Tungsten Oxide Nanorods Research Articles

Room temperature NO2-sensing properties of hexagonal tungsten oxide nanorods**Project supported by the National Natural Science Foundation of China (Grant Nos. 60771019, 61271070, and 61274074) and the Tianjin Key Research Program of Application Foundation and Advanced Technology, China (Grant No. 11JCZDJC15300).

Hexagonal WO3 nanorods were synthesized through a facile hydrothermal method. The nanorods properties were investigated by scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD). The NO2-sensing performances in terms of sensor response, response/recovery times and repeatability at room temperature were optimized by varying the heat treatment temperature of WO3 nanorods. The optimized NO2 sensor (400-°C-annealed WO3 nanorods) showed an ultra-high sensor response of 3.2 and short response time of 1 s to 5-ppm NO2. In addition, the 400-°C-annealed sample exhibited more stable repeatability. Furthermore, dynamic responses measurements of annealed samples showed that all the annealed WO3 nanorods sensors presented p-type behaviors. We suppose the p-type behavior of the WO3 nanorods sensor to be that an inversion layer is formed in the space charge layer when the sensor is exposed to NO2 at room temperature.Therefore, the 400-°C-annealed WO3 nanorods sensor is one of the most energy conservation candidates to detect NO2 at room temperature.

Direct Growth of Crystalline Tungsten Oxide Nanorod Arrays by a Hydrothermal Process and Their Electrochromic Properties

Transparent crystalline tungsten oxide nanorod arrays for use as an electrochromic layer have been directly prepared on fluorine-doped tin oxide-coated glass via a facile tungsten film-assisted hydrothermal process using aqueous tungsten hexachloride solution. X-ray diffraction analysis and field-emission scanning electron microscopy were used to characterize the phase and morphology of the grown nanostructures. Arrays of tungsten oxide nanorods with diameter of ∼22 nm and length of ∼240 nm were obtained at 200°C after 8 h of hydrothermal reaction. We propose a growth mechanism for the deposition of the monoclinic tungsten oxide phase in the hydrothermal environment. The tungsten film was first oxidized to tungsten oxide to provide seed sites for crystal growth and address the poor connection between the growing tungsten oxide and substrate. Aligned tungsten oxide nanorod arrays can be grown by a W thin film-assisted heterogeneous nucleation process with NaCl as a structure-directing agent. The fabricated electrochromic device demonstrated optical modulation (coloration/bleaching) at 632.8 nm of ∼41.2% after applying a low voltage of 0.1 V for 10 s, indicating the potential of such nanorod array films for use in energy-saving smart windows.

Micro-contacted self-assembled tungsten oxide nanorods for hydrogen gas sensing

Electron beam lithography was used to fabricate platinum μ-contacts over tungsten oxide nanorods formed on a mica substrate. This made possible the measurement of sensorial response of these self-assembled tungsten oxide nanorods to hydrogen gas for the first time. The nanorods were prepared by thermal evaporation from an oxide source. Consequently, two types of conductometric sensors were assembled: a) percolating network of nanorods and b) set of individually contacted WO3 nanorods. The preparation procedures are described in detail and the comparison of response of both types of assemblies is given. The first sensorial measurements revealed a good response of the b) type of sensor and the minimum repeatedly detected concentration of H2 was 50 ppm.

Branched Tungsten Oxide Nanorod Arrays Synthesized by Controlled Phase Transformation for Solar Water Oxidation

AbstractBranched (NH4)0.33⋅WO3 nanorods have been successfully synthesized by applying a simple hydrothermal approach with ammonium oxalate as capping agents. After annealing them in different atmospheres, the (NH4)0.33⋅WO3 nanorods were converted to WO3 nanorods. Atmosphere‐dependent morphology evolution and phase transformation by annealing of (NH4)0.33⋅WO3 were observed. The branched WO3 nanorods produce a higher photoelectrochemical activity than the bare WO3 nanorods without branches obtained under similar conditions, and the improvement of efficiency by branched nanorods was ascribed to the better light‐trapping characteristics and increase in surface area, which facilitates holes transfer at the WO3/electrolyte interface. The possible formation mechanisms of branched nanorods have been explored and the results show that the synergistic interaction between ammonium ion (NH4+) and acetate ion (−COO−) contributes to the formation of branched (NH4)0.33⋅WO3 nanorods.

Three-dimensional nitrogen-doped graphene frameworks anchored with bamboo-like tungsten oxide nanorods as high performance anode materials for lithium ion batteries

Bamboo-like WO3 nanorods were anchored on three-dimensional nitrogen-doped graphene frameworks (r-WO3/3DNGF) by a facile one-step hydrothermal synthesis plus heating processes. There is a strong dependence of the obtained r-WO3/3DNGF nanostructures on the content of 3DNGF. The composite with 20 wt% 3DNGF content shows the most favorable structure where bamboo-like WO3 nanorods lie flat on the surface of fungus-like 3DNGF, and exhibits a high discharge capacity of 828 mAh g−1 over 100 cycles at 80 mA g−1 with the largest capacity retention of 73.9% for WO3 and excellent rate capacities of 719, 665, 573, 453 and 313 mAh g−1 at 80, 160, 400, 800 and 1600 mA g−1, respectively. The electrochemical performance is better than most of reported WO3-based carbonaceous composites, which can be attributed to the synergistic effects of the following actions: i) WO3 nanorods effectively shorten the diffusion path of Li+; ii) mechanically strong 3DNGF alleviates the huge volume change of WO3 upon Li+ intercalation/extraction; and iii) nitrogen-doping in 3D graphene frameworks improves electronic conductivity and provides large numbers of lithium ion diffusion channels.

Synthesis of high aspect ratio WO2 nanostructures

Tungsten oxide (WO2) nanorods and nanowires were prepared by the heat treatment of WO2 nanocrystalline powders in the presence of Ar. Nanocrystalline powders produced by a simple water-assisted route at the room temperature were annealed at different temperatures for different durations, which yielded orthorhombic and monoclinic WO2 crystals. Annealing the powders at 700 °C and above resulted in orthorhombic WO2 nanorods/nanowires with an average diameter of 60–70 nm beside the monoclinic WO2 nanocrystalline powders with a diameter of ~5 nm. The lengths of the nanorods increased from several 100 nm up to several 10 µm with increasing temperature while their diameters did not change. With increased length, nanowires became more elastic in nature having a cotton-like fabric. The prepared nanostructures have been characterized by X-ray powder diffraction measurements, scanning electron microscopy, transmission electron microscopy, and high-resolution transmission electron microscopy. An oriented attachment mechanism leading to root growth from a parent structure was proposed. High aspect ratio WO2 nanostructures with an average diameter of 60–70 nm and lengths changing from several 100 nm up to several 10 µm were synthesized by a simple annealing process under atmospheric pressure from WO2 nanoparticles (~ 30 nm) and a possible formation mechanism was proposed.

TUNGSTEN OXIDE NANOSTRUCTURES FOR ENERGY STORAGE AND FIELD EMISSION APPLICATIONS

Single crystalline tungsten oxide (WO3) nanorods were synthesized over flexible carbon fabric via simple hydrothermal method and used directly as supercapacitor (SC) electrode. WO3 nanorods with their peculiar structural features over the fabric exhibited excellent electrochemical performance with a high specific capacitance of 481.34 F/g at scan speed 10 mV s -1 . Additionally, these nanorods showed propitious as electron field emitters where the registered turn on field and estimated field enhancement values were 3.6 V/µm and 4550 respectively. These results indicate that present unique WO3 nanorods arrays over flexible carbon cloth substrate are a promising candidate for constructing high-performance solid state electronics and electrochromic devices.

Synthesis of Tungsten Oxide Nanorod, Its Application on Textile Material, and Study of Its Functional Properties

Nanomaterial and its application in textiles are emerging as vast and diverse field due to enhanced functionalized characteristics. This study emphasizes the fabrication of tungsten trioxide nanostructured rods and analyzes its electrostatic and ultraviolet resistance properties. These nanorods are synthesized by hydrothermal method. Through hydrothermal method rod like nanostructures were grown on polyester fabric as it withstands curing temperature easily. The growth mechanism of the film is investigated. Electrostatic analysis of treated polyester fabric was failed but the analysis of seeded solution revealed that it has tunable transmittance modulation under different voltages and repetitive cyclic between the clear and blue states. Ultraviolet resistance of 100% seeded polyester fabric was higher than untreated fabric with respect to increasing concentration of nanorods. Results show that although the seeded solution is perfect, the conductivity of tungsten trioxide cannot be achieved on textiles.

Near-infrared selective dynamic windows controlled by charge transfer impedance at the counter electrode.

Recent developments in the exploitation of transparent conductive oxide nanocrystals paved the way to the realization of a new class of electrochemical systems capable of selectively shielding the infrared heat loads carried by sunlight and prospected the blooming of a key enabling technology to be implemented in the next generation of "zero-energy" building envelopes. Here we report the fabrication of a set of electrochromic devices embodying an engineered nanostructured electrode made by high aspect-ratio tungsten oxide nanorods, which allow for selectively and dynamically controlling sunlight transmission over the near-infrared to visible range. Varying the intensity of applied voltage makes the spectral response of the device change across three different optical regimes, namely fully transparent, near-infrared only blocking and both visible and near-infrared blocking. It is demonstrated that the degree of reversible modulation of the thermal radiation entering the glazing element can approach a remarkable 85%, accompanied by only a modest reduction in the luminous transmittance.

Thermal oxidation of seeds for the hydrothermal growth of WO3 nanorods on ITO glass substrate

This work reports a simple seed formation method for the hydrothermal growth of tungsten oxide (WO3) nanorods. A WO3 seed layer was prepared by thermal oxidation, where a W-sputtered substrate was heated and oxidized in a furnace. Oxidation temperatures and periods were varied at 400–550°C and 5–60min, respectively, to determine an appropriate seed layer for nanorod growth. Thermal oxidation at 500°C for 15min was found to produce a seed layer with sufficient crystallinity and good adhesion to the substrate. These properties prevented the seed from peeling off during the hydrothermal process, thereby allowing nanorod growth on the seed. The nanorod film showed better electrochromic behavior (higher current density of −1.11 and +0.65mAcm−2) than compact film (lower current density of −0.54 and +0.28mAcm−2).

Enhancement of gas-sensing characteristics of hydrothermally synthesized WO3 nanorods by surface decoration with Pd nanoparticles

Enhancement of the gas-sensing characteristics of nanostructured metal oxides by surface decoration with noble metal nanoparticles has recently received significant attention as a powerful technique for improving gas-sensing performance. Herein, we introduce a facile method of decorating tungsten oxide (WO3) nanorods by palladium (Pd) nanoparticles to achieve enhanced NH3 gas-sensing performance. The WO3 nanorods were synthesized by a scalable hydrothermal method followed by thermal calcination to activate surface bonding for Pd decoration. Decoration of Pd nanoparticles on the surface of WO3 was achieved through reduction of the complex Na2PdCl4 using Pluronic as both surfactant and reducing agent. The materials obtained were characterized by SEM, EDS, HRTEM, and XRD. The gas-sensing characteristics of bare WO3 and Pd–WO3 nanorods were tested for NH3, CO, H2, CO2, and CH4 detection at different temperatures. Results revealed that decoration of Pd nanoparticles on the surface of WO3 nanorods significantly enhances the NH3 gas-sensing characteristics of the metal oxide. The transient stability of the sensor determined after 10 on/off cycles of switching from air to gas demonstrated the effective reusability of the fabricated device.

Synthesis and Characterization of Sphere-like Pt Nanoparticles Supported on DWCNT/WO3 Nanorods with Electrocatalytic Activity

Tungsten oxide (WO3) nanorods, which were used to load platinum (Pt) nanoparticles, were investigated. H2WO4 nanorods with diameters from 10 to 50 nm were obtained when tungsten precursor was added into homogenous double-walled carbon nanotubes (DWCNT) and ethylene glycol (EG) solution. Nanosized rod-like WO3 were achieved after calcination of the DWCNT/H2WO4 composite. Sphere-like Pt nanoparticles were loaded on the surface of the nanorods by EG in-situ reduction. Pt particles were isolated by DWCNT/WO3 nanorods and secondary accumulation could be prevented when Pt particles appeared in the DWCNT/WO3 nanorod/EG dispersion solution. Therefore, Pt nanoparticles with mean diameters of 2–6 nm could be obtained. Pt-deposited on DWCNT/WO3 nanorods exhibited high electrochemical activity, which could facilitate the low-cost mass production of Pt catalyst.

Rapid Preparation of WO3 Nanorods Via Microwave-Assisted Solvothermal Synthesis

A simple and efficient method enables to synthesize nanorods WO3 with high quantity by microwave-solvothermal. It has been found that ethanol involves the features of inexpensive and environmentally friendly, which is more suitable to be considered as a solvent. Tungsten oxide is an important transition metal oxide material, which possesses some unique properties such as electrochromic, opochromic, and gaschromic properties. The authors used the microwave assisted solvothermal method, which has the advantages of both microwave and solvothermal methods, to synthesize 1-D tungsten oxide such as nanorods. The synthesis of tungsten oxide nanorods has been investigated under microwave-solvothermal conditions at 240°C, and uses WCl6 and poly(ethylene glycol) (PEG) 20000 as a precursor and dispersant in 20% ethanol. The average diameter of WO3 nanorods were 80–100 nm and 3 μm long. The crystallized products were characterized by X-ray diffraction, thermogravimetric analysis, dynamic light scattering, X-ray fluorescence, scanning electron microscopy with energy-dispersive X-ray spectroscopy, and transmission electron microscopy.

The Effects of Growth Time on WO3 Nanostructure Synthesized by HFCVD Method

Tungsten oxide (WO3−x) is a magic material and has attracted great interest due to its wide-ranging application to various fields of technology, including electrochromic device, photo catalysis, gas sensors, water splitting, photoelectric sensors, and lithium-ion batteries. In this paper, we report the controllable synthesis of tungsten oxide (WO3) nanorods on silicon substrates by the Hot Filament Chemical Vapor Deposition (HFCVD) method. The growth mechanism of thin films is related to many parameters like time, pressure work, concentration of gases that are used, substrate temperature and etc. The most important parameter of the growth mechanism is time. The growth time is an important factor to control the size and shape of the final WO3 structure. The effect of the growth time on structural, morphological and optical properties of the films has been investigated by X-ray diffraction (XRD), Field Emissive Scanning Electron Microscopy (FESEM) and UV–Vis spectrophotometer, respectively. The XRD spectra showed a single-crystal structure with a monoclinic structure at the shortest growth time. From FESEM images, it can be seen that by increasing the growth time the nanorods structure appears and continues to grow larger. Their diameter and length are 80 and 500 nm, respectively. The change of the morphology due to changing the growth time can be highlighted. Our results showed that the controlled growth of nanorods ranging from a thin to a larger diameter can be realized by appropriate choice at the deposition time. Finally, by increasing the deposition time to 180 s the reflection of the WO3 was increased. The results can be used to guide a better understanding about the growth behavior of WO3 nanorods and can contribute towards the development of the novel nanodevices.

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.

Syntheses and Applications of Inorganic Electrochromic Materials and Devices

Tungsten oxide (WO3) nanorods were synthesized by a hydrothermal process. Transparent film was fabricated by an aggregation-deposition process of WO3 nanorods on transparent conductive glass. The as-synthesized WO3 nanorods exhibit good electrochromic properties. WO3 nanopaltes were directly grown on transparent conductive glasses by a crystal-assisted hydrothermal process, showing enhanced response and coloration efficiency. These properties of WO3 nanostructures endow their promising application in large-area smart windows. Prussian blue films were fabricated and used in color-changeable betteries. A bi-functional Prussian blue/Aluminium device has been demonstrated for self-powered electrochromic window and self-rechargeable transparent battery. Without applying any external bias, Prussian blue can be bleached to Prussian white by connecting the Prussian blue and Al electrodes. After bleaching, the blue colour of the device can be spontaneously recovered. In this regard, the designed device can be used as an energy-saving electrochromic smart window which requires no external power source. The as-prepared device can also be used as a battery with a high output voltage. Most importantly, after discharging, such a battery can automatically self-recharge. The as-prepared device exhibits promising applications in broad fields, for example, as a self-powered smart window to perform indoor light and heat management, and as a self-rechargeable transparent battery to power some delicate devices.

Visible light photoinactivation of bacteria by tungsten oxide nanostructures formed on a tungsten foil

Antibacterial activity of tungsten oxide nanorods/microrods were studied against Escherichia coli bacteria under visible light irradiation and in dark. A two-step annealing process at temperatures up to 390°C and 400–800°C was applied to synthesize the tungsten oxide nanorods/microrods on tungsten foils using KOH as a catalyst. Annealing the foils at 400°C in the presence of catalyst resulted in formation of tungsten oxide nanorods (with diameters of 50–90nm and crystalline phase of WO3) on surface of tungsten foils. By increasing the annealing temperature up to 800°C, tungsten oxide microrods with K2W6O19 crystalline phase were formed on the foils. The WO3 nanorods showed a strong antibacterial property under visible light irradiation, corresponding to >92% bacterial inactivation within 24h irradiation at room temperature, while the K2W6O19 microrods formed at 800°C could inactivate only ∼45% of the bacteria at the same conditions.

Gas Sensitivity Enhancement of WO3 Nano-rods by Gold Nanoparticles

Tungsten oxide nano-rods were prepared by acidic precipitation from sodium tungstate solution and sensitized with gold nanoparticles prepared by Turkevich method from tetrachloroauric acid trihydrate. In order to prevent aggregation the gold nanoparticles of 18nm characteristic size were stabilized by methoxy-polyethylene glycol (mPEG-SH) and mixed with hexagonal WO3. Suspension drops of doped and non-doped WO3 were deposited on micro-hotplates with interdigitated gold electrodes to measure sensing layer conductivity. Sensor responses of pure and doped WO3 were measured for NH3 and H2S in synthetic air up to 100ppm at the operation temperature of 140-200°C. The presence of gold nanoparticles significantly increased the sensitivity for H2S, whereas for NH3 the response was not affected. Test results are compared in terms of sensitivity, response time and operating temperature.

Complex three-dimensional tungsten oxide nanowire networks: controllable synthesis and growth mechanism

Tungsten oxide nanorods, nanowires, and three-dimensional (3D) nanowire networks have been synthesized just by controlling an oxygen flow rate which was introduced in thermal evaporation. X-ray diffraction (XRD) results show that the crystal structures of the nanostructural film are indexed to WO2, W18O49, and WO3 phases in sequence with the increase in the oxygen flow rate. High-resolution scanning electron microscopy, transmission electron microscopy, and XRD spectroscopy are applied to investigate the formation process of 3D nanowire networks, and the results indicate that the nanorods grow into nanotrees, and then the nanowire networks grow along a preferred orientation from the bottom to the top with the increase in evaporation time. The growth mechanism for complex 3D-WO3 nanowire networks is also discussed in detail.