WO3 Nanocrystals Research Articles

Solvothermal, Chloroalkoxide-based Synthesis of Monoclinic WO3 Quantum Dots and Gas-Sensing Enhancement by Surface Oxygen Vacancies

We report for the first time the synthesis of monoclinic WO3 quantum dots. A solvothermal processing at 250 °C in oleic acid of W chloroalkoxide solutions was employed. It was shown that the bulk monoclinic crystallographic phase is the stable one even for the nanosized regime (mean size 4 nm). The nanocrystals were characterized by X-ray diffraction, High resolution transmission electron microscopy, X-ray photoelectron spectroscopy, UV-vis, Fourier transform infrared and Raman spectroscopy. It was concluded that they were constituted by a core of monoclinic WO3, surface covered by unstable W(V) species, slowly oxidized upon standing in room conditions. The WO3 nanocrystals could be easily processed to prepare gas-sensing devices, without any phase transition up to at least 500 °C. The devices displayed remarkable response to both oxidizing (nitrogen dioxide) and reducing (ethanol) gases in concentrations ranging from 1 to 5 ppm and from 100 to 500 ppm, at low operating temperatures of 100 and 200 °C, respectively. The analysis of the electrical data showed that the nanocrystals were characterized by reduced surfaces, which enhanced both nitrogen dioxide adsorption and oxygen ionosorption, the latter resulting in enhanced ethanol decomposition kinetics.

Solvent-regulated solvothermal synthesis and morphology-dependent gas-sensing performance of low-dimensional tungsten oxide nanocrystals

This paper reports a solvent-regulated, surfactant-free hydrothermal process at 180°C for 24h to synthesize tungsten oxide (WO3) nanocrystals with various morphologies (urchin-like, rod-like and plate-like WO3) using WCl6 as the W source and just changing the solvents (from ethanol, ethanol/water (1:1 in v/v) to water); then those WO3 nanocrystals are used as the active materials to fabricate gas sensors. The results of XRD, SEM, TEM and N2 adsorption-desorption isotherms indicate that the sample obtained in ethanol takes on a urchin-like morphology (U-WO3) with apparent diameters of 1-2 μm derived by radially aggregating WO3 nanorods (∼5nm in diameter), different from the sample obtained in the mixture of ethanol/water (1:1 in v/v), showing a rod-like morphology (R-WO3 with diameters of ∼5nm), and the product synthesized in water, showing a plate-like shape (P-WO3) with dimensions of (100–200nm)×(100–200nm)×(20–30nm). Common volatile organic compounds (acetone, methanol, methanal, ethanol and benzene) and some gases (H2, CO and H2S) are used as the target substances to evaluate the gas-sensing performance of the WO3 sensors obtained. The U-WO3 sensor is highly sensitive to acetone vapors, whereas the R-WO3 and P-WO3 sensors are highly sensitive to H2S. The morphology-dependent gas-sensing performance can be attributed to the difference in microstructures and crystalline states of the WO3 nanocrystals obtained in different solvents.

Microwave-Assisted Synthesis and Gas-Sensing Performance of Hollow-Spherical WO<sub>3</sub> Nanocrystals

Hollow-spherical WO3 nanocrystals were obtained by calcining an organic-inorganic W-C precursor containing C and W in a microwave oven or in a conventional muffle furnace, and the W-C precursor hybrid precursor was synthesized via a hydrothermal method. The samples were characterized by XRD, TG-DTA, FTIR and SEM. The morphologies of the WO3 samples obtained by conventionally heating (C-WO3) and microwave-assisted heating (M-WO3) were compared. The average diameter and shell thickness of C-WO3 hollow spheres is about 450 and 200 nm, respectively. The average diameter and shell thickness of M-WO3 hollow spheres is about 500 and 50 nm, respectively. The M-WO3 has a loose and multilayered shell, and their nanoparticles are smaller than those of C-WO3. The improved structure of M-WO3 is due to shorter heating time and the unique heating style in a microwave oven. The gas-sensing performances of the WO3 sensors were investigated. The M-WO3 sensor has better response to ethanol vapors than the C-WO3.

In Situ Total X‐Ray Scattering Study of WO3 Nanoparticle Formation under Hydrothermal Conditions

Pair distribution function analysis of in situ total scattering data recorded during formation of WO3 nanocrystals under hydrothermal conditions reveal that a complex precursor structure exists in solution. The WO6 polyhedra of the precursor cluster undergo reorientation before forming the nanocrystal. This reorientation is the critical element in the formation of different hexagonal polymporphs of WO3.

In Situ Total X‐Ray Scattering Study of WO3 Nanoparticle Formation under Hydrothermal Conditions

AbstractPair distribution function analysis of in situ total scattering data recorded during formation of WO3 nanocrystals under hydrothermal conditions reveal that a complex precursor structure exists in solution. The WO6 polyhedra of the precursor cluster undergo reorientation before forming the nanocrystal. This reorientation is the critical element in the formation of different hexagonal polymporphs of WO3.

WO3 Nanoparticles Synthesized Through Ion Exchange Route as Pt Electrocatalyst Support for Alcohol Oxidation

AbstractTungsten oxide (WO3) nanocrystals with the diameter <5 nm supported on porous carbonized resin (denoted as C‐WO3) are synthesized. The WO3 precursors are dispersed at ion level through ion exchange route, then reduced to WO3 nanocrystals. Pt nanoparticles are loaded on the porous C‐WO3 matrix (denoted as Pt/C‐WO3) and used as electrocatalyst in fuel cell for alcohol oxidation, in which WO3 is found efficient promotion effect on Pt electrocatalyst in the electrochemical activity and stability. Thereinto, Pt/C‐WO3 gives 1.63 times higher current densities than the commercial Pt/C (TKK) for methanol oxidation at the same Pt loadings. Moreover, Pt/C‐WO3 electrocatalyst shows excellent properties in mass transfer than Pt/C (TKK). The present method can be readily scaled up for the production of other nanomaterials as well as WO3.

Surface interaction of WO3 nanocrystals with NH3. Role of the exposed crystal surfaces and porous structure in enhancing the electrical response

We report on the surface interaction between NH3 and WO3 nanoparticles having different exposed surfaces or different porous structure, to identify the relative importance of exposed crystal surfaces, porous architecture, and specific surface area in the oxide sensing properties. WO3 nanocrystals with tailored morphology and definite prominent surfaces were synthesized by hydrothermal reactions. In parallel, inverted opal macroporous WO3 films have been prepared by a one-step sol–gel procedure, and WO3 hierarchical layers have been obtained by an innovative one-step dual-templating strategy which leads to macropores and mesopores simultaneously. The performances of WO3 samples in NH3 sensing, indicate that high-energy surfaces result in a significant improvement of the electrical response. Enhanced porous structure and high surface area are not enough to produce high electrical response, while their synergistic combination with tailored crystal faceting appears effective. XPS survey performed on shape controlled WO3 nanocrystals demonstrated that, upon interaction with NH3, oxidized nitrogen atoms represent the prevalent species on the surface of rectangular (WO3-RE) nanocrystals with highly exposed high-energy {020} and {002} facets. Conversely, in the case of rectangular platelets (WO3-SS) and square platelets (WO3-RS) with very low surface area of high-energy surfaces, N–H surface groups are predominant. These results suggest that {020} and {002} crystal surfaces provide privileged reactive sites for ammonia oxidation and therefore they play a key role in driving the sensing properties of the WO3 layers.

Enhancing photoelectrochemical performance with a bilayer-structured film consisting of graphene–WO3nanocrystals and WO3vertically plate-like arrays as photoanodes

A bilayer-structured film photoelectrode, containing an under-layer of reduced graphene oxide–WO3 nanocrystals (G-WNC) or WO3 nanocrystals (WNC) and a top layer of WO3 vertically plate-like arrays (WP), was designed and fabricated by solution-based and hydrothermal methods. The morphologies, structural and optical properties of as-prepared composites were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and ultraviolet visible (UV-Vis) spectrometry. Moreover, the photoelectrochemical properties were studied through linear scan voltammetry, electrochemical impedance spectroscopy (EIS) and incident photon to current conversion efficiency (IPCE). It was found that the photocurrents of WP, WNC/WP and G-WNC/WP films were 0.68, 0.80 and 1.23 mA cm−2 at 1.2 V (vs. Ag/AgCl) under AM 1.5G illumination, respectively, whereas the IPCE values were 28.12%, 31.33% and 39.52% successively at the wavelength of 355 nm. EIS data illustrated that the charge transfer resistance was clearly reduced by introducing graphene in the underlayer of the composite film. The enhancement of G-WNC/WP film could be ascribed to the synergic effects of the bilayer structure and the limitation of charge recombination.

Photoinduced electron transfer from phenanthroimidazole to nano WO3, CuO ZrO2 and Al2O3. LUMO – CB energy binding efficiency relationship

A new kind of fluorophore 2-(4-fluorophenyl)-1-phenyl-1H-phenanthro [9,10-d] imidazole has been synthesized and characterized by 1H NMR, 13C NMR and mass spectral studies. The dynamics of photoinduced electron injection and energy transfer from phenanthroimidazole to CuO, ZrO2, WO3 and Al2O3 nanocrystals has been studied by Fourier transform infrared spectroscopy (FT-IR), absorption, fluorescence and lifetime spectroscopic methods. The association between nanocrystals and phenanthroimidazole is explained by both absorption and fluorescence quenching data. The distance between phenanthroimidazole and nanocrystals as well as the critical energy transfer distance has been obtained. The free energy change (ΔGet) for electron injection has also been deduced. Qualitative relationship exists between the efficiency of binding with the LUMO – CB energy gap and also the efficiency of binding with the HOMO – CB energy gap.

Low-temperature and highly selective NO-sensing performance of WO3 nanoplates decorated with silver nanoparticles

For improving the low-temperature response and selectivity of WO3-based sensors, Ag nanoparticles (AgNPs) have been used to modify the WO3 nanoplates. Ag@plate-WO3 nanocomposites with various amounts of AgNPs were synthesized by growing AgNPs on WO3 nanoplates. XRD, SEM, TEM and XPS spectrum were used to characterize the Ag@plate-WO3 samples. The gas-sensing properties were evaluated at r.t. −250°C using NO gases with various concentrations (0.5–50ppm). AgNPs enhance the low-temperature response and selectivity of the Ag@plate-WO3 sensors for NO detection, and the amounts of AgNPs influence the NO-sensing performance of the Ag@plate-WO3 sensors. The sample with 0.5% AgNPs shows the best performance. Its optimum operating temperature is around 170°C, but it shows response even at room temperature. The Ag@plate-WO3 sensors have a high selective response to NO gas, among various gases (i.e., H2 and CO) and organic vapors (i.e., alcohol, acetone, methanol and benzene). The morphologies of WO3 nanocrystals also influence the NO-sensing properties of the Ag@WO3 sensors, and the plate-like WO3 samples are better than the particle-like WO3 samples in improving NO-sensing performance. The NO-sensing enhancement should result from the synergistic effects of AgNPs and the loose house-of-card structure of plate-like WO3 aggregates.

Photoinduced electron transfer from benzimidazole to nano WO3, CuO and Fe2O3. A new approach on LUMO–CB energy-binding efficiency relationship

The dynamics of photoinduced electron injection from 2-(4-fluorophenyl)-1-phenyl-1H-benzimidazole (FPPBI) to CuO, Fe2O3 and WO3 nanocrystals has been studied by FT-IR, absorption, fluorescence and lifetime spectroscopic methods. The association between nanoparticles and FPPBI is explained from both absorption and fluorescence quenching data. The distance between FPPBI and nanoparticles as well as the critical energy transfer distance has been obtained. The free energy change (ΔGet) for electron injection has also been deduced. Energy gap between LUMO and CB is correlated with the binding efficiency of the FPPBI with nanoparticles. The electron transfer between the FPPBI with nanosemiconductors is explained in detail.

Hierarchical structured tungsten oxide nanocrystals via hydrothermal route: microstructure, formation mechanism and humidity sensing

Hierarchical structured tungsten oxide nanocrystals were synthesized via the hydrothermal route assisted by a capping agent of ammonium benzoate (AB). The products were characterized using scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. The experimental results show that the crystal microstructures could be changed from flower-shape to star-shape by changing the mole ratio of ammonium benzoate to sodium tungstate (AB/ST). The crystal phases were changed from orthorhombic WO3⋅0.33H2O to hexagonal WO3 with the increase in the concentration of AB. Based on the results from Fourier transform infrared spectroscopy and time-dependent growth analysis, a self-assembly growth mechanism has been proposed for the formation of flower, spherical, and star-netted microstructures at different mole ratios of the AB/ST. The star-netted WO3 nanocrystals were applied as a sensitive layer for humidity sensing performed using a Love-mode ZnO/36∘ Y-cut LiTaO3 surface acoustic wave device, and a stable and sensitive response to the change of relative humidity was obtained.

Preparation and Photocatalytic Performance of Nanostructural WO<sub>3</sub> Induced by Ion in Hydrothermal Synthesis

WO3 nanocrystals have been successfully synthesized via an ion induced auxiliary hydrothermal method. The experiment products were characterized by powder X-ray diffraction (XRD) and the Photocatalytic oxidation performance of products were characterized by using the photocatalytic oxidation of methyl orange under the condition of hydrogen peroxide. The optimal amount of catalyst required for catalytic oxidation experiments and the concentration of methyl orange (MO) were determined. The experimental results indicated that with he enhancement of metallicity in alkali main group, the photocatalytic activity of the WO3 induced by alkali metal cation increased; The SO42- has higher induce catalytic activity than Cl- when the type and quantity of cation are the same; however, the ion induction on the impact of surface area is quite different, and it founded that they comply with this regular pattern, that is Na+> K+> Li+ and SO42-> Cl-.

Ultrathin, Single-Crystal WO3 Nanosheets by Two-Dimensional Oriented Attachment toward Enhanced Photocatalystic Reduction of CO2 into Hydrocarbon Fuels under Visible Light

An ultrathin, single-crystal WO3 nanosheet of ∼4-5 nm in thickness, corresponding to six repeating unit cells of monoclinic WO3 along the c axis, was synthesized with laterally oriented attachment of tiny WO3 nanocrystals formed using a solid-liquid phase arc discharge route in an aqueous solution. Size-quantization effects in this ultrathin nanostructure alter the WO3 band gap to enable the nanosheet to exhibit enhanced performance for photocatalytic reduction of CO2 in the presence of water in hydrocarbon fuels that do not exist in its bulk form.

WO3 nanocrystals with tunable percentage of (0 0 1)-facet exposure

Orthorhombic WO3 nanocrystals with tunable percentage of (001)-facet exposure were synthesized by hydrothermal reaction using BF4− as the guiding agent. The preferable exposure of the high-energy (001) facets endows the nanocrystals with significantly enhanced photocatalytic activity as probed by photodegradation of Rhodamine B and the electron spin resonance techniques.

Morphologically different WO3 nanocrystals in photoelectrochemical water oxidation

Different morphologies of WO3 nanocrystals such as nanorods and nanoplates have been obtained under hydrothermal conditions using ammonium metatungstate as the precursor in presence of different organic acids such as citric, oxalic, and tartaric acid in the reaction medium. Detailed characterization of the crystal structure, particle morphology, and optical band gap of the synthesized powders have been done by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and solid-state UV–visible spectroscopy study. The as-synthesized materials are WO3 hydrates with orthorhombic phase which transform to the hexagonal WO3 through dehydration upon heating at 350 °C. The resultant products are crystalline with nanoscale dimensions. Finally, the photoactivity of the synthesized materials annealed at 500 °C has been compared employing in photoelectrochemical water oxidation under the illumination of AM 1.5G simulated solar light (100 mWcm−2). The photocurrent measurements upon irradiation of light exhibit obvious photocatalytic activity with a photocurrent of about 0.77, 0.61, and 0.65 mAcm−2 for the WO3 film derived with the oxalic acid, tartaric, and citric acid assisting agents, respectively, at 1.8 V versus Ag/AgCl electrode.

Hierarchically plasmonic photocatalysts of Ag/AgCl nanocrystals coupled with single-crystalline WO3 nanoplates

The hierarchical photocatalysts of Ag/AgCl@plate-WO₃ have been synthesized by anchoring Ag/AgCl nanocrystals on the surfaces of single-crystalline WO₃ nanoplates that were obtained via an intercalation and topochemical approach. The heterogeneous precipitation process of the PVP-Ag⁺-WO₃ suspensions with a Cl⁻ solution added drop-wise was developed to synthesize AgCl@WO₃ composites, which were then photoreduced to form Ag/AgCl@WO₃ nanostructures in situ. WO₃ nanocrystals with various shapes (i.e., nanoplates, nanorods, and nanoparticles) were used as the substrates to synthesize Ag/AgCl@WO₃ photocatalysts, and the effects of the WO₃ contents and photoreduction times on their visible-light-driven photocatalytic performance were investigated. The techniques of TEM, SEM, XPS, EDS, XRD, N₂ adsorption-desorption and UV-vis DR spectra were used to characterize the compositions, phases and microstructures of the samples. The RhB aqueous solutions were used as the model system to estimate the photocatalytic performance of the as-obtained Ag/AgCl@WO₃ nanostructures under visible light (λ ≥ 420 nm) and sunlight. The results indicated that the hierarchical Ag/AgCl@plate-WO₃ photocatalyst has a higher photodegradation rate than Ag/AgCl, AgCl, AgCl@WO₃ and TiO₂ (P25). The contents and morphologies of the WO₃ substrates in the Ag/AgCl@plate-WO₃ photocatalysts have important effects on their photocatalytic performance. The related mechanisms for the enhancement in visible-light-driven photodegradation of RhB molecules were analyzed.

Electronic Structure of <i>h</i>-WO<sub>3</sub> and CuWO<sub>4</sub> Nanocrystals, Harvesting Materials for Renewable Energy Systems and Functional Devices

— The electronic structure of hexagonal WO3 and triclinic CuWO4 nanocrystals, prospective materials for renewable energy production and functional devices, has been studied using the X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES) methods. The present XPS and XES results render that the W 5d-and O 2p-like states contribute throughout the whole valence-band region of the h-WO3 and CuWO4 nanocrystalline materialls, however maximum contributions of the O 2p-like states occur in the upper, whilst the W 5d-like states in the lower portions of the valence band, respectively.

Sol–gel derived mesoporous Pt and Cr-doped WO3 thin films: the role played by mesoporosity and metal doping in enhancing the gas sensing properties

Mesoporous Cr or Pt-doped WO3 thin films to be employed as ammonia gas sensors were prepared by a fast one-step sol–gel procedure, based on the use of triblock copolymer as templating agent. The obtained films were constituted by aggregates of interconnected WO3 nanocrystals (20–50 nm) separated by mesopores with dimensions ranging between 2 and 15 nm. The doping metals, Pt and Cr, resulted differently hosted in the WO3 mesoporous matrix. Chromium is homogeneously dispersed in the oxide matrix, mainly as Cr(III) and Cr(V) centers, as revealed by EPR spectroscopy; instead platinum segregated as Pt (0) nanoparticles (4 nm) mainly included inside the WO3 nanocrystals. The semiconductor layers containing Pt nanoclusters revealed, upon exposure to NH3, remarkable electrical responses, much higher than Cr-doped and undoped layers, particularly at low ammonia concentration (6.2 ppm). This behavior was attributed to the presence of Pt nanoparticles segregated inside the semiconductor matrix, which act as catalysts of the N–H bond cleavage, decreasing the activation barrier in the ammonia dissociation. The role of the mesoporous structure in influencing the chemisorption and the gas diffusion in the WO3 matrix appeared less decisive than the electronic differences between the two examined doping metals. The overall results suggest that a careful combination between mesoporous architecture and metal doping can really promote the electrical response of WO3 toward ammonia.

Size effects of WO3 nanocrystals for photooxidation of water in particulate suspension and photoelectrochemical film systems

Abstract Monoclinic WO3 nanocrystals were synthesized by a hydrothermal reaction and post calcination. Their particle sizes were varied from 30 nm to 500 nm by changing calcination temperature from 500 °C to 800 °C. Photooxidation of water was studied in particulate suspension (PS) system and photoelectrochemical (PEC) film system. For PS system, WO3 nanocrystals were suspended in 50 mM AgNO3 solution to measure O2 evolution rate. For PEC system, WO3 films were fabricated by doctor blade method using synthesized nanocrystals. Photocurrent density was measured at AM 1.5 G (1 sun) solar condition in 0.5 M H2SO4. In PS system, the sample calcined at the highest temperature generated the largest amount of oxygen, whereas in PEC system the sample calcined at 600 °C showed the maximum photocurrent. The two systems also showed opposite response to deposition of the Pt co-catalyst. These different behaviors were attributed to different mechanisms of charge separation in the two systems.