In situ etching WO3 nanoplates: Hydrothermal synthesis, photoluminescence and gas sensor properties

A novel hydrothermal process using p-nitrobenzoic acid as structure-directing agent has been employed to synthesize plate-shaped WO₃ nanostructures containing holes. The p-nitrobenzoic acid plays a critical role in the synthesis of such novel WO₃ nanoplates. The morphology, structure and optical property of the WO₃ nanoplates have been characterized by transmission electron microcopy (TEM), scanning electron microcopy (SEM), X-ray diffraction (XRD) and photoluminescence (PL). The lateral size of the nanoplates is 500-1000 nm, and the thickness is about 80 nm. The formation mechanism of WO₃ nanoplates is discussed briefly. The gas sensitivity of WO₃ nanoplates was studied to ethanol and acetone at different operation temperatures and concentrations. Furthermore, the WO₃ nanoplate-based gas sensor exhibits high sensitivity for ethanol and acetone as well as quick response and recovery time at low temperature.     

Fabrication of a room-temperature NO2 gas sensor based on WO3 films and WO3/MWCNT nanocomposite films by combining polyol process with metal organic decomposition method

Polyol process was combined with metal organic decomposition (MOD) method to fabricate a room-temperature NO₂ gas sensor based on a tungsten oxide (WO₃) film and another a nanocomposite film of WO₃/multi-walled carbon nanotubes (WO₃/MWCNTs). X-ray diffractometry (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to analyze the structure and morphology of the fabricated films. Comparative gas sensing results indicated that the sensor that was based on the WO₃/MWCNT nanocomposite film exhibited a much higher sensitivity than that based on a WO₃ film in detecting NO₂ gas at room temperature. Microstructural observations revealed that MWCNTs were embedded in the WO₃ matrix. Therefore, a model of potential barriers to electronic conduction in the composite material was used to suggest that the high sensitivity is associated with the stretching of the two depletion layers at the surface of the WO₃ film and at the interface of the WO₃ film and the MWCNTs when detected gases are adsorbed at room temperature. The sensor that is based on a nanocomposite film of WO₃/MWCNT exhibited a strong response in detecting very low concentrations of NO₂ gas at room temperature and is practical because of the ease of its fabrication.     

Synthesis and characterization of novel nanorod superstructures and twin octahedral morphologies of WO3 by hydrothermal treatment

Controlled WO₃ morphologies, such as nanorods and octahedral structures, were synthesized by the hydrothermal technique using sulfate salts based structure directing agents (SDAs). The role of the sulfate salts’ cation in controlling the shape, size and phase of WO₃ nanomaterials was investigated by choosing sulfate salts whose cations are from d-bloc elements (FeSO₄, (NH₄)₂Fe(SO₄)₂, CoSO₄, CuSO₄, ZnSO₄), an alkaline earth metal (MgSO₄) and a non-metal ((NH₄)₂SO₄). In addition chloride (MnCl₂) and acetate (Zn(CH₃CO₂)₂) anion based SDAs were also used in order to clarify the role of sulfate ions in the growth of WO₃ nanostructures. We controlled the pH of the reaction medium with oxalic acid. The obtained WO₃ samples were investigated by SEM, micro-Raman, and XRD. At pH = 1, the WO₃ samples exhibit novel superstructures consisting of aligned hexagonal nanorods, whereas at pH = 5.25, novel twin octahedral morphology with a cubic structure is obtained. The results demonstrate that the phase and morphology change is influenced by the pH and both the anion and the cation of the SDA. A growth mechanism for the obtained novel WO₃ morphologies is presented.     

Nanocrystalline samarium tungstate: facile morphology-controlled preparation,characterization and investigation of optical and photocatalytic properties

In the current study, samarium tungstate Sm₂(WO₄)₃ nanoparticles were synthesized via a new route based on the reaction between samarium nitrate hexahydrate and sodium tungstate dihydrate in an aqueous solution. Besides, three amino acids such as cysteine, alanine, and glycine were introduce as capping agents and their effects on the morphology and particle size of Sm₂(WO₄)₃ were investigated. It was found that size and morphology could be easily realized by changing the type of amino acids. XRD, SEM, EDS, and UV–Vis spectroscopy were employed to characterize structural, morphological, and optical properties of Sm₂(WO₄)₃ nanoparticles. Sm₂(WO₄)₃ nanoparticles indicated a paramagnetic behavior at room temperature, as evidenced by using vibrating sample magnetometer. Furthermore, the photocatalytic properties of as synthesized Sm₂(WO₄)₃ were evaluated by degradation of methyl orange as water contaminant.     

Synthesis and electrochromic application of surfactants tailored WO3 nanostructures

As WO₃ is excellent material for electrochromic displays more investigation is needed to find the good and low cost method for preparation of WO₃ nanostructures with uniform morphology and narrow distributions using a surfactant mediated method. In this study, the synthesis of WO₃ nanostructures was accomplished using various surfactants such as polyethylene glycol, sodium chloride and sodium dodecyl sulfate and sodium tungstate and diethyl sulfate as the inorganic precursors. All samples were characterized for their opto-structural and morphological studies by UV-Vis spectrophotometer, X-ray diffractometer and scanning electron microscopy techniques. The electrochromic performance of these samples was studied in LiClO₄/PC as electrolyte for Li⁺ insertion/extraction. The use of surfactants has been employed to enhance the uniformity of WO₃ samples.     

Structural and electrochemical studies of tungsten oxide (WO<Subscript>3</Subscript>) nanostructures prepared by microwave assisted wet-chemical technique for supercapacitor

Tungsten oxide (WO₃) nanostructures were synthesised using the microwave-assisted wet chemical method without any addition of surfactant for three different microwave irradiation times (10, 20 and 30 min). Then as-prepared nanostructures were characterised using various characterization techniques to know their structural, morphology and optical properties. The monoclinic and orthorhombic (WO₃) crystal structure was obtained from the as-prepared nanostructures by using X-ray diffraction analysis and its calculated crystalline size were found to be increased from 14 to 18 nm on increasing microwave irradiation time. The functional groups were investigated by using Fourier transform infrared spectroscopy analysis and the W–O chemical bonding nature was confirmed. The surface morphology was unclear using scanning electron microscope analysis, and a careful observation in high resolution transmission electron microscope studies shows that rod shaped structure. A blue-shifted optical absorption spectrum was observed by ultraviolet–visible spectroscopy analysis analysis and the bandgap energy value of WO₃ was calculated as approximately 3.62 eV. The emission behaviours were investigated by using photoluminescence spectrofluorometer analysis and an enhanced defect reduced emission was obtained. Finally, the electrochemical properties were analyzed by using cyclic voltammogram and galvanostatic charge–discharge analysis analyses. The maximum capacitance was recorded at 264 F/g which was declined to 149 F/g with the growth of WO₃ nanostructures size. Hence, the effect of microwave on structure and morphology, and consequent supercapacitor performances of WO₃ were discussed in details.     

Preparation of platinum-loaded cubic tungsten oxide: A highly efficient visible light-driven photocatalyst

Multistructural tungsten oxide samples were prepared using the hydrothermal method in the presence of different sulfates. In this paper, we present WO₃ nanorods, WO₃ toothpicks and cubic WO₃ samples prepared in the presence of Na₂SO₄, Li₂SO₄ and FeSO₄, respectively. These catalysts were characterized by XRD, SEM, TEM, EDS and UV-vis DR. It is found that Fe₂O₃ was impregnated in the cubic WO₃ which is different from other two samples. After Pt loading, Pt-loaded WO₃ with different morphology acting as novel visible light-driven photocatalysts showed remarkably high photocatalytic activity under visible light radiation. Significantly, the maximum efficiency of photodegradation was observed at 1 wt.% Pt loading amount in the cubic WO₃ sample. The highest photocatalytic activity of the cubic Pt/Fe₂O₃/WO₃ photocatalyst is attributed to the synergistic action of Pt nanoparticles and Fe₂O₃.     

Enhanced photovoltaic performance of WO3 nanoparticles added dye sensitized solar cells

This study investigates the effect of various concentrations of Tungsten Oxide (WO₃) nanoparticles on the performance of dye sensitized solar cells. For that WO₃ nanoparticles are synthesized by using solvothermal method and annealed at room temperature, 100 and 400 °C. The crystalline and morphology of the synthesized WO₃ nanoparticles was characterized by X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM). Solar cell with 1 % WO₃ nanoparticles (annealed at 100 °C) added TiO₂ electrode exhibits an enhanced short-circuit current density of 12.94 mA cm^?2, open-circuit photo voltage of 0.67 V, fill factor of 0.57, and overall power conversion efficiency of 5.03 %.     

Thin film deposition and characterization of pure and iron-doped electron-beam evaporated tungsten oxide for gas sensors

Pure tungsten oxide (WO₃) and iron-doped (10 at.%) tungsten oxide (WO₃:Fe) nanostructured thin films were prepared using a dual crucible Electron Beam Evaporation (EBE) technique. The films were deposited at room temperature under high vacuum onto glass as well as alumina substrates and post-heat treated at 300 °C for 1 h. Using Raman spectroscopy the as-deposited WO₃ and WO₃:Fe films were found to be amorphous, however their crystallinity increased after annealing. The estimated surface roughness of the films was similar (of the order of 3 nm) to that determined using Atomic Force Microscopy (AFM). As observed by AFM, the WO₃:Fe film appeared to have a more compact surface as compared to the more porous WO₃ film. X-ray photoelectron spectroscopy analysis showed that the elemental stoichiometry of the tungsten oxide films was consistent with WO₃. A slight difference in optical band gap energies was found between the as-deposited WO₃ (3.22 eV) and WO₃:Fe (3.12 eV) films. The differences in the band gap energies of the annealed films were significantly higher, having values of 3.12 eV and 2.61 eV for the WO₃ and WO₃:Fe films respectively. The heat treated films were investigated for gas sensing applications using noise spectroscopy. It was found that doping of Fe to WO₃ produced gas selectivity but a reduced gas sensitivity as compared to the WO₃ sensor.     

Effect of annealing temperature on the characteristics and sensing properties of WO<Subscript>3</Subscript> nanowires

A uniform WO₃ nanowire structure was prepared by two-step thermal oxidation method on Si substrate. WO₃ nanowires show different morphology and crystal structures after annealing at different temperatures. The influence of annealing temperature on WO₃ nanowires was investigated by SEM, TEM and XRD. Higher crystallization property and lower surface state was obtained with higher annealing temperature. The gas sensing properties of the WO₃ nanowires with various annealing temperatures to NO₂ with the concentration ranging from 1 to 4 ppm were examined at different temperatures ranging from room temperature to 200 °C. The results indicate that WO₃ nanowires can greatly lower the working temperature of sensors and sensors based on WO₃ nanowires show p-type or n-type sensing behaviors depending on annealing temperatures. Possible sensing mechanism of p-type WO₃ nanowires and the influence of annealing temperature on sensing types was explained. This work might supply new ideas about gas sensing mechanisms and open a new way to develop p-type WO₃ sensing materials.     

Hydrothermal Sm-doped tungsten oxide vertically plate-like array photoelectrode and its enhanced photoelectrocatalytic efficiency for degradation of organic dyes

Samarium doped tungsten oxide film was synthesized by a hydrothermal method with sodium tungstate as W precursor and samarium oxide as dopant. After annealing at 450 °C for 0.5 h, the morphology and structural characterization of as-prepared films were determined with scanning electron microscopy, X-ray diffraction and high-resolution transmission electron microscope. For the pure and Sm-doped WO₃ films serving as the photoanodes, photoelectrocatalytic properties were demonstrated by degrading methyl orange and methylene blue solution, showing that Sm-doped WO₃ film has faster degrading rate than pure WO₃ film. Photoelectrochemical properties were investigated using linear sweep voltammetry, electrochemical impedance spectroscopy, Mott–Schottky and incident photon to current conversion efficiency. Sm-doped WO₃ achieves a high photocurrent of 1.50 mA cm^?2 at 1.4 V versus. Ag/AgCl, which is 1.8 times as high as that of pure WO₃ film (0.83 mA cm^?2). Moreover, photogenerated hole injection efficiency was improved by retarding the recombination at the interface of electrode/electrolyte. The results indicate the Sm₂O₃ formed by excess doping led to a better photoelectrocatalytic and photoelectrochemical activities of Sm-doped WO₃ film, suggesting that the doping of Sm is a favorable strategy to improve the performance of WO₃ film photoanode.     

Nanostructured tungsten and tungsten trioxide films prepared by glancing angle deposition

Nanostructured tungsten (W) and tungsten trioxide (WO₃) films were prepared by glancing angle deposition using pulsed direct current magnetron sputtering at room temperature with continuous substrate rotation. The chemical compositions of the nanostructured films were characterized by X-ray photoelectron spectroscopy, and the film structures and morphologies were investigated using X-ray diffraction and high resolution scanning electron microscopy. Both as-deposited and air annealed tungsten trioxide films exhibit nanostructured morphologies with an extremely high surface area, which may potentially increase the sensitivity of chemiresistive WO₃ gas sensors. Metallic W nanorods formed by sputtering in a pure Ar plasma at room temperature crystallized into a predominantly simple cubic β-phase with <100> texture although evidence was found for other random grain orientations near the film/substrate interface. Subsequent annealing at 500 °C in air transformed the nanorods into polycrystalline triclinic/monoclinic WO₃ structure and the nanorod morphology was retained. Substoichiometric WO₃ films grown in an Ar/O₂ plasma at room temperature had an amorphous structure and also exhibited nanorod morphology. Post-deposition annealing at 500 °C in air induced crystallization to a polycrystalline triclinic/monoclinic WO₃ phase and also caused a morphological change from nanorods into a nanoporous network.     

Synthesis and characterization of WO3 thin films by surfactant assisted spray pyrolysis for electrochromic applications

Thin films of WO₃ were prepared by surfactant assisted spray pyrolysis on F-doped SnO₂ (FTO) conductive glass by using hexadecyltrimethylammonium bromide (HTAB) and polyethylene glycol (PEG400):HTAB as growth controlling agents. The surface tension of the spraying solutions was experimentally evaluated and was correlated with the deposition processes (nucleation and growth) of very smooth and homogenous films. The effect of the surfactant, alone and associated with PEG, on the structure (XRD), morphology (AFM), surface composition (XPS), FTIR and hydrophilicity (contact angle) were investigated and their influence on the electrochromic activity was discussed. Using surfactants and PEG, the coloration efficiency, transmission modulation and cycling stability of the WO₃ thin films can be enhanced.     

CVD Grown Tungsten Oxide for Low Temperature Hydrogen Sensing: Tuning Surface Characteristics via Materials Processing for Sensing Applications

The intrinsic properties of semiconducting oxides having nanostructured morphology are highly appealing for gas sensing. In this study, the fabrication of nanostructured WO₃ thin films with promising surface characteristics for hydrogen (H₂) gas sensing applications is accomplished. This is enabled by developing a chemical vapor deposition (CVD) process employing a new and volatile tungsten precursor bis(diisopropylamido)-bis(tert-butylimido)-tungsten(VI), [W(N^tBu)₂(NⁱPr₂)₂]. The as-grown nanostructured WO₃ layers are thoroughly analyzed. Particular attention is paid to stoichiometry, surface characteristics, and morphology, all of which strongly influence the gas-sensing potential of WO₃. Synchrotron-based ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), X-ray photoelectron emission microscopy (XPEEM), low-energy electron microscopy (LEEM) and 4-point van der Pauw (vdP) technique made it possible to analyze the surface chemistry and structural uniformity with a spatially resolved insight into the chemical, electronic and electrical properties. The WO₃ layer is employed as a hydrogen (H₂) sensor within interdigitated mini-mobile sensor architecture capable of working using a standard computer's 5 V 1-wirebus connection. The sensor shows remarkable sensitivity toward H₂. The high, robust, and repeatable sensor response (S) is attributed to the homogenous distribution of the W⁵⁺ oxidation state and associated oxygen vacancies, as shown by synchrotron-based UPS, XPS, and XPEEM analysis.     

Structural and optical properties of Ag doped tungsten oxide (WO<Subscript>3</Subscript>) by microwave-assisted chemical route

The present study explores, the pure and silver (Ag) doped WO₃ nanoparticles synthesized by microwave irradiation method. Powder X-ray diffraction results reveal that the WO₃ doped with Ag concentration from 0 to 10 wt% crystallizes in monoclinic structure. TEM analysis shows both pristine and silver doped WO₃ nanoparticles. They are having spherical morphology with a average size from 30 to 40 nm. Scanning electron microscopy studies depicts that both the pristine and Ag doped WO₃ form in spherical shaped morphology with an average diameter of 40–30 nm, which is in proper agreement with the average crystallite sizes calculated by Scherrer’s formula. A considerable red shift in the absorbing band edge and a decrease in the band gap energy from 3.00 to 2.85 eV for Ag doped samples were observed by using UV–DRS spectra analysis. The defects in crystal and oxygen deficiencies were analyzed by photoluminescence spectra analysis.     

Controllable synthesis of gadolinium tungstate nanoparticles with different surfactants by precipitation route

In this paper, gadolinium tungstate Gd₂(WO₄)₃ nanoparticles were synthesized via a new approach based on the reaction between gadolinium nitrate hexahydrate and Na₂WO₄.2H₂O in water.   Besides, three surfactants such as ethylene diaminetetraacetic acid (EDTA), sodium dodecyl sulfate (SDS), and polyvinylpyrrolidone (PVP) were used to investigate their effects on the morphology and particle size of Gd₂(WO₄)₃ nanoparticles. According to the vibrating sample magnetometer, gadolinium tungstate Gd₂(WO₄)₃ nanoparticles indicated a paramagnetic behavior at room temperature. The as-synthesized nanoparticles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance spectroscopy (UV–Vis), and energy dispersive X-ray microanalysis (EDX). To evaluate the catalytic properties of nanocrystalline gadolinium tungstate, the photocatalytic degradations of methyl orange under ultraviolet light irradiation were carried out.     

Facile hydrothermal crystallization of NaLn(WO4)2 (Ln=La-Lu,and Y), phase/morphology evolution,and photoluminescence

Abstract

Hydrothermal reaction of Ln nitrate and Na₂WO₄ at pH=8 and 200 °C for 24 hours, in the absence of any additive, has directly produced the scheelite-type sodium lanthanide tungstate of NaLn(WO₄)₂ for the larger Ln³⁺ of Ln=La-Dy (including Y, Group I) and an unknown compound that can be transformed into NaLn(WO₄)₂ by calcination at the low temperature of 600 °C for the smaller Ln³⁺ of Ln=Ho-Lu (Group II). With the successful synthesis of NaLn(WO₄)₂ for the full spectrum of Ln, the effects of lanthanide contraction on the structural features, crystal morphology, and IR responses of the compounds were clarified. The temperature- and time-course phase/morphology evolutions and the phase conversion upon calcination were thoroughly studied for the Group I and Group II compounds with Ln=La and Lu for example, respectively. Unknown intermediates were characterized by elemental analysis, IR absorption, thermogravimetry, and differential scanning calorimetry to better understand their chemical composition and coordination. The photoluminescence properties of NaEu(WO₄)₂ and NaTb(WO₄)₂, including excitation, emission, fluorescence decay, and quantum efficiency of luminescence, were also comparatively studied for the as-synthesized and calcination products.     

Effect of hydrothermal temperature on structure and photochromic properties of WO3 powder

Tungsten trioxide (WO₃) powders were prepared via a simple hydrothermal method. The morphology, structure and photochromic activity of the synthesized WO₃ powders were studied by X-ray diffraction, scanning electron microscopy and UV–vis spectrophotometer combined with color difference meter. The results showed the synthesized WO₃ powders with hexagonal phase got much better photochromic properties than the WO₃ powders with cubic phase, the ones not appear until about 160 °C. Besides, the WO₃ powder synthesized at 120 °C exhibited the best photochromic properties of the samples prepared below 160 °C, the particles of which formed a shape of clusters of cactus with uniform size and good dispersion.     

Effect of hydrothermal duration on synthesis of WO<Subscript>3</Subscript> nanorods

Among different type of transition metal oxides, tungsten trioxide (WO₃) is a suitable candidate for electronic device fabrication due to its n-type property and wide band gap. Herein, one-dimensional tungsten trioxide (WO₃) nanorods were achieved from an aqueous solution of sodium tungstate dihydrate (Na₂WO₄·2H₂O) and sodium chloride (NaCl) in an acidic media by a time-optimized hydrothermal synthesis in autoclave at 180°C or different synthesis durations. For studying morphology and size of obtained powder, X-ray diffraction (XRD), scanning electron microscope (SEM), and high resolution transmission electron microscope (HRTEM) were applied. Finally, WO₃ nanorods of about 2–3 μm in length and 100–200 nm in diameter were obtained during 3 h hydrothermal process.     

High‐Performance Photoelectrochemical‐Type Self‐Powered UV Photodetector Using Epitaxial TiO2/SnO2 Branched Heterojunction Nanostructure

TiO₂/SnO₂ branched heterojunction nanostructure with TiO₂ branches on electrospun SnO₂ nanofiber (B‐SnO₂ NF) networks serves as a model architecture for efficient self‐powered UV photodetector based on a photoelectrochemical cell (PECC). The nanostructure simultaneously offers a low degree of charge recombination and a direct pathway for electron transport. Without correcting 64.5% loss of incident photons through light absorption and scattering by the F‐doped tin oxide (FTO) glass, the incident power conversion efficiency reaches 14.7% at 330 nm, more than twice as large as the nanocrystalline TiO₂ (TiO₂ NC, 6.4%)‐film based PECC. By connecting a PECC to an ammeter, the intensity of UV light is quantified using the output short‐circuit photocurrent density (J_sc) without a power source. Under UV irradiation, the self‐powered UV photodetector exhibits a high responsivity of 0.6 A/W, a high on/off ratio of 4550, a rise time of 0.03 s and a decay time of 0.01 s for J_sc signal. The excellent performance of the B‐SnO₂ NF‐based PECC type self‐powered photodetector will enable significant advancements for next‐generation photodetection and photosensing applications.