Plasma synthesis of various polymorphs of tungsten trioxide nanoparticles using gliding electric discharge in humid air:characterization and photocatalytic properties

A gliding electric arc(glidarc)discharge generates a low-temperature plasma at atmospheric pressure.When the discharge occurs in humid air as the feed gas,the chemistry of a glidarc plasma consists of in situ formation of HO°and NO°as the primary chemical species.Tungsten trioxide(WO_(3))nanoparticles were successfully prepared by exposure of a liquid precursor to glidarc plasma.The WO_(3)samples were calcined at three different temperatures(300℃,500℃and 800℃),resulting in different pure polymorphs:γ-WO_(3)(at 300℃),β-WO_(3)(at 500℃)andα-WO_(3)(at 800℃)according to x-ray diffraction analysis.The identification of WO_(3)compounds was also confirmed by attenuated total reflection Fourier transform infrared spectroscopy analysis.Increase in the calcination temperature of WO_(3)induced a decrease in its specific surface area according to Brunauer–Emmett–Teller nitrogen physisorption analysis.The UV-visible results showed that the absorption bands of plasma-WO_(3)samples were more intense than those of WO_(3)samples obtained by a precipitation route,a classical method used for comparison.Consequently,this parameter can improve the photocatalytic properties of WO_(3)under visible light.The photodegradation(in sunlight conditions)of gentian violet,chosen as a model pollutant,confirmed the photocatalytic properties of plasma-WO_(3)samples.This novel synthesis method has great potential to improve the efficiency of advanced tungsten trioxide-based functional material preparation,as well as in pollution-reducing and energy-saving tungsten extractive metallurgy.

Giant saturation absorption of tungsten trioxide film prepared based on the seedless layer hydrothermal method

Nonlinear materials have gained wide interest as saturable absorbers and pulse compression for pulsed laser applications due to their unique optical properties.This work investigates the third-order nonlinear phenomenon of tungsten trioxide(WO_(3))thin films.The giant nonlinear absorption and nonlinear refractive index of WO_(3)thin films were characterized by Z-scan method at 800 nm.We experimentally observed the giant saturable absorption(SA)and nonlinear refractive index of WO_(3)thin films prepared by the seedless layer hydrothermal method,with SA coefficient being as high as-2.59×105cm·GW^(-1).The SA coefficient is at least one order of magnitude larger than those of the conventional semiconductors.The nonlinear refractive index n_(2)of WO_(3)film has been observed for the first time in recent studies and the corresponding coefficient can be up to 1.793 cm^(2)·GW^(-1).The large third-order nonlinear optical(NLO)response enables WO_(3)thin films to be promising candidates for optoelectronic and photonic applications in the near-infrared domain.

A Simple Method of Electrospun Tungsten Trioxide Nanofibers with Enhanced Visible-Light Photocatalytic Activity

The present study involves the fabrication of tungsten trioxide(WO3) nanofibers by an electrospinning technique using polyvinyl pyrrolidone(PVP)/citric acid/tungstic acid as precursor solution. It was found that the PVP concentration was one of the most crucial processing parameters determining the final properties of WO3 nanofibers. The optimum concentration of PVP was from 75 to 94 g L-1. The average diameter of the nanofibers increases with increasing the PVP concentration, whereas it is decreased after sintering and orthorhombic structure were formed at 500 °C. The photocatalytic properties of the as-synthesized nanofibers were also investigated by degrading methylene blue and twofold efficiency was obtained compared with that of commercial WO3 microparticles.

An Aqueous Asymmetric Supercapacitor Based on Activated Carbon and Tungsten Trioxide Nanowire Electrodes

An asymmetric supercapacitor （ASC） was assembled by using an activated carbon as positive electrode and WO3 nanowire as negative electrode, and its electrical performances were tested in voltage windows ranging from 0 to 1.5 V. A high specific capacitance of 51 Fog-1 could be achieved at the current density of 0.25 A·g-1 . Moreover, the ASC displays a good cycling stability with 86% of capacitance retention after 800 cycles, its energy density can be up to 11.9 Wh·kg-1 at the power density of 210 W·kg -1, and remains 7.7 Wh·kg-1 at a power density of 1250 W· kg-1. The excellent electrical performance is perhaps due to the crystal orientation of （001） planes for the WO3 nanowire, which favors the rapid reaction between W（VI） and H＋ cations. This aqueous asymmetric WO3//AC supercapacitor is promising for practical applications due to its easy preparation of WO3.

Solid-state Z-scheme assisted hydrated tungsten trioxide/ZnIn_(2)S_(4) photocatalyst for efficient photocatalytic H_(2) production

Efficient water splitting for H_(2) evolution over semiconductor photocatalysts is highly attractive in the field of clean energy.It is of great significance to construct heterojunctions,among which the direct Z-scheme nanocomposite photocatalyst provides effective separation of photo-generated carriers to boost the photocatalytic performance.Herein,Z-scheme hydrated tungsten trioxide/ZnIn_(2)S_(4) is fabricated via an in-situ hydrothermal method where ZnIn_(2)S_(4) nanosheets are grown on WO_(3)·xH_(2)O.The close contact between WO_(3)·0.5H_(2)O and WO_(3)·0.33H_(2)O as well as ZnIn_(2)S_(4) improve the charge carrier separation and migration in the photocatalyst,where the strong reducing electrons in the conduction band of ZnIn_(2)S_(4) and the strong oxidizing holes in the valence band of WO_(3)·0.33H_(2)O are retained,leading to enhanced photocatalytic hydrogen production.The obtained WO_(3)·xH_(2)O/ZnIn_(2)S_(4) shows an excellent H_(2) production rate of 7200μmol g^(−1) h^(−1),which is 11 times higher than pure ZnIn_(2)S_(4).To the best of our knowledge,this value is higher than most of the WO_(3)-based noble metal-free semiconductor photocatalysts.The improved stability and activity are attributed to the formation of the Z-scheme heterojunction,which can markedly accelerate the interfacial charge separation for surface reaction.This work offers a promising strategy towards the design of an efficient Z-scheme photocatalyst to suppress electron–hole recombination and optimize redox potential.

Origin of varistor properties of tungsten trioxide (WO_3) ceramics

To study the physical origin of the non-ohmic behavior of WO3 ceramics,the effects of heat treatment in different atmospheres on WO3 varistors were investigated.Experiments showed that there was a dependence of the nonlinear coefficient on thermal treatment under different atmospheres.Thermal treatments in argon and oxygen atmospheres at 900℃proved this dependence,and indicated that the nonlinear coefficient got significantly lower when the samples were thermally treated under argon atmosphere.Subsequent exposure to oxygen atmosphere at the same temperature led to the restoration of electrical properties.The result shows that the physical origin of the non-ohmic behavior of WO3 ceramics is oxygen on the grain surfaces adsorbed by intrinsic defects.

3D Tungsten Trioxide Nanosheets as Optoelectronic Materials for On-chip Quantification of Global Antioxidant Capacity

The photoelectrochemical properties of semiconductors mainly depend on the size and shape of the corresponding nanoparticles.Herein,3D WO3 nanosheets were controllably synthesized with the aid of polyethyleneimine,which presents enhanced photocurrent responses.Based on this excellent photoelectrochemical property,a photoelectrochemical chip was prepared by lithography technology for the smart monitoring of the antioxidant capacity(AC)in red wine and exhibits a series of advantages including rapid response time,high sensitivity,and long-lasting stability.The mechanism of the present photoelectrochemical sensing was explored and shows a single electron transfer reaction.Furthermore,only 200µL of samples are required for one testing,which demonstrates that the present photoelectrochemical chip can be potentially integrated with a portable commercial device(such as a mobile phone)for further research and development of food and drug supervision.

Innovative approach to boosting the chemical stability of AZ31 magnesium alloy using polymer-modified hybrid metal oxides

Meeting the demands of complex and advanced applications requires the development of high-performance hybrid materials with unique properties.However,the integration of polymeric frameworks with MgO/WO_(3) composite layers faces challenges due to the lack of understanding of the formation mechanism and the challenge of determining the impact of self-assembled architecture on anticorrosive properties.In this study,we aimed to enhance the corrosion resistance of the MgO layer produced by plasma electrolysis(PE)of AZ31 Mg alloy by incorporating WO_(3) with partially phosphorated poly(vinyl alcohol)(PPVA).Two types of porous MgO layers were produced using the PE process with an alkaline-phosphate electrolyte,one with and one without WO_(3) nanoparticles,which were subsequently immersed in an aqueous solution of PPVA.Incorporating PPVA into the WO_(3)-MgO layer resulted in hybrids being deposited in a fragmented manner,creating a“laminar reef-like structure”that sealed most of the structural defects in the layer.The PPVA-sealed WO_(3)-based coating exhibited superior corrosion resistance compared to the other samples.Computational analyses were employed to explore the mechanism underlying the formation of PPVA/WO_(3) hybrids on the MgO layer.These findings suggest that PPVA-WO_(3)-MgO hybrid coatings can potentially improve corrosion resistance in various fields.

Preparation of Ultrafine Tungsten Powder by Sol-Gel Method

Sol-gel method was employed for the preparation of nanoscale tungsten powder. The effects of different preparation conditions on particle size were discussed and the optimum preparation condition was found. The products were characterized by X-ray diffraction, scan electron microscopy and so on. The results show that the intermediate is monoclinic WO3, its particle shape is approximately spherical, and the particle size distribution is narrow. The average particle size is about 60 nm. After deoxidization, WO3 turns into cubic tungsten powder with small particle size （average particle size about 120 nm） and narrow size distribution.

Degradation of BPB in photocatalysis enhanced by photosensitizer

The treatment of wastewater containing bromphenol blue （BPB） by WO3/α-Fe203 was studied. Reaction mechanism of photocatalysis enhanced by photosensitizer was probed. The relationships between the composition of heterogeneous photocatalyst, the starting concentration of BPB, amount of photocatalyst, pH, amount of H2O2, illumination time and the decoloring rate of BPB were discussed. The results show that the decoloring rate of BPB can reach 99.1% by using WO3/α-FeaO3 as heterogeneous photocatalyst, with the composition of m（WO3）：m（α-Fe2O3 ）=3：1, the initial concentration of BPB = 15mg/L, the amount of catalyst = 0.300 g, pH= 6.3, the amount of H2O2 =0.2 mL, and the illumination time = 6 h.

Morphology dependent photocatalytic activity of WO_3 nanostructures

The shape of nanostructure has important effects on their properties, therefore in this study, we have prepared and characterized three different morphologies of WO_3 nanostructures i.e. nanorod, nanosphere and nanoplate for surveying shape effect on their photocatalytic properties toward degradation of Rhodamine B （RhB） dye. Obtained results show that nanoplate WO_3 in comparison with others has the best photocat- alytic activity. According to SEM, and photocatalytic degradation results, the reason for this behavior is the sharp edges and corners of WO_3 nanoplates. Because of their low coordination number, atoms located in the edges and comers of the WO_3 nanoplates have more activity, adsorb more RhB and therefore give more photocatalytic activity to the WO_3 nanoplates. Using of different scavengers showed that hydroxyl radicals are mainly responsible for photocatalytic activity of WO_3 nanoplates and nangspheres but for WO_3 nanorods, superoxide radicals are the main photocatalytic degradation agents.

Simple point contact WO_3 sensor for NO_2 sensing and relevant impedance analysis

A simple and new point contact tungsten trioxide （WO3） sensor, which can be prepared by the oxidation of tungsten filaments via in-situ induction heating, likely detects low concentration （ppm level） environmental pollutants such as NO2. X-ray diffraction （XRD） and field emission scanning electron microscopy （FE-SEM） were applied to characterize the phase and the microstructure of the samples, respec-tively. It was found that the synthesized WO3 films exhibited a monoclinic phase and were composed of hierarchical microcrystals and nanocrystals. The point contact WO3 sensor （W-WO3-W） showed rectifying characteristics and an ideal sensing performance of about 110 C. A single semicircle in Nyquist plots was recorded by electrochemical impedance spectroscopy （EIS） at a relatively low temperature of 150 C but faded away above 200 C, which revealed that the sensing process was governed by a determining factor, i.e., grain boundaries at the contact site.

WO_3 Anodic Oxide Film——I.Electrochromism and Auto-bleaching mechanism

Electrochromic and auto-bleaching processes at the WO2 anodic film in 0. 5 mol/L H2SO4 solution were investigated by cyclic voltammetry, a. c. impedance technique and photocurrent spectrometry. The colouration mechanism consists of hydrogen adsorption on the WO2 surface and the transport of H atoms in the WO, lattice. The bleaching process involves at least two steps: transport of interstitial H atoms and hydrogen desorption on the W surface, resulting in interstitial H+ ions; then extration of the H+ ions driven by the external electric field. The auto-bleaching arises from the hydroxylation due to both partial interstitial H atoms and a little of water contained in the film.

Theoretical Investigations on the Structural and Electronic Properties of WO3 Polymorphs

The structural characteristic and electronic properties of seven WObulk polymorphs have been studied by first-principles density functional theory(DFT) calculations. Our calculation results indicate that according to the energy of WOper unit, the stability of different WOsystems reduces in turn in the following order: monoclinic WO, orthorhombic WO, tetragonal WO, triclinic WO, hexagonal WOand cubic WO. And when the unit cell is distorted from the cubic framework, the alternatively long and short W–O bonds are formed, implying an improvement of bonding-antibonding splitting associated with the interactions between W and O atoms. The deformation of the WO6 octahedron enhances the stability of the system to some extent though the energy difference between various WOphases is small(< 0.1 eV/per WOunit). Besides, the band gap tends to enlarge accompanied with the symmetry decrease of WObulk. According to the band structures, the minimum band gaps for those WOpolymorphs are direct at the Γ point except for the simple cubic and hexagonal structures. In addition, for all the WOphases, the compositions of the valance band maximum(VBM) and conduction band minimum(CBM) are the same, which are relative to the O 2 p and W 5 d orbitals, respectively.

A Kinetic Approach to Photomineralization of Methane in Air by Membranes Based on TiO2/WO3

Photomineralization of methane in air(10.0-1,000 ppm(mass/volume)of C)at 100%relative humidity(dioxygen as oxygen donor),was systematically studied at 318±3 K,in an annular laboratory-scale reactor,by photocatalytic membranes immobilising titanium dioxide and tungsten trioxide as co-photocatalysts.Kinetics of both substrate disappearance,to yield intermediates,and total organic carbon(TOC)disappearance,to yield carbon dioxide,were followed.A kinetic model was employed,from which,by a set of differential equations,four final optimised parameters,k1 and K1,k2 and K2,were calculated,able to fit the whole kinetic profile adequately.Modelling of quantum yields,as a function of substrate concentration and irradiance,as well as of concentration of photocatalysts,was carried out very satisfactorily.Kinetics of hydroxyl radicals reacting between themselves,leading to hydrogen peroxide,other than with substrate or intermediates leading to mineralization,were considered,paralleled by second competition kinetics involving superoxide radical anion.When using appropriate blends of the two photocatalysts,limiting quantum yieldsF∞values increase considerably and approach the maximum allowable value for the investigated molecule,in a much wider range of irradiances than that shown by the single catalysts mainly at low irradiances.This may be interpreted by strong competition kinetics of superoxide radicals generated by the catalyst defects,in the corresponding range of high irradiances.By this way,operation at high irradiance values is possible,without losing any efficiency for the mineralization process.

Enhanced photocatalytic performance of tungsten-based photocatalysts for degradation of volatile organic compounds: a review

Photocatalytic oxidation process for the degradation of volatile organic compounds(VOCs)contaminants is a promising technology.But until now,the low photocatalytic activity of the conventional TiO_(2) photocatalyst under visible-light irradia-tion hinders the deployment of this technique for VOCs degradation.WO_(3) has been proved to be a suitable photocatalytic material for degradation of various VOCs as its appropriate band-gap,high stability and great capability.Nevertheless,the actual implementation of WO_(3) is still restricted by short lifetime of photoexcited charge carriers and low light energy conver-sion efficiency:its photocatalytic performance is needed to be improved.This review discusses the process of tungsten-based photocatalyst for removal of VOCs and summarizes a variety of strategies to improve the VOCs oxidation performances of WO_(3),such as controlling the morphology structure,engendering chemical defects,coupling heterojunction,doping suitable dopants and loading a co-catalyst.In addition,the practical application of tungsten-based photocatalyst is discussed.

Ultrasensitive room-temperature geranyl acetone detection based on Fe@WO_(3−x) nanoparticles in cooked rice flavor analysis

In the assessment of food quality,geranyl acetone plays a crucial role as a volatile organic compound(VOC)biomarker for diverse agricultural products,while the ultralow concentration detection meeting application requirements has been barely studied.Herein,an iron(Fe)-doped WO_(3−x) gas sensor was employed for greatly sensitive,selective,and scalable geranyl acetone detection.The results proved that precisely-regulated oxygen vacancy(OV)and sophisticatedly-active electron transition of Fe-doped WO_(3−x) nanoparticles were fulfilled by modifying the doping amount of Fe^(3+),leading to the prominently enhanced sensitivity(23.47 at 6 ppm),low limit of detection(LOD)(237 ppb),optimal selectivity,and outstanding long-term stability.Furthermore,the enhancing mechanism of gas sensing performance was substantiated through density functional theory(DFT)calculation,while the practical application for the evaluation of spoiled cooked rice was conducted as well.This study demonstrates a reliable method for detecting a VOC biomarker in cooked rice,which can ensure food security and improve palatability of cooked rice.

Gold/WO3 nanocomposite photoanodes for plasmonic solar water splitting

A facile electron-charging and reducing method was developed to prepare Au/＇VVO3 nanocomposites for plasmonic solar water splitting. The preparation method involved a charging step in which electrons were charged into WO3 under negative bias, and a subsequent reducing step in which the stored electrons were used to reductively deposit Au on the surface of WO3. The electron-charged WO3 （c-WO3） exhibited tunable reducibility that could be easily controlled by varying the charging parameters, and this property makes this method a universal strategy to prepare metalAVO3 composites. The obtained Au/VVO3 nanocomposite showed greatly improved photoactivity toward the oxygen evolution reaction （OER） when compared with WO3. After Au decoration, the OER photocurrent was improved by a percentage of over 80% at low potentials （〈0.6 V vs. SCE）, and by a percentage of over 30% at high potentials （〉1.0 V vs. SCE）. Oxygen evolution measurements were performed to quantitatively determine the Faraday efficiency for OER, which reflected the amount of photocurrent consumed by water splitting. The Faraday efficiency for OER was improved from 74% at the WO3 photoanode to 94% at the Au-8/＇vVO3 composite photoanode, and this is the first direct evidence that the Au decoration significantly restrained the anodic side reactions and enhanced the photoelectrochemical （PEC） OER efficiency. The high photoactivity of the composite photoanode toward OER was ascribed to the plasmon resonance energy transfer （PRET） enhancement and the catalytic enhancement of Au nanoparticles （NPs）.

Silicotungstic acid-derived WO_(3) composited with ZrO_(2) supported on SBA-15 as a highly efficient mesoporous solid acid catalyst for the alkenylation of p-xylene with phenylacetylene

Highly dispersed silicotungstic acid-derived WO_(3) composited with ZrO_(2) supported on SBA-_(15) (WZ/SBA-_(15)) as an ordered mesoporous solid acid catalyst was prepared via a facile incipient wetness impregnation (IWI) method that active ingredients, ZrO_(2) and WO_(3), were impregnated into the channels of SBA-_(15) simultaneously with a subsequent calcination process. The relationship between catalyst nature and performance was explored by high resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), FT-IR, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), N_(2) adsorption-desorption, NH_(3) temperature-programmed desorption (NH_(3)-TPD), and FT-IR of pyridine adsorption (Py-IR) characterization techniques. The catalytic performance of W_(12)Z_(15)/SBA-_(15) is not only greater than that of single component solid acid catalysts, WO_(3)/SBA-_(15) and ZrO_(2)/SBA-_(15), but also W_(12)/Z_(15)/SBA-_(15) prepared by impregnating active ingredients, ZrO_(2) and WO_(3), into SBA-_(15) in sequence. The outstanding performance of W_(12)Z_(15)/SBA-_(15) is derived from the strong interaction between ZrO_(2) and WO_(3), which results in more acid sites, and relatively high specific surface area, large pore volume, and ordered mesoporous structure of SBA-_(15). The characterization and reaction results clearly demonstrate that the synergy of ZrO_(2) and WO_(3) has a clear boost for the alkenylation. The optimized W_(12)Z_(15)/SBA-_(15)-500 achieves a 99.4% conversion of phenylacetylene and a 92.3% selectivity of main product α-arylstyrene for the alkenylation of p-xylene with phenylacetylene, with very low level of oligomers producing at the same time. Moreover, W_(12)Z_(15)/SBA-_(15)-500 shows excellent catalytic stability and regeneration. Therefore, W_(12)Z_(15)/SBA-_(15)-500 is a promising solid acid catalyst for the alkenylation.

ITO/Cu multilayer electrodes for high-brightness electrochromic displays

Fabry-Perot(F-P)nanocavity-type electrochromic devices with multicolor tunability have attracted significant interest in the past two years for their potential uses in a wide variety of applications,such as electronic display,military camouflage,and dynamic decoration.However,challenges such as insufficient brightness,lengthy switching times,and poor cycling stability have yet to be overcome.Herein,we demonstrate electrochromic electrodes based on ITO/Cu as both the current collector and the reflective layer,with WO_(3 )as the electrochromic material,forming a unique three-layered structure.The constituted WO_(3 )/ITO/Cu films present a high brightness value of about 84%before coloration,and a decent brightness value of 48%after coloration at−0.8​V.Moreover,a fast switching time between different coloration states(tc​=​2.2​s;tb​=​1​s)is recorded,attributed to the extremely low sheet resistivity of the ITO/Cu current collector.The films also reveal excellent electrochemical cycling stability across 2000 cycles.