Ion-beam reduction of the surface of higher niobium oxide

X-ray photoelectron spectroscopy is used to study the process of reduction of the surface of the higher oxide Nb₂O₅ upon bombardment with inert gas ions (Ar⁺) and reactive gas ions (O₂⁺) with an energy of 1 and 3 keV in high vacuum at room temperature. It is found that, upon bombardment with Ar⁺ ions, the lower oxide NbO and the intermediate oxide NbO₂ are formed in the surface layers of the oxide Nb₂O₅. Bombardment with O₂⁺ ions leads to the formation of an extremely insignificant amount of the intermediate oxide NbO₂ in the surface layers of the oxide Nb₂O₅. It is revealed that the process of ion-beam reduction of the surface of the oxide Nb₂O₅ depends on the ion type, dose, and energy of exposure.     

Ion-beam reduction of the surface of tantalum higher oxide

The process of reduction of the surface of higher oxide Ta₂O₅ under irradiation by inert gas (Ar⁺) and chemically active gas (O₂⁺) ions with an energy of 3 keV in high vacuum is investigated by X-ray photoelectron spectroscopy at room temperature. It is found that intermediate oxide TaO₂, lower oxide TaO, and metallic Ta form in the surface layers of Ta₂O₅ under Ar⁺ ion bombardment. An insignificant amount of intermediate oxide TaO₂ forms in the surface layers of Ta₂O₅ under O₂⁺ ion bombardment. Ion-beam-induced reduction of the Ta₂O₅ surface is shown to depend on the type of ion and irradiation dose.     

Lattice dynamics of molybdenum and tungsten

Summary The lattice dynamics of molybdenum and tungsten has been re-examined within the framework of the transition metal model potential by incorporating the local-field correction to the dynamical matrix for phonon dispersion relation. It is observed that the local-field correction can be represented by an attractive short-range potential which tends to cancel or ?screen? the Born-Mayer repulsion. The numerical calculations for molybdenum and tungsten show that the local-field correction plays a significant role and leads to a better agreement between the theoretically computed phonon dispersion curves and the experimental data.     

Ion-beam modification of track membrane surface

An ion-beam method to modify the track membrane surface is suggested. An ion gun based on a magnetron sputterer is developed. This gun provides ion energies in the range of 5 eV–1 keV, ion current density up to 0.8 mA/cm², and an ion beam aperture of 90 mm. After the track membrane surface has been irradiated by argon ions with an energy of 50–100 eV, the angle of surface-water contact decreases from 65°–75° to 10°–25°. If the irradiating ion energy is 300–800 eV, the angle of contact increases from 65°–75° to 90°–100°.     

含钼涂层钨材料在氦等离子体作用下的前期表面形貌演变研究

为了研究纳米绒毛的生成过程及内在的物理机制,采用磁控溅射镀膜的方式在钨片上镀有钼涂层(Mo-W)。借助微观结构观察的手段分析了钨、钼块材以及钨-钼(Mo-W)样品在773K、1073K氦等离子体作用条件下的表面形貌演变过程。结果表明:钨、钼以及钨-钼(Mo-W)样品在氦等离子体作用下表面形貌演变规律基本一致;纳米绒毛及其珊瑚状前驱体是在尺寸相对较大的锥状结构上演化而来的。     

Infrared spectra of matrix-isolated tungsten oxides

The infrared spectra of the WO, WO₂, and WO₃ molecules were observed in Kr and Ar matrices at 14 K. The vibrational constants of W¹⁶O, ω_e and ω_eχ_e, in both Ar and Kr matrices were derived from the measured frequencies of W¹⁶O and W¹⁸O. The ν₁ and ν₃ mode absorptions of three WO₂ isotopomers were identified in krypton matrices and an upper limit to the bond angle of 119.4 ± 0.5° was determined. The true bond angle is estimated to be 114 ± 3°. The different effects of tungsten isotopes on the linewidths of the ν₁ and ν₃ absorption peaks of WO₂ were observed and are consistent with the calculated effects. The WO₃ molecule, whose spectrum was observed in Ar matrices and possibly in Kr, was found to be planar symmetric (D_3h). The observed lineshapes of WO₃ absorption peaks are consistent with the expected effects of tungsten isotopes in natural abundance.     

Electronic structure of tetragonal tungsten bronzes and electrochromic oxides

X-ray photoemission spectra of the band structures of WO₃, crystalline H _x WO₃ and the tetragonal and cubic bronzes M _x WO₃ (M=Li, Na) exhibit great similarity. In the bronzes tungsten 5d conduction band states are occupied. The tungsten 4f core level spectra of these materials have an unusual, but characteristic structure attributed to a combination of final state screening and hydrogen or alkali ion neighbor effects. The band structure of amorphous electrochromic WO₃ films differs in characteristic ways from that of the crystalline bronzes.     

Laser photolytic deposition of molybdenum and tungsten thin film microstructures

We have deposited thin metal films of Mo and W with sub-m lateral resolution using the photodissociation of the carbonyls by the frequency doubled radiation of an Arion laser at cw and ps-pulse operation, respectively. In both regimes similar curves are obtained for the intensity dependence of the deposition rate. No difference was observed for the lateral resolution using cw or ps-pulse irradiation. Measurements of the deposition rate as a function of the spot size indicate that the decomposition occurs in the adsorbed phase.     

Field emission studies of selenium adsorption on tungsten and molybdenum

The adsorption of selenium on tungsten and molybdenum field emitters has been studied. Despite the topographical similarity of the two substrates there are marked differences in the migration and localised adsorption of the first two monolayers of selenium. Anomalous behaviour of the emission current and Fowler-Nordheim parameters is found for adsorption on both substrates. Measurements on single crystal planes indicate that this behaviour is not caused by localised growth or clustering effects. It is suggested that the model proposed by Nicolaou and Modinos to explain similar effects during germanium adsorption on tungsten is also applicable in the present case.     

ESCA and IR study of cyclopentadienyl tungsten and molybdenum carbonyl compounds

The ESCA spectra of a selection of cyclopentadienyl tungsten and molybdenum carbonyls are reported. The combination of ESCA and IR provides data of a c     

Ion-beam formation of the active surface of methanol oxidation electrocatalysts on the tantalum substrates

The active surfaces of electrocatalysts are formed by the ion-beam-assisted deposition of one of the rare-earth metals and platinum onto tantalum substrates from a neutral vapor fraction and the vacuum-arc discharge plasma of a pulsed ion source. Deposited metal ions are used as agents to aid deposition. The composition and microstructure of the formed surface layers are investigated via scanning electron microscopy, electron probe microanalysis, electron backscatter diffraction, and Rutherford backscattering spectrometry. Electrocatalytic activity in the electrochemical oxidation of methanol, which underlies the operation of low-temperature fuel cells, is investigated via cyclic voltamperometry.     

Theoretical photoelectronic spectra of molybdenum, tungsten, and rhenium disilicides

Photoelectronic spectra of volume samples of molybdenum, tungsten, and rhenium disilicides are theoretically investigated as functions of the excitation energy (hv = 12–1486 eV) by the method of linearized attached plane waves. The main contribution to the spectrum at low (hv = 12–52 eV) and very high (hv = 1486 eV) excitation energies is given by the metal d-states, while the contribution of the Si p-states is dominant at intermediate excitation energies. An analysis of the calculated results demonstrates that the main peak of the photoelectronic spectra is displaced by 0.2 eV toward the bottom of the valence band when going from MoSi₂ to WSi₂; it is displaced by 0.3 eV when going from WSi₂ to ReSi_1.75. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 75–80, July, 2006.     

Electrochromic properties of tungsten oxides synthesized from aqueous solutions

Tungsten oxides are known to exhibit interesting electrochemical properties. Ion insertion (Li⁺, H⁺) within the oxide network is highly reversible. It leads to a blue coloration and WO₃ thin films can be used as electrochromic layers in display devices or smart windows. Tungsten oxide thin films can be conveniently deposited from aqueous solutions of tungstic acid. However polytungstic acids are not stable and tend to precipitate into hydrated tungsten oxide WO₃⋅2H₂O. The condensation of polytungstic species can be chemically controlled by adding foreign ions in the solution. Precipitation is no more observed in the presence of H₂O₂. Peroxopolytungstic acids are formed in which chelating [O₂]²⁻ ligands prevent the formation of an oxide network. Such solutions are specially convenient for the deposition of optically transparent thin films. Mixed oxides WO₃-MoO₃ are obtained when condensation is performed in the presence of Mo⁶⁺ cations. This paper shows how the condensation of tungstic acid can be chemically controlled and describes the electrochemical properties of the films deposited from such solutions. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Electrochemical behaviour of tungsten oxides in lithium-organic electrolyte cells

The electrochemical behaviour of various tungsten oxides in lithium-organic electrolyte cells has been studied by voltage-composition curves, micropolarization and cycling tests. The change in structure of the oxides which follows the lithium incorporation has been investigated by X-rays showing marked differences between WO₃, the typical crystallographic shear oxide W₂₀O₅₈, and the tunnel structure oxide W₁₈O₄₉.     

Study of the electronic structure of rhenium and tungsten oxides on the O K-edge

Oxygen K-edge x-ray absorption spectra were studied on the electrochromic amorphous thin film a-WO₃ in the comparison with crystalline oxides having variable electronic (d⁰, d¹, d²) and atomic structure: monoclinic m-WO₃ (insulator - d⁰), cubic Na_0.6WO₃ (metal - d¹), cubic ReO₃ (metal - d¹), layered-type hexagonal h-WO₃, WO₃H₂O and with intercalated H_xReO₃ (metal - d²), H_xWO₃ oxides having a metal-isulating transition. The changes in the XANES range 10–15 eV above the absorption edge are interpreted based on the known band-structure calculations. The high-energy features are related to the multiple-scattering processes (EXAFS) on the nearest atoms. The intensity of the feature at 550–560 eV is attributed for the first time to the value of the metal-oxygen-metal bond angle. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

The partial catalytic oxidation of toluene on vanadium and molybdenum oxides

The partial catalytic oxidation of toluene on pure and mixed vanadium and molybdenum oxides was studied over the temperature range 300–500°C. The main reaction products were maleic anhydride (MA), benzaldehyde (BA), and carbon oxides (CO _x ) depending on the catalyst composition and reactor temperature. The samples containing more than 50% vanadium were characterized by conversion and selectivity close to those of pure vanadium oxide V₂O₅. Reaction temperature was found to influence the amount of products formed, primarily the amounts of MA and BA. The role played by the generation of the singlet molecular oxygen form in the samples and its influence on the selectivity of the reaction is considered.     

Spectral investigation of the state of water in crystalline heteropoly tungsten and molybdenum compounds

A conclusion, based on literature data and infrared spectral measurements, is drawn about the unique nature and role of water in heteropoly compounds. With the help of spectral and gravimetric measurements it is shown that only molecules of water are present in the middle salts of heteropoly acids. Hydroxonium ions found in crystalline heteropoly acids play the role of cations and therefore are in no way connected with the structure of the heteropoly anions. These data are in complete agreement with the model of the heteropoly anion calculated by Keggin. The proposed interpretation of the spectra of different hydrates and the resulting explanation of their dehydration mechanism is in complete agreement with known x-ray structural data.In conclusion, I sincerely thank M. E. Zubrilin for assistance in conducting these series of experiments.