Hexagonal Ammonium Tungsten Bronze Research Articles

Influence of the Precursor Structure on the Formation of Tungsten Oxide Polymorphs.

Understanding material nucleation processes is crucial for the development of synthesis pathways for tailormade materials. However, we currently have little knowledge of the influence of the precursor solution structure on the formation pathway of materials. We here use in situ total scattering to show how the precursor solution structure influences which crystal structure is formed during the hydrothermal synthesis of tungsten oxides. We investigate the synthesis of tungsten oxide from the two polyoxometalate salts, ammonium metatungstate, and ammonium paratungstate. In both cases, a hexagonal ammonium tungsten bronze (NH4)0.25WO3 is formed as the final product. If the precursor solution contains metatungstate clusters, this phase forms directly in the hydrothermal synthesis. However, if the paratungstate B cluster is present at the time of crystallization, a metastable intermediate phase in the form of a pyrochlore-type tungsten oxide, WO3·0.5H2O, initially forms. The pyrochlore structure then undergoes a phase transformation into the tungsten bronze phase. Our studies thus experimentally show that the precursor cluster structure present at the moment of crystallization directly influences the formed crystalline phase and suggests that the precursor structure just prior to crystallization can be used as a tool for targeting specific crystalline phases of interest.

In situ facile one-step solvothermal synthesizing hexagonal ammonium tungsten bronze (NH4)0.33WO3 to tap NIR light absorption ability by controlling crystallinity

Tungsten bronze (TB) has potentially high visible light transparency and excellent near-infrared (NIR) shielding ability. To fully dig out the concealed optical properties of TB with specific chemical components, in this work, hexagonal ammonium tungsten bronze (h-ATB) (NH4)0.33WO3 was fabricated by a one-step solvothermal reaction. Through X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and ultraviolet–visible–near infrared (UV–vis–NIR) spectroscopy to characterize h-ATB, which unraveled that controlling crystallinity could adjust the optical property of h-ATB. In particular, when the crystallinity of the h-ATB is over 95%, it exhibits the attractive NIR light shielding capability, which is in line with the predicted optical property via the first principle calculation based on density functional theory (DFT). Given the result, this work can provide a feasible research idea to explore the hidden optical properties of TB.

Vacuum pyrolysis of ammonium paratungstate: Study on reaction mechanism and morphology changes of product

At present, tungsten oxide is mainly produced by atmospheric decomposition of ammonium paratungstate (APT), which usually causes uneven particle size and morphology. In this paper, non-agglomerated tungsten oxide was prepared by vacuum pyrolysis (vacuum pressure is about 20 Pa) of APT. The vacuumpyrolysis mechanism and characterization changes of APT were investigated. The results showed that the pyrolysis of APT in a vacuum could be divided into five main stages based on thermal effect, crystallization water removing (25−170 °C), ammonia removing (170−250 °C), (NH4)6.84H3.16[H2W12O42] dissociation (250−390 °C), hexagonal ammonium tungsten bronze and WO3 part reduction (390−500 °C), h-WO3 crystal transformation (500−600 °C). The effects of pyrolysis temperature, heating rate, and holding time on the morphology and particle size of products were further studied. With increasing temperature, the particle size of the products decreased, and there was no agglomeration. Coarse tungsten oxide (62.43 μm) was obtained at a temperature increase rate of 1 °C/min. The shorter heat preservation time (10 min) was more conducive to the preparation of fine tungsten oxide (32.41 μm). The crystal morphology during APT decomposition into tungsten oxide was "hereditary". Compared with atmospheric decomposition, the vacuum environment is conducive to the preparation of narrow tungsten oxide products.

WO3 nanopaticles and PEDOT:PSS/WO3 composite thin films studied for photocatalytic and electrochromic applications

WO3 is a widely studied material for electrochromic and photocatalytic applications. In the present study, WO3 nanoparticles with a controlled structure (monoclinic or hexagonal) were obtained by controlled thermal decomposition of hexagonal ammonium tungsten bronze in air at 500 °C and 600 °C, respectively. The formation, morphology, structure and composition of the as-prepared nanoparticles were studied by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM-EDX). The photocatalytic activity of the monoclinic and hexagonal WO3 nanoparticles was studied by decomposing methyl orange in aqueous solution under UV light irradiation. In order to study the electrochromic properties of the WO3 nanoparticles, as well to introduce them for self-cleaning photocatalytic surface applications, thin films were prepared from the WO3 particles together with a conductive polymer. For this, PEDOT:PSS was used, which gives excellent opportunities for obtaining transparent and conductive thin films, suitable for both electrochromic and photocatalytic applications. By spin-coating, transparent PEDOT:PSS/WO3 composite thin films were prepared, on which cyclic voltammetry measurements were performed, and the coloring and bleaching states were studied. Our initial results for the PEDOT:PSS/WO3 composite thin films are promising, suggesting that such composites, after further development, might be successfully used in electrochromic devices and photocatalysis.

Study about the morphology effect on the photo-efficiency of WO 3

The photocatalytic activity of hexagonal (h-) WO3 nanoparticles (NPs) and nanowires (NWs) was investigated and compared with the performance of monoclinic (m-) WO3 nanoparticles in the aqueous photo-bleaching reaction of methyl orange (MO) under UV irradiation. It has been known that the m-WO3 is better photocatalyst than the h-WO3, but due to the advantageous morphology we investigated whether the h-WO3 NWs can reach the photo-efficiency of the m-WO3 NPs. The h-WO3 was successfully synthesized in NW and NP morphology using a microwave (MW) assisted hydrothermal procedure starting from Na2WO4 and thermal annealing of hexagonal ammonium tungsten bronze, (NH4)0.33-xWO3, respectively. We found that the h-WO3 NWs exhibited almost three times higher photoactivity than the h-WO3 NPs. The improved performance of the NWs can be attributed to the enlarged surface area and the good charge carrier ability of the nanowire morphology. The catalytic tests also confirmed that the morphological effect could lead to as high photoactivity in the case of h-WO3 NWs as exhibited by the m-WO3 NPs.

Gas sensing selectivity of hexagonal and monoclinic WO 3 to H 2S

Hexagonal and monoclinic tungsten oxide (h- and m-WO 3) samples were produced by annealing hexagonal ammonium tungsten bronze, (NH 4) 0.07(NH 3) 0.04(H 2O) 0.09WO 2.95 at 470 and at 600 °C, respectively. Their structure, composition and morphology were analyzed by XRD, Raman, XPS, 1H-MAS NMR and SEM. In order to study the effect of crystal structure on the gas sensitivity of tungsten oxides, h- and m-WO 3 were tested as gas sensors to CH 4, CO, H 2, NO and H 2S (1000 and 10 ppm) at 200 °C. Monoclinic WO 3 responded to all gases, but its gas sensing signal was two magnitudes greater to 10 ppm H 2S than to other gases, and it also detected H 2S even at 25 °C. Hexagonal WO 3 responded only to 10 ppm H 2S. Its sensitivity was smaller compared to m-WO 3, however, the response time of h-WO 3 was significantly faster. The gas sensing tests showed that while m-WO 3 had relative selectivity to H 2S in the presence CH 4, CO, H 2, NO; h-WO 3 had absolute selectivity to H 2S in the presence these gases.

Phase transformations of ammonium tungsten bronzes

This article discusses the formation and structure of ammonium tungsten bronzes, (NH4) x WO3−y . As analytical tools, TG/DTA-MS, XRD, SEM, Raman, XPS, and 1H-MAS NMR were used. The well-known α-hexagonal ammonium tungsten bronze (α-HATB, ICDD 42-0452) was thermally reduced and around 550 °C a hexagonal ammonium tungsten bronze formed, whose structure was similar to α-HATB, but the hexagonal channels were almost completely empty; thus, this phase was called reduced hexagonal (h-) WO3. In contrast with earlier considerations, it was found that the oxidation state of W atoms influenced at least as much the cell parameters of α-HATB and h-WO3, as the packing of the hexagonal channels. Between 600 and 650 °C reduced h-WO3 transformed into another ammonium tungsten bronze, whose structure was disputed in the literature. It was found that the structure of this phase—called β-HATB, (NH4)0.001WO2.79—was hexagonal.

The effect of K+ ion exchange on the structure and thermal reduction of hexagonal ammonium tungsten bronze

This paper discusses the changes in the structure and thermal reduction of nanosize hexagonal ammonium tungsten bronze (HATB), (NH4)0.33−xWO3−y, which were caused by K+ ion exchange (doping) and studied by XRD, XPS, 1H-MAS NMR, FTIR, SEM and TG/DTA-MS. Comparison of the cell parameters of undoped and doped HATB revealed that both a and c cell parameters decreased after the ion exchange reaction, which showed that smaller K+ ions partly replaced the larger NH 4 + ions in the hexagonal channels of HATB. After the reaction, from the hexagonal channels less NH3 evolved, which also supported the incorporation of K+ ions into the hexagonal channels.

Thermal stability of hexagonal tungsten trioxide in air

We studied the thermal stability of different hexagonal tungsten trioxide, h-WO3 samples, which were prepared either by annealing hexagonal ammonium tungsten bronze, (NH4)0.33−xWO3−y, or by soft chemical synthesis from Na2WO4. The structure and composition of the samples were studied by powder XRD, SEM-EDX, XPS and 1H-MAS NMR. The thermal properties were investigated by simultaneous TG/DTA, on-line evolved gas analysis (TG/DAT-MS), SEM and in situ powder XRD. The preparative routes influenced the thermal properties of h-WO3 samples, i.e. the course of water release, the exothermic collapse of the hexagonal framework and the phase transformations were all affected.

Preparation of hexagonal WO 3 from hexagonal ammonium tungsten bronze for sensing NH 3

Hexagonal tungsten oxide (h-WO 3) was prepared by annealing hexagonal ammonium tungsten bronze, (NH 4) 0.07(NH 3) 0.04(H 2O) 0.09WO 2.95. The structure, composition and morphology of h-WO 3 were studied by XRD, XPS, Raman, 1H MAS (magic angle spinning) NMR, scanning electron microscopy (SEM), and BET-N 2 specific surface area measurement, while its thermal stability was investigated by in situ XRD. The h-WO 3 sample was built up by 50–100 nm particles, had an average specific surface area of 8.3 m 2/g and was thermally stable up to 450 °C. Gas sensing tests showed that h-WO 3 was sensitive to various levels (10–50 ppm) of NH 3, with the shortest response and recovery times (1.3 and 3.8 min, respectively) to 50 ppm NH 3. To this NH 3 concentration, the sensor had significantly higher sensitivity than h-WO 3 samples prepared by wet chemical methods.

Controlling the Composition of Nanosize Hexagonal WO<sub>3</sub> for Gas Sensing

Hexagonal (h-) WO3 was prepared through heating hexagonal ammonium tungsten bronze (HATB), (NH4)0.07(NH3)0.04(H2O)0.09WO2.95. By adjusting the heating temperature and atmosphere of HATB, we could control the oxidation state of tungsten atoms and the residual NH3/NH4 + content in h-WO3. The as-produced h-WO3 nanoparticles with different composition were tested as gas sensors and the effect of composition on gas sensing properties was studied. Our results showed that oxidized h-WO3 had the best sensitivity to H2S.

Stability and Controlled Composition of Hexagonal WO3

This paper discusses the formation of nanosized hexagonal tungsten oxide (h-WO3) during the annealing of hexagonal ammonium tungsten bronze (HATB), (NH4)0.33−xWO3−y. This process was investigated by TG/DTA-MS, XRD, SEM, Raman, XPS, and 1H-MAS NMR analyses. Through adjusting the temperature and atmosphere of annealing HATB, the composition (W oxidation state, residual NH4+ and NH3 content) of h-WO3 could be controlled. The effect of composition on the conductivity and gas sensitivity of h-WO3 was studied. New structural information was obtained about both HATB and h-WO3. It was found that NH4+ and NH3 could be situated at three different positions in HATB. Residual NH4+ and NH3 in the hexagonal channels seemed to be vital for stabilizing h-WO3: when they were completely released, the hexagonal framework collapsed. We propose that the structure of h-WO3 cannot be maintained without some stabilizing ions or molecules in the hexagonal channels.

In situ HT‐XRD Study on the Formation of Hexagonal Ammonium Tungsten Bronze by Partial Reduction of Ammonium Paratungstate Tetrahydrate

AbstractHexagonal ammonium tungsten bronze, (NH4)0.33–xWO3–y, is a possible constituent of the intermediate “tungsten blue oxide” in non‐sag (NS) tungsten production and a material of chromogenic and sensor interest. Its formation has been studied by in situ high‐temperature powder X‐ray diffraction (HT‐XRD) accompanied by semiquantitative phase analysis through the partial thermal reduction of ammonium paratungstate tetrahydrate, (NH4)10[H2W12O42]·4H2O, in flowing 10 % H2/He. The effect of the heating program on the formation of the title compound has been discussed. Highly crystalline, monophasic hexagonal ammonium tungsten bronze prepared by partial reduction of ammonium paratungstate tetrahydrate has been characterised by high‐precision powder XRD, X‐ray photoelectron spectroscopy (XPS), chemical analysis and solid‐state 1H NMR spectroscopy (1H‐MAS‐NMR). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Solid-state 1H NMR studies of different tungsten blue oxides and related substances

The doping of ‘tungsten blue oxide’ (TBO) with K, Si and Al leads to the outstanding high-temperature creep resistance of the so-called non-sag (NS) tungsten wire. The composition of the blue-colored TBO depends on the reduction conditions of ammonium paratungstate, (NH 4) 10[H 2W 12O 42]-4H 2O, such as temperature, atmosphere, type of rotary kiln or pusher-type furnace used and rate of feed through the furnace. Besides crystalline compounds (WO 3, hexagonal tungsten bronze phase, W 20O 58, W 18O 49, WO 2) the industrially produced TBO powders may contain up to 50% of amorphous phases. The wide-line and especially the high-resolution solid-state 1H NMR investigation of five industrially manufactured TBOs and seven reference substances show a distinction between five proton-containing species. Now, together with chemical, quantitative X-ray powder diffraction and NH 4 +/K + exchange analyses, we are able to give a complete and consistent characterization of different TBOs. Only TBOs with a certain content of hexagonal ammonium tungsten bronze, h-(NH 4) η,WO 3 (η <- 0.33), reveal the incorporation of K + via an ion exchange mechanism during the doping process.

Formation of hexagonal ammonium tungsten bronze during thermal decomposition of APT: Y. Yamamoto et al (Tokyo Tungsten Co Ltd, Tokyo, Japan) J.Japan Soc. Powder and Powder Metallurgy, vol 40, no 11, 1993, 1126–1130. (In Japanese)

Formation of hexagonal ammonium tungsten bronze during thermal decomposition of APT: Y. Yamamoto et al (Tokyo Tungsten Co Ltd, Tokyo, Japan) J.Japan Soc. Powder and Powder Metallurgy, vol 40, no 11, 1993, 1126–1130. (In Japanese)

酸化タングステンの水素還元機構

Hexagonal ammonium tungsten bronze (ATB, (NH4)xWO3) and tungsten trioxide obtained by the thermal decomposition from monoclinic ammonium paratungstate (APT) and tungstic acid were reduced in dry hydrogen. Monoclinic WO2.90, tetragonal WO2.90, monoclinic WO2, and cubic β-W (i.e. W3O) for intermediate were produced. The decomposition of β-W was investigated by DTA in argon atmosphere. The reduction process are as follows.WO3(1)monoclinic WO2.90(2)monoclinic WO2(3)cubic W (α-W)(4)(NH4)xWO3(5)tetragonal WO2.90(6)(7)cubic W3O (β-W) (8)Reduction path of the WO3 that accompanied by heat evolution while thermal decomposition of APT is (1)(2)(3). Reduction of ATB of endothermic compound proceeds as (5)(6) and β-W form. The WO3 from tungstic acid of endothermic compound is reduced path as (4)(5) and β-W also form. The β-W decompose to WO2 and α-W accompanied with heat evolution with elevation of reduction temperature. It is conceivable that endothermic energy during thermal decomposition go down activation energy to produce tungsten metal.

Insertion of hydrogen into hexagonal ammonium tungsten bronze (NH4)0.3WO3

Insertion of hydrogen into the hexagonal ammonium tungsten bronze was carried out by electrochemical and “hydrogen spill-over” techniques. The coefficient x = 0.06 in the hydrogen bronze Hx(NH4)0.3WO3 and the diffusion coefficient of hydrogen DH = 2.29 × 10−17 m2s−1 were found by the hydrogen insertion in the absence of water.

パラタングステン酸アンモニウムの熱分解過程

During thermal decomposition of ammonium paratungstate (Monoclinic and Orthorhombic AP T), study on formation process of very fine grain tungsten oxide particle was carried out by thermobalance in nitrogen atmosphere. Thermogravimetric analysis, X-ray diffraction, scanning electron microscopy and specific surface area measurement have been used to make clear the thermal decomposition process. The results obtained are as follows.(1) Both monoclinic and orthorhombic APT decompose into WO3 through the following order:5(NH4)2O⋅12WO3⋅5H2O→X(NH4)2O⋅12WO3→12WO35(NH4)2O⋅12WO3⋅11H2O→X(NH4)2O.12WO3→12WO3(2) Crystal system of X(NH4)2O⋅12WO3 changed along the following step.APT→Amorphous→Hexagonal ammonium tungsten bronze (→Monoclinic WO3)(3) Ultrafine individual particles are formed on the original APT particle changed like spongy at around 773K, and the particles were uniform in size and shape.

パラタングステン酸アンモニウムの熱分解における超微粒タングステン酸化物の生成機構

The determing factor of tungsten oxide particle size during thermal decomposition of monoclinic ammonium paratungstate (APT) has been studied. APT was decomposed by thermobalance in nitrogen, oxygen, air and mixture of ammonia and nitrogen atmosphere. Formation mechanism of ultrafine tungsten oxide particle was investigated using X-ray diffraction analysis, scanning electron microscopy and specific surface area measurement. The relationship of particle size between hexagonal ammonium tungsten bronze (ATB) and tungsten oxide was discussed. Formation of ATB was controlled by parameters such as heating rate, sample mass and kind of atomosphere. Particle size of tungsten oxide was affected with amount of ATB formed in the intermediate, that is, particle size was very fine when ATB was formed in large quantities. Ultarfine individual particle of tungsten oxide was formed by a decomposing reaction from ATB to W03 or WO2.9, and grain growth of its particle took place at temperatures above 873 K.

パラタングステン酸アンモニウムの熱分解におけるアンモニウムタングステンブロンズの生成条件

パラタングステン酸アンモニウムの熱分解におけるアンモニウムタングステンブロンズの生成条件