Review on electrochromic property for WO3 thin films using different deposition techniques

Raman Study of Novel Nanostructured WO3 Thin Films Grown by Spray Deposition

The present communication reports on the effect of the sprayed solution volume variation (as a thickness variation element) on the detailed Raman spectroscopy for WO3 thin films with different thicknesses grown from precursor solutions with two different concentrations. Walls-like structured monoclinic WO3 thin films were obtained by the spray deposition method for further integration in gas sensors. A detailed analysis of the two series of samples shows that the increase in thickness strongly affects the films' morphology, while their crystalline structure is only slightly affected. The Raman analysis contributes to refining the structural feature clarifications. It was observed that, for 0.05 M precursor concentration series, thinner films (lower volume) show less intense peaks, indicating more defects and lower crystallinity, while thicker films (higher volume) exhibit sharper and more intense peaks, suggesting improved crystallinity and structural order. For higher precursor concentration 0.1 M series, films at higher precursor concentrations show overall more intense and sharper peaks across all thicknesses, indicating higher crystallinity and fewer defects. Differences in peak intensity and presence reflect variations in film morphology and structural properties due to increased precursor concentration. Further studies are ongoing.

Innovations in WO3 gas sensors: Nanostructure engineering, functionalization, and future perspectives

This review critically examines the progress and challenges in the field of nanostructured tungsten oxide (WO3) gas sensors. It delves into the significant advancements achieved through nanostructuring and composite formation of WO3, which have markedly improved sensor sensitivity for gases like NO2, NH3, and VOCs, achieving detection limits in the ppb range. The review systematically explores various innovative approaches, such as doping WO3 with transition metals, creating heterojunctions with materials like CuO and graphene, and employing machine learning models to optimize sensor configurations. The challenges facing WO3 sensors are also thoroughly examined. Key issues include cross-sensitivity to different gases, particularly at higher temperatures, and long-term stability affected by factors like grain growth and volatility of dopants. The review assesses potential solutions to these challenges, including statistical analysis of sensor arrays, surface functionalization, and the use of novel nanostructures for enhanced performance and selectivity. In addition, the review discusses the impact of ambient humidity on sensor performance and the current strategies to mitigate it, such as composite materials with humidity shielding effects and surface functionalization with hydrophobic groups. The need for high operating temperatures, leading to higher power consumption, is also addressed, along with possible solutions like the use of advanced materials and new transduction principles to lower temperature requirements. The review concludes by highlighting the necessity for a multidisciplinary approach in future research. This approach should combine materials synthesis, device engineering, and data science to develop the next generation of WO3 sensors with enhanced sensitivity, ultrafast response rates, and improved portability. The integration of machine learning and IoT connectivity is posited as a key driver for new applications in areas like personal exposure monitoring, wearable diagnostics, and smart city networks, underlining WO3's potential as a robust gas sensing material in future technological advancements.

Fabrication of Tungsten Oxide Nanowalls through HFCVD for Improved Electrochemical Detection of Methylamine

In this study, well-defined tungsten oxide (WO3) nanowall (NW) thin films were synthesized via a controlled hot filament chemical vapor deposition (HFCVD) technique and applied for electrochemical detection of methylamine toxic substances. Herein, for the thin-film growth by HFCVD, the temperature of tungsten (W) wire was held constant at ~1450 °C and gasification was performed by heating of W wire using varied substrate temperatures ranging from 350 °C to 450 °C. At an optimized growth temperature of 400 °C, well-defined and extremely dense WO3 nanowall-like structures were developed on a Si substrate. Structural, crystallographic, and compositional characterizations confirmed that the deposited WO3 thin films possessed monoclinic crystal structures of high crystal quality. For electrochemical sensing applications, WO3 NW thin film was used as an electrode, and cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were measured with a wide concentration range of 20 μM~1 mM of methylamine. The fabricated electrochemical sensor achieved a sensitivity of ~183.65 μA mM-1 cm-2, a limit of detection (LOD) of ~20 μM and a quick response time of 10 s. Thus, the fabricated electrochemical sensor exhibited promising detection of methylamine with considerable stability and reproducibility.

Dual-Functional Electrochromic Smart Window Using WO3·H2O-rGO Nanocomposite Ink Spray-Coated on a Low-Cost Hybrid Electrode

Electrochromic windows have gained growing interest for their ability to change their optical state in the visible and NIR ranges with minimal input power, making them energy-efficient. However, material processing costs, fabrication complexity, and poor electrochromic properties can be barriers to the widespread adoption of this technology. To address these issues, electrochromic material and fabrication processes are designed to realize their potential as a cost-effective and energy-efficient technology. In this work, an electrochromic composite material-based ink is synthesized consisting of WO3·H2O nanoplates supported on rGO (reduced graphene oxide) nanosheets (WH-rGO), wherein an optimized amount of rGO (0.05 to 0.5 wt %) is introduced for providing a higher conduction pathway for efficient charge transport without sacrificing the electrochromic performance of WO3·H2O nanoplates. The stable ink dispersion prepared in the study is deposited by spray coating on transparent conducting electrodes over large areas (25 cm2). The WH-rGO nanocomposite (0.4 wt %) results in 43% optical modulation at 700 nm, with bleaching and coloration times of 6 and 8 s, respectively. Interestingly, the device also possesses an electrochemical energy storage capability with an areal capacitance of 16.3 mF/cm2. The electrochromic composite material is successfully translated on tin doped indium oxide (ITO)-coated Al metal mesh hybrid electrodes (T = 80%, Rs = 40 Ω/□) to replace ITO. Finally, an electrochromic device of 5 × 5 cm2 is fabricated by spray-coating the ink on cost-effective ITO/Al-mesh hybrid electrodes. The device displays blue to colorless modulation with an excellent bleaching time of 0.43 s and a coloration time of 2.16 s, making it one among the fast-operating devices fabricated by complete solution processing. This work showcases the economical production of a dual-function electrochromic device, which can be a feasible option as an alternative to existing ITO-based devices in both automotive and infrastructure applications.

Multicolor Tunable Electrochromic Materials Based on the Burstein-Moss Effect

Inorganic electrochromic (EC) materials, which can reversibly switch their optical properties by current or potential, are at the forefront of commercialization of displays and smart windows. However, most inorganic EC materials have challenges in achieving multicolor tunability. Here, we propose that the Burstein-Moss (BM) effect, which could widen the optical gap by carrier density, could be a potential mechanism to realize the multicolor tunable EC phenomenon. Degenerated semiconductors with suitable fundament band gaps and effective carrier masses could be potential candidates for multicolor tunable EC materials based on the BM effect. We select bulk Y₂CF₂ as an example to illustrate multicolor tunability based on the BM effect. In addition to multicolor tunability, the BM effect also could endow EC devices with the ability to selectively modulate the absorption for near infrared and visible light, but with a simpler device structure. Thus, we believe that this mechanism could be applied to design novel EC smart windows with unprecedented functions.

Enhanced Coloration Time of Electrochromic Device Using Integrated WO3@PEO Electrodes for Wearable Devices

Electrochromic technologies that exhibit low power consumption have been spotlighted recently. In particular, with the recent increase in demand for paper-like panel displays, faster coloration time has been focused on in researching electrochromic devices. Tungsten trioxide (WO₃) has been widely used as an electrochromic material that exhibits excellent electrochromic performance with high thermal and mechanical stability. However, in a solid film-type WO₃ layer, the coloration time was long due to its limited surface area and long diffusion paths of lithium ions (Li-ions). In this study, we attempted to fabricate a fibrous structure of WO₃@poly(ethylene oxide) (PEO) composites through electrospinning. The fibrous and porous layer showed a faster coloration time due to a short Li-ion diffusion path. Additionally, PEO in fibers supports Li-ions being quickly transported into the WO₃ particles through their high ionic conductivity. The optimized WO₃@PEO fibrous structure showed 61.3 cm²/C of high coloration efficiency, 1.6s fast coloration time, and good cycle stability. Lastly, the electrochromic device was successfully fabricated on fabric using gel electrolytes and a conductive knitted fabric as a substrate and showed a comparable color change through a voltage change from -2.5 V to 1.5 V.

Towards an All-Solid-State Electrochromic Device: A Review of Solid-State Electrolytes and the Way Forward

In order to curb high electricity usage, especially in commercial buildings, smart windows, also known as “switchable” or “smart” glasses, have attracted a significant amount of attention in an effort to achieve energy savings in eco-friendly buildings and transportation systems. Smart windows save energy by regulating the input of solar heat and light and hence cutting down air-conditioning expenses, while maintaining indoor comfort. This is achieved by electrochromism, which is defined as the reversible colour change in electrochromic (EC) materials from transparent to dark blue and vice versa under a small applied voltage. Recent state-of-the-art electrochromic devices (ECD) adopt liquid-based electrolytes as the main source of energy for basic operations. While this has resulted in much success in ECDs as reported in past studies, there remain several drawbacks to this aspect, such as liquid electrolyte leakage and evaporation, not to mention safety concerns related to the harmful nature of electrolyte materials. This paper aims to review the recent advances in various solid electrolytes that are potential solutions to the mentioned problems.

An Electrochromic Ag-Decorated WO3-x Film with Adjustable Defect States for Electrochemical Surface-Enhanced Raman Spectroscopy

Electrochemical surface-enhanced Raman scattering (EC-SERS) spectroscopy is an ultrasensitive spectro-electrochemistry technique that provides mechanistic and dynamic information on electrochemical interfaces at the molecular level. However, the plasmon-mediated photocatalysis hinders the intrinsic electrochemical behavior of molecules at electrochemical interfaces. This work aimed to develop a facile method for constructing a reliable EC-SERS substrate that can be used to study the molecular dynamics at electrochemical interfaces. Herein, a novel Ag-WO_3-x electrochromic heterostructure was synthesized for EC-SERS. Especially, the use of electrochromic WO_3-x film suppresses the influence of hot-electrons-induced catalysis while offering a reliable SERS effect. Based on this finding, the real electrochemical behavior of p-aminothiophenol (PATP) on Ag nanoparticles (NPs) surface was revealed for the first time. We are confident that metal-semiconductor electrochromic heterostructures could be developed into reliable substrates for EC-SERS analysis. Furthermore, the results obtained in this work provide new insights not only into the chemical mechanism of SERS, but also into the hot-electron transfer mechanism in metal-semiconductor heterostructures.

WO3 Films Grown by Spray Pyrolysis for Smart Windows Applications

WO3 nanostructured thin films were grown using spray deposition on FTO coated glass. The effect of the precursor concentration and the solution quantity, which determines the deposition time, on the electrochemical, electrochromic and optical properties of the WO3 films was investigated. The films were found to exhibit a good electrochromic activity with a reasonably good durability of charge exchange and optical modulation under harsh electrochemical cycling in Li-ion-conducting electrolyte. Associated compositional and structural characteristics were probed by several techniques, indicating that the observed improved durability may be due to the unique WO3 thin films’ structuring, the surface of the films consisting of wall-like structures combined with bubble-like islands on a polycrystalline WO3 granular background, that requires further study in greater detail.

A Reversible Moisture-Responsive Plasmonic Color-Raman and Transmittance Modulation Device by Dispersing Hyaluronan-Functionalized Ag into Nanofibers

Plasmonic physical color generation, which mostly depends on selective absorption, creates unique colors by light transmission and scattering. Based on this, regulating plasmon and transparency with external stimulation is a promising approach for fabricating optical devices with enhanced visual displays; however, few studies have addressed the implementation of dual-optical modulation. In addition, developing a color response to environmental stimuli through the highly shape-sensitive plasmon depth modulation has long remained a significant challenge once the nanostructure is determined. Some stimulations also require high amounts of electricity, which can be costly. In this study, strategically designed hyaluronan-functionalized triangular silver nanoparticles (AgNPs) were embedded in polyvinyl alcohol-polyethylene nanofiber films to achieve a breakthrough in the moisture-responsive dual-optical modulation of the plasmonic color-Raman and transparency. Switchable colors that are reversible were induced in plasmon-resonance-modulation AgNPs via moisture stimulation, adjusting the expansion-tunable dielectric constant of hyaluronan-functionalized AgNPs and varying the electron density due to electron transfer. Furthermore, a moisture gradient was used to decrease the Raman scattering and increase the photoluminescence, which is a significant demonstration of smart-plasmonic evolution. This effect occurred due to the gradual transition from plasmon-driven photoluminescence quenching to photoluminescence enhancement as the interval of the Ag and hyaluronic acid molecules was increased. The transparency of the composite film was also dynamically regulated by turning moisture on/off. This occurred because of the significant difference in hygroscopic expansion between hyaluronan and the nanofibers, which generated a large variation in the total refractive index and caused changes in the surface roughness.

Recent advances in development of nanostructured photodetectors from ultraviolet to infrared region: A review

Herein, we aim to evaluate the photodetector performance of various nanostructured materials (thin films, 2-D nanolayers, 1-D nanowires, and 0-D quantum dots) in ultraviolet (UV), visible, and infrared (IR) regions. Specifically, semiconductor-based metal oxides such as ZnO, Ga₂O₃, SnO₂, TiO₂, and WO₃ are the majority preferred materials for UV photodetection due to their broad band gap, stability, and relatively simple fabrication processes. Whereas, the graphene-based hetero- and nano-structured composites are considered as prominent visible light active photodetectors. Interestingly, graphene exhibits broad band spectral absorption and ultra-high mobility, which derives graphene as a suitable candidate for visible detector. Further, due to the very low absorption rate of graphene (2%), various materials have been integrated with graphene (rGO-CZS, PQD-rGO, N-SLG, and GO doped PbI₂). In the case of IR photodetectors, quantum dot IR detectors prevails significant advantage over the quantum well IR detectors due to the 0-D quantum confinement and ability to absorb the light with any polarization. In such a way, we discussed the most recent developments on IR detectors using InAs and PbS quantum dot nanostructures. Overall, this review gives clear view on the development of suitable device architecture under prominent nanostructures to tune the photodetector performance from UV to IR spectral regions for wide-band photodetectors.

Photocatalytic Nanofiber Membranes for the Degradation of Micropollutants and Their Antimicrobial Activity: Recent Advances and Future Prospects

This review paper systematically evaluates current progress on the development and performance of photocatalytic nanofiber membranes often used in the removal of micropollutants from water systems. It is demonstrated that nanofiber membranes serve as excellent support materials for photocatalytic nanoparticles, leading to nanofiber membranes with enhanced optical properties, as well as improved recovery, recyclability, and reusability. The tremendous performance of photocatalytic membranes is attributed to the photogenerated reactive oxygen species such as hydroxyl radicals, singlet oxygen, and superoxide anion radicals introduced by catalytic nanoparticles such as TiO₂ and ZnO upon light irradiation. Hydroxyl radicals are the most reactive species responsible for most of the photodegradation processes of these unwanted pollutants. The review also demonstrates that self-cleaning and antimicrobial nanofiber membranes are useful in the removal of microbial species in water. These unique materials are also applicable in other fields such as wound dressing since the membrane allows for oxygen flow in wounds to heal while antimicrobial agents protect wounds against infections. It is demonstrated that antimicrobial activities against bacteria and photocatalytic degradation of micropollutants significantly reduce membrane fouling. Therefore, the review demonstrates that electrospun photocatalytic nanofiber membranes with antimicrobial activity form efficient cost-effective multifunctional composite materials for the removal of unwanted species in water and for use in various other applications such as filtration, adsorption and electrocatalysis.

Advances in Electrochemical Energy Devices Constructed with Tungsten Oxide-Based Nanomaterials

Tungsten oxide-based materials have drawn huge attention for their versatile uses to construct various energy storage devices. Particularly, their electrochromic devices and optically-changing devices are intensively studied in terms of energy-saving. Furthermore, based on close connections in the forms of device structure and working mechanisms between these two main applications, bifunctional devices of tungsten oxide-based materials with energy storage and optical change came into our view, and when solar cells are integrated, multifunctional devices are accessible. In this article, we have reviewed the latest developments of tungsten oxide-based nanostructured materials in various kinds of applications, and our focus falls on their energy-related uses, especially supercapacitors, lithium ion batteries, electrochromic devices, and their bifunctional and multifunctional devices. Additionally, other applications such as photochromic devices, sensors, and photocatalysts of tungsten oxide-based materials have also been mentioned. We hope this article can shed light on the related applications of tungsten oxide-based materials and inspire new possibilities for further uses.

Guiding synthetic targets of anodically coloring electrochromes through density functional theory

Electrochromic devices offer many technological applications, including flexible displays, dimmable mirrors, and energy-efficient windows. Additionally, adsorbing electrochromic molecular assemblies onto mesoporous metal-oxide surfaces facilitates commercial and manufacturing potential (i.e., screen-printing and/or roll-to-roll processing). These systems also demonstrate synthetic versatility, thus making a wide array of colors accessible. In this work, using Time-Dependent Density Functional Theory (TD-DFT), we investigated ten different bi-aryl type molecules of 3,4-ethylendioxythiophene (EDOT) conjugated to various phenyl derivatives as potential anodically coloring electrochromes (ACEs). The non-substituted phenylene, hexylthiol-EDOT-phenyl-phosphonic acid, PA1, was synthesized and characterized as a means of model validity. PA1 absorbs in the UV region in its neutral state and upon oxidation absorbs within the visible, hence showcasing its potential as an ACE chromophore. The properties of PA1 inspired the designs of the other nine structural derivatives where the number and position of methoxy groups on the phenylene were varied. Using our DFT treatment, we assessed the impact of these modifications on the electronic structures, geometries, and excited-state properties. In particular, we examined stabilization intermolecular interactions (S-O and O-H) as they aid in molecule planarization, thus facilitating charge transport properties in devices. Additionally, destabilizing O-O forces were observed, thereby making some chromophores less desirable. A detailed excited state analysis was performed, which linked the simulated UV-Vis spectra to the dominant excited state transitions and their corresponding molecular orbitals. Based on these results, the nine chromophores were ranked ergo providing an ordered list of synthetic targets.

Boosting Transport Kinetics of Ions and Electrons Simultaneously by Ti3C2Tx (MXene) Addition for Enhanced Electrochromic Performance

Electrochromic technology plays a significant role in energy conservation, while its performance is greatly limited by the transport behavior of ions and electrons. Hence, an electrochromic system with overall excellent performances still need to be explored. Initially motivated by the high ionic and electronic conductivity of transition metal carbide or nitride (MXene), we design a feasible procedure to synthesize the MXene/WO_3-x composite electrochromic film. The consequently boosted electrochromic performances prove that the addition of MXene is an effective strategy for simultaneously enhancing electrons and ions transport behavior in electrochromic layer. The MXene/WO_3-x electrochromic device exhibits enhanced transmittance modulation and coloration efficiency (60.4%, 69.1 cm² C^-1), higher diffusion coefficient of Li⁺ and excellent cycling stability (200 cycles) over the pure WO_3-x device. Meanwhile, numerical stimulation theoretically explores the mechanism and kinetics of the lithium ion diffusion, and proves the spatial and time distributions of higher Li⁺ concentration in MXene/WO_3-x composite electrochromic layer. Both experiments and theoretical data reveal that the addition of MXene is effective to promote the transport kinetics of ions and electrons simultaneously and thus realizing a high-performance electrochromic device. This work opens new avenues for electrochromic materials design and deepens the study of kinetics mechanism of ion diffusion in electrochromic devices.

Full-Color-Tunable Nanophotonic Device Using Electrochromic Tungsten Trioxide Thin Film

Color generation based on strategically designed plasmonic nanostructures is a promising approach for display applications with unprecedented high-resolution. However, it is disadvantageous in that the optical response is fixed once the structure is determined. Therefore, obtaining high modulation depth with reversible optical properties while maintaining its fixed nanostructure is a great challenge in nanophotonics. In this work, dynamic color tuning and switching using tungsten trioxide (WO₃), a representative electrochromic material, are demonstrated with reflection-type and transmission-type optical devices. Thin WO₃ films incorporated in simple stacked configurations undergo dynamic color change by the adjustment of their dielectric constant through the electrochromic principle. A large resonance wavelength shift up to 107 nm under an electrochemical bias of 3.2 V could be achieved by the reflection-type device. For the transmission-type device, on/off switchable color pixels with improved purity are demonstrated of which transmittance is modulated by up to 4.04:1.

Optimized oxygen deprived low temperature sputtered WO3 thin films for crystalline structures

We report a detailed analysis on the effects of processing parameters for sputtered tungsten trioxide (WO₃) thin nanoscale films on their structural, vibrational and electrical properties. The research aims to understand the fundamental aspects of WO₃ sputtering at relatively low temperatures and in an oxygen deprived environment targeting applications of temperature and oxygen sensitive substrates. Structural analysis indicates that films deposited at room temperature, or substrate temperatures at or below 400 °C with low oxygen partial pressure are amorphous. Crystallization of the films was observed with distinct Raman peaks when the films were annealed at 300 °C or above using rapid thermal annealing for 10 min. Films revealed monoclinic phases of WO₃ with the presence of W-O-W stretching, bending and lattice vibrational modes in the Raman spectra. Interestingly, a change of transport behavior from insulating to semiconducting was observed for as deposited films on post annealing. Annealed films revealed stoichiometric WO₃ phases with no external defects detected. The present study adopts a route to intercalate WO₃ in a variety of applications from electrochromic coloration to a nanocrystalline thin film for electronic devices sensitive to higher temperatures and gas flow in the sputtering system.

Study of the Thermal Annealing on Structural and Morphological Properties of High-Porosity A-WO3 Films Synthesized by HFCVD

High-porosity nanostructured amorphous tungsten OXIDE (a-WO₃) films were synthesized by a Hot Filament Chemical Vapor Deposition technique (HFCVD) and then transformed into a crystalline WO₃ by simple thermal annealing. The a-WO₃ films were annealed at 100, 300, and 500 °C for 10 min in an air environment. The films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), micro-Raman spectroscopy, high-resolution transmission electron microscopy (HR-TEM), and UV–vis spectroscopy. Results revealed that the a-WO₃ films were highly porous, composed of cauliflower-like structures made of nanoparticles with average sizes of 12 nm. It was shown that the effect of annealing on the morphology of the a-WO₃ films leads to a sintering process. However, the morphology is conserved. It was found that at annealing temperatures of 100 °C, the a-WO₃ films are of an amorphous nature, while at 300 °C, the films crystallize in the monoclinic phase of WO₃. The calculated bandgap for the a-WO₃ was 3.09 eV, and 2.53 eV for the film annealed at 500 °C. Finally, the results show that porous WO₃ films preserve the morphology and maintain the porosity, even after the annealing at 500 °C.

Electrochromic Sensors Based on Conducting Polymers, Metal Oxides, and Coordination Complexes

Electrochromic sensors offer multi-mode registration of analytical signal based on combination of electrochemical and optical techniques. This emerging direction of analytical chemistry is relatively new; therefore, it has very high potential for various applications in chemical and biochemical analysis. Properties of sensors based on various electrochromic materials such as polymers, polymer derivatives, polymer composites, metal oxides, metal oxide complexes, phthalocyanines, porphyrins, and dyes are critically overviewed, evaluated, and compared. The most promising directions in analytical application of electrochromic polymers are highlighted.

Matrix Assisted and/or Laser Desorption Ionization Quadrupole Ion Trap Time-of-Flight Mass Spectrometry of WO3 Clusters Formation in Gas Phase. Nanodiamonds, Fullerene, and Graphene Oxide Matrices

The formation of W _x O _y^+●/-● clusters in the gas phase was studied by laser desorption ionization (LDI) and matrix assisted laser desorption ionization (MALDI) of solid WO₃. LDI produced (WO₃) _n^{+ ●/- ●} (n = 1-7) clusters. In MALDI, when using nano-diamonds (NDs), graphene oxide (GO), or fullerene (C₆₀) matrices, higher mass clusters were generated. In addition to (WO₃) _n^-● clusters, oxygen-rich or -deficient species were found in both LDI and MALDI (with the total number of clusters exceeding one hundred ≈ 137). This is the first time that such matrices have been used for the generation of(WO₃) _n^+●/-● clusters in the gas phase, while new high mass clusters (WO₃) _n^-● (n = 12-19) were also detected. Graphical Abstract.