Effect of pH on the electrochromic and photoluminescent properties of Eu doped WO3 film

Environmental Engineering Applications of Electronic Nose Systems Based on MOX Gas Sensors

Nowadays, the electronic nose (e-nose) has gained a huge amount of attention due to its ability to detect and differentiate mixtures of various gases and odors using a limited number of sensors. Its applications in the environmental fields include analysis of the parameters for environmental control, process control, and confirming the efficiency of the odor-control systems. The e-nose has been developed by mimicking the olfactory system of mammals. This paper investigates e-noses and their sensors for the detection of environmental contaminants. Among different types of gas chemical sensors, metal oxide semiconductor sensors (MOXs) can be used for the detection of volatile compounds in air at ppm and sub-ppm levels. In this regard, the advantages and disadvantages of MOX sensors and the solutions to solve the problems arising upon these sensors' applications are addressed, and the research works in the field of environmental contamination monitoring are overviewed. These studies have revealed the suitability of e-noses for most of the reported applications, especially when the tools were specifically developed for that application, e.g., in the facilities of water and wastewater management systems. As a general rule, the literature review discusses the aspects related to various applications as well as the development of effective solutions. However, the main limitation in the expansion of the use of e-noses as an environmental monitoring tool is their complexity and lack of specific standards, which can be corrected through appropriate data processing methods applications.

Review on recent progress in WO3-based electrochromic films: preparation methods and performance enhancement strategies

Transition metal oxides have drawn tremendous interest due to their unique physical and chemical properties. As one of the most promising electrochromic (EC) materials, tungsten trioxide (WO₃) has attracted great attention due to its exceptional EC characteristics. This review summarizes the background and general concept of EC devices, and key criteria for evaluation of WO₃-based EC materials. Special focus is placed on preparation techniques and performance enhancement of WO₃ EC films. Specifically, four methods - nanostructuring, regulating crystallinity, fabricating hybrid films, and preparing multilayer composite structures - have been developed to enhance the EC performance of WO₃ films. Finally, we offer some important recommendations and perspectives on potential research directions for further study.

Tailoring the Emission Behavior of WO3 Thin Films by Eu3+ Ions for Light-Emitting Applications

The article reports the successful fabrication of Eu³⁺-doped WO₃ thin films via the radio-frequency magnetron sputtering (RFMS) technique. To our knowledge, this is the first study showing the tunable visible emission (blue to bluish red) from a WO₃:Eu³⁺ thin film system using RFMS. X-ray diffractograms revealed that the crystalline nature of these thin films increased upto 3 wt% of the Eu³⁺ concentration. The diffraction peaks in the crystalline films are matched well with the monoclinic crystalline phase of WO₃, but for all the films', micro-Raman spectra detected bands related to WO₃ monoclinic phase. Vibrational and surface studies reveal the amorphous/semi-crystalline behavior of the 10 wt% Eu³⁺-doped sample. Valence state determination shows the trivalent state of Eu ions in doped films. In the 400-900 nm regions, the fabricated thin films show an average optical transparency of ~51-85%. Moreover, the band gap energy gradually reduces from 2.95 to 2.49 eV, with an enhancement of the Eu³⁺-doping content. The doped films, except the one at a higher doping concentration (10 wt%), show unique emissions of Eu³⁺ ions, besides the band edge emission of WO₃. With an enhancement of the Eu³⁺ content, the concentration quenching process of the Eu³⁺ ions' emission intensities is visible. The variation in CIE chromaticity coordinates suggest that the overall emission color can be altered from blue to bluish red by changing the Eu³⁺ ion concentration.

Enhancement of electrochromic properties using nanostructured amorphous tungsten trioxide thin films

Electrochromic technologies have recently attracted attention due to their energy-saving performance for reducing green gas emissions. The materials design and preparation of electrochromic materials with sufficient microstructure and crystallographic features for suitable ion intercalation/deintercalation are essential for high performance and efficiency. In the present work, nanostructured amorphous tungsten trioxide (WO3) films are electrodeposited to enhance electrochromic properties by controlling the pH of electrolytes. Electron microscopy and spectroelectrochemical analysis demonstrate that smaller grain sizes result in larger electrochemical reactive surface areas and shorter ion diffusion lengths. Consequently, the ions efficiently intercalated and deintercalated during the coloring and bleaching states, respectively. In particular, prepared WO3 films at electrolyte pH 1.4 demonstrate high optical modulation (74.83%) and good transmittance switching speeds (1.56 and 2.06 s during coloring and bleaching, respectively) at 650 nm, as well as comparable coloration efficiency (61.92 cm2 C-1 at 650 nm).

Reversible hydrogen spillover in Ru-WO3-x enhances hydrogen evolution activity in neutral pH water splitting

Noble metal electrocatalysts (e.g., Pt, Ru, etc.) suffer from sluggish kinetics of water dissociation for the electrochemical reduction of water to molecular hydrogen in alkaline and neutral pH environments. Herein, we found that an integration of Ru nanoparticles (NPs) on oxygen-deficient WO_3-x manifested a 24.0-fold increase in hydrogen evolution reaction (HER) activity compared with commercial Ru/C electrocatalyst in neutral electrolyte. Oxygen-deficient WO_3-x is shown to possess large capacity for storing protons, which could be transferred to the Ru NPs under cathodic potential. This significantly increases the hydrogen coverage on the surface of Ru NPs in HER and thus changes the rate-determining step of HER on Ru from water dissociation to hydrogen recombination.

While water splitting electrolysis offers an appealing means to produce H₂ fuel, catalysts show sluggish reaction rate in neutral media. Here, authors utilize hydrogen spillover from oxygen-deficient tungsten oxides to Ru nanoparticles to boost the neutral-pH H₂ evolution performances.

Novel High-Performance and Low-Cost Electrochromic Prussian White Film

Prussian white (PW), due to its low cost, easy synthesis, open structure, and fast ion extraction/interaction, is introduced to the electrochromic field. The PW films were successfully grown on indium tin oxide (ITO) glass by a facial hydrothermal method. Impressively, the PW film exhibits excellent electrochemical cycling stability without obvious decay over 10 000 cycles and a high coloration efficiency of 149.3 cm² C^-1. The film also provides the large optical transmittance contrast (over 70%) in a wide wavelength range of 650-800 nm. Furthermore, the PW film shows the rapid coloration and bleaching response. These results suggest that PW is a promising practical candidate of high-performance electrochromic material.

Boosting light harvesting and charge separation of WO3 via coupling with Cu2O/CuO towards highly efficient tandem photoanodes

Photoanodes based on semiconductor WO₃ have been attractive due to its good electron mobility, long hole-diffusion length, and suitable valence band potential for water oxidation. However, the semiconductor displays disadvantages including a relatively wide bandgap, poor charge separation and transfer, and quick electron–hole recombination at the interface with the electrolyte. Here we present a significantly improved photoanode with a tandem structure of ITO/WO₃/Cu₂O/CuO, which is prepared first by hydrothermally growing a layer of WO₃ on the ITO surface, then by electrodepositing an additional layer of Cu₂O, and finally by heat-treating in the air to form an exterior layer of CuO. Photocurrent measurements reveal that the prepared photoanode produces a maximum current density of 4.7 mA cm⁻², which is, in comparison, about 1.4 and 5.5 times the measured values for ITO/WO₃/Cu₂O and ITO/WO₃ ones, respectively. These enhancements are attributed to (1) harvested UV, visible, and NIR light of the solar spectrum, (2) accelerated charge separation at the heterojunction between WO₃ and Cu₂O/CuO, (3) better electrocatalytic activity of formed Cu_xO than pure Cu₂O, (4) formation of a protective layer of CuO. This study thus may lead to a promising way to make high-performance and low-cost photoanodes for solar energy harvesting.

A highly efficient photoanode with a tandem structure of ITO/WO₃/Cu₂O/CuO produces a photocurrent about 1.4 and 5.5 times the measured values for ITO/WO₃/Cu₂O and ITO/WO₃ ones, respectively.

Synthesis and photocatalytic activity study of S-doped WO3 under visible light irradiation

In this study, a photocatalyst S-doped WO₃ was successfully synthesized by the hydrothermal method. The prepared undoped and S-doped WO₃ samples were then characterized by XRD, SEM, XPS, and UV-vis DRS. The results showed that the band gap energy of S-doped WO₃ was lower than that of the undoped WO_3, which led to a better absorption of visible light. Furthermore, the results of XPS analysis suggested that the doping with S element resulted in an increase in lattice oxygen vacancies on the surface of S-WO₃, which could effectively improve the photocatalytic activity. The photocatalytic performance of the S-WO₃ samples were evaluated by the measurement of methylene blue (MB) degradation under visible light irradiation. The experimental results demonstrated that S-doped WO₃ sample exhibited a much better photodegradation performance compared to undoped WO₃, with the maximum MB removal efficiency of 78.7% for the 5% S-WO₃ sample. Based on the above results, the mechanisms of photodegradation of MB by S-WO₃ were discussed.

Enhanced electrochromic switches and tunable green fluorescence based on terbium ion doped WO3 films

Multifunctional WO₃-based materials have been increasingly attracting attention due to their unique physical and electrochemical nature. In this work, the luminescent species, terbium (Tb) ions, were first successfully doped into WO₃ films by a hydrothermal method to incorporate their electrochromic and photoluminescent functions. The amorphous state and porous net structure are introduced, which can be attributed to the inhibited orientation growth caused by the occupation of Tb ions in the WO₃ lattice. Here, the optimal 13% Tb-WO₃ film exhibits enhanced electrochromic properties: a high transmittance modulation of 66.71%; a fast response speed of less than 10 s; an improved CE value of 48.33 cm² C^-1 at 680 nm; and cycling stability over 600 cycles without obvious degradation, arising from its larger active surface area. Meanwhile, its green-colored emission could be realized under 260 nm UV light and is electro-switchable upon applying voltage.