Modeling the optical properties of WO3 and WO3-SiO2 thin films

The optical properties and surface morphology of sol-gel spin coated WO₃ and WO₃-SiO₂ composite films annealed at 250 and are investigated. For the purpose of extracting the optical parameters of the films, a novel form for the dielectric function is introduced, consisting of two Tauc-Lorentz oscillators and an Urbach tail component, which is suited for amorphous multi-transition materials with substantial subgap absorption. The evolution of the refractive indices, transmittances, and band gaps with doping is marked by sizable shifts at 2.0-2.5% SiO₂ doping for the films, and 4.0-4.5% doping for the films. In addition, pronounced changes in the surface roughness of the films occur at these doping values.     

Pt nanoparticles supported on WO3/C hybrid materials and their electrocatalytic activity for methanol electro-oxidation

A simple and novel method for the preparation of WO₃/C is presented. This method includes the adsorption and decomposition of phosphotungstic acid (PWA) on carbon. For the purpose of comparison, WO₃/C is also prepared by a conventional method using sodium tungstate as the precursor. These two WO₃/C species are denoted as WO₃/C-1 and WO₃/C-2, respectively. It is shown from transmission electron microscopy (TEM) that the WO₃ particles in WO₃/C-1 present a more even distribution and smaller particle size than those in WO₃/C-2. Pt particles dispersed on WO₃/C-1 display the characteristic diffraction peaks of Pt in the face-centered cubic phase. Cyclic voltammetry and chronoamperometry show that the Pt-WO₃/C-1 catalyst exhibits much better methanol oxidation activity than the Pt-WO₃/C-2 and Pt/C catalysts. This significant improvement in catalytic performance may be attributed to the hydrogen spillover effect and the uniform distribution of Pt and WO₃ particles.     

Layered WO3/TiO2 nanostructures with enhanced photocurrent densities

Layered WO₃/TiO₂ nanostructures, fabricated by magnetron sputtering, demonstrate significantly enhanced photocurrent densities compared to individual TiO₂ and WO₃ layers. First, a large quantity of compositions having different microstructures and thicknesses were fabricated by a combinatorial approach: diverse WO₃ microstructures were obtained by adjusting sputtering pressures and depositing the films in form of wedges; later layers of TiO₂ nanocolumns were fabricated thereon by the oblique angle deposition. The obtained photocurrent densities of individual WO₃ and TiO₂ films show thickness and microstructure dependence. Among individual WO₃ layers, porous films exhibit increased photocurrent densities as compared to the dense layer. TiO₂ nanocolumns show length-dependent characteristics, where the photocurrent increases with increasing film thickness. However, by combining a WO₃-wedge type layer with a layer of TiO₂ nanocolumns, PEC properties strikingly improve, by about two orders of magnitude as compared to individual WO₃ layers. The highest photocurrent that is measured in the combinatorial library of porous WO₃/TiO₂ films is as high as 0.11 mA/cm². Efficient charge-separation and charge carrier transfer processes increase the photoconversion efficiency for such films.     

Wet chemical preparation and characterization of electrochromic WO3

Thin films of electrochromic WO₃ were prepared via wet chemical deposition. Precursor solutions containing WOCl₄ in isopropanol were used and films were deposited by spin coating. Various techniques were used for characterization of the films such as Rutherford backscattering spectroscopy (RBS), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), UV-VIS spectroscopy and electrochemical methods. Composition, structural characteristics and electrochromic properties were studied as function of the curing temperature, in the range 80–500°C.     

Stability of gasochromic WO3 films

Gasochromic films consist of an electrochromic layer such as WO₃ and a very thin catalyst coating, like Pt. Hydrogen is dissociated on the catalyst into H atoms, which color the electrochromic film. A complete bleaching can be achieved in oxygen, whereas flushing with argon or evacuating the sample leads to a comparatively slow and incomplete bleaching. We discuss two kinds of aging processes, i.e. catalysed poisoning by reactants in air and a change in the water content of the WO₃. Poisoning by air increases the time needed for coloring in H₂ and bleaching in O₂ or in Ar. From results with variable WO₃ film thicknesses, we conclude that poisoning results from adsorption of a blocking species on the interior surfaces of the WO₃ pores and not on the catalyst. The adsorption process is accelerated by the catalyst. After drying the device at 100°C in vacuum, there was a severe decrease in the coloring and bleaching rates due to a reduction of the diffusion in the WO₃. Furthermore, the coloration at steady state was more intense. The variation of the water content of the WO₃ was attempted by exposing it to dry or humid atmospheres and was investigated by IR spectroscopy. No changes in water content could be detected, and no significant change in the coloration velocity could be found. To demonstrate the long-term stability of the film, a 1.1 m×0.6 m large window was switched 20,000 times at 20°C over 2 yr without any significant change in performance.     

Hydrogen evolution reaction on single crystal WO3/C nanoparticles supported on carbon in acid and alkaline solution

Single crystal tungsten oxide (WO₃) nanoparticles were prepared via a microwave-assisted method. Electrochemical activity for hydrogen evolution reaction (HER) on WO₃ supported on carbon black (WO₃/C) electrocatalyst was first studied in acid solution (0.5 M H₂SO₄) and alkaline solution (1.0 M KOH) at room temperature. The overall experimental results revealed that the electrocatalytic activity for HER on WO₃/C is one order magnitude higher than those obtained with carbon black in 0.5 M H₂SO₄ and is six times than in the case of carbon black in 1.0 M KOH. These results demonstrated that WO₃ could enhance the electrocatalytic activity for hydrogen evolution reaction in acid solution (0.5 M H₂SO₄) and alkaline solution (1.0 M KOH). On the other hand, the kinetic reaction mechanisms were discussed on WO₃/C electrocatalysts and carbon black in acid solution and alkaline solution for HER. Consequently, the rate-determining step changed from Tafel to Volmer due to the incorporation of WO₃ in acid solution. However, the rate-determining step carries through Tafel reaction on both electrocatalysts in alkaline solution though WO₃ was introduced.     

Study on photocurrent of bilayers photoanodes using different combination of WO3 and Fe2O3

Bilayer photoanodes were prepared onto glass substrates (FTO) in order to improve generated photocurrents using UV-vis light by water splitting process. A comparative study of photocatalytic was performed over the films surface using Fe₂O_3, WO₃ and mixture of bicomponents (Fe₂O₃:WO₃). Different types of films were prepared using Fe₂O_3, WO₃ and bicomponents (mixture) on FTO substrates. The films were grown by sol gel method with the PEG-300 as the structure-directing agent. The photo-generated of the samples were determined by measuring the currents and voltages under illumination of UV-vis light. The morphology, structure and related composition distribution of the films have been characterized by SEM, XRD and EDX respectively. Photocurrent measurements indicated surface roughness as the effective parameter in this study. The deposited surfaces by bicomponents or mixture are flat without any feature on the surface while the deposited surfaces by WO₃ appears rough surface as small round (egg-shaped particles) and cauliflower-like. The surface deposited by Fe₂O₃ show rough no as well as WO₃ surface. The deposited surfaces by WO₃ reveal the higher value of photocurrent measurement due to surface roughness. Indeed, the roughness can be effective in increasing contact surface area between film and electrolyte and diffuse reflection (light scattering effect). The solution (Fe₂O₃:WO₃) shows the low photocurrent value in compare to WO₃ and Fe₂O₃ hat it may be due to decomposition the compound at 450 ± 1 °C to iron-tungstate Fe₂(WO₄)₃.     

Yb-doped WO3 photocatalysts for water oxidation with visible light

Yb-doped WO₃ photocatalysts were prepared by co-sputtering WO₃ and Yb, followed by annealing in air for water oxidation with visible light. All the obtained photocatalysts were monoclinic with sputtering power of Yb up to 10 W and displayed no optical absorption red shift. In photoelectrochemical (PEC) studies, the photocurrent densities were improved with up to 0.34 at.% Yb in WO_3, with the highest photocurrent of 1.3 mA/cm² (1.2 V vs. Ag/AgCl) achieved with <0.1 at.% Yb. Electrochemical impedance spectroscopy (EIS) measurements showed that optimized Yb doping reduced charge transfer resistance and increased donor density of WO₃ photocatalyst. The improvement in photocurrent density was attributed to enhanced conductive carrier path, increased oxygen vacancies and 4f¹³ orbital configuration due to Yb³⁺ substitution of W⁶⁺.     

The character of WO3 film prepared with RF sputtering

The effects of preparation conditions on WO₃ films using RF reactive sputtering were investigated in order to prepare a high efficiency semiconductor electrode. The properties of the electrodes were measured in the solution of H₂SO₄. We found the optimum condition for the photocurrent in our system. The photocurrent is independent of O₂ concentration in the range of 20–50%. We suppose that a photocurrent of WO₃ depends on an orientation and a grain size. The result of XRD spectra corresponded well with SEM image. From the SEM images and the absorption spectra it was considered that the thicker the WO₃ films were the rougher the surface became.     

Pd nanoparticles supported on WO3/C hybrid material as catalyst for oxygen reduction reaction

Pd nanoparticles supported on WO₃/C hybrid material have been developed as the catalyst for the oxygen reduction reaction (ORR) in direct methanol fuel cells. The resultant Pd–WO₃/C catalyst has an ORR activity comparable to the commercial Pt/C catalyst and a higher activity than the Pd/C catalyst prepared with the same method. Based on the physical and electrochemical characterizations, the improvement in the catalytic performance may be attributed to the small particle sizes and uniform dispersion of Pd on the WO₃/C, the strong interaction between Pd and WO₃ and the formation of hydrogen tungsten bronze which effectively promote the direct 4-electron pathway of the ORR at Pd.     

A facile one-step synthesis of single crystalline hierarchical WO3 with enhanced activity for photoelectrochemical solar water oxidation

Hierarchical architectures consisting of one-dimensional (1D) nanostructures are of great interest for potential use in energy and environmental applications in recent years. In this work, hierarchical tungsten oxide (WO₃) has been synthesized via a straightforward, template-free, hydrothermal route from ammonium metatungstate hydrate and implemented in photoanode fabrication for solar water oxidation in photoelectrochemical cells and photocatalytic oxidation of organic pollutant. The flower-like WO₃ micro-patterns are constructed by self-organized nanoscale length 1D building blocks, which are single-crystalline in nature, grown along (001) direction and confirm an orthorhombic crystal phase. Time-dependent experiments have been conducted to demonstrate their morphology evolution. The hierarchical architecture based photoanodes produce higher photocurrent (2-fold high) than the nanoparticles based photoanodes from solar water oxidation. The photon to current conversion efficiency achieved with the hierarchical architectures is 45% at 400 nm. The enhanced activity can be attributed to improved charge-separation by superior charge transportation through single-crystalline 1D building blocks.     

Preparation of supported SrCeO3-based membrane by spin coating method

SrCe_0.9Y_0.1O_3−δ (SCY10) powder with a pure perovskite phase is prepared by solid-state reaction method. NiO is dispersed uniformly in SCY10 powder to fabricate NiO-SCY10 anode substrate. The starting powder, the mixture of SrCO₃, CeO₂ and Y₂O₃, is deposited directly on the green substrate instead of SCY10 powder by spin coating. After co-firing at 1300 °C for 3 h, the starting powder reacts to form SCY10 top layer on the substrate. SEM micrographs show that the top layer is defect-free and adheres well with the anode substrate without any delamination. A single fuel cell is assembled with anode-supported SCY10 membrane as electrolyte membrane and Ag as cathode. The electrochemical property of the fuel cell is tested with hydrogen as fuel in the temperature range of 600-800 °C. The open circuit voltage (OCV) reaches 1.05 V at 800 °C, and the maximum power density is 50 mW cm⁻², 155 mW cm⁻², 200 mW cm⁻² at 600 °C, 700 °C, 800 °C, respectively.     

Highly selective hydrogen sensing of Pt-loaded WO3 synthesized by hydrothermal/impregnation methods

Unloaded and 0.25–1.0 wt% Pt-loaded WO₃ nanoparticles were synthesized by hydrothermal method using sodium tungstate dihydrate and sodium chloride as precursors in an acidic condition and impregnated using platinum acetylacetonate. Pt-loaded WO₃ films on an Al₂O₃ substrate with interdigitated Au electrodes were prepared by spin-coating technique. The response of WO₃ sensors with different Pt-loading concentrations was tested towards 0.01–1.0 vol% of H₂ in air as a function of operating temperature (200–350 °C). The 1.0 wt% Pt-loaded WO₃ sensing film showed the highest response of ∼2.16 × 10⁴ to 1.0 vol% H₂ at 250 °C. Therefore, an operating temperature of 250 °C was optimal for H₂ detection. The responses of 1.0 wt% Pt-loaded WO₃ sensing film to other flammable gases, including C₂H₅OH, C₂H₄ and CO, were considerably less, demonstrating Pt-loaded WO₃ sensing film to be highly selective to H₂.     

Electrochromic properties of nanocomposite WO3 films

A new nanocomposite WO₃ (NWO) film-based electrochromic layer was fabricated by a spray and electroplating technique in sequence. An indium–tin oxide (ITO) nanoparticle layer was employed as a permanent template to generate the particular nanostructure. The structure and morphology of the NWO film were characterized. The optical and electrochromic properties of the NWO films under lithium intercalation are described and compared to the regular WO₃ film. The NWO films showed an improved cycling life and an improved contrast with compatible bleach-coloration transition time, owing to the larger reactive surface area. The nanocomposite WO₃ film-based electrochromic device (NWO-ECD) was also successfully fabricated. Most importantly, the NWO film can be prepared on a large scale directly onto a transparent conductive substrate, which demonstrates its potential for many electrochromic applications, especially, smart windows, sunroof and displays.     

Application of WO3 thin films for enhancement of photolysis in AgCl

A double-layer AgCl–WO₃ structure was employed to produce photochemical hydrogen for doping of an AgCl film. Atomic photochemical hydrogen, detached under the action of light from hydrogen donor molecules, previously adsorbed on the WO₃ surface, migrated through the WO₃ film into the AgCl film, which provided doping of the AgCl surface and yielded hydrogen sensitization simultaneous to illumination and yielded the enhancement of photochromism in the AgCl films. The atomic hydrogen played the role of a reducing agent and triggered the formation of sensitization centers on the halide surface, which in turn facilitated the growth of silver clusters and colloids under the action of light. The double-layer AgCl–WO₃ structure realizes the idea of two-stage catalysis: first the oxide surface catalyses hydrogen production under the action of light, then the photochemical hydrogen atoms act as catalysts during the photolysis of the halide.     

Investigation of WO3/ZnO thin-film heterojunction-based Schottky diodes for H2 gas sensing

A comparative study of Schottky diode hydrogen gas sensors based on Pd/WO₃/Si and Pd/WO₃/ZnO/Si structure is presented in this work. Atomic force microscopy and X-ray photoelectron spectroscopy reveal that the WO₃ sensing layer grown on ZnO has a rougher surface and better stoichiometric composition than the one grown on the Si substrate. Analysis of the I–V characteristics and dynamic response of the two sensors when exposed to different hydrogen concentrations and various temperatures indicate that with the addition of the ZnO layer, the diode can exhibit a larger voltage shift of 4.0 V, 10 times higher sensitivity, and shorter response and recovery times (105 s and 25 s, respectively) towards 10,000-ppm H₂/air at 423 K. Study on the energy band diagram of the diode suggests that the barrier height is modulated by the WO₃/ZnO heterojunction, which could be verified by the symmetrical sensing properties of the Pd/WO₃/ZnO/Si gas sensor with respect to applied voltage.     

Direct solar water splitting cell using water, WO3, Pt, and polymer electrolyte membrane

A solar water splitting cell composed of WO₃, Polymer Electrolyte Membrane (PEM) and Pt was constructed for producing hydrogen from deionized water in sunlight. Spectral responsivity measurements under various temperatures and bias voltages were conducted for the cell using the Incident Photon to Current Efficiency (IPCE) method. For comparison, a known WO₃ Photo Electro Chemical (PEC) cell containing H₃PO₄ electrolyte, WO₃/H₃PO₄/Pt, was tested using the same test method. The WO₃/PEM–H₂O/Pt cell showed better Quantum Efficiency (QE) performance compared to that obtained from the cell with the chemical electrolyte. For the first time, spectral responsivity of photo water splitting process without bias power was unveiled in the new WO₃ cell, demonstrating the self-sustained photo electrolysis capability. Bias voltage effect on Solar to Hydrogen (STH) conversion efficiency was dramatic in the range from 0.2 V to 1.2 V and suppressions of STH were observed when high bias voltages were applied. In addition, a strong temperature effect on the energy conversion efficiency at high bias voltage was observed in the cell containing PEM–H₂O, revealing that the STH at 54 °C is nearly five times that at 14 °C.     

Residual charges and infrared absorption in electrochromic WO3 films prepared by hydrogen-introduced sputtering

Electrochromic WO₃ films were prepared by rf−sputtering in atmosphere consisting of Ar/H₂/O₂ mixed gas. The as-sputtered films require several times of injection/extraction of ions (the aging) for obtaining reversible coloration/bleaching. After the aging, there are ions (protons) remaining in the films, namely residual charges. From the results of IR absorption of the as-sputtered and aged films, the residual charges contribute to create OH and HOH bonds. Hydrogen introduced in the films during sputtering is transformed only into OH bonds combining with unstable oxygen in the films. The introduced hydrogen is considered to suppress the growth of grain in WO₃ films from AFM observation.     

Electrochromic WO3 thin films active in the IR region

Herein, we investigate the electrochromic performances in the infrared (IR) region, in particular in the midwavelength (MW, 3–5 μm) and long wavelength (LW, 8–12 μm) bands, of WO₃ thin films grown by RF-sputtering and pulsed laser deposition. For an optimized voltage window, 200 nm room temperature thin films are the most efficient in the MW band, with the highest contrast in reflection, namely 35%, whereas thicker films, typically 500 nm, are required in the LW band. At 400 °C films show contrasts in reflection lower than 10%, surprisingly associated with a reasonable insertion amount of 0.20. Indeed, no straight correlation between the electrochemical and the optical properties in the IR region was established.     

Preparation and characterization of Pt supported on graphene with enhanced electrocatalytic activity in fuel cell

Pt nanoparticles are deposited onto graphene sheets via synchronous reduction of H₂PtCl₆ and graphene oxide (GO) suspension using NaBH₄. Lyophilization is introduced to avoid irreversible aggregation of graphene (G) sheets, which happens during conventional drying process. Pt/G catalysts reveal a high catalytic activity for both methanol oxidation and oxygen reduction reaction compared to Pt supported on carbon black (Pt/C). The performance of Pt/G catalysts is further improved after heat treatment in N₂ atmosphere at 300 °C for 2 h, and the peak current density of methanol oxidation for Pt/G after heat treatment is almost 3.5 times higher than Pt/C. Transmission electron microscope (TEM) images show that the Pt particles are uniformly distributed on graphene sheets. X-ray photoelectron spectroscopy (XPS) results demonstrate that the interaction between Pt and graphene is enhanced during annealing. It suggests that graphene has provided a new way to improve electrocatalytic activity of catalyst for fuel cell.