Mesoporous WO3 Films Research Articles

Simplified All‐Solid‐State WO3 Based Electrochromic Devices on Single Substrate: Toward Large Area, Low Voltage, High Contrast, and Fast Switching Dynamics

AbstractElectrochromic devices (ECDs) represent one of the most promising energy saving and solar control technology for the market of energy‐efficient building and optoelectronic devices. A continuous and intense effort is currently devoted to the development of effective solid‐state ECDs and their integration in multifunctional systems, such as photoelectrochromics. Here, the fabrication of simplified all‐solid‐state WO3 based ECDs on single‐substrate is reported, demonstrating how the rational design of highly interconnected WO3 columnar nanostructures with Nafion polymer matrix remarkably decreases the charge transport barrier at the hybrid electrolyte/electrochromic interface (EEI), thus determining an impressive improvement of overall device performances. The soft polymer substrate of the electrolyte plays a key role on the formation of WO3 pillar‐like structures and on the increase of interfacial contact area by affecting the vacuum‐deposition WO3 growth. Apart from providing higher transmittance in bleached state, the resulting device, entirely manufactured at room temperature by bottom‐up process, exhibits lower activation voltages (0.5–3 V) and faster switching kinetics (5–10 s) compared with monolithic ECDs based on both bulk and mesoporous WO3 films. Furthermore, the enhanced EEI enables the scale‐up on large area and flexible substrate ensuring simultaneously a wide optical contrast (ΔT = 70%), and high coloration efficiency.

Fast-switching quasi-solid state electrochromic full device based on mesoporous WO3 and NiO thin films

Mesoporous tungsten trioxide (WO3) and nickel oxide (NiO) electrochromic (EC) thin films have been fabricated by sol-gel method with assistant of tri-block copolymer [PEO132PEO50PEO132] (F108). By controlling weight ratio of F108 and calcination temperatures, uniform and transparent mesoporous films are fabricated. Both WO3 and NiO films are comprised by nanoparticles with diameters of ca.10 nm, mesopores with pore size of approximately 6 nm are distributed in the films. The thickness of the films are ca. 400 nm. The as-prepared films have ultra-fast switching time during EC redox reactions. Applied with positive and negative voltages of 40 s respectively, the coloration/bleaching time of WO3 and NiO films are 5.7 s/2.3 s and 0.9 s/3.8 s, which is faster than that of comparative EC films fabricated by sol-gel method without F108 (coloration/bleaching time of WO3 and NiO films are 17.5 s/14.5 s and 13.6 s/4.9 s) and EC films fabricated by magnetron sputtering method (coloration/bleaching time of WO3 and NiO films are 29.4 s/21.3 s and 9.6 s/7.5 s). Quasi-solid state electrochromic device assembled with mesoporous WO3 and NiO thin films has excellent EC performance, such as fast switching speed (3.2/1.1 s for coloration/bleaching time), large optical modulation of about 45% (at 600 nm), high coloration efficiency (130.8 cm2/C at the wavelength of 600 nm) and good cycle stability. These excellent properties are derived from mesoporous channels in the films. For the mesoporous channels can shorten the diffusion paths and make it easier for ions to transport in films.

Fast-switching electrochromic properties of mesoporous WO3 films with oxygen vacancy defects.

In this study, mesoporous WO3 films with oxygen vacancy defects have been fabricated using the camphene-assisted sol-gel method. By controlling the optimized weight ratio of camphene on the WO3 films, we developed a unique film structure of the WO3 phase with both mesoporous morphology and oxygen vacancy defects due to the distinctive effect of camphene. The mesoporous WO3 films with oxygen vacancy defects fabricated using 10 wt% camphene showed superb multifunctional electrochromic (EC) properties with both fast switching speeds (5.8 s for coloration speed and 1.0 s for bleaching speed) and high coloration efficiency (CE, 51.4 cm2 C-1), which include the most prominent properties, particularly for switching speeds among WO3-based films reported so far. The attractive EC properties are due to the synergistic effects of the mesoporous morphology and oxygen vacancy defects on the WO3. The fast switching speeds are mainly caused by the reduced Li+ diffusion pathway due to the mesoporous morphology and increased electrical conductivity due to the oxygen vacancy defects. In addition, the increased CE value is due to the large transmittance modulation as a result of a more effective electrostatic contact of the mesoporous morphology and an increased optical bandgap of the oxygen vacancy defects on the WO3. Therefore, this unique film structure of the mesoporous WO3 films with oxygen vacancy defects can be potentially regarded as a novel EC material for high-performance EC devices.

Unassisted photoelectrochemical water splitting beyond 5.7% solar-to-hydrogen conversion efficiency by a wireless monolithic photoanode/dye-sensitised solar cell tandem device

Achieving the spontaneous evolution of hydrogen from photoelectrochemical (PEC) cells in water using solar light is a desirable but difficult goal. Here, we report a highly efficient wireless monolithic tandem device composed of bipolar highly transparent BiVO4-sensitised mesoporous WO3 films/Pt and a porphyrin-dye-based photoelectrode achieving 5.7% without any external bias. A sandwich infiltration process was used to produce a thin BiVO4 layer coated onto mesoporous WO3 films while preserving high transparency, enabling high photonic flux into the second dye-sensitised photoanode. In addition, the porphyrin-dye-sensitised photoanode with a cobalt electrolyte generated sufficient bias, realising highly efficient unassisted solar water splitting in the tandem cells. By combining the highly transparent BiVO4-sensitised mesoporous WO3 films with the state-of-the-art water oxidation catalyst and a single dye-sensitised solar cell with a high open circuit potential in a monolithic tandem configuration, an extraordinarily high solar-to-hydrogen (STH) conversion efficiency with spontaneous hydrogen evolution was obtained.

The influence of sol–gel processing on the electrochromic properties of mesoporous WO3 films produced by ultrasonic spray deposition

High performance mesoporous tungsten oxide films were deposited by ultrasonic spray deposition using templated sol–gel chemistry. The dynamics of both sol preparation and hydrolysis were investigated by UV–vis spectroscopy. A stable sol was formed after 12h, while optimum electrochromic performance was obtained for 12h of hydrolysis. Electrochromic performance is strongly correlated to the annealing conditions, with optimized films displaying coloration efficiency >50cm2/C and switching times <10s. FTIR spectroscopy revealed that WO3 produced under optimized conditions were free of hydroxyl and carbonate impurities. Performance scaled with the specific surface area and nanoscale morphology. The WO3 films display good long term cycling durability up to 2500 cycles, which was attributed to the high degree of film crystallinity.

Fabrication Mediated by Self-Assembly of Block Copolymer and Photoelectrochemical Properties of Mesoporous WO3 Films

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Sensitive slab optical waveguides composed of mesoporous metal-oxide thin films on the tin-diffused layers of float glass substrates

The application of uniform transparent mesoporous thin films for waveguide-based optical chemical and biological sensors could result in ultrahigh sensitivity due to a great increase in the depth of interaction between evanescent wave and analyte molecules adsorbed within the mesoporous film. For this purpose, we designed a low-loss, prism-coupling-compatible slab optical waveguide that can be prepared by deposition of sol-gel surfactant templated mesoporous metal-oxide thin films on the tin-diffused glass layers being intrinsic to float glass slides. In this study, mesoporous WO3 and SnO2 thin films were fabricated and the resulting elements were demonstrated to keep the waveguiding capability and to allow for in situ detection of molecular adsorption within the mesoporous film by single-beam polarimetric interferometry. Induced changes in the phase difference between the fundamental transverse electric and magnetic modes were detected by two methods: (a) monitoring of an interference fringe on the waveguide surface and (b) detection of the light emitting from the prism coupler.

Novel low density mesoporous WO3 films prepared by electrodeposition

Novel mesoporous tungsten trioxide (WO 3 ) films were deposited on ITO-glass by continuous, cathodic electrodeposition. The electrolytes for the deposition were prepared by using the peroxy-tungstic acid (50 mM) in 50:50 water:2-propanol solution with varying amounts of sulfuric acid. It is observed that the morphology of the films could be controlled by varying the sulfuric acid concentration in the electrolytes. The most porous films were obtained at a pH of 0.8, and this had a thickness of 0.6 μm, average particle size of 12 nm, and was quasi-transparent. The surface morphology and the structure of the films were characterized by X-ray diffraction, SEM, TEM, and cyclic voltammetry for the hydrogen intercalation reaction. Using a chopped light source, the present mesoporous WO 3 films generated a photocurrent that was three times higher than transparent films.

Electrochromic and structural characteristics of mesoporous WO3 films prepared by a sol-gel method

The preparation of electrochromic films of mesoporous tungsten trioxide from tungstic acid and tungstic hexaethoxide precursors with the addition of an organic stabiliser via a sol-gel method is reported. These films have been structurally characterised and both the film morphology and crystalline composition of the films were found to be significantly dependent on the temperature at which the films were annealed and upon the choice of precursor. Films annealed at lower temperatures consisted of amorphous and hexagonal tungsten trioxide, whereas films annealed above 500 °C comprised solely of monoclinic WO3. The electrochromic activity of the films was found to be equally dependent on method of preparation, and both the composition and the structure of the WO3 films were shown to clearly influence the colouration efficiency of the films.

Electrochromic and structural characteristics of mesoporous WO3 films prepared by a sol-gel method

Electrochromic and structural characteristics of mesoporous WO3 films prepared by a sol-gel method

Enhancement of Photocatalytic and Electrochromic Properties of Electrochemically Fabricated Mesoporous WO3 Thin Films

Mesoporous WO3 films with a lamellar structure (see Figure) have been synthesized by electrodeposition using sodium dodecyl sulfate (SDS) as a templating agent. Compared to non‐porous WO3 prepared with isopropanol instead of SDS, lamellar phase mesoporous WO3 showed higher photocatalytic activity and greater current density for hydrogen intercalation, probably due to the larger surface area of mesoporous WO3 and more facile charge transport.