Mesoporous Tungsten Oxide Research Articles

XRD studies of thermally stable mesoporous tungsten oxide synthesised by a templated sol–gel process from tungstic acid precursor

Thermally stable mesoporous tungsten oxide film was prepared by a surfactant templated sol–gel method using tungstic acid as tungsten source. By careful selection of suitable non-ionic surfactant and heating processes, thermally stable mesostructured tungsten oxide films can be obtained, capable of withstanding heat treatment at temperatures of more than 400 °C. The results show that tungstic acid is a suitable alternative precursor to the tungsten hexachloride normally reported in the literature for the preparation of mesostructured tungsten oxide. This work opens a new pathway for the preparation of mesoporous tungsten oxide films using tungstic acid precursor with many advantages including reduced cost, easy handling, and insensitivity to moisture.

Field-Emission Performance of Wormhole-Like Mesoporous Tungsten Oxide Nanowires

We prepared wormhole-like mesoporous tungsten oxide nanowires on a Cu-tape/Si substrate, and explored the field-emission performances. The wormhole-like mesoporous tungsten oxide nanowires of 20 nm diameter exhibited excellent field-emission properties with extremely low turn-on and threshold fields (emission current density of 10 μA/cm2 and 10 mA/cm2) of 0.083 V/μm and 1.75 V/μm, respectively, as well as current stability of about 1400 μA/cm2 at a fixed field of 0.67 V/μm. This approach provides an efficient methodology for fabricating a field emitter that is expected to work at low voltage and can be used in field-emission displays.

Effect of Calcination on Crystallinity for Nanostructured Development of Wormhole‐Like Mesoporous Tungsten Oxide

The effects of calcination on the crystallinity and grain growth model for microstructure development of wormhole‐like mesoporous tungsten oxide are investigated in this study. We found that residual mesopores of wormhole‐like tungsten oxide can be seen calcined above 500°C, as evidenced by retention of a wormhole‐like mesostructure of tungsten oxide. A model was proposed to combine grain growth with wormhole‐like mesoporous behavior and to investigate whether the formation of wormhole‐like mesoporous tungsten oxide can retard the collapse rate of mesopores during the calcination process.

Electroless coating of tungsten oxide on the surface of copper powder

Tungsten oxide was successfully deposited on the surface of copper powder and the thickness of coating layer was dependent on deposition time. Because a spontaneous reaction occurred on the interface between copper and tungsten-peroxo electrolyte, there was a maximum thickness that could be obtained, as confirmed from XRD and EDX results. Mesoporous tungsten oxide was also deposited using SDS as a structure directing agent. As-synthesized tungsten oxide was amorphous and, after calcination at 450 °C, crystallized tungsten oxide was produced. Compared to pure tungsten oxide, the tungsten oxide coated copper oxide showed enhanced absorption in the visible region.

Synthesis and Characterization of Chromium‐Doped Mesoporous Tungsten Oxide for Gas Sensing Applications

AbstractSBA‐15 (2D hexagonal structure) and KIT‐6 (3D cubic structure) silica materials are used as templates for the synthesis of two different crystalline mesoporous WO3 replicas usable as NO2 gas sensors. High‐resolution transmission electron microscopy (HRTEM) studies reveal that single‐crystal hexagonal rings set up the atomic morphology of the WO3 KIT‐6 replica, whereas the SBA‐15 replica is composed of randomly oriented nanoparticles. A model capable of explaining the KIT‐6 replica mesostructure is described. A small amount of chromium is added to the WO3 matrix in order to enhance sensor response. It is demonstrated that chromium does not form clusters, but well‐distributed centers. Pure WO3 KIT‐6 replica displays a higher response rate as well as a lower response time to NO2 gas than the SBA‐15 replica. This behavior is explained by taking into account that the KIT‐6 replica has a higher surface area as demonstrated by Brunauer–Emmett–Teller analyses and its mesostructure is fully maintained after the screen‐printing step involved in sensors preparation. The presence of chromium in the material results in a shorter response time and improved sensor response to the lowest NO2 concentrations tested. Electrical differences related to mesostructure are reduced as a result of additive introduction.

Comprehensive understanding on the formation of highly ordered mesoporous tungsten oxides by X-ray diffraction and Raman spectroscopy

Highly ordered mesoporous tungsten oxides (WO 3) have been synthesized via the hard templating method using mesoporous silica as a hard template and phosphotungstic acid as a precursor. The influences of the calcination temperature, the impregnation ratio of the precursor to the silica hard template and the hydrothermal treatment time of the silica template on the structure of products have been systematically investigated. Both X-ray diffraction and Raman spectroscopy were performed to study the crystalline phases of the resultant mesoporous WO 3 as a function of the calcination temperature. The ideal calcination temperature for the synthesis of highly ordered mesoporous WO 3 is found to be between 823 and 1073 K. Only a small fraction of crystalline WO 3 can be detected in the composites when the calcination temperature is below 773 K. When the calcination temperature is increased from 773 to 1173 K, the monoclinic and triclinic phases of WO 3 coexist in the products. Moreover, the ratio of the monoclinic to triclinic phase changes with the variation of the calcination temperature, and it reaches a maximum at about 923 K. Increasing the hydrothermal treatment time of the silica template KIT-6 to 5 days at 373 K favors the synthesis of highly ordered mesoporous WO 3. When KIT-6 prepared without hydrothermal treatment is used as the hard template, a symmetry transformation from Ia 3 ¯ d to I4 1/ a can be observed. The present work provides a convenient and reproducible method for the synthesis of highly ordered mesoporous WO 3 materials with crystalline walls, which may find potential applications in different areas.

Atmospheric plasma-calcination of mesoporous tungsten oxide utilizing plasma dielectric barrier discharge

The fabrication of mesoporous tungsten oxide films by spin-coating method followed by an atmospheric plasma-calcination method is described and discussed here. For the calcination process the dielectric barrier discharge system generating homogenous atmospheric plasma discharge was designed and used. By this method, large surface area mesoporous films, with disordered pores of average diameter size of about 4 to 5 nm were synthesized. All the plasma-calcined films exhibit amorphous structure. The process of calcination was evaluated by FT-IR spectroscopy and in comparison with other methods a very high speed of calcination process was achieved.

Hydrolysis reaction on the characterization of wormhole-like mesoporous tungsten oxide

Wormhole-like mesostructures have promising applications in heterogeneous catalysis because channel branching within the wormhole-like mesostructures can facilitate access to active sites on the framework walls. We adopt the poly(alkylene oxide) triblock copolymer of L62 (BASF Pluronic EO 8PO 30EO 8) as a template to form wormhole-like mesoporous tungsten oxide. In the hydrolysis experiment, 10 or 20% anhydrous ethanol was replaced with distilled deionized water (H 2O). The crystallinity of wormhole-like mesoporous tungsten oxide decreases when the anhydrous ethanol solvent is replaced with H 2O. Such a decrease in the relative strength of O–W–O binding attributes to the hydrolysis of the wormhole-like mesoporous tungsten oxide from Raman spectra. Specific surface area and average pore size of wormhole-like mesoporous tungsten oxide decrease with the amount of H 2O replacement. The phenomenon can also be confirmed by nitrogen adsorption–desorption isotherms. These results show that the hydrolysis reactions have great significance and application for the development of wormhole-like mesoporous tungsten oxide in the future.

New templated method to synthesize electrochromic mesoporous tungsten oxides

Mesoporous tungsten oxides were synthesized with a new nonionic gemini surfactant template, which was then removed by calcination. The effect of calcination on the as-synthesized product was studied with TG–DSC and Fourier-transform infrared spectroscopy. The results of X-ray diffractions, high-resolution transmission electron microscopy, and nitrogen sorption analyses show that the mesoporous tungsten oxide calcined at 300 °C preserved accessible vermicular mesoporosity with specific surface area of 145.5 m 2/g. Enhanced electrochromic response was found for the mesostructured sample from the simultaneous voltammetric and spectrophotometric studies.

Nanostructured mesoporous tungsten oxide films with fast kinetics for electrochromic smartwindows

A potential driven self-assembly of sodium dodecyl sulfate/tungsten oxideaggregates at the electrolyte–electrode interface followed by template extractionand annealing yielded mesoporous thin films of electrochromic tungsten oxide(WO3). Electron microscopy images revealed that the films are characterized by a hitherto unreportedhybrid structure comprising nanoparticles and nanorods with a tetragonal crystalline phase ofWO3 with the measuredlattice parameters: a = 0.53 nm and c = 0.37 nm. In addition to pentagonal voids characteristic of the tetragonalWO3 phase at the lattice scale, open channels and pores of 5–10 nm in diameterlie between the nanoparticles, which cumulatively promote rapid chargetransport through the film. This resulted in colouration efficiency(ηmax∼90 cm2 C−1 atλ = 900 nm) and switchingkinetics (colouration time = 3 s and bleaching time = 2 s for a 50% change in transmittance) higher and faster than previously reported values for mesoporousWO3 films. Repetitive cycling between the clear and blue states has no deleterious effect on theelectrochromic performance of the film, which is suggestive of its potential as a cathode inpractical electrochromic windows.

Fabrication of one-dimensional mesoporous tungsten oxide

Mesoporous tungsten oxide nanofibres and tube-like nanofibres are fabricated by thegas-filled assistant sol–gel immersion method with porous anodic alumina membraneconfinement. Tube-like nanofibres are obtained at an immersion time of less than 3 min, butnanofibres are obtained at an immersion time of more than 5 min under 50 psinitrogen gas pressure. The results show that the gas-filled method has a higherefficiency to induce mesoporous materials into membrane channels. One-dimensional(1D) mesoporous tungsten oxide nanowire is characterized as a triclinic crystalwith discontinuous lattice morphology at the mesopores. The one-dimensionaltungsten oxide with disordered mesopores obtained indicates that the liquid-crystalmechanism should be applied for the formation of the mesoporous structure. Thegas-filled immersion method provides a convenient and low-cost route for the fastproduction of mesoporous tungsten oxide nanofibres and tube-like nanofibres.

Direct electron transfer and electrocatalysis of hemoglobin adsorbed onto electrodeposited mesoporous tungsten oxide

A novel hemoglobin (Hb) electrode has been presented through adsorbed Hb onto mesoporous tungsten oxide via one-step electrodeposition. Positions of Soret absorption band showed Hb retained its natural configuration on the resulting films. Electrochemical impedance spectroscopy and cyclic voltammetry were employed to study the electrochemical behavior of adsorbed Hb. The immobilized Hb showed a pair of stable and well-defined peaks at about −284 and −174 mV at 100 mV s −1 in pH 6.0 buffers and displayed excellent electrocatalytical responses to the reduction of H 2O 2, NO 2 - and trichloroacetic acid. The presence of meso-WO 3 film greatly enhances the protein loadings, accelerates interfacial electron transfer and improves thermal stability of the adsorbed protein.

Preparation and characterization of three-dimensional mesoporous crystals of tungsten oxide

Three-dimensional (3D) mesoporous tungsten oxide single crystals has been prepared by using aminosilylized 3D mesoporous SBA-15 as the template and characterized by X-ray diffraction, transmission electron microscopy, and selected area electron diffraction. The product is an array of hexagonally ordered nanorods linked by nanosized bridges to form a 3D open framework as an inverse replica of SBA-15.

Effect of copolymer and additive concentrations on the behaviors of mesoporous tungsten oxide

Nanostructured tungsten oxide films exhibit a series of interesting behaviors due to their excellent properties of photochromic reaction and gas sensing. Mesoporous films with a large surface area are desirable to improve their performance. The main purpose of this study is to elucidate the effects of the copolymer and additive concentrations on the microstructure of the mesoporous tungsten oxide prepared by a sol–gel method. The typical calcined tungsten oxide samples synthesized by L62 and P123 exhibit a Brunauer–Emmett–Teller (BET) surface area of 156 and 138 m 2/g, respectively. The pore size of 4–10 nm in the sample increases with increasing copolymer concentration. For different additive conditions synthesized by a triblock copolymer, HCl or distilled deionized water (DDW) additive could further promote the BET surface area of mesoporous tungsten oxide than an additive of NaOH, which can be attributed to the stabilization of the micellization in acid media. The sol–gel tungsten oxide involving the use of nonionic triblock copolymer as the templates with disorder pores exhibits outstanding sensitivity in photochromic and gas-sensing properties. Mesoporous tungsten oxide powder synthesized by triblock copolymer exhibits a high transmittance with an indirect E G of 3.73 eV. Gas-sensing measurement reveals excellent recovery and sensitivity under low gas concentration, low operation temperature for mesoporous tungsten oxide film and a sensitivity of 25 as exposed to 20 ppm NO gas at 40 °C.

Oxidation of geraniol and other substituted olefins with hydrogen peroxide using mesoporous, sol–gel-made tungsten oxide–silica mixed oxide catalysts

The preparation of a series of mesoporous tungsten oxide–silica mixed oxides by sol–gel methods under basic conditions is reported. Surface modification with methyl and 3-chloropropyl groups is possible in an amount between 10 and 40 mol% with respect to the silane precursor. The amount of polar organic functional groups controls the surface area, the porosity, and the catalytic activity of the solids in the oxidation of different substrates with hydrogen peroxide. The oxidation of geraniol is studied in detail. The catalysts are active and produce epoxides in good yields. The latter are influenced by the presence of polar organic groups. The preparation method allows the preparation of catalysts that are resistant to leaching and can be recycled several times without appreciable loss of activity.

Mesoporous sol–gel WO 3 thin films via poly(styrene-co-allyl-alcohol) copolymer templates

In this study a copolymer poly(styrene-co-allyl-alcohol), [–CH 2CH(C 6H 5)–] x [–CH 2CH–(CH 2OH)–] y has been employed as a novel template in a sol–gel synthesis process to direct the formation of mesoporous tungsten oxide. The copolymer, due to its rigid hydrophobic block of polystyrene, is a more effective surfactant in an alcohol solution than polypropylene oxide-polyethylene oxide based compounds. The films have been prepared by a spin-coating technique from an ethanol solution of tungsten hexachloride. A room temperature, ultraviolet illumination method has been found to be very suitable for removing the polymer template, leading to the formation of a high-quality mesoporous structure. The electrochromic and optical properties of the mesoporous films are described and compared to standard sol–gel tungsten oxide films. Mesoporous materials exhibit superior high-rate ion-insertion performance when used as electrochromic layers.

Synthesis of Tungsten Oxide on Copper Surfaces by Electroless Deposition

Tungsten oxide films were successfully synthesized by electroless deposition on copper substrates from tungsten−peroxo electrolytes. The reducing electrons were provided by the copper substrate during oxidation to CuO. Mesoporous tungsten oxide was also fabricated by this method using SDS as a templating agent. This method can be used to fabricate molybdenum oxide, niobium oxide, zirconium oxide, and titanium oxide from metal−peroxo electrolytes.

Synthesis and electrochromic properties of mesoporous tungsten oxide

The electrochromic properties of mesoporous tungsten oxide are described and compared to standard sol–gel films of WO3. The block copolymer template used in the synthesis was removed by solvent extraction and calcination methods, leading to mesoporous tungsten oxides which range from amorphous to crystalline depending on the removal treatment. The pores are not ordered but do exhibit a fairly regular diameter with an average size of 4 to 5 nm. The mesoporous tungsten oxides exhibit a well behaved electrochromic response and their electrochemical and optical reversibilities are better than those of the analogous sol–gel film. In addition, the mesoporous films exhibit higher rates for coloration and bleaching. The high surface area of these materials is responsible for this behavior and also leads to multiple peaks in the voltammetric scans.