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.     

Electrochromic characteristics of fibrous reticulated WO3 thin films prepared by pulsed spray pyrolysis technique

The electrochromic (EC) behavior of fibrous reticulated WO₃ films prepared from ammonium tungstate precursor by pulsed spray pyrolysis method was investigated. All the films were prepared using identical technological parameters and a thorough investigation of the electrochromic properties of the films deposited at 300 °C is reported. The structural properties were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electrochromic and optical properties were measured using cyclic voltammetry and ultraviolet (UV)-visible spectrophotometry. The films are amorphous and have a fibrous reticulate-like morphology having micron-size circular rings. The films show high transparency in the visible range and the optical band gap energy is about 3.1 eV. Electrical measurements show that the resistivity monotonically decreases as temperature increases, which indicates thermal hopping transport. The activation energy for hopping transport is of the order 4×10⁻⁴ eV. The electrochromic coloration efficiency (CE) is found to be 34 cm²/C at 630 nm.     

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.     

Photoelectrochemical evaluation of undoped and C-doped CdIn2O4 thin film electrodes

Undoped and C-doped cadmium indate (CdIn₂O₄) thin films and powders were synthesized, characterized, and evaluated for photoelectrochemical water splitting. Both undoped and C-doped CdIn₂O₄ samples have cubic lattices, and the presence of carbonate-type species was confirmed in the C-doped sample by XPS. Doping C into CdIn₂O₄ leads to a red shift (but no separate peak) in light absorption and band gap narrowing. The photocurrent densities of CdIn₂O₄ electrodes are at least three-fold greater than either CdO or In₂O₃ electrodes with equivalent film thickness. Carbon doping further improved the photocurrent densities by 33%. The photoelectrochemical performance of C-doped CdIn₂O₄ was optimized with respect to several synthetic parameters, including the C:In molar ratio and glucose concentration in the spray precursor solution, the calcination temperature, and the film thickness. The present work shows that CdIn₂O₄ is a promising photocatalyst and can be suitably doped to improve the electrochemical properties for solar conversion applications.     

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.     

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.     

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⁺.     

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.     

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.     

Efficient visible-light-driven photocatalytic H2 production over Cr/N-codoped SrTiO3

Visible-light-response Cr/N-codoped SrTiO₃ was prepared by a sol–gel hydrothermal method. The comparison studies indicate that Cr-doped and Cr/N-codoped SrTiO₃ can be synthesized by this means, but not the N-doped SrTiO₃. The theoretical calculations exhibit the defect formation energy of the Cr/N codoping into SrTiO₃ is much smaller than that of the N doping into SrTiO₃, illuminating that the incorporation of Cr can promote the N doping into the O sites in the SrTiO₃. Compared to the Cr-doped SrTiO₃, the Cr/N-codoped SrTiO₃ photocatalyst shows the high photocatalytic activities for hydrogen production with the quantum efficiency of 3.1% at 420 nm, due to the smaller band gap and much less vacancy defects.     

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 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.     

The photoinduced evolution of O2 and H2 from a WO3 aqueous suspension in the presence of Ce/Ce

This is a report on the production of O₂ and H₂ from photocatalytic and photochemical processes in the WO₃–H₂O–Ce⁴⁺_aq–hν system. The photoproduction of O₂ and H₂ was studied over the range of WO₃ concentrations from 2 to 8 g dm⁻³, and conduction band electron scavenger concentrations 1–20 mM Ce_aq⁴⁺. Medium and high concentrations of the electron scavenger gave mainly O₂ as the main product. Dilute solutions of [Ce_aq⁴⁺]< 2 mM initially produced dioxygen, and then hydrogen after an induction period of 3–4 h. Yields of 140–250 μmol O₂  h⁻¹ and 1–7 μmol H₂ h⁻¹ were obtained and were found to depend on the physical properties and content of WO₃, the concentration of the electron scavenger, illumination period and wavelength, and the radiation geometry. The photoactivity of the suspension was correlated to the level of crystallinity of WO₃ powders. The studied system utilizes WO₃ to accomplish the initial light absorption, charge separation, and production of O₂ and H⁺ from the interaction of water molecules with photogenerated WO₃ valence band holes, in the presence of Ce⁴⁺_aq species as a scavenger of conduction band electrons. This is followed by the evolution of H₂ from a homogeneous photochemical reduction of H⁺ and/or H₂O by photoexcited Ce³⁺_aq, formed from the earlier reduction of Ce⁴⁺_aq. The obtained results show that, with an appropriate design, tungsten trioxide is a promising material that can be used as a photoactive component in energy conversion systems or in environmental photocatalysis, using artificial or solar light.     

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.     

CuInS2/In2S3 thin film solar cell using spray pyrolysis technique having 9.5% efficiency

Copper indium sulfide (CuInS₂)/In₂S₃ solar cells were fabricated using spray pyrolysis method and high short circuit current density and moderate open circuit voltage were obtained by adjusting the condition of deposition and thickness of both the layers. Consequently, a relatively high efficiency of 9.5% (active area) was obtained without any anti-reflection coating. The cell structure was ITO/CuInS₂/In₂S₃/Ag. We avoided the usual cyanide etching and CdS buffer layer, both toxic, for the fabrication of the cell.     

Ag grid induced photocurrent enhancement in WO3 photoanodes and their scale-up performance toward photoelectrochemical H2 generation

The hydrogen generation from photoelectrochemical (PEC) water splitting under visible light was investigated using large area tungsten oxide (WO₃) photoanodes. The photoanodes for PEC hydrogen generation were prepared by screen printing WO₃ films having typical active areas of 0.36, 4.8 and 130 cm² onto the conducting fluorine-doped tin oxide (FTO) substrates with and without embedded inter-connected Ag grid lines. TiO₂ based dye-sensitized solar cell was also fabricated to provide the required external bias to the photoanodes for water splitting. The structural and morphological properties of the WO₃ films were studied before scaling up the area of photoanodes. The screen printed WO₃ film sintered at 500 °C for 30 min crystallized in a monoclinic crystal structure, which is the most useful phase for water splitting. Such WO₃ film revealed nanocrystalline and porous morphology with grain size of ∼70-90 nm. WO₃ photoanode coated on Ag grid embedded FTO substrate exhibited almost two-fold degree of photocurrent density enhancement than that on bare FTO substrate under 1 SUN illumination in 0.5 M H₂SO₄ electrolyte. With such enhancement, the calculated solar-to-hydrogen conversion efficiencies under 1 SUN were 3.24% and ∼2% at 1.23 V for small (0.36 cm²) and large (4.8 cm²) area WO₃ photoanodes, respectively. The rate of hydrogen generation for large area photoanode (130.56 cm²) was 3 mL/min.     

Unique porous thick Sm0.5Sr0.5CoO3 solid oxide fuel cell cathode films prepared by spray pyrolysis

Ultrasonic spray pyrolysis assisted by an electrostatic field was used to deposit thick Sm_0.5Sr_0.5CoO₃ (SSC) films (>40 μm) as solid oxide fuel cell (SOFC) cathodes with a unique porous columnar structure. The high porosity and great thickness provided many active sites for reduction reaction. The space between columns, as well as the large pores (∼100 nm) inside the columns allowed gas molecules to diffuse quickly to the reaction sites; thus, very low interfacial resistance values (0.20 and 0.035 Ω cm² at 600 and 700 °C, respectively) were obtained. Moreover, the high deposition rate, ease of operation in open air and low cost make the ultrasonic spray pyrolysis assisted by an electrostatic field a particularly useful method for preparation of films ideal for SOFC operation.     

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.     

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.     

Ultrasonic spray pyrolysis synthesis of Ag/TiO2 nanocomposite photocatalysts for simultaneous H2 production and CO2 reduction

Simultaneous photocatalytic hydrogen production and CO₂ reduction (to form CO and CH₄) from water using methanol as a hole scavenger were investigated using silver-modified TiO₂ (Ag/TiO₂) nanocomposite catalysts. A simple ultrasonic spray pyrolysis (SP) method was used to prepare mesoporous Ag/TiO₂ composite particles using TiO₂ (P25) and AgNO₃ as the precursors. The material properties and photocatalytic activities were compared with those prepared by a conventional wet-impregnation (WI) method. It was found that the samples prepared by the SP method had a larger specific surface area and a better dispersion of Ag nanoparticles on TiO₂ than those prepared by the WI method, and as a result, the SP samples showed much higher photocatalytic activities toward H₂ production and CO₂ reduction. The optimal Ag concentration on TiO₂ was found to be 2 wt%. The H₂ production rate of the 2% Ag/TiO₂–SP sample exhibited a six-fold enhancement compared with the 2% Ag/TiO₂–WI sample and a sixty-fold enhancement compared with bare TiO₂. The molar ratio of H₂ and CO in the final products can be tuned in the range from 2 to 10 by varying the reaction gas composition, suggesting a viable way of producing syngas (a mixture of H₂ and CO) from CO₂ and water using the prepared Ag/TiO₂ catalysts with energy input from the sun.