Synthesis of hydrated tungsten(VI) oxide sols by peptization

A method has been developed for the synthesis of hydrated tungsten oxide hydrosols, with this method being based on potassium tungstate hydrolysis followed by peptization of the formed precipitate. The influence of the conditions of precipitation, aging, and washing of the precipitate on the particle phase composition and shape and the degree of precipitate peptization has been studied. Hydrosol-particle sizes have been determined by different methods. It has been found that the dispersed phase of the hydrosols consists mainly of platelike particles of hydrated tungsten oxide WO₃ · 2H₂O with a number-average size of 52 nm. The sols are stable to aggregation in a pH range of 3.0–4.5. The zeta potential of the particles ranges from–33 to–38 mV.     

Products of carbothermal reduction of tungsten oxides in argon flow

The investigation into carbothermal reduction of tungsten oxides has shown that this process involves several steps: WO₃ → WO_2.72 → WO₂ → W. The resulting oxide is rapidly reduced to tungsten metal at 950°C. The carbidization process has a diffusion mechanism. The staged character of carbothermal reduction of tungsten oxides and subsequent carbidization of tungsten are confirmed by scanning electron microscopy, which made it possible to determine the phase composition and size of the resulting particles.     

Colloid chemical properties of hydrated tungsten trioxide hydrosols

Hydrosols of hydrated tungsten trioxide WO₃ ? nH₂O have been synthesized via peptization of a precipitate obtained by hydrolysis of potassium tungstate. It has been shown that the resulting sols are stable to aggregation in a pH range of 3.0–4.5. Hydrodynamic diameters and ζ potentials of WO₃ ? nH₂O particles have been determined as functions of dispersion medium pH. In addition, their density and degree of hydration have been found for the stable sols. The thickness of particle surface layers has been estimated.     

钨氧化物纳米结构的合成与表征

采用溶剂热法以WCl₆作为前体合成出了一维和二维的钨氧化物纳米结构,研究了反应溶剂和前体浓度对钨氧化物物相和形貌的影响并评价了各种钨氧化物纳米结构对NO₂气体的敏感性能。XRD、SEM、TEM和XPS的表征结果表明,通过改变溶剂和调整WCl₆浓度,可分别获得单斜的W₁₈O₄₉纳米棒、W₁₈O₄₉纳米线和WO₃纳米片结构。气敏性能测试结果表明,钨氧化物纳米结构对NO₂气体表现出良好的可逆性,与W₁₈O₄₉纳米棒和WO₃纳米片相比,W₁₈O₄₉纳米线对NO₂具有更高的灵敏度。     

Photoelectrochemical properties of tungsten trioxide thin film electrodes prepared from facet-controlled rectangular platelets

The control of anisotropic crystal growth is critical for directing the orientation of crystal lattice planes, and it plays a key role towards understanding the effects of different planes on chemical reactions. Here, we report on the photoelectrochemical properties of plate-structured tungsten trioxide (WO₃) thin films prepared from facet-controlled rectangular platelets of hydrotungstite (WO₃·2H₂O) and tungstite (WO₃·H₂O), which are directly grown on tungsten substrates. The WO₃ thin films, prepared via WO₃·2H₂O platelets, show relatively stable current for photoelectrochemical water splitting and methanol oxidation. On the other hand, the photocurrent of the WO₃ thin films prepared via WO₃·H₂O platelets was significantly decreased during the photoelectrochemical oxidation of water, which is likely due to the accumulation of partially oxidized intermediates such as peroxo species on the surface. These results indicate that the surface nanostructures of WO₃ may have a significant influence on photoelectrode efficiency and selectivity for the catalytic oxygen evolution reaction.     

High-Pressure Synthesis of Fully Occupied Tetragonal and Cubic Tungsten Bronze Oxides

A high-pressure reaction yielded the fully occupied tetragonal tungsten bronze K₃W₅O₁₅ (K_0.6WO₃). The terminal phase shows an unusual transport property featuring slightly negative temperature-dependence in resistivity (dρ/dT<0) and a large Wilson ratio of R_W=3.2. Such anomalous metallic behavior possibly arises from the low-dimensional electronic structure with a van Hove singularity at the Fermi level and/or from enhanced magnetic fluctuations by geometrical frustration of the tungsten sublattice. The asymmetric nature of the tetragonal tungsten bronze K_xWO₃-K_0.6−yBa_yWO₃ phase diagram implies that superconductivity for x≤0.45 originates from the lattice instability because of potassium deficiency. A cubic perovskite KWO₃ phase was also identified as a line phase—in marked contrast to Na_xWO₃ and Li_xWO₃ with varying quantities of x (<1). This study presents a versatile method by which the solubility limit of tungsten bronze oxides can be extended.     

Phase transformations of ammonium tungsten bronzes

This article discusses the formation and structure of ammonium tungsten bronzes, (NH₄) _x WO_3−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)_0.001WO_2.79—was hexagonal.     

Structure of small defects in nonstoichiometric WO3−x

Structural models are derived for the hitherto “unresolved” small defects which should occur in the nonstoichiometric phase WO_3?x. The mechanisms of aggregation or interaction of these small defects to produce extended defect structures (i.e., crystallographic shear planes and pentagonal and/or hexagonal tungsten bronze-type columns) are discussed next. Linear defects, consisting of two pairs of edge-shared octahedra, are proposed. These readily explain the large number of electron microscope observations of precipitation and dissolution phenomena reported for reduced WO_3?x and doped tungsten trioxide specimens.     

Solid-state pH ultramicrosensor based on a tungstic oxide film fabricated on a tungsten nanoelectrode and its application to the study of endothelial cells

In this paper, preparation of a novel pH ultramicrosensor and its physiological application has been discussed. A tungsten nanoelectrode was produced by an etching method in 0.1 mol/l NaOH solution at the potential of +0.4 V (versus Ag/AgCl reference electrode) for about 100 s and the diameters ranged from 500 to 800 nm. The pH ultramicrosensor was fabricated by producing WO₃ at W nanoelectrode surface by electrooxidation in 2.0 mol/l H₂SO₄ solution between 1.0 and 2.0 V. At last, Nafion was coated on the surface of WO₃ to protect the pH ultramicrosensor. The W/WO₃ pH ultramicrosensor exhibited a good pH linear region from 2.0 to 12.0 with a super-Nernstian slope of −53.5 ± 0.5 mV/pH unit. Response times ranged from 3 s at about pH 6.0-7.0 up to 15 s at high pH. An interference of various ions to the pH measurement was also studied in this paper. We also studied the lifetime, stability and reproducibility of the W/WO₃ pH ultramicrosensor. In order to testing the performance of W/WO₃ ultramicrosensor, we applied it to measure the extracellular pH values and a pH variation was also given about the normal, damaged and recovery endothelial cells.     

Phase relations in the SnWO ternary system near to WO3

Phase relations in the SnWO system for compositions near to WO₃ and temperatures up to 1173 K have been determined by electron microscopy and X-ray diffraction. The phase limits for the bronzes previously reported in this system have been determined. For the orthorhombic I bronzes the phase limits are from Sn_0.04WO₃ to Sn_0.06WO₃. Two orthorhombic II bronze phases form, one at a composition of Sn_0.13WO₃ to Sn_0.15WO₃, and another at Sn_0.16WO₃. These bronzes have structures which consist of lamellae of WO₃ united by fault planes. The other bronze phase to form, with the tetragonal tungsten bronze structure, has a lower composition limit of Sn_0.21WO₃.     

Phase transitions in tungsten trioxide at low temperatures

Capacitance and electrical resistivity measurements have been made on stoichiometric and on oxygen-deficient tungsten trioxide crystals from 4.2 to 300°K. X ray oscillation and rotation photographs were made on single crystals of both materials near 200°K and near 300°K. Capacitance and resistivity anomalies identify phase transitions near 40, 65, 130, 220, and 260°K in stoichiometric WO₃. Resistivity anomalies occur near 80, 130, 220, and 260°K in oxygen-deficient tungsten trioxide. Capacitance measurements suggest that the transformation at 130°K of a low-temperature phase to a high-temperature phase of stoichiometric WO₃ is associated with a doubling of thec-parameter of the unit cell. Resistivity measurements establish probable conduction mechanisms in each phase of stoichiometric and of oxygen-deficient tungsten trioxide, and show that oxygen-deficient tungsten trioxide undergoes a semiconductor-to-metal transition near 220°K. Electronic phenomena that do not appear to be associated with structural phase transformations are observed near 16°K in stoichiometric WO₃.     

Effect of tungsten oxide loading on metathesis activity of ethene and 2-butene over WO<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> catalysts

The metathesis of ethene and 2-butene to propene was studied over WO₃/SiO₂ catalysts with various WO₃ loadings (2, 4, 8, 12, 16, and 24 wt%). The 2-butene conversion and propene selectivity increased greatly with WO₃ loading increasing from 2 to 8 wt%, reached maximum at 8–12 wt% WO₃ loading, and then decreased when the WO₃ loading was higher than 12 wt%. From the above results and taking the economics into account, the optimal amount of WO₃ loading was ~8 wt%. The catalysts were characterized by physico-chemical and spectroscopic techniques to elucidate the effect of different tungsten oxide loadings on the metathesis reactivity of ethene and 2-butene. The characterization data indicated that three types of tungsten species (i.e., surface tetrahedral tungsten species, surface octahedral polytungstate species, and WO₃ crystallites) were present in the catalysts. It was found that WO₃ was not the active centers, and surface tetrahedral tungsten species might be more active than octahedral polytungstate species in metathesis reaction. The reduced form of tungsten species [W⁺⁴, W⁺⁵, and W^+(6−y) (0 < y < 1)] may be the suitable state of W species acting as metathesis active centers.     

Nanostructured tungsten oxide as photochromic material for smart devices,energy conversion,and environmental remediation

The reversible photochromic response of tungsten oxide (WO₃) holds promise for solar-related applications as it is capable of photo charging during illumination (color-switching) and spontaneous discharging post-illumination (self-bleaching). Advances in WO₃-based nanostructures synthesis via micro/nanofabrication techniques have created remarkable potential application opportunities. Smart windows represent a typical energy-saving technology; ultraviolet indicators can sense radiation safety limits, and the around-the-clock photocatalysts can be used for pollutant degradation and bacterial disinfection applications. These materials, their distinct properties, and the effects of their application must be comprehensively understood prior to commercialization. In this work, we first summarize the affiliation between the crystallographic properties-optical features-photochromic behavior of WO₃. Several photochromic models and kinetic equations are then presented, accompanied by the related characterization techniques and evaluation methods. The factors affecting photochromic efficiency (e.g., light absorption, surface reaction, and carrier migration) are delineated to clarify the advantages of the specific nanostructured WO₃ and the most efficient available strategies for constructing WO₃-based nanomaterials. The theory, technique, and performance associated with chromogenic applications in smart devices, energy conversion, and environmental remediation are deliberated in detail. Finally, we outline the challenges and emerging trends in this area calling for further innovation to fill various gaps.     

A green route for microwave synthesis of sodium tungsten bronzes NaxWO3 (0<x<1)

A green route has been developed for microwave synthesis of sodium tungsten bronzes Na_xWO₃ (0<x<1) from Na₂WO₄, WO₃ and tungsten powder. The hybrid microwave synthesis was carried out in argon atmosphere using CuO powder as the heating medium. Tungsten powder is used as the reducing agent instead of the alkali metal iodides previously used for the microwave synthesis of oxide bronzes. The prepared samples were characterized by powder X-ray diffraction, energy-dispersive X-ray analysis and scanning electron microscopy, and their phase constitutions, crystal structures and morphologies are in consistence with that in the literature. This synthesis method is simple, green and atom economic, and promising for preparation of other oxide bronzes and related compounds.     

Influence of Pt and Au nanophases on electrochromism of WO3 in nanostructure thin-film electrodes

We report electrochromic properties of WO₃ in Au–WO₃ and Pt–WO₃ nanostructure thin-film electrodes prepared by co-sputtering deposition method. The nanostructure electrodes consisted of Au or Pt metallic nanophase and a tungsten oxidative phase, indicating the formation of crystalline metallic nanophases in the amorphous oxide matrix. In particular, due to metallic nanophases, the modified electrochromic properties of WO₃ were observed in the Au–WO₃ and Pt–WO₃. The nanostructure electrodes showed a reverse optical modulation with respect to applied potentials in H₂SO₄ solution compared to that of pure WO₃ electrode. However, due to an excellent electrocatalytic activity of platinum for methanol electrooxidation at 25 °C, the electrochromism of the Pt–WO₃ in contrast with that of the Au–WO₃ was affected by the potentials for methanol electrooxidation in 2 M CH₃OH and 0.5 M H₂SO₄.     

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−xWO_3−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.     

Effect of Structure of Pd/WO3-ZrO2 Catalyst on Its Activity for Direct Oxidation of Ethylene to Acetic Acid

A series of 1%Pd/WO₃-ZrO₂ catalysts with different W/Zr ratios and calcination temperatures of WO₃-ZrO₂ were prepared by an impregnation method. Their crystal structure, surface state, and acidity were determined using X-ray diffraction, N₂ adsorption, NH₃ temperature-programmed desorption, pyridine infrared spectroscopy, and temperature-programmed reduction. Special attention was paid to the surface states of tungsten and palladium under different preparation conditions. The results revealed that WO_x surface species underwent a transformation from polytungstate species to coexistent polytungstate/crystalline WO₃ and further to crystalline WO₃ particles with increase of W/Zr ratio and calcination temperature. The W/Zr = 0.2 sample calcined at 1 073 K showed the maximum amount of polytungstates, which were responsible for the excellent activity. Moreover, the state of palladium was only dependent on the calcination temperature. Well-dispersed Pd species were responsible for high selectivity to acetic acid, and large metallic Pd particles were favorable for ethylene combustion.     

Inside Cover: Mechanistic Understanding of Efficient Polyethylene Hydrocracking over Two-Dimensional Platinum-Anchored Tungsten Trioxide (Angew. Chem. Int. Ed. 40/2023)

Chemical upcycling of polyethylene (PE) can convert plastic waste into valuable resources. However, engineering a catalyst that allows PE decomposition at low temperatures with high activity remains a significant challenge. Herein, we anchored 0.2 wt.% platinum (Pt) on defective two-dimensional tungsten trioxide (2D WO₃) nanosheets and achieved hydrocracking of high-density polyethylene (HDPE) waste at 200–250 °C with a liquid fuel (C_5–18) formation rate up to 1456 g_products ⋅ g_{metal species}⁻¹ ⋅ h⁻¹. The reaction pathway over the bifunctional 2D Pt/WO₃ is elucidated by quasi-operando transmission infrared spectroscopy, where (I) well-dispersed Pt immobilized on 2D WO₃ nanosheets trigger the dissociation of hydrogen; (II) adsorption of PE and activation of C−C cleavage on WO₃ are through the formation of C=O/C=C intermediates; (III) intermediates are converted to alkane products by the dissociated H. Our study directly illustrates the synergistic role of bifunctional Pt/WO₃ catalyst in the hydrocracking of HDPE, paving the way for the development of high-performance catalysts with optimized chemical and morphological properties.     

The structures of intergrowth tungsten bronzes of Ba,Sn, Pb,and Sb

The structures of intergrowth tungsten bronzes (ITB) of compositions Ba_0.04WO₃, Sn_0.04WO₃, Pb_0.04WO₃, Sn_0.18WO₃, and Sb_0.25WO₃ have been deduced from high-resolution electron microscope images. Both the Pb_0.04, Sn_0.04, and Ba_0.04, ITB phases consist of single rows of hexagonal tunnels occupied by Pb, Sn, or Ba atoms intergrown in a WO₃-like matrix. The Sb_0.25, ITB phase is composed of similar rows of Sb-containing single hexagonal tunnels, the centers of which are separated by a WO₃-like matrix only two octahedra in thickness. The structure of the Sn_0.18, ITB phase consists of double rows of hexagonal tunnels containing Sn atoms joined by a single strip of WO₃-like octahedra. The structures are compared with the structures of other known ITB phases and the nonstoichiometric behavior of these phases is discussed.     

Structure and Thermal Stability of La0.10WO3+y; A Hexagonal Tungsten Bronze Related Phase Formed at High Pressure

The structure and thermal stability of a hexagonal tungsten bronze (HTB) related compound, La_xWO_3+y with x≈0.10 and y≈0.15, has been studied by X-ray diffraction, thermal analysis, and electron microscopy. The structure was refined by the Rietveld method from X-ray powder diffractometer data of a La_0.10WO₃ sample prepared at T=1250°C and P=25 kbar, which consisted of two tungsten bronze related phases in 1:1 proportion. The unit cell dimensions are as follows: La_0.108WO_3+y (y≈0.16), a=7.40890(5), and c=3.79329(4) Å (HTB-related structure); La_0.091WO₃, a=3.82458(6) Å (cubic perovskite tungsten bronze (PTB) structure). The lanthanum atoms in La_0.108WO_3+y are located on the hexagonal axis and statistically distributed on two sites close to the tungsten atom plane. Thermal stability studies of the La_0.10WO₃ sample in an argon atmosphere under ambient pressure conditions revealed that the HTB-related compound is metastable, decomposing to the stable PTB-type structure and WO₃. It was also found from the TG experiments in argon and oxygen that additional oxygen atoms (y) are present in the structure, thus forming a lanthanum tungsten oxide of the above composition. The electron diffraction and microanalysis studies confirmed that crystals of the HTB- and PTB-type structures were formed, with a lanthanum content of x≈0.1.