Tungsten Oxide Nanostructures Research Articles

A Dissolution‐Regeneration Route to Synthesize Blue Tungsten Oxide Flowers and their Applications in Photocatalysis and Gas Sensing

Tungsten oxide (WO3) has unique physicochemical properties. Although various synthetic methods have been proposed to fabricate tungsten oxide with different morphologies, methods of tailoring the tungsten oxide nanostructure toward new morphologies are still needed. In this work, a self‐assembled WO3·0.33H2O nanoflower is synthesized using a dissolution and regeneration method. In such a two‐step synthetic process, temperature, solvent, and reaction times are found to be the main parameters affecting the morphology of WO3·0.33H2O. The synthesized WO3·0.33H2O nanoflower exhibits a localized surface plasmon resonance effect and a high photocatalytic activity, which can be explained by the doping of W5+ and its morphology. Furthermore, this WO3·0.33H2O nanoflower can also be used as a gas sensor and has good selectivity and linearity towards acetone vapor in the range of 50–500 ppm.

Synthesis, Characterization and Photocatalytic Activity of Tungsten Oxide Nanostructures

Synthesis, Characterization and Photocatalytic Activity of Tungsten Oxide Nanostructures

Synthesis and optical properties of Au decorated colloidal tungsten oxide nanoparticles

In this study, colloidal tungsten oxide nanoparticles were fabricated by pulsed laser ablation of tungsten target using the first harmonic of a Nd:YAG laser (1064nm) in deionized water. After ablation, a 0.33g/lit HAuCl4 aqueous solution was added into as-prepared colloidal nanoparticles. In this process, Au3+ ions were reduced to decorate gold metallic state (Au0) onto colloidal tungsten oxide nanoparticles surface. The morphology and chemical composition of the synthesized nanoparticles were studied by AFM, XRD, TEM and XPS techniques. UV–Vis analysis reveals a distinct absorption peak at ∼530nm. This peak can be attributed to the surface plasmon resonance (SPR) of Au and confirms formation of gold state. Moreover, X-ray photoelectron spectroscopy reveals that Au ions’ reduction happens after adding HAuCl4 solution into as-prepared colloidal tungsten oxide nanoparticles. Transmission electron microscope shows that an Au shell has been decorated onto colloidal WO3 nanoparticles. Noble metal decorated tungsten oxide nanostructure could be an excellent candidate for photocatalysis, gas sensing and gasochromic applications. Finally, the gasochromic behavior of the synthesized samples was investigated by H2 and O2 gases bubbling into the produced colloidal Au/WO3 nanoparticles. Synthesized colloidal nanoparticles show excellent coloration contrast (∼80%) through NIR spectra.

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.

Detection of volatile organic compounds using flexible gas sensing devices based on tungsten oxide nanostructures functionalized with Au and Pt nanoparticles

Flexible gas sensor devices are fabricated and optimized by integrating directly, via a single-step vapor-phase deposition method, highly crystalline tungsten oxide nanostructures functionalized with either gold or platinum nanoparticles. Gas tests of these devices show significant improvements with respect to flexible gas sensors based on non-functionalized structures, including greater responses to various volatile organic compounds (ethanol, acetone, methanol and toluene) and better selectivity towards ethanol and methanol, as demonstrate results for the sensors based on platinum-functionalized structures. The method presented here, which includes the fabrication of the whole flexible gas sensing device and the integration of functional nanostructures without the use of transfer methods, provides a simpler, faster and inexpensive method for the fabrication of highly functional flexible microsystems for gas sensing.

PH-controllable synthesis of unique nanostructured tungsten oxide aerogel and its sensitive glucose biosensor

This work presents a controllable synthesis of nanowire-networked tungsten oxide aerogels, which was performed by varying the pH in a polyethyleneimine (PEI)-assisted hydrothermal process. An enzyme–tungsten oxide aerogel co-modified electrode shows high activity and selectivity toward glucose oxidation, thus holding great promise for applications in bioelectronics.

Tungsten oxide thin films obtained by anodisation in low electrolyte concentration

Tungsten oxide nanostructured films were grown on tungsten substrates by anodisation under a fixed voltage and with sodium fluoride as electrolyte. The effect of the anion chloride and the influence of the modifying agent disodium hydrogen phosphate in the tungsten oxide films were also investigated. The structural characterisation of the films was performed by scanning electron microscopy, atomic force microscopy and Raman spectroscopy. The band gap was determined through diffuse reflectance spectroscopy. The thin films were photoluminescent and emitted in the range of 300 to 630nm when irradiated at 266nm. The synthesised films efficiently degraded of methyl orange dye in the presence of hydrogen peroxide and 250nm radiation. The modifying agent was responsible for the improvement of the photocatalytic activity. Films with similar photocatalytic performance were obtained when the system sodium fluoride and disodium hydrogen phosphate were replaced by sodium chloride. The porous structure and low band gap values were responsible for the photocatalytic behaviour.

Morphology-controlled preparation of tungsten oxide nanostructures for gas-sensing application**Project supported by the National Natural Science Foundation of China (Grant Nos. 61274074 and 61271070) and the Natural Science Foundation of Tianjin, China (Grant No. 11JCZDJC15300).

A novel three-dimensional (3D) hierarchical structure and a roughly oriented one-dimensional (1D) nanowire of WO3 are selectively prepared on an alumina substrate by an induced hydrothermal growth method. Each hierarchical structure is constructed hydrothermally through bilateral inductive growth of WO3 nanowire arrays from a nanosheet preformed on the substrate. Only roughly oriented 1D WO3 nanowire can be obtained from a spherical induction layer. The analyses show that as-prepared 1D nanowire and 3D hierarchical structures exhibit monoclinic and hexagonal phases of WO3, respectively. The gas-sensing properties of the nanowires and the hierarchical structure of WO3, which include the variations of their resistances and response times when exposed to NO2, are investigated at temperatures ranging from room temperature (20 °C) to 250 °C over 0.015 ppm–5 ppm NO2. The hierarchical WO3 behaves as a p-type semiconductor at room temperature, and shows p-to-n response characteristic reversal with the increase of temperature. Meanwhile, unlike the 1D nanowire, the hierarchical WO3 exhibits an excellent response characteristic and very good reversibility and selectivity to NO2 gas at room temperature due to its unique microstructure. Especially, it is found that the hierarchical WO3-based sensor is capable of detecting NO2 at a ppb level with ultrashort response time shorter than 5 s, indicating the potential of this material in developing a highly sensitive gas sensor with a low power consumption.

Room temperature selective oxidation of aniline to azoxybenzene over a silver supported tungsten oxide nanostructured catalyst

Heterogeneous catalysts comprising silver nanoparticles supported on nanostructured tungsten oxide were applied for room temperature oxidative coupling of aniline to azoxybenzene, an important chemical intermediate and a chemical of industrial interest.

Exposed facet and crystal phase tuning of hierarchical tungsten oxide nanostructures and their enhanced visible-light-driven photocatalytic performance

Tungsten oxide hierarchical nanostructures controllably assembled with one dimensional nanostructures which exhibit different exposed facets and crystal phases were synthesized via a facile hydrothermal reaction assisted by urea.

Tailoring nanoscale properties of tungsten oxide for inkjet printed electrochromic devices.

This paper focuses on the engineering procedures governing the synthesis of tungsten oxide nanocrystals and the formulation of printable dispersions for electrochromic applications. By that means, we aim to stress the relevancy of a proper design strategy that results in improved physicochemical properties of nanoparticle loaded inks. In the present study inkjet printable nanostructured tungsten oxide particles were successfully synthesized via hydrothermal processes using pure or acidified aqueous sol-gel precursors. Based on the proposed scheme, the structure and morphology of the nanoparticles were tailored to ensure the desired printability and electrochromic performance. The developed nanomaterials with specified structures effectively improved the electrochemical response of printed films, resulting in 2.5 times higher optical modulation and 2 times faster coloration time when compared with pure amorphous films.

Determination of gas sensing properties of thermally evaporated WO3 nanostructures

Thin films of tungsten oxide (WO3) nanostructures were grown on indium tin oxide coated glass substrates by thermal evaporation technique under oxygen and argon (O2/Ar) mixed gas atmosphere. The films were characterized by X-ray diffraction, Raman spectroscopy and field emission scanning electron microscopy to study the structural and morphological properties of the grown films. Three different shapes of nanostructures (one dimensional nanorods, two dimensional nanosheets and three dimensional nanosized orthorhombic structures) were formed due to the variations of growth conditions such as substrate temperature and oxygen/argon flow rate. In particular, the flow rate of oxygen plays an important role in controlling the nucleation and growth of WO3 nanostructures. The ethanol gas sensing properties of the films were investigated under different concentrations (10–50 ppm) at room temperature, which reveals that the sensitivity of the sensor was greatly enhanced with the increasing gas concentration. This result indicates that the WO3 films are good candidate for sensing ethanol gas in low concentration at room temperature.

Single-step co-deposition of nanostructured tungsten oxide supported gold nanoparticles using a gold–phosphine cluster complex as the gold precursor

The use of a molecular gold organometallic cluster in chemical vapour deposition is reported, and it is utilized, together with a tungsten oxide precursor, for the single-step co-deposition of (nanostructured) tungsten oxide supported gold nanoparticles (NPs). The deposited gold-NP and tungsten oxide supported gold-NP are highly active catalysts for benzyl alcohol oxidation; both show higher activity than SiO2 supported gold-NP synthesized via a solution-phase method, and tungsten oxide supported gold-NP show excellent selectivity for conversion to benzaldehyde.

Hetero- and homogeneous three-dimensional hierarchical tungsten oxide nanostructures by hot-wire chemical vapor deposition

We present the synthesis of three-dimensional tungsten oxide (WO3−x) nanostructures, called nanocacti, using hot-wire chemical vapor deposition. The growth of the nanocacti is controlled through a succession of oxidation, reduction and re-oxidation processes. By using only a resistively heated W filament, a flow of ambient air and hydrogen at subatmospheric pressure, and a substrate heated to about 700°C, branched nanostructures are deposited. We report three varieties of simple synthesis approaches to obtain hierarchical homo- and heterogeneous nanocacti. Furthermore, by using catalyst nanoparticles site-selection for the growth is demonstrated. The atomic, morphological and crystallographic compositions of the nanocacti are determined using a combination of electron microscopy techniques, energy-dispersive X-ray spectroscopy and electron diffraction.

Synthesis of tungsten oxide nanoparticles using a hydrothermal method at ambient pressure

Abstract

Exposed facets induced enhanced acetone selective sensing property of nanostructured tungsten oxide

WO3 nanorods with exposed (100) and (002) facets were fabricated via a hydrothermal route using different directing agents. Microspheres that were assembled randomly by WO3 nanorods with exposed (002) facets were porous with a specific surface area of 62.2 m2 g−1. Acetone gas sensing properties of the as-prepared sensors were investigated for the breath diagnosis of diabetes. The acetone detection limit of WO3 microspheres was 0.25 ppm, and the response (Ra/Rg) to 1 ppm acetone was 3.53 operated at 230 °C with a response and recovery time of 9 and 14 s, respectively. WO3 samples with exposed (002) facets exhibited better acetone sensitivity and selectivity than those with (100) facets. The sensing mechanism was discussed in detail. Because of the asymmetric distribution of unsaturated coordinated O atoms in the O-terminated (002) facets, the surface had an extent of polarization. Induced local electric dipole moment contributed to easy adsorption and reaction between the (002) facets and acetone molecules with a large dipole moment at low working temperatures. Room temperature PL spectra clearly displayed that the WO3 samples exposed with (002) facets had numerous oxygen vacancies and defects, resulting in excellent acetone sensing properties.

Selective Oxidation of Propylene to Propylene Oxide over Silver-Supported Tungsten Oxide Nanostructure with Molecular Oxygen

Propylene oxide (PO) is a versatile chemical intermediate, and by volume it is among the top 50 chemicals produced in the world. The catalytic conversion of propylene to PO by molecular oxygen with minimum waste production is of high significance from an academic as well as an industrial point of view. We have developed a new synthesis strategy to prepare 2–5 nm metallic silver nanoparticles (AgNPs) supported on tungsten oxide (WO3) nanorods with diameters between 30 and 40 nm, in the presence of cationic surfactant (cetyltrimethylammonium bromide: CTAB), capping agent (polyvinylpyrrolidone: PVP), and hydrazine. The synergy between the surface AgNPs and WO3 nanorods facilitates the dissociation of molecular oxygen on the metallic Ag surface to produce silver oxide, which then transfers its oxygen to the propylene to form PO selectively. The catalyst exhibits a PO production rate of 6.1 × 10–2 mol gcat–1 h–1, which is almost comparable with the industrial ethylene-to-ethylene oxide production rate.

Micromachined gas sensors based on tungsten oxide nanoneedles directly integrated via aerosol assisted CVD

Micromachined gas sensors based on tungsten oxide nanostructures (NSs) in the form of polycrystalline films, non-aligned and quasi-aligned nanoneedles (NNs) films are fabricated by integrating various microsystems technology steps with aerosol assisted chemical vapour deposition (AACVD). Measurement of the performance of these structures to various analytes, including NO2, H2, EtOH, H2S, CO, C6H6, demonstrate enhanced sensing properties for gas sensors based on films comprised of NNs as opposed to polycrystalline, with non-aligned NNs films showing the greatest sensor responses. A marked contribution of the NNs diameter to the sensor response is noticed, particularly for sensors composed of NNs with diameters between 25 and 50nm. In addition, fabrication variables such as the NNs arrangement and the electrode geometry suggest their contribution to the sensor response is connected with the effects of the gaseous analytes at the surface, i.e. whether these analytes act as reducing or oxidizing agents.

Synthesis and Characterization of Nanostructured Tungsten Oxide by Hard Template Method

Nanostructured WO3 was synthesized by a hard template method using platelet-shaped mesoporous silica SBA-15. The nanostructured WO3 consisted of nanorod arrays comprised of mixed crystalline tetragonal and monoclinic phases with the diameter of 8.5 nm and the length of 170-300 nm. The specific surface area was 30 m2/g, which was about 6 times higher compared to that synthesized without the template.