Photochromic Inorganic/Organic Thermoplastic Elastomers

Photochromic materials are an important class of “smart materials” and are broadly utilized in technological devices. However, most photochromic materials reported so far are composed of inorganic compounds that are challenging to process and suffer from poor mechanical performance, severely limiting their applications in various markets. In this paper, inorganic photochromic tungsten trioxide (WO₃) nanocrystals are conveniently grafted with polymers to hurdle the deficiency in processability and mechanical properties. This new type of photochromic material can be thermally processed into desired geometries like disks and dog‐bone specimens. Fully reversible photochromic response under UV light is also achieved for WO₃‐graft polymers, exhibiting tunable response rate, outperforming the pristine WO₃ nanocrystals. Notably, the resulted graft polymers show extraordinary mechanical performance with excellent ductility (≈800% breaking strain) and relatively high breaking strength (≈2 MPa). These discoveries elucidate an effective pathway to design smart inorganic/organic hybrid thermoplastic elastomers endowed with outstanding photochromic and mechanical properties as well as exceptional processability.     

Scalable Preparation of Photochromic Composite Foils with Excellent Reversibility for Light Printing

Photochromic inks for repeatable light‐printed media have attracted increasing attention owing to the fact that they may be widely applied to reduce the consumption of papers and plastics and conserve the environment. Therefore, it is of practical significance to develop convenient photochromic inks with a low cost and on a large scale. In this study, a simple one‐step hydrothermal route was used to prepare tungsten trioxide (WO₃) nanoparticles, which were further used to make photochromic inks and transparent photochromic films. The obtained transparent photochromic film could rapidly respond to UV light within tens of seconds, then return to its initial state, with different recovery times at different temperatures, and also exhibit good reversible coloration–bleaching effect. A typical polyethylene terephthalate (PET) foil coated with the photochromic ink could also be repeatedly printed with various patterns and displayed excellent rewritable performance over tens of cycles. This study proposes a simple method for widespread applications of WO₃‐based photochromic inks.     

Nanoparticles from the Vapor Phase: Synthesis and Characterization of Si, Ge, MoO3, and WO3Nanocrystals

In this paper, we present two studies of nanocrystals prepared by the pulsed laser vaporization–controlled condensation (LVCC) technique. In the first study, we present a comparison between the surface oxidation, morphology and the photoluminescence (PL) properties of Si and Ge nanocrystals. In the second study, we compare the photochromic properties of Mo and W oxides nanoparticles with the properties of the corresponding bulk materials. We also present evidence for a novel photoreduction of the white WO₃ nanoparticles into the blue W₂O₅ following the irradiation of the particles with the second harmonic of the Nd:YAG laser in air. These studies illustrate the novel properties of the nanoscale particles, which could lead to significant practical and technological applications.     

Detection of liquid petroleum gas using mixed nanosized tungsten oxide-based thick-film semiconductor sensor

The thick-film semiconductor sensor for liquid petroleum gas (LPG) detection was fabricated using a mixed WO₃-based sensor. We present the characterization of both their structural properties by means of XRD measurements and the electrical characteristics by using gas-sensing properties. The sensing characteristics such as sensitivity, working range, cross-sensitivity and response time were studied by using nanosized WO₃-based mixed with different metal oxides (SnO₂, TiO₂ and In₂O₃) and doped with noble metals (Au, Pd and Pt). The WO₃-based mixed with 5 wt.% In₂O₃ and 0.5 wt.% Pd showed the higher sensing characteristic at low concentration of LPG sensor at an operating temperature 225 °C.     

Synthesis and characterization of chemically grown electrochromic tungsten oxide

Tungsten oxide thin films, which are cathodic coloration materials that are used in electrochromic devices, were prepared by a chemical growth method and their electrochromic properties were investigated. The thin films of WO₃ were deposited onto electrically conducting substrates: fluorine doped tin oxide coated glass (FTO) with sheet resistance of about 10 Ω/cm. Transparent, uniform and strongly adherent thin film samples of WO₃ were studied for their structural, morphological, optical and electrochromic properties. The XRD data confirmed the monoclinic crystal structure of WO₃ thin films. The direct band gap Eg for the films was found to be 2.95 eV which is good for electrochromic device application. The electrochromism of WO₃ thin film was evaluated in 0.5 M LiClO₄/propylene carbonate for Li⁺ intercalation. Electrochromic properties of WO₃ thin films were studied with the help of Cyclic Voltammetry (CV), Chronoamperometry (CA) and Chronocoulometry (CC) techniques.     

Na0.33WO3溶胶及薄膜的制备与光致变色性质研究

无机光致变色材料在图像显示、光记录、信息存储和光转换方面有着巨大的潜在应用前景,引起了材料工作者的广泛重视[1￣3]。氧化钨是一种重要的无机光致变色材料,目前对氧化钨的研究多以无定     

Synthesis of WOn‐WX2 (n=2.7, 2.9; X=S,Se) Heterostructures for Highly Efficient Green Quantum Dot Light‐Emitting Diodes

Preparation of two‐dimensional (2D) heterostructures is important not only fundamentally, but also technologically for applications in electronics and optoelectronics. Herein, we report a facile colloidal method for the synthesis of WO_n ‐WX₂ (n =2.7, 2.9; X=S, Se) heterostructures by sulfurization or selenization of WO_n nanomaterials. The WO_n ‐WX₂ heterostructures are composed of WO_2.9 nanoparticles (NPs) or WO_2.7 nanowires (NWs) grown together with single‐ or few‐layer WX₂ nanosheets (NSs). As a proof‐of‐concept application, the WO_n ‐WX₂ heterostructures are used as the anode interfacial buffer layer for green quantum dot light‐emitting diodes (QLEDs). The QLED prepared with WO_2.9 NP‐WSe₂ NS heterostructures achieves external quantum efficiency (EQE) of 8.53 %. To our knowledge, this is the highest efficiency in the reported green QLEDs using inorganic materials as the hole injection layer.     

Synthesis of WOn‐WX2 (n=2.7, 2.9; X=S,Se) Heterostructures for Highly Efficient Green Quantum Dot Light‐Emitting Diodes

Preparation of two‐dimensional (2D) heterostructures is important not only fundamentally, but also technologically for applications in electronics and optoelectronics. Herein, we report a facile colloidal method for the synthesis of WO_n ‐WX₂ (n =2.7, 2.9; X=S, Se) heterostructures by sulfurization or selenization of WO_n nanomaterials. The WO_n ‐WX₂ heterostructures are composed of WO_2.9 nanoparticles (NPs) or WO_2.7 nanowires (NWs) grown together with single‐ or few‐layer WX₂ nanosheets (NSs). As a proof‐of‐concept application, the WO_n ‐WX₂ heterostructures are used as the anode interfacial buffer layer for green quantum dot light‐emitting diodes (QLEDs). The QLED prepared with WO_2.9 NP‐WSe₂ NS heterostructures achieves external quantum efficiency (EQE) of 8.53 %. To our knowledge, this is the highest efficiency in the reported green QLEDs using inorganic materials as the hole injection layer.     

溶胶-凝胶法制备纳米WO3气致变色材料

本文主要阐述了纳米WO₃气致变色材料的溶胶-凝胶制备方法、气致变色机理及其应用的研究新进展,重点评述了溶胶-凝胶法中各种制备条件诸如掺杂、模板剂、溶剂、热处理温度等对这种材料的结构和性能的影响,最后展望了纳米WO₃气致变色材料的研究和应用前景。我们认为,在未来的溶胶-凝胶法制备纳米WO₃气致变色材料领域,如何进行最优化的掺杂设计和选择高效模板剂、如何降低气体检测温度、以及气致变色机理等将是该项研究的重点。     

Solution Processing of Topochemically Converted Layered WO3 for Multifunctional Applications

Solution processing of nanomaterials is a promising technique for use in various applications owing to its simplicity and scalability. However, the studies on liquid-phase exfoliation (LPE) of tungsten oxide (WO₃) are limited, unlike others, by a lack of commercial availability of bulk WO₃ with layered structures. Herein, a one-step topochemical synthesis approach to obtain bulk layered WO₃ from commercially available layered tungsten disulfide (WS₂) by optimizing various parameters like reaction time and temperature is reported. Detailed microscopic and spectroscopic techniques confirmed the conversion process. Further, LPE was carried out on topochemically converted bulk layered WO₃ in 22 different solvents; among the solvents studied, the propan-2-ol/water (1 : 1) co-solvent system appeared to be the best. This indicates that the possible values of surface tension and Hansen solubility parameters for bulk WO₃ could be close to that of the co-solvent system. The obtained WO₃ dispersions in a low-boiling-point solvent enable thin films of various thickness to be fabricated by using spray coating. The obtained thin films were used as active materials in supercapacitors without any conductive additives/binders and exhibited an areal capacitance of 31.7 mF cm⁻² at 5 mV s⁻¹. Photo-electrochemical measurements revealed that these thin films can also be used as photoanodes for photo-electrochemical water oxidation.     

Detection of nitrogen dioxide using mixed tungsten oxide-based thick film semiconductor sensor

The thick film semiconductor sensor for NO₂ gas detection was fabricated by screen-printing method using a mixed WO₃-based as sensing material. The sensing characteristics, such as response time, response linearity, sensitivity, working range, cross sensitivity, and long-term stability were further studied by using a WO₃-based mixed with different metal oxides (SnO₂, TiO₂ and In₂O₃) and doped with noble metals (Au, Pd and Pt) as sensing materials was observed. The highest sensitivity for low concentrations (<16 mg l⁻¹) was observed using WO₃-based mixed with In₂O₃ or TiO₂. The NO₂ gas sensor showing the fastest response and recovery time (both within 2 min), good linearity (Y=0.606X+0.788 R²=0.991) for gas concentrations from 3 to 310 mg l⁻¹, low resistance (3 MΩ), high sensitivity, undesirable cross sensitivity effect and good long-term stability (at least 120 days) using WO₃-SnO₂-Au as sensing material.     

WO3/MO2 (M = Zr, Sn, Ti) heterogeneous acid catalysts: Synthesis, study, and use in cumene hydroperoxide decomposition

Thirty (5–40)% WO₃/MO₂ (M = Zr, Ti, Sn), heterogeneous acidic catalysts have been synthesized by two methods, specifically, via homogeneous acid solutions and from solutions brought to pH 9 with ammonia, both followed by calcination at 600–900°C. The catalysts have been characterized by IR spectroscopy and scanning electron microscopy, and their aqueous washings have been analyzed. Their acidity has been determined by the thermal analysis of samples containing adsorbed pyridine, and in terms of the proton affinity scale. Catalytic activities have been compared for cumene hydroperoxide (CHP) decomposition at 40°C in cumene and acetone. For all M, the catalysts are one type and contain W in strongly and weakly bound states, the latter being a polyoxometalate that can be washed off. Both tungstate phases are active in acid catalysis. Brønsted acid sites with a broad strength distribution have been found. The strongest of them are heteropolyacid protons. The catalysts 30% WO₃/SnO₂ and 20% WO₃/ZrO₂ (in acetone) and 10–20% WO₃/TiO₂ (in cumene) are the most active in CHP decomposition, and their activity is not related to their total acidity. Phases containing W⁶⁺ that form during the high-temperature synthesis are responsible for the high acidity, and additional protons that may appear owing to W⁶⁺ reduction can play only a minor role.     

Porous Tungsten Oxide: Recent Advances in Design,Synthesis, and Applications

Tungsten oxide (WO₃) has received ever more attention and has been highly researched over the last decade due to its being a low-cost transition metal semiconductor with tunable, yet widely stable, band gaps. This minireview briefly highlights the challenges in the design and synthesis of porous WO₃ including methods, precursors, solvent effects, crystal phases, and surface activities of the porous WO₃ base material. These topics are explored while also drawing a connection of how the morphology and crystal phase affect the band gap. The shifts in band gap not only impact the optical properties of tungsten but also allow tuning to operate on different energy levels, which makes WO₃ highly desirable in many applications such as supercapacitors, batteries, solar cells, catalysts, sensors, smart windows, and bioapplications.     

Understand the Degradation Mechanism of Electrochromic WO3 Films by Double‐step Chronoamperometry and Chronocoulometry Techniques Combined with in situ Spectroelectrochemical Study

In this paper, we demonstrate that the double‐step chronoamperometry and chronocoulometry techniques are efficient tools for characterizing the basic electrochromic performance of WO₃ films (i. e., coloration/bleaching conversion, charge capacity and coloration efficiency). In combination with in situ spectroelectrochemical study, the variations in the optical modulation and charge capacity of the WO₃ film under different potential windows were attributed to different ion diffusion depths and the quantity of WO₃ sites participating in the redox reaction. Moreover, the double‐step techniques have distinctive advantages for analyzing the cyclic mechanism of the WO₃ film. When the inserted Li⁺ ions in the coloration process cannot be completely extracted from the film in the next bleaching process, these ions accumulate in the film upon cycling, leading to the degradation of the electrochromic performance. Here the accumulated ions are referred to as “unrecoverable ions”. The abundant formation of unrecoverable ions may be due to incomplete reduction of a portion of Li_x WO₃, which is caused by collapse of the Li⁺ ion diffusion channels in the deep of film after repeated cycles. All these results support that the double‐step chronoamperometry and chronocoulometry techniques have significant advantages to analyze the cyclic stability and explore the degradation mechanism of electrochromic WO₃ films.     

TiO<Subscript>2</Subscript>/WO<Subscript>3</Subscript> photoanodes with enhanced photocatalytic activity for air treatment in a polymer electrolyte cell

The application of electrochemically enhanced photocatalysis in air treatment using a Nafion-based photoelectrochemical cell and TiO₂/WO₃ photoanodes for organic vapor photooxidation under both UV and visible light irradiation is briefly presented. In that direction, the obtained results regarding the preparation and characterization of the TiO₂/WO₃ photoanodes with enhanced photocatalytic activity are reviewed. Particular emphasis is given in the comparison of the photocatalytic behavior of bilayer TiO₂/WO₃ coatings, electrosynthesized on stainless steel mesh and powder C + mixed (WO₃ + TiO₂) photoanodes. The advantages of using a high surface area C + mixed (WO₃ + TiO₂) powder catalysts as photoanodes against their plain TiO₂ + C and WO₃ + C analogues are discussed.     

Electrochromic responses of low-temperature-annealed tungsten oxide thin films in contact with a liquid and a polymeric gel electrolyte

Thin films of tungsten trioxide (WO₃) for electrochromic application were synthesized by potentiostatic method by using a peroxytungstic acid as a solution precursor. The morphology of the films with and without postthermal annealing was analyzed by atomic force microscopy. When they were in contact with the liquid electrolyte (LiI in propylene carbonate, PC) and under alternatively applied negative (−1.5 V) and positive (+1.0 V) potentials, the transient optical transmittance modulations at wavelength of 650 nm of the as-deposited and 60 °C annealed WO₃ samples were higher than that of 100 °C annealed WO₃ films, and the switching times between the colored and bleached states were related to the surface morphology of the WO₃ films. In polymeric gel electrolyte (LiI and polymethyl methacrylate in PC) devices, longer time was required for complete coloration as well as bleaching process compared with the liquid one. A parametric analysis was made for each of the transient optical transmittance curves of WO₃-based electrochromic devices to extract the values of the response time in coloration (reduction) and bleaching (oxidation) processes. It concludes that the coloration process was determined by the exchange of current density at the electrolyte–WO₃ interface and a possible inhomogeneous interfacial potential for ion intercalation retards the effective coloration time. The bleaching process seems to be controlled by the space charge-limited lithium ion diffusion in WO₃ electrode and the ionic conductivity of the electrolyte as well.     

Polyoxometalate-Based Photochromic Supramolecular Hydrogels with Highly Ordered Spherical and Cylindrical Micellar Nanostructures

Polyoxometalates (POMs) have attracted much attention in the field of photochromic materials. However, POM-based photochromic supramolecular hydrogels with high transparency and good photochromic properties are seldom reported. In this work, a homogenous, optically transparent, injectable, and photochromic supramolecular hydrogel was fabricated through the coassembly of ammonium heptamolybdate (Mo₇) and an imidazolium-based zwitterionic amphiphile (3-(1-hexadecyl-3-imidazolio)propanesulfonate (C₁₆IPS)). The balance between electrostatic attraction and repulsion of Mo₇ clusters and zwitterionic amphiphiles enables them to coassemble into a homogenous and transparent supramolecular hydrogel. By adjusting the molar ratio of C₁₆IPS/Mo₇, ordered spherical micelle-based hydrogels and aligned wormlike micelle-based hydrogels can be obtained. The incorporation of Mo₇ into hydrogels endows these hydrogels with excellent photochromic properties. Specifically, after coassembly with C₁₆IPS, the photochromic ability of hydrogels is significantly enhanced compared with that of a pure aqueous solution of Mo₇. These hydrogels exhibit great potential applications as photochromic materials for the recording of rewritable information.     

Micelle‐Directing Synthesis of Ag‐Doped WO3 and MoO3 Composites for Photocatalytic Water Oxidation and Organic‐Dye Adsorption

In this paper, an Ag‐doped WO₃ (and MoO₃) composite has been prepared by following a simple micelle‐directed method and high‐temperature sintering route. The as‐prepared samples were characterized by X‐ray diffraction, inductively coupled plasma, transmission electron microscopy, X‐ray photoelectron spectroscopy, UV/Vis diffuse reflectance spectroscopy, Brunauer–Emmett–Teller, photoluminescence spectroscopy, and electrochemical impedance spectroscopy techniques. The photocatalytic experiments reveal that their oxygen‐production rates are up to 95.43 μmol (75.45 μmol) for Ag‐doped WO₃ (MoO₃), which is 9.5 (7.3) times higher than that of pure WO₃: 9.012 μmol (MoO₃: 9.00 μmol) under visible‐light illumination (λ ≥420 nm), respectively. The improvement of their photocatalytic activity is attributed to the enhancement of their visible‐light absorption and the separation efficiency of photogenerated carriers by Ag doping. Moreover, Ag‐doped WO₃ (MoO₃) also shows excellent adsorption of rhodamine B (RhB) and methylene blue (MB) in aqueous solution, with maximum adsorption capacities towards RhB and MB of 822 and 820 mg g⁻¹ for Ag‐doped WO₃, and 642 and 805 mg g⁻¹ for Ag‐doped MoO₃, respectively.     

Planarization-Induced Activation Wavelength Red-Shift and Thermal Half-Life Acceleration in Hydrazone Photoswitches

The optimization and modulation of the properties of photochromic compounds, such as their activation wavelengths and thermal relaxation half-lives (τ_1/2), are essential for their adaptation in various applications. In this work, we studied the effect of co-planarization of the rotary fragment of two photochromic hydrazones with the core of the molecule on their switching properties. The Z and E isomers of both compounds exhibit red-shifted absorption bands relative to their twisted versions, allowing for their photoswitching using longer wavelengths of light. Additionally, the thermal half-lives of both hydrazones are drastically shortened from hundreds of years to days.     

Recent progress of luminescent metal complexes with photochromic units

Photo-responsive molecules have been studied extensively because of their light irradiation abilities that enable modulation of certain physical and chemical properties in emerging molecular electronic and photonic devices. For advanced photonic applications, photochromic metal complexes that have photochromic units as the photo-responsive ligand are highly desirable, as they allow improvement of the photochromic properties and their photo-switching functionality. This article focuses on recent progress in luminescent metal complexes with photochromic units. Luminescence-switching properties of photochromic metal complexes depend on characteristic electronic transitions. The electronic transitions of photochromic metal complexes can be divided into three categories: (1) π–π* transition of the ligand, (2) metal to ligand charge transfer (MLCT) in transition-metal complex, and (3) f–f transition in lanthanide complex. Luminescence modulation using various metal complexes with photochromic units has been studied extensively in recent years, and various applications for future molecular switching devices are expected in the field of advanced photonics. Based on the literature and our studies on luminescent metal complexes with photochromic units, we report on the recent progress of luminescent metal complexes with photochromic units.