Peroxopolytungstic Acid Research Articles

WO3.1/3H2O nanorods/nanoplates: Growth mechanism and CO2 uptake

The paper reports a facile solvothermal synthesis of WO3.1/3H2O nanorods/nanoplates using solid peroxopolytungstic acid as a precursor. The crystallographic origin of reaction-time induced solvent-assisted growth and morphological evolution from anisotropic 1d structures to 2d plate-like morphologies were thoroughly investigated. Furthermore, the potential of these nanostructures for CO2 adsorption was explored. The larger CO2 uptake was observed for 1d rod-like structures than the plate-like morphologies. The CO2 uptake variations with morphological differences were correlated with their growth behavior and available free surfaces.

One-step controllable synthesis of three-dimensional WO3 hierarchical architectures with different morphologies decorated with silver nanoparticles: enhancing the photocatalytic activity

Hierarchical architecture self-assembled three-dimensional WO3 and WO3·Ag with high uniformity have been successfully obtained using peroxopolytungstic acid as precursor and silver in a morphological engineering approach under moderate hydrothermal conditions.

Niobium incorporated WO3 nanotriangles: Band edge insights and improved photoelectrochemical water splitting activity

In this work, a facile hydrothermal method is proposed to fabricate Nb-doped WO3 nanotriangle thin films, and their band edge properties and photoelectrochemical water splitting activity were explored. The process of doping and thin film formation was simultaneously achieved in a single step hydrothermal condensation of peroxopolytungstic acid solution containing Nb precursor. The crystallographic study reveals that doping of Nb into WO3 lattice obstructs the reconstructive transformation of orthorhombic WO3·0.33H2O during annealing consequently producing hexagonal phase instead of the monoclinic phase. The insertion of Nb causes the increase in band gap and induces oxygen vacancies in WO3. Uniform distribution of Nb in WO3 was observed containing majorly Nb5+ valence and a small amount of Nb4+ state. PEC characterization showed the increase in photocurrent, (at AM 1.5G illumination) incident photon to current and photoconversion efficiency values of WO3 upon Nb doping. Band edge analysis revealed that both conduction and valence band edge of WO3 show downward shift towards higher potential vs. RHE whereas the Fermi level show upward shift as a result of Nb doping.

Electrochromic devices based on tungsten oxide layers modified with polyethylene glycol

For the synthesis of tungsten-oxide-based electrochromic layers, the effect of the addition of polyethylene glycol (PEG) in different concentrations into peroxopolytungstic acid sol followed by thermal treatment at different temperatures on the performance of electrochromic devices (ECDs) is studied. The modification of the tungsten-oxide-layer sol-gel synthesis under optimal conditions (PEG: WO3 ratio 1∶2, temperature 450°C) provides a significant improvement of the ECD optical contrast and coloration efficiency due to the formation of a highly developed surface of the tungsten oxide layer, enhancing its contact with electrolyte while retaining continuity of the electrochromic layer.

Intercalation of barium into monoclinic tungsten oxide nanoplates for enhanced photoelectrochemical water splitting

Intercalating alkali earth metals into WO3 lattice is a promising strategy to tune its optical and electrical properties. However, hollow cubic tunnels present in monoclinic WO3 do not provide sufficient space for the diffusion and intercalation of large ions into its interstitial cavities. Therefore, a unique synthesis strategy is required to intercalate large ions into the monoclinic WO3 lattice without causing structural distortion or damage. In this study, a single-step hydrothermal method is proposed for the fabrication of barium-intercalated monoclinic WO3 thin films for efficient photoelectrochemical water splitting applications. The intercalation of barium into WO3 is achieved by the hydrothermal condensation of peroxopolytungstic acid solution. The proposed method yields hydrated orthorhombic WO3, which provides enough space in the hexagonal interstitial cavities between its [001] planes for the stable intercalation of barium. Subsequently, stable barium-intercalated monoclinic WO3 is obtained via reconstructive transformation without causing any significant structural distortion or damage. The intercalation of barium affects the preferential direction of crystal growth and the morphology of WO3. Furthermore, barium intercalation reduces the bandgap of WO3 and slightly increases its carrier density. As a result, higher photocurrent and incident photon-to-current efficiency values are achieved. Based on spectroscopic and electrochemical results, a probable band diagram of barium-intercalated WO3 is proposed. Importantly, the proposed method can be efficiently extended to fabricate monoclinic WO3 thin films intercalated with large alkali metal and alkali earth metal ions for various applications.

Enhancement of photoelectrochemical water splitting response of WO3 by Means of Bi doping

Doping is a very effective strategy for tailoring the electronic band structure and improving the charge transport properties of WO3, which could in turn enhance its photoelectrochemical (PEC) activity. In this work, we report a facile hydrothermal route to synthesize Bi doped WO3 thin films on fluoride-doped tin oxide (FTO) glass substrate and investigated the insights of its band alignment. Systematic doping of Bi into WO3 was achieved during the condensation of peroxopolytungstic acid (PTA) in the course of hydrothermal synthesis. The effect of Bi doping on the morphology and crystal structure was investigated and the approximate amount of Bi incorporated into WO3 was estimated using energy dispersive spectroscopy (EDS). Results of UV–Vis spectroscopy, Mott-Schottky analysis and valence-band (VB) X-ray photoelectron spectroscopy revealed that the insertion of Bi into the lattice of WO3 changes the band gap, valence band maximum, and the conduction band minimum of WO3. PEC measurements displayed remarkable enhancement in photocurrent values from 0.401 mA cm−2 for un-doped WO3 to ∼1.511 mA cm−2 (ca. 4-fold increase) for optimized Bi doped WO3 samples at 1.23 V vs. RHE under simulated AM 1.5 G sunlight without the addition of catalysts. The results of electrochemical impedance spectroscopy confirmed that doping WO3 with Bi lead to low charge transfer resistance across the electrode/electrolyte interface and an increase in charge-carrier density. This work suggests that Bi doping has the potential to significantly improve the PEC water splitting efficiency of WO3.

Silver-controlled evolution of morphological, structural, and optical properties of three-dimensional hierarchical WO3 structures synthesized from hydrothermal method

Hierarchical structures of self-assembled three-dimensional (3D) WO3–Ag were synthesized via hydrothermal growth using precursor solutions of peroxopolytungstic acid with different amounts of Ag. The as-grown samples were analyzed by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, ultraviolet–visible spectroscopy, and X-ray photoelectron spectroscopy. The XRD and Raman studies showed that as the amount of Ag was varied from 0 to 10 wt% in the hydrothermal growth solution, the crystal phase gradually changed from orthorhombic WO3·0.33H2O to hexagonal WO3. The FTIR and TGA studies revealed different hydration levels, supporting the XRD and Raman results. By controlling the amount of Ag in the precursor solution, platelet-like building blocks and hexagonal building blocks were obtained, highlighting the role of Ag in the hydrothermal growth of 3D WO3·0.33H2O and WO3 microcrystals. In addition, high-magnification FESEM images showed that the Ag nanoparticles were anchored on the surface of the 3D hierarchical WO3–Ag structures, and the UV–vis measurements demonstrated that the 3D hierarchical structures gradually absorbed more light when the Ag content was increased. Moreover, the band-gap energy decreased when the Ag content was increased from 0 wt% (Eg = 2.65 eV) to 10 wt% (Eg = 2.26 eV). These experimental results demonstrate that the amount of Ag played a crucial role in determining the building blocks' morphology, and the hydration level, optical properties, and crystal phase of the WO3·nH2O microcrystals.

Fundamental investigation of Ti doped WO3 photoanode and their influence on photoelectrochemical water splitting activity

Herein, we utilize a facile hydrothermal method for the fabrication of Ti doped WO3 thin films on fluorine-doped tin oxide (FTO) for the efficient photoelectrochemical water splitting activity. Doping of Ti into WO3 is achieved during the condensation of a stable aqueous precursor solution of peroxopolytungstic acid (PTA) at 150°C. Characterization results show that, doping of Ti into WO3 suppresses the crystal growth along (200) facet and shift the 2 θ values to higher degree with a corresponding decrease in d spacing. Elemental mapping along with EDS measurements show the uniform distribution and approximate at% of Ti doped in WO3. The WO3 photoanode doped with 1.16 at% Ti displayed cathodic shift in onset potential with a maximum photocurrent density of 1.139mAcm−2 (at 1.23 vs. RHE), which is found to be 3.5 times higher than undoped WO3 (0.335mAcm−2) under simulated 1.5 AM sunlight. Spectroscopic and impedance measurements show that, the substitution of W with Ti, widens the band gap and shifts the conduction band edge (CBE) and flat band potential upwards to higher energies keeping the relative energy difference between valence band edge (VBE) and Fermi level constant. Such a change in band edge positions of WO3 after Ti doping, shifts the conduction band minimum (CBM) towards the H+/H2 redox potential. The study presented herein demonstrates a simple but systematic and efficient approach for the design and fabrication of band edge-tailored WO3 photoanodes via Ti doping for the efficient photoelectrochemical water splitting.

Facile fabrication of bitter-gourd-shaped copper (II) tungstate thin films for improved photocatalytic water splitting

Copper (II) tungstate (CuWO4) has better light absorption and selectivity for a photochemical water-splitting reaction than those of its binary oxide counterpart. In this work, we report a facile single-step hydrothermal method to grow CuWO4 with a bitter gourd shape directly on a transparent conductive substrate for the first time. CuWO4 was synthesized via the condensation of a stable aqueous precursor solution of peroxopolytungstic acid and a copper precursor. The proposed method uniformly deposited CuWO4 on fluorine-doped tin oxide with strong adhesion. The obtained CuWO4 films showed a superior photocurrent density of 0.6mA/cm2 at 1.23Vvs. a reversible hydrogen electrode in a 0.1M Na2SO4 electrolyte, which was relatively higher than the values recently reported for CuWO4 photoanodes. The optimized CuWO4 exhibited an incident photon-to-current conversion efficiency of 30% at 350nm for the photoelectrochemical oxidation of water. An X-ray photoelectron spectroscopy valence band edge analysis revealed an increase in the valence band edge position in the CuWO4, along with a decrease in the band gap compared to those in WO3. The procedure proposed here provides a promising approach for the design of an efficient CuWO4 photoanode for water splitting.

Preparation of aligned W18O49 nanowire clusters with high photocatalytic activity

The aligned W18O49 nanowire clusters were synthesized via a facile and economic ethanol-assisted hydrothermal method using peroxopolytungstic acid as precursor. Results show that the as-prepared W18O49 exhibits a high yield and ultrathin structure with preferential growth direction along [010]. The amount of peroxopolytungstic acid and reaction time play significant role on the morphology of W18O49 nanowires. The nanowires have unique structure, high degree of crystallinity, large specific surface area, and large number of defects such as oxygen vacancies, which are responsible for their high photocatalytic performance for degradation of methylene blue. The photocatalytic conversion of methylene blue can reach above 98% after degradation. W18O49 also exhibits good photodegradation stability after five cycles of reuse. The results demonstrate that the as-prepared W18O49 nanowire clusters are expected to be a promising material for applications in the field of environment.

Structure and Morphologic Influence of WO3 Nanoparticles on the Electrochromic Performance of Dual‐Phase a‐WO3/WO3 Inkjet Printed Films

The optimization of tungsten trioxide (WO3) nanoparticles produced via hydrothermal synthesis for application in electrochromic (EC) devices is reported. The structure and morphology of the nanoparticles are controlled by changing the acidity of the aqueous solvent added to the sol‐gel precursor (peroxopolytungstic acid) during synthesis. Orthorhombic hydrated WO3 nanorods or monoclinic WO3 nanoslabs are obtained when HCl is added, while synthesis only in aqueous medium results in a mixture of both types of polymorphs. Dual‐phase thin films are processed by inkjet printing deposition of the nanoparticles in flexible polyethylene terephthalate substrate with indium tin oxide coating (ITO PET) followed by the deposition of the precursor solution. When compared with purely amorphous tungsten oxide films, the dual phase ones present higher optical densities and improved capacity, and cyclability stability. The best results, obtained for orthorhombic hydrated nanoparticles (ortho‐WO3·0.33H2O), are due to its high surface area and improved conductivity. Additionally, the ex situ X‐ray diffraction (XRD) lithiation studies show evidence of a higher distortion of the monoclinic when compared with the orthorhombic crystallographic structure, which contribute to the inferior EC performance. These results validate the use of inkjet printing deposition with low processing temperatures for EC dual‐phase thin films containing optimized nanoparticles which are compatible with low‐cost substrates.

Inkjet printing of sol–gel derived tungsten oxide inks

Tungsten (VI) oxide−WO3 is a widely studied functional inorganic semiconductor material with exceptional chromogenic properties. It is used in energy efficient systems such as smart windows, sensors, displays, storage units, photocatalysts and solar cells. Layers of WO3 are generally produced using expensive vacuum deposition techniques. In this paper, the inkjet printing of sol–gel derived tungsten inks on glass and transparent conductive oxide is reported. Peroxo sol–gel synthesis was used to prepare peroxopolytungstic acid sols which were then modified using different solvents to obtain a suitable jetting ink. Described are the rheological and structural properties of WO3 inks, the dynamics of WO3 droplets and the morphology and quality of WO3 printouts. The functionality of these transparent WO3 layers is successfully demonstrated in an electrochromic device.

Monodisperse WO3 hierarchical spheres synthesized via a microwave assisted hydrothermal method: time dependent morphologies and gas sensing characterization

Monodisperse WO3 hierarchical spheres were successfully synthesized via a microwave assisted hydrothermal method and a subsequent annealing process. The synthesis was performed using peroxopolytungstic acid as a precursor in the presence of sodium sulfate. The effects of hydrothermal reaction time on the microstructure were also investigated and discussed. It is found that the reaction time influences the morphologies in terms of particle sizes and dispersities. A gas sensor based on the as-prepared WO3 was fabricated and tested. The results revealed that the sensor showed relatively good selectivity and repeatability to acetone vapour. When the acetone concentration was in the range of 100 to 1000 ppm, the relationship between the response and the acetone concentration exhibited a good linearity.

Ordered mesoporous WO3film with outstanding gasochromic properties

Highly ordered mesoporous WO3 film was synthesized by employing a template-assisted peroxopolytungstic acid sol–gel method and applied for the first time in gasochromic smart windows. The results demonstrated that this mesoporous microstructured sample with semicrystalline pore walls exhibits excellent performance, including wide optical modulation ability, fast coloring/bleaching response and reliable cyclic stability.

Fabrication of Pd Doped WO3 Nanofiber as Hydrogen Sensor

Pd doped WO3 fibers were synthesized by electro-spinning. The sol gel method was employed to prepare peroxopolytungstic acid (P-PTA). Palladium chloride and Polyvinyl pyrrolidone (PVP) was dissolved in the sol Pd:WO3 = 10% molar ratio. The prepared sol was loaded into a syringe connected to a high voltage of 18.3 kV and electrospun fibers were collected on the alumina substrates. Scanning electron microscope (SEM), X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques were used to analyze the crystal structure and chemical composition of the fibers after heat treatment at 500 °C. Resistance-sensing measurements exhibited a sensitivity of about 30 at 500 ppm hydrogen in air, and the response and recovery times were about 20 and 30 s, respectively, at 300 °C. Hydrogen gas sensing mechanism of the sensor was also studied.

The Process Optimization and Structural Analysis of Gasochromic Thin Films Derived by Peroxopolytungstic Acid

The sol-gel method with a combination of dip-coating process was employed to prepare peroxopolytungstic acid gasochromic thin films. The influence of preparation process on the structural and gasochromic durability was then discussed. We found that hydrogen peroxide content shown a significant impact on the structure of tungsten oxide thin films, which directly determined the gasochromic durability

The Characterization and Synthesis of Polyaniline Doped by Peroxo–Polytungstic Acid

Polyaniline nanorods, nanoplates and urchin-like structures have been synthesized successfully by the template-free method of aniline and peroxo–polytungstic acid using ammonium persulphate as the oxidant in aqueous solution. The peroxo–polytungstic acid concentration and reaction temperature play important role in the formation process of polyaniline nanostructures. As the precursor concentration increased to 0.29mol/L, nanorods and urchin structures were obtained. With the decrease of temperature, the amount of PANI nanoplates with radially aligned dendritic surface structures will increase. The radially aligned dendritic surface is probably due to the molecular structures of the dopant, the linear nature of PANI chains and the second nucleation formation.

Fabrication of WO3-Based Composite Films and Improvement Its Photochromic Properties by Copper Doping

Abstract Tungsten-based photochromic composite films were fabricated using peroxopolytungstic acid methanol solution and transparent urethane resin. The composite films showed photochromic properties by UV–vis light irradiation. Photochromic properties of the films were improved with addition of the other elements of Cu. The reaction rate constants k of the films with the Cu/W ratio of 0, 0.1, 0.5, and 1 were 2.68 × 10−2, 3.33 × 10−2, 2.57 × 10−2, and 1.51 × 10−2 min−1, and the constant of the film with Cu/W of 0.1 was larger than that of the nondoped film.

Morphology-controlled synthesis of tungsten oxide hydrates crystallites via a facile, additive-free hydrothermal process

Tungsten oxide hydrates crystallites with a diversity of morphologies were successfully synthesized by employing peroxo-polytungstic acid as the precursor via a facile and additive-free hydrothermal process. The crystal structure, morphology and orientation relationship were investigated by using X-ray diffractometer, scanning electron microscopy and transmission electron microscopy. Results show that with the increase of the hydrothermal reaction time, the product morphology shows an obvious evolution from hexagonal plates to round angular blocks and to hexagonal gears, through irregular nanosheets and to cuboid rods finally. Furthermore, the driving force of this morphology evolution was found to be correspondent with the interesting change of the orientation growth and the phase transition of tungsten oxide hydrates under the hydrothermal process.

Microstructure control of dual-phase inkjet-printed a-WO3/TiO2/WOX films for high-performance electrochromic applications

The microstructural aspects related to crystalline or amorphous structure of as-deposited and annealed films of sol–gel-derived WO3 are shown in the literature to be critical for electrochromic (EC) performance. In consideration of ion insertion materials, there is a need for developing light and at the same time nanocrystalline structures to improve both coloration efficiency and switching kinetics. By controlling microstructure and morphology, one could design a material with optimal EC performance. This report compares the microstructural and morphological characteristics of standard WO3 wet deposition techniques versus inkjet printing technology (IPT), correlating these features with their optical and electrochemical performances, emphasizing the importance of the dual-phase a-WO3/TiO2/WOX film composition proposed in this work for high-performance EC applications. The effect of the type and content of metal oxide nanoparticles in the precursor sols formulated in various peroxopolytungstic acid (PTA) and oxalic acid (OAD) proportions on film properties is comprehensively studied using multi-factorial design of experiment (DOE). To the authors' knowledge, no other report on sol–gel deposition of inorganic EC materials via the inkjet printing technique exists, in which furthermore the film crystallinity can be controlled under low-temperature process conditions. The proposed method enables development of EC films which irrespective of their composition (a-WO3, a-WO3/TiO2 or a-WO3/TiO2/WOX) outperform their amorphous or nanocrystalline analogues presented as the state-of-the-art due to their superior chemical and physical properties.