of Nanoribbons of Blue Potassium Molybdenum Bronze

Abstract We have successfully prepared nanoribbons of blue potassium molybdenum bronze (K0.3MoO3) from hydrogen molybdenum bronze (H0.3MoO3) by using crystallite splitting induced by solid-phase transformation and subsequent selective growth under hydrothermal conditions. The obtained nanoribbons were single crystals with a size of ca. 100 µm × 100 nm × 10 nm, well-elongated to the crystal b axis that corresponds to the conduction direction as a quasi-one-dimensional metal. A simple deaeration procedure necessary for preparing a single phase of blue potassium bronze is also given.

Electrochemical Reduction of Molybdate on Glassy Carbon Electrode in Acidic Media

The reduction of molybdate on glassy carbon electrodes was studied in a solution of 0.5 M H2SO4 + 3.8 x 10-3 M MoO4 2- using potentiostatic polarization, cyclic voltammetry and chronoamperometry. The results indicate that molybdate reduction processes depends on potential and polarization time. Initial states correspond to reversible adsorption of reduced molybdate species. At -700 mV, molybdate reduction is influenced by the hydrogen evolution reaction. Under these conditions, polarization promotes the formation of hydrogen molybdenum bronzes.

Luttinger liquid behaviour of Li0.9Mo6O17 studied by scanning tunnelling microscopy

Scanning tunnelling spectroscopy (STS) was used to study the Luttinger liquid behaviour of the purple bronze Li0.9Mo6O17 in the temperature range 5 K < T < 300 K. In the entire temperature range the suppression of the density of states at the Fermi energy can be fitted very well by a model describing the tunnelling into a Luttinger liquid at ambient temperature. The power-law exponent extracted from these fits reveals a significant increase above 200 K. It changes from α = 0.6 at low temperature to α = 1.0 at room temperature.

Photoemission spectroscopy and the unusually robust one-dimensional physics of lithium purple bronze

Temperature-dependent photoemission spectroscopy in Li0.9Mo6O17 contributes to evidence for one-dimensional (1D) physics that is unusually robust. Three generic characteristics of the Luttinger liquid are observed: power law behavior of the k-integrated spectral function down to temperatures just above the superconducting transition, k-resolved lineshapes that show holon and spinon features, and quantum critical (QC) scaling in the lineshapes. Departures of the lineshapes and the scaling from expectations in the Tomonaga–Luttinger model can be partially described by a phenomenological momentum broadening that is presented and discussed. The possibility that some form of 1D physics obtains even down to the superconducting transition temperature is assessed.

Electrochemical deposition and behavior of mixed-valent molybdenum oxide film at glassy carbon and ITO electrodes

The effect of solution composition and the type of the anionic species on the electrochemical formation of mixed-valent molybdenum oxide on a glassy carbon and ITO electrode surfaces was elucidated. Susccessive recording of the voltammograms has shown that anionic species display different stabilizing effect on the reductive formation of hydrogen molybdenum bronzes [MoO3−x (OH)x] and chloroacetic acid buffer has given the best results. The deposit was built upon cycling the potential between 0 and −0.9V (vs. Ag/AgCl) via reduction of Mo(VI) to Mo(V) on the electrode surface in pH 3.0 chloroacetic acid solution. Electrochemical impedance measurements carried out in this medium revealed a shift in potential zero charge values from -0.2V to -0.55V after the potential of the GCE had been cycled for 30min. An establishment of mixed-valent molybdenum oxide deposit by time on the gold electrode surface was proved by quartz crystal microbalance measurements. Atomic force and scanning electron microscopy techniques were made use of so as to characterize the surface structures of the electrodes. X-ray photoelectron spectroscopy studies confirmed that the deposit contains both Mo(V) and Mo(VI). The deposited films exhibited unique catalytic activity towards nitrite oxidation consistent with the change in peak characteristics.

Room-temperature solution synthesis of Ag nanoparticle functionalized molybdenum oxide nanowires and their catalytic applications

A simple chemical solution route for the synthesis of large-scale high-quality Ag nanoparticle functionalized molybdenum oxide nanowire at room temperature has been developed. In the synthesis, the protonated amine was intercalated into the molybdenum bronze layers to reduce the electrostatic force of the lamellar structures, and then the Ag nanoparticle functionalized long nanowires could be easily induced by a redox reaction between a molybdenum oxide–amine intermediate and Ag+ at room temperature. The intercalation lamellar structures improved the nucleation and growth of the Ag nanoparticles, with the result that uniform Ag nanoparticles occurred on the surface of the MoO3 nanowire. In this way Ag nanoparticles with average sizes of around 6 nm, and high-purity nanowires with mean diameter of around 50 nm and with typical lengths of several tens to hundreds of micrometers were produced. The heteronanostructured nanowires were intricately and inseparably connected to each other with hydrogen bonds and/or bridge oxygen atoms and packed together, forming a paper-like porous network film. The Ag–MoO3 nanowire film performs a promoted catalytic property for the epoxidation of cis-cyclooctene, and the heteronanostructured nanowire film sensor shows excellent sensing performance to hydrogen and oxygen at room temperature.

Luttinger-liquid theory of purple bronze Li0.9Mo6O17in the charge regime

Molybdenum purple bronze Li$_{0.9}$Mo$_{6}$O$_{17}$ is an exceptional material known to exhibit one dimensional (1D) properties for energies down to a few meV. This fact seems to be well established both in experiments and in band structure theory. We use the unusual, very 1-dimensional band dispersion obtained in \emph{ab-initio} DFT-LMTO band calculations as our starting point to study the physics emerging below 300meV. A dispersion perpendicular to the main dispersive direction is obtained and investigated in detail. Based on this, we derive an effective low energy theory within the Tomonaga Luttinger liquid (TLL) framework. We estimate the strength of the possible interactions and from this deduce the values of the TLL parameters for charge modes. Finally we investigate possible instabilities of TLL by deriving renormalization group (RG) equations which allow us to predict the size of potential gaps in the spectrum. While $2k_F$ instabilities strongly suppress each other, the $4k_F$ instabilities cooperate, which paves the way for a possible CDW at the lowest energies. The aim of this work is to understand the experimental findings, in particular the ones which are certainly lying within the 1D regime. We discuss the validity of our 1D approach and further perspectives for the lower energy phases.

Salt-flux synthesis of complex oxides: Cs0.33MoO3, CsFe(MoO4)2, and the inverse salt-inclusion phase Cs2Mo0.65O0.21Cl5.44

Recent work on the growth of complex oxides in salt fluxes is reviewed. Single crystals of Cs0.33MoO3, CsFe(MoO4)2 and the new phase, Cs2Mo0.65O0.21Cl5.44, have been grown from the reaction of metal oxides Fe2O3, MoO3 and molybdenum powder in a eutectic CsCl/NaCl flux. Cs0.33MoO3 is one of the known molybdenum bronzes, featuring mixed valent molybdenum ions. CsFe(MoO4)2 has a layered KAl(MoO4)2 structure, which consists of infinite slabs of corner-sharing MoO4 and FeO6 polyhedra separated by layers of Cs+ cations. Cs2Mo0.65O0.21Cl5.44 forms with a new structure type (orthorhombic, Cmcm, a = 7.434(1) Å, b = 17.330(3) Å, c = 8.074(1) Å, Z = 4) which is comprised of isolated [Mo(O/Cl)Cl5] octahedra embedded in a distorted CsCl matrix. Magnetic susceptibility measurements indicate antiferromagnetic ordering for both CsFe(MoO4)2 (T N = 4.5 K) and Cs2Mo0.65O0.21Cl5.44 (T N = 7 K).

Upper Critical Magnetic Field far above the Paramagnetic Pair-Breaking Limit of Superconducting One-DimensionalLi0.9Mo6O17Single Crystals

The upper critical field H(c2) of purple bronze Li0:9Mo6O17 is found to exhibit a large anisotropy, in quantitative agreement with that expected from the observed electrical resistivity anisotropy. With the field aligned along the most conducting axis, H(c2) increases monotonically with decreasing temperature to a value 5 times larger than the estimated paramagnetic pair-breaking field. Theories for the enhancement of H(c2) invoking spin-orbit scattering or strong-coupling superconductivity are shown to be inadequate in explaining the observed behavior, suggesting that the pairing state in Li0:9Mo6O17 is unconventional and possibly spin triplet.

Transport and thermodynamic properties of quasi-two-dimensional purple bronzesA0.9Mo6O17(A=Na, K)

We report a comparative study of the specific heat, electrical resistivity and thermal conductivity of the quasi-two-dimensional purple bronzes Na$_{0.9}$Mo$_6$O$_{17}$ and K$_{0.9}$Mo$_6$O$_{17}$, with special emphasis on the behavior near their respective charge-density-wave transition temperatures $T_P$. The contrasting behavior of both the transport and the thermodynamic properties near $T_P$ is argued to arise predominantly from the different levels of intrinsic disorder in the two systems. A significant proportion of the enhancement of the thermal conductivity above $T_P$ in Na$_{0.9}$Mo$_6$O$_{17}$, and to a lesser extent in K$_{0.9}$Mo$_6$O$_{17}$, is attributed to the emergence of phason excitations.

Voltammetric Study of the Molybdate Reduction and Subsequent Oxidation on Glassy Carbon Electrode in Acid Media

The electroreduction of molybdate and subsequent electro-oxidation on glassy carbon electrodes was studied by cyclic voltammetry in acid medium. The results show that the reduction of molybdate depends on applied potential and molybdate concentration in solution. The molybdate reduction process is characterized by cathodic peaks Ic (-0.143 mV), IIc (-0.270 mV) and the cathodic band (Bc) around -400 mV. Cathodic peaks Ic and IIc correspond to the early stages of reduction of molybdate. Both peaks show some degree of reversibility with respect to the anodic peaks Ia and IIa. As the cathode switching potential is more negative than -400 mV and the concentration of molybdate increases, the band Bc is the dominant factor in the reduction process, resulting in the formation of the anodic peaks IIIa (-217 mV) and IVa (477 mV) on voltammogram. This behaviour can be justified through the formation and oxidation of hydrogen molybdenum bronzes on the surface of glassy carbon.

Breaking the Law

Physics Conduction of heat in solids is often achieved through the same means as electrical conduction—via charge carriers—and materials that are good electrical conductors (such as metals) also tend to conduct heat efficiently. For metals, the ratio between the thermal and electrical conductivities is determined by the Wiedemann-Franz (WF) law. However, in one-dimensional systems, a distinct state known as the Tomonaga-Luttinger (TL) liquid is predicted to occur, wherein the spin and charge degrees of freedom of an electron separate into spinons and holons; because heat is conducted by both but electricity only by holons, a violation of the WF law may occur. Wakeham et al. report such a violation in the purple bronze Li0.9Mo6O17, which is a conductor consisting of weakly interacting one-dimensional chains. The violation they find is as large as a factor of 100,000 at the lowest temperatures accessed and consistent with the scenario of spin-charge separation in a TL liquid, even though the material is only approximately one-dimensional. If, as the authors speculate, its TL liquid nature is a result of strong correlations within the chains, it may be possible to tune the dimensionality of the system, and thus effectively recombine spinons and holons, by appropriate chemical substitution. Nat. Commun. 2 , 10.1038/ncomms1406 (2011).

Neutron diffraction study of quasi-one-dimensional lithium purple bronze: Possible mechanism for dimensional crossover

The crystallographic structure of quasi-one-dimensional lithium purple bronze (LPB) was investigated using neutron powder diffraction (NPD) at temperatures $T$ in the range 5 $&lt;T&lt;$ 295 K. It has a monoclinic symmetry with space group $P$2${}_{1}$/$m$, the lattice parameters at 295 K are $a$ $=$ 12.7530(4) \AA{}, $b$ $=$ 5.5239(1) \AA{}, $c$ $=$ 9.4909(2) \AA{}, and $\ensuremath{\beta}$ $=$ 90.588(2)${}^{\ensuremath{\circ}}$. The sample stoichiometry was determined, through chemical analysis and refinement of the NPD data, to be Li${}_{0.924}$Mo${}_{6}$O${}_{17}$. The linear thermal expansion of the lattice parameters agrees with previously reported high-resolution dilatometry measurements. The bond-valence-sum method was applied to calculate the valence of each Mo ion as a function of $T$, which allows discussion of the mechanism by which charge is transferred between the double one-dimensional conducting chains.