Facile synthesis of nanocrystalline hexagonal tungsten trioxide from metallic tungsten powder and hydrogen peroxide

A hexagonal form of tungsten trioxide (h‐WO₃, particle size: 15.9‐57.1 nm) was found to be formed by a direct reaction between metallic tungsten powder (W, particle size: 0.45‐0.59 μm) and 15%‐30% hydrogen peroxide (H₂O₂) aq solution. Oxide film on the powder surface having the similar crystal structure as h‐WO₃ was essential for the formation, and the surface oxide film was formed by aging the powder in air at 45°C, a relative humidity of 100% (P_H2O 96 hPa) for 3‐28 days or in ambient atmosphere at room temperature for 12 years. The Rietveld analysis performed in the space group P6₃/mcm (Z = 6) indicated the crystal structures were the same as those of the reported h‐WO₃ and that the crystallographic characteristic was as follows: a = 0.74219 nm, c = 0.77198 nm for h‐WO₃ from the 28‐day aged powder, and a = 0.74538 nm, c = 0.77194 nm for h‐WO₃ from the 12‐year aged powder.     

Mn‐Loaded Mesoporous Silica Nanocomposite: A Highly Efficient Humidity Sensor

We report a relative humidity sensor based on manganese‐nanoparticle‐loaded mesoporous silica SBA‐15 using a facile hydrothermal route. The as‐developed nanocomposite material (Mn/SBA‐15) possesses a high surface area and a high pore volume. The obtained samples were characterized by using low‐angle X‐ray Diffraction (XRD), Fourier‐transform infrared spectroscopy (FTIR), N₂ adsorption–desorption, high‐resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), and energy‐dispersive X‐ray (EDX) spectroscopy techniques. The Mn/SBA‐15 exhibited, improved humidity response and recovery time as compared to pure SBA‐15 in the 11%–92% RH range. Optimal results were obtained for the 5 wt% Mn‐loaded SBA‐15 sample, which displayed excellent linearity, low hysteresis, and high humidity response. A change in ~5 orders of magnitude in resistance was observed over 11%–92% RH range. The investigation of humidity sensing properties of Mn/SBA‐15 nanocomposite shows that this material has good prospects as humidity sensor.     

New Methods for Preparing Submicrometer Powders of The Tungstate‐Ion Conductor Sc2(WO4)3 and its Al and In Analogs

Two new methods for preparing submicrometer powders of M₂(WO₄)₃, M = Sc, In, and Al via combustion synthesis are reported. Stoichiometric combinations of trivalent metal nitrates, ammonium metatungstate, and either urea or carbohydrazide as the fuel were reacted at 550°C, producing amorphous or poorly crystallized powders with an average particle size ranging from 164 to 350 nm. Calcining the powders at 800°C for 1 h produced well‐crystallized, phase‐pure powders with an average particle size ranging from 210 to 711 nm. Powders sintered at 1000°C for 14 h resulted in pellets that were 87%–95% of the theoretical density, which is notably higher than typically obtained from powders prepared by solid‐state reaction. Whereas there was little difference in the microstructure of Al₂(WO₄)₃ pellets prepared with the two different powders, the carbohydrazide‐derived powders resulted in In₂(WO₄)₃ and Sc₂(WO₄)₃ pellets with a larger grain size than those prepared with urea‐derived powders. The electrical conductivity of the sintered pellets, while comparable to that reported for polycrystalline M₂(WO₄)₃ prepared by solid‐state reaction, was strongly influenced by grain‐boundary effects.     

A novel resistive‐type humidity sensor based on poly(p‐diethynylbenzene)

Poly(p‐diethynylbenzene) (PDEB) synthesized with nickel catalyst Ni(CC ○ CCH)₂(PPh₃)₂ (Ni C) in dioxane–toluene mixed‐solvent system at 25°C shows a rich trans structure with pendant‐group ( ○ CCH) content of about 35% having higher molecular weight and good solubility. A novel resistive‐type humidity sensor based on PDEB is presented. Its humi‐sensing characteristics are described and discussed. The impedance of the sensor changed from ∼ 10³–10⁷ Ω in almost the whole humidity range [∼ 15–92% relative humidity (RH)], which is low compared with sensors based on other humi‐sensitive conjugate polymers, and hysteresis of no more than 3% RH was observed. The sensor prepared by Langmuir–Blodgett (LB) deposition method shows the best humidity response. An explanation of humi‐sensing behavior of PDEB is attempted by taking into account the interaction between hydrogen protons and super π‐conjugate orbits in PDEB. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2010–2015, 1999     

Cr-doped WO3 nanosheets: Structural,optical and formaldehyde sensing properties

Co-precipitated undoped and Cr-doped WO₃ nanosheets have been investigated by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM) in order to study the influence of Cr doping on their structural and morphological properties. XRD analyses confirm the monoclinic structure of nanocrystalline WO₃, whereas the FESEM and TEM images exhibit nanosheet-like morphology of the as-synthesized WO₃ materials. Among all the samples examined, the 1.5 at% Cr-doped WO₃ nanosheets exhibit the selective maximum response (~82%) to formaldehyde over methanol, ethanol, propan-2-ol and acetone at the operating temperature of 200 °C for 50 ppm concentration in air. The sensing mechanism has been explained based on chemisorption of oxygen on the WO₃ surface and the subsequent reaction between the adsorbed oxygen species and the formaldehyde molecules.     

Low Temperature Synthesis of Needle‐like α‐FeOOH and Their Conversion into α‐Fe2O3 Nanorods for Humidity Sensing Application

In this study, we report template and surfactant‐free, low temperature (70°C) synthesis of needle‐like α‐FeOOH and its conversion at 400°C into α‐Fe₂O₃ nanorods using Fe(+2) and Fe(+3) chlorides and urea as a hydrolysis‐controlling agent. The isolated needle‐like α‐FeOOH indicates asparagus‐type growth pattern having length ca. 600 nm with 80 nm diameter at base and apex diameter of around 10 nm. The sample on heating (α‐Fe₂O₃) shows nanorod‐like morphology. The samples were characterized using various physicochemical characterization techniques such as XRD, Raman spectroscopy, UV‐Vis spectroscopy, particle size distribution analysis, Field Emission Scanning Electron Microscopy (FE‐SEM), and humidity sensing performance. The humidity sensing behavior of both α‐FeOOH and α‐Fe₂O₃ was studied. The α‐FeOOH shows quicker (10 s) and higher response toward change in humidity from 20%RH to 90%RH as compared with α‐Fe₂O₃ (60 s). Their typical morphology and crystalline structure plays an important role in humidity sensing behavior.     

Excellent humidity sensing properties of cadmium titanate nanofibers

We report humidity sensing characteristics of CdTiO₃ nanofibers prepared by electrospinning. The nanofibers were porous having an average diameter and length of ～50–200 nm and ～100 μm, respectively. The nanofiber humidity sensor was fabricated by defining aluminum electrodes using photolithography on top of the nanofibers deposited on glass substrate. The performance of the CdTiO₃ nanofiber humidity sensor was evaluated by AC electrical characterization from 40% to 90% relative humidity at 25 °C. The frequency of the AC signal was varied from 10^?1 to 10⁶ Hz. Fast response time and recovery time of 4 s and 6 s were observed, respectively. The sensor was highly sensitive and exhibited a reversible response with small hysteresis of less than 7%. Long term stability of the sensor was confirmed during 30 day test. The excellent sensing characteristics prove that the CdTiO₃ nanofibers are potential candidate for use in high performance humidity sensors.     

Solvothermally grown WO3·H2O film and annealed WO3 film for wide-spectrum tunable electrochromic applications

Tungsten trioxide (WO₃) is a classical electrochromic (EC) material with advantages of abundant reserves, high coloration efficiency and cyclic stability. However, WO₃ films are often accompanied by a narrow spectrum of modulation due to a single-color change from transparent to blue. In this work, we report a wide-spectrum tunable WO₃·H₂O nanosheets EC film solvothermally grown on fluorine-doped tin oxide (FTO) glass. Interestingly, the crystalline WO₃·H₂O nanosheets film is transformed into amorphous WO₃ after annealing at 250 °C for 1 h. The amorphous film can be transformed into crystalline WO₃ film by increasing the annealing temperature to 450 °C. After annealing at 250 °C, the WO₃ film exhibits an optical modulation of 75.8% in a broad solar spectrum range of 380–1400 nm and blocks 88.9% of solar irradiance. Fast switching responses of 4.9 s for coloration and 6.0 s for bleaching, and a coloration efficiency of 86.4 cm² C⁻¹ are also achieved. Additionally, the WO₃ film annealed at 250 °C also demonstrates an excellent cyclic stability, where 99.6% of the initial optical modulation can be retained after 1500 cycles. This simple and mild solvothermal method used in this work provides a new idea for the preparation of wide-spectrum tunable WO₃ EC films.     

Plasmonic Effects of Infiltrated Silver Nanoparticles Inside TiO2 Film: Enhanced Photovoltaic Performance in DSSCs

The plasmonic effects of infiltrated silver (Ag) nanoparticles, with different contents, inside a nanostructured TiO₂ film on the photovoltaic performance of dye‐sensitized solar cells (DSSCs) are explored. The synthesized Ag nanoparticles are immobilized onto deposited TiO₂ nanoparticles by a new strategy using 3‐mercaptopropionic acid (MPA), a bifunctional linker molecule. Transmission electron microscope (TEM) images show that monodispersed Ag and polydispersed TiO₂ nanoparticles have an average diameter of 12 ± 3 nm and 5 ± 1 nm, respectively. Moreover, Fourier transform infrared spectroscopy (FTIR) analysis reveals that Ag nanoparticles were successfully functionalized and capped with MPA. Optical studies on the MPA‐capped Ag nanoparticles inside TiO₂ film show an increase in the total absorbance of the electrode. Moreover, EIS measurements confirm that MPA‐capped Ag nanoparticles inhibit the charge recombination and improve the stability of nanoparticles in I₃^?/I^? electrolyte. The DSSC assembled with optimal content of MPA‐capped Ag nanoparticles demonstrated an enhanced power conversion efficiency (8.82% ± 0.07%) compared with the pure TiO₂ (7.30% ± 0.05%). The increase in cell efficiency was attributed to the enhanced dye light absorption in strength and spectral range due to the surface plasmon resonance of MPA‐capped Ag nanoparticles in the photoanode.     

Liquid flame spray fabrication of WO3-reduced graphene oxide nanocomposites for enhanced O3-sensing performances

WO₃ is one of the inspiring sensing materials that show high response to O₃; an efficient fabrication of WO₃ film with incorporation of complementary additives is essential for enhanced sensitivity. Here we report film deposition by liquid flame spraying, characterization of nanostructured WO₃-reduced graphene oxide (rGO) composites and their gas-sensing activities to O₃. The starting feedstock was prepared from WC_l6 and rGO for pyrolysis synthesis by flame spraying. Nano-porous WO₃-rGO films were successfully fabricated and characterized by transmission electron microscopy, field emission scanning electron microscopy, Raman spectrometry, thermal analyses and X-ray diffraction. Nanosized WO₃ grains exhibited oriented nucleation on rGO flakes whereas rGO retained intact its nano-structural features after spraying. Constrained grain growth of WO₃ of 60–70 nm in size was realized in the rGO-containing films with as compared to ~220 nm in the pure WO₃ film. The WO₃-rGO film sensors showed quicker response to O₃ and faster recovery than rGO-free WO₃ film sensors. Addition of rGO in 1.0 wt% or 3.0 wt% in the films caused a significantly reduced effective working temperature of the film sensors from ~ 250 °C to ~ 150 °C.     

Fabrication of Ag/WO3 nanobars for Baeyer–Villiger oxidation using hydrogen peroxide

We report the preparation of Ag/WO₃ nanobars, mediated by cationic surfactant CTAB through hydrothermal route. XRD revealed the formation of metallic Ag supported on monoclinic WO₃ phase and TEM diagram showed the formation of bar-like structure, where supported Ag nanoparticles are in the range between 2 and 7 nm. The catalyst exhibited high activity for selective oxidation of cyclohexanone to caprolactone with H₂O₂. A cyclohexanone conversion of 97% with 99% caprolactone selectivity was achieved over this catalyst at 80 °C temperature. Moreover, the catalyst did not show any significant activity loss even after 5 reuses and proved its efficiency in the oxidation of other cycloalkanones also.     

Enhanced performance of γ-Fe2O3:WO3 nanocomposite towards selective acetone vapor detection

Metal oxide nanocomposite sensors based on γ-Fe₂O₃ and WO₃ were investigated in acetone vapor of various concentrations (1–100 ppm) at operating temperatures between 250 and 350 °C. The composites were prepared by simple solid state mixing and porous thick-film gas sensors were fabricated on alumina substrates. The γ-Fe₂O₃:WO₃ (50:50) nanocomposite showed a marked enhancement in sensing response down to 1 ppm acetone vapor detection at 300 °C. The response was ~2-fold better compared to pure WO₃ or pure γ-Fe₂O₃ with a very fast response (1 s) and very short recovery time (3 s). No appreciable sensitivity was observed towards alcohol vapor (an interfacing agent for diabetics) and in moisture (present in breath). The enhanced performance was due to n–n heterojunction effect.     

Anti-thermal quenching of Eu3+ luminescence in negative thermal expansion Zr(WO4)2

Thermal quenching of luminescence is the most critical problem for rare earth doped phosphors used in light-emitting diodes (LEDs). Herein, we demonstrate that thermal quenching can be considerably suppressed via the negative thermal expansion effect in Zr(WO₄)₂ that serves as host for Eu³⁺ red emission. The photoluminescence (PL) intensity is surprisingly enhanced by 130% when the temperature is raised from room temperature to 100 °C. As temperature further increases to 160 °C, the PL intensity turns to reduce, which is still 1.4 times of that at room-temperature. Moreover, Zr(WO₄)₂:15%Eu phosphor has good durability, which still exhibits strong red luminescence (only 13% loss) after being kept in 85 °C/85% relative humidity chamber for 240 h. The anti-thermal quenching of Eu³⁺ luminescence can be ascribed mainly to the following two factors: first one is the thermal-enhanced energy transfer between Eu³⁺ ions induced by the contraction of Zr(WO₄)₂ unit-cell volume that leads to the strong structural rigidity of host lattice; second one would be electron traps in the host that favors the increase of electrons on the excited energy levels. This important anti-thermal quenching effect induced from the negative thermal expansion of the host matrix may stimulates a novel and efficient approach to design highly thermal stable phosphors for next-generation LEDs.     

Ag-decorated novel h’-WO3 nanostructures for sustainable applications

Recently, octahedral molecular sieve hexagonal structure (called h’) of WO₃ was discovered for enhancing the physical and electrical properties, which is a promising candidate for electrochemical and energy storage applications. Nevertheless, the number of research on this new structure is still limited. In this study, novel h’-WO₃ nanoflakes and nanopolygons decorated with Ag nanoparticles were synthesized using a one-step hydrothermal method. Ag decoration of the h’-WO₃ nanostructure did not affect its crystal structure and morphology but remarkably enhanced its physicochemical properties. Ag decoration narrowed the optical bandgap, increased the oxygen deficiency, and increased the charge transfer between WO₃ and the Ag NPs, which consequently increased the antibacterial and photocatalytic activity. The sample decorated with 5.0 wt% Ag showed the highest methylene blue degradation efficiency and antibacterial activity with S. aureus, while the sample decorated with 7.0 wt% Ag exhibited the highest antibacterial activity with E. coli. The sample decorated with 5.0 wt% Ag displayed the highest electrochromic performance with high optical contrast (ΔT ～70% at 800 nm, 57.3% at 633 nm, and 35.5% at 550 nm with a ±3 V applied potential for coloring/bleaching) and fast coloring/bleaching times (13.5/7.5 s). This work provides a facile approach for preparing high-performance materials for a variety of eco-friendly applications.     

Microwave Dielectric Ceramics Li2MO4−TiO2 (M = Mo,W) with Low Sintering Temperatures

In this work, novel series of (1 ? x)Li₂MO₄–xTiO₂ (M = Mo, W; x = 0.3, 0.4, 0.45, 0.5, 0.6) ceramics were developed for microwave dielectric application. They were prepared via the mixed‐oxide process and the phase composition, microstructures, sintering behaviors, and microwave dielectric properties were investigated. The X‐ray diffraction (XRD) pattern and scanning electron microscope analysis indicated that the Li₂MO₄ (M = Mo, W) did not react with rutile TiO₂ and a stable two‐phase composite system Li₂MO₄–TiO₂ (M = Mo, W) was formed. The XRD pattern of cofired ceramics revealed that some parts of Li₂MoO₄ phase and very small part of Li₂WO₄ phase react with Ag to form Ag₂MoO₄ phase and Ag₂WO₄ phase, respectively. At x = 0.45–0.5, temperature stable microwave dielectric materials with low sintering temperature (700°C–730°C) were obtained: ε_r = 10.6–11.0, Qf = 30 060–32 800 GHz, and temperature coefficient of resonant frequency ~0 ppm/°C.     

Preparation of hierarchical WO3 dendrites and their applications in NO2 sensing

Hierarchical WO₃ dendrites were synthesized via low-cost and environmental-friendly solvothermal strategy. Characterization results indicated that WO₃ dendrites were composed of several multi-directional dendritic nanosheets. To further understand the formation of WO₃ dendrites, time-dependent experiments were carried out and formation mechanism was investigated. Since such dendritic structures rarely occurred in the field of gas sensing, the synthesized WO₃ dendrites were subjected to detailed NO₂ sensing tests. Results demonstrated that WO₃ dendrites based sensors had low detection limit (200 ppb) and fast response and recovery (7 s, 12 s to 5 ppm NO₂). Moreover, the sensor was also highly sensitive, selective and stable at low optimal operating temperature of 140 °C.     

Microstructural characterization and crystallization behaviour of (1 − x)TeO2–xWO3 (x = 0.15, 0.25, 0.3 mol) glasses

DTA, XRD and SEM investigations were conducted on the (1 − x)TeO₂–xWO₃ glasses (where x = 0.15, 0.25 and 0.3). Whereas the 0.75TeO₂–0.25WO₃ and 0.7TeO₂–0.3WO₃ glasses show no exothermic peaks, an indication of no crystallization in their glassy matrices, two crystallization peaks were observed on the DTA plot of the 0.85TeO₂–0.15WO₃ glass. On the basis of the XRD measurements of the 0.85TeO₂–0.15WO₃ glass samples heated to 510 °C and 550 °C (above the peak crystallization temperatures), α-TeO₂ (paratellurite), γ-TeO₂ and WO₃ phases were detected in the sample heated to 510 °C and the α-TeO₂ and WO₃ phases were present in the sample heated to 550 °C. SEM micrographs taken from the 0.85TeO₂–0.15WO₃ glass heated to 510 °C showed that centrosymmetrical crystals were formed as a result of surface crystallization and were between 3 μm and 15 μm in width and 12 μm and 30 μm in length. On the other hand, SEM investigations of the 0.85TeO₂–0.15WO₃ glass heated to 550 °C revealed the evidence of bulk massive crystallization resulting in lamellar crystals between 1 μm and 3 μm in width and 5 μm and 30 μm in length. DTA analyses were carried out at different heating rates and the Avrami constants for the 0.85TeO₂–0.15WO₃ glass heated to 510 °C and 550 °C were calculated as 1.2 and 3.9, respectively. Using the modified Kissinger equation, activation energies for crystallization were determined as 265.5 kJ/mol and 258.6 kJ/mol for the 0.85TeO₂–0.15WO₃ glass heated to 510 °C and 550 °C, respectively.     

A Novel Highly Sensitive Humidity Sensor Based on ZnO/SBA‐15 Hybrid Nanocomposite

This work deals with the study of hydrothermally synthesized zinc oxide (ZnO) loaded mesoporous SBA‐15 hybrid nanocomposite for relative humidity sensing (RH) at room temperature. The sensor exhibits an excellent ~5 orders impedance change along with excellent linearity, quick response time (17 s), rapid recovery time (18 s), negligible hysteresis (1.2%), good repeatability, and stability (1.8%) in 11%–98% RH range. In addition, complex impedance spectra of the sensor at different RHs were analyzed to understand the humidity sensing mechanism. Our study can open a new way for realizing ZnO/SBA‐15 hybrid nanocomposite for fabrication of high‐performance RH sensors.     

Electrical characterization of room temperature humidity sensors in La0.8Sr0.2Fe1−xCuxO3 (x = 0, 0.05, 0.10)

Semiconducting oxide gas sensors based on La_0.8Sr_0.2Fe_1?xCu_xO₃ (x = 0, 0.05, 0.10) (LSF, LSFC05, and LSFC10, respectively) were prepared by screen-printing for humidity detection at room temperature.The thick-films were heat-treated at 800, 900 and 1000 °C for 1 h and all the compositions proved to be effective in humidity sensing and presented a good reproducibility between several measurements. However, the best results were obtained with LSFC10 fired at 800 °C which showed a detection limit of 15% relative humidity and a maximum sensor response of about 87%, higher than the previous results. Copper addition to lanthanum strontium ferrites proved to be effective in lowering the sensors’ detection limit.     

Vacuum-Deposited Thin Film of Aniline–Formaldehyde Condensate/WO3·nH2O Nanocomposite for NO2 Gas Sensor

A hydrated tungsten oxide (WO₃·nH₂O)-embedded aniline–formaldehyde condensate (AFC/WO₃·nH₂O) nanocomposite thin film was prepared on an indium tin oxide (ITO)-coated glass surface via vacuum-deposition technique. The resulting AFC/WO₃·nH₂O/ITO thin film was characterized using ultraviolet–visible spectroscopy (UV–Vis), Fourier transform infrared spectroscopy (FTIR), X-ray diffractometry (XRD), and electron microscopy (SEM). The composite thin film exhibited a crystalline surface morphology containing nanocrystals of WO₃·nH₂O with a diameter ranging from 15 to 20 nm. The AFC/WO₃·nH₂O/ITO film allowed for the low potential detection of NO₂ gas at a concentration range from 0 to 9000 ppm. The NO₂ gas sensing characteristics were studied by measuring the change in the current with respect to concentration and time. The current of the AFC/WO₃·nH₂O/ITO film linearly increased with an increase in concentration of NO₂ gas with a response of ~20 s.