Non-stoichiometric Tungsten Oxide Research Articles

Laser-induced the enhancement of Raman scattering performance in WO3-x/Ag composite films

Non-stoichiometric tungsten oxide (WO3-x) has controllable defects, high charge density, and good synergy with other materials to exhibit good surface-enhanced Raman scattering (SERS) properties. Its heterojunction structure provides an opportunity to develop high-quality and low-cost SERS substrates. This study obtained WO3-x/Ag composite thin films were obtained by Nd: YAG fiber pulsed laser modification at ambient conditions. The effects of interactions between heterojunctions and laser modification on the samples’ morphology, composition, and optical properties were investigated. The absorption peaks exhibited a red shift by varying the laser scan speeds, and the SERS properties of the sample were evaluated by methylene blue (MB) dye. The results show that the laser-modified WO3-x/Ag films have good stability as SERS substrates. The characteristic peaks of MB can still be detected after 90 days in the air. The WO3-x/Ag films also have good homogeneity and a low detection limit, with a limit of detection (LOD) as low as 10−7 M, and an enhancement factor as high as 1.34 × 104. The simulated results by the finite difference in time domain (FDTD) showed substantial agreement with those of the experimental ones.

Enhancement of photoresponse for InGaAs infrared photodetectors using plasmonic WO3-x/CsyWO3-xnanocrystals.

Fast and accurate detection of light in the near-infrared (NIR) spectral range plays a crucial role in modern society, from alleviating speed and capacity bottlenecks in optical communications to enhancing the control and safety of autonomous vehicles through NIR imaging systems. Several technological platforms are currently under investigation to improve NIR photodetection, aiming to surpass the performance of established III-V semiconductor p-i-n (PIN) junction technology. These platforms include in situ-grown inorganic nanocrystals and nanowire arrays, as well as hybrid organic-inorganic materials such as graphene-perovskite heterostructures. However, challenges remain in nanocrystal and nanowire growth, large-area fabrication of high-quality 2D materials, and the fabrication of devices for practical applications. Here, we explore the potential for tailored semiconductor nanocrystals to enhance the responsivity of planar metal-semiconductor-metal (MSM) photodetectors. MSM technology offers ease of fabrication and fast response times compared to PIN detectors. We observe enhancement of the optical-to-electric conversion efficiency by up to a factor of ~2.5 through the application of plasmonically-active semiconductor nanorods and nanocrystals. We present a protocol for synthesizing and rapidly testing the performance of non-stoichiometric tungsten oxide (WO3-x) nanorods and cesium-doped tungsten oxide (CsyWO3-x) hexagonal nanoprisms prepared in colloidal suspensions and drop-cast onto photodetector surfaces. The results demonstrate the potential for a cost-effective and scalable method exploiting tailored nanocrystals to improve the performance of NIR optoelectronic devices.

Single-atom palladium on nonstoichiometric tungsten oxide as bifunctional electrocatalyst for zinc-air battery

Searching for an excellent bifunctional electrocatalyst used in zinc-air batteries (ZABs) is a long-term and challenging task. Single-atom catalysts have been becoming potential bifunctional electrocatalysts because of their maximum metal atom utilization and low content of metal itself. Herein, noble metal palladium (Pd) is uniformly anchored in the supported WO2.72 to form single-atom Pd/WO2.72 owing to the special 4d orbit of Pd and nonstoichiometric 2D planar structure of WO2.72. Single-atom 5 % Pd/WO2.72 is prepared in this work, showing excellent bifunctional electrocatalytic behaviors with high E1/2 of 0.86 V for ORR and low Ej=10 of 1.59 V for OER in alkaline medium, and the ∆E is 0.73 V, which outperforms the Pt/C and RuO2. Furthermore, the 5 % Pd/WO2.72-ZAB has a high open-circuit voltage (OCV) value of 1.491 V, a large power density of 168 mW cm−2, and a large specific capacity of 633 mAh g−1. Besides, the 5 % Pd/WO2.72-ZAB also has good cycling stability.

Room-Temperature Preparation of Platinized Nonstoichiometric Tungsten Oxide via Platinum Photodeposition Followed by Chemical Reduction: Kinetic Enhancement of Photocatalytic Oxidation and Disinfection under Low-Intensity Visible-Light Irradiation

Room-Temperature Preparation of Platinized Nonstoichiometric Tungsten Oxide via Platinum Photodeposition Followed by Chemical Reduction: Kinetic Enhancement of Photocatalytic Oxidation and Disinfection under Low-Intensity Visible-Light Irradiation

Electronic structure modulation of Ru/W20O58 catalyst via interfacial Ru–O–W bridging bond for high-performance Li–O2 batteries

The development of advanced catalysts with high activity is crucial to addressing the issues of sluggish reaction kinetics at the cathode of Li–O2 batteries. Herein, the authors propose a facile strategy to synthesize Ru nanoparticles anchored on non-stoichiometric tungsten oxide nanosheets (Ru/W20O58) hybrids. The non-stoichiometric lattice structure of W20O58 provides abundant surface nucleation sites, and the defects-induced restriction effect inhibits the agglomeration of Ru nanoparticles. The interfacial covalent coupling between Ru and W20O58 facilitates charge transfer at the interface via Ru–O–W bridging bond and optimizes the electronic structure of catalysts surface. As the obtained Ru/W20O58 employed as cathode catalysts, Li–O2 batteries exhibit high specific discharge capacity of 6346.4 mAh g−1, improved rate performance and a cycling life of 260 cycles. The density functional theory calculation demonstrate that the interfacial coupling enhances the adsorption energy of intermediates on the surface and regulates the discharge reaction pathway, leading to the restraint of O2– nucleophilic attack and the formation of amorphous and decomposable discharge products. This work provides an insight on the correlation between electrochemical activity and interfacial coupling, shedding lights on a promising cathode catalyst for advanced Li–O2 batteries.

Significant effect on the photo conversion performance of non-stoichiometric tungsten oxide with various reaction solvents

Photothermal therapy (PTT) is a new emerging treatment method in recent years which has the advantages of non-invasive and wide adaptability. The key to PTT is the development of effective photothermal agents. Herein, the non-stoichiometric tungsten oxides were prepared in a facile solvothermal method with various reaction solvents (ethanol, n-propanol, isopropanol) and time. The obtained samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscope, photoluminescence and UV–vis–NIR adsorption spectroscopy. The kind of solvent had a significant effect on the phase, microstructure, photo conversion efficiency. Once n-propanol was used as solvent, the samples possessed high crystallinity, uniform fusiform morphology, good dispersibility, obvious absorbance of NIR and the photothermal conversion efficiency of 35.6%. Under the 808 nm near-infrared light irradiation, in vitro cell tests were further conducted on L929 normal and MCF-7 tumor cells for comparative experiments. The outstanding properties make the samples highly potential agents for PTT.

Toward Effective CO2 Reduction in an Acid Medium: Electrocatalysis at Cu2O-Derived Polycrystalline Cu Sites Immobilized within the Network of WO3 Nanowires.

A hybrid catalytic system composed of copper (I)-oxide-derived copper nanocenters immobilized within the network of tungsten oxide nanowires has exhibited electrocatalytic activity toward CO2 reduction in an acid medium (0.5 mol dm-3 H2SO4). The catalytic system facilitates conversion of CO2 to methanol and is fairly selective with respect to the competing hydrogen evolution. The preparative procedure has involved voltammetric electroreduction of Cu2O toward the formation and immobilization of catalytic Cu sites within the hexagonal structures of WO3 nanowires which are simultaneously partially reduced to mixed-valence hydrogen tungsten (VI, V) oxide bronzes, H x WO3, coexisting with sub-stoichiometric tungsten (VI, IV) oxides, WO3-y . After the initial loss of Cu through its dissolution to Cu2+ during positive potential scanning up to 1 V (vs RHE), the remaining copper is not electroactive and seems to be trapped within in the network of hexagonal WO3. Using the ultramicroelectrode-based probe, evidence has also been provided that partially reduced nonstoichiometric tungsten oxides induce reduction of CO2 to the CO-type reaction intermediates. The chronocoulometric data are consistent with the view that existence of copper sites dispersed in WO3 improves electron transfers and charge propagation within the hybrid catalytic layer. The enhanced tolerance of the catalyst to the competitive hydrogen evolution during CO2R should be explained in terms of the ability of H x WO3 to consume protons and absorb hydrogen as well as to shift the proton discharge at Cu toward more negative potentials. However, the capacity of WO3 to interact with catalytic copper and to adsorb CO-type reaction intermediates is expected to facilitate removal of the poisoning CO-type adsorbates from Cu sites.

Synergizing Inter and Intraband Transitions in Defective Tungsten Oxide for Efficient Photocatalytic Alcohol Dehydration to Alkenes.

Photocatalysis under mild conditions is an intriguing avenue for organic chemical manufacturing to confront the serious fossil energy crisis. Herein, we report a direct light-driven alkene production through alcohol dehydration, using nonstoichiometric tungsten oxide of W18O49 nanowires with abundant lattice defects as a photocatalyst. A representative ethylene (C2H4) production rate of 275.5 mmol gcat–1 h–1 is achieved from ethanol (C2H5OH) dehydration, together with excellent selectivity up to 99.9%. The universality of our approach is further demonstrated with other alcohol dehydration. Combining ultrafast transient absorption spectroscopy with in situ X-ray photoelectron spectroscopy, we underline that the inter- and intraband transitions synergistically contribute to such excellent activity. In particular, the intraband transition excites the electrons in defect bands into an energetically “hot” state, largely alleviating the charge recombination. As a result, the C–OH bond of chemisorbed C2H5OH molecules can be effectively dissociated to furnish the formation of C=C bonds. Our work offers a fresh insight into sustainable alkene production with renewable energy input under mild conditions.

Enhanced electrochromism from non-stoichiometric electrodeposited tungsten oxide thin films

Nanocrystalline structure tungsten oxide (nc-WO3; WO3·2H2O) thin films were deposited on fluorine-doped tin oxide (FTO) substrates via a facile electrochemical deposition (ECD) method. The X-ray diffraction (XRD), Fourier Transform Infrared (FT-IR) spectrum, and scanning electron microscope (SEM) results confirmed the gradual temperature dependent structural transitions from nanocrystalline tungsten oxide (nc-WO3) to amorphous tungsten oxide (a-WO3) at 200 °C. Electrochromic (EC) performances of the films were examined in an aqueous solution of 0.5 M H2SO4 by cyclic voltammetry tests integrated in-situ transmittance measurements. The results showed that the EC properties of WO3 films were closely correlated to its structural arrangements of –OH groups and the structural water molecules present in the WO3 matrix. A fast-switching speed (5 s for coloration and 3 s for bleaching), a significant optical modulation (above 89 % at 632 nm), a high coloration efficiency of 51.1 cm2/C and cycling stability of more than 1000 cycles were achieved for nc-WO3 film. Additionally, an EC device was successfully assembled using NiO film prepared by chemical bath deposition as a complementary electrode and exhibited fascinating performance.

Formation mechanism of nanocrystalline W derived cubic-H0.5WO3

In general, thermal treatment of W produces non-stoichiometric tungsten oxides under suitably reducing atmosphere. Anomalously, the present work reports the cubic-H0.5WO3 by heat treatment of nanocrystalline W under H2O(v) atmosphere. The nanocrystalline W readily oxidizes at lower temperatures due to its enhanced oxidation kinetics compared to the as-received form. The W reacts with H2O(v), which forms γ-WO3 along with hydrogen as a byproduct. Followingly, the H intercalation to the interstitial sites of WO3 results in nanocrystalline cubic-H0.5WO3. The crystallographic relationship of this formation pathway follows (110)W//(010)WO3//(010)H0.5WO3. As per the unit cell volumes of WO3 and H0.5WO3, a single unit cell of WO3 turns into 8 unit cells of H0.5WO3 with H atom intercalation.

WO3 and WO3-x thin films prepared by DC hollow cathode discharge

Photoactive semiconducting crystalline films of non-stoichiometric tungsten oxides (WO3-x, 0 ≤ x ≤ 3) were deposited by DC hollow cathode discharge from a tungsten nozzle in a reactive Ar + O2 atmosphere. The ratio of tungsten to oxygen in the layer was driven by controlling the O2 flow. All layers were prepared on soda-lime glass substrate (SLG) and on glass substrates coated with an FTO thin film and after deposition were annealed in air or argon atmosphere. The properties of WO3-x layers were analyzed with respect to their potential applications. Surface morphology was analyzed by SEM, qualitative phase analysis by XRD and Raman spectroscopy, chemical composition by EDX and photoactivity of individual samples by linear voltammetry. The layers stoichiometrically closest to WO3 and annealed in an argon atmosphere showed the highest photoactivity response.

Bulk synthesis of tungsten-oxide nanomaterials by a novel, plasma chemical reactor configuration, studies on their performance for waste-water treatment and hydrogen evolution reactions

We report a novel, yet simple, DC non-transferred thermal plasma torch assisted, green, plasma-chemical reactor configuration, for high-rate synthesis of tungsten-oxide nanomaterials, up to two hundred grams per hour. An expanded argon/oxygen plasma beam is produced with a collimated, frozen structure, which interacts uniformly and in a controlled manner, over a large area of the remotely placed, tungsten target, to produce superfine particles in bulk amounts and without mixed with large aggregates. The target surface gets oxidized to form volatile tungsten-oxide, from which nanoparticles are nucleated by gas phase condensation process. Optical emission spectroscopic studies reveal that a massive argon plasma ion density leads to the production of reactive atomic oxygen radicals that were responsible for the rapid oxidation kinetics. Non-stoichiometric tungsten-oxide in pale blue colour and nanometer sizes were deposited on the water-cooled walls of the vacuum chamber. This sample was annealed to produce nanocrystalline, tungsten-oxide in yellow colour, which was characterized with narrow size distribution and fine material crystallinity. Functional properties of the synthesized nanomaterials were also measured, their photocatalytic characteristics for the degradation of the dye Rhodamine-B and antibacterial properties against E. coli bacteria. The annealed powder sample could remove the dye almost completely in about three hours. The same nanopowders demonstrated promising antibacterial properties, which may therefore lead to multi-functionality during the treatment of wastewater. Their hydrogen evolution reaction activity is also investigated that demonstrates moderate performance.

Modulation of resistive switching properties of non-stoichiometric WO3−x based asymmetric MIM structure by interface barrier modification

The impact of device operation condition and ambient moisture on the interface-type resistive switching (RS) characteristics of a non-stoichiometric polycrystalline tungsten oxide (WO3−x) based metal–insulator–metal device with an Au top electrode and a Pt bottom electrode has been investigated. The device exhibits rectification and stable bipolar RS characteristics without the need for any forming step, where the switching is primarily dominated by the Schottky type Au/WO3−x interface. DC conduction characteristics of the device have been investigated at different temperature, bias stress, and relative humidity conditions. Current conduction through the active layer has been found to be dominated by Schottky emission at low electric field and Poole–Frenkel emission at high electric field. An increase in current and a strong reduction in the rectification characteristic have been observed on subjecting the device to DC bias stress of appropriate polarity as well as increasing ambient moisture. Modification of the Schottky barrier due to defect redistribution when DC bias stress is applied and due to the dipoles induced at the Au/WO3−x interface by water molecules with increasing ambient moisture content have been discussed as a possible mechanism of the observed RS modulation.

In situ interfacial optimization of CoOOH/W18O49/NF heterojunction for boosted water-splitting performance

The development of heterostructure electrocatalysts with abundant active sites as binderless electrodes is crucial for the improvment of the catalytic efficiency. Herein, the CoOOH/W18O49 heterojunction grown on nickel foam (abbreviated as Pre-CoOOH/W18O49/NF) was fabricated via a facile solvothermal method. It is found that in situ electrochemical reconstruction induces the amorphization of the non-stoichiometric tungsten oxide and effectively changes the interfacial interaction, which enables the electrons at the heterojunction interfaces to be redistributed. The Co2+/Ni2+ ions successfully optimize the valence configuration of tungsten (W) sites, leading to a high proportion of W4+ and Co3+/Ni3+ active sites. Simultaneously, the potential of the flat band becomes more negative than the reduction potential of hydrogen for the water splitting, so as to achieve enhanced intrinsic hydrogen evolution activity. As expected, the reconstruction contributes to more efficient electrocatalytic activity compared with that of the direct hydrogen/oxygen evolution in alkaline solution. The activated CoOOH/W18O49/NF electrode (abbreviated as Act-CoOOH/W18O49/NF) with the optimized amorphous-crystalline interface shows low overpotentials of 89 and 198 mV at 10 and 20 mA cm−2 for HER and OER, respectively. It only needs 1.55 V (10 mA cm−2) decomposition voltage for overall water splitting and retains a high stability at least 50 h.

Plasmonic semiconductor photocatalyst: Non-stoichiometric tungsten oxide

Semiconductor photocatalysis has attracted increasing attention due to its potential application in solving the problems related to energy crisis and environmental pollution. As a typical plasmonic semiconductor, non-stoichiometric tungsten oxide (WO3-X) has invoked significant interest for its unique property and excellent photocatalytic performance. In this review, we briefly introduce the fundamental properties of the WO3-x, and then summarize the synthesis methods such as solvothermal reaction, solid phase reduction and exfoliation treatment, together with the modification strategies such as doping and constructing homo-/hetero-junctions. Additionally, we emphasize the practical applications of WO3-x in hydrogen evolution, nitrogen fixation, carbon dioxide reduction, and pollutant degradation. Finally, comprehensive conclusions and perspectives on the fabrication of WO3-x photocatalyst leading to satisfactory performance are given as well.

Hydrogen permeability of non-stoichiometric tungsten oxides

This paper presents results from an investigation of the permeation rate of gaseous hydrogen through a double-layer membrane by the classic gas accumulation method at 400 °C. Dense 8-micrometer thick tungsten films were grown on highly permeable 40 mm diameter Eurofer substrates by the combined magnetron sputtering and high energy ion bombardment method. By in-situ exposure of the tungsten side to pure oxygen at 0.9 bar for 2 hours, a noticeable decrease in permeation rates at 400°C was achieved, expressed as the permeation rate reduction factor, which ranged from 3.8 to 38. Oxide characterisation followed after permeation rate tests. Focused ion beam cross-section after the oxidation/permeation rate cycle revealed an oxide layer thickness between 80 and 100 nm. X-ray diffraction and Raman analyses revealed a mixture of many non-stoichiometric oxide types after the oxidation/permeation rate cycles. Their corresponding hydrogen permeability at 400°C in the range from 8.1 × 10−18 – 5.7 × 10−17 mole H2/s/m/Pa0.5 was calculated from measured data.

СІНТЕЗ І ФОТОКАТАЛІТИЧНІ ВЛАСТИВОСТІ КОМПОЗИЦІЙНИХ ПОКРИТТІВ НА ОСНОВІ КОБАЛЬТУ З ТУГОПЛАВКИМИ КОМПОНЕНТАМИ

The possibility of electrosynthesis and control of the surface composition and morphology of the electrolytic cobalt coatings with refractory metals by varying the parameters of electrolysis has been proved. It was found that oxygen and carbon are included in the composition of the coatings as well as the main components, thus such systems can be considered as composite. The coatings deposited by pulsed current can be considered as composite materials the oxide phase for which is formed directly in the electrode process as an intermediate of incomplete reduction of tungstates and hydrolysis of zirconium (IV) salts. The topography of the films is distinguished by the presence of elliptical and spherical grains with crystallite sizes of 80 - 180 nm. On the surface of the coatings, there are hills (large grains) with a diameter of 1 - 3 μm. The fractal dimension of the surface is 2.77, which indicates the 3D mechanism of crystal growth during the formation of coatings. In terms of phase composition, composites are predominantly amorphous materials that contain nanocrystalline cobalt and the intermetallic compound Co3W and Zr3Co. The study of the morphology and topography of the composite coatings surface, as well as its quantitative and phase composition, indicates the possibility of photocatalytic activity of the Co-Mo-WOх, Co-Mo-ZrO2 and Co-W-ZrO2 coatings. Investigation of the photodegradation of the azo dye methyl orange found that the efficiency of MO removal from the solution was 24%, 18%, and 10% for 30 min of ultraviolet irradiation in the presence of Co-Mo-WOх, Co-Mo-ZrO2 and Co-W-ZrO2 on composite coatings, respectively. The higher photoactivity of Co-Mo-WOx composite coatings can be explained by the presence of non-stoichiometric molybdenum and tungsten oxides.

Review: Oxygen-deficient tungsten oxides

Oxygen-deficient/non-stoichiometric tungsten oxides are a distinct class between two only stoichiometric tungsten oxides, i.e., WO3 and WO2. The WO3 could be termed as a parent structure while understanding the non-stoichiometric tungsten oxides. The oxygen vacancies (Ovacs) induced in the parent (WO3) structures cause the re-adjustment of atomic arrangement to compensate for the oxygen deficiency and follow the crystal chemistry. The degree of Ovacs determines the range of WO3-x. The W18O49 (WO2.722) is the highest oxygen-deficient stable phase. These non-stoichiometric oxides proved superior to WO3 for various applications. Thus, the composition, chemistry, crystallography, synthesis, and formation mechanisms of such materials have been elaborated. Further, the superiority of the materials in view of few applications is well discussed. Finally, the importance of non-stoichiometry/Ovacs in metal oxides for various functional applications can be understood.

Advanced developments in nonstoichiometric tungsten oxides for electrochromic applications

This review summarizes the recent progress of WO3−x for switchable smart devices, focusing on the synthesis, film fabrication, and strategies to improve EC properties, followed by challenges and future development in multifunctional devices.

Microwave-assisted in-situ incorporation of nanostructured HyWO3-x on cotton cellulose for enhanced ultraviolet shielding applications

Nanostructured materials modified cotton cellulose could be a versatile and cost-effective solution for developing wide range of protective fabrics. In this background, cotton cellulose was surface modified with hydrated form of non-stoichiometric tungsten oxide (HyWO3-x) using microwave reduction technique for the development of UV radiation blocking and thermo-chromic fabrics. Herein, the surface modification and characterization of cotton cellulose having ligand to metal charge transfer (LMCT) transition via reduction process with enhanced UV blocking features are presented. X-ray diffraction patterns confirmed the poly-crystalline and amorphous nature of unmodified and surface modified cotton fabrics. Scanning electron micrographs revealed the growth of nanostructured HyWO3-x on cotton cellulose and surface analysis showed the existence of tungsten, carbon, oxygen elements. Differential scanning calorimetry of treated fabric exhibited a change of exothermic behavior from endothermic at the characteristic onset temperature. Thermogravimetric analysis revealed the least weight loss up to 600°C of the surface treated cotton cellulose. Further, Ultraviolet Protection Factor (UPF) of the modified fabric was measured using an in-vitro method following the AATCC 183:2004 standard employing UV transmittance analyzer. The UPF was estimated to be 2000, which is excellent for protective clothing applications according to UPF standard for fabrics. Also, the ligand to metal charge transfer characteristics of WO3-x modified cotton cellulose revealed the selective transition towards microwave irradiation at different reduction power.