Non-stoichiometric Tungsten Oxide Research Articles

Studies of Buried Layers and Interfaces of Tungsten Carbide Coatings on the MWCNT Surface by XPS and NEXAFS Spectroscopy

Currently, X-ray photoelectron spectroscopy (XPS) is widely used to characterize the nanostructured material surface. The ability to determine the atom distribution and chemical state with depth without the sample destruction is important for studying the internal structure of the coating layer several nanometers thick, and makes XPS the preferable tool for the non-destructive testing of nanostructured systems. In this work, ultra-soft X-ray spectroscopy methods are used to study hidden layers and interfaces of pyrolytic tungsten carbide nanoscale coatings on the multi-walled carbon nanotube (MWCNT) surfaces. XPS measurements were performed using laboratory spectrometers with sample charge compensation, and Near Edge X-ray Absorption Fine Structure (NEXAFS) studies using the Russian–German dipole beamline (RGBL) synchrotron radiation at BESSY-II. The studied samples were tested by scanning and transmission electron microscopy, X-ray diffractometry, Raman scattering and NEXAFS spectroscopy. It was shown that the interface between MWCNT and the pyrolytic coating of tungsten carbide has a three-layer structure: (i) an interface layer consisting of the outer graphene layer carbon atoms, forming bonds with oxygen atoms from the oxides adsorbed on the MWCNT surface, and tungsten atoms from the coating layer; (ii) a non-stoichiometric tungsten carbide WC1-x nanoscale particles layer; (iii) a 3.3 nm thick non-stoichiometric tungsten oxide WO3-x layer on the WC1-x/MWCNT nanocomposite outer surface, formed in air. The tungsten carbide nanosized particle’s adhesion to the nanotube outer surface is ensured by the formation of a chemical bond between the carbon atoms from the MWCNT upper layer and the tungsten atoms from the coating layer.

Induced C[sbnd]C coupling in CO2 photocatalytic reduction via carbothermally reduced nonstoichiometric tungsten oxide

Photocatalytic conversion of carbon dioxide (CO2) is a promising strategy for both renewable solar energy storage and carbon emission reduction. Forming multicarbon products in CO2 photocatalytic reduction remains very difficult, due to the kinetic barriers of CC coupling. In this study, we introduce surface pentavalent tungstic (W5+) species in nonstoichiometric tungsten oxides via carbothermal reduction of commercial WO3 powder, achieving photocatalytic CO2 conversion with an aldehyde selectivity of 35% at a total normal CO2 conversion rate of 1.8 µmol·gcat−1·h−1. The as-prepared nonstoichiometric tungsten oxides exhibit improved band structures and electron transport properties, and the W5+ surface species with oxygen vacancies play a pivotal role in facilitating CC coupling of key intermediates. In-depth carbothermal reduction at an elevated temperature (880 °C) delivers more tetravalent W4+ species, decreasing the CO and aldehyde production rates. This work provides fundamental information to facilitate CC coupling in CO2 photoreduction via the introduction of surface active species.

Plasma-liquid synthesis of MoOx and WO3 as potential photocatalysts.

Plasmas in contact with liquids represent a green chemistry method for the synthesis of metal oxides. In this work, underwater plasma was used for the synthesis of molybdenum and tungsten oxides. The obtained samples were analyzed by various techniques. Results showed that underwater plasma with Mo electrodes allows obtaining non-stoichiometric molybdenum oxide (MoOx). In the case of tungsten electrodes, monoclinic WO3 was formed. The synthesized oxides have a wide band gap (3.21 eV for MoOx and 3.27 eV for WO3). The photocatalytic and sorption activities of the synthesized oxides towards the decomposition of cationic and anionic dyes (Methylene Blue, Rhodamine B, and Reactive Red 6C) were studied. MoOx shows excellent photocatalytic performance under UV and visible light irradiation. The photocatalytic activity of WO3 under visible light is less than that under UV irradiation.

The 1G4 - 3H6 electron transition process of Tm3+ promoted by nonmetallic plasmon

The coupling of rare-earth ions doped upconversion nanoparticles (UCNPs) with localized surface plasmon resonance (LSPR) of noble metal has attracted more interest because of improving luminescent efficiency of UCNPs. The nonstoichiometric semiconductor tungsten oxides can provide a possibility for the achievement of effective upconversion luminescence enhancement owning to concentrating and transferring the NIR-plasmonic energy in upconversion system. In this work, Yb3+/Tm3+-doped NaYF4 nanoparticles (NaYF4:Yb-Tm NPs)/ W18O49 nanowires (NWs) composite film which can tremendously improve the upconversion luminescence of Tm3+ as high as 140-fold has been fabricated. Investigations show that the LSPR-induced plasmonic energy transfer process produced by W18O49 NWs is the primary reason for enhancing the upconversion luminescence of NaYF4:Yb-Tm NPs, originating from much more intense excitation electric-field to generate more electrons on the excited state energy-levels of Tm3+ ions. It is anticipated that the development of non-metallic plasmon-sensitized upconversion luminescence materials may provide a platform for widely exploring application in biomedical imaging, therapeutics, and energy conversion.

Nonstoichiometric tungsten oxide: structure, synthesis, and applications

Stoichiometric tungsten oxide (WO3) and tungsten bronzes have received much attention due to the energy-related functional performances. In addition, as a transition metal oxide, substoichiometric tungsten oxides have manifested their unique properties and exceptional potential to lots of applications; however, no other studies have concentrated on the oxygen-deficient tungsten oxides till now. Herein, this article presents the origin of structure and synthesis of W18O49 nanomaterials chiefly, and then the general introduction of different applications, including electrochemistry, photocatalysis, near-infrared shielding and phototherapy and sensing.

Defect-induced ferromagnetism in W18O49 nanowires

In this paper, strong room temperature ferromagnetism was observed in non-stoichiometric tungsten oxide nanostructure, sea urchin-like single crystalline W18O49 nanowires prepared by a solvothermal approach. The prepared W18O49 nanowires exhibited a 39-fold enhancement in saturation magnetic moment as compared to the transformed WO3 by annealing in air. By combining x-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), x-ray photoelectron spectroscopy (XPS), and electron-spin resonance (EPR) spectroscopy, a defect complex, i.e., W5+ ions accompanied by oxygen vacancies, has been identified in the prepared W18O49. The observed ferromagnetism in W18O49 is qualitatively explained through a model involving bound magnetic polarons which were induced by the coupling between the electron trapped by oxygen vacancy and the unpaired 5d electron of W5+ ion. The results provide us better insight into the defect engineering of room-temperature ferromagnetism in tungsten oxide.

Producing oxygen-rich WO3 thin films by post-deposition thermal annealing

Non-stoichiometric oxygen-rich monoclinic tungsten oxide (WO3+x) thin films have been produced by RF sputtering on SiO2 substrates. The films were annealed at temperatures between 300 °C and 700 °C for 4 h in a pure oxygen atmosphere after deposition. The crystal structure and chemical composition of the films were studied in detail via grazing incidence x-ray diffraction (GIXRD), Rutherford backscattering spectrometry (RBS), energy dispersive x-ray spectroscopy (EDX) and Raman spectroscopy. Their surface properties were analyzed by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Optical properties were accessed via ellipsometry. Results indicate that the annealing procedure relaxes the crystal structure and increases the surface roughening. An increase of bandgap, surface texturization and fragments of a triclinic phase are observed as consequences.

The study of the tribological, thermal and physical properties of phenylone C-2 based composites containing nonstoichiometric molybdenum and tungsten oxides

The given paper lays bare the preparation and study of multifunctional tribocomposites with matrix based on Phenylone C-2 and the antifriction fillers. Besides, it contains a comparative evaluation of physical, mechanical, tribological and thermophysical properties of multifunctional tribocomposites to assess the effect of nanodimensional fillers.

Nonstoichiometric tungsten oxide residing in a 3D nitrogen doped carbon matrix, a composite photocatalyst for oxygen vacancy induced VOC degradation and H2 production

An oxygen vacancy rich WO3−x based composite material with 3D nitrogen doped carbon as a matrix is synthesized and it exhibits photocatalytic VOC removal and H2 production activities.

Using Intrinsic Intracrystalline Tunnels for Near‐Infrared and Visible‐Light Selective Electrochromic Modulation

Dual‐band electrochromic composite materials are of utmost importance in advancing the electrochromic field toward achieving the ideal smart window with independent control over visible and near‐infrared (NIR) radiation. However, such composites usually need deliberate architecting of their mesoscale structure (e.g., via block copolymer‐templating method) to make the electrolyte contact with both NIR and visible‐light modulating components. Herein, instead of arduously making exterior pores, the intrinsic structural tunnels are utilized directly in electrochromic materials to facilitate the accommodation and transportation of insertion ions, which permit the infiltration of the electrolyte to be in contact with both visible (Prussian blue) and NIR‐light modulating components (nonstoichiometric tungsten oxide). Such simple‐fabricated composite materials exhibit excellent dual‐band electrochromic performance with an unprecedented dynamic optical range for modulation of visible and NIR light, up to 71.2% at 633 nm and 64.8% at 1600 nm, respectively.

Vacancy-Induced Semiconductor–Insulator–Metal Transitions in Nonstoichiometric Transition Metal Oxides

Metal–insulator transitions in strongly correlated transition metal oxides (TMOs) induced by electrochemical charging have been attributed to formation of vacancy defects. However, the role of native defects in affecting these transitions is not well understood. This presentation will discuss a new type of phase transition in p-type, nonstoichiometric nickel oxide involving a semiconductor-to-insulator-to-metal transition along with the complete reversal of conductivity from p- to n-type at room temperature induced by the interaction of native vacancy defects with redox species such as Li+ during electrochemical charging. Direct observation of vacancy-ion interactions using in-situ near-infrared photoluminescence spectroscopy show that this transition is a result of passivation of native nickel (cationic) vacancy defects and subsequent formation of oxygen (anionic) vacancy defects driven by Li+ insertion into the lattice. Changes in the oxidation states of nickel due to defect interactions probed by X-ray photoemission spectroscopy support the above conclusions. In contrast, n-type, nonstoichiometric tungsten oxide shows only insulator-to-metal transition, which is a result of oxygen vacancy formation. The defect-property correlations shown here in these model systems can be extended to other oxides to enable rational engineering of a new robust class of nanoscale, low temperature TMO catalysts and sensors having tailored functionality and selectivity.

Vacancy-Induced Semiconductor-Insulator-Metal Transitions in Nonstoichiometric Nickel and Tungsten Oxides.

Metal-insulator transitions in strongly correlated oxides induced by electrochemical charging have been attributed to formation of vacancy defects. However, the role of native defects in affecting these transitions is not clear. Here, we report a new type of phase transition in p-type, nonstoichiometric nickel oxide involving a semiconductor-to-insulator-to-metal transition along with the complete reversal of conductivity from p- to n-type at room temperature induced by electrochemical charging in a Li+-containing electrolyte. Direct observation of vacancy-ion interactions using in situ near-infrared photoluminescence spectroscopy show that the transition is a result of passivation of native nickel (cationic) vacancy defects and subsequent formation of oxygen (anionic) vacancy defects driven by Li+ insertion into the lattice. Changes in the oxidation states of nickel due to defect interactions probed by X-ray photoemission spectroscopy support the above conclusions. In contrast, n-type, nonstoichiometric tungsten oxide shows only insulator-to-metal transition, which is a result of oxygen vacancy formation. The defect-property correlations shown here in these model systems can be extended to other oxides.

Novel Non-Stoichiometric Tungsten Oxide Based Catalyst Support for the Increased CO Tolerance in PEMFC

Cost and durability are two major factors that delay large-scale production and commercialization of PEMFC’s. One of the technologically ready application of the proton exchange membrane fuel cells (PEMFC) is in the combined heat and power systems (µCHP), which are used in the individual households or buildings. The performance of the µCHP systems greatly depends on the purity of the hydrogen stream, which is produced via methane reforming process. To overcome low CO tolerance of the commercially used Pt electrocatalyst and to lower the catalyst content we have prepared non-stoichiometric tungsten oxide as a Pt based catalyst support. We have prepared several catalysts designated as 10% Pt/WO3-C, 20% Pt/WO3-C, 40% Pt/WO3-C. The structure and morphology characteristics of the prepared catalysts were investigated using XRD, TEM and SEM/EDX techniques. Investigations concerning electroactivity of these catalysts towards the hydrogen oxidation reaction (HOR) were performed using cyclic voltammetry, linear sweep voltammetry, forming an ultra thin catalyst layer onto RDE. Mechanism and the kinetics of the prepared catalysts towards HOR were evaluated and if was found that increased mass activity of the 10% Pt/WO3-C could be attributed to the interactive naure of the WO3 catalyst support. Obtained results clearly show increased CO tolerance of Pt/WO3-C catalyst compared to commercial Pt/C, which was confirmed by lowering the stripping potential of the CO, adsorbed on the surface of the 10% Pt/WO3-C. catalyst is more facile than that on commercial 40% Pt/C. These catalysts were employed as anode catalyst in the MEA, and the performance of single cell PEMFC were compared to commercial catalyst.

Low Platinum Content Catalyst on the Non-Stoichiometric Tungsten Oxide Based Catalyst Support for the Increased CO Tolerance in PEMFC

Cost and durability are two major factors that delay large-scale production and commercialization of PEMFC’s. One of the technologically ready application of the proton exchange membrane fuel cells (PEMFC) is in the combined heat and power systems (µCHP), which are used in the individual households or buildings. The performance of the µCHP systems greatly depends on the purity of the hydrogen stream, which is produced via methane reforming process. To overcome low CO tolerance of the commercially used Pt electrocatalyst and to lower the catalyst content we have prepared non-stoichiometric tungsten oxide as a Pt based catalyst support. We have prepared several catalysts designated as 10% Pt/WO3-C, 20% Pt/WO3-C, 40% Pt/WO3-C. The structure and morphology characteristics of the prepared catalysts were investigated using XRD, TEM and SEM/EDX techniques. Investigations concerning electroactivity of these catalysts towards the hydrogen oxidation reaction (HOR) were performed using cyclic voltammetry, linear sweep voltammetry, forming an ultra thin catalyst layer onto RDE. Mechanism and the kinetics of the prepared catalysts towards HOR were evaluated and if was found that increased mass activity of the 10% Pt/WO3-C could be attributed to the interactive naure of the WO3 catalyst support. Obtained results clearly show increased CO tolerance of Pt/WO3-C catalyst compared to commercial Pt/C, which was confirmed by lowering the stripping potential of the CO, adsorbed on the surface of the 10% Pt/WO3-C. catalyst is more facile than that on commercial 40% Pt/C. These catalysts were employed as anode catalyst in the MEA, and the performance of single cell PEMFC were compared to commercial catalyst.

Noble metal-comparable SERS enhancement from semiconducting metal oxides by making oxygen vacancies

Surface-enhanced Raman spectroscopy (SERS) represents a very powerful tool for the identification of molecular species, but unfortunately it has been essentially restricted to noble metal supports (Au, Ag and Cu). While the application of semiconductor materials as SERS substrate would enormously widen the range of uses for this technique, the detection sensitivity has been much inferior and the achievable SERS enhancement was rather limited, thereby greatly limiting the practical applications. Here we report the employment of non-stoichiometric tungsten oxide nanostructure, sea urchin-like W18O49 nanowire, as the substrate material, to magnify the substrate–analyte molecule interaction, leading to significant magnifications in Raman spectroscopic signature. The enrichment of surface oxygen vacancy could bring additional enhancements. The detection limit concentration was as low as 10−7 M and the maximum enhancement factor was 3.4 × 105, in the rank of the highest sensitivity, to our best knowledge, among semiconducting materials, even comparable to noble metals without ‘hot spots'.

Decorating of Graphene-Supported Palladium Nanoparticles with Nanostructured Tungsten Oxide Towards More Efficient Electrocatalytic Oxidation of Formic Acid

By depositing tungsten oxide nanostructures (nanorods) over palladium nanoparticles supported onto graphene nanosheets and dispersed onto glassy carbon substrate, enhancement of the overall electrocatalytic activity towards oxidation of formic acid in acid medium was observed. The effect was particularly evident from the increase of chronoamperometric currents at a fairly low potential of 0.04 V vs. RHE. It is reasonable to expect that introduction of graphene nanosheets to the system facilitates distribution of charge at the electrocatalytic interface. Under such conditions, tungsten oxide nanorods seem to be partially and reversibly reduced not only to nonstoichiometric hydrogen tungsten(VI,V) oxide bronzes (HxWO3, 0<x<1) but also substoichiometric lower tungsten (VI,IV) oxides (WO3-y, 0<y<1). Mutual interactions between tungsten oxides or graphene and Pd nanoparticles are likely to affect electrochemical characteristics of all components. The metal-oxide interactions, as well as high population of hydroxyl groups at the electrocatalytic interface (favoring oxidative removal of passivating CO adsorbates), are presumably responsible for the overall enhancement effect. In comparison to conventional electrodeposited microporous tungsten oxides, WO3 nanorods, despite their small dimensions, are more robust and less hydrated. Electrochemical diagnostic experiments were supported with microscopic measurements aiming at monitoring morphology of catalytic surfaces with use of transmission (TEM) and scanning (SEM) electron microscopies. The palladium nanoparticles were of the sizes 10 to 20 nm, and the tungsten oxide nanorods had diameters of 50–70 nm while being approximately 5 μm long. Our ultimate goal is to minimize loading of noble metal nanoparticles while keeping the electrocatalytic activity at the high level.

Structure and Electroactivity of Tungsten Oxides in Acid Medium: Enhancement of Charge Propagation Rates through Nanostructuring and Formation of Mixed-Metal Oxide Systems

Electrochromism in tungsten oxides and related systems has been recognized for long time and has been still of interest for display applications. Redox transitions and related reversible coloration of thin WO3 films are believed to be controlled by simultaneous injection or removal of electrons and protons. The dynamics of partial reduction (in acid medium) of transparent tungsten(VI) oxides to blue hydrogen tungsten(VI,V) oxide bronzes or lower tungsten(VI,IV) oxides is largely dependent of the oxide morphology, nanostructuring, degree of hydration, or population of colored W(VI,V) or W(VI,IV) sites. For example, by depositing nanorods of tungsten oxide, rather than conventional tungsten oxide, on the ITO conducting glass (or the inert glassy carbon) substrate, the enhancement in the degree of the system's voltammetric reduction in acid medium (0.5 M H2SO4) has been observed. Tungsten oxide nanorods seem to be partially and reversibly reduced not only to nonstoichiometric hydrogen tungsten(VI,V) oxide bronzes (HxWO3, 0<x<1) but also substoichiometric lower tungsten (VI,IV) oxides (WO3-y, 0<y<1). Another important issue is the mobility and availability of protons within the tungsten oxide film. We have found that by controlled admixing of tungsten oxide with zirconia nanoparticles both nanostructured morphology and high mobility of protons can be adjusted. The resulting mixed (hybrid or composite) system is well-behaved: redox reactions (which are characteristics of WO3) are fast and reversible despite the presence of semiconducting zirconia (ZrO­2). While ZrO2 is not electroative, WO3 undergoes redox reactions leading to formation of partially reduced hydrogen bronzes (H x WO3) in which mobility of H+ is coupled to electron transfers. The composite material is stabilized through electrostatic attraction between positively charged zirconium oxo cations (ZrO2+, ZrOH3+, [Zr3(OH)4]3+) and anionic tungstates (WO4 2- in WO3*H2O or H2WO4). Among important issues are: high population of hydroxyl groups and high mobility of protons at surfaces of zirconia (ZrO2) nanostructures in addition to the existence of fast electron transfers coupled to proton displacements in partially reduced tungsten oxides (HxWO3). In a different series of chronocoulometric experiments, diffusion coefficients (for charge propagation) have been determined, and they have been found typically higher for hybrid mixed metal (tungsten(VI)/zirconium(IV) oxide films (close to 10-7 cm s-1). Mechanistic considerations and the results of spectroelectrochemical and diagnostic spectroscopic (e.g. FTIR) experiments will also be provided. We acknowledge collaboration with Christian Perruchot and Mohamed Jouini from ITODYS, Université Paris Diderot, France.

Admixing palladium nanoparticles with tungsten oxide nanorods toward more efficient electrocatalytic oxidation of formic acid

By depositing nanorods of tungsten oxide over catalytic palladium nanoparticles (dispersed onto glassy carbon substrate), enhancement of the overall electrocatalytic activity toward oxidation of formic acid in acid medium was observed. The effect was particularly evident from the increase of chronoamperometric currents at a fairly low potential of 0.04V vs. RHE. Under such conditions, tungsten oxide nanorods seem to be partially and reversibly reduced not only to nonstoichiometric hydrogen tungsten(VI,V) oxide bronzes (HxWO3, 0<x<1) but also substoichiometric lower tungsten (VI,IV) oxides (WO3−y, 0<y<1). Mutual interactions between tungsten oxides with Pd nanoparticles affect electrochemical characteristics of both components. The metal–oxide interactions, as well as high population of hydroxyl groups at the electrocatalytic interface (favoring oxidative removal of passivating CO adsorbates), are most likely responsible for the overall enhancement effect. In comparison to conventional electrodeposited microporous tungsten oxides, WO3 nanorods, despite their small dimensions, are more robust and less hydrated. Electrochemical diagnostic experiments were supported with microscopic measurements aiming at monitoring morphology of catalytic surfaces with use of transmission (TEM) and scanning (SEM) electron microscopies. The palladium nanoparticles were of the sizes 10–20nm, and the tungsten oxide nanorods had diameters of 50–70nm while being approximately 5μm long.

Room temperature NO2-sensing properties of Ti-added nonstoichiometric tungsten oxide nanowires

Solvothermally synthesized W18O49 nanowires were modified by addition of titanium. The effect of the concentration and the introduction procedure of Ti additive on the microstructure and NO2 sensing properties of W18O49 nanowires were investigated. In order to study the influence of the additive introduction procedure, Ti addition was carried out by physical impregnation process and chemical synthesis process, in which the additive was introduced after and during nanowire synthesis, respectively. Introduction of Ti additive by chemical synthesis process suppressed the growth of one-dimensional nanowires along their axis direction and resulted in the secondary assembly of nanowires bundles forming unfavorable structure for gas sensing. By contrast, Ti addition preformed to as-synthesized nanowires with a proper additive concentration (2at% in the present study) could create a favorable spotty configuration of tiny additive particles on the nanowire surface. The Ti-added W18O49 nanowires showed temperature-dependent response characteristic reversal, which behave as a p-type semiconductor at operating temperature below 100°C. Ti addition not only lowered the optimal operating temperature of W18O49 nanowires sensors from 150°C to room temperature but also increased the maximum value of sensor response. At room temperature, the Ti-added nanowires exhibited rapid response characteristic to NO2 gas.

High-yield fabrication of W18O49@TiO2 core–shell nanoparticles: microstructures and optical-thermal properties

In the present study, high-yield W18O49@TiO2 core–shell nanoparticles were prepared by modified plasma arc gas condensation without any catalysts or substrates. All the as-prepared samples were characterized by FEG-SEM, XRD, FEG-STEM, and HAADF analytic techniques. The results of the structural analysis show that the as-prepared nanoparticles presenting a core–shell morphology with an average diameter of 43.5 ± 8.0 nm were composed of non-stoichiometric tungsten oxide (W18O49 phase) as the core (20–40 nm) and rutile-phase TiO2 as the shell with non-uniform thickness (10–20 nm). For the optical properties of the as-prepared W18O49@TiO2 core–shell nanoparticles, Raman spectroscopy and photoluminescence (PL) spectra were used. Compared with pure TiO2 and W18O49 nanocrystals, the experimental results reveal that the defects in the lattice between the core and shell layers induced the board and shifted peaks in Raman spectra. Also, W18O49@TiO2 core–shell nanoparticles exhibited green emission at 483 nm wavelength observed in PL spectrum. Thermal gravimetric analyzer (TGA) results indicate that the TiO2 shell served a stable layer and prevented further oxidation from the atmosphere of the W18O49 core, thereby improving the thermal stability of W18O49 nanoparticles.