Enhanced electrochromic performance of macroporous WO3 films formed by anodic oxidation of DC-sputtered tungsten layers

Self-organized macroporous tungsten trioxide (WO₃) films are obtained by anodic oxidation of DC-sputtered tungsten (W) layers on 10 mm × 25 mm indium tin oxide (ITO)-coated glass. Under optimized experimental conditions, uniformly macroporous WO₃ films with a thickness of ca. 350 nm are formed. The film shows a connected network with average pore size of 100 nm and a pore wall thickness of approximately 30 nm. The anodized film becomes transparent after annealing without significant change in macroporous structure. In 0.1 M H₂SO₄, the macroporous WO₃ films show enhanced electrochromic properties with a coloration efficiency of 58 cm² C⁻¹. Large modulation of transmittance (∼50% at 632.8 nm) and a switching speed of about 8 s are also achieved with this macroporous film.     

Visible light photoelectrochemical responsiveness of self-organized nanoporous WO3 films

Visible light-responsive WO₃ nanoporous films with preferential orientation of the (0 0 2) planes were prepared by anodization in neutral F⁻-containing strong electrolytes. The pore diameter of the self-organized structure was estimated to be in the region of 70-90 nm. Voltages were applied by stepping, which positively influenced passivity breakdown and played a significant role in the formation of self-organized nanoporous films. Under visible light irradiation, the photocurrent density (at 1.6 V vs. Ag/AgCl) and maximum photoconversion efficiency generated by the annealed nanoporous film were 3.45 mA/cm² and 0.91%, respectively. The annealed nanoporous WO₃ films show maximum incident photon-to-current conversion efficiency of 92% at 340 nm at 1.2 V vs. Ag/AgCl. These values are higher than that of annealed compact WO₃ film due to the large interfacial heterojunction area. The photoelectrochemical activities and electronic conductivities were also enhanced by annealing crystallization, which removed the recombination centers.     

Template assisted fabrication of TiO2 and WO3 nanotubes

Anodic aluminum oxide (AAO) templates with diameters of 200–500 nm were generated by anodizing a commercial aluminum (Al) substrate (99.7%) in 1 vol% phosphoric acid (H₃PO₄), with an applied voltage of 195 V. Titania and tungsten oxide nanotubes (NTs) were successfully grown on AAO template by the sol–gel process. Thermal gravimetric analyzer (TGA) curves showed that gel can be transfered to nanocrystalline particles after 19% weight loss of water molecule by evaporation. The results showed that the nanocrystalline TiO₂ NTs presented at 200 °C, and grains grew as temperature increased. At a temperature of 550 °C, the (101), (103), (004), (112), (200), (105), and (211) planes of anatase TiO₂ were detected clearly, whereas tungsten oxide NTs are amorphous after heat treatment at 200 °C or 300 °C. But the (110), (111), (002), (022), (222), and (004) planes of γ-WO₃ phase can be observed obviously after the heat treatment at 400 °C.     

Electrochemical synthesis of WO3/PANI composite for electrocatalytic reduction of iodate

Composite film of polyaniline (PANI) and tungsten oxide (WO₃) was electrodeposited by cyclic voltammetric technique from a solution of aniline and tungstic acid. The obtained WO₃/PANI film displayed a significant enhancement of electrocatalytic activity for iodate reduction and a better stability than that of pure WO₃ and PANI films. Result of amperometric experiment revealed a good linear relationship with concentration of IO₃⁻ from 20 to 500 μM, with a high sensitivity of 0.54 μA/μM and a detection limit of 2.7 μM for the determination of iodate. This composite film was also successfully applied in determination of iodate in commercial table salt.     

Porous niobium oxide films prepared by anodization in HF/H3PO4

Ordered porous niobium oxide with the diameter of less than 10 nm and the aspect ratio of more than 20 is prepared by anodization of niobium foils at 2.5 V in the mixture of 1 wt% HF and 1 M H₃PO₄ for 1 h. In this study, the effects of the mixed electrolytes, anodic potential and anodization time on the preparation of porous niobium oxide are described based on the current-time transients during anodization and morphological observations. It is founded that a single HF electrolyte leads to the formation of pores as well as the fast dissolution of formed pores at the surface. The dissolution of the formed oxide is significantly retarded by the addition of appropriate amount of H₃PO₄.     

Effects of precursor concentration on crystalline morphologies and particle sizes of electrospun WO3 nanofibers

Tungsten oxide (WO₃) nanofibers with different crystalline morphologies and various particle sizes were fabricated using an electrospinning technique. The nanofibers were prepared from mixtures of polyvinyl alcohol (PVA) and ammonium metatungstate hydrate (AMH) of various concentrations ranging from 4.2%w/v to 50.0%w/v. After calcination at 500 °C for 2 h, the nanofibers were observed to have a monoclinic crystal structure with diameters ranging from 30 to 250 nm. AMH concentration had a large influence on the resulting nanofiber morphology. Very low AMH concentration of 4.2%w/v led to the formation of WO₃ nanofibers having a very large area of monocrystalline structure. Higher AMH concentrations result in polycrystalline WO₃ nanofibers with joined nanoparticles along the fiber axis. The average particle size within the nanofibers increased from 29 to 66 nm as the AMH concentration increased from 8.3%w/v to 50%w/v. At these precursor concentration levels, primary particles were formed before PVA was completely burnt off, resulting in agglomeration of primary particles along the nanofiber axis.     

Semiconducting properties of anodic WO3 amorphous films

The semiconducting properties of amorphous WO₃ anodic films grown in different solutions and at different current densities have been investigated. The Mott—Schottky plots have shown that the donor density, N_d, of the grown films is strongly dependent on the films thickness and is not influenced by the nature of the anodizing solutions. The possible influence of the kinetics of anodization on N_d is discussed. The intersection voltages at 1/C²_sc = 0 in the Mott—Schottky plots show a complex dependence on the film thickness. The possibility of obtaining the flat band potential from these plots is also discussed.A linear relationship between the square of the photocurrent and the electrode potential has been observed as previously reported for single crystals. The dissolution rate of anodic WO₃ films is increased under illumination and strongly decreased in presence of Fe^{$\text{2+}\text{3+}$} couple. The kinetics of electron transfer between the amorphous WO₃ anodic films and the Fe^{$\text{2+}\text{3+}$} redox couple in the electrolyte seems to occur in accordance with the theory developed for single crystal semiconductors.     

Formation of active sites on Ir/WO3–SiO2 for selective catalytic reduction of NO by CO

Pretreatment conditions for the activation of Ir/WO₃–SiO₂ for the selective catalytic reduction of NO by CO in the presence of excess O₂ were studied. Sequential treatment involving calcination in the presence of O₂ and H₂O followed by reduction and then re-oxidation under mild conditions was found to effectively activate Ir/WO₃–SiO₂. Temperature-programmed desorption during calcination, X-ray diffraction, and temperature-programmed reduction by H₂ revealed that calcination was necessary for oxidative removal of the NH₃ ligands from the iridium precursor, that reduction produced metallic iridium and partially reduced tungsten oxide, and that re-oxidation produced tungsten oxide with low reducibility. Transmission electron microscopy revealed that Ir was supported on finely dispersed tungsten oxide and that the iridium particle size after the sequential activation was 1–1.5 nm.     

The photoelectrochemical properties of anodic Bi2O3 films

Anodic oxide films on bismuth are amphoteric semiconductors with n-type and p-type behavior and an optical band gap of 2.8 eV at room temperature. The semiconducting properties were analyzed in situ during the study of electrochemical and photoelectrochemical reactions at the phase boundary oxide—electrolyte.The photopotential, photoconductivity and capacity measurements together with the electrochemical measurements have been shown to be valuable tools in the connection of bulk (electrophysical) properties with surface (electrochemical) properties.Thermodynamic stability is discussed and data are given which refer to the mechanism and kinetics of the cathodic decomposition and photodecomposition of the Bi₂O₃ layer.     

Stability-enhanced indium hexacyanoferrate electrodes: Morphological characterization, in situ EQCM analysis in nonaqueous electrolytes and application to a WO3 electrochromic device

This paper presents a promising transparent counterelectrode system for a WO₃ electrochromic device (ECD) on the basis of a stability-enhanced indium hexacyanoferrate (InHCF) electrode and a NaClO₄/propylene carbonate (PC) electrolyte. Through SEM characterization it was found that clusters of granular InHCF nanoparticles (ca. 80-140 nm) were deposited on ITO substrates in HCl and KCl-stabilized plating solutions, and uniform micrometer thick films with high charge capacity could be obtained. From in situ electrochemical quartz crystal microbalance study, it was discovered that Na⁺ would enter or move out from the InHCF film in the “desolvated” form during the redox process in a PC electrolyte. Besides, NaClO₄/PC resulted in higher electrochemical activity and reversibility than LiClO₄/PC. With these discoveries, a durable WO₃-InHCF ECD featuring blue-to-colorless electrochromism was fabricated successfully. The device remained 73.6 and 88.7% of its initial ΔT values at 600 and 800 nm after 40,000 rapid and successive coloring/bleaching cycles, respectively. Moreover, the cycling-induced loss of electrochromic performance almost completely restored after 1-month rest and kept unchanged for another month. Thus, the applicability of this nonaqueous InHCF counterelectrode system to ECDs was verified.     

A macrokinetic study of the WO3/Zn reaction diluted with NaCl

In this work, experiments with stoichiometric WO₃ + 3Zn mixture, diluted with NaCl, were conducted for nanostructured tungsten synthesis. The reaction samples, preheated until 720 K, were self-ignited and reacted in the steady combustion regime. The temperature–time profiles in the combustion wave were collected over the NaCl interval from 1 to 6 mol, and the values of the combustion parameters (T_c, U_c) were evaluated. From these profiles the spatial distributions of heat generation rate (x) and degree of conversion η(x) in the combustion wave were received at different k values. The calculated activation energy for the combustion process was E = 55 ± 2 kJ mol⁻¹. After the reduction experiments, pure tungsten nanopowder with particle size from 200 to 50 nm was obtained depending on NaCl concentration.     

Selective reduction of NO with CO in the presence of O2 with Ir/WO3 catalysts: Influence of preparation variables on the catalytic performance

Several Ir/WO₃ catalysts were prepared, which were different in the loading of Ir (up to 10 wt.%) and in the pretreatment conditions, and used for the reduction of NO (500 ppm) with CO (3000 ppm) in the presence of excess O₂ (5%), SO₂ (2 ppm), and H₂O (1%). The rate of NO conversion per unit mole of Ir was found to be maximal at a small Ir loading of 0.5–1.0 wt.%. The good catalytic performance was achieved when the Ir precursors (IrO_x) supported on WO₃ were reduced under mild conditions by He at 400 °C or H₂/He at a lower temperature of 130 °C. The catalysts were characterized by XRD, FTIR after CO adsorption, and H₂-TPR. The characterization results show that the active sites are exposed zero-valent Ir species, in accordance with previous works using Ir on WO₃ and other supports, and that the fraction of these active species is larger for a higher degree of Ir dispersion (smaller Ir particles). The preparation conditions for highly active Ir/WO₃ catalysts were confirmed and possible reaction mechanisms were discussed.     

Structural effects of WO3 incorporation on USY zeolite and application to free fatty acids esterification

Zeolites have occupied a distinguished position due to their unique properties as solid acids and catalytic results achieved in several industrial reactions. This work studied the influence of supported WO₃ on USY zeolite structure, acidity and activity towards an esterification reaction. High dispersion of WO₃ species on USY was achieved, but at higher loading (?11.4%), microcrystalites of WO₃ were detected below the theoretical monolayer coverage (∼32%). Tungsten species were deposited preferentially inside the zeolite structure and interacted with the Brønsted sites of USY as well as on silanol surface groups with the formation of small aggregates. In addition, dealumination took place, especially in the samples with high WO₃ loading. USY had the most and the strongest acidic sites (Brønsted type), but the incorporation of WO₃ decreased the amount and the strength of the new sites. However, all WO₃/USY catalysts were more active than USY in the esterification of oleic acid with ethanol (conversion above 74%, 2 h at 200 °C). The calculation of the TOF for a 1 h reaction demonstrated that 11.4% WO₃/USY was the most active catalyst. Furthermore, it had the lowest rate of deactivation of acid sites after the reaction (∼13% after four cycles). The better performance of the 11.4% WO₃/USY sample was also attributed to a better distribution of strength of the acidic sites and a more hydrophobic character of the synthesized material.     

Nb2O5-supported WO3: a comparative study with WO3/Al2O3

WO₃/Nb₂O₅-supported samples prepared by impregnation are characterised by X-ray diffraction (XRD), Raman spectroscopy and X-ray absorption spectroscopy (XAS) at the W–L₃ absorption edge, as well as temperature programmed reduction (TPR) and FT-IR monitoring of pyridine adsorption. Results are compared with those obtained for WO₃/Al₂O₃ samples prepared in the same conditions, showing that niobia is able to disperse tungsta better than alumina does. Formation of a crystalline WO₃ needs larger tungsten contents on niobia than on alumina, since tungsten solution into niobia is easier than into alumina. Raman and XAS spectra recorded under ambient conditions suggest that similar WO_x species are formed on both supports at tungsten contents 0.5–1 theoretical monolayers; however, TPR results for the low tungsten loaded samples indicate that, when reduction starts (always at temperatures higher than 700 K under H₂/Ar flow) there is a larger concentration of tetrahedral [WO₄] species on alumina, than on niobia. Samples with low tungsten loading have been tested in isopropanol decomposition and ethylene oxidation, following both processes by FT-IR of adsorbed species up to 673 K. Results show that adsorption of ethylene on WO₃/Nb₂O₅ yields acetaldehyde and acetate at 473 K, while this adsorption is non-reactive either on the supports or on WO₃/Al₂O₃. Isopropanol adsorbs dissociatively on both supports, leading to acetone and propene formation on tungsta–niobia, but only propene on tungsta–alumina, probably due to the larger reducibility of the tungsten-containing phases.     

Preparation of tungsten borides by combustion synthesis involving borothermic reduction of WO3

An experimental study on the preparation of two tungsten borides, WB and W₂B₅, was conducted by self-propagating high-temperature synthesis (SHS), during which borothermic reduction of WO₃ and elemental interaction of W with boron proceeded concurrently. Powder mixtures with two series of molar proportions of WO₃:B:W = 1:5.5:x (with x = 1.16–2.5) and 1:7.5:y (with y = 0.5–1.33) were adopted to fabricate WB and W₂B₅, respectively. The starting stoichiometry of the reactant compact substantially affected the combustion behavior and the phase composition of the final product. The increase of metallic tungsten and boron reduced the overall reaction exothermicity, leading to a decrease in both combustion temperature and reaction front velocity. The initial composition of the reactant compact was optimized for the synthesis of WB and W₂B₅. In addition to small amounts of W₂B and W₂B₅, the powder compact of WO₃ + 5.5B + 2 W produced WB dominantly. Optimum formation of W₂B₅ was observed in the sample of WO₃ + 7.5B + 0.85W. Experimental evidence indicates that an excess amount of boron about 10–13% is favorable for the formation of WB and W₂B₅.     

Fabrication and characterization of WO3 flocky microspheres induced by ethanol

The novel tungsten trioxide flocky microspheres induced by ethanol were successfully obtained via a simple and convenient hydrothermal route. The as-prepared products were characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, BET surface area measurement and UV-Vis diffuse reflectance spectroscopy. WO₃ powder prepared with volume ratio (v_ethanol/v_water) of 40% revealed a complete flocky microsphere structure with particle size of 3.0-4.0 μm and exhibited good photochromic property. The possible formation mechanism of the flocky microspheres was proposed and the improved photochromic properties were also investigated.     

WO_3负载量对V_2O_5/WO_3-TiO_2催化剂脱硝性能的影响

采用V_2O_5/WO_3-TiO_2作为脱硝催化剂,考察活性组分V_2O_5和助剂WO_3负载量对催化剂脱硝活性和抗硫抗水性能的影响。结果表明,3%V_2O_5/x WO_3-TiO_2催化剂(x=3%、4%、5%、6%、7%、8%、9%、10%)上NOx转化率随着WO_3负载量增加而升高,催化剂反应温度窗口不断拓宽。单独通水蒸汽及同时通SO2和水蒸汽对催化剂的毒害作用均较强,表明H2O和NH3的竞争吸附是催化剂抗硫抗水性能较差的重要原因。SO_2与H_2O和NH_3反应生成亚硫酸铵盐和硫酸铵盐,导致催化剂孔隙堵塞,催化活性降低。     

Influencing factors on electrochromic hysteresis performance of ruthenium purple produced by a WO3/Tris(2,2′-bipyridine)ruthenium(II)/polymer hybrid film

A [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine)/WO₃ hybrid (denoted as Ru-WO₃) film was prepared as a base layer on an indium tin oxide electrode by electrodeposition from a colloidal solution containing peroxotungstic acid, [Ru(bpy)₃]²⁺ and poly(sodium 4-styrenesulfonate). A ruthenium purple (RP, Fe^III₄[Ru^II(CN)₆]₃, denoted as Fe^III-Ru^II) layer was electrodeposited on a neat WO₃ film or a Ru-WO₃ film from an aqueous RP colloid solution to yield a WO₃/RP bilayer film or a Ru-WO₃/RP bilayer film, respectively. The spectrocyclic voltammetry measurement reveals that Fe^II-Ru^II is oxidized to Fe^III-Ru^II by a geared reaction of [Ru(bpy)₃]^2+/3+ and Fe^III-Ru^II is reduced by a geared reaction of H_xWO₃/WO₃ in the Ru-WO₃/RP film. These geared reactions produced electrochromic hysteresis of the RP layer. However, the absorbance change in the hysteresis was smaller than that for the Ru-WO₃/Prussian blue bilayer film reported previously, resulting from the lower electroactivities of any redox component for the Ru-WO₃/RP film. The lower electroactivities could be explained by the specific interface between the Ru-WO₃ and RP layers. It might contribute to either an increase of the interfacial resistance between the Ru-WO₃ and RP layers, or formation of the physically precise interface between the layers to make it difficult for counter ions to be transported in the interfacial liquid phase involved in the redox reactions in the film. The specific interface at the Ru-WO₃ and RP layers could be formed possibly by the electrostatic interaction between [Ru(bpy)₃]²⁺ and terminal [Ru(CN)₆]⁴⁻ moieties of RP. It could be suggested by the decreased redox potential of [Ru(bpy)₃]²⁺ in the Ru-WO₃ layer from 1.03 to 0.61 V by formation of the RP layer.     

Electrodeposition of tungsten from ZnCl2-NaCl-KCl-KF-WO3 melt and investigation on tungsten species in the melt

The electrodeposition of tungsten in ZnCl₂-NaCl-KCl-KF-WO₃ melt at 250 °C was further studied to obtain a thicker deposit. In the ordinary electrolysis at 0.08 V vs. Zn(II)/Zn, the current density decreased from 1.2 mA cm⁻² to 0.3 mA cm⁻² in 6 h. A thickness of the obtained tungsten layer was 2.1 μm and the estimated current efficiency was 93%. A supernatant salt and a bottom salt were sampled after 6 h from the melting and were analyzed by ICP-AES and XRD. It was found that the soluble tungsten species slowly changes to insoluble ones in the melt. The soluble species was suggested to be WO₃F⁻ anion. One of the insoluble species was confirmed to be ZnWO₄ and the other one was suggested to be K₂WO₂F₄. Electrodeposition was carried out under the same condition as above except for the intermittent addition of WO₃ every 2 h. The current density was kept at the initial value and the thickness was 4.2 μm. The intermittent addition of WO₃ was confirmed to be effective to obtain a thicker tungsten film.