Analysis of the physical structure of nanometric WOx/ZrO2 using electrical measurements

The structure of WO_x/ZrO₂ was studied by X-ray diffraction, laser Raman spectroscopy and measurement of electrical properties using impedance spectroscopy. Results from classical analysis were consistent with a structure comprising nanometric ZrO₂ particles covered by a WO_x surface layer. Based on this information we modelled the impedance spectra as the superposition of two contributions. The values of the electrical properties estimated from our model indicated the presence of a dielectric and a semiconductor. The first phase had electrical properties closely matching the reported values for ZrO₂, whereas the semiconductor phase was assigned to a non-stoichiometric WO_x phase. The tungsten-bearing species had temperature-dependent properties and play an important role in the ac response of the studied system and also in oxidation–reduction processes. The activation energy is 1.3 eV for ZrO₂, whereas WO_x has two slightly different energy values (2.4 and 2.1 eV) in different temperature ranges. Use of impedance spectroscopy provides valuable information about the surface structure as well as the contribution of the bulk, which may be important in catalysis. PACS 68.35.Bs; 81.05.Ys; 82.65.Dp     

Thermally-induced morphological transition in WOx deposited on a ZrO2(1 0 0) substrate

A combined atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) study of tungsten oxide model catalysts is presented. The model catalysts were prepared by applying the real preparation method to a ZrO₂(1 0 0) single crystal support. AFM imaged several granular structures of scattered dimensions on the surface of ZrO₂(1 0 0) in the as prepared samples. After heating, at low loading the tungsten species rearranged into small WO_x particles strongly interacting with the substrate. At high tungsten content large WO₃ aggregates also formed. XPS analysis confirmed these changes. The estimated surface density of the interacting W-containing species closely matched that of real catalysts.     

Thermal effects on the structural properties of tungsten oxide nanoparticles

Tungsten oxide nanoparticles are prepared by evaporating and oxidizing the tungsten boat in helium and oxygen atmosphere and then quenched to the liquid nitrogen temperature. The as-prepared tungsten oxide nanoparticles are porous-free with uniform size. The morphology and particle size distribution of the as-prepared and after sinter treatments tungsten oxide nanoparticles are revealed by TEM and AFM. The long-range order of these nanoparticles can be examined by X-ray diffraction technique. The as-prepared nanoparticles exhibit a mixture structure of monoclinic and hexagonal crystals. Preliminary X-ray diffraction results indicate that the hexagonal structure is transformed to monoclinic structure after annealing to above 600°C. In order to better distinguish the structural properties of the tungsten oxide (WO_{3− x}) nanoparticles before and after annealing, the X-ray absorption spectrum technique is utilized; thus, the detailed local atomic arrangement of oxygen and/or tungsten can be determined. According to the XAS result, the shape of the W L₃-edge undergoes no considerable changes. This infers that structural transformation of tungsten oxide nanoparticle may be caused by the migration of oxygen after sintering. From the O K-edge of absorption spectrum, it suggests that a mixture phase structure is obtained when sintered below 300°C. And this result indicates that heat treatment to approximately 600°C produces a stable structure of a monoclinic crystal of WO₃.     

Thermal spreading of WO3 onto zirconia support

This study focused on preparation of tungsten oxide supported on zirconia by thermal spreading. The prepared samples were characterized by infrared spectroscopy, UV-vis diffuse reflection spectroscopy, X-ray diffraction, and also by methanol dehydration reaction. It was observed that isolated octahedral tungsten dispersed species and dispersed polytungstate were formed on zirconia surface, although some WO₃ that remained after the thermal treatment could also be detected. The presence of these species led to an increase of the number of Lewis sites and the generation of Brönsted acid sites. High calcination temperatures promoted the creation of Brönsted sites as a consequence of polytungstate species formation. The activity on methanol dehydration was also determined by the concentration of these species, whereas the isolated WO_x species were found poorly active. The correlation observed between the catalytic performance and the tungsten dispersed species, as revealed by spectroscopic techniques, evidenced the occurrence of thermal spreading of WO₃ on ZrO₂. The results presented in this work show that WO₃ thermal spreading on ZrO₂ may be effectively accomplished as predicted by thermodynamics.     

Surface properties of palladium catalysts supported on ternary ZrO2–Al2O3–WOx oxides prepared by the sol–gel method: Study of the chemical state of the support

The surface properties of Pd and Pd–Pt catalysts supported on binary ZrO₂–WOx and ternary ZrO₂–Al₂O₃–WOx oxides prepared by the sol–gel method were studied. Special attention was paid to the study of the texture of the catalysts as well as the chemical state of tungstated zirconia and tungstated zirconia promoted with alumina in the palladium catalysts. The catalysts were tested in the isomerization of n-hexane and were characterized by N₂ physisorption, XRD, TPR, Raman spectroscopy, XPS and FT-IR of adsorbed pyridine. The catalysts had bimodal pore size distributions with mesopores in the range 55–70 Å and macropores of 1000 Å in diameter. The catalysts had a surface WOx coverage (4.4–6.0 W nm^?2) lower than that of the theoretical monolayer (7.0 W nm^?2). A lower acidity of the ternary ZrO₂–Al₂O₃–WOx oxide as compared to the binary ZrO₂–WOx oxide was found. Higher activity in the isomerisation of n-hexane was obtained in the Pd–Pt catalysts supported on ternary ZrAlW oxides prepared by sol–gel that is correlated with the coexistence on the surface of W⁴⁺ (WO₂) or W⁰ and W⁶⁺ (Al₂(WO₄)₃) species, ZrO₂ in the tetragonal phase and a high amount of ZrOx suboxides species in a low oxidation state (Zr³⁺ and Zr²⁺).     

High-resolution electron microscopy of heterostructures and interfaces

High-resolution transmission electron microscopy (HREM) allows to study a wide range of device-relevant topics in heteroepitaxial layer structures. Quantitative HREM may be used to obtain chemical information on a near-atomic scale from interfacial transition zones. The physical background is described and demonstrated on several examples in the Al _x Gal_1–x As/GaAs system. The HREM contrast of antiphase boundaries in InP grown on Si was studied by image simulations and has been compared to experimental images. Silicon carbide precipitates were identified by HREM at the homoepitaxial Si/Si interface. They stem from carbon contamination prior to Si layer growth.     

Metal-support interactions in systems palladium deposited on oxidized tungsten surfaces

The morphology of the palladium (Pd) overlayers on oxidized tungsten (W) tips has been studied by Field Emission Microscopy (FEM). The effect of thermal treatment on the interaction of Pd with the support and chemisorption of CO on variously treated Pd-containing samples has been investigated. The results are discussed in relation to complementary macroscopic experiments by synchrotron radiation excited photoelectron spectroscopy (SRPES) and thermally programmed desorption (TPD) of carbon monoxide (CO) on a polycrystalline W foil. A distinct influence of support pre-oxidation on the Pd layer growth has been demonstrated. Two types of oxidized supports have been used: tungsten with oxygen pre-adsorbed at room temperature (RT) and then heated to 700 K (WO_x/W (RT) system) and tungsten oxidized at 1300 K (WO_x/W (1300 K) system) in situ. The surface of WO_x/W (1300 K) sample is fully oxidized in contrast to WO_x/W (RT), where the presence of un-oxidized patches has been demonstrated by SRPES measurements. A Pd layer grows on the WO_x/W (RT) surface mostly on the densely populated planes (1 1 0) and (2 1 1) of the W tip. Heating of this system up to 700 K results in disaggregation of the original Pd layer. Pd clusters on the tungsten tip oxidized at 1300 K are localized on the atomically rough (1 1 1) plane. The observed differences in CO adsorption on the aforementioned types of investigated samples can be attributed to differences in the chemical nature of their surfaces.     

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.     

Atomic imaging of oxygen desorption from tungsten trioxide

We report direct observations by high-resolution electron microscopy of oxygen desorption from tungsten trioxide. Clear evidence is found for layer epitaxial growth of metallic tungsten with (110)W|(100)WO₃ and [001]W and [001]WO₃ parallel to the electron beam, consistent with low-energy electron diffraction data on the low-temperature epitaxy of WO₃ on W. W[001] was always observed parallel to the electron beam independent of the surface normal. The results suggest that bulk and surface damage, and displacement processes, are similar.     

Application of Black Pearl carbon-supported WO3 nanostructures as hybrid carriers for electrocatalytic RuSex nanoparticles

RuSe_x electrocatalytic nanoparticles were deposited onto hybrid carriers composed of Black Pearl carbon-supported tungsten oxide; and the resulting system's electrochemical activity was investigated during oxygen reduction reaction. The tungsten oxide-utilizing and RuSe_x nanoparticle-containing materials were characterized using transmission electron microscopy, X-ray diffraction and electrochemical diagnostic techniques such as cyclic voltammetry and rotating ring-disk voltammetry. Application of Black Pearl carbon carriers modified with ultra-thin films of WO₃ as matrices (supports) for RuSe_x catalytic centers results during electroreduction of oxygen in 0.5 mol dm⁻³ H₂SO₄ (under rotating disk voltammetric conditions) in the potential shift of ca. 70 mV towards more positive values relative to the behavior of the analogous WO₃-free system. Also the percent formation (at ring in the rotating ring-disk voltammetry) of the undesirable hydrogen peroxide has been decreased approximately twice by utilizing WO₃-modified carbon carriers. The results are consistent with the bifunctional mechanism in which oxygen reduction is initiated at RuSe_x centers and the hydrogen peroxide intermediate is reductively decomposed at reactive WO₃-modified Black Pearl supports. The electrocatalytic activity of the system utilizing WO₃-modified Black Pearl supports has been basically unchanged upon addition of acetic acid, formic acid or methyl formate to the sulfuric acid supporting electrolyte.     

NMR relaxation times and proton motion in HxWO3

The relaxation timesT ₁,T _1q,T _1D, andT ₂ for¹H in the tetragonal-A phase of H_xWO₃ have been measured over the temperature range 190 to 490 K. The¹H relaxation behaviour appears to be governed by diffusion over inequivalent jump distances, approximating to a short range planar diffusion and a long range isotropic diffusion. Parameters for the latter motion are estimated asE _a = 68 kJ/mol and ₀=2.5×10^–13 s. The powder X-ray diffraction pattern for this phase of H_xWO₃ has been studied over the temperature range 300–470 K. The tetragonal distortion diminishes with temperature and H_0.43 WO₃ becomes cubic at about 435 K. Volumetric studies of hydrogen evolution show that decomposition accelerates at approximately this temperature.     

A structural analysis of W-Sb mixed oxide catalyst

An investigation on the structure of W-Sb mixed oxide catalyst, W₁₂Sb_xO_y (x = 1, 3, 5), is proposed. The W-Sb mixed oxide powders were prepared by the calcination of aqueous precursors, antimony tartrate and ammoniummetatungstate, and characterized with scanning electron microscope, X-ray diffractometer, and transmission electron microscope. At low content of Sb (x = 1), the W-Sb mixed oxide powder consisted of polyhedral particles, and their crystal structure was triclinic WO₃. At higher content (x = 3, 5), majority of the oxide powders were bar-shaped particles, consisting of triclinic WO₃ and tetragonal WO₃. With electron diffraction pattern and simulation, Sb incorporation into the cuboctahedral sites of perovskite-like WO₃ was proved and its effect on the phase transition from triclinic to tetragonal was discussed.     

Formation of combustible gases during interaction of tungsten and zirconium with supercritical fluid H2O/CO2

Oxidation of bulk samples of tungsten (923 K) and zirconium (773 and 873 K) by H₂O/CO₂ supercritical fluid (molar ratio [CO₂]/[H₂O] = 0.17–0.26) at a pressure of about 300 atm is investigated. Oxidation produces monoclinic WO₃, monoclinic W₁₉O₅₅, monoclinic ZrO₂, H₂, CO, CH₄, and carbon (on the surface of tungsten oxide). Differences in oxidation mechanisms for tungsten and zirconium are revealed. CO₂ molecules take part in the oxidation of tungsten only after oxide formation in reaction with H₂O. Zirconium is oxidized fully, and oxidation of tungsten terminates in the formation of the oxide layer at the metal surface.     

Surface composition modification of tungsten higher oxide under He<Superscript>+</Superscript> ion bombardment

The surface reduction of higher oxide WO₃ under irradiation by He⁺ ions with the energies 1 and 3 keV in a high vacuum is investigated by X-ray photoelectron spectroscopy. It is found that lower WO₂ and intermediate WO _x (2 < x < 3) oxides form first in WO₃ surface layers under He⁺ ion bombardment, and with an increase in the irradiation dose metallic tungsten forms. It is shown that the degree of irradiated oxide surface metallization increases with an increase in the energy of the bombarding He⁺ ions. A comparison of WO₃ oxide surface composition modification under He⁺ and Ar⁺ ion irradiation is presented.     

Micro‐Raman spectroscopic characterization of a tunable electrochromic device for application in smart windows

An asymmetric electrochromic (EC) device based on an active EC tungsten oxide–titanium oxide (WO₃–TiO₂) layer was constructed. The EC active layer consisted predominantly of monoclinic WO₃ nanocrystallites with a minor additional component of hexagonal WO₃ and amorphous TiO₂. Detailed micro‐Raman spectroscopic studies of the intercalation and deintercalation of lithium in the EC active layer of the EC device as a function of the applied voltage were performed. Three significant structural stages occur upon intercalating Li into the WO₃–TiO₂ layer when coloration potentials of 1.0, 1.5, 2.0, and 3.0 V are applied to the EC device. In the first stage (applied potential of 1.0 V), the m‐Li_x WO₃ phase is retained. In the second stage, (applied potential of 1.5 and 2.0 V) the m‐Li_x WO₃ transforms to a tetragonal phase. In the third stage, (applied potential of 3.0 V) the Raman spectrum exhibits no spectral bands, showing that Li_x WO₃ has attained the highest‐symmetry cubic phase. This phase sequence is confirmed by the X‐ray diffraction (XRD) measurement. These phase transitions can be reversed and, upon complete deintercalation, m‐WO₃ with traces of h‐WO₃ is recovered. Optical transmission studies were performed in conjunction with Raman and XRD studies. A shift of the optical transmittance peak position from 639 to 466 nm and reduction in the width of the transmittance curve with increasing applied potential opens up the possibility of smart window applications for the nanocrystalline WO₃‐based EC device. Copyright © 2008 John Wiley & Sons, Ltd.     

插层化合物钨青铜单晶的物理研究

本工作利用X射线衍射分析、电导测量、红外、喇曼散射等手段,对用熔盐电解法生长的K_xWO₃,Na_yWO₃和K_xNa_yWO₃单晶进行了深入的研究,得出K_xNa_yWO₃是钠离子插入到K_xWO₃的结论,提出了K_xNa_{y关键词：}     

Enhancement of the superconducting transition temperature near a phase instability in NaxWO3

The superconducting transition temperature of tetragonal I sodium tungsten bronze (Na_xWO₃) has been found to increase rapidly as the x-value is decreased to the metal-semiconductor phase transition. It is suggested that a soft mode instability is responsible for the increased electron-phonon interaction near the phase transition.     

Synthesis of tailored WO3 and WOx (2.9 < x < 3) nanoparticles by adjusting the combustion conditions in a H2/O2/Ar premixed flame reactor

Flame synthesis of WO₃ and WO_x (2.9 < x < 3) nanoparticles is carried out by adding a dilute concentration of WF₆ as precursor in a low-pressure H₂/O₂/Ar premixed flame reactor. The reactor is equipped with molecular-beam sampling and particle mass spectroscopy (PMS) to determine particle composition and sizes as a function of height above burner. Varying the H₂/O₂ ratio allowed us to tune the stoichiometry of the product. With a H₂/O₂ ratio of 0.67 white colored stoichiometric WO₃ is formed, whereas the H₂/O₂ ratio >0.8 yields blue colored non-stoichiometric WO_x (2.9 < x < 3) nanoparticles. The size of nanoparticles can be controlled by varying the residence time in the high-temperature zone of the reactor as observed by molecular-beam sampling with subsequent analysis using PMS. Transmission electron microscopy (TEM) images of as-synthesized nanoparticles show that particles are non-agglomerated and have an almost spherical morphology. The X-ray diffraction (XRD) pattern of the as-synthesized material indicates that the powders exhibit poor crystallinity, however, subsequent thermal annealing of the sample in air changes its structure from amorphous to crystalline phase. It is observed that particles with sub-stoichiometric composition (WO_x) show higher conductivity compared to the stoichiometric WO₃ sample.     

Titania surface modification and photovoltaic characteristics with tungsten oxide

WO₃-coated TiO₂ film was prepared by depositing TiO₂ suspension containing small amounts of ammonium tungstate solution. The morphology and structure of the samples were characterized with high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and photoluminescence (PL) emission spectrum. The results showed that WO₃ formed a coating layer on surface of TiO₂ and significantly reduced the surface traps of TiO₂ nanoparticles. Transient photovoltage and electrochemical impedance measurements (EIS) were employed to study the charge separation/recombination process. The results revealed that the charge recombination was greatly retarded and the electron lifetime was increased due to the coating layer of WO₃. These observations showed good correlation with current-voltage analyses of dye-sensitized solar cell fabricated from these films, with WO₃ overlayer resulting in an increase in open-circuit voltage of up to 37 mV and 11% improvement in overall device efficiency.     

Preparation, characterization and photocatalytic activity of multi-walled carbon nanotube-supported tungsten trioxide composites

Multi-walled carbon nanotube (MWCNT)-supported tungsten trioxide (WO₃) composite catalysts were prepared by liquid-phase process. WO₃ nanoparticles grew on the inner and outer surface of MWCNTs. Their photocatalytic activities in the degradation of the Rhodamine B Dye were studied. The effects of mass ratio of MWCNTs to WO₃ were discussed. X-ray diffraction, field emission transmission electron microscopy, thermogravimetric-differential thermal analysis and ultraviolet-visible light absorption spectra were carried out to characterize the composite catalysts. The results indicated that the optimum mass ratio of MWCNTs to WO₃ is 5:100.